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GEOSAT altimeter observations of the surface circulation of the Southern Ocean
Abstract
Using Geosat altimeter data for 26 months from November 1986 to December 1988 and a newly developed technique for the analysis of height data, the variability of the sea level and the surface geostrophic currents in the Southern Ocean is investigated. The processed Geosat data are used to examine the relationship between the mesoscale variability and the values of mean circulation, determined from historical hydrographic data. It is shown that the geographical patterns of both the mean flow and the mesoscale variability are correlated. An efficient objective-analysis algorithm for generating smoothed fields from observations randomly distributed in time and two space dimensions is developed and applied to 26 months of Geosat data. The smoothed fields are then used to investigate the large-scale low-frequency variability of the sea level and the surface geostrophic velocity in the Southern Ocean, in order to identify the mode of the observed variations.
... The separation of the Brazil Current from the coast is observed at about 38° S, then the southern boundary of the South Atlantic subtropical gyre is established. This area also oscillates in latitude and depending on how intense the Malvinas Current is; higher intensity, lower latitude [5][6][7][8]. ...
... The BMC has been studied by means of classical oceanographic surveys [3,4], by Ge-oSat altimetry data [5], and by satellite sea surface imagery [13,15]. There are well-known global models and local inverse models [3,4], all of them based on experimental data. ...
... The separation of the Brazil Current from the coast is observed at about 38 • S, then the southern boundary of the South Atlantic subtropical gyre is established. This area also oscillates in latitude and depending on how intense the Malvinas Current is; higher intensity, lower latitude [5][6][7][8]. ...
The Brazil-Malvinas Confluence (BMC) is one of the most complex oceanic areas in the Earth's oceans and the prediction of high frequency structures tends to fail. The authors studied the BMC using Multiscale Ultrahigh Resolution (MUR) imagery for the Sea Surface Temperature (SST) to address why the predictions are not as good as expected. The studies were carried out by means of two approaches. The first approach is the non-linear fitting of a harmonic model keeping the frequencies as parameters pixel by pixel. The second approach is from fractal geometry. The three first q-order Rényi dimensions were computed. At the same time, an inverse fractal interpolation was carried out to compute the contraction factor. Both of them are related to the chaotic behavior of nature. This work has three relevant contributions. The correlation between the harmonic models and the SST data is quite poor in general, implying the low harmonicity, and low harmonic predictability, of the pixel-by-pixel time series. It is verified that the quasi-annual and quasi-semiannual waves have periods of about 420 and 210 days, respectively. The second one is the confirmation of the high complexity of the BMC area because the three Rényi dimensions are equal. This has the strong finding of the monofractality of the dynamic of the SST in the BMC. Finally, the contraction factor, one of the parameters of the fractal interpolation, is relatively high, implying the presence of highly complex internal structures in the SST temporal evolution.
... From the sea surface height variability it is possible to calculate parameters such as the eddy kinetic energy of the oceans, statistics which can be compared with model output (Colton and Chase, 1983) or with other data sets such as that of Wyrtki et al. (1976). Similar studies have been carried out on smaller areas such as the Gulf of Mexico (Cheney and Marsh, 1981b;Robinson et al., 1983) and on the Antarctic Circumpolar Current (Colton and Chase, 1983;Chelton et al., 1990). ...
... The reduced spatial resolution of the Seasat data (600 km track separation) also results in a somewhat smoothed version of the variability field. Chelton et al. (1990) use two years of the Geosat data in an along-track technique and obtain results much closer to those of the present study. Their peak variability is 6 cm lower than the 51 cm found here but this may be due to their regridding of data to a 2° grid with some associated smoothing. ...
... The main areas of high variability are coincident with the East Madagascar Current, the Agulhas Current and the Agulhas Return Current, with maximum variability in the Agulhas Retroflection area. Chelton et al. (1990) compare Geosat variability for the whole Southern Ocean with dynamic heights from Gordon and Molinelli (1982) and show that variability maxima generally occur along the cores of major currents. The exact relationship of the sea surface height variability observed by Geosat to the current systems is dependant on the system dynamics. ...
p>The aims of the project are outlined and the satellite altimeter is introduced as a new method for the physical oceanographer to study ocean dynamics. The principles of the satellite altimeter are explained and a description of the sources of error found in the data is given, together with two possible methods for removal of residual errors. The principles of primitive equation ocean circulation models are outlined and the Fine Resolution Antarctic Model (FRAM) is introduced. Methods of hydrographic data collection are discussed and historical data on the circulation of the Southern Ocean are reviewed. The methods for Geosat data extraction, quality control and error correction used in the study are discussed. Data processed by collinear techniques have been used to obtain height profiles along tracks in two regions of the Southern Ocean; South of Africa and in the Central South Pacific. Mesoscale variability fields calculated from the along track altimeter data are presented and explained in terms of historical in situ current data and bottom topography. Geostrophic current speeds calculated from the residual height profiles yield velocity anomalies of the order of 20 cm s"1 for the Antarctic Circumpolar Current (ACC) and up to 1.38 m s1 for the Agulhas Current System, similar to values obtained by in situ measurements. Autocorrelation function calculations show that the characteristic length scales of height anomalies in the ACC are smaller than those of the Agulhas Current System. The FRAM data set is used as a source of artificial sea surface heights to give model altimeter data which are processed and analyzed for the Geosat study regions. The resultant variability fields are compared with the Geosat results. The effects of Geosat's sampling strategy and altimeter processing on the FRAM data are analyzed and are given as an indication of the possible reliability of Geosat variability fields. The results of feature recognition and autocorrelation function analyses are compared to those from Geosat both as a verification for the FRAM mesoscale circulation and as an aid to altimeter data interpretation.</p
... Two parameters of large importance to ocean circulation, and measurable by satellite, are sea surface height (e.g. Chelton et al., 1990) (SSH) and SST (e.g. Olson et al., 1988). ...
... (1991) also show that the use of weighted least squares method (where the weighting is the reciprocal of the mesoscale variability) gives more accurate results than that of the conventional least squares method. Chelton et al. (1990) in a study of the Southern Ocean, use a collinear analysis. ...
... The surface can be any specified function, but is typically a linear or quadratic surface (e.g. Chelton et al., 1990). ...
Satellites provide a unique semi-synoptic view of the world's oceans. In recent years, two forms of remotely sensed data have been particularly useful in providing information about ocean circulation, namely altimetric measurements of sea surface height (SSH) and infrared radiometric measurements of sea surface temperature (SST). However, in order to interpret new types of data correctly and obtain meaningful results, new techniques must be developed. In this thesis, techniques to process TOPEX/POSEIDON radar altimeter SSH data and Along-Track Scanning Radiometer (ATSR) SST data are developed. These techniques are tested in the South Atlantic Ocean. The effectiveness of an existing technique to correct for across-track variations in altimeter sampling and the associated SSH errors due to across-track mean sea surface variation is studied. The effects of orbit error removal and interpolation on altimeter data are investigated using ocean model data from the Parallel Ocean Climate Model (POCM). A technique to obtain absolute velocities from altimetry alone is implemented and its accuracy assessed through use of the POCM data. Remnant cloud contamination in the ATSR 0.5° night SST data is discovered and a new technique to remove the cloud contamination is proposed and tested. The seasonality of this cloud contamination is investigated and is found to coincide with the occurrence of marine stratiform clouds. Finally, the relationship between SST and SSH data is examined. It is found that spatial cross-correlations between SST and SSH are surprisingly high (~0.7) in regions associated with fronts and mesoscale variability such as the Agulhas, the Antarctic Circumpolar Current and the Brazil/Falkland regions. In these areas, coherency analysis reveals that the cross-correlations peak at wavelengths of 400-600 km. The strength of the cross-correlations is found to be seasonal, peaking in the winter and minimising in summer.
... The BMC is also one of the most energetic oceanic areas with intensified instability (Chelton et al. 1990) due to eddy-meandering activities reported in several studies (Campagna et al. 2006, Chelton et al. 1990). According to Karabashev & Evdoshenko (2018), mesoscale activity occurs in the region due to the interaction of currents, the influence of the slope, and the effect of oceanic islands on the MC. ...
... The BMC is also one of the most energetic oceanic areas with intensified instability (Chelton et al. 1990) due to eddy-meandering activities reported in several studies (Campagna et al. 2006, Chelton et al. 1990). According to Karabashev & Evdoshenko (2018), mesoscale activity occurs in the region due to the interaction of currents, the influence of the slope, and the effect of oceanic islands on the MC. ...
The Brazil-Malvinas Confluence (BMC) is a significant biological frontier where distinct currents meet, fostering optimal conditions for phytoplankton development. In this study we tested the hypothesis that eddys promote an increase in phytoplankton biomass at the Brazil-Malvinas Confluence (BMC), altering species diversity. Phytoplankton were collected with Niskin bottles and nutrient concentrations assessed at two depths (Surface and Deep Chlorophyll Maximum Layer - DCML) in areas outside and under the influence of Cold-Core (CCE) and Warm-Core (WCE) Eddies. Environmental variables were determined in situ using a CTD profiler. Four regions were separated based on environmental variables and phytoplankton species, namely, the Brazil Current (BC), Malvinas Current (MC), CCE, and WCE. Species diversity was higher in the eddies. The conditions of the WCE were different from those of the CCE, with low temperature and salinity and high cell density values in the latter. The phylum Bacillariophyta was predominant in terms of species richness in all regions and was responsible for the higher cell density in the MC, while dinoflagellates were dominant in the BC and eddies. Therefore, eddy activity alters the structure, diversity and biomass of the phytoplankton community in the BMC.
... In this area, referred to as the Brazil/Malvinas Confluence, both flows veer offshore and form large amplitude meanders and eddies (Fig. 1). Steep gradients in salinity, temperature and dissolved nutrients characterize the Confluence, which is one of the most energetic regions in the world ocean (Olson et al., 1988;Chelton et al., 1990). ...
... A few stations from the Northern Offshore fall in the region dominated by the Southern Offshore ecoregion (Fig. 3). These stations may represent sporadic southward penetrations of the Brazil Current which frequently develop into large mesoscale eddies that characterize the Brazil/Malvinas Confluence (e.g., Chelton et al., 1990;Gordon, 1981). The Subtropical Front separates the Subantarctic Water and the Subtropical Water masses. ...
... In this area, referred to as the Brazil/Malvinas Confluence, both flows veer offshore and form large amplitude meanders and eddies (Fig. 1). Steep gradients in salinity, temperature and dissolved nutrients characterize the Confluence, which is one of the most energetic regions in the world ocean (Olson et al., 1988;Chelton et al., 1990). ...
... A few stations from the Northern Offshore fall in the region dominated by the Southern Offshore ecoregion (Fig. 3). These stations may represent sporadic southward penetrations of the Brazil Current which frequently develop into large mesoscale eddies that characterize the Brazil/Malvinas Confluence (e.g., Chelton et al., 1990;Gordon, 1981). The Subtropical Front separates the Subantarctic Water and the Subtropical Water masses. ...
We compiled an extensive dataset of copepod presence/absence data to define ecoregions in the southwestern South Atlantic (33°S–55°S) and to examine their relationships to water mass characteristics. We also investigated the role of mesoscale fronts in determining the boundaries of the ecoregions. Finally, we compared copepod distributions with previously defined biogeographical provinces in the southwestern South Atlantic. The regional copepod community is organized into six assemblages that occupy distinct areas of the shelf, the shelfbreak, and the oceanic realm. These areas are characterized by different water masses. In general, the spatial configuration of the ecoregions matches that of the previously defined regional biogeographic provinces and marine fronts seem to act as boundaries between the ecoregions.
... The Brazil-Malvinas Confluence (BMC) is generated at around 38 • S and extends poleward to around 45 • S [10]. The BMC is a dynamically active region [11]. Previous studies documented that the climate modes such as the Southern Annular Mode (SAM) [13,14], the Pacific-South American pattern (PSA) [15] and the Antarctic Dipole (AD) [16] are associated with the variability of the ACC. ...
... Sustainability 2019, 11,4853 ...
The Antarctic Circumpolar Current (ACC) plays an important role in the climate as it balances heat energy and water mass between the Pacific and Atlantic Oceans through the Drake Passage. However, because the historical measurements and observations are extremely limited, the decadal and long-term variations of the ACC around the western South Atlantic Ocean are rarely studied. By analyzing reconstructed sea surface temperatures (SSTs) in a 147-year period (1870-2016), previous studies have shown that SST anomalies (SSTAs) around the Antarctic Peninsula and South America had the same phase change as the El Niño Southern Oscillation (ENSO). This study further showed that changes in SSTAs in the regions mentioned above were enlarged when the Pacific Decadal Oscillation (PDO) and the ENSO were in the same warm or cold phase, implying that changes in the SST of higher latitude oceans could be enhanced when the influence of the ENSO is considered along with the PDO.
... To investigate the influence of bathymetry on cross-front transport, we plot in Figure 7 Simulations and observational studies have shown that 'hot spots' of enhanced eddy kinetic energy (EKE) and eddy-induced cross-jet transport can occur downstream of shallow bathymetric obstacles (Chelton et al. 1990;Frenger et al. 2015;Patara et al. 2016;Barthel et al. 2022;Foppert et al. 2017). Consistent with this, we expect to see enhanced eddy kinetic energy in locations of large total cross-front transport. ...
The Southern Ocean plays an integral role in the global climate system, exchanging heat, salt, and carbon throughout the major ocean basins via the deep, fast-flowing Antarctic Circumpolar Current. The Antarctic Circumpolar Current is bounded by spatiotemporally varying fronts that partition distinct water masses. Locating and quantifying cross-front transport is crucial for understanding global patterns of inter-basin exchange; however, this is challenging because fronts are typically defined by hydrographic properties, such as temperature or salinity, which are subject to external sources and sinks, rather than by properties of the flow itself. Here we show that Southern Ocean fronts can be characterized by material contours that minimize deformation and cross-contour mixing over a prescribed time window. By tracking material contours in a sequence of such windows, we quantify cross-front transport and show that the fronts exhibit a global pattern of alternating poleward and equatorward transport caused by frontal meandering in regions downstream of prominent sea-floor obstacles. These results highlight the importance of bathymetric features in controlling Southern Ocean dynamics and inter-basin exchange.
... The BC, a branch of the South Equatorial Current (SEC) (Willson and Rees 2000), is a western boundary current according to the classification of wind-driven currents, and exhibits high mesoscale variability, with intermittent formation of fronts and eddies (Gordon and Greengrove 1986;Schwarz and Perez 2010). The complex ocean structure produced by the confluence of the MC and BC is one of the important factors in the high-productivity fishing area in the Southwest Atlantic (Chelton et al. 1990;Piola and Matano 2001). ...
Exploring the impacts of climate variability on the marine fishery ecosystems in the Southwest Atlantic Ocean is conducive to establishing an ecosystem‐based approach for the protection and rational utilization of fishery resources. In this study, long‐term fisheries data, 23 environmental data from the entire Southwest Atlantic, and 25 global climate data have been used to explore the regime shift of the fishery ecosystem and the response of fishery resources to climate change from 1950 to 2018. The results indicated that changes in the Southwest Atlantic fishery ecosystem exhibited a significant nonstationary trend, and there were three noteworthy regime shifts in 1976/1977, the late 1980s, and the late 20th century. The temperature, sea surface height, water runoff, and cloudiness were the environmental variables with the greatest impact on fishery resources within the Southwest Atlantic Fishery Ecosystem, while zonal wind speed and air temperature yielded a more significant impact on low latitude areas. In terms of climate indices, fishery resources have the most obvious response to the Global Mean Land‐Ocean Temperature Index and Antarctic Sea Ice Extent, and the Atlantic Multidecadal Oscillation had an intense impact on low latitude areas concurrently. The study highlights the climate‐related nonstationary changes in the Southwest Atlantic fishery ecosystem.
... In the opposite direction to the BC, the Malvinas Current is a fast (40 to 70 Sv), barotropic current formed from a branch of the Antarctic Circumpolar Current, which moves north parallel to the break in the Argentine Shelf, reaching approximately 38° S (Matano, Palma, and Piola 2010). There, it converges with the Brazil Current at the Brazil-Malvinas Confluence (BMC) (Chelton et al. 1990;de Freitas et al. 2017;Frey, Piola, and Morozov 2023). The BMC is located between 36° S and 39° S (Reid, Junior, and Patzert 1977;Artana et al. 2021) and is characterised by the high energy potential of the thermohaline gradient, responsible for the meridional exchanges in the Atlantic Ocean between tropical and polar regions. ...
The Brazil current (BC) is a westerly current that flows south along the Brazilian coast, being part of the Southern Brazilian Continental Shelf (SBCS). Recent studies have indicated a trend towards intensification and a shift to the south of the region where this current predominates. We analysed the seasonality of the relationship between atmospheric variables and sea surface temperature (SST) with potential trends and changes in the spatio‐temporal pattern of these variables from 1980 to 2020 over the BCs area in the South Atlantic Ocean. For this purpose, monthly data on SST, air temperature at 2 m above the surface (T2M), mean sea level pressure (MSLP), zonal wind (U10) and meridional wind (V10), obtained from ERA5 reanalysis, were used. Descriptive statistical analyses, trends using the Mann‐Kendall test, correlation matrices and Pettitt's test revealed a significant spatial correlation between the variables, with temporal trends of variation, especially over the BCs area. The meridional (zonal) wind predominantly exhibited a north–south (west–east) direction, supporting the hypothesis that the study region was displaced. Additionally, statistically significant positive trends were observed for SST (0.02°C ∙ dec⁻¹ in austral autumn, winter and spring and 0.01°C ∙ dec⁻¹ in austral summer), T2M (0.02°C ∙ dec⁻¹ in austral winter and spring), MSLP (0.05°C ∙ dec⁻¹ in austral autumn) and negative for U10 (−0.01°C ∙ dec⁻¹ in austral spring). Pettitt's test results confirm significant changes in the behaviour of most analysed variables from the late 1990s to the early 2000s. The post‐breakpoint periods of the variables consistently showed above‐average values compared to the pre‐breakpoint periods, supporting positive upward trends consistent with literature findings. In particular, they highlight the notable upward slope in SST in the BC region.
... The shores of the Brazilian coast are one of the most dynamic regions of the global ocean (Chelton et al, 1990); (Piola and Matano, 2001). The South Atlantic Ocean is characterized by large thermohaline contrasts, which is an ideal scenario for the extraction of thermal energy and intense mesoscale activity (Gordon, 1989). ...
The search for energy alternatives is becoming more important in the strategic development of countries, as global energy demand accelerates. The thermal energy in the oceans, the highest exploitation potential, is renewable and has the ability to remove heat from the oceans and carbonic gas from the atmosphere. In this study, we used the temperature results from the numerical model HYCOM for resoucers evaluation of Ocean Thermal Energy Conversion (OTEC). Based on these data, daily, monthly, seasonal and annual averages were calculated, allowing to evaluate the thermal gradient variability in the Blue Amazon, region where Brazil has sovereign rights for exploration, conservation and management of living and nonliving natural resources. Three thermal energy sites were obtained the definition of their viability and operational of his conversion by teorical OTEC plant throughout the period of the average. The brazilian northern region stands out with the higher energy potential between the surface and a depth range of 600m ≤ z ≥ 1000m, from the results we indicate that the Brazilian Ocean Thermal Energy Park (OTEP) is technically feasible for the operations of OTEC. However, form the economic perspective, results show that only portions closest to the continental shelf, have the capacity to operate.
... Положения фронтов АЦТ можно определить как по данным измерений вдоль треков спутников, так и по интерполированным на регулярную сетку данным. Альтиметрия показывает, что географическое распределение течений и их изменчивость сильно зависят от рельефа дна [12,22]. Авторы работ [27,42] изучили струк-туру АЦТ в проливе Дрейка, используя альтиметрические данные и гидрофизические измерения, что позволило обнаружить связь между сильными течениями и максимумами градиента АДТ. ...
The Antarctic Circumpolar Current plays the key role in the circulation of the Southern Ocean and affects the distribution of heat by the ocean on the global scale. The study of the dynamics and structure of this current becomes especially important in a changing climate. The current is well revealed by satellite altimetry data, which makes possible to study temporal and spatial variability of its structure at different scales. In these studies, the methods for determining the position of individual fronts of the Antarctic Circumpolar Current based on satellite altimetry data become especially important. In this work, we compare various approaches for detection of front locations. The structure of the northernmost branch of the Antarctic Circumpolar Current, the Subantarctic Front, and its spatial and temporal variability was studied based on satellite altimetry data from 1993 to 2020 and the results of a hydrographical section occupied by the R/V “Akademik Mstislav Keldysh” in the northern part of the Drake Passage in February 2020. We selected a 350 km long part of the TOPEX/Poseidon and Jason-1,2,3 satellite altimeter track from Tierra del Fuego to the south for the analysis of the dynamics of the front. Criteria for determining the position of the northern and main branches of the Subantarctic Front are presented based on satellite altimetry data. A long-term shift of the position of the fronts relative to the previously accepted levels of absolute dynamic topography has been found. It was found that the accuracy of determining the position of fronts using fixed values of dynamic topography may decrease with time, in particular due to changes in the mean sea level. A statistically significant long-term trend of sea level rise in the region of the Subantarctic Front was found. This trend is 4 mm/yr for the northern branch and 2.5 mm/yr for the main branch.
... This species can adapt to different environmental conditions and occurs in subtropical and temperate tropical regions (Castilho et al., 2008), but it is commonly found in colder regions (Boschi, 1969). However, at certain times of the year, it shows a greater abundance and distribution along the northern coast of Rio de Janeiro due to the confluence of ocean currents from Brazil and Malvinas, causing seasonal upwelling (Chelton et al., 1990;Campos et al., 2000). Consequently, this species can seasonally change its distributional pattern. ...
Here, we used distribution models to predict the size of the environmentally suitable area for shrimps of fishing interest that were impacted by the tailing plume from the collapse of the Fundão Dam, one of the largest ecological disasters ever to occur in Brazil. Species distribution models (SDMs) were generated for nine species of penaeid shrimp that occurred in the impacted region. Average temperature showed the highest percentage of contribution for SDMs. The environmental suitability of penaeids varied significantly in relation to the distance to the coast and mouth river. The area of environmental suitability of shrimps impacted by tailings plumes ranged from 27 to 47 %. Notably, three protected areas displayed suitable conditions, before the disaster, for until eight species. The results obtained by the SDMs approach provide crucial information for conservation and restoration efforts of coastal biodiversity in an impacted region with limited prior knowledge about biodiversity distribution.
... In this region, a front called the Brazil Malvinas Confluence (BMC), formed by the encounter between the warm waters of the BC and cold waters of the Malvinas Current (MC). It is considered one of the most dynamic regions on the planet (Chelton et al., 1990;Cavalcanti and Ambrizzi, 2009). The study area includes the SAO region (Fig. 1). ...
... On average, this front intersects the 1,000 m isobath at 38°30′S in summer and north of 37°S in winter (Artana et al., 2019;Saraceno et al., 2004). Further downstream, both currents retroflect, and instabilities generate prominent mesoscale structures (Chelton et al., 1990;Zyrjanov & Severov, 1979). The circulation of the BMC and dynamics of eddies are greatly influenced by the presence of the Zapiola Rise (ZR), a sediment mount that extends zonally along 45°S between 36° and 51°W. ...
The goal of this work is to study the dynamical structure of eddies of the Brazil‐Malvinas Confluence zone (BMC eddies) using direct velocity measurements carried out by Shipborne Acoustic Doppler Current Profiler during five oceanographic cruises performed in 2016–2022. In total, in situ data of 13 BMC eddies, including nine anticyclones and four cyclones are available. These data show that the orbital velocity in such eddies can reach 189 cm/s and their vertical structure is highly barotropic. In several eddies, the velocities exceeding 100 cm/s are observed down to a depth of 560 m and at a depth of 800 m they are still higher than 80 cm/s. The spatial structure of velocity and horizontal shear in the eddies is strongly asymmetric, with higher velocities in the southern part near the intense thermohaline BMC front. Altimetry data show qualitative agreement with in situ data, but underestimate the horizontal velocity shear and the maximum velocities at the periphery of the BMC eddies. We also use satellite altimetry and Argo float measurements to study these eddies, and estimate their impact on the thermohaline structure. The analysis shows that the eddies with orbital velocities exceeding 100 cm/s cause intense temperature and salinity anomalies reaching 7–9°C and 1 psu in anticyclones and −4°C and 0.8 psu in cyclones at 100–300 m depth.
... The collision generates a thermohaline front called the Brazil-Malvinas Confluence zone (Deacon, 1937). The BMC creates instabilities generating prominent (sub-)mesoscale structures, including eddies, making the Southwestern Atlantic one of the most energetic regions of the world ocean (Chelton et al., 1990). ...
In recent decades, southern elephant seals (SES) have become a species of particular importance in ocean data acquisition. The scientific community has taken advantage of technological advances coupled with suitable SES biological traits to record numerous variables in challenging environments and to study interactions between SES and oceanographic features. In the context of big dataset acquisition, there is a growing need for methodological tools to analyze and extract key data features while integrating their complexity. Although much attention has been paid to study elephant seal foraging strategies, the continuity of their surrounding three-dimensional environments is seldom integrated. Knowledge gaps persist in understanding habitat use by SES, while the representativeness of a predator-based approach to understanding ecosystem structuring is still questioned. In this study, we explore SES habitat use by using a functional data analysis approach (FDA) to describe the foraging environment of five female elephant seals feeding in the Southwestern Atlantic Ocean. Functional principal component analysis followed by model-based clustering were applied to temperature and salinity (TS) profiles from Mercator model outputs to discriminate waters sharing similar thermohaline structures. Secondly, in situ TS profiles recorded by the SES were employed to determine the habitat visited within the range of potential environments identified from the model data. Four Functional Oceanographic Domains (FOD) were identified in the Brazil-Malvinas Confluence, all visited, in varying proportion, by four of the five females studied. We found that the females favored areas where all the FODs converge and mix, generating thermal fronts and eddies. Prey-capture attempts increased in such areas. Our results are in accordance with previous findings, suggesting that (sub-)mesoscale features act as biological hotspots. This study highlights the potential of coupling FDA with model-based clustering for describing complex environments with minimal loss of information. As well as contributing to better understanding of elephant seal habitat use and foraging strategies, this approach opens up a wide range of applications in oceanography and ecology.
... The positions of the ACC fronts can be determined both from measurements along the satellite tracks and from the data interpolated onto a regular grid. Altimetry shows that the geographic distribution of currents and their variability strongly depend on the bottom topography [12,22]. The authors of [27,42] studied the structure of the ACC in the Drake Passage using altimetry data and hydrographic measurements, which made it possible to reveal relationship between strong currents and the maxima of the ADT gradient. ...
... The BMC, located in the SWA (Figure 1), is characterized by a strong thermal gradient due to the confluence of the Brazil and Malvinas currents, which makes the BMC region extremely dynamic and is considered one of the most energetic in the global ocean (Chelton et al., 1990;Pezzi et al., 2009;Piola and Matano, 2019). Additionally, in this area, subtropical and subantarctic fronts are also characterized by horizontal temperature gradients (Orsi et al., 1995;Dong et al., 2006;Chadwick et al., 2020). ...
Hydrostatic adjustment and vertical mixing are the two main mechanisms used to describe the stability of the marine atmospheric boundary layer (MABL) in oceanic regions with intense horizontal temperature gradients. To analyze the occurrence of these mechanisms, we performed simulations using an active coupled regional ocean–atmosphere numerical model in the southwestern Atlantic Ocean (SWA) region in October 2014 in the presence of an atmospheric frontal system. A novel in‐situ dataset was sampled by radiosondes in the Brazil–Malvinas Confluence (BMC) region and used with the model dataset. The vertical mixing mechanism and a prefrontal warm‐air temperature advection were identified, which modulated the shallower and more stable MABL. The hydrostatic adjustment mechanism was not evident because of the near‐surface wind convergence field modification caused by the large‐scale atmospheric system observed in our experiment. The coupled model simulation presented good agreement compared to in‐situ and satellite data. This contribution to the knowledge of the ocean–atmosphere interaction processes at the SWA reinforced that coupled models can be a helpful tool to investigate the air–sea interactions and physical mechanisms that explain MABL stability.
... In the study area, the BMC is associated with mesoscale eddies (Gordon, 1989;Berden et al., 2020), meanders, and is considered one of the most energetic regions of the world (Chelton et al., 1990). Internal waves have been reported to occur during spring and summer, propagating along the Patagonian shelf break and continental slope upstream of the Malvinas Current, but few have been observed in the vicinity of our study area (Magalhães and da Silva, 2017). ...
Seafloor depressions (SD) are features commonly observed on the ocean floor. They often occur as circular, small-sized (up to 10 s of m) incisions caused by fluid expulsion. Larger depressions (100s m to km) are considerably less abundant, and their origin and development have been scarcely studied. This study investigated two giant morphological depressions (>5 km) using recently acquired multibeam bathymetry and backscatter, sediment echosounder, and high-resolution seismic data. An arc-shaped (SD-N) and a sub-circular depression (SD-S) are located on the Ewing Terrace at the Argentine Continental Margin north and south of the Mar del Plata Canyon, respectively. The study area is influenced by the Brazil-Malvinas Confluence, where major counterflowing ocean currents affect sedimentation, and northward flowing currents form a large contourite depositional system. Using an existing seismo-stratigraphy, the onset of SD-N was dated to the middle Miocene (∼15–17 Ma), whereas SD-S started developing at the Miocene/Pliocene boundary (∼6 Ma). Acoustic anomalies indicate the presence of gas and diffuse upward fluid migration, and therefore seafloor seepage is proposed as the initial mechanism for SD-S, whereas we consider a structural control for SD-N to be most likely. Initial depressions were reworked and maintained by strong and variable bottom currents, resulting in prograding clinoform reflection patterns (SD-N) or leading to the build-up of extensive cut-and-fill structures (SD-S). Altogether, this study highlights the evolution of two unique and complex seafloor depressions throughout the geologic past under intense and variable bottom current activity in a highly dynamic oceanographic setting.
... Overall, our data at the BMC show very high mixing rates in the upper 400 m of the water column, which are one or two orders of magnitude greater than the canonical value in the thermocline of 10 − 5 m 2 s − 1 , which makes sense considering that the BMC is among the most energetic regions in the world ocean (Chelton et al., 1990). The quantitative and qualitative results found in this study are in agreement with those obtained from seismic data in the BMC by Gunn et al. (2021). ...
... The BMC is one of the most energetic regions in the South Atlantic (Chelton et al. 1990), mainly due to the activity of cyclonic and anticyclonic eddies that are important in the distribution of marine organisms over long distances, when they leave the original current, and in the increase of productivity (cyclonic eddies) through introduction of nutrients in the surface layer (Angel-Benavides et al. 2016;Jagadeesan et al. 2019). ...
The Brazil-Malvinas Confluence (BMC) is characterized by high environmental variability and represents a hotspot of phytoplankton biodiversity. This study has investigated the phytoplankton composition in the BMC and a Cold Core Eddy (CCE), with particular reference to the life forms and trophic modes of the species. Vertical hauls were performed between the maximum chlorophyll depth and the surface with a 20-μm mesh plankton net at six sampling points in the BMC (4 hauls) and CCE (2 hauls) during the austral spring (November 2019). Temperature and salinity were determined in situ . The Brazil Current and Malvinas Current in the BMC had different species compositions, and the CCE had greater similarity with the Malvinas Current. Autotrophic organisms (mainly diatoms) and marine planktonic oceanic and/or neritic life forms were predominant in the BMC and the CCE. Heterotrophic dinoflagellates were predominant in the Brazil Current and the CCE, while mixotrophic dinoflagellates in the Malvinas Current were associated with low temperatures and salinities. A high diversity of species was confirmed in the region, mainly associated with marine planktonic oceanic and/or neritic species, and there was a predominance of autotrophic organisms. The temperature conditions in the CCE promoted an increase in species richness in the region.
... The Brazil Malvinas Confluence (BMC, Fig. 1) region is considered an extremely dynamic region and one of the most energetic of the global ocean (Chelton et al., 1990;Gordon, 1981;Pezzi et al., 2009;Piola and Matano, 2001). The BMC, located in the SWA, presents intense thermal Fig. 1. ...
This study presents an overview of oceanic mesoscale eddies and their influence on the overlying atmosphere and surrounding waters, with a focus on the southwestern Atlantic Ocean. The high values of eddy kinetic energy (EKE) in this region are related to the mesoscale eddies observed, which transport dynamic and thermodynamic specific properties away from their origin point, during their life cycle. This transport capacity interferes with several atmospheric and oceanic processes, such as the local marine atmospheric boundary layer (MABL) stability, heat fluxes, and primary production. In the Southern Hemisphere, cyclonic (anticyclonic) eddies are related to clockwise (counterclockwise) circulation and negative (positive) anomalies of the sea surface temperature (SST) and the latent and sensible heat fluxes. It has been observed that over the clockwise and cold (counterclockwise and warm) eddies, there is a decrease (increase) in the near-surface wind, as well as stable (unstable) conditions on the MABL, vertical downward (upward) movements in the atmosphere, and decreased (increased) precipitation. SST anomalies influence the curl, divergence, and magnitude of the surface wind and wind stress, while mesoscale surface oceanic currents mainly affect the stress and wind curls. The atmospheric response to SST anomalies can be explained using two main mechanisms: hydrostatic adjustment and vertical mixing. However, as will be shown, the feedback among the SST anomalies, wind, and oceanic currents is far more complex. This work compiles and provides a theoretical basis for future work concerning air-sea interactions of mesoscale oceanic structures.
... Past idealised and observational studies have suggested that, like upwelling, eddy kinetic energy is enhanced downstream of topography (Chelton et al., 1990), but that there is a spatial offset between eddy kinetic energy and upwelling (Barthel et al., 2022), eddy heat fluxes (Abernathey and Cessi, 2014;Foppert et al., 2017), and baroclinic activity (Bischoff and Thompson, 2014;Chapman et al., 2015). We investigate this phenomenon using ACCESS-OM2-01. ...
The upwelling of cold water from the depths of the Southern Ocean to its surface closes the global overturning circulation and facilitates uptake of anthropogenic heat and carbon. Upwelling is often conceptualised in a zonally averaged framework as the result of isopycnal flattening via baroclinic eddies. However, upwelling is zonally non-uniform and occurs in discrete hotspots near topographic features. The mechanisms that facilitate topographically confined eddy upwelling remain poorly understood and thus limit the accuracy of parameterisations in coarse-resolution climate models.
Using a high-resolution global ocean sea-ice model, we calculate spatial distributions of upwelling transport and energy conversions associated with barotropic and baroclinic instability, derived from a thickness-weighted energetics framework. We find that five major topographic hotspots of upwelling, covering less than 30% of the circumpolar longitude range, account for up to 76% of the southward eddy upwelling transport. The conversion of energy into eddies via baroclinic instability is highly spatially correlated with upwelling transport, unlike the barotropic energy conversion, which is also an order of magnitude smaller than the baroclinic conversion. This result suggests that eddy parameterisations that quantify baroclinic energy conversions could be used to improve the simulation of upwelling hotspots in climate models. We also find that eddy kinetic energy maxima are found on average 110 km downstream of upwelling hotspots in accordance with sparse observations. Our findings demonstrate the importance of localised mechanisms to Southern Ocean dynamics.
... Both boundary currents encounter each other at the Brazil-Malvinas Confluence (BMC), a region delimited to the south by the subantarctic front and to the north by the subtropical front (Saraceno et al., 2004). This region is characterized by large variability, high kinetic energy and the development of large eddies (i.e., Chelton et al., 1990). ...
The present study addresses the importance of the water exchanges between the Northern Argentinean Shelf and the open ocean at interannual timescales. Based on the ocean reanalysis ORAP5.0 data set the main objective of this work is to identify which mechanisms drivers these exchanges and also estimate the interchanges of waters across the shelf‐break. In order to estimate the interchanges in the region we compute the velocity field normal to shelf‐break and perform an empirical orthogonal function analysis to unveil potential modes of variability. A leading mode that explains the largest percentage of variance is found. This mode presents a dipole structure associated to increments and decrements in the shelf water exportation in the Argentinian and southern Brazilian shelves. Wavelet analysis of the principal component associated to the mode suggest that power concentrates in 2, 4 and 10‐years periods. These timescales coincide with those of the latitude of separation of the Brazil Current timeseries, which is suggested to modulate the mode. Statistical analysis techniques are employed to strengthen confidence in the proposed hypothesis linking the mode to the potential driver. Satellite chlorophyll‐a maps are explored to evaluate potential implications of the mode on biology. This independent data set is employed to further validate the results derived from the ocean reanalysis at 2‐years timescales. At these timescales the chlorophyll‐a concentration field exhibits positive and negative interannual anomalies close to the shelf‐break and in the neighboring open ocean that are consistent with the circulation patterns characteristics of biennial modulations.
... The Southwestern Atlantic Ocean (SWA) is considered one of the most active regions of the World Ocean (Chelton et al., 1990). This region comprises different ocean regimes mainly related to the Brazil-Malvinas Confluence (BMC) and to coastal waters mainly originated from the La Plata river. ...
Satellite-derived SST data from January l985 to December 2004 were obtained from the NOAA Pathfinder Project at a spatial resolution of 4 km. Monthly data together with 20-year long climatological means were used to produce sea surface temperature anomaly (ASST) images for the Southwestern Atlantic Ocean. The variability of the ASST fields was analyzed using Complex Empirical Orthogonal Function (CEOF) and wavelet methods to account for the main modes of variability. The most energetic peaks found in the series were centered at ~1 year, ~3.5 years, ~6 years, and ~7 years. While the first is a known dominant peak for the SST fields of the region, the last ones are not very well documented and may be related to large-scale forcing. Some ASST peaks are coincident in time with the El Niño events of 1987 and 1998. The first five CEOF modes, however, summed up to only ~35% of the total variance of the ASST fields of the region: a sign that our study area is very inhomogeneous as previously reported in the literature.
... The dynamically active Brazil-Malvinas Confluence (BMC) region [1] is characterized by the confluence of the Brazil (warm, saltier) and Malvinas (cold, less salty) currents and has its variability mostly dominated by the ocean's mesoscale, especially eddies and meanders. The importance of the BMC eddies on the across-front distribution of ocean's properties such as heat, salt, nutrients, chlorophyll and others, is recently becoming well understood [2][3][4]. ...
The Brazil–Malvinas Confluence (BMC) is one of the most dynamical regions of the global ocean. Its variability is dominated by the mesoscale, mainly expressed by the presence of meanders and eddies, which are understood to be local regulators of air-sea interaction processes. The objective of this work is to study the local modulation of air-sea interaction variables by the presence of either a warm (ED1) and a cold core (ED2) eddy, present in the BMC, during September to November 2013. The translation and lifespans of both eddies were determined using satellite-derived sea level anomaly (SLA) data. Time series of satellite-derived surface wind data, as well as these and other meteorological variables, retrieved from ERA5 reanalysis at the eddies’ successive positions in time, allowed us to investigate the temporal modulation of the lower atmosphere by the eddies’ presence along their translation and lifespan. The reanalysis data indicate a mean increase of 78% in sensible and 55% in latent heat fluxes along the warm eddy trajectory in comparison to the surrounding ocean of the study region. Over the cold core eddy, on the other hand, we noticed a mean reduction of 49% and 25% in sensible and latent heat fluxes, respectively, compared to the adjacent ocean. Additionally, a field campaign observed both eddies and the lower atmosphere from ship-borne observations before, during and after crossing both eddies in the study region during October 2013. The presence of the eddies was imprinted on several surface meteorological variables depending on the sea surface temperature (SST) in the eddy cores. In situ oceanographic and meteorological data, together with high frequency micrometeorological data, were also used here to demonstrate that the local, rather than the large scale forcing of the eddies on the atmosphere above, is, as expected, the principal driver of air-sea interaction when transient atmospheric systems are stable (not actively varying) in the study region. We also make use of the in situ data to show the differences (biases) between bulk heat flux estimates (used on atmospheric reanalysis products) and eddy covariance measurements (taken as “sea truth”) of both sensible and latent heat fluxes. The findings demonstrate the importance of short-term changes (minutes to hours) in both the atmosphere and the ocean in contributing to these biases. We conclude by emphasizing the importance of the mesoscale oceanographic structures in the BMC on impacting local air-sea heat fluxes and the marine atmospheric boundary layer stability, especially under large scale, high-pressure atmospheric conditions.
... However, the collision (confluence) of the southbound subtropical Brazil Current and the northbound sub-Antarctic MFC pivots seasonally around a fixed point at 38.98S-54.28W for nearly 300 km (Gordon, 1981;Chelton et al., 1990;Saraceno et al., 2005), and after collision the main flow of the MFC describes a sharp cyclonic loop, forming the Malvinas Return Flow (MRF) (Palma et al., 2004; see figure 1 in Saraceno et al., 2005). Despite the influence of the MFC on the occurrence of seals in tropical waters of Brazil the behaviour of that current also hardly justifies the northern presence of seals by itself. ...
The present knowledge of pinnipeds’ distribution is principally based on the location of breeding colonies. Several species of pinnipeds have shown the ability to undertake long travels to different and unusual locations, where they are frequently interpreted as vagrants. A total of seven species of pinnipeds are known to occur along the Brazilian coast: four belong to the family Otariidae and three to the family Phocidae. From 1954 to 2008 a total of 54 records of pinnipeds were reviewed for the coast of Rio de Janeiro State, ten of them were new records, representing 18.2% of the total. The most common species registered in the study area was Arctocephalus tropicalis (49.1%; N = 27) followed by Mirounga leonina (20%; N = 11). The other species recorded were Lobodon carcinophaga, Otaria flavescens, Arctocephalus australis and Hydrurga leptonyx. From 51 pinniped specimens with information on periods of occurrence, 76.5% (N = 39) were reported during winter, and the other 12 specimens were equally distributed over the three other seasons. Most sub-Antarctic fur seals (88%) and southern elephant seals (83%) were males. The majority of the species were classified as sexually immature. The seasonal pattern of the pinnipeds found on the coast of Rio de Janeiro State is related to the intensive northward flow of the Malvinas/Falkland Current during winter. In addition, the potential to swim long distances together with the lack of physical barriers in the marine environment could help the dispersion of the seals to distant regions far from their traditional breeding or feeding regions.
... Neste ítem são avaliados os impactos regionais entre os experimentos (GORDON, 1989;CHELTON et al., 1990), variando seu posicionamento entre cerca de 35ºS e 36ºS (LEGECKIS e GORDON, 1982;CAMPOS et al., 1999) e cerca de 38ºS e 40ºS (CIOTTI et al., 1995;CAMPOS et al., 1999). ...
his study investigates, through computational modeling, the climatic effects of a possible scenario of prolonged solar minimum of activity, such as the Maunder Minimum, and the increase of CO2 in the terrestrial climate system. This research is conducted using the Community Earth System Model (CESM), which is the state-of-the-art in climate modeling, and is also one of the CMIP/IPCC models. The results reinforce that a future scenario of a prolonged solar minimum of activity has greater regional climate impacts than global ones. The effects of a future prolonged minimum of solar activity indicate that this scenario is only capable of slowing down the temperature increase caused by the increased concentration of CO2 in the earth's atmosphere. The climate change caused by these scenarios suggests that the most affected regions in South America, even with the deceleration of temperature increase, are northern and northeastern Brazil.
... The continental shelf is one of the widest in the world, and two distinct boundary currents influence it: The Malvinas Current, that circulates northward carrying cold nutrient rich and the relatively freshwater of sub-antarctic origin, and the Brazil Current, that circulates southward, carrying warmer, nutrient-poor and saltier waters (Piola et al. 2000;Palma et al. 2008;Matano et al. 2010;Marrari et al. 2017). These currents flow in opposite directions and meet, in average, at 36° S (Acha et al. 2004), in a region known as the Brazil-Malvinas Confluence, which is one of the most energetic globally (i.e., Chelton et al. 1990;Garzoli 1993;Piola and Matano 2001). These currents and the association with several shelf and shelf-break fronts controlled by the strong winds, large-amplitude tides and freshwater discharges, makes this ecosystem one of the most productive in the world (150-300 gcm −2 year −1 ) (Acha et al. 2004;Romero et al. 2006;Matano and Palma 2008;Palma et al. 2008;Matano et al. 2010;Marrari et al. 2017;Martinetto et al. 2019). ...
The Argentine Sea (Southwestern Atlantic) is one of the most productive ecosystem in the Southern Hemisphere. Research on metals in this region is scarce or null. In this study, we evaluated the concentrations of some metals in the suspended particulate matter (SPM), to provide baseline data that would enable us to understand the role of the SPM in the transport of metals in the Argentine Sea. Sampling was carried out during the austral summer 2016 at 20 stations distributed in the Argentine Sea. Surface seawater samples were collected and then filtered by vacuum through Millipore® HAWP 04,700 filters (0.45 µm). The samples were acid-digested (HNO3 and HClO4, 5:1) and the metal concentrations were determined with ICP-OES Optima 2100 DV (Perkin Elmer). Significant spatial variations were detected due to the extension of the study area, with the highest levels of metals in the stations next to large urban centers (Cd, Cu, Cr, Fe, Mn, Ni, Pb and Zn: 13.9, 154.9, 48.7, 54,470, 7646, 49.2, 58.6 and 509.5 μg g⁻¹ d.w. respectively), which was supported by the nMDS and Cluster analyses. According to PCA analysis, two groups of metals that could have similar behavior were stablished: one group integrated by Cu, Zn, Pb and Ni, and the other group integrated by Cr, Fe and Mn. Metals that act as micronutrients and the toxic ones were present in all the sampling stations, highlighting the need to reinforce the study of these elements in this extensive and productive area of the South Atlantic Ocean.
... Using a 19-year-long time series of reanalysis data, Leyba et al. (2017) described the H and Hl anomalies in the SWAO and their association with both cyclonic and anticyclonic (oceanic) mesoscale eddies at the BMC and at the Subtropical and Subantarctic Fronts in the South Atlantic Ocean as a whole. Large H and Hl anomalies were observed in the BMC region, considered one of the more energetic regions of the global ocean (Chelton et al., 1990), and related to the large oceanic instabilities caused by the meeting of the Brazil and Malvinas currents at approximately 38 • S. ...
Turbulent air–sea heat fluxes were computed from in situ high‐frequency micrometeorological data during two research cruises performed in the southwestern Atlantic Ocean (SWAO) occurring in June 2012 and October 2014. Two different and dynamical areas were covered by the cruises: the Brazil‐Malvinas Confluence (BMC) and the Southern Brazilian Continental Shelf (SBCS). The eddy covariance (EC) method was used to estimate the air–sea sensible‐ and latent‐heat fluxes. This article compares these novel high‐frequency estimates of heat fluxes with bulk parametrizations made at the same location and time from independent measurements taken on board the ships. When comparing the EC and bulk‐estimated time series of sensible‐heat fluxes, we found a good agreement both in their magnitude and variability, with small bias (generally <20 W·m⁻²) between the datasets from the two study areas in the SWAO. However, the EC and bulk latent‐heat flux comparisons show large biases ranging from 75 W·m⁻² to 100 W·m⁻² in the SBCS and BMC, respectively. These biases were always associated with short‐term, high‐frequency environmental perturbations occurring either in the atmosphere or in the ocean with the majority related to strong wind burst events and large air–sea temperature gradients. The short period changes in atmospheric conditions were mostly related to the passage of transient synoptic systems over the two study areas. The large air–sea temperature gradients were mostly linked to the surface characteristics of the BMC and SBCS regions, where sharp oceanographic fronts are located. Our results are able to contribute to improving weather and climate simulations of the mid‐ to high latitudes of South America, a region largely influenced by the sea‐surface temperature patterns of the SWAO in combination with the frequent propagation of transient atmospheric systems.
... The Malvinas Current (MC) detaches from the northernmost branch of the ACC, defined by the SAF, and travels northward along the shelf-break of the Argentine Continental Shelf until it collides with the southward BC at about 38 degrees south ( Figure 1). The confluence of these two currents is called the Brazil-Malvinas Confluence (BMC) region and it is characterized for being one of the most energetic regions of the world (e.g., [24]) and for generating a large number of eddies. ...
The understanding of the physical drivers of sea level trend is crucial on global and regional scales. In particular, little is known about the sea level trend in the South Atlantic Ocean in comparison with other parts of the world. In this work, we computed the South Atlantic mean sea level (SAMSL) trend from 25 years of satellite altimetry data, and we analyzed the contributions of steric height (thermosteric and halosteric components) and ocean mass changes for the period 2005-2016 when all the source data used (Argo, GRACE and satellite altimetry) overlap. The SAMSL trend is 2.65 ± 0.24 mm/yr and is mostly explained by ocean mass trend, which is 2.22 ± 0.21 mm/yr. However, between 50° S-33° S, the steric height component constitutes the main contribution in comparison with the ocean mass component. Within that latitudinal band, three regions with trend values higher than the SAMSL trend are observed when considering 25 years of satellite SLA. In the three regions, a southward displacement of the Subtropical, Subantarctic, and Polar Fronts is observed. The southward shift of the fronts is associated with the strengthening and polar shift of westerly winds and contributes to a clear thermosteric trend that translates to the SLA trend observed in those regions.
... Three typical frontal regimes are known to exist in the Southern Ocean and have been defined as merging, shoaling, and lee meandering, depending on their position relative to the bathymetry (Sallée et al., 2008). Topography influences the pathway of the fronts as seen in previous studies (Chelton et al., 1990;Gille, 1994;Moore et al., 1999;Sokolov and Rintoul, 2009a;Dong et al., 2006), but also influences the mean intensity of the jet associated with the fronts. In areas where topography is not prominent, fronts are especially subject to large meandering due to mesoscale activity and atmospheric forcing (Sallée et al., 2008). ...
The subantarctic Prince Edward Islands lie in the Antarctic Circumpolar Current, between the Subantarctic Front (SAF) and the Polar Front (PF). These fronts positions were found to be highly variable at interannual and monthly time scales and revealed a significant long-term southward trend in the region. The intense mesoscale activity, observed upstream the islands at the South West Indian Ridge, also showed an interannual and intra-annual variability as well a decrease in eddy kinetic energy over 24 years. At a more local scale, we highlighted that the archipelago’s environment was impacted by the mesoscale features produced upstream.Tides appeared to be another important driver of variability of the circulation in between the two islands.An idealised model configuration was designed for the Prince Edward Islands region to study the mesoscale eddy properties and the physical mechanisms of their formation at the ridge. The Eddy Available Potential Energy revealed a maximum of energy around 800 m depth, confirming the deep reaching characteristic of the eddy originated in the region and suggested the presence of a local energy source at this depth. This eddies activity was shown to be the result of a combination of barotropic and baroclinic instabilities occurring at the ridge.Finally, we investigated on the potential consequences of a southward shift of the SAF in the region of the islands.Because the model was idealised, it allowed us to simulate an SAF southward shift by shifting the initial and boundary conditions. The main result was the clear decrease of mesoscale activity in the region which could potentially impact the ecosystems of the Prince Edward Islands.
Plain Language Summary
The United Nations has identified 2021–2030 as the Decade of Ocean Science, with a goal to improve predictions of ocean and climate change. Improved understanding of the Southern Ocean is crucial to this effort, as it is the central hub of the global ocean. The Southern Ocean is the formation site for much of the dense water that fills the deep ocean, sequesters the majority of anthropogenic heat and carbon, and controls the flux of heat to Antarctica. The large‐scale circulation of the Southern Ocean is strongly influenced by interactions with sea ice and ice shelves, and is mediated by smaller scale processes, including eddies, waves, and mixing. The complex interplay between these dynamic processes remains poorly understood, limiting our ability to understand, model and predict changes to the Southern Ocean, global climate and sea level. This article provides a holistic review of Southern Ocean processes, connecting the smallest scales of ocean mixing to the global circulation and climate. It seeks to develop a common language and knowledge‐base across the Southern Ocean physical science community to facilitate knowledge‐sharing and collaboration, with the aim of closing loops on our understanding of one of the world's most dynamic regions.
A holistic review is given of the Southern Ocean dynamic system, in the context of the crucial role it plays in the global climate and the profound changes it is experiencing. The review focuses on connections between different components of the Southern Ocean dynamic system, drawing together contemporary perspectives from different research communities, with the objective of “closing loops” in our understanding of the complex network of feedbacks in the overall system. The review is targeted at researchers in Southern Ocean physical science with the ambition of broadening their knowledge beyond their specific field and facilitating better-informed interdisciplinary collaborations. For the purposes of this review, the Southern Ocean dynamic system is divided into four main components: large-scale circulation; cryosphere; turbulence; and gravity waves. Overviews are given of the key dynamical phenomena for each component, before describing the linkages between the components. The reviews are complemented by an overview of observed Southern Ocean trends and future climate projections. Priority research areas are identified to close remaining loops in our understanding of the Southern Ocean system.
Bottom‐current related sediments have been commonly used for paleoceanographic reconstructions. However, the strength and variability of bottom currents are poorly understood and thus the processes that control sedimentation in deep environments are not clear. In this study, we focus on the Drake Passage, which is connected to the Antarctic Circumpolar Current, that has a major impact on the global climate. We studied the intensity and variability of bottom currents and how they are related to sedimentary processes. For this purpose, we used 27‐years from GLORYS12 Mercator Ocean reanalysis at high resolution to evaluate the bottom current dynamics. Geophysical data and surface grain size measurements were used to identify the type of sediment deposits. Our results show that the dynamics of bottom currents is disconnected from the sea surface dynamics, and bottom circulation is strongly controlled by the rough topography of the Drake Passage. The patterns for the first modes of bottom‐current variability are related to the local topography and seem to generally control the distribution of contourites. The second and third EOF modes show patterns in the bottom currents that differ from the mean field, and they may affect the rate of erosion and deposition differently. Time series of bottom currents reveals multiple high‐speed current events, but contourite drifts seem to accumulate preferentially in zones of slow and stable bottom currents. Our study highlights the potential of using ocean reanalysis to better constrain bottom currents in zones of scarce data and to plan future campaigns of direct measurements.
The Brazil-Malvinas Confluence (BMC) is the region where opposing and intense western boundary currents meet along the Southwestern Atlantic slope at about 38°S, generating one of the most energetic mesoscale regions of the global ocean. Based on shipborne observations acquired within the Uruguayan Economic Exclusive Zone (EEZ), combined with satellite data and an eddy tracking algorithm, we analyze the cross-shelf exchanges during May 2016, when the BMC was in an anomalous northern position. Two types of shelf water export were observed triggered by mesoscale dynamics: one was the export of shallow Rio de la Plata Plume waters driven off-shelf by the retroflection of the Brazil Current. This export formed a 70 km wide, 20 m deep filament that propagated offshore at 0.3 m s–1, with a transport of 0.42 Sv. It lasted about 10 days before being mixed with ambient Confluence waters by strong winds. An additional type of off-shelf transport consisted of a subsurface layer of Subantarctic Shelf Waters (SASW) about 60 m thick that subducted at the BMC reaching 130 m deep and transporting 0.91 ± 0.91 Sv. We show that geostrophic currents derived from satellite altimetry over the slope can be useful to track this subsurface off-shelf export as they are significantly correlated with absolute velocity measurements at this depth. Argo temperature and salinity profiles show evidence of these two types of shelf water export occurring between the BMC front and the separation of the Brazil Current from the shelf-break, suggesting this is a relatively frequent phenomenon, in agreement with previous observations.
Plain Language Summary
The Argentine Basin is a turbulent region in the Southern Ocean that is not well understood. We want to determine how well we can make forecasts of the region given some level of error in our starting point, and additionally investigate the dependence of model resolution on our ability to forecast. We design a series of numerical experiments where we make a change in the winds at the start of the model run and quantify how the heat and carbon in the surface ocean change compared to model runs without changing the winds. These differences are measured as time progresses. We determine that in the coarser resolution models, the differences decay for up to 45 days before increasing, whereas in the finer resolution model the differences decay for two weeks before increasing. This gives us a time frame during which small errors in the forecast models do not compound. However, a caveat is that though the coarser model will be less sensitive to errors our results also suggest that it may be less accurate in forecasting the true ocean dynamics.
The Patagonian slope is the region where Subantarctic waters and bathymetry give raise to physical and ecological processes that support a rich biodiversity and a large-scale industrial fisheries. Unique among the species that depend on this region is the deep diving southern elephant seal, Mirounga leonina. We report here on changes in the foraging behavior of a female seal explained by the combined effect of a cold and high salinity water mass and a decrease in surface chlorophyll-a concentration. Behavioral and oceanographic data from about 5000 profiles of temperature, conductivity, pressure, light and prey encounters were collected within an area ranging 59.5–61°W and 46–47.5°S, at depths of 300–700 m, on the Patagonian slope, during November–December 2018. A decrease in temperature (0.15 °C) and an increase in salinity (0.03) was found below the mixed layer, during December. Light data revealed a significant increase of irradiance in December (almost reaching the ocean bottom) associated with a decrease of chlorophyll-a in the upper levels. Concomitantly, the seal had a different diving behavior in December, foraging near the surface at night and close to the bottom during daylight hours. Also, the seal doubled the prey capture attempts in December compared to November. This study reveals the importance of ocean physical properties on seal's diving and foraging behavior, and how this changes, although small, can impact on seals diet and body composition during their post-breeding trips.
Although the Malvinas Current (MC) plays a key role over the Patagonian shelf in sustaining an extremely rich ecosystem it remains rather undocumented. In this thesis, we combined in situ, satellite data, and operational model outputs to study the MC. The MC is an offshoot of the Antarctic Circumpolar Current (ACC). It flows northward up to 38°S where it turns southward forming the Malvinas Return Flow (MRF). We show that a substantial portion of the mesoscale activity from the ACC is dissipated over the Malvinas Plateau. We discovered the occurrence of recurrent blocking events cutting the MC from its source, the ACC. However, the MC does not collapse as a recirculation cell is established. Criteria in potential density and dynamic topography were defined to study the MC system fronts (Polar Front, Subantarctic Front and Brazil Current Front). Waters from the South of the Polar Front (PF) are recurrently injected into the MC as pulses or feeding events. Polar waters accumulate in the recirculation region between the MC and the MRF. Variations in the water characteristics of the recirculation region are consistent with changes in the occurrence of blocking and feeding events. Combining mooring and satellite altimetric data, a 24-year long time series of the MC transport at 41 ° S was constructed. Maxima and minima of the Malvinas current transport at 41 ° S are not associated with the ACC, rather with eddies coming from the Argentine Basin. Transport maxima appear to be related with cyclonic eddied detached from the Polar Front and transport minima with large anticyclonic anomalies from the Brazil Current.
Direct velocity measurements of the Malvinas Current (MC) were carried out on multiple occupations of five transects across the flow using a Shipborne Acoustic Current Profiler (SADCP) on the R/V Akademik Sergey Vavilov and Akademik Mstislav Keldysh. These data are used to determine local features of the three‐dimensional velocity field of the current. The occupations covered the northern branch of the Antarctic Circumpolar Current (ACC) and the southern part of the MC. Five transects across the flow were located at 350–550 km from each other from the Drake Passage to the western Argentine Basin at 46°S. The new observations reveal that the current is organized in two branches, namely, an inshore branch extending to a depth of 200–300 m and a main offshore branch, which flows approximately over the 1,400 m isobath. This two‐branch structure is observed on each of the cross‐flow transects. The observed velocities of the inshore branch exceed 40 cm/s on each studied crossing of the current. The MC is a cold western boundary current that follows the Subantarctic Front. This flow originates as an offshoot of the northern branch of the ACC and continues over the Falkland/Malvinas Plateau and along the western slope of the Argentine Basin. Volume transports of the upper 700 m of the MC calculated for each crossing range between 1.4 and 4.4 Sv for the inshore branch and between 21.2 and 25.5 Sv for the offshore (main) branch.
Meteorological data collected using ocean buoys are very important for weather forecasting. In addition, they provide valuable information on ocean–atmosphere interaction processes that have not yet been explored. Accordingly, data collection using ocean buoys is well established around the world. In Brazil, ocean buoy data are obtained by the Brazilian Navy through a monitoring network on the Brazilian coast, which has high potential for wind power generation. In this context, the present study aimed to analyze the scaling behavior of wind speed on the Brazilian coast (continental shelf), South Atlantic Ocean and coast of Africa in order to determine long-range correlations and acquire more information on the crossover phenomenon at various scales. For this purpose, the detrended fluctuation analysis technique and numerical simulation with the Weather Research and Forecasting mesoscale model were used. The results from buoys show that wind speed exhibits a scaling behavior, but without the crossover phenomenon in the Brazilian coast, South Atlantic Ocean and coast of Africa, indicating the dependence of the phenomenon by the terrestrial surface, suggesting influence on the wind power generation. Buoy data from the South Atlantic Ocean and coast Africa showed a subdiffusive behavior (α≥1), whereas those from the Brazilian coast indicated persistence (0.5<α<1), except for the confluence region of Porto Seguro, which indicated anti-persistence (α=0.48). The numerical simulations using the WRF mesoscale model showed a subdiffusive behavior on the Brazilian coast, and no persistence was reproduced in most coastal buoys. The findings support deeper understanding of the wind regime and its associated properties, with particular attention to the Brazilian continental shelf, suggesting that the methodology is useful for assessing the potential of offshore wind power generation.
This study presents an analysis of the formation and development of an extratropical cyclone formed in the southern coast of Brazil and how the ocean-atmosphere coupling affects this phenomenon. Was simulated one extratropical cyclonecase in the SouthwestAtlantic occurredat 02to 05 september2006 withthe coupled numerical model system Coupled Ocean Atmosphere Wave Sediment Transport(COAWST) and another experiment with the atmospheric numerical model Weather Research Forecast (WRF), to analyze the coupling effect between numerical models, as well theatmospheric and oceanic dynamics and the air-sea interaction processes during the passage of the extratropical cyclone. Was observed how the Atmospheric-Marine Boundary Layer Instabilityand the mesoscale effects of the event impacttheheat fluxes between the ocean and atmosphere. One area with intense heat fluxes associated to thatinstabilitywas formed in the cold sector of the extratropical cyclone. This moisture from the ocean to theatmosphere is elevated at high levels of the atmosphere by the action of the wind convergence in low levels and it was precipited in the form of rain. The active Sea Surface Temperature inthe COAWST provided similar results when compared withthe Climate Forecast System Reanalysis (CFSR) data, while the WRF model results showed a more intense heat fluxes than the CFSR data.
O papel do Oceano Atlântico Sul na manutenção do clima global é de extrema importância uma vez que compõe uma das partes fundamentais da circulação Termohalina Global. Nesse contexto, o Sudoeste do Atlântico Sul é uma região complexa e energética, onde duas correntes de contorno oeste de sentidos opostos se encontram formando a Confluência Brasil-Malvinas. O objetivo deste trabalho é investigar mudanças de longo perı́odo na variabilidade da latitude de separação da Corrente do Brasil (CB) da Plataforma Continental, considerada um delimitador do limite sul do Giro Subtropical do Atlântico Sul na região do Sudoeste do Atlântico Sul. Para tal, foi usado resultados do modelo HYCOM em um experimento datado denominado ATIa0,25. O modelo foi forçado por dados da reanálise do NCEP e seus resultados abrangem o perı́odo de 1960 a 2010. A latitude de separação da CB da costa foi definida neste trabalho como o ponto em que a isoterma de 18ºC cruza a isóbata de 1000 metros de profundidade. A variabilidade da latitude de separação da CB pode ser dividida em três momentos principais: no começo a latitude de separação está ao sul de sua média, na década de 70 está a norte de sua posição média até a década de 90; depois dos anos 90 a posição começa a se deslocar para sul até 2010. As tendências encontradas são 0,37º ± 0,02º /década entre 1960 e o final da década de 70 e 0,10º ± 0,01º /década de 1980 a 2010. Os transportes das correntes do Brasil e das Malvinas, as latitudes dos rotacionais nulo e máximo do vento e o ı́ndice do Modo Anular Sul são também analisados no perı́odo de 1960 a 2010, como possı́veis forçantes da variação da latitude da separação da CB. Todas a séries obtidas apresentam mudança ao intensificação das tendências entre as décadas de 70 e 80 - assim como na série de separação da CB. Esse comportamento pode estar relacionado ao Modo Anular Sul. Este modo de variabilidade climática afeta as forçantes de separação da CB em escalas de tempo diferentes. O deslocamento para sul do limite sul do Giro Subtropical do Atlântico Sul observado nesse trabalho inicia-se no inı́cio da década de 90 e está relacionado com mudança nos regimes de vento da bacia causadas provavelmente por mudanças nos campos de temperatura de superfı́cie do mar.
In the southern oceans, only a few cold-water coral mound sites have so far been reported compared to the extensively surveyed North Atlantic. Here, we document for the first time the widespread occurrence of exposed and buried coral mounds at the northern Argentine margin named Northern Argentine Mound Province (NAMP). The integration of multibeam bathymetry,
sediment echosounder, Acoustic Doppler Current Profiler (ADCP), and CTD data allowed the characterisation of its morphosedimentary, stratigraphic, and hydrodynamic setting. The NAMP, covering at least 2000 km², represents the largest coral mound province in terms of areal extent reported so far and a major site in the southern oceans. The coral mound distribution is directly linked to morphosedimentary features of the regional Contourite Depositional System (CDS) influenced by the Antarctic Intermediate Water (AAIW), the Upper Circumpolar Deep Water (UCDW), and their interface, forming a complex bottom-current controlled environment here termed Coral-Contourite System. Coral mounds are most abundant in the lower AAIW between 900 and 1050 m, just above the Ewing Terrace Moat, which has been shaped by high-velocity currents associated with the AAIW-UCDW boundary. This clustering appears to be the result of an increased concentration of suspended sediment and food particles, transported along the AAIW-UCDW density gradient and delivered upward by turbulent hydrodynamic processes, which are enhanced by the irregular moat topography.
Within the Ewing Terrace Moat, coral mounds are restricted to topographic obstacles, modifying the otherwise highly erosive, destructive hydrodynamic regime and thereby locally creating favourable conditions for coral growth. The low-velocity currents of the UCDW, currently influencing the Ewing Terrace, promote extensive contourite deposition, which has led to a progressive demise and burial of pre-existing mounds. The presence of these buried mounds suggests past mound aggradation 100 to 150 m deeper likely due to a downward shift of the AAIW during glacials as a consequence of sea-level lowering. Overall, the mound formation in the NAMP seems to be mainly controlled by the availability of suspended particles, in combination with a sufficient bottom-current strength, keeping material in suspension while preventing destructive erosion and excessive sedimentation.
Our study highlights (1) the importance of high-energy processes at water-mass boundaries for the delivery of sediment and food particles to corals, (2) the role of the AAIW as a crucial water mass for coral mound development, (3) the variability of mounds in response to glacial interglacial cycles, and (4) the potential of CDSs in general and the Argentine margin in particular as hotspots for coral mounds and, thus, ideal site for future studies.
Future wide-swath altimetry missions will provide high-resolution information about ocean surface elevation, and facilitate the characterization of meso- and sub-mesoscale ocean activities. In this study, the demand analysis of three-dimensional (3D) oceanic state reconstruction on wide-swath SSH data features was evaluated using a data assimilation strategy. Three groups of experiments were performed to determine if the wide-swath altimetry observations would improve the three-dimensional (3D) field estimates of ocean temperature-salinity-velocity (T-S-V), and to evaluate how the spatial and temporal resolution and accuracy of the wide-swath altimetry observations affected the ocean state estimation. The Regional Ocean Modeling System and the four-dimensional variational data assimilation method were used in the experiments, with numerical simulation for the Taiwan region at a resolution of 1/10° as the example. The sensitivity of the 3D ocean state construction to the wide-swath altimetry measurements was also investigated. The results showed that the wide-swath sea surface height (SSH) measurements would have an overall positive impact on the 3D T-S-V field and that the positive effect would increase as the resolution and accuracy of the observations increased, but the net benefits would gradually decrease. Among the three examined features of the wide-swath altimetry observations, the temporal resolution had the most influence on the 3D ocean state analysis.
Radar altimeter is an active microwave remote sensor, which can be used to measure the sea surface height (SSH), the significant wave height (SWH) and the backscatter coefficient by transmitting electromagnetic waves and receiving echoes returned from the surface. The backscatter coefficient can be further used to obtain the wind speed. The data measured by the radar altimeter can be further used for study of marine gravity anomalies, geoids, sea topography, seabed topography, ocean tides, wind-wave fields and ocean dynamics. With the development of technology, radar altimetry data can be used not only for oceanographic research but also for inland water level changes, land dynamics, sea ice thickness changes, glacier mass balance and abnormal climate impacts.
In order to meet the application requirements, the measurements derived from the radar altimeter must reach a very high precision, which sets extremely high demands on the operational mechanism, system design and data processing technology for the radar altimeter. Error correction and echo signal processing technologies are important parts of the radar altimeter data processing technology, and they are of great significance to improve the accuracy of range measurements for radar altimetry.
The HY-2A radar altimeter is the first spaceborne radar altimeter of China. Since its operation in 2011, a large amount of observation data have been accumulated. The analysis, calibration, modeling and application of these data are the goal of satellite engineering.This paper focuses on the HY-2A radar altimeter and studies the error correction and sea-land echo signal processing technology. The HY-2A radar altimeter data are reprocessed. The reprocessed SSH and SWH products are evaluated and preliminary applications are conducted using the reprocessed altimeter data. The main work and innovations are as follows:
(1) Based on the fourth order Stokes wave theory, the relationship between the sea state bias and the wave period is discussed. The mean wave period from the ECMWF reanalysis ERA-interim is introduced to the sea state bias estination. A method based on the crossover data and a three-dimensional nonparametric model is proposed. Compared with the traditional two-dimensional sea state bias estimates based on the wind speed and the significant wave height, the three-dimensional sea state bias estimates based on the wind speed, significant wave height and mean wave period can increase the accuracy of the SSH products by 1.15 cm.
(2) An improved local linear regression estimator is proposed for the sea state bias estimation. The improved algorithm reduces the time complexity from O(n2) to O(n).
(3) The time tag biases of dual-frequency range measurements from the HY-2A radar altimeter are estimated and corrected. The corrected range measuements are applied to calculate the HY-2A dual-frequency ionosphere correction, significantly improving the accuracy of the HY-2A dual-frequency ionosphere correction.
(4) The whole process of data processing from the original waveform processing, error correction to product quality assessment has been conducted for the HY-2A radar altimeter. The accuracy of the sea surface height is better than 5.5 cm and the accuracy of the significant wave height is better than 0.2 m.
(5) The HY-2A radar altimter data are used to obtain global mean sea level change, El Niño, elevation maps of the Antarctica and Greenland ice sheets, and water level change of the Qinghai Lake, and these lay the foundation for subsequent application of the HY-2A radar altimeter data.
A nonlinear model is devloped and analytical results obtained to discuss the response of the Antarctic Circumpolar Current to wind forcing over a wide range of frequencies. The main results are as follows: (i) The nonlinear equations of motion can be conveniently separated into one ''baroclinic'' and one ''barotropic'' mode. (ii) For forcing with period T equal to less than a few years, wind-driven fluctuations in the Antarctic Circumpolar Current are barotropic and governed by the linearized Laplace tidal equations. Theory suggests that fluctuations in the transport should lag, and be most strongly correlated with, the circumpolar-averaged wind stress. These theoretical results are consistent with recent measurements made in Drake Passage. An interested untested theoretical prediction is that the sea level fluctuations measured at the southern side of Drake-Passage with T between one month and a few years should be coherent at zero lag with sea level fluctuations at the same latitude around the earth. (iii) For longer period forcing, baroclinic fluctuations are important. The baroclinic pressure, current and associated density variations all decrease exponentially with depth. Exponential depth decay of these baroclinic fields is in fact observed, the decay scale being about 1 km. (iv) The theory indicates thatmore » significant large-scale, wind-driven fluctuations in the strength of the baroclinic Antarctic Circumpolar Current can only occur at frequencies with periodicity > or approx. =70 years. Climatic changes associated with such variability must therefore consist of oscillations of similar or longer period. This is consistent with limited observations which suggest that wind and sea-surface temperature in the region of the Antarctic Circumpolar Current have fluctuated through one ''cycle'' over the last 100 years.« less
The dynamics of a wind-driven current in a zonal channel is reviewed and investigated with a quasigeostrophic β-plane model with two layers and eddy resolution. The model structure is similar to the one used by McWilliams, Holland and Chow (1978) and the channel has dimensions of 4000 km × 1500 km. The experiments with this model address the problem of the relative role of bottom friction and bottom form drag in the balance of a current driven by a steady eastward surface windstress. The response of the system is investigated for different values of the friction parameter and various locations of topographic barriers in the bottom layer of the channnel. The principal momentum balance emerging from these experiments support the concept of Munk and Palmén (1951) for the dynamics of the Antarctic Circum-polar Current proposing that the momentum input by the windstress is transferred to the deep ocean - in the present model by vigorous eddy activity - where it may leave the system by bottom form drag. Frictional effects in the balance of the circumpolar flow may thus be of minor importance.
A region of transition of surface water characteristics from
subantarctic to antarctic and an associated eastward flowing Antarctic
Circumpolar Current (ACC) have long been recognized to exist as a band
around Antarctica. In this review we summarize the most important
observational and theoretical findings of the past decade regarding the
ACC, identify gaps in our knowledge, and recommend studies to address
these. The nature of the meridional zonation of the ACC is only now
being revealed. The ACC seems to exist as multiple narrow jets imbedded
in, or associated with, density fronts (the Subantarctic and Polar
fronts) which appear to be circumpolar in extent. These fronts meander,
and current rings form from them; lateral frontal shifts of as much as
100 km in 10 days have been observed. The volume transport of the ACC
has been estimated many times with disparate results. Recently, yearlong
direct measurements in Drake Passage have shown the mean transport to be
approximately 134 × 106 m³/s, with an uncertainty
of not more than 10%. The instantaneous transport can vary from the mean
by as much as 20%, with most of the variation associated with changes in
the reference flow at 2500 m rather than in the vertical shear.
Meridional exchanges of heat across the ACC are known to be important to
the heat balance of the abyssal ocean and consequently to global
climate. The most likely candidate process for the required poleward
heat exchange seems to be mesoscale eddies, though narrow abyssal
boundary currents may also be important. Observations from ships,
drifters, and satellites reveal surface mean kinetic energy to be at a
maximum along the axis of the ACC and eddy kinetic energy to be large
mainly in western boundary regions and off the tip of Africa. Eddy
variability in the open ocean is consistent with baroclinic instability
of the narrow jets. Calculations using data from Drake Passage show that
the necessary conditions for baroclinic and barotropic instabilities are
met in the ACC. The basic dynamical balance of the ACC is still not well
known, although bottom and lateral topography and dynamic instabilities
are shown to be important in balancing wind forcing. The ACC is
generally conceded to be driven by the wind, but the coupling of wind
and thermohaline circulations have not yet been adequately investigated.
The mechanism responsible for the multiple cores of the ACC has not been
identified in detail. It is suggested that future studies address: (1)
the circumpolar structure and temporal behavior of the Subantarctic
Front and Polar Front; (2) the general dynamical balance of the ACC and
specific mechanisms for creation and maintenance of the major fronts;
(3) the representativeness to the entire ACC of the existing estimates
of meridional exchanges of heat and other properties, as well as
kinematic and dynamic quantities, made in Drake Passage; (4) the
variability of the ACC transport in several places and coherence of its
variability; (5) the climatology of fields of atmosphere-ocean forcing
over the southern ocean; and (6) the possibility of identifying and
using simple indices as good indicators of the behavior of the ACC or
parts thereof.
Daily averaged positions from about 280 satellite-tracked drifting buoys launched during the first GARP Global Experiment are used to estimate the sea surface current velocity field south of 20°S. The time-averaged surface velocity field based on velocity averages over 4°×4° areas is used to construct the surface stream function. The gross features of the mean circulation are generally in agreement with other representations of the time-averaged circulation although they do differ in details from the climatological relative surface dynamic topographies in the southern hemisphere subtropical gyres. Mean and eddy horizontal momentum fluxes based on a 2°×2° grid are estimated for the region south of 40°S. The zonally averaged momentum flux at the sea surface for the region of the Antarctic Circumpolar Current, from 40° to 60°S, leads to a momentum flux divergence of 60 cm2 s-2 indicating a meridional flux of eastward momentum away from the current axis. Given the relatively large errors associated with the zonally averaged momentum fluxes, the sign of the flux divergence remains uncertain.
Three months of vector wind observations from the Seasat-A satellite scatterometer (SASS) were used to construct gridded fields of monthly average wind stress and wind stress curl over the global ocean. These fields were examined to identify features either poorly resolved or not present in wind stress fields constructed from conventional data. Particular attention was focused on the spatial structures in the high southern latitudes and the tropical regions. The SASS wind stress fields are compared globally with the Hellerman and Rosenstein climatological monthly average surface wind stress fields and with monthly averages of wind stress computed from contemporaneous 1000 mb wind analyses produced by the National Meterological Center. The potential for satellite scatterometry improving the present knowledge of the global wind stress field and making important contributions to ocean modelling is demonstrated by comparison of global maps of the Sverdrup circulation computed from 3-month average SASS and Hellerman and Rosenstein climatological wind stress curl fields. (from authors' abstract)
Bottom friction is an important sink of energy in the ocean. Indeed, high-resolution ocean models need bottom friction to achieve a satisfactory kinetic energy level at equilibrium. However, bottom friction has also subtle and discriminating effects on the different energy transfers and therefore on the 3D structure of the flow, some of which have to be clarified. In this study, those effects on an unstable baroclinic jet are reexamined using a primitive equation model. As in previous studies using quasigeostrophic models, it was found that bottom friction strongly affects the barotropic mode whereas the baroclinic modes are weakly changed. The new result is that bottom friction yields a significant space-scale selection. Analysis of the dynamics reveals strong agreement with previous quasigeostrophic studies at the mesoscale in the interior but differences in the eddy field at small scales close to the surface. A rationalization of these results is proposed by a comparison with preceding atmospheric studies. It is shown that the ''barotropic governor'' of James and Gray is not active in ocean simulations and that the scale selection induced by bottom friction is primarily induced by nonlinear interactions in the three-dimensional structure of the eddy field.
The surface response of the Southern Hemisphere's oceans to the large spatial scale, interseasonal changes in wind forcing during the FGGE year of 1979 is investigated. The primary data are the analyzed daily wind fields, and the trajectories of the FGGE drifting buoy array. The zonal wind forcing is characterized by large spatial patterns of low frequency (annual and semiannual) variability. particular attention is paid to the second harmonic, which has amplitude peaks at 35°–40° S with solstitial maxima, and amplitude peaks at 60°S with equinoctial maxima. The distinct phase change occurs at 50°S. The analysis of the drifting buoy data is guided by the wind patterns, but first the question of the current-following characteristics of the FGGE buoys is addressed. Compared to the wind, the buoy drift has even larger spatial scales, and more low frequency contributions to its intra-annual variance. Like the wind, amplitude peaks in the second harmonic of monthly mean zonal drift are found in each o...
The eddy kinetic energy distribution in the Southern Ocean has been estimated from Seasat altimeter data available on repetitive orbits during 24 days in September-October 1978 and from 192 free-drifting buoy trajectories obtained during the First GARP Global Experiment (FGGE) from November 1978 to the first months of 1980. A good spatial coherence is found between the results of these two independent methods in the Southern Ocean from 30°S to 65°S. The distribution shows strong, eddy kinetic activity near the western boundaries (Argentine Basin, Mozambique Plateau, Tasman Sea, etc.) and near topographic features (ridges and continental shelves: Drake passage, Crozet and Kerguelen plateaus, Macquarie Rise, etc.). At first the ratio between both estimates was found to be very high, with kinetic energy distribution from FGGE drifters being 3 times larger than from the altimeter. After an investigation of the effect of the wind on eddy kinetic energy estimates from drifting buoys it seems that the wind cannot be the main reason for such a large discrepancy. Next, the eddy kinetic energy distribution from drifters was recomputed and processed through a band-pass filter, in the range 1/6 to 1/24 cycles per day, in order to achieve a better match to the temporal scales explored by Seasat altimeter measurements. Although there still remain some differences between the two distributions, comparable values for mean eddy kinetic energy estimates over the whole Southern Ocean are then found. The remaining differences might be attributed to various sources, one of them being the nonsimultaneity of the two data bases.
Techniques for the synoptic analysis, vertical inference, dynamical adjustment and forecast of altimetric and deeper in situ data are presented as a first step towards the design of continuous assimilation schemes in limited area oceanic domains. A year long time series of stream function maps, denoted Mark 2, drawn in the POLYMODE area of the western North Atlantic was used as a benchmark for various tests and simulations. An original projection/extension scheme using empirical modes of density and/or pressure anomalies was used to obtain a first guess of the three dimensional structure of the currents, starting from surface topography only.
The separation of the Brazil and Malvinas (Falkland) Currents from the western boundary is explored with the use of satellite and drifter data. The location of the separation of these boundary currents from the continental margin over a multiyear period is determined by digitizing the crossing of the surface thermal front indicative of each feature with the 1000 m isobath. Three years (July 1984 to June 1987) of 1 km resolution AVHRR data collected by the Argentina Meteorological Service and 4 years of lower resolution Global Retrieval Tape (GRT) data were used to generate a total time series extending from November 1981 to June 1987; i.e. years.The mean latitudes of separation from the shelf break are 35.8 ± 1.1° for the Brazil Current and 38.6 ± 0.9° for the Malvinas Current. The along-coast ranges of the separation positions, 930 and 850 km, respectively, are quite large relative to similar statistics for the Gulf Stream or Kuroshio. Observed temporal variability suggests cyclical excursions of the currents along the coast at semi-annual and annual periods, although there is considerable interannual variation in the signal. Drifter trajectories overlaid on satellite images demonstrate events associated with the annual transition in 1984–1985. Shorter time-scale perturbations in the currents' separation latitudes occur in the 30–60 day band, which corresponds to the mesoscale eddy field. The connection of the variation in the separation with various possible forcing mechanisms is briefly discussed, along with the problem of gaining a theoretical understanding of this dynamic situation. Finally, the extensions of the two currents into the South Atlantic interior are described statistically using the high resolution data set.
The annual mean depth-integrated steric height P and stream function ψ of the world ocean are calculated from a Sverdrup model with Hellerman and Rosenstein's (1983) annual mean wifids. The parameterization of friction is unspecified, but friction is assumed to be important only along western boundaries. A simple rule, based on Sverdrup interior flow and geostrophy of longshore flow along western boundary currents, is used to calculate P at the inshore edge of the western boundary current. The substantial predicted longshore gradients of P drive the model western boundary currents against friction, independent of frictional parameterization. One corollary is an “Island Rule”–an explicit expression for circulation around an island, in terms of wind stress, again independent of frictional parameterization. The circulations around Australasia, New Zealand and Malagasy are calculated as 16±4 Sv, 29±v, and 4±3 Sverdrups respectively. Inclusion of island effects result in more accurate flow estimates in the Southern Hemisphere than have previously been obtained from Sverdrup models.
In this analysis of satellite-tracked drifting surface buoys released in the Southern Ocean, buoy velocities are averaged along trajectories for 90 days to determine the mean circulation, and eddy kinetic energy is computed using perturbations about the 90-day mean. The 90-day mean is based upon wind-driven circulation theory, Southern Ocean wind spectra, cumulative averages of mean buoy velocity and energy, and plots of the resulting mean circulation. Excluding the Antarctic Circumpolar Current (ACC), the spatial distribution of the eddy kinetic energy is consistent with previous distributions, based upon spatially averaged ship drift and surface drifting buoy data, reported in the literature. In the ACC, values of eddy kinetic energy are generally lower than found elsewhere. The analysis indicates that the spatial structure of perturbations about the 90-day mean is zonally extended and meridionally compressed. The meridional compression may be due to β, which acts as the restoring force for pla...
The yearly wave (first harmonic) in the sea level mean pressure on the southern hemisphere is largest at 30° S over and near the three continents, where it also explains most of the mean annual variance. In middle and high latitudes, the half-yearly wave dominates so that even in the Antarctic the yearly wave is mostly secondary. In single years, the amplitude of the yearly wave south of 40° S may be large, but as the phase varies widely from one year to another, the mean wave becomes small.The yearly wave in the mean zonal geostrophic wind at sea level is largest in the tropics and subtropics. Like the pressure wave, it is second to the half-yearly wave over most of middle and high southern latitudes. The phase of the yearly wave appears in four concentric belts as follows: maximum westerlies/minimum easterlies in summer in the tropics outside the eastern Pacific Ocean; maximum westerlies/minimum easterlies in winter in the subtropics; maximum westerlies in summer in the latitudes round 50° S (unlike the northern hemisphere); and maximum westerlies/minimum easterlies in winter over most of the area covered by sea ice. Just as the pressure wave, the yearly wave in the zonal wind has a large interannual variation.
We outline the half-yearly oscillations in sea-level pressure gradients and zonal winds on the Southern Hemisphere by means of daily synoptic maps for several years. The pressure oscillation has amplitude peaks near 50° and the Antarctic coast, separated by a minimum along 60° where the phase changes from maxima in the extreme seasons in the south to maxima in the transitional season in the north. The zonal geostrophic wind consequently has a half-yearly wave with equinoctial maxima in the latitudes near 60°, and one with solstitial maxima north of 50°.We describe the interannual variability of the half-yearly oscillations in the pressure gradients and winds, and give examples of the oscillations during FGGE and the IGY.
This review article highlights the three-century development of our scientific understanding of ocean tides, culminating through myths, paradoxes, and controversies in a global tide model that now permits the prediction of the instantaneous total tide anywhere in the open oceans with an accuracy of better than 10 cm. All major aspects of tidal research, including empirical, mathematical, and empirical-mathematical methods, are considered. Particular attention is drawn to the most recently developed computerized techniques comprehending hydrodynamical dissipation and secondary tide-generating forces, finite-differencing schemes, geometric boundary and bathymetry modeling, and hydrodynamical interpolation of properly selected empirical tide data. Numerous computer experiments are mentioned that were carried out by various researchers in order to evaluate the magnitudes of the featured effects. Further possible improvements are mentioned, especially in nearshore areas, in the Arctic Sea, and near Antarctica, where empirical tide and bathymetry data are either rough or marginal.
Monthly charts of the approximately zonal Tasman Front between the Australian coast, (near 152°E) and 164°E, and between latitudes, 31° to 37°S have been constructed from infrared satellite imagery for 29 months from March 1982 to April 1985. From these it was found that one to three meanders, but usually two, are present between 152° and 164°E, and that the second meander pinches off at times to form a warm-core ring in the same manner as is known to occur for the first meander. Both second meander and ring tend to propagate westwards. The mean zonal widths of the first and second meander are similar and are 3.6° to 3.7° longitude (330–345 km). The average and the most likely (modal) positions of the Tasman Front were determined and the two positions are most different when the Front meanders northward. The zonal length scale of frontal perturbations increases eastward from 140 near 155°E to 300 km near 161°E, and perturbations propagate westward at approximately 0.1 m s−1.
A time series of net transport during 1979 of the Antarctic Circumpolar
Current (ACC) through the upper 2500 m of Drake Passage was obtained as
part of the International Southern Ocean Studies program by Whit worth
(1983), and was presented in a revised form by Whitworth and Peterson
(1985). Spectral analyses are performed on records comprising the
revised series to determine the major sources of transport variability.
Multiyear records from bottom pressure gauges at each side of the
passage are then compared with fields of wind stress over the Southern
Ocean. The net transport series is dominated in the subseasonal time
scales by the lunar fortnightly and monthly tides and by baroclinic
activity in the northern passage, processes that are independent of
wind. For seasonal time scales, transport variability appears to be
mainly barotropic, thus being well represented by bottom pressure
records from each side of the passage. Comparisons of multiyear bottom
pressure records with time series of the zonally averaged curl of wind
stress in narrow bands along the perimeters of the ACC suggest that
seasonal fluctuations in the strengths of the subtropical and subpolar
atmospheric pressure systems flanking the ACC induce changes in the
slope of the sea surface across the current. The dominant annual cycle
in curl associated with the subtropical highs appears to influence sea
level across the entire ACC, whereas the effects of the strong
semiannual cycle in curl associated with the subpolar lows are confined
to the southern portions of the current.
The last decade of research in the Southern Ocean has shown that the Antarctic Circumpolar Current (ACC) is a complex system composed of narrow, high-speed currents separated by broad, quiescent zones. The circumpolar nature of this structure was examined using position and velocity data obtained from approximately 300 surface-drifting buoys deployed in the Southern Ocean during the First GARP Global Experiment (FGGE). The distribution of buoys on 1°×1° squares shows that in some regions, most notably south of Australia, the buoys form three ccoherent bands of high buoy density which are separated by regions of low buoy density. The latitudes of these bands coincide with those of the Subtropical Front, Subantarctic Front, and Polar Front. The further examine the relationship between these fronts and buoy distribution, locations of the three fronts, determined from historical hydrographic data, were used to partition the buoys into zonal bands corresponding to front and nonfront regions. A mean buoy density and mean near-surface speed were then computed for each zonal band. High buoy densities were associated with all three fronts in the region south of Australia. Other regions also showed a tendency, although not as pronounced, for buoys to accumulate in fronts. The mean near-surface speeds suggest that the Subantarctic and Polar Fronts are circumpolar. Moreover, the mean near-surface speeds associated with the three frontal regions differ. Speeds within the Subantarctic and Polar Front regions are approximately twice that associated with the Subtropical Front.
The book is planned as one of a series to reflect growing interest in polar research and is largely intended for students moving into oceanography or polar studies from other scientific backgrounds. The first part deals with the early ideas and evidence of a great southern continent, the pioneering observations of the early explorers and of the sealers who profited from the new discoveries, the observations made by polar explorers on their way to the continent, and the systematic studies of oceanographic expeditions. The second part summarizes present knowledge of the water movements and their probable effects on temperature and salinity distributions, biological productivity, distributions of marine plants and animals, climate and ice cover. It tries to show how the present knowlede has grown from earlier findings, and to indicate its relevance to economic problems, such as the conservation of marine living resources. -from Author
Meausrements from bottom pressure gauges located at the north and south sides of Drake Passage are used to extend the one-year time series of volume transport of the Antarctic Circumpolar Current (ACC) given previously by Whitworth. A small error in that paper is corrected, and a revised transport time series is presented which shows the importance of including the transport in the northern and southern margins of Drake Passage. Direct measurements of vertical shear averaged across the passage, are in good agreement with geostrophic shear and suggest that the ACC is in geostrophic balance. Although most of the ACC transport is in the baroclinic field, transport fluctuations are mainly barotropic. Transport estimates based on pressure difference alone differ from the estimates of Whitworth by less than 10 × 106 m3 s−1. Fluctuations in transport of almost half of the mean value occur over periods as short as two weeks.
Yearlong measurements of current and temperature are used to examine the characteristics of eddy heat flux and its distribution in Drake Passage. This seemingly straightforward calculation can produce misleading results when measurement errors are not corrected and when careful consideration is not given to the nature of the time series under study and the coordinate system used. When moorings blow over during periods of high current speeds, eddy heat fluxes produced from the resulting records may be overestimated by as much as 20% if not corrected by using concomitant pressure time series and vertical temperature gradients. Although short-time-scale processes (such as tides, internal waves, and inertial oscillations) are routinely eliminated from heat flux estimates by low-pass filtering, low-frequency contamination of eddy heat fluxes is not usually considered. Long-period events can impose unwanted, dominating cross correlation (eddy heat flux) between fluctuation temperature and velocity. In Drake Passage, low-frequency variability in some current records is associated with sporadic lateral shifts of the fronts within the Antarctic Circumpolar Current. Defining an effective eddy time scale (40 hours to 90 days) and band-passing the current records before calculating eddy heat flux results in values that are more homogeneous (consistent with one another) in direction and have higher statistical significance. Finally, we consider the coordinate system used for the eddy heat flux calculations relative to the dyanamic processes under study. For the Circumpolar Current we define poleward eddy heat flux as that component perpendicular to the axis of the current that forms a continuous, but distorted, band encircling Antarctica. Since the current direction varies in time, across-stream eddy heat flux is calculated relative to the 90-day low-pass direction. The resulting heat fluxes from all deep instruments are small (about 1 kW/m2), whereas those at depths less than 1000 m in the northern passage are an order of magnitude larger.The average across-stream flux for all available instruments is 3.7 kW/m2 and directed to the right (poleward) of the 90-day low-pass current. Cospectra of fluctuation temperature and velocity show the heat flux is separated, on the average, into two frequency bands with corresponding periods between 100 and 40 days and 16 and 10 days. This pattern is strongly influenced by the records from shallow instruments and those in the northern passage and compares favorably with the cospectrum from a 5-year bridged record from the central passage. Comparison with previous estimates in this region shows considerable differences, partly as a result of the use of different techniques. However, analysis of five 1-year records from a single location shows the interannual variability is large, which urges caution when interpreting the significance of isolated short-term estimates of eddy heat flux.
Values of poleward heat flux due to low-frequency current fluctuations in Drake Passage are presented for 19 long-term current meter records obtained during 1975, 1976 and 1977. Most of the measurements (10) are in the center of the passage near the historical location of the Polar Front where the flux is found to average 1.7 W cm/sup -2/. While the variability is large at this location (range of 0.9--2.8 W cm/sup -2/ and a standard deviation of 0.7 W cm/sup -2/), all measurements are within the same order of magnitude. There is no obvious depth dependence in the measured flux.
Locally weighted regression, or loess, is a way of estimating a regression surface through a multivariate smoothing procedure, fitting a function of the independent variables locally and in a moving fashion analogous to how a moving average is computed for a time series. With local fitting we can estimate a much wider class of regression surfaces than with the usual classes of parametric functions, such as polynomials. The goal of this article is to show, through applications, how loess can be used for three purposes: data exploration, diagnostic checking of parametric models, and providing a nonparametric regression surface. Along the way, the following methodology is introduced: (a) a multivariate smoothing procedure that is an extension of univariate locally weighted regression; (b) statistical procedures that are analogous to those used in the least-squares fitting of parametric functions; (c) several graphical methods that are useful tools for understanding loess estimates and checking the assumptions on which the estimation procedure is based; and (d) the M plot, an adaptation of Mallow's Csubp/sub procedure, which provides a graphical portrayal of the trade-off between variance and bias, and which can be used to choose the amount of smoothing.
In April 1985 the U.S. Navy satellite GEOSAT began generating a
remarkable data set that may change the way in which physical
oceanographers view the global oceans. GEOSAT (Figure 1) carries a radar
altimeter that provides a continuous record of sea level along the
satellite ground track. Such records enable determination of sea level
variability and have application in many areas of ocean dynamics.
Experience with GEOS 3 (Geodynamics Experimental Ocean Satellite 3) and
Seasat in the 1970s demonstrated the enormous potential of altimetry for
oceanography. Seasat, for example, gathered sufficient altimeter data in
its last 25 days alone to yield a global description of the mesoscale
eddy field [Cheney et al., 1983], wave number spectra of sea level
variability [Fu, 1983], and a global model of the M2 tide [Mazzega,
1985].
Trajectories of approximately 300 satellite-tracked drifting buoys deployed throughout the Southern Hemisphere oceans during the Fiat GARP Global Experiment (FGGE) have been analyzed to infer the mean surface circulation and kinetic energy distributions of the surface flow. The resulting picture of mean surface circulation is consistent with expectations based on compilations of historical observations of currents and the density field. The distribution of kinetic energy of the mean flow is highly zonal in character with high values in the persistent zonal flows of the Antarctic Circumpolar Current and the South Equatorial Currents and low values in the broad intervening zone. Using statistics based on 5° squares, the western boundary currents do not emerge as dominant features of the mean flow. On the other hand, these currents do appear to be a major source of eddy kinetic energy. Most eddy kinetic energy appears to be due to fluctuations with periods less than one month. Both mean and eddy kin...
In March 1985, the U.S. Navy altimeter satellite GEOSAT began generating
a data set with unprecedented spatial and temporal coverage of the
global oceans. The satellite carries a radar altimeter that provides a
profile of sea level along the satellite ground track. Such records
enable determination of sea level and its variability, and have
application in many areas of geodesy and ocean dynamics. As suggested by
the acronym GEOSAT, the satellite's primary purpose was improvement of
the marine gravity field. Because of the value of this information to
the U.S. military, the first 18 months of observations are classified.
However, the geodetic mission ended September 30, 1986, and on October
1, a series of maneuvers was begun to alter the orbit to produce sea
surface profiles within a few kilometers of previous released SEASAT
data tracks. This orbit has a repeat period of about 17 days, and the
mission is now referred to as the GEOSAT Exact Repeat Mission (ERM). The
ERM officially became operational November 8, 1986. The contents of the
report include the following: the GEOSAT altimeter mission; geophysical
data records (ocean); geophysical data records (land and ice); and data
analysis considerations.
Over 35 million surface observations covering the world ocean from 1870–1976 have been processed for the purpose of calculating monthly normals and standard errors of the eastward and northward components of the wind stress and work done by the winds in the lower 10 m of the atmosphere. The fields are intended to serve as boundary conditions for models of the ocean circulation. Wind and air-minus-sea temperatures are calculated in a form suitable for determining stress by any bulk aerodynamics model in which the drag coefficient can be represented by six or less coefficients of a second-degree polynomial in wind speed and stability. The particular case of the wind speed and stability dependent drag coefficient discussed by Bunker is selected for analysis. January and July charts of wind stress, curl of the wind stress, mass transport stream-function, divergence of the Ekman transport and the rate of mechanical energy transfer are illustrated and discussed.
Data from three years of pressure gauge measurements in the Drake Passage are interpreted in terms of fluctuations in transport of the Antarctic Circumpolar Current. The pressure data are compared with wind-stress data calculated from gridded southern hemisphere atmospheric-pressure data. Transport fluctuations with periods longer than 30 days are highly correlated with fluctuations in wind stress integrated over the Southern Ocean, with transport lagging wind by about nine days. The results are consistent with the rate at which momentum is supplied to the current system by wind stress.
Two-year time series measurements of current and temperature were made at the northern edge of the Antarctic Circumpolar Current in the southwestern Pacific from 1978 to 1980 to investigate eddy heat and momentum fluxes in the circumpolar region. The measurement site near 49° 30′S 170°W is at a transition zone or front between subantarctic and subtropical waters in which meridional temperature and salinity gradients and, hence, vertical shear in eastward velocity extend down to 2,000 m depth. Energetic eddies are found to have typical amplitudes of 20 cm s−1 at 1,000 m depth, to be vertically coherent over the depth range from 1,000 to 5,000 m, to vary over temporal scales of 20 days and horizontal scales of 60 km and to propagate southeastward at about 12 cm s−1. Observed eddy kinetic energy of 169 cm2 s− at 1,000 m depth is larger than that at comparable depths in the Gulf Stream and Kuroshio extension regions and in central Drake Passage. Like Drake Passage eddies, these eddies transport a significant amount of heat poleward in the band of periods between 20 and 50 days, but this band does not dominate the spectrum the way it does in Drake Passage. Energetic fluctuations with periods longer than 50 days effect an eddy heat flux whose sign is ambiguous so that the net meridional heat flux is not statistically significant. Eddy momentum fluxes transport eastward momentum northward away from the Circumpolar Current. These fluxes are statistically significant, but are only about one fourth of the size required to balance the eastward momentum put into the water column by the wind. An estimate of the standing eddy momentum flux due to large spatial scale variations in the Circumpolar Current is also too small so that pressure forces against bottom topography is likely to be the mechanism by which the wind input is balanced.
A method that is independent of choice of reference level is advanced for calculating, from standard hydrographic data, advective oceanic heat flux by mean geostrophic motions. The method depends on the ability to choose a path of constant vertically averaged potential temperature (reference temperature). It was applied to a collection of historical hydrographic data from the Southern Ocean. A circumpolar path with reference temperature 1.3°C that closely follows the mean position of the Antarctic Polar Front was chosen. The advective geostrophic heat flux across this path was calculated to be 0±23×1013 W. The standard error (and bias, which was found to be small) of this calculation was estimated from a statistical model of correlation of ocean variability that seems appropriate to the way the data are sampled in space and time. The wind-driven Ekman heat flux was calculated at −15×1013 W, that is, equatorward. To balance Gordon's estimate of sea to air heat transfer of 30×1013 W south of the Polar Front, a compensating poleward flux of +45±30×1013 W is postulated. Eddy heat flux seems a prime candidate for accomplishing this flux.
A sequence of numerical calculations has been made for the equilibrium balances of eddies and mean currents in open and partially blocked, periodic channels. The physical model employed is a two-layer, quasigeostrophic, wind-driven one, with important bottom friction and weak lateral friction. The resolved eddies provide the interior fluxes of momentum and potential vorticity which allow the mean state to be a balanced one. The set of calculations does not provide a parameter study as such, but does provide examples of the influences of alternative physical processes and geometrical constraints. These alternatives include the presence or absence of a partial barrier across the channel, the length of the channel, the addition of a transient component to the wind-driving, and the addition of a topographic sill across the channel gap. Particular attention is focused upon the steadily driven general circulation of a β-plane channel, because of the structural simplicity of the solution. The results may be broadly summarized as follows. The eddies are generated by a baroclinic instability of the mean flow. They act to intensity the upper layer mean jet and mean cross-jet potential vorticity gradient (through eddy horizontal Reynolds stress and relative vorticity flux divergence, respectively) and to transfer downwards mean zonal momentum, energy, and potential vorticity gradient (through eddy interfacial pressure drag, vertical pressure work, and vortex stretching flux divergence, respectively). In the case of a zonally uniform channel, the meridional heat flux is found not to conform closely to previously proposed parameterizations. The presence of a partial meridional barrier and a topographic obstacle are found to strongly influence the equilibrium solution, while neither a change in the basin length nor the presence of a transient wind component appear to importantly alter the solution.
In June of 1974 Jim Pool, then director of the research section of the Applied Mathematics Division of Argonne National Laboratory, initiated a series of informal meetings to consider the possibility of producing a package of quality programs for the solution of linear systems and related problems. The participants included members of the Laboratory staff, visiting scientists, and various consultants and speakers at the AMD colloquium series. It was decided that there was a need for a LINPACK and that there was a secure technological basis for its production. Accordingly, a proposal was submitted to the National Science Foundation, which agreed to fund the project for three years beginning January 1976;, the Department of Energy also provided support at Argonne.
The LINPACK project had a number of objectives. In the first place, it represented research into the mechanics of software production; to our knowledge no package has been produced in quite the way we have done it, and some of the many things we have learned from the effort will appear in subsequent publications. Secondly, we hope that we have provided a yardstick against which future mathematical software projects can be measured. Third, we hoped to produce a package that would be used, both as library subroutines and by people who wish to modify or extend the code to handle special problems (however, to protect the innocent, we request that anyone who modifies a LINPACK code also change the name of the subroutine and comment the modifications). Finally, we hope that LINPACK and this guide will be of value in the classroom.
These goals have imposed constraints on the code and its documentation. We have tried to make the code both machine independent and efficient. Although these two objectives are often at odds, we feel that in LINPACK we have gone far toward achieving both. The code itself has to be in Fortran, which is the language of scientific programming in the United States. We have tried to mitigate the unclarity of Fortran codes by carefully structuring our programs and adopting indentation conventions that reflect the structure. In addition the programs share a common nomenclature and uniform typing and commenting conventions. In documenting LINPACK we have tried to serve both the casual user and the person who must know the technical details of our programs. We have done this by segregating user oriented material for any particular set of programs into the first three sections of each chapter of the documentation, leaving the technical material for later.
Copyrighted by American Geophysical Union. A new method is developed for studying large-scale temporal variability of ocean currents from satellite altimetric sea level measurements at intersections (crossovers) of ascending and descending orbit ground tracks. Using this method, sea level time series can be constructed from crossover sea level differences in small sample areas where altimetric crossovers are clustered. The method is applied to Seasat altimeter data to study the temporal evolution of the Antarctic Circumpolar Current (ACC) over the 3-month Seasat mission (July-October 1978). The results reveal a generally eastward acceleration of the ACC around the Southern Ocean with meridional disturbances which appear to be associated with bottom topographic features. This is the first direct observational evidence for large-scale coherence in the temporal variability of the ACC. It demonstrates the great potential of satellite altimetry for synoptic observation of temporal variability of the world ocean circulation.
Direct estimation of the absolute dynamic topography from satellite altimetry has been confined to the largest scales (basically the basin-scale) owing to the fact that the signal-to-noise ratio is more unfavorable everywhere else. But even for the largest scales, the results are contaminated by the orbit error and geoid uncertainties. Recently a more accurate Earth gravity model (GEM-T1) became available, providing the opportunity to examine the whole question of direct estimation under a more critical limelight. It is found that our knowledge of the Earth's gravity field has indeed improved a great deal. However, it is not yet possible to claim definitively that our knowledge of the ocean circulation has improved through direct estimation. Yet, the improvement in the gravity model has come to the point that it is no longer possible to attribute the discrepancy at the basin scales between altimetric and hydrographic results as mostly due to geoid uncertainties. A substantial part of the difference must be due to other factors; i.e., the orbit error, or the uncertainty of the hydrographically derived dynamic topography.
Eight to nine sets of global collinear altimeter data with a cross-track grid spacing of approximately 900 km at the equator and 600 km at midlatitude have been obtained from the last 25 days of the 1978 SEASAT mission. Since the geoid is time-invariant, such observations can reveal sea surface height variability caused by dynamic ocean phenomena. Variations due to deep ocean mesoscale features are presently solved for by eliminating the longer wavelength deviations, yielding sea height profiles with an accuracy of a few cm, and a global variability map constructed from these data which reveal an exceptionally realistic perspective of mesoscale energetics. The dominance of exceedingly small variability over extensive regions of the oceans is noted. In both the Atlantic and the Pacific, the North Equatorial Current systems were clearly expressed as zonal bands of higher variability.
An account is given of the pulse-compression technique, in which a radar altimeter transmits a relatively long pulse and processes the returned signal in a way equivalent to transmitting a very short one and measuring the returned power in a sequence of range gates. The effective short pulse enhances the range resolution that would be obtained from the actual long pulse. Pulse compression and sea-level tracking are important to the overall error budget for altimetric estimates of sea level. Attention is presently given to the high degree of accuracy required for the NASA TOPEX altimeter scheduled for launch in mid-1992.
The accuracy of large-scale functions to approximate the orbit error in satellite altimetry is examined. Functions are presented for the approximation of bias, bias and tilt, bias tilt, and curvature. The method for selecting the best approximation and performing a rigorous error analysis is described.
Results are reported from an extensive investigation of orbit-determination strategies for the Geosat Exact Repeat Mission (ERM). The goal is to establish optimum geodetic parameters and procedures for the computation of the most accurate Geosat orbits possible and to apply these procedures for routine computation during the ERM for the following purposes: (1) to enhance the value of the Geosat oceanographic investigations by providing the user community with improved ephemerides, (2) to develop orbit determination techniques for the upcoming altimetric mission Topex/Poseidon, and (3) to assess the radial orbit accuracy obtainable with recently developed gravity models. To this end, ephemerides for the entire first year of the ERM have been computed using the GEODYN II orbit program on the Cyber 205 supercomputer system at the NASA Goddard.
Precise ephemerides have been determined for the U.S. Navy Geosat Exact Repeat Mission (ERM) using an improved gravity-field model, PTGF-4A (Shum et al. 1989). The Geosat orbits were computed in a terrestrial reference system which is tied to the reference system defined by satellite laser ranging (SLR) to Lageos through a survey between the Tranet Doppler receiver and the SLR system located at Wettzell, FRG. The remaining Doppler tracking station coordinates were estimated simultaneously with the geopotential in the PTGF-4A solution. In this analysis, three continuous 17-day Geosat orbits, which were computed using the 46-station Tranet data and global altimeter crossover data, have a crossover residual rms of 20 cm, indicating that the Geosat radial orbit error is of the order of 20 cm. The orbits computed based on data collected by a 7-station OPNET tracking network and crossover data have the same level of accuracy.
Time changes in global mesoscale sea level variances were observed with satellite altimetry between November 1986 and March 1988, showing significant, geographically coherent seasonal patterns. The NE Pacific and NE Atlantic variances show the most reliable patterns, higher than their yearly averages in both the fall and winter. The response to wind forcing appears as the major contributor to the NE Pacific and Atlantic signals; errors in the estimated inverse barometer response due to errors in atmospheric pressure, residual orbit errors, and errors in sea state bias are evaluated and found to be negligible contributors to this particular signal. The equatorial regions also show significant seasonal patterns, but the uncertainties in the wet tropospheric correction prevent definitive conclusions. The western boundary current changes are very large but not statistically significant. Estimates of the regression coefficient between sea level and significant wave height, an estimate of the sea state bias correction, range between 2.3 and 2.9 percent and vary with the type of orbit correction applied.
Mesoscale eddies constitute the most energetic component of the variability of ocean currents. Sea level variations measured by the Geosat radar altimeter are used to study the spatial and temporal scales of the eddy motion. An attempt is also made to map the temporal evolution of the eddy field in the region of the Agulhas Current south of Africa, where the eddy motions are among the strongest in the world. The results demonstrate that Geosat has provided an unprecedented opportunity to map from space the temporal evolution of sea level variability associated with the energetic eddies in the ocean.
The choice of an orbit for satellite altimetric studies of the ocean's circulation and tides requires an understanding of the orbital characteristics that influence the accuracy of the satellite's measurements of sea level and the temporal and spatial distribution of the measurements. The orbital characteristics that influence accurate calculations of the satellite's position as a function of time are examined, and the pattern of ground tracks laid down on the ocean's surface as a function of the satellite's altitude and inclination is studied. The results are used to examine the aliases in the measurements of surface geostrophic currents and tides. Finally, these considerations are used to specify possible orbits that may be useful for the upcoming Topex/Poseidon mission.
The detection of tides on the Patagonian shelf off Argentina by the SEASAT satellite radar altimeter is reported. The satellite data was obtained during two passes of the satellite over the shelf on August 1 and August 4, 1978, and compared with empirical estimates of the tidal elevation along the satellite ground tracks based on coastal tide gage data, an estimate of tidal variation over the shelf, and the tidal prediction equations of Shureman (1958). Good agreement was observed between the satellite data of sea surface height with respect to a standard ellipsoid and the empirical estimate for the near-shore pass, with agreement degraded slightly for the pass further off shore. It is concluded that the generally good agreement found demonstrates the applicability of satellite radar altimeter data to the determination of tides and that small length scale fluctuations in the geoid are minor along the two ground tracks.
The influence of the seasonal cycle on the sample correlation between two time series is discussed. It is shown that any narrow band energetic signal common to both time series can dramatically reduce the statistical reliability of time-lagged sample cross correlation values by reducing the effective number of degrees of freedom in the records.The results are used to discuss the statistical analysis of wind-forced transport of the Antarctic Circumpolar Current through Drake Passage given in a recent paper by Wearn and Baker (Deep-Sea Research, 27, 875–888, 1980). It is suggested that their high correlation between Drake Passage transport and circumpolar averaged zonal wind stress may largely be due to the presence of a strong semi-annual signal in both time series. This reduces the statistical significance of their conclusion that this reflects a dynamical coupling between the atmosphere and the ocean. Their conclusion that the southern hemisphere analyzed with fields are of high quality (based on the high correlation with Drake Passage transport) is also questioned.