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Tidally-induced submesoscale features in the atlantic jet and Western Alboran Gyre. A study based on HF radar and satellite images
Abstract
An analysis is presented of the submesoscale tidally-induced anticyclonic eddies formed at the eastern mouth of the Strait of Gibraltar and their role in explaining mesoscale features of the Atlantic Jet – Western Alboran Gyre (AJ-WAG) system. These eddies have been identified for the first time using an observational approach based on the analysis of High Frequency Radar (HFR) derived surface currents on the eastern mouth of the Strait which has been complemented with the analysis of Sea Surface Temperature (SST) satellite images. It is found that the major source of positive vorticity to the AJ is provided by its interaction with these anticyclonic eddies which in turn help maintain the water feeding of the upper branch of the WAG. This new role in maintaining the WAG is added to others previously recognized by other authors based on the supply of negative vorticity that these eddies provide to the whole WAG. In addition, these tidally-induced eddies may contribute to the destabilization of the AJ-WAG system when strong and persistent easterly winds coincide with periods of high tidal current amplitude. In these cases, meteorological forcing produces a weak and southward deflected AJ almost disconnected from a displaced eastward WAG. The WAG in turn is being flattened by an overdeveloped North Western Cyclonic Gyre (NWCG) that may be favored by the action of the mentioned eddies.
... The high frequency (HF) radar [12][13][14][15][16] is another example for ocean surface current measurements. There are about 150 HF radars along the coast of the U.S., including the Great Lakes, and the data are reported in real-time to the U.S. Integrated Ocean Observing System [17]. ...
Acoustic Doppler current profilers (ADCP) are quasi-remote sensing instruments widely used in oceanography to measure velocity profiles continuously. One of the applications is the quantification of land–ocean exchange, which plays a key role in the global cycling of water, heat, and materials. This exchange mostly occurs through estuaries, lagoons, and bays. Studies on the subject thus require that observations of total volume or mass transport can be achieved. Alternatively, numerical modeling is needed for the computation of transport, which, however, also requires that the model is validated properly. Since flows across an estuary, lagoon, or bay are usually non-uniform and point measurements will not be sufficient, continuous measurements across a transect are desired but cannot be performed in the long run due to budget constraints. In this paper, we use a combination of short-term transect-based measurements from a vessel-mounted ADCP and relatively long-term point measurements from a moored ADCP at the bottom to obtain regression coefficients between the transport from the vessel-based observations and the depth-averaged velocity from the bottom-based observations. The method is applied to an Arctic lagoon by using an ADCP mounted on a buoyant platform towed by a small inflatable vessel and another ADCP mounted on a bottom deployed metal frame. The vessel-based measurements were performed continuously for nearly 5 h, which was sufficient to derive a linear regression between the datasets with an R2-value of 0.89. The regression coefficients were in turn applied to the entire time for the moored instrument measurements, which are used in the interpretation of the subtidal transport variations.
... This southward displacement of 38 the AJ results in the increase of the horizontal size of the CsCG and the de-39 velopment of an offshore upwelling (Sarhan et al., 2000;Macías et al., 2008a). 40 On the other hand, the high energetic tidal processes that strongly influence 41 the Strait of Gibraltar could also increase/decrease the magnitude of the AJ 42 during the eastward/westward phase of the tidal cycle (e.g., García-Lafuente 43 et al., 2002a;Romero-Cózar et al., 2021). Additionally, tides can destabilise 44 and disengage the AJ-WAG system by enhancing the positive vorticity in 45 the easternmost side of the Strait (Sánchez-Garrido et al., 2013). ...
The temporal evolution of the physical and biogeochemical properties of the Atlantic Jet (AJ) along the first ∼ 75 m of the water column during a 4-day journey was analysed by following the trajectory of a drifter dragged by the jet from the Strait of Gibraltar towards the Alborán Sea. Three stages were differentiated based on the evolution of several variables (e.g., velocity, temperature, nutrients, fluorescence). (i) Within the Strait of Gibraltar, the water column was primarily influenced by the tidal cycle, leading to a nutrient-enrichment of surface waters. However, due to the short residence time, the phytoplankton community that was mainly dominated by diatoms, did not demonstrate significant changes. (ii) Once outside the Strait, the drifter trajectory was mainly influenced by the frontal dynamics associated with the AJ. The drifter moved forward along the jet but also laterally across it and was continuously attracted to the mainstream (maximum current speed) or detached to its southern edge (minimum current speed). Due to the associated upwelling processes induced by the intensification of the current along the mainstream, the water column was characterized by colder, nutrient-richer water and lower fluorescence values. Conversely, along the southern edge of the jet, the water column was characterized by higher temperature, low nutrient concentration, and higher fluorescence. Along the first stations of this stage, diatom total abundance and biovolume continuously increased, reaching values ∼ 12-times higher than the initial concentrations. (iii) In the last stage, the water parcel was still influenced by the frontal dynamics but with less intensity. Additionally, the colder and denser water of the AJ and the associated phytoplankton community subducted progressively as it moved into the region surrounded by warmer waters. Concomitantly, fluorescence and diatoms total abundance and biovolume decreased and were influenced by the decline of nutrient availability and the increase of mesozooplankton. Our results reveal the coupled processes induced by the entrance of the AJ in the Alborán Sea and highlight the strong control of the physical environment over the ecological processes in this region.
... Sánchez-Garrido et al. (2013) studied the collapse of the WAG through a numerical approach and, consequently, the enhancement of positive vorticity in the easternmost Strait as the cause for the AJ-WAG system breakdown. The AJ positive vorticity can also be raised by both tides (Romero-Cózar et al., 2021) and flows driven by atmospheric pressure. The water mass composition of the Mediterranean outflow (e.g., Millot, 2014;Millot & García-Lafuente, 2011;Naranjo et al., 2015) is affected by the state of the WAG (García-Lafuente et al., 2017). ...
The main objective of the present study is to gain insight into the response of the Atlantic Jet (AJ) to the atmospheric forcing, in order to understand the hydrodynamic processes behind its disconnection from the Western Alboran Gyre (WAG) and the eastward displacement of the latter. An analysis was carried out based on observations of High Frequency Radar (HFR) derived currents and sea level, numerical model simulations and satellite imagery, with particular focus on the events of intense and endured easterlies. Among the main factors that characterize the atmospheric forcing linked to intense and persistent easterly wind events, the conjugation of the direct local wind forcing effect on the Atlantic Inflow (AI) on the eastern side of the Strait of Gibraltar and the response of the coastal waters in the northwestern (NW) corner of the Alboran Sea to the wind forcing over the Alboran Sea are of particular interest. In addition, the effect of the elevation surges in the Strait coming from the western Mediterranean Sea on reducing the AI intensity must be taken into account. These surges appear in coincidence with the establishment of persistent and intense easterlies over the Alboran Sea. The alteration of the AJ-WAG system in response to this forcing causes important changes in the phytoplankton distributions in the Alboran Sea. After the events of persistent and intense easterlies the phytoplankton accumulations in the NW Alboran Sea are displaced offshore and the phytoplankton transported with the AJ is injected towards the central zone of the WAG.
High-frequency coastal radars (HFRs) have proved to be excellent tools for monitoring coastal circulation, providing synoptic, high spatial and temporal resolution surface current data in real time. They may also give detailed information on the surface wave field of coastal areas. An HFR has been operating in the Gulf of Naples (south-eastern Tyrrhenian Sea) since 2004. In this paper we show the results of their utilisation in the study of the Gulf dynamics and sea state, with a focus on potential applications in the context of operational oceanography.
Mesoscale circulation patterns in the adjacent basins of the Strait of Gibraltar were investigated by means of altimetry data. In the Gulf of Cadiz, the pattern is relatively stable with two gyres: a cyclonic gyre close to the southern Iberian coast and an anticyclonic one on the western side of the Strait of Gibraltar. Both structures are located in the right place to convey the surface circulation towards the Strait and feed the Atlantic inflow. In the Alboran Sea, our results confirm that the western anticyclonic gyre is the most stable feature observed, while the eastern cyclonic gyre is subject to great variability. The mesoscale structures fluctuate at seasonal and interannual frequencies, but they may also undergo great changes in a very short time scale. A simple correlation analysis suggests that changes in the upstream Gulf of Cadiz basin may be transmitted through the Strait of Gibraltar to the Alboran Sea with a time delay of around one week.
An assessment of accuracy of a three site short-range (27 MHz) CODAR SeaSonde HF radar network deployed in the Strait of Gibraltar is attempted by comparing its surface current estimates with measurements from a moored point current meter. Radial and total current vectors are compared for a 47 day period from 19 October to 4 December 2013, yielding angular offsets, root mean square errors and correlations in the range 2?–30?, 8–22 cm.s–1 and 0.31–0.81, respectively. Statistics improve when the measured antenna pattern is used, except at one radar site. A self-consistency check in overwater baseline reveals that the dominant source of velocity differences is HF radar variance error.
In this study, for the first time at regional scale, the combined use of
remote sensing data (altimetry and sea surface temperature records)
provides a description of the persistent, recurrent and transient
circulation regimes of the Alborán Sea circulation. The analysis
of 936 altimeter-derived weekly absolute dynamic topography (ADT) and
surface geostrophic current maps for 1993-2010 reveals the presence of a
dominant annual signal and of two interannual modes of variability. The
winter-spring phase is characterized by two stable gyral scale features;
the well-known Western Anticyclonic Gyre within the western area and the
Central Cyclonic Gyre, a new structure not identified in former studies,
occupying the central and eastern parts of the Alborán Sea. A
double anticyclonic gyre regime constitutes the stable circulation
system of the summer-autumn period when the Eastern Anticyclonic Gyre is
formed within the eastern Alborán basin. In this case, the
Central Cyclonic Gyre is narrower and located closer to the Western
Anticyclonic Gyre. They represent two stable states of the system,
robust at the decadal time scale, whereas transient changes reflect
perturbations on these stable states and are mainly observed at an
interannual scale. The circulation variability and the gyral features
development may be dynamically linked to the corresponding changes of
the Gibraltar transport rates.
A case of extreme meteorologically forced fluctuation of net flow
through the Strait of Gibraltar is analysed. The Atlantic water inflow
was interrupted during some days and the net flow reached a peak of -1.5
Sv towards the Atlantic Ocean. In spite of the rapid increase of
atmospheric pressure that triggered this episode, the expected inflow
contribution to the net flow induced by pressure variation should not
exceed 75% of the mean inflow, insufficient to reverse the inflow. Wind
stress acting on the upper layer can induce important inflow and
therefore net flow fluctuations. A simple model is proposed in which
wind stress intensity as a function of frequency determines the inflow
response. For low frequency fluctuations and moderate wind speeds the
model predicts a gain that, if added to the atmospheric pressure effect,
could bring the inflow fluctuation peak beyond the mean, thus explaining
the inflow interruption.
Empirical orthogonal function (EOF) analysis has been widely used in meteorology and oceanography to extract dominant modes of behavior in scalar and vector datasets. For analysis of two-dimensional vector fields, such as surface winds or currents, use of the complex EOF method has become widespread. In the present paper, this method is compared with a real-vector EOF method that apparently has previously been unused for current or wind fields in oceanography or meteorology. It is shown that these two methods differ primarily with respect to the concept of optimal representation. Further, the real-vector analysis can easily be extended to three-dimensional vector fields, whereas the complex method cannot. To illustrate the differences between approaches, both methods are applied to Ocean Surface Current Radar data collected off Cape Hatteras, North Carolina, in June and July 1993. For this dataset, while the complex analysis "converges" in fewer modes, the real analysis is better able to isolate flows with wide cross-shelf structures such as tides.
The inflow of Atlantic water through the strait of Gilbraltar usually exhibits a stationary wavelike pattern in the Alboran sea. We use an objective analysis technique for quantitative scale separation to investigate the interaction of scales in the region. The large scale is clearly dominated by an anticyclonic gyre. Smaller scale analysis shows the existence of several mesoscale cyclonic eddies along the northern boundary of the western Alboran sea anticyclonic gyre. The relationship between the large-scale/mesoscale variability and the induced ageostrophic vertical motion is established using the Q vector formulation. We find that on the macroscale, upward motion occurs upstream of the anticyclonic gyre (upstream of a wave crest) while downward motion takes place downstream (upstream of a trough). We also show that the vertical motion associated with the mesoscale eddies is an order of magnitude higher than the large-scale vertical motion. These patterns of large-scale and mesoscale vertical m...
We present new experimental results and assemble them with previous results in order to develop an improved picture of the upper layer circulation in the Alboran Sea. It is suggested that the key idea to understanding this upper layer circulation is the tendency of the Atlantic jet (AJ) to have negative curvature. Local interactions with the western Alboran gyre (WAG) or the African coast can, however, counterbalance this tendency and modify the anticyclonic path of the AJ. It is also proposed that the density gradients in the WAG can be maintained in part by means of an intermittent surface cross-gyre current which results in an input, mixing, and renewal of Atlantic water. The static stability at the bottom of the gyre increases because of the mixing of Atlantic water with Western Mediterranean Deep Water which is uplifted close to the African coast. This mixing process thereby acts as a local source of potential vorticity. We also report the existence in the Alboran Sea of subsurface anticyclonic eddies (located between 100 and 400 m) of relatively cold water that appear to be detached from the Iberian shelf. Regarding the large-scale variability of the AJ-WAG system, we present evidence of an eastward migration of the WAG and the subsequent formation of a new anticyclonic gyre in the western Alboran basin on a timescale of 1 month. This eastward gyre migration process temporarily allows the simultaneous presence of three anticyclonic gyres in the Alboran Sea.
The circulation and dynamics of the Modified Atlantic Water have been studied using data from an intensive field experiment carried out between 22 September and 7 October 1992. Data included 134 CTD casts, ADCP, and satellite imagery. A well-defined wavelike front was observed with two significant anticyclonic gyres in the western and eastern Alboran Sea. Smaller-scale cyclonic eddies were also observed. The front separates the more saline older modified Atlantic water (5 > 38) in the northern region from the fresher, more recent modified Atlantic water (S < 36.8) in the south. The associated baroclinic jet had a mean transport of 1 Sv and maximum geostrophic velocities of 1.0 m s-1. The three-dimensional structure and spatial scales of both gyres were similar, that is, 90 km long and 220 m deep. In the eastern Alboran, northeast of Oran, the origin of the Algerian Current was also detected with an eastward transport of 1.8 Sv. The general picture can be presented as a structure formed by a wavelike front coupled with two large anticyclonic gyre-small cyclonic eddy systems. The relative importance of stratification, relative vorticity, and Froude number in the distribution of Ertel's potential vorticity has been examined, and potential vorticity conservation is used to infer vertical motion. The vertical velocity forcing has been computed using the quasigeostrophic Q vector formulation of the omega equation. It is found that the differential vorticity advection due to mesoscale phenomena in the western Alboran plays a main role in this forcing. The vertical velocities associated with these mesoscale structures reach maximum absolute values of 15 m day-1.
A three-dimensional primitive equation model is used to investigate the physical processes governing the exchanges of water between the Mediterranean Sea and the Atlantic Ocean at the level of the Strait of Gibraltar. The circulation is driven by connecting two reservoirs filled with waters of different densities. The motion starts from rest and is initiated by removing the dam separating them at the initial time. The results are in agreement with observations. In the strait, the light Atlantic water flows into the Mediterranean Basin in the surface layer while the denser Mediterranean water moves toward the ocean as a deep current. After a spinup time interval of three days, the flow transport in the strait reaches a quasi-stationary value of 0.9 Sv (Sv ≡ 106 m3 s-1). As it enters the Alboran Sea, the flow of Atlantic water experiences a transition regime where the surface current is transformed into a shallow buoyant jet. After several days of integration, this flow intrudes into the Alboran Basin and forms a large anticyclonic gyre while the head of the plume propagates along the southern coast of the sea as a coastal Kelvin front. The shape and dimensions of that anticyclonic gyre are comparable with the western Alboran Sea anticyclonic gyre as observed by satellite imagery and in situ data. Sensitivity experiences show the dependence of the gyre formation on the interaction between the two masses of water (i.e., Atlantic and Mediterranean) at the strait exit. The transition experienced by the flow of Atlantic water at its arrival in the Alboran Basin is due to a sharp variation of the meridional pressure gradient, which produces a strong divergence of the flow. This generates an abrupt variation of the interface slope and a gain of negative relative vorticity by the surface flow as a consequence of the potential vorticity conservation. The negative vorticity is then advected into the Alboran Sea and generates the anticyclonic gyre. Therefore, the anticyclonic Alboran gyre is triggered by the transition regime due to the intrusion of a shallow buoyant jet into the denser Mediterranean waters, which produces a divergence of the flow and a bump of the interface. This mechanism is related to the three-dimensional aspect of the problem and can only be reproduced with three-dimensional models, Nonlinearities, Coriolis acceleration, and production of negative vorticity by vortex stretching play a major role in this physical process.
The performance of a network of five CODAR (Coastal Ocean Dynamics Application Radar) SeaSonde high- frequency (HF) radars, broadcasting near 13 MHz and using the Multiple Signal Classification (MUSIC) algorithm for direction finding, is described based on comparisons with an array of nine moorings in the Santa Barbara Channel and Santa Maria basin deployed between June 1997 and November 1999. Eight of the moorings carried vector-measuring current meters (VMCMs), the ninth had an upward-looking ADCP. Coverage areas of the HF radars and moorings included diverse flow and sea-state regimes. Measurement depths were ;1 m for the HF radars, 5 m for the VMCMs, and 3.2 m for the ADCP bin nearest to the surface. Comparison of radial current components from 18 HF radar-mooring pairs yielded rms speed differences of 7-19 cm s21 and correlation coefficients squared (r 2) in the range of 0.39-0.77. Spectral analysis showed significant coherence for frequencies below 0.1 cph (periods longer than 10 h). At higher frequencies no significant coherence was found, and noise levels corresponding to 6 cm s 21 rms were evident in the radar data. Errors in the radar bearing determination were found in 10 out of 18 comparisons, with a typical magnitude of 58-108, and a maximum of 198. The effects of bearing errors on total vector currents were evaluated using a simple flow field and measured bearing errors, showing up to 15% errors in computed flow speeds, and up to ;98 errors in flow directions.
1] Two high-frequency radar stations (CODAR) were installed along the northern California coast in May 2001. Comparisons of radar data with acoustic Doppler current profiler (ADCP) current data and near-surface drifter tracks indicate considerable agreement, with minimum RMS differences of order 0.05–0.15 m/s and average drifter-HF-radar track separation rates of 5 ± 3 km/d. Radar data resolve the three main sources of intraseasonal current variability in the area: (1) upwelling/relaxation dynamics, (2) tidal and diurnal forcing, and (3) inertial currents. Subtidal fluctuations are the largest component of variability, accounting for 45–75% of the variance. Wind-driven dynamics are the dominant source of this subtidal variability (67% of subtidal variability). Both upwelling and relaxation periods exhibit consistent patterns of surface velocity, with nearshore currents being slower and more poleward than offshore currents, which are strongly equatorward. Analysis of tidal and inertial variability indicate that current fluctuations are polarized toward clockwise rotation and are generally weaker and more linearly polarized near the coast. M 2 tidal current ellipses switch direction of rotation at the shelf break, suggesting the presence of internal tidal waves. Currents in all frequency bands are deflected and accelerated around Pt. Reyes, and there are indications of increased current variability and changes in flow direction near Cordell Bank. The presence of considerable cross-shore and alongshore gradients in the strength and direction of surface flow patterns, and in particular weak poleward currents over the inner shelf, could have important consequences for plankton retention in the area.
Ocean models based on consistent hydrostatic, quasi-hydrostatic, and nonhydrostatic equation sets are formulated and discussed. The quasi-hydrostatic and nonhydrostatic sets are more accurate than the widely used hydrostatic primitive equations. Quasi-hydrostatic models relax the precise balance between gravity and pressure gradient forces by including in a consistent manner cosine-of-latitude Coriolis terms which are neglected in primitive equation models. Nonhydrostatic models employ the full incompressible Navier Stokes equations; they are required in the study of small-scale phenomena in the ocean which are not in hydrostatic balance. We outline a solution strategy for the Navier Stokes model on the sphere that performs efficiently across the whole range of scales in the ocean, from the convective scale to the global scale, and so leads to a model of great versatility. In the hydrostatic limit the Navier Stokes model involves no more computational effort than those models which assume strict hydrostatic balance on all scales. The strategy is illustrated in simulations of laboratory experiments in rotating convection on scales of a few centimeters, simulations of convective and baroclinic instability of the mixed layer on the 1- to 10-km scale, and simulations of the global circulation of the ocean.
This study analyses the water mass exchanges at subinertial scale between Algeciras Bay and the adjacent Strait of Gibraltar. The mechanisms triggering this exchange process is investigated with the aid of recently-acquired data on surface currents obtained using a system of HF coastal radars deployed on the eastern side of the Strait, and remotely-sensed images of sea surface temperature (SST) and chlorophyll from the MODIS sensor of the Aqua satellite. HF radar data on surface currents are analyzed by the application of real empirical orthogonal function (EOF) decomposition, which produces three EOF modes explaining more than 70% of the variance of the surface currents at the mouth of the Bay (modes 2, 3, and 6). Mode 2 is related to the fluctuations of the Atlantic Jet in the central zone of the Strait, mainly due to a combined effect of the atmospheric pressure fluctuations in the Western Mediterranean Sea and local wind in the eastern side of the Strait; mode 3 is related to the coastal currents induced by zonal wind forcing on the north-western coast of the Strait and Alboran Sea; and mode 6 seems to be related to water transport induced by winds blowing with a significant north component into and out of the Bay.
Observations from a high frequency radar system and outputs from a high resolution operational ocean model working at the Strait of Gibraltar have been analyzed and compared during the period February 2013 – September 2014 in order to evaluate their capability to resolve the surface circulation of the region. The description of the mean circulation patterns has been statistically assessed, showing good agreement, particularly in the central region of the strait corresponding with the Atlantic Jet (AJ) stream, although some short scale features are not reproduced by the model. In the frequency domain very high concordance is observed. Tidal maps of diurnal and semi-diurnal constituents are in good agreement with previous observations. The analysis of the model and radar response to the wind forcing reveals that the low resolution of the model wind – forcing field and its deeper superficial level smoothes the wind effect on the simulated currents. The first three EOF modes account for the 86% of model and radar variances. The coincidence between the observed and simulated patterns is very significant for the first two modes, which account for the mean velocity field and the latitudinal shifting of the AJ consequence of the flow-topography interaction. The third mode captures the wind – induced circulation, and greater discrepancies are found in this case. Results underline the complementary character of both systems: radar observations improve the model description, resolving short scale processes, while the model completes the radar information when the time or spatial coverage is poorer. This article is protected by copyright. All rights reserved.
The Atlantic Jet (AJ) is the inflow of Atlantic surface waters into the Mediterranean Sea. This geostrophically adjusted jet fluctuates in a wide range of temporal scales from tidal to subinertial, seasonal, and interannual modifying its velocity and direction within the Alboran Sea. At seasonal scale, a clearly defined cycle has been previously described, with the jet being stronger and flowing towards the northeast during the first half of the year and weakening and flowing more southwardly towards the end of the year. Different hypothesis have been proposed to explain this fluctuation pattern but, up to now, no quantitative assessment of the importance of the different forcings for this seasonality has been provided. Here, we use a 3D hydrodynamic model of the entire Mediterranean Sea forced at the surface with realistic atmospheric conditions to study and quantify the importance of the different meteorological forcings on the velocity and direction of the AJ at seasonal time scale. We find that the direct effects of local zonal wind variations are much more important to explain extreme collapse events when the jet dramatically veers southward than to the seasonal cycle itself while sea level pressure variations over the Mediterranean seem to have very little direct effect on the AJ behavior at monthly and longer time scales. Further model results indicate that the annual cycle of the thermohaline circulation is the main driver of the seasonality of the AJ dynamics in the model simulations. The annual cycles in local wind forcing and SLP variations over the Mediterranean have no causal relationship with the AJ seasonality.
During the ICTIOALBORAN-0793 multidisciplinary oceanographic survey carried out in July 1993 by the Instituto Español de Oceanografı́a (IEO) in the Alboran Sea, some anomalous features were detected. One was the presence of a small cyclonic eddy in the western Alboran Basin, close to the African coast. The upper layer of the eddy consisted of Mediterranean Surface Water and was separated from its supposed source (the northern Alboran Sea) by the Atlantic Jet. Another feature was the probable temporary interruption of the flow of fresh Atlantic Water (S≈36.5) into the eastern Alboran Basin and its replacement by a modified (saltier) Atlantic Water. These features can be explained assuming a time evolution of the surface circulation in the Alboran Sea forced by speed variations in the inflowing Atlantic Water through the Strait of Gibraltar. A collection of satellite images covering the survey period and across-strait sea level difference data, indicative of the geostrophic velocity of the inflow through the Strait, were used to check this assumption. Both sets of data supplied independent but compatible information in the sense that they complemented each other and gave support to the proposed evolving model. Finally, some speculative ideas attempting to correlate the inferred variability in the Alboran Sea with the state of the baroclinic water exchange through the Strait of Gibraltar (maximal or submaximal) are discussed.
The stability of the mean state of the surface circulation of the
Western Alboran Sea, characterized by a jet of Atlantic water (AJ)
surrounding the Western Alboran Gyre (WAG), is investigated with a
submesoscale-resolving operational ocean model. It is shown that this
circulation state collapses through the interaction between the AJ-WAG
system and a growing cyclonic eddy that arises close to the Spanish
coast. This eddy develops as the WAG partially blocks the positive
potential vorticity (PV) flux coming from the Strait of Gibraltar. It is
found that tides increase the positive PV flux with respect to a
non-tidal simulation, thus making the destabilization of the mean
circulation state more likely. Atmospheric pressure driven flows are
also shown to have the potential to destabilize the circulation by
rising dramatically the PV flux within a time scale of some days. This
provides an explanation as to why the circulation of the Western Alboran
Sea exhibits more variability toward winter time, when meteorological
fluctuations are enhanced.
High Frequency (HF) radar stations have been working operationally in the southeastern part of the Bay of Biscay since 2009. The (2) systems provide hourly surface currents, with 5 km spatial resolution and a radial coverage lying close to 180 km. The detailed and quantitative description of the spatial patterns observed by the HF radar offers new evidence on the main ocean processes, at different time scales, affecting a study area where surface currents show marked temporal and spatial variability. A clear seasonality in terms of sea surface currents and along-slope circulation is observed, with cyclonic and anticyclonic patterns during the winter and summer months, respectively. From the analysis of low-pass filtered currents, a key component of this seasonal variability is associated with the surface signature of the slope current (Iberian Poleward Current (IPC)). Clearly intensified over the upper part of the slope, this current circulates eastward off the Spanish coast and northward over the French shelves in winter.
Examination of the HF radar current fields reveals the presence of mesoscale structures over the area. At higher frequencies, an EOF (Empirical Orthogonal Function) analysis of the inertial band-pass filtered data is used to study the complex spatial and temporal patterns associated with these processes and to evaluate quantitatively the relative contribution of the high frequency to the total variability, in space and time. Overall, inertial currents represent between 10 and 40% of the total variability; their contribution is significantly greater in summer and over the deeper part of the slope. Tides contribute much less than the total Kinetic Energy (KE), although their contribution over the shelf can be higher than that of the inertial oscillations, during winter.
We analyze the Alboran Sea mesoscale variability in a 2-decade high resolution (2 km) simulation. The circulation modes and eddies are described from a statistical perspective. The double-gyre quasi-steady state is confirmed as the most common circulation mode in the Alboran Sea (48% of the time), followed by single-gyre mode in about 24% of the situations. These persistent modes are compared in terms of structure, frequency of occurrence and seasonality, energetics, and their links to inflow variability and wind forcing. The double-gyre state is the most stable situation with higher kinetic and potential energy and lower eddy kinetic energy and seems to be limited by a critical value of the Rossby radius (∼12 km). The transitions between the quasi-steady states are studied and typified with a particular focus on the Western Alboran Gyre migration which stands out as the major contributor for flow transitions. A typical sequence of flow types and transients is proposed: The double-gyre is usually canceled after a migration event occurring in late summer or autumn. The Western Alboran Gyre is not immediately replaced by a new born anticyclone. Instead, there is a cycle of migration-merging events with the Eastern Alboran Gyre which eventually force the latter to move westward and form a single-gyre situation. The generation of a second Eastern Alboran Gyre in spring-early summer sets the beginning of a new double-gyre stable period. An analysis of coherent mesoscale eddies is conducted and their statistics, sites and processes of generation are described. Cyclones (anticyclones) mean radius are in the range of ∼12 km (∼15 km), they are much more frequent in winter time and cyclones significantly outnumber the anticyclones. The link of mesoscale processes and transients (in particular of migrations) with the inflow is analyzed, and no clear relationship of transient events with inflow magnitude is disclosed. On the other hand, the large majority of transitions (specially migrations) are associated with a clear shift of the angle and latitude of the Atlantic Jet to the south. The stratification and wind variability are also important suggesting a significant role of eddy dynamics and intrinsic variability in determining the stable modes and transitions. Despite the seasonality of the flow types sequence, there are periods of long stability with the double-gyre blocking situations resisting the winter periods. The time variability is analyzed and compared with observations.
An extensive dataset collected in the Alboran Sea during the 1982 Donde Va experiment is used to characterize the kinematics and dynamics of the inflow of Atlantic water into the Mediterranean Sea. The veering of the inflow toward the ENE after leaving the Strait of Gibraltar and the existence of an anticyclonic gyre that fills much of the western Alboran Sea in the upper 200 m are confirmed in the mean. Inflow variability with periods of 2 to 10 days is described. Particularly striking is one interval of about nine days during which the gyre was not present and the inflow adopted a southerly course after leaving the strait. The observations are interpreted in terms of vorticity conservation and in the light of earlier theoretical and numerical results. (A)
During the period 6–18 October 1982, six flights by a U.S. Navy research aircraft were made over the Alboran Sea Gyre as part of the Donde Va? experiment. During five of these flights sonobuoys were dropped in a linear pattern designed to show the temporal and spatial variability of the currents that constitute the gyre. A normal development consisted of approximately 15 sonobuoys dropped along the longitudinal line 4°45′W from the Spanish coast to within 30 km of the Moroccan coast. During the following 4 hours, the sonobuoys were relocated twice, their positions noted, and their individual drift speeds and directions derived. Aircraft precision radiation temperature and aircraft expendable bathythermograph data were collected concurrently with the sonobuoy drops to determine the thermal structure of the water during the drift period. Inertial navigation winds, (at the 300-m altitude of the aircraft) as well as other meteorological data, were also collected. Analysis of these data shows that at ...
The performances of a shore-based high-frequency (HF) radar network deployed along the coast of the Venice lagoon (northern Adriatic Sea) are discussed based on a comparison with a single bottom-mounted ADCP deployed in the shallow-water area offshore of the lagoon for a 40-day period in August–September 2005.
The analyses, carried out using currents representative of the first meter for the HF radars and 2.5 m for the ADCP, gave rms differences of radial currents in the range of 8.7–14.7 cm s−1 (correlation 0.37– 0.82) for the ideal pattern and 8.4–20.5 cm s−1 (correlation 0.14–0.84) for the measured pattern. Good correlation was found between surface current vectors and moored data (scalar correlation up to R = 0.83, vector correlation ρ = 0.78, veering angle 6°). Comparison metrics were improved for the low-passed currents. Angular offsets ranged between +6° and +11°. Differences depended primarily on the geophysical variability within the water column. Bearing offsets also contributed because they lead to comparisons with radial velocities at erroneous angular sectors.
Radar performances were severely affected by strong northeasterly wind pulses in their early stages. An increased broadband noise, spread over the entire Doppler spectrum across all ranges to the antennas, masked the Bragg peaks and determined the loss in radar coverage, introducing gross underestimations of both radial velocities and total currents.
A survey of relevant numerical, laboratory, and observational studies, combined with our own laboratory experiments, suggests that the surface outflow from a strait generally forms a gyre in the adjacent sea if the exit corner is sharp. The gyre grows from an initial ``separation bubble'' of radius u/f, where u is the current speed and f the Coriolis frequency. The criterion for the maintenance of a coastal jet along a curved coast, instead of separation and gyre formation, seems to be that the Rossby number u(fRw)-1
Data collected in the 1960's and recent data indicate that the flow in the Strait of Gibraltar does not move in the form of continuous currents but as tidal-induced pulses. A descriptive model based on these data indicates that the pulses are a result of increases in the speed of the tidal streams as they encounter the constrictions of the regional bathymetry (especially the Camarinal Sill and between the Camarinal Sill and Tarifa). Periodic increases modify the regional flow so that during each tidal cycle, the eastward-flowing surface Atlantic water and westward-flowing deep Mediterranean water are alternately emitted as large pulses in the Mediterranean Sea (Atlantic water) and Atlantic Ocean (Mediterranean water). These periodic pulses vary in the amount of water they contain according to the daily and monthly variation in tidal current strength. Besides the pulses generated at the intervals of the semidiurnal tide, it appears that short-period pulses of flow are generated on the Camarinal Sill. Occurring near the time of the greatest local variations of the tidal current, these short-period pulses are able to trigger very strong internal waves and current fronts in the upper layer, which are propagated eastward into the Alboran Sea. These continuous periodic pulses are a permanent feature of the waters of the Strait of Gibraltar that should be taken into account in any study of the region. Reprints.
a b s t r a c t Tidal forcing and its fortnightly variation are known to be one of the main regulating agents of physical and biogeochemical signatures in the Strait of Gibraltar and surrounding areas. Samples obtained during spring and neap tides in the region were analyzed to determine the influence of this tidal variation on the submesoscale distribution of water masses and biological elements. During spring tides, strong and intermittent mixing processes between Mediterranean and Atlantic waters occur in the vicinity of the Camarinal Sill together with an eastward advection of those mixed waters into the Alboran Sea. Furthermore, the intense suction of surface coastal waters into the main channel of the strait was detected as chlorophyll patches in the Alboran Sea during spring tides. In contrast, the most charac-teristic phenomenon during neap tides was the arrival of pulses of relatively nutrient-rich North Atlantic Central Waters to the surface regions of the Alboran Sea. In addition, traces of the suction of coastal waters were observed for the first time during neap tides. Therefore, our results show that submesoscale processes are crucial in the dynamics of the Strait of Gibraltar, and they must be considered for the correct description of the biogeochemical features of Alboran Sea, especially during an inactive period of the coastal upwelling.
In the Western Alborán Sea light Atlantic water and dense Mediterranean waters meet. This gives rise to intense density fronts and energetic mesoscale features such as two semi-permanent anticyclonic vortices: the western and eastern Alborán gyres. However, the circulation in the Alborán Sea is strongly varying in time, with episodes where neither the western Alborán gyre nor the eastern Alborán gyre are present. In October 1996 four consecutive high-resolution surveys of the western Alborán gyre (WAG) were carried out and Lagrangian floats were released. During the first survey the western Alborán Sea was occupied by the WAG, surrounded by the Atlantic jet. The following surveys showed the WAG drifting eastward, beginning a WAG migration event, as confirmed 15 days later by the Lagrangian float tracks and the SST images. To replace the old gyre, one month later a new gyre was formed. Analysis of ADCP velocities, SST images and TS properties show a change, between the first and the second survey, in the Atlantic inflow through the Strait of Gibraltar. The relationships between this change in the Atlantic inflow, a drop in the atmospheric pressure at Ceuta, the tidal regime and the WAG migration are discussed. The existence of a high resolution sampling in space together with the possibility of addressing the temporal variability has allowed us to describe, in detail and for the first time, the time evolution of the Western Alborán Sea during a WAG migration event, relating the migration to changes in the inflow through the Strait of Gibraltar.
Hydrographic and current meter data, obtained during June to October 1982, and numerical model experiments are used to study the distribution and flow of Mediterranean waters in the western Alboran Sea. The Intermediate Water is more pronounced in the northern three-fourths of the sea, but its distribution is patchy as manifested by variability of the temperature and salinity maxima at scales ≤10 km. Current meters in the lower Intermediate Water showed mean flow toward the Strait at 2 cm s−1. A reversal of this flow lasted about 2 weeks. A rough estimate of the mean westward Intermediate Water transport was 0.4 × 106 m3 s–1, about one-third of the total outflow, so that the best estimates of the contributions of traditionally defined Intermediate Water and Deep Water account for only about one-half of the total outflow. The Deep Water was uplifted against the southern continental slope from Alboran Island (3°W) to the Strait. There was also a similar but much weaker banking against the Spanish slope, but a deep current record showed that the eastward recirculation implied by this banking is probably intermittent. Two-layer numerical model experiments simulated the Intermediate Water flow with a flat bottom and the Deep Water with realistic bottom topography. Both experiments replicated the major circulation features, and the Intermediate Water flow was concentrated in the north because of rotation and the Deep Water flow in the south because of topographic control.
During October 2003 an intensive oceanographic survey (BIOMEGA) was carried out in the Alboran Sea, coinciding with a migration event of the Western Alboran Sea Gyre (WAG). The observations gathered during that cruise constitute the first field evidence of a migrated stage of the WAG. In this work we present the main differences between the 3D hydrodynamic fields observed during BIOMEGA and those corresponding to a WAG located at its usual position. The migration of the gyre was followed by satellite (altimetry and sea surface temperature) imagery. The causes of the gyre migration are explored in terms of the quasi-geostrophic tendency equation, in particular of the dynamics governing scales larger than the Rossby radius of deformation. It is shown that the steady state gyre must be almost equivalent barotropic and that the key process to break down the stationarity would be a density advection at gyre scale. The mechanisms to explain the migration of the WAG proposed by previous authors are discussed in light of the explanation proposed in this work.
This paper investigates the transport structure of surface currents around the Monterey Bay, CA region. Currents measured by radar stations around Monterey Bay indicate the presence of strong, spatial and temporal, nonlinear patterns. To understand the geometry of the flow in the bay, Lagrangian coherent structures (LCS) are computed. These structures are mobile separatrices that divide the flow into regions of qualitatively different dynamics. They provide direct information about the flow structure but are geometrically simpler than particle trajectories themselves. The LCS patterns were used to reveal the mesoscale flow conditions observed during the 2003 Autonomous Ocean Sampling Network (AOSN-II) experiment.
Drifter paths from the AOSN experiment were compared to the patterns induced by the LCS computed from high-frequency radar data. We verify that the fate of the drifters can be better characterized based on the LCS than direct interpretation of the current data. This property can be exploited to optimize drifter deployment.
Data from a mooring line deployed midway between the Alboran Island and Cape Tres Forcas are used to study the time variability of the Alboran Sea from May 1997 to May 1998. The upper layer salinity and zonal velocity present annual and semiannual cycles characterised by a minimum in spring and autumn and a maximum in summer and winter. Temperature has the opposite behaviour to that of salinity indicating changes in the presence of the Atlantic water within the Alboran Passage. A large set of SST images is used to study these cycles. The decrease of salinity and velocity in our mooring location in spring and autumn seems to be related to the eastward drifting of the Western Alboran Gyre (WAG). The increase of salinity and velocity is caused by the Atlantic current flowing south of the Alboran Island and its associated thermohaline front. Conductivity–temperature–depth (CTD) data from two cruises along the 3°W are coherent with current meters and SST interpretations.During the period analysed, summer months are characterised by the stability of the two-gyre system, while in winter, the circulation is characterised by a coastal jet flowing close to the African shore. We use sea level differences across the Strait of Gibraltar for studying the variability of the Atlantic inflow. We discuss the changes in the Alboran Sea circulation and its relation with the variability of the inertial radius of the Atlantic inflow. Though our results are speculative, we find a possible relation between the disappearance of the two-gyre system and a reversal of the circulation in Gibraltar. Longer time series are needed to conclude, but comparison with previous works makes us think that the seasonal cycle described from May 1997 to May 1998 could be the most likely one for the Alboran Sea upper layer.
This paper focuses on the validation of remotely sensed ocean surface currents from SeaSonde-type high-frequency (HF) radar systems. Hourly observations during the period July 22, 2003 through September 9, 2003 are used from four separate radar sites deployed around the shores of Monterey Bay, CA. Calibration of direction-finding techniques is addressed through the comparisons of results obtained using measured and ideal (i.e., perfect) antenna patterns. Radial currents are compared with observations from a moored current meter and from 16 surface drifter trajectories. In addition, four overwater baselines are used for radar-to-radar comparisons. Use of measured antenna patterns improves system performance in almost all cases. Antenna-pattern measurements repeated one year later at three of the four radar locations exhibit only minor changes indicating that pattern distortions are stable. Calibrated results show root-mean-square (rms) radial velocity differences in the range of 9.8-13.0 cm/s, which suggest radar observation error levels in the range of 6.9-9.2 cm/s. In most cases, clear evidence of bearing errors can be seen, which range up to 30deg for uncalibrated radar-derived radial currents and up to 15deg for currents obtained using measured antenna patterns. Bearing errors are not, however, constant with angle. The results recommend use of measured antenna patterns in all SeaSonde-type applications. They also recommend an expanded simulation effort to better describe the effects of antenna-pattern distortions on bearing determination under a variety of ocean conditions
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Modelado de alta resolución para el estudio de la respuesta oceánica al forzamiento del viento en el estrecho de Gibraltar
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al forzamiento del viento en el estrecho de Gibraltar. Universidad de Cádiz.
A Study of 10-day Forecast (A Synthetic Report)
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