# Dimitris Menemenlis's research while affiliated with California Institute of Technologyand other places

## Publications (182)

Recent studies highlight that oceanic motions associated with horizontal scales smaller than 50 km, defined here as submesoscales, lead to anomalous vertical heat fluxes from colder to warmer waters. This unique transport property is not captured in climate models that have insufficient resolution to simulate these submesoscale dynamics. Here, we use an ocean model with an unprecedented resolution that, for the first time, globally resolves submesoscale heat transport. Upper-ocean submesoscale turbulence produces a systematically-upward heat transport that is five times larger than mesoscale heat transport, with winter-time averages up to 100 W/m^2 for mid-latitudes. Compared to a lower-resolution model, submesoscale heat transport warms the sea surface up to 0.3 °C and produces an upward annual-mean air–sea heat flux anomaly of 4–10 W/m^2 at mid-latitudes. These results indicate that submesoscale dynamics are critical to the transport of heat between the ocean interior and the atmosphere, and are thus a key component of the Earth’s climate.
We propose a satellite mission that uses a near-nadir Ka-band Doppler radar to measure surface currents, ice drift and ocean waves at spatial scales of 40 km and more, with snapshots at least every day for latitudes 75 to 82°, and every few days for other latitudes. The use of incidence angles of 6 and 12° allows for measurement of the directional wave spectrum, which yields accurate corrections of the wave-induced bias in the current measurements. The instrument's design, an algorithm for current vector retrieval and the expected mission performance are presented here. The instrument proposed can reveal features of tropical ocean and marginal ice zone (MIZ) dynamics that are inaccessible to other measurement systems, and providing global monitoring of the ocean mesoscale that surpasses the capability of today's nadir altimeters. Measuring ocean wave properties has many applications, including examining wave–current interactions, air–sea fluxes, the transport and convergence of marine plastic debris and assessment of marine and coastal hazards.
The Sea Surface KInematics Multiscale monitoring (SKIM) mission proposes to use Doppler-based measurements of velocities to provide global estimates of surface currents and ice drift at spatial scales of 40 km and more, with snapshots at least every day for latitudes 75 to 82, and every few days otherwise. Given the contribution of wave motion to Doppler measurements we have chosen to favor near-nadir incidence angles, between 6 and 12 degrees, in order to measure the directional wave spectrum and perform an accurate correction of the wave-induced bias. The resulting instrument design, algorithm for current velocity and mission performance are presented here. We find that a Ka-band near-nadir instrument can reveal features on tropical ocean and marginal ice zone dynamics that are inaccessible to other measurement system, as well as a global monitoring of the ocean mesoscale that surpasses the capability of today's nadir altimeters. Measuring ocean wave properties allows many applications from wave-current interactions and air-sea fluxes to microplastics convergence and coastal hazards.
The transition scale Lt from balanced geostrophic motions to unbalanced wave motions, including nearinertial flows, internal tides, and inertia-gravity wave continuum, is explored using the output from a global 1/488 horizontal resolution Massachusetts Institute of Technology general circulation model (MITgcm) simulation. Defined as the wavelength with equal balanced and unbalanced motion kinetic energy (KE) spectral density, Lt is detected to be geographically highly inhomogeneous: it falls below 40 km in the western boundary current and Antarctic Circumpolar Current regions, increases to 40-100 km in the interior subtropical and subpolar gyres, and exceeds, in general, 200 km in the tropical oceans. With the exception of the Pacific and Indian sectors of the Southern Ocean, the seasonal KE fluctuations of the surface balanced and unbalanced motions are out of phase because of the occurrence of mixed layer instability in winter and trapping of unbalanced motion KE in shallow mixed layer in summer. The combined effect of these seasonal changes renders Lt to be 20 km during winter in 80% of the Northern Hemisphere oceans between 258 and 458N and all of the Southern Hemisphere oceans south of 258S. The transition scale's geographical and seasonal changes are highly relevant to the forthcoming Surface Water and Ocean Topography (SWOT) mission. To improve the detection of balanced submesoscale signals from SWOT, especially in the tropical oceans, efforts to remove stationary internal tidal signals are called for.
Sea ice models with the traditional viscous-plastic (VP) rheology and very small horizontal grid spacing can resolve leads and deformation rates localized along Linear Kinematic Features (LKF). In a 1-km pan-Arctic sea ice-ocean simulation, the small scale sea-ice deformations are evaluated with a scaling analysis in relation to satellite observations of the Envisat Geophysical Processor System (EGPS) in the Central Arctic. A new coupled scaling analysis for data on Eulerian grids is used to determine the spatial and temporal scaling and the coupling between temporal and spatial scales. The spatial scaling of the modeled sea ice deformation implies multi-fractality. It is also coupled to temporal scales and varies realistically by region and season. The agreement of the spatial scaling with satellite observations challenges previous results with VP models at coarser resolution, which did not reproduce the observed scaling. The temporal scaling analysis shows that the VP model, as configured in this 1-km simulation, does not fully resolve the intermittency of sea ice deformation that is observed in satellite data.
Almost all heat reaching the bases of Antarctica's ice shelves originates from warm Circumpolar Deep Water in the open Southern Ocean. This study quantifies the roles of mean and transient flows in transporting heat across almost the entire Antarctic continental slope and shelf using an ocean/sea ice model run at eddy- and tide-resolving (1/48 degree) horizontal resolution. Heat transfer by transient flows is approximately attributed to eddies and tides via a decomposition into time scales shorter than and longer than one day, respectively. It is shown that eddies transfer heat across the continental slope (ocean depths greater than 1500 meters), but tides produce a stronger shoreward heat flux across the shelf break (ocean depths between 500 meters and 1000 meters). However, the tidal heat fluxes are approximately compensated by mean flows, leaving the eddy heat flux to balance the net shoreward heat transport. The eddy-driven cross-slope overturning circulation is too weak to account for the eddy heat flux. This suggests that isopycnal eddy stirring is the principal mechanism of shoreward heat transport around Antarctica, though likely modulated by tides and surface forcing.
Supporting Information S1
The wavenumber spectrum of sea surface height (SSH) is an important indicator of the dynamics of the ocean interior. While the SSH wavenumber spectrum has been well studied at mesoscale wavelengths and longer, using both in situ oceanographic measurements and satellite altimetry, it remains largely unknown for wavelengths less than ~70 km. The Surface Water Ocean Topography (SWOT) satellite mission aims to resolve the SSH wavenumber spectrum at 15-150-km wavelengths, which is specified as one of the mission requirements. The mission calibration and validation (CalVal) requires the ground truth of a synoptic SSH field to resolve the targeted wavelengths, but no existing observational network is able to fulfill the task. A high-resolution global ocean simulation is used to conduct an observing system simulation experiment (OSSE) to identify the suitable oceanographic in situ measurements for SWOT SSH CalVal. After fixing 20 measuring locations (the minimum number for resolving 15-150-km wavelengths) along the SWOT swath, four instrument platforms were tested: pressure-sensor-equipped inverted echo sounders (PIES), underway conductivity-temperature-depth (UCTD) sensors, instrumented moorings, and underwater gliders. In the context of the OSSE, PIES was found to be an unsuitable tool for the target region and for SSH scales 15-70 km; the slowness of a single UCTD leads to significant aliasing by high-frequency motions at short wavelengths below ~30 km; an array of station-keeping gliders may meet the requirement; and an array of moorings is the most effective system among the four tested instruments for meeting the mission's requirement. The results shown here warrant a prelaunch field campaign to further test the performance of station-keeping gliders.
The upwelling system off Peru/Chile is characterized by significant mesoscale to submesoscale surface variability that results from the instability of the coastal currents (due to the strong vertical and horizontal shears) and to the marked density cross-shore gradients (associated with the mean upwelling). Here we investigate to what extent upwelling intensity can be inferred from sea surface temperature (SST) derived from remote sensing. As a first step in validation, a comparison between SST observations is performed, which indicates that the 1 km gridded multi-scale ultra-high-resolution (MUR) SST data set is defining a zone of maximum SST gradients closer to shore than the low-resolution National Centers for Environmental Information 0.25° resolution data set. Two model versions, at nominal resolutions of 2 km and 4 km, of the Massachusetts Institute of Technology general circulation model are analysed. A high-resolution version at 2 km is examined for the period 13 September 2011–23 January 2013, while a 4 km version is examined for 6 March 2011–22 April 2013. MUR shows maxima SST gradients in the range of 0.03 ± 0.02 K km⁻¹ while the model showed higher gradients around 0.05 ± 0.02 K km⁻¹. Based on coherence spectra, the relationship between upwelling rate (as inferred from the vertical velocity) and SST gradient is documented in the model from intraseasonal to annual timescales. It suggests that changes in SST gradient magnitudes are related to changes in the intensity of coastal upwelling off Peru and Chile. Such a relationship between SST gradients and vertical velocity would allow for the use of satellite-derived SSTs to monitor the intensity of coastal upwelling from the intraseasonal to annual timescales.
The 2015–2016 El Niño led to historically high temperatures and low precipitation over the tropics, while the growth rate of atmospheric carbon dioxide (CO2) was the largest on record. Here we quantify the response of tropical net biosphere exchange, gross primary production, biomass burning, and respiration to these climate anomalies by assimilating column CO2, solar-induced chlorophyll fluorescence, and carbon monoxide observations from multiple satellites. Relative to the 2011 La Niña, the pantropical biosphere released 2.5 ± 0.34 gigatons more carbon into the atmosphere in 2015, consisting of approximately even contributions from three tropical continents but dominated by diverse carbon exchange processes. The heterogeneity of the carbon-exchange processes indicated here challenges previous studies that suggested that a single dominant process determines carbon cycle interannual variability.
Fine-scale genetic structure (FSGS) is common in plants, driven by several ecological and evolutionary processes, among which is gene flow. Mangrove trees rely on ocean surface currents to spread their hydrochorous propagules through space. Since pollen dispersal is generally restricted to local scales, high level of short-distance propagule dispersal is expected to result in FSGS in Rhizophora spp. We investigated FSGS, recent bottleneck events, as well as historical and contemporary expansion patterns in Rhizophora racemosa populations from the entire coast of Cameroon, using 11 polymorphic microsatellite markers. Populations of the Cameroon Estuary complex (CEC) showed significant FSGS and significant reduction in effective population sizes (recent bottlenecks), compared to the other areas. Additionally, our results indicate stark differences between historical and contemporary expansion models. These suggest that contemporary processes such as restricted propagule dispersal, bottleneck events from high indirect and direct anthropogenic pressure, and recolonization by founders from ancient local pockets/refugia most plausibly shape the patterns of FSGS in the CEC.
The El Niño Modoki in 2010 led to historic droughts in Brazil. In order to understand its impact on carbon cycle variability, we derive the 2011–2010 annual carbon flux change (δF↑) globally and specifically to Brazil using the NASA Carbon Monitoring System Flux (CMS-Flux) framework. Satellite observations of CO2, CO, and solar-induced fluorescence (SIF) are ingested into a 4D-variational assimilation system driven by carbon cycle models to infer spatially resolved carbon fluxes including net ecosystem production, biomass burning, and gross primary productivity (GPP). The global 2011–2010 net carbon flux change was estimated to be δF↑=−1.60 PgC, while the Brazilian carbon flux change was −0.24 ± 0.11 PgC. This estimate is broadly within the uncertainty of previous aircraft-based estimates restricted to the Amazon basin. The 2011–2010 biomass burning change in Brazil was −0.24 ± 0.036 PgC, which implies a near-zero 2011–2010 change of the net ecosystem production (NEP): The near-zero NEP change is the result of quantitatively comparable increases GPP (0.31 ± 0.20 PgC) and respiration in 2011. Comparisons between Brazilian and global component carbon flux changes reveal complex interactions between the processes controlling annual land-atmosphere CO2 exchanges. These results show the potential of multiple satellite observations to help quantify and spatially resolve the response of productivity and respiration fluxes to climate variability.
Two global ocean models ranging in horizontal resolution from 1/12° to 1/48° are used to study the space- and time-scales of sea surface height (SSH) signals associated with internal gravity waves (IGWs). Frequency-horizontal wavenumber SSH spectral densities are computed over seven regions of the world ocean from three simulations of the HYbrid Coordinate Ocean Model (HYCOM) and two simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). High-wavenumber, high-frequency SSH variance follows the predicted IGW linear dispersion curves. The realism of high-frequency motions (>0.87cpd) in the models is tested through comparison of the frequency spectral density of dynamic height variance computed from the highest resolution runs of each model (1/25° HYCOM and 1/48° MITgcm) with dynamic height variance frequency spectral density computed from 9 in-situ profiling instruments. These high-frequency motions are of particular interest because of their contributions to the small-scale SSH variability that will be observed on a global scale in the upcoming Surface Water and Ocean Topography (SWOT) satellite altimetry mission. The variance at supertidal frequencies can be comparable to the tidal and low-frequency variance for high-wavenumbers (length scales smaller than ∼50km), especially in the higher resolution simulations. In the highest resolution simulations, the high-frequency variance can be greater than the low-frequency variance at these scales.
We update observationally-based estimates of subaqueous melt, Qm, beneath Petermann Glacier Ice Shelf (PGIS), Greenland, and model its sensitivity to oceanic thermal forcing, TF, and subglacial runoff, Qsg, using the Massachusetts Institute of Technology general circulation model (MITgcm), in a two-dimensional domain, with 20-m-vertical and 40-m horizontal resolution at the grounding line. We adjust the drag coefficient to match the observationally-based Qm. With the inclusion of Qsg, the maximum melt rate ( Qmmax) is two times larger in summer and 1/3 larger annually than in winter. Qmmax increases above-linear with TF and below-linear with Qsg. We estimate that Qmmax increased by 24% (+8.1 m/yr) beneath PGIS from the 1990s to the 2000s from a 0.21° C warming in ocean temperature and a doubling in Qsg, hence contributing to its thinning. If the PGIS is removed, we estimate that the modeled melt rate near the grounding line will increase 13–16 times.
A realistic representation of sea-ice deformation in models is important for accurate simulation of the sea-ice mass balance. Simulated sea-ice deformation from numerical simulations with 4.5, 9, and 18 km horizontal grid spacing and a viscous–plastic (VP) sea-ice rheology are compared with synthetic aperture radar (SAR) satellite observations (RGPS, RADARSAT Geophysical Processor System) for the time period 1996–2008. All three simulations can reproduce the large-scale ice deformation patterns, but small-scale sea-ice deformations and linear kinematic features (LKFs) are not adequately reproduced. The mean sea-ice total deformation rate is about 40 % lower in all model solutions than in the satellite observations, especially in the seasonal sea-ice zone. A decrease in model grid spacing, however, produces a higher density and more localized ice deformation features. The 4.5 km simulation produces some linear kinematic features, but not with the right frequency. The dependence on length scale and probability density functions (PDFs) of absolute divergence and shear for all three model solutions show a power-law scaling behavior similar to RGPS observations, contrary to what was found in some previous studies. Overall, the 4.5 km simulation produces the most realistic divergence, vorticity, and shear when compared with RGPS data. This study provides an evaluation of high and coarse-resolution viscous–plastic sea-ice simulations based on spatial distribution, time series, and power-law scaling metrics.
Mangrove forests are systems that provide ecosystem services and rely on floating propagules of which the dispersal trajectories are determined by ocean currents and winds. Quantitating connectivity of mangrove patches is an important conservation concern. Current estimates of connectivity, however, fail to integrate the link between ocean currents at different spatial scales and dispersal trajectories. Here, we use high-resolution estimates of ocean currents and surface winds from meteorological and oceanographic analyses, in conjunction with experimental data on propagule traits (e.g., density, size, and shape) and dispersal vector properties (e.g., strength and direction of water and wind currents). We incorporate these data in a dispersal model to illustrate the potential effect of wind on dispersal trajectories of hydrochorous propagules from different mangrove species. We focus on the Western Indian Ocean, including the Mozambique Channel, which has received much attention because of its reported oceanic complexity, to illustrate the effect of oceanic features such as eddy currents and tides. In spite of the complex pattern of ocean surface currents and winds, some propagules are able to cross the Mozambique Channel. Eddy currents and tides may delay arrival at a suitable site. Experimentally demonstrated differences in wind sensitivity among propagule types were shown to affect the probability of departure and the shape of dispersal trajectories. The model could be used to reconstruct current fluxes of mangrove propagules that may help explain past and current distributions of mangrove forests and assess the potential for natural expansion of these forests.
Tidal currents and large oceanic currents, such as the Agulhas, Gulf Stream and Kuroshio, are known to modify ocean wave properties, causing extreme sea states that are a hazard to navigation. Recent advances in the understanding and modeling capability of ocean currents at subme-soscales have revealed the ubiquitous presence of fronts and filaments. Based on realistic numerical models, we show that these structures can be the main source of variability in significant wave heights at scales less than 200 km, including important variations down to 10 km. Model results are consistent with wave height variations along satellite altimeter tracks, resolved at scales larger than 50 km. The spectrum of significant wave heights is found to be roughly proportional to Hs 2 /(g 2 Tm0,−1 2) times the current spectrum, where Hs is the spatially-averaged significant wave height, Tm0,−1 is the average energy period, and g is the gravity acceleration. This variability induced by currents has been largely overlooked in spite of its relevance for extreme wave heights and remote sensing.
Recent studies suggest that the thickness of Winter Water (WW), that is, water with potential temperature below ∼-1°C located below Antarctic Surface Water and above Circumpolar Deep Water (CDW) is critical in determining the ice shelf melt rate, especially for the Pine Island Glacier (PIG). Existing model studies, however, misrepresent WW thickness and properties in the Amundsen Sea (AS). Here, we adjust a small number of model parameters in a regional Amundsen and Bellingshausen Seas configuration of the Massachusetts Institute of Technology general circulation model in order to reproduce properties and thickness of WW and CDW close to observations, with significant improvement for WW compared to previous studies. The cost, which is defined as weighted model-data difference squared, is reduced by 23%. Although a previous modeling study points out that the local surface heat loss upstream from Pine Island Polynya could be the reason for the observed 2012 PIG melt decline and WW thickening, they did not show WW freshening, which was observed at the same time. Model sensitivity experiments for surface heat loss, PIG melt rate, and precipitation fail to replicate WW freshening concurrent with PIG melt decline, implying that these processes can not fully explain the observed PIG melt decrease.
The Amundsen Sea sector is experiencing the largest mass loss, glacier acceleration, and grounding line retreat in Antarctica. Enhanced intrusion of Circumpolar Deep Water onto the continental shelf has been proposed as the primary forcing mechanism for the retreat. Here, we investigate the dynamics and evolution of Thwaites Glacier with a novel, fully-coupled, ice-ocean numerical model. We obtain a significantly improved agreement with the observed pattern of glacial retreat using the coupled model. Coupled simulations over the coming decades indicate a continued mass loss at a sustained rate. Uncoupled simulations using a depth-dependent parameterization of sub-ice-shelf melt significantly overestimates the rate of grounding line retreat compared to the coupled model as the parameterization does not capture the complexity of the ocean circulation associated with the formation of confined cavities during the retreat. Bed topography controls the pattern of grounding line retreat, while oceanic thermal forcing impacts the rate of grounding line retreat. The importance of oceanic forcing increases with time as Thwaites grounding line retreats farther inland.
The El Ni\~{n}o Modoki in 2010 lead to historic droughts in Brazil. We quantify the global and Brazilian carbon response to this event using the NASA Carbon Monitoring System Flux (CMS-Flux) framework. Satellite observations of CO$_2$, CO, and solar induced fluorescence (SIF) are ingested into a 4D-variational assimilation system driven by carbon cycle models to infer spatially resolved carbon fluxes including net ecosystem exchange, biomass burning, and gross primary productivity (GPP). The global net carbon flux tendency, which is the flux difference 2011-2010 and is positive for net fluxes into the atmosphere, was estimated to be -1.60 PgC between 2011-2010 while the Brazilian tendency was -0.24 $\pm$ 0.11 PgC. This estimate is broadly within the uncertainty of previous aircraft based estimates restricted to the Amazonian basin. The biomass burning tendency in Brazil was -0.24 $\pm$ 0.036 PgC, which implies a near-zero change of the net ecosystem production (NEP). The near-zero change of the NEP is the result of quantitatively comparable increase in GPP (0.34 $\pm$ 0.20) and respiration in Brazil. Comparisons of the component fluxes in Brazil to the global fluxes show a complex balance between regional contributions to individual carbon fluxes such as biomass burning, and their net contribution to the global carbon balance, i.e., the Brazilian biomass burning tendency is a significant contributor to the global biomass burning tendency but the Brazilian net flux tendency is not a dominant contributor to the global tendency. These results show the potential of multiple satellite observations to help quantify the spatially resolved response of productivity and respiration fluxes to climate variability.
Recent studies show that the vigorous seasonal cycle of the mixed layer modulates upper-ocean submesoscale turbulence. Here we provide model-based evidence that the seasonally-changing upper-ocean stratification in the Kuroshio Extension also modulates submesoscale (here 10-100 km) inertia-gravity waves. Summertime re-stratification weakens submesoscale turbulence but enhances inertia-gravity waves near the surface. Thus, submesoscale turbulence and inertia-gravity waves undergo vigorous out-of-phase seasonal cycles. These results imply a strong seasonal modulation of the accuracy of geostrophic velocity diagnosed from submesoscale sea-surface height delivered by the Surface Water and Ocean Topography (SWOT) satellite mission.
High-resolution, three-dimensional simulations from the Massachusetts Institute of Technology general circulation model ocean model are used to calculate the subaqueous melt rate of the calving faces of Umiamako, Rinks, Kangerdlugssup, Store, and Kangilerngata glaciers, west Greenland, from 1992 to 2015. Model forcing is from monthly reconstructions of ocean state and ice sheet runoff. Results are analyzed in combination with observations of bathymetry, bed elevation, ice front retreat, and glacier speed. We calculate that subaqueous melt rates are 2-3 times larger in summer compared to winter and doubled in magnitude since the 1990s due to enhanced subglacial runoff and 1.6 ± 0.3°C warmer ocean temperature. Umiamako and Kangilerngata retreated rapidly in the 2000s when subaqueous melt rates exceeded the calving rates and ice front retreated to deeper bed elevation. In contrast, Store, Kangerdlugssup, and Rinks have remained stable because their subaqueous melt rates are 3-4 times lower than their calving rates, i.e., the glaciers are dominated by calving processes.
Mangroves are seafaring taxa through their hydrochorous propagules that have the potential to disperse over long distances. Therefore, investigating their patterns of gene flow provides insights on the processes involved in the spatial genetic structuring of populations. The coastline of Cameroon has a particular geomorphological history and coastal hydrology with complex contemporary patterns of ocean currents, which we hypothesize to have effects on the spatial configuration and composition of present-day mangroves within its spans. A total of 982 trees were sampled from 33 transects (11 sites) in 4 estuaries. Using 11 polymorphic SSR markers, we investigated genetic diversity and structure of Rhizophora racemosa, a widespread species in the region. Genetic diversity was low to moderate and genetic differentiation between nearly all population pairs was significant. Bayesian clustering analysis, PCoA, estimates of contemporary migration rates and identification of barriers to gene flow were used and complemented with estimated dispersal trajectories of hourly released virtual propagules, using high-resolution surface current from a mesoscale and tide-resolving ocean simulation. These indicate that the Cameroon Volcanic Line (CVL) is not a present-day barrier to gene flow. Rather, the Inter-Bioko-Cameroon (IBC) corridor, formed due to sea level rise, allows for connectivity between two mangrove areas that were isolated during glacial times by the CVL. Genetic data and numerical ocean simulations indicated that an oceanic convergence zone near the Cameroon Estuary complex (CEC) presents a strong barrier to gene flow, resulting in genetic discontinuities between the mangrove areas on either side. This convergence did not result in higher genetic diversity at the CEC as we had hypothesized. In conclusion, the genetic structure of Rhizophora racemosa is maintained by the contrasting effects of the contemporary oceanic convergence and historical climate change-induced sea level rise.
A realistic representation of sea ice deformation in models is important for accurate simulation of the sea ice mass balance. In this study, model ice strength sensitivity experiments show an increase in Arctic Basin sea ice volume of 7 % and 35 % for a decrease in ice strength of, respectively, 30 % and 70 %, after 8 years of model integration. This volume increase is caused by a combination of dynamic and thermodynamic processes. On the one hand, a weaker ice cover initially produces more ice due to increased deformation and new ice growth. The thickening of the ice, on the other hand, increases the ice strength and decreases the sea ice volume export out of the Arctic Basin. The balance of these processes leads to a new equilibrium Arctic Basin ice volume. Simulated sea ice deformation strain rates from model simulations with 4.5, 9, and 18-km horizontal grid spacing are compared with synthetic aperture radar satellite observations (RGPS). All three model simulations can reproduce the large-scale ice deformation patterns but they do not reproduce all aspects of the observed deformation rates. The overall sea ice deformation rate is about 50 % lower in all model solutions than in the satellite observations, especially in the seasonal sea ice zone. Small scale sea ice deformation and linear kinematic features are not adequately reproduced. A decrease in model grid spacing, however, produces a higher density and more localized ice deformation features. Overall, the 4.5-km simulation produces the lowest misfits in divergence, vorticity, and shear when compared with RGPS data. Not addressed in this study is whether the differences between simulated and observed deformation rates are an intrinsic limitation of the viscous-plastic sea ice rheology that was used in the sensitivity experiments, or whether it indicates a lack of adjustment of existing model parameters to better represent these processes. Either way, this study provides new quantitative metrics for existing and new sea ice rheologies to strive for.
This study discusses the upper-ocean (0-200 m) horizontal wavenumber spectra in the Drake Passage from 13 yr of shipboard ADCP measurements, altimeter data, and a high-resolution numerical simulation. At scales between 10 and 200 km, the ADCP kinetic energy spectra approximately follow a k-3 power law. The observed flows are more energetic at the surface, but the shape of the kinetic energy spectra is independent of depth. These characteristics resemble predictions of isotropic interior quasigeostrophic turbulence. The ratio of across-track to along-track kinetic energy spectra, however, significantly departs from the expectation of isotropic interior quasigeostrophic turbulence. The inconsistency is dramatic at scales smaller than 40 km. A Helmholtz decomposition of the ADCP spectra and analyses of synthetic and numerical model data show that horizontally divergent, ageostrophic flows account for the discrepancy between the observed spectra and predictions of isotropic interior quasigeostrophic turbulence. In Drake Passage, ageostrophic motions appear to be dominated by inertia-gravity waves and account for about half of the near-surface kinetic energy at scales between 10 and 40 km. Model results indicate that ageostrophic flows imprint on the sea surface, accounting for about half of the sea surface height variance between 10 and 40 km.
Essential components and technical details of regional ocean forecasting systems configured from the Regional Ocean Modeling System are discussed with the goal of bridging the gap between user and ocean modeling communities. Recent development of these systems and applications are also surveyed. Design considerations of such a system for the South China Sea are discussed, based on regional dynamic characteristics and potential applications.
We study the impact of synthesizing ocean and sea ice concentration data with a global, eddying coupled sea ice-ocean configuration of the Massachusetts Institute of Technology general circulation model with the goal of reproducing the 2004 three-dimensional time-evolving ice-ocean state. This work builds on the state estimation framework developed in the Estimating the Circulation and Climate of the Ocean consortium by seeking a reconstruction of the global sea ice-ocean system that is simultaneously consistent with (1) a suite of in situ and remotely-sensed ocean and ice data and (2) the physics encoded in the numerical model. This dual consistency is successfully achieved here by adjusting only the model’s initial hydrographic state and its atmospheric boundary conditions such that misfits between the model and data are minimized in a least-squares sense. We show that synthesizing both ocean and sea ice concentration data is required for the model to adequately reproduce the observed details of the sea ice annual cycle in both hemispheres. Surprisingly, only modest adjustments to our first-guess atmospheric state and ocean initial conditions are necessary to achieve model-data consistency, suggesting that atmospheric reanalysis products remain a leading source of errors for sea ice-ocean model hindcasts and reanalyses. The synthesis of sea ice data is found to ameliorate misfits in the high latitude ocean, especially with respect to upper ocean stratification, temperature, and salinity. Constraining the model to sea ice concentration modestly reduces ICESat-derived Arctic ice thickness errors by improving the temporal and spatial evolution of seasonal ice. Further increases in the accuracy of global sea ice thickness in the model likely require the direct synthesis of sea ice thickness data.
The NASA Carbon Monitoring System (CMS) Flux Project aims to attribute changes in the atmospheric accumulation of carbon dioxide to spatially resolved fluxes by utilizing the full suite of NASA data, models, and assimilation capabilities. For the oceanic part of this project, we introduce ECCO2-Darwin, a new ocean biogeochemistry general circulation model based on combining the following pre-existing components: (i) a full-depth, eddying, global-ocean configuration of the Massachusetts Institute of Technology general circulation model (MITgcm), (ii) an adjoint-method-based estimate of ocean circulation from the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project, (iii) the MIT ecosystem model "Darwin", and (iv) a marine carbon chemistry model. Air-sea gas exchange coefficients and initial conditions of dissolved inorganic carbon, alkalinity, and oxygen are adjusted using a Green's Functions approach in order to optimize modeled air-sea CO2 fluxes. Data constraints include observations of carbon dioxide partial pressure (pCO2) for 2009-2010, global air-sea CO2 flux estimates, and the seasonal cycle of the Takahashi et al. (2009) Atlas. The model sensitivity experiments (or Green's Functions) include simulations that start from different initial conditions as well as experiments that perturb air-sea gas exchange parameters and the ratio of particulate inorganic to organic carbon. The Green's Functions approach yields a linear combination of these sensitivity experiments that minimizes model-data differences. The resulting initial conditions and gas exchange coefficients are then used to integrate the ECCO2-Darwin model forward. Despite the small number (six) of control parameters, the adjusted simulation is significantly closer to the data constraints (37% cost function reduction, i.e., reduction in the model-data difference, relative to the baseline simulation) and to independent observations (e.g., alkalinity). The adjusted air-sea gas exchange parameter differs by only 3% from the baseline value and has little impact (-0.1%) on the cost function. The particulate inorganic to organic carbon ratio was increased more than threefold and reduced the cost function by 22% relative to the baseline integration, indicating a significant influence of biology on air-sea gas exchange. The largest contribution to cost reduction (35%) comes from the adjustment of initial conditions. In addition to reducing biases relative to observations, the adjusted simulation exhibits smaller model drift than the baseline. We estimate drift by integrating the model with repeated 2009 atmospheric forcing for seven years and find a volume-weighted drift reduction of, for example, 12.5% for nitrate and 30% for oxygen in the top 300 m. Although there remain several regions with large model-data discrepancies, for example, overly strong carbon uptake in the Southern Ocean, the adjusted simulation is a first step towards a more accurate representation of the ocean carbon cycle at high spatial and temporal resolution.
The structure of the Antarctic Slope Front (ASF) and the associated Antarctic Slope Current (ASC) on the Scotia Sea side of the Weddell‐Scotia Confluence (WSC) is described using data from a hydrographic survey and three 1 year long moorings across the continental slope. The ASC in this region flows westward along isobaths with an annual mean speed of ∼0.2 m s−1, with time variability dominated by the K 1 and O 1 tidal diurnal constituents, a narrowband oscillation with ∼2‐week period attributable to the spring/neap tidal cycle, and seasonal variability. Realistic and idealized high‐resolution numerical simulations are used to determine the contribution of tides to the structure of the ASF and the speed of the ASC. Two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured ASF/ASC cross‐slope structure, the time variability at our moorings, and the reduced stratification within the WSC. Two idealized simulations run with tide‐only forcing, one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous, show that tides can generate the ASC and ASF through volume flux convergence along the slope initiated by effects including the Lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior. Climate models that exclude the effects of tides will not correctly represent the ASF and ASC or their influence on the injection of intermediate and dense waters from the WSC to the deep ocean. Tides set mean properties of Antarctic Slope Front in Weddell‐Scotia ConfluenceVolume convergence by tidal rectification, plus mixing, creates Slope CurrentShelf water properties in Weddell‐Scotia Confluence modified by tides
Most regional ocean models that use discharge as part of the forcing use relatively coarse river discharge data sets (1°, or ∼110 km) compared to the model resolution (typically 1/4° or less), and do not account for seasonal changes in river water temperature. We introduce a new climatological data set of river discharge and river water temperature with 1/6° grid spacing over the Arctic region (Arctic River Discharge and Temperature; ARDAT), incorporating observations from 30 Arctic rivers. The annual mean discharge for all rivers in ARDAT is 2817 ± 330 km3 yr-1. River water temperatures range between 0 °C in winter to 14.0 - 17.6 °C in July, leading to a long-term mean monthly heat flux from all rivers of 3.2 ± 0.6 TW, of which 31% is supplied by Alaskan rivers and 69% is supplied by Eurasian rivers. This riverine heat flux is equivalent to 44% of the estimated ocean heat flux associated with the Bering Strait throughflow, but during the spring freshet can be ∼10 times as large, suggesting that heat flux associated with Arctic rivers is an important component of the Arctic heat budget on seasonal time scales.
The impact of data assimilation on the transports of eastward-flowing Equatorial Undercurrent (EUC) and North Equatorial Countercurrent (NECC) in the Pacific Ocean from 1458E to 958W during 2004–05 and 2009– 11 was assessed. Two Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), solutions were analyzed: one with data assimilation and one without. Assimilated data included satellite observations of sea surface temperature and ocean surface topography, in which the sampling patterns were approximately uniform over the 5 years, and in situ measurements of subsurface salinity and temperature profiles, in which the sampling patterns varied considerably in space and time throughout the 5 years. Velocity measurements were not assimilated. The impact of data assimilation was considered significant when the difference between the transports computed with and without data assimilation was greater than 5.5 3 10 6 m 3 s 21 (or 5.5 Sv; 1 Sv [ 10 6 m 3 s 21) for the EUC and greater than 5.0 Sv for the NECC. In addition, the difference of annual-mean transports computed from 3-day-averaged data was statistically significant at the 95% level. The impact of data assimilation ranged from no impact to very substantial impact when data assimilation increased the EUC transport and decreased the NECC transport. The study's EUC results had some correspondence with other studies and no simple agreement or disagreement pattern emerged among all studies of the impact of data assimilation. No comparable study of the impact of data assimilation on the NECC has been made.
NASA's Carbon Monitoring System (CMS) Flux Pilot Project (FPP) was designed to better understand contemporary carbon fluxes by bringing together state-of-the art models with remote sensing datasets. Here we report on simulations using NASA's Goddard Earth Observing System Model, version 5 (GEOS-5) which was used to evaluate the consistency of two different sets of observationally informed land and ocean fluxes with atmospheric CO2 records. Despite the observation inputs, the average difference in annual terrestrial biosphere flux between the two land (NASA Ames CASA and CASA-GFED) models is 1.7 Pg C for 2009-2010. Ocean models (NOBM and ECCO2-Darwin) differ by 35% in their global estimates of carbon flux with particularly strong disagreement in high latitudes. Based upon combinations of terrestrial and ocean fluxes, GEOS-5 reasonably simulated the seasonal cycle observed at northern hemisphere surface sites and by the Greenhouse gases Observing SATellite (GOSAT) while the model struggled to simulate the seasonal cycle at southern hemisphere surface locations. Though GEOS-5 was able to reasonably reproduce the patterns of XCO2 observed by GOSAT, it struggled to reproduce these aspects of AIRS observations. Despite large differences between land and ocean flux estimates, resulting differences in atmospheric mixing ratio were small, typically less than 5 ppm at the surface and 3 ppm in the XCO2 column. A statistical analysis based on the variability of observations shows that flux differences of these magnitudes are difficult to distinguish from inherent measurement variability, regardless of the measurement platform.
Using an Observing System Simulation Experiment (OSSE), we investigate the impact of JAXA Greenhouse gases Observing SATellite ‘IBUKI’ (GOSAT) sampling on the estimation of terrestrial biospheric flux with the NASA Carbon Monitoring System Flux (CMS-Flux) estimation and attribution strategy. The simulated observations in the OSSE use the actual column carbon dioxide (XCO2 ) b2.9 retrieval sensitivity and quality control for the year 2010 processed through the Atmospheric CO2 Observations from Space algorithm. CMS-Flux is a variational inversion system that uses the GEOS-Chem forward and adjoint model forced by a suite of observationally constrained fluxes from ocean, land and anthropogenic models. We investigate the impact of GOSAT sampling on flux estimation in two aspects: 1) random error uncertainty reduction and 2) the global and regional bias in posterior flux resulted from the spatiotemporally biased GOSAT sampling. Based on Monte Carlo calculations, we find that global average flux uncertainty reduction ranges from 25% in September to 60% in July. When aggregated to the 11 land regions designated by the phase 3 of the Atmospheric Tracer Transport Model Intercomparison Project, the annual mean uncertainty reduction ranges from 10% over North American boreal to 38% over South American temperate, which is driven by observational coverage and the magnitude of prior flux uncertainty. The uncertainty reduction over the South American tropical region is 30%, even with sparse observation coverage. We show that this reduction results from the large prior flux uncertainty and the impact of non-local observations. Given the assumed prior error statistics, the degree of freedom for signal is ~1132 for 1-yr of the 74 055 GOSAT XCO2 observations, which indicates that GOSAT provides ~1132 independent pieces of information about surface fluxes. We quantify the impact of GOSAT's spatiotemporally sampling on the posterior flux, and find that a 0.7 gigatons of carbon bias in the global annual posterior flux resulted from the seasonally and diurnally biased sampling when using a diagonal prior flux error covariance.
On 21 March 1960, sounds from three 300-lb depth charges deployed at 5.5-min. intervals off Perth, Australia were recorded by the SOFAR station at Bermuda. The recorded travel time of these signals, about 13,375 s, is a historical measure of the ocean temperature averaged across several ocean basins. The 1960 travel time measurement has about 3-s precision. High-resolution global ocean state estimates for 2004 from the “Estimating the Circulation and Climate of the Ocean, Phase II” (ECCO2) project were combined with ray tracing to determine the paths followed by the acoustic signals. The acoustic paths are refracted geodesics that are slightly deflected by either small-scale topographic features in the Southern Ocean or the coast of Brazil. The refractive influences of intense, small-scale oceanographic features, such as Agulhas Rings or eddies in the Antarctic Circumpolar Current, greatly reduce the necessary topographic deflection and cause the acoustic paths to meander in time. The ECCO2 ocean state estimates, which are constrained by model dynamics and available data, were used to compute present-day travel times. Measured and computed arrival coda were in good agreement. Based on recent estimates of warming of the upper ocean, the travel-time change over the past half-century was nominally expected to be about minus 9 s, but little difference between measured (1960) and computed (2004) travel times was found. Taking into account uncertainties in the 1960 measurements, the 2004 ocean state estimates, and other approximations, the ocean temperature averaged along the sound channel axis over the antipodal paths has warmed at a rate less than about 4.6 m °C yr−1 (95% confidence). Ultimately, however, the estimated uncertainties are comparable in size to the expected warming signal.
[1] The rapid recent decline of Arctic Ocean sea ice area increases the flux of solar radiation available for primary production and the area of open water for air-sea gas exchange. We use a regional physical-biogeochemical model of the Arctic Ocean, forced by the National Centers for Environmental Prediction/National Center for Atmospheric Research atmospheric reanalysis, to evaluate the mean present-day CO2 sink and its temporal evolution. During the 1996–2007 period, the model suggests that the Arctic average sea surface temperature warmed by 0.04°C a−1, that sea ice area decreased by ∼0.1 × 106 km2 a−1, and that the biological drawdown of dissolved inorganic carbon increased. The simulated 1996–2007 time-mean Arctic Ocean CO2 sink is 58 ± 6 Tg C a−1. The increase in ice-free ocean area and consequent carbon drawdown during this period enhances the CO2 sink by ∼1.4 Tg C a−1, consistent with estimates based on extrapolations of sparse data. A regional analysis suggests that during the 1996–2007 period, the shelf regions of the Laptev, East Siberian, Chukchi, and Beaufort Seas experienced an increase in the efficiency of their biological pump due to decreased sea ice area, especially during the 2004–2007 period, consistent with independently published estimates of primary production. In contrast, the CO2 sink in the Barents Sea is reduced during the 2004–2007 period due to a dominant control by warming and decreasing solubility. Thus, the effect of decreasing sea ice area and increasing sea surface temperature partially cancel, though the former is dominant.
The rapid recent decline of Arctic Ocean sea ice area increases the flux of solar radiation available for primary production and the area of open water for air-sea gas exchange. We use a regional physical-biogeochemical model of the Arctic Ocean, forced by the National Centers for Environmental Prediction/National Center for Atmospheric Research atmospheric reanalysis, to evaluate the mean present-day CO[subscript 2] sink and its temporal evolution. During the 1996-2007 period, the model suggests that the Arctic average sea surface temperature warmed by 0.04°C a[superscript -1], that sea ice area decreased by ∼0.1 × 106 km2 a][superscript -1], and that the biological drawdown of dissolved inorganic carbon increased. The simulated 1996-2007 time-mean Arctic Ocean CO[subscript 2] sink is 58 ± 6 Tg C a[superscript -1]. The increase in ice-free ocean area and consequent carbon drawdown during this period enhances the CO[subscript 2] sink by ∼1.4 Tg C a[superscript -1], consistent with estimates based on extrapolations of sparse data. A regional analysis suggests that during the 1996-2007 period, the shelf regions of the Laptev, East Siberian, Chukchi, and Beaufort Seas experienced an increase in the efficiency of their biological pump due to decreased sea ice area, especially during the 2004-2007 period, consistent with independently published estimates of primary production. In contrast, the CO[subscript 2] sink in the Barents Sea is reduced during the 2004-2007 period due to a dominant control by warming and decreasing solubility. Thus, the effect of decreasing sea ice area and increasing sea surface temperature partially cancel, though the former is dominant. Keywords: Arctic Ocean, sea ice, ocean productivity
[1] We present three-dimensional, high-resolution simulations of ice melting at the calving face of Store Glacier, a tidewater glacier in West Greenland, using the Massachusetts Institute of Technology general circulation model. We compare the simulated ice melt with an estimate derived from oceanographic data. The simulations show turbulent upwelling and spreading of the freshwater-laden plume along the ice face and the vigorous melting of ice at rates of meters per day. The simulated August 2010 melt rate of 2.0±0.3 m/d is within uncertainties of the melt rate of 3.0±1.0 m/d calculated from oceanographic data. Melting is greatest at depth, above the subglacial channels, causing glacier undercutting. Melt rates increase proportionally to thermal forcing raised to the power of 1.2–1.6 and to subglacial water flux raised to the power of 0.5–0.9. Therefore, in a warmer climate, Store Glacier melting by ocean may increase from both increased ocean temperature and subglacial discharge.
Receptions on three vertical hydrophone arrays from basin-scale acoustic transmissions in the North Pacific during 1996 and 1998 are used to test the time-mean sound-speed properties of the World Ocean Atlas 2005 (WOA05), of an eddying unconstrained simulation of the Parallel Ocean Program (POP), and of three data-constrained solutions provided by the estimating the circulation and climate of the ocean (ECCO) project: a solution based on an approximate Kalman filter from the Jet Propulsion Laboratory (ECCO-JPL), a solution based on the adjoint method from the Massachusetts Institute of Technology (ECCO-MIT), and an eddying solution based on a Green's function approach from ECCO, Phase II (ECCO2). Predictions for arrival patterns using annual average WOA05 fields match observations to within small travel time offsets (0.3-1.0 s). Predictions for arrival patterns from the models differ substantially from the measured arrival patterns, from the WOA05 climatology, and from each other, both in terms of travel time and in the structure of the arrival patterns. The acoustic arrival patterns are sensitive to the vertical gradients of sound speed that govern acoustic propagation. Basin-scale acoustic transmissions, therefore, provide stringent tests of the vertical temperature structure of ocean state estimates. This structure ultimately influences the mixing between the surface waters and the ocean interior. The relatively good agreement of the acoustic data with the more recent ECCO solutions indicates that numerical ocean models have reached a level of accuracy where the acoustic data can provide useful additional constraints for ocean state estimation.
Considerable effort is presently being devoted to producing high-resolution sea surface temperature (SST) analyses with a goal of spatial grid resolutions as low as 1 km. Because grid resolution is not the same as feature resolution, a method is needed to objectively determine the resolution capability and accuracy of SST analysis products. Ocean model SST fields are used in this study as simulated "true'' SST data and subsampled based on actual infrared and microwave satellite data coverage. The subsampled data are used to simulate sampling errors due to missing data. Two different SST analyses are considered and run using both the full and the subsampled model SST fields, with and without additional noise. The results are compared as a function of spatial scales of variability using wavenumber auto-and cross-spectral analysis. The spectral variance at high wavenumbers (smallest wavelengths) is shown to be attenuated relative to the true SST because of smoothing that is inherent to both analysis procedures. Comparisons of the two analyses (both having grid sizes of roughly 1/20 degrees) show important differences. One analysis tends to reproduce small-scale features more accurately when the high-resolution data coverage is good but produces more spurious small-scale noise when the high-resolution data coverage is poor. Analysis procedures can thus generate small-scale features with and without data, but the small-scale features in an SST analysis may be just noise when high-resolution data are sparse. Users must therefore be skeptical of high-resolution SST products, especially in regions where high-resolution ( similar to 5 km) infrared satellite data are limited because of cloud cover.
Two high-resolution (~1 km grid spacing) numerical model simulations of the Amundsen Sea, West Antarctica, are used to study the role of the ocean in the mass loss and grounding line retreat of Pine Island Glacier. The first simulation uses BEDMAP bathymetry under the Pine Island ice shelf, and the second simulation uses NASA IceBridge-derived bathymetry. The IceBridge data reveal the existence of a trough from the ice-shelf edge to the grounding line, enabling warm Circumpolar Deep Water to penetrate to the grounding line, leading to higher melt rates than previously estimated. The mean melt rate for the simulation with NASA IceBridge data is 28m/yr, much higher than previous model estimates but closer to estimates from remote sensing. Although the mean melt rate is 25% higher than in the simulation with BEDMAP bathymetry, the temporal evolution remains unchanged between the two simulations. This indicates that temporal variability of melting is mostly driven by processes outside the cavity. Spatial melt rate patterns of BEDMAP and IceBridge simulations differ significantly, with the latter in closer agreement with satellite-derived melt rate estimates of 50m/yr near the grounding line. Our simulations confirm that knowledge of the cavity shape and its time evolution are essential to accurately capture basal mass loss of Antarctic ice shelves.
The largest dischargers of ice in Greenland are glaciers that terminate in the ocean and melt in contact with sea water. Studies of ice-sheet/ocean interactions have mostly focused on melting beneath near-horizontal floating ice shelves. For tidewater glaciers, melting instead takes place along the vertical face of the calving front. Here we modify the Massachusetts Institute of Technology general circulation model (MITgcm) to include ice melting from a calving face with the freshwater outflow at the glacier grounding line. We use the model to predict melt rates and their sensitivity to ocean thermal forcing and to subglacial discharge. We find that melt rates increase with approximately the one-third power of the subglacial water flux, and increase linearly with ocean thermal forcing. Our simulations indicate that, consistent with limited field data, melting ceases when subglacial discharge is shut off, and reaches several meters per day when subglacial discharge is high in the summer. These results are a first step toward a more realistic representation of subglacial discharge and of ocean thermal forcing on the subaqueous melting of tidewater glaciers in a numerical ocean model. Our results illustrate that the ice-front melting process is both complex and strongly time-dependent.
We examine the pattern of spreading of warm subtropical-origin waters around Greenland for the years 1992–2009 using a high-resolution (4 km horizontal grid) coupled ocean and sea-ice simulation. The simulation, provided by the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project, qualitatively reproduces the observed warming of subsurface waters in the subpolar gyre associated with changes of the North Atlantic atmospheric state that occurred in the mid-1990s. The modeled subsurface ocean temperature warmed by 1.5°C in southeast and southwest Greenland during 1994–2005 and subsequently cooled by 0.5°C; modeled subsurface ocean temperature increased by 2–2.5°C in central and then northwest Greenland during 1997–2005 and stabilized thereafter, while it increased after 2005 by
At the Last Glacial Maximum (LGM), the salinity contrast between northern source deep water and southern source bottom water was reversed with respect to the contrast today. Additionally, Glacial Southern Source Bottom Water (GSSBW) was saltier than Antarctic Bottom Water (AABW), over and above the difference implied by the mean sea level change. This study examines to what extent cold temperatures, through their effect on ice formation and melting, could have caused these differences. Computational sensitivity experiments using a coupled ice shelf cavity-sea ice-ocean model are performed in a Weddell Sea domain, as a representative case study for bottom water formation originating from Antarctic continental shelves. Ocean temperatures at the domain open boundaries are systematically lowered to determine the sensitivity of Weddell Sea water mass properties to a range of cool ocean temperatures. The steady state salinities differ between experiments due to temperature-induced responses of ice shelf and sea ice melting and freezing, evaporation and open boundary fluxes. The results of the experiments indicate that reduced ocean temperature can explain up to 30% of the salinity difference between GSSBW and AABW, primarily due to decreased ice shelf melting. The smallest and most exposed ice shelves, which abut narrow continental shelves, have the greatest sensitivity to the ocean temperature changes, suggesting that at the LGM there could have been a shift in geographical site dominance in bottom water formation. More sea ice is formed and exported in the cold ocean experiments, but the effect of this on salinity is negated by an equal magnitude reduction in evaporation.
A coupled ocean and sea ice model is used to investigate dense water (DW) formation in the Chukchi and Bering shelves and the pathways by which this water feeds the upper halocline. Two 1992-2008 data-constrained solutions at 9- and 4-km horizontal grid spacing show that 1) winter sea ice growth results in brine rejection and DW formation; 2) the DW flows primarily down Barrow and Central-Herald Canyons in the form of bottom-trapped, intermittent currents to depths of 50-150 m from the late winter to late summer seasons; and 3) eddies with diameters similar to 30 km carry the cold DW from the shelf break into the Canada Basin interior at depths of 50-150 m. The 4-km data-constrained solution does not show eddy transport across the Chukchi Shelf at shallow depths; instead, advection of DW downstream of polynya regions is driven by a strong (similar to 0.1 m s(-1)) mean current on the Chukchi Shelf. Upper halocline water (UHW) formation rate was obtained from two methods: one is based on satellite data and on a simple parameterized approach, and the other is computed from the authors' model solution. The two methods yield 5740 +/- 61420 km(3) yr(-1) and 4190-4860 +/- 61440 km(3) yr(-1), respectively. These rates imply a halocline replenishment period of 10-21 yr. Passive tracers also show that water with highest density forms in the Gulf of Anadyr and along the eastern Siberian coast immediately north of the Bering Strait. These results provide a coherent picture of the seasonal development of UHW at high spatial and temporal resolutions and serve as a guide for improving understanding of water-mass formation in the western Arctic Ocean.
At any given point the atmospheric column inventory of CO2 over the ocean can change due to divergence in lateral transport of CO2 in the atmosphere and air-sea exchanges at the sea surface. As part of the NASA Carbon Monitoring System pilot project we have diagnosed air-sea exchanges of CO2 from an eddying ocean model constrained with observations from 2009 and 2010. The resulting flux patterns show temporal and spatial variation on time scales ranging from those of synoptic meteorology to interannual. Decomposing the variation of fluxes into different temporal bands reveals the contributions of different driving mechanisms. We focus our analysis on the North Atlantic region from a global MITgcm ocean data assimilation experiment, with an explicitly resolved ecosystem and carbonate chemistry module, undertaken as part of the ECCO2 project. Sub-seasonal fluxes correlate strongly with forcing by synoptic meteorology, but are modulated by the underlying ocean state. Magnitudes are modulated somewhat by seasonal variations in sea-surface conditions and by standing patterns of ocean circulation. The results show characteristic spatial and temporal scales. The magnitude of the sub-seasonal terms (equivalent to ± 0.25 g C m-2 day-1) is below the current observing threshold of NASAs upcoming remote sensing mission, OCO-2 (approximately a 1ppmv column integral change between 16 day windows and over a rough area of 500 km ± 500 km). Sub-seasonal anomalies are, however, significant relative to estimates of global average ocean uptake of 1.7±10-2 g C m-2 day-1 and of the same order of magnitude as the mean regional fluxes estimated from climatology (which range from 0 to 0.36 g C m-2 day-1) for the North Atlantic. Patterns are distinctive with strong spatial and temporal correlations -- suggesting that knowledge of time-dependent atmosphere and ocean processes could provide additional information on the expected variability between sequences of retrievals over the ocean.
The objective of the NASA Carbon Monitoring System (CMS) Surface Carbon Flux Pilot Project is to use NASA observational constraints and state-of-the-art assimilating models to estimate CO2 fluxes between the land, ocean, and atmosphere. Here we describe the two ocean components of this pilot study and evaluate impact of estimated ocean fluxes on the time-evolving atmospheric CO2 concentration. The first ocean component is the NASA Ocean Biogeochemistry Model (NOBM), which is comprised of a biogeochemical processes model, coupled to the Poseidon ocean model, and driven at the surface by the Modern Era Retrospective-analysis for Research and Applications (MERRA). Ocean color data is assimilated using the Ocean-Atmosphere Spectral Irradiance Model (OASIM). The second ocean component is based on a global, eddying physical ocean solution provided by the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), which is coupled to the Massachusetts Institute of Technology ecosystem model (Darwin), and to a marine carbon chemistry model. The ECCO2 ocean solution assimilates a variety of satellite and in-situ data, including Jason altimetry, AMSRE-E sea surface temperature, and Argo temperature and salinity profiles. The two CMS ocean components generate independent air-sea CO2 flux estimates and hence provide a means to probe system uncertainties. We document the similarities and differences between the two solutions and compare them to in-situ observations and to the Takahashi air-sea CO2 flux climatology. The two ocean solutions are used to drive two atmospheric carbon chemistry models, GEOS-5 and GEOS-Chem, in order to investigate impact of air-sea CO2 flux variability on atmospheric CO2 concentrations. Preliminary results, available at the time of writing of this abstract, suggest that large-scale biases between the two ocean solutions have a large, measurable impact on atmospheric CO2 variability. Impact of high frequency and wavenumber differences between the two solutions on atmospheric CO2 variability will also be investigated.
The goal of NASA Carbon Monitoring Study (CMS) Flux Pilot Project is to incorporate the full suite of NASA observational, modeling, and assimilation capabilities to assess the role NASA satellite remote sensing can play in the attribution of changes in globally distributed CO2 concentrations to the carbon cycle and anthropogenic emissions. To that end, CMS has initiated a coordinated effort between NASA land surface, ocean, and atmospheric scientists and their collaborators to provide global estimates of CO2 constrained by satellite observations and informed by a prior contemporaneous estimates of "bottom-up" fluxes from land surface and ocean models. The ocean carbon fluxes are calculated from the MITgcm-ECCO2 physical and biogeochemical adjoint ocean state estimation system and the NASA Ocean Biochemical Model (NOBM). The terrestrial fluxes are calculated from the CASA and CASA-GFED models. We present preliminary satellite-constrained a posteriori CO2 [|#29#|]fluxes using a priori fluxes derived from the model outputs for the 2009-2010 period. We compare these fluxes to upscaled FLUXNET eddy-covariance sites and a posteriori CO2 concentrations to the Total Column Observing Network (TCCON) networks, surface measurements, and the Tropospheric Emission Spectrometer mid-tropospheric CO2.
Many components of the carbon cycle are constrained by a variety of remote sensing measurements. Observations of land surface parameters constrain estimates of carbon flux from terrestrial biosphere models while estimates of oceanic carbon fluxes are informed by satellite observations of ocean color and ocean properties. Atmospheric CO2 concentrations, which are governed by the balance of terrestrial, oceanic, and anthropogenic fluxes, are observed from space by an expanding suite of instruments (AIRS, TES, and GOSAT) in addition to being monitored by an extensive global network of surface stations. Additionally, atmospheric transport patterns simulated by NASA's GEOS-5 data analysis system are strongly influenced by observations of atmospheric state variables. NASA's Carbon Monitoring System Flux Pilot Project was created to quantify the constraints placed on carbon flux estimates by the current observing system and to assess what additional observational needs are required for future monitoring and attribution efforts. To this end, we have conducted an ensemble of GEOS-5 modeling studies using different combinations of two sets of land (NASA-CASA, CASA-GFED) and two sets of ocean (NOBM, ECCO2/Darwin) fluxes. Results from this ensemble of simulations are sampled at locations consistent with NOAA GMD and TCCON surface networks as well as locations of AIRS, TES, and GOSAT overpasses to quantify how surface flux uncertainty may be observed by different observing systems. Additionally, an ensemble of GEOS-5 simulations with alterations to subgrid-scale transport parameterizations is analyzed to compare model transport uncertainty with flux uncertainty. Our results indicate that uncertainty in both land and ocean flux estimates can introduce a large degree of variability into atmospheric CO2 distributions and that the magnitude of these differences is observable by existing satellite and in situ platforms. In contrast, transport uncertainty introduced by subgrid-scale parameterizations has a much smaller impact on CO2 mixing ratios because the assimilated observations of state variables place a strong constraint on atmospheric transport patterns.
A three dimensional model of Arctic Ocean circulation and mixing, with a horizontal resolution of 18 km, is overlain by a biogeochemical model resolving the physical, chemical and biological transport and transformations of phosphorus, alkalinity, oxygen and carbon, including the air-sea exchange of dissolved gases and the riverine delivery of dissolved organic carbon. The model qualitatively captures the observed regional and seasonal trends in surface ocean PO4, dissolved inorganic carbon, total alkalinity, and pCO2. Integrated annually, over the basin, the model suggests a net annual uptake of 59 Tg C a-1, within the range of published estimates based on the extrapolation of local observations (20-199 Tg C a-1). This flux is attributable to the cooling (increasing solubility) of waters moving into the basin, mainly from the subpolar North Atlantic. The air-sea flux is regulated seasonally and regionally by sea-ice cover, which modulates both air-sea gas transfer and the photosynthetic production of organic matter, and by the delivery of riverine dissolved organic carbon (RDOC), which drive the regional contrasts in pCO2 between Eurasian and North American coastal waters. Integrated over the basin, the delivery and remineralization of RDOC reduces the net oceanic CO2 uptake by ˜10%.
The acceleration of Pine Island Glacier in the last decade correlates significantly with an increase in ocean temperatures in the Amundsen Sea during the same period. Although studies have been carried out to try and link both phenomenons, the demonstration of a significant link between sub-cavity ice shelf melting and ice flow acceleration remains an open question. Here, we present a new coupled ocean circulation/ice flow model, based on the MITgcm and ISSM models, that includes significant offline coupling between the sub-ice shelf cavity ocean circulation and the glacier ice flow. Computed melting rates are used to constrain ice flow, which in turn is used to constrain geometry of the sub-ice shelf cavity. The model is applied to the Amundsen Sea/Pine Island Glacier and Thwaites Glacier, to try and assess the sensitivity of ice flow acceleration to a scenario of increased melting under the ice shelf. The results show significant ice flow acceleration on a short term basis (10 to 100 years), as well as modification of the ocean circulation under the ice shelf, in response to a changing sub-ice shelf cavity geometry. These results demonstrate that there are significant links between changing ocean circulation patterns in the Amundsen Sea and sudden ice flow acceleration of Pine Island Glacier in the last decade. This work was performed at the California Institute of Technology's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration's Cryosphere Science Program.
We examine the spatial trends in Arctic sea ice drift speed from satellite data and the role of wind forcing for the winter months of October through May. Between 1992 and 2009, the spatially averaged trend in drift speed within the Arctic Basin is 10.6% ± 0.9%/decade, and ranges between -4% and 16%/decade depending on the location. The mean trend is dominated by the second half of the period. In fact, for the five years after a clear break point in March 2004, the average trend increased to 46% ± 5%/decade. Over the 1992-2009 period, averaged trends of wind speed from four atmospheric reanalyses are only 1% to 2%/decade. Regionally, positive trends in wind speed (of up to 9%/decade) are seen over a large fraction of the Central Arctic, where the trends in drift speeds are highest. Spatial correlations between the basin-wide trends in wind and drift speeds are moderate (between 0.40 and 0.52). Our results suggest that changes in wind speed explain a fraction of the observed increase in drift speeds in the Central Arctic but not over the entire basin. In other regions thinning of the ice cover is a more likely cause of the increase in ice drift speed.
North Pacific Subtropical Mode Water (NPSTMW) is an essential feature of the North Pacific subtropical gyre imparting significant influence on regional SST evolution on seasonal and longer time scales and, as such, is an important component of basin-scale North Pacific climate variability. This study examines the seasonal-to-interannual variability of NPSTMW, the physical processes responsible for this variability, and the connections between NPSTMW and basin-scale climate signals using an eddy-permitting 1979-2006 ocean simulation made available by the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2). The monthly mean seasonal cycle of NPSTMW in the simulation exhibits three distinct phases: (i) formation during November March, (ii) isolation during March June, and (iii) dissipation during June November each corresponding to significant changes in upper-ocean structure. An interannual signal is also evident in NPSTMW volume and other characteristic properties with volume minima occurring in 1979, 1988, and 1999. This volume variability is correlated with the Pacific decadal oscillation (PDO) with zero time lag. Further analyses demonstrate the connection of NPSTMW to the basin-scale ocean circulation. With this, modulations of upper-ocean structure driven by the varying strength and position of the westerlies as well as the regional air sea heat flux pattern are seen to contribute to the variability of NPSTMW volume on interannual time scales.
We present an optimized 1992–2008 coupled ice-ocean simulation of the Arctic Ocean. A Green's function approach adjusts a set of parameters for best model-data agreement. Overall, model-data differences are reduced by 45%. The optimized simulation reproduces the negative trends in ice extent in the satellite records. Volume and thickness distributions are comparable to those from the Ice, Cloud, and land Elevation Satellite (2003–2008). The upper cold halocline is consistent with observations in the western Arctic. The freshwater budget of the Arctic Ocean and volume/heat transports of Pacific and Atlantic waters across major passages are comparable with observation-based estimates. We note that the optimized parameters depend on the selected atmospheric forcing. The use of the 25 year Japanese reanalysis results in sea ice albedos that are consistent with field observations. Simulated Pacific Water enters the Bering Strait and flows off the Chukchi Shelf along four distinct channels. This water takes ∼5–10 years to exit the Arctic Ocean at the Canadian Arctic Archipelago, Nares, or Fram straits. Atlantic Water entering the Fram Strait flows eastward, merges with the St Ana Trough inflow, and splits into two branches at the southwest corner of the Makarov Basin. One branch flows along Lomonosov Ridge back to Fram Strait. The other enters the western Arctic, circulates cyclonically below the halocline, and exits mainly through the Nares and Fram straits. This work utilizes the record of available observations to obtain an Arctic Ocean simulation that is in agreement with observations both within and beyond the optimization period and that can be used for tracer and process studies.
This presentation describes a global ocean state estimation system and explores the role that sustained basin?scale acoustic thermometry can play in evaluating and constraining the resulting ocean state estimates. The state estimation system is that developed by the ECCO2 project. Solutions are based on an eddying, full?depth ocean, and sea ice configuration of the Massachusetts Institute of Technology general circulation model. Data constraints include altimetry, hydrography, sea surface temperature, and satellite observations of sea ice extent, thickness, and velocity. Green?s functions and the adjoint method are used to adjust initial and surface boundary conditions and empirical parameters such as vertical diffusivity, albedos, and drag coefficients, in order to reduce model/data discrepancies. Early prototype basin?scale estimation systems that used acoustic data were deployed in the Eastern Mediterranean for THETIS?2 and in the North Pacific for ATOC. More recently, 1 decade of North Pacific acoustic thermometry data was compared with ocean simulation and estimation results. The comparisons with acoustic data provide stringent tests of the time mean hydrography and of the large?scale temperature variability in the models. The differences are sometimes substantial, indicating that acoustic thermometry data can provide significant additional constraints for numerical ocean models. Of particular interest is the deployment of a basin?scale acoustic array for monitoring changes in the deep ocean interior.
This paper describes the sea ice component of the Massachusetts Institute of Technology general circulation model (MITgcm); it presents example Arctic and Antarctic results from a realistic, eddy-admitting, global ocean and sea ice configuration; and it compares B-grid and C-grid dynamic solvers and other numerical details of the parameterized dynamics and thermodynamics in a regional Arctic configuration. Ice mechanics follow a viscous-plastic rheology and the ice momentum equations are solved numerically using either line-successive-over-relaxation (LSOR) or elastic–viscous-plastic (EVP) dynamic models. Ice thermodynamics are represented using either a zero-heat-capacity formulation or a two-layer formulation that conserves enthalpy. The model includes prognostic variables for snow thickness and for sea ice salinity. The above sea ice model components were borrowed from current generation climate models but they were reformulated on an Arakawa C grid in order to match the MITgcm oceanic grid and they were modified in many ways to permit efficient and accurate automatic differentiation. Both stress tensor divergence and advective terms are discretized with the finite-volume method. The choice of the dynamic solver has a considerable effect on the solution; this effect can be larger than, for example, the choice of lateral boundary conditions, of ice rheology, and of ice-ocean stress coupling. The solutions obtained with different dynamic solvers typically differ by a few cm s−1 in ice drift speeds, 50 cm in ice thickness, and order 200 km3 yr−1 in freshwater (ice and snow) export out of the Arctic.
The importance of the Antarctic Circumpolar Current (ACC), and the Southern Ocean, in the Climate System, has become apparent in the last decades. Water masses formed at high latitudes are linked to the shallow and deep branches of the meridional overturning circulation. Antarctic Intermediate Water (AAIW), as the main water mass ventilating the base of the worlds ocean thermocline, play a central role not only in the climate system dynamics, but also in the global cycles of carbon dioxide and other bio-geochemical tracers. We used a set of sensitivity experiments of the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project to study water mass formation. The global high-resolution data syntheses are obtained by an orthogonal projection of the MITgcm onto all available satellite and in-site data. Sensitivity experiments include various surface boundary conditions (ERA40, JRA25), which in turn are important for the air-sea interaction in the area of water mass formation. These sensitivity studies are used to investigate the mechanisms of water mass formation such as AAIW formation and its relationship with ACC frontal locations, as well as to determine the sensitivity of the model solution to available atmospheric forcing products. Deep mixed layers formed as a result of deep convection in winter are the nursery grounds for the Subantarctic Mode Water (SAMW), the lighter precursor of the AAIW. Diapycnal transformation of AAIW, due to internal mixing and air-sea interactions, is found to occur mainly in the South Pacific Ocean (ERA40). Significant AAIW transformation is also observed in the South Indian Ocean. While the heaviest AAIW (27.4-27.8 kg.m-3) is formed in the South Indian Ocean, reaching a peak production of about 40Sv during late July, the lighter version of the AAIW (27-27.4 kg.m-3) is mainly produced in the South Pacific Ocean.
Recent airborne campaigns under NASA's Operation Icebridge have provided a significant increase in the availability of new observations of glaciers and ice sheets. Here, we focus on new ice thickness data and how these data help us constrain glacier ice mass fluxes; and new bathymetry data used to better constrain freshwater fluxes resulting from ice-ocean interactions at the underside of floating ice shelves. Icebridge ice thickness acquired in Greenland provide new thickness gates to estimate ice discharge and complete the circumnavigation of the island; yet, significant gaps remain in places not covered by radio echo sounding or where radio echo sounding is challenged by difficult environmental conditions. In the Antarctic, Icebridge collected ice thickness data along the Bellingshausen Sea sector where hardly any data had been collected in the past, ice thickness was inferred solely based on ice surface elevation, grounding line position and assumptions of hydrostatic equilibrium, and prior mass balance results indicated a large imbalance which was not entirely consistent with GRACE data and laser altimetry data. We are now resolving these differences with the new data. Finally, underneath ice shelves, new bathymetry data of Pine Island Glacier and Larsen C ice shelf derived from airborne gravity combined with other dat sets have had a large impact on our general knowledge and understanding of sub-ice-shelf cavities and the associated ice-shelf/ocean interactions. The new data reveal unknown seafloor ridges, seabed troughs, sills and over-deepenings that affect the pattern of sub-ice-shelf ocean circulation, the access of ocean heat to sub-ice-shelf cavities and glacier grounding lines, and rates of submarine melting. We demonstrate this by comparing estimates of submarine melting obtained with old and new bathymetry in the regional MITgcm ocean model configuration for Larsen C and Pine Island Ice Shelves, in Antarctica. In addition, we compare the ECCO2 estimates of submarine melting with independent estimates of submarine melting calculated from remote sensing data (ice velocity, thickness and surface mass balance). The first ECCO2 results, in progress, will be presented at the meeting.
The traditional view on the mass balance of the Greenland Ice Sheet is that interior snowfall accumulation is balanced by discharge of surface runoff and icebergs at the periphery. Most Greenland glaciers however terminate in the ocean, and melt in contact with the warm ocean waters to produce glacial melt before detaching into icebergs. Underneath floating ice shelves, the melting process is governed by the buoyancy associated with the melting of glacier ice at the seawater-ice interface. Under tidewater glaciers, the melting process is also forced by the strongly buoyant influx of subglacial freshwater near the grounding line. In August 2008, we collected bathymetry, temperature, salinity and current velocity data in front of 4 west Greenland glaciers (Eqip Sermia, Kangilerngata Sermia, Sermeq Kujatdleq and Sermeq Avangnardleq) to calculate the rates of submarine melting of the calving faces. The results revealed large rates of melting (meters per day), and large spatial variations from fjord to fjord as well as across the calving faces. In August 2010, we returned to Eqip Sermia, Sermeq Avangnardleq and visited Store and Little glaciers to conduct similar measurements. Strong outflows of subglacial water were detected on Avangnardleq, Lille and Store glaciers, and high rates of submarine melting were deduced from the data. We find that the sea bed in front of the calving faces (100 to 500 m) are much shallower than in the bulk of the glacial fjords (800 to 900 m), and the sill depth at the fjord entrance (~300 m ) is confirmed to be the major control on the access of warm ocean waters to the submerged calving faces. In the presence of heavy brash ice, our data suggest a conceivably weakened submarine circulation. Finally, we combine our summer data with long-term records of temperature and salinity, at the depth relevant to submarine melting, from the ECCO2 ocean state estimation project to examine seasonal to long-term trends in thermal forcing from the ocean. We observe a strong seasonality and large inter-annual variations in glacial fjords of interest. This enables a quantification of thermal forcing of the ocean on the calving faces of Greenland, its potential impact on submarine melting, which in turn effects glacier un-grounding, glacier velocity, glacier mass balance, and ultimately ice sheet mass balance as a whole.
The acceleration of Greenland tidewater glaciers has increased the mass loss from the Greenland Ice Sheet. Submarine melting is one of the possible drivers for glacier acceleration. Enhanced submarine melting could result from ocean warming, changes in ocean current, and increase in sub-glacial runoff. We use a combination of numerical modeling and field data to understand the mechanism of submarine melting in Greenland. Specifically, oceanographic data (temperature, salinity, and current velocity) were collected in August 2008 and 2010 near the calving fronts of the Lille Gletscher, Store Gletscher, Eqip Sermia, Kangilerngata Sermia, Sermeq Kujatdleq and Sermeq Avangnardleq glaciers in central West Greenland. These data are compared to high-resolution regional ocean simulations carried out using the Massachusetts Institute of Technology general circulation model (MITgcm). MITgcm includes submarine melting at the base of an ice shelf and we have added a new module to simulate the melting process along the vertical calving face of Greenland tidewater glaciers. We integrate the MITgcm with JRA25 atmospheric and ECCO2 oceanic boundary conditions and compare the simulation results with the West Greenland data. We also conduct model sensitivity studies for ocean temperature, sub-glacial runoff, and fjord. The preliminary results show a quadratic increase in submarine melting with warmer ocean temperature and a role of sub-glacial runoff in changing ocean circulation. This study could help us evaluate the impact of ocean warming and enhanced runoff on submarine melting and in turn on glacier mass balance. This work is performed at UCI under a contact with NASA Cryosphere Science Program.
Sea ice movement is driven by surface wind and ocean currents. The spatial inhomogeneity of these forces causes internal sea ice stress gradients, which eventually cause ice to ridge or break up. This sea ice deformation is an important process for (1) the sea ice mass balance, (2) brine rejection into the ocean, (3) regulation of ocean-to-air heat and gas fluxes, and (4) altering the air and water drag coefficients and transfer of momentum at the ice ocean interface.. Sea ice deformation occurs across a broad range of spatial scales. Most noticeable are linear kinematic features (LKFs) that have lengths of hundreds to thousands of kilometers and a typical lifetime of days to weeks. In addition, as inferred from data, sea ice deformation a) has a spatial distribution with higher deformation rates in the seasonal ice zone than for perennial sea ice, and b) does not scale linearly with the length scale over which it is integrated but follows a power law. Consecutive observations provided by RADARSAT Synthetic Aperture Radar (SAR) are used to derive sea ice velocity fields by a maximum-cross-correlation method. From these velocity fields the fields of divergence, shear and vorticity are obtained. These datasets are products of the RADARSAT Geophysical Processor System (RGPS). These RGPS sea ice deformation fields are compared to solutions of a coupled sea ice-ocean model. Arctic sea ice-ocean simulations from the MIT general circulation model (MITgcm) with 4.5, 9, and 18 km horizontal grid spacing were carried. The model setup uses a viscous-plastic sea ice rheology with an elliptical yield curve. Such models can reproduce some aspects of sea ice drift but it remains unclear whether the model physics are suitable to reproduce the observed sea ice deformation features. First comparisons with satellite remote sensing data reveal big differences in the shape, frequency of occurrence, and spatial distribution of LKFs. In this study three main questions are addressed: (1) How does the spatial distribution and shape of LKFs depend on model resolution? (2) Is the observed power law scaling of sea ice deformation also present in the modeled deformation fields? and (3) Can the model parametrization be improved to better represent the observed large-scale sea ice deformation? In general all three model solutions produce less LKFs compared to RGPS observations and the overall modeled deformation rate is lower than the observed one. However, an increase in model resolution produces more and clearer confined ice deformation features. The observed power law scaling of sea ice deformation is reproduced in the model. Noticeable is that the power law scaling exponent is not constant but heavily depends on sea ice concentration, thickness and time of year. Additional model runs with a changed sea ice strength formulation were performed. If the sea ice strength formulation is changed from a linear to a cubic dependence on ice thickness, the difference between modeled and observed deformation fields is reduced.
Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 23 (2009): GB4006, doi:10.1029/2008GB003396. The spatial distribution and fate of riverine dissolved organic carbon (DOC) in the Arctic may be significant for the regional carbon cycle but are difficult to fully characterize using the sparse observations alone. Numerical models of the circulation and biogeochemical cycles of the region can help to interpret and extrapolate the data and may ultimately be applied in global change sensitivity studies. Here we develop and explore a regional, three-dimensional model of the Arctic Ocean in which, for the first time, we explicitly represent the sources of riverine DOC with seasonal discharge based on climatological field estimates. Through a suite of numerical experiments, we explore the distribution of DOC-like tracers with realistic riverine sources and a simple linear decay to represent remineralization through microbial degradation. The model reproduces the slope of the DOC-salinity relationship observed in the eastern and western Arctic basins when the DOC tracer lifetime is about 10 years, consistent with published inferences from field data. The new empirical parameterization of riverine DOC and the regional circulation and biogeochemical model provide new tools for application in both regional and global change studies. I.M.M. and M.J.F. are grateful to National Science Foundation for financial support.
The Arctic sea ice in many respects is an important component of the Earth's climate system, e.g., sea ice governs the ocean to atmosphere heat flux, freezing and melting influences the upper ocean salinity and density, and sea ice dynamics act as a latent energy transport. During recent years substantial changes of the Arctic sea ice cover have been observed. Many of these aspects of sea ice and its recent changes can be reproduced by coupled sea ice-ocean models. In part this can be attributed to the fact that model parameters are tuned to produce observed ice concentration (extent) and drift distributions. Detailed comparisons between satellite remote sensing data with model results, however, reveal big differences in certain aspects of the sea ice cover, e.g., for fracture zones and for small scale dynamic processes. It remains unclear whether the model physics are suited to reproduce these observed sea ice features. Accurate modeling of leads and fracture zones is important for realistic (1) new ice production estimates, (2) ocean to air heat flux, and (3) brine rejection into the ocean. In this study we use satellite remote sensing data to compare with and to improve results of the MIT general circulation model (MITgcm) as used in the framework of the ECCO2 project (http://ecco2.org). Model integrations in an Arctic domain at horizontal grid spacing of 18, 9, and 4.5 km using two different atmospheric forcing datasets (ERA40 and JRA-25) were carried out. Sea ice motion, deformation, and estimates of ice production are obtained from Synthetic Aperture Radar (SAR) using the RADARSAT Geophysical Processor System (RGPS). Even though the viscous-plastic dynamic sea ice model with elliptical yield curve is able to produce what appears to be linear kinematic features (LKFs), the orientation and spatial density are far from that which is observed. In addition the LKFs occur less frequently in the simulations. Figure 1 shows an example of the fractional number each grid cell was deformed (divergence > 0.02/day or shear > 0.03/day) during a two-month period (Nov./Dec. 1999). While the RGPS data shows a clear discrimination between the thinner seasonal sea ice with more deformation (left and upper part of Figure 1a) and the perennial sea ice, the MITgcm deformation zones are mainly confined to a few LKFs and to the marginal ice zones. Higher model resolutions result in more small scale deformation features but do not change the general deformation distribution. These comparisons are used to address model uncertainties. Figure 1: Fractional number of times (in percent) a grid cell was active (magnitude divergence > 0.02/day or shear > 0.03/day) between Nov.-Dec. 1999.
The halocline in the Arctic Ocean plays an important role in regulating heat exchange at the bottom of the mixed layer and it has a direct effect on the ocean sea ice energy balance and sea ice mass balance. Modeling the halocline, however, remains a challenge in current state-of-the-art coupled ocean sea ice models including those that participated in the Arctic Ocean Model Intercomparison Project. In this study, we successfully reproduce a cold halocline in the Canada Basin by implementing a subgrid-scale brine rejection parameterization in an ocean general circulation model. The brine rejection scheme improves the solution by redistributing surface salts rejected during sea ice formation to their neutral buoyancy depths. The depths are based on salt plume physics and published laboratory and numerical experiments. Compared with hydrographic data from 1993 to 2004, distribution of most of the rejected salt to the bottom of the mixed layer seems to yield the lowest model-data misfits. We also show that the model's mixed layer depth is sensitive to the background diffusivity ν used in the k-profile parameterization vertical mixing scheme. A background diffusivity of 10−6 m2/s in combination with brine rejection scheme described herein yield the best simulation of the Arctic halocline.
The authors investigate the response of the Arctic Ocean freshwater budget to changes in the North Atlantic Oscillation (NAO) using a regional-ocean configuration of the Massachusetts Institute of Technology GCM (MITgcm) and carry out several different 10-yr and 30-yr integrations. At 1/6° (~18 km) resolution the model resolves the major Arctic transport pathways, including Bering Strait and the Canadian Archipelago. Two main calculations are performed by repeating the wind fields of two contrasting NAO years in each run for the extreme negative and positive NAO phases of 1969 and 1989, respectively. These calculations are compared both with a control run and the compiled observationally based freshwater budget estimate of Serreze et al. The results show a clear response in the Arctic freshwater budget to NAO forcing, that is, repeat NAO negative wind forcing results in virtually all freshwater being retained in the Arctic, with the bulk of the freshwater content being pooled in the Beaufort gyre. In contrast, repeat NAO positive forcing accelerates the export of freshwater out of the Arctic to the North Atlantic, primarily via Fram Strait (~900 km[superscript 3] yr[superscript −1]) and the Canadian Archipelago (∼500 km[superscript 3] yr[superscript −1]), with a total loss in freshwater storage of ~13 000 km[superscript 3] (15%) after 10 yr. The large increase in freshwater export through the Canadian Archipelago highlights the important role that this gateway plays in redistributing the freshwater of the Arctic to subpolar seas, by providing a direct pathway from the Arctic basin to the Labrador Sea, Gulf Stream system, and Atlantic Ocean. The authors discuss the sensitivity of the Arctic Ocean to long-term fixed extreme NAO states and show that the freshwater content of the Arctic is able to be restored to initial values from a depleted freshwater state after ~20 yr.
As part of an ongoing effort to obtain a best possible, time-evolving analysis of most available ocean and sea ice data, a dynamic and thermodynamic sea-ice model has been coupled to the Massachusetts Institute of Technology general circulation model (MITgcm). Ice mechanics follow a viscous-plastic rheology and the ice momentum equations are solved numerically using either line-successive-over-relaxation (LSOR) or elastic-viscous-plastic (EVP) dynamic models. Ice thermodynamics are represented using either a zero-heat-capacity formulation or a two-layer formulation that conserves enthalpy. The model includes prognostic variables for snow and for sea-ice salinity. The above sea ice model components were borrowed from current-generation climate models but they were reformulated on an Arakawa C grid in order to match the MITgcm oceanic grid and they were modified in many ways to permit efficient and accurate automatic differentiation. This paper describes the MITgcm sea ice model; it presents example Arctic and Antarctic results from a realistic, eddy-permitting, global ocean and sea-ice configuration; it compares B-grid and C-grid dynamic solvers and the effects of other numerical details of the parameterized dynamics and thermodynamics in a regional Arctic configuration; and it presents example results from coupled ocean and sea-ice adjoint-model integrations.
High-resolution retrievals of recent changes in the Earth's gravity field using GRACE (Gravity Recovery and Climate Experiment) offer insights into Earth system processes encompassing the atmosphere, ocean, land water hydrology, and land ice. With a focus in particular on the Pan-Arctic region, where water/ice mass fluxes and sea ice cover are rapidly changing, we utilize this space geodetic data set along with estimates of the atmospheric precipitation and evaporation fluxes from re-analysis data sets (e.g., NCEP and JRA-25) to determine variations and trends in freshwater river discharge from the entire Pan-Arctic river basins into the Arctic Ocean from 2003 to 2008. As GRACE has now completed more than 6 years of time-variable gravity measurements, this analysis yields new quantitative insights into annual to inter-annual dynamics of the Pan-Arctic hydrologic mass-balance, including regions of ungauged freshwater drainage. Since freshwater river discharge into the Arctic Ocean is a major source of Arctic Ocean freshwater, we will also discuss the relative importance of these freshwater fluxes on climate and circulation changes in the Arctic Ocean by using numerical simulations with an ocean model (ECCO-2).
Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 23 (2009): GB1005, doi:10.1029/2008GB003349. We synthesize estimates of the contemporary net air-sea CO2 flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO2 (pCO2) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO2 for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a−1. This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in thehigh latitudes. Both estimates point toward a small (∼ −0.3 Pg C a−1) contemporary CO2 sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO2 and a strong uptake of anthropogenic CO2. A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO2-based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO2 is estimated to be −1.7 ± 0.4 Pg C a−1 (inversion) and −1.4 ± 0.7 Pg C a−1 (pCO2-climatology), respectively, consisting of an outgassing flux of river-derived carbon of ∼+0.5 Pg C a−1, and an uptake flux of anthropogenic carbon of −2.2 ± 0.3 Pg C a−1 (inversion) and −1.9 ± 0.7 Pg C a−1 (pCO2-climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between −0.2 and −0.3 Pg C a−1 across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO2 fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink. Core financial support for this study came from the National Aeronautics and Space Administration under grant NAG5-12528 to NG and SMF, with additional support by the U.S. National Science Foundation. M. Gloor was supported by the EBI nd EEE institutes at the University of Leeds. M. Gerber, SM, FJ, and AM thank the European Commission for support through CarboOcean (511176-2) and the NOCES project (EVK2-CT-2001- 00134). TT has been supported by NOAA grant NAO30AR4320179P27.
Analyzing ocean variability, understanding its importance for the climate system, and quantifying its socio-economic impacts are among the primary motivations for obtaining ongoing global ocean observations. There are several possible approaches to address these tasks. One with much potential for future ocean information services and for climate predictions is called ocean synthesis, and is concerned with merging all available ocean observations with the dynamics embedded in an ocean circulation model to obtain estimates of the changing ocean that are more accurate than either system alone can provide. The field of ocean synthesis has matured over the last decade. Several global ocean syntheses exist today and can be used to investigate key scientific questions, such as changes in sea level, heat content, or transports. This CWP summarizes climate variability as “seen” by several ocean syntheses, describes similarities and differences in these solutions and uses results to highlight developments necessary over the next decade to improve ocean products and services. It appears that multi-model ensemble approaches can be useful to obtain better estimates of the ocean. To make full use of such a system, though, one needs detailed error information not only about data and models, but also about the estimated states. Results show that estimates tend to cluster around methodologies and therefore are not necessarily independent from each other. Results also reveal the impact of a historically under-sampled ocean on estimates of inter-decadal variability in the ocean. To improve future estimates, we need not only to sustain the existing observing system but to extend it to include full-depth ARGO-type measurements, enhanced information about boundary currents and transports through key regions, and to keep all important satellite sensors flying indefinitely, including altimetry, gravimetry and ice thickness, microwave SST observations, wind stress measurements and ocean color. We also need to maintain ocean state estimation as an integral part of the ocean observing and information system. Published Venice, Italy 3.7. Dinamica del clima e dell'oceano
Oceanic mixed layer heat budgets are crucial for climate modeling efforts because they govern the evolution of sea surface temperatures (SST), the most important oceanic parameter driving atmospheric circulation. Mixed layer heat budget variability is controlled by surface heat fluxes, entrainment, advection, and diffusion. The respective role of these processes varies as a function of region, spatial scale, and frequency. We present results from an analysis of mixed layer heat budgets in a 1992-2007 ECCO2 ocean state estimate. We quantify the various contributions to changes in mixed layer temperature and their associated errors for three oceanic regions in the North Pacific Ocean: the subtropical gyre, an upwelling region off the US West Coast, and a dynamically active area in the Kuroshio region. This regionalization allows for a simplification of the heat budget and the identification of various regimes and frequency/wavenumber bands that are dominated by one or two of the heat budget terms. An integral part of this investigation is the mixed layer heat budget error analysis that allows us to work towards improving the representation of mixed layer processes in ocean models.
It is estimated that riverine sources of terriginous DOC may represent a source of carbon to the Arctic Ocean as large as ca 30 Tg C a-1; a significant flux in the regional budget. We use a numerical model to explore the role of Pan-Arctic riverine fluxes of dissolved organic carbon (DOC) in contributing to air-sea fluxes of carbon dioxide in the Arctic Ocean. The model is based on the Arctic sector of the eddy-permitting ECCO2 configuration of the MIT ocean model, forced by time-varying NCEP re-analysis products with an explicit representation of fresh water run-off in the Arctic region. Open boundary conditions are taken from the ECCO2 global ocean state estimates. The biogeochemical model explicitly represents the cycling of dissolved inorganic carbon, alkalinity, dissolved oxygen, phosphate and dissolved organic matter of marine origin, and riverine DOC. We parametrize riverine DOC fluxes to the Arctic Ocean using a combination of freshwater river runoff and DOC data collected at the mouth of main Arctic rivers. The DOC tracer is given a half-life of 10 years based on estimates from field data and supported by a previous, idealized modeling study. We will discuss the role of terriginous DOC sources in regional air-sea carbon fluxes through a comparison of model integrations, which include and exclude this source.
An investigation using a combined numerical modeling and theoretical approach is followed to better resolve the role of Subtropical Mode Water (STMW) in the exchange of information between the atmosphere and the ocean linked to climate variability in the North Pacific Ocean. In this, a high resolution MIT General Circulation Model (MITgcm) simulation is analyzed to study the formation, isolation and dispersal of STMW and identify correlations between STMW variations and established climatic signals in the Pacific basin. During a 171- month time period (from January 1992 to March 2006), the seasonal variability is the dominant temporal variation observed. From climatological model fields, STMW exhibits distinct features in time and space. In addition to seasonality there is also an interannual signal observed in STMW variability. This interannual variation pattern is connected closely to the climate shifts of North Pacific, with further investigation showing that there is a high correlation between the STMW variability and the Pacific Decadal Oscillation index. To identify the mechanisms responsible for this interannual STMW variability, classical ocean thermocline theories are reviewed and STMW connections to large scale ocean circulation patterns are explored. A planetary geostrophic ocean model (PGOM) is employed as a theoretical platform for this purpose. Specifically, numerical PGOM experiments are performed to isolate and examine in further detail, the influence of variations of the large scale wind stress pattern and large scale air-sea heat flux on STMW variability. It may be gathered from these experiments that variability in this large scale wind stress is seen to affect the variability pattern of model STMW. Yet, results also indicate that the amplitude of seasonal and interannual variability of STMW volume is primarily dominated by the variability in the air-sea heat flux.
1] Sea ice drift and deformation from coupled ice-ocean models are compared with high-resolution ice motion from the RADARSAT Geophysical Processor System (RGPS). In contrast to buoy drift, the density and extent of the RGPS coverage allows a more extensive assessment and understanding of model simulations at spatial scales from \$10 km to near basin scales and from days to seasonal timescales. This work illustrates the strengths of the RGPS data set as a basis for examining model ice drift and its gradients. As it is not our intent to assess relative performance, we have selected four models with a range of attributes and grid resolution. Model fields are examined in terms of ice drift, export, deformation, deformation-related ice production, and spatial deformation patterns. Even though the models are capable of reproducing large-scale drift patterns, variability among model behavior is high. When compared to the RGPS kinematics, the characteristics shared by the models are (1) ice drift along coastal Alaska and Siberia is slower, (2) the skill in explaining the time series of regional divergence of the ice cover is poor, and (3) the deformation-related volume production is consistently lower. Attribution of some of these features to specific causes is beyond our current scope because of the complex interplay between model processes, parameters, and forcing. The present work suggests that high-resolution ice drift observations, like those from the RGPS, would be essential for future model developments, improvements, intercomparisons, and especially for evaluation of the small-scale behavior of models with finer grid spacing.

## Citations (3,766)

... Even at the time, it was known that shots as weak as 4-lb TNT could be detected at distances of 10,000 km or more [Ewing and Worzel, 1948; Bryan et al., 1963]. During the Acoustic Thermometry of Ocean Climate (ATOC) program [The ATOC Consortium, 1998] 20-min coded acoustic signals from a broad-band, 250-W acoustic source located off California were detected at New Zealand (10,000 km range) by a single hydrophone (M. Dzieciuch, personal communication , 2008). ...
... At the same time, local wind forcing in the Strait of Gibraltar has a far reaching impact in terms of spatial and time scales. Garcia-Lafuente et al. (2002) estimated that the wind forcing contribution to exchange flows through the Strait of Gibraltar is of 0.3 Sv, and Fukumori et al. (2007) suggested that altimetry observed basin-wide sea-level interannual variability in the Mediterranean Sea was linked to winds in the Strait of Gibraltar area. The driving mechanism, according to Menemenlis et al. (2007), is the Atlantic Ocean to Mediterranean Sea sea-level difference reaction to the along-strait wind set up. ...
... Air-sea exchange of CO 2 injects relatively old carbon from the surface ocean into the atmo- sphere (see overviews in Dutta 2016; Turnbull et al. 2016), an effect that is strongest in the Southern Ocean surrounding Antarctica ( Anderson et al. 2009;Franke et al. 2008;Rodgers et al. 2011). Here, strong winds induce upwelling of "old" deep water to the surface and into the atmosphere, to be subsequently mixed northward throughout the Southern Hemisphere ( Krakauer et al. 2006;Rodgers et al. 2011). On a global scale, predominantly zonal winds in both hemispheres result in east-west zonal bands of different atmospheric 14 C concentrations. ...
... Open boundary conditions ( Marchesiello et al., 2001) were imposed using monthly averaged data from the Estimating the Circulation and Climate of the Ocean project (ECCO; Heimbach et al., 2006), a Chapman (1985) condition for the barotropic variables, and clamped conditions (Dirichlet boundary conditions) for the baroclinic variables. In the future, improvement of the transport across and along the open boundaries could be expected with the use of relaxation to the open boundary forcing fields in their vicinity (Lermusiaux, 2007). ...
... Navy SOSUS receivers and autonomous vertical line arrays were used as receivers. The intent was to determine large-scale ocean tem- perature changes by measuring changes in travel times (dividing known distances between sources and receivers by travel time gives sound speed, which is pro- portional to temperature changes) (ATOC Consortium 1998;Dushaw et al. 2009). ...
... Since the advent of the industrial revolution, humankind has inadvertently relocated a significant volume of carbon to the troposphere, where it now resides as a greenhouse gas, warming the earth via radiative forcing (IPCC, 2013). Global warming, however, is not the sole consequence of surplus atmospheric CO 2 : the surface ocean has absorbed approximately 30% of anthropogenic CO 2 emissions (Mikaloff Fletcher et al., 2006;Le Quéré et al., 2010), contributing to ocean acidification (Caldeira & Wickett, 2003). While this absorption is an important sink, abating the greenhouse effect (IPCC, 2013), it has consequences for marine ecosystems. ...
... The dispersive properties of a wave which propagates partly in the water column and partly by causing the ice sheet to flex elastically were first derived by Greenhill (1887) and experimentally confirmed by Ewing and Crary (1934). The slow decay of such waves in ice was ascribed to creep losses incurred in ice flexure, a thickness-dependent process which offers a method of deriving mean ice thickness by measuring attenuation along the wave propagation path (Menemenlis et al., 1995; Wadhams, 1973; Wadhams, 1986). It was later suggested that the ice cover 'selects' a preferred " resonant " frequency, at which a wave travels at the same speed in the ice as in the water, and that observations of this resonant wave might be used to infer path-integrated ice thickness without reference to mechanical parameters (Johannessen et al., 2004; Nagurny et al., 1999; Nagurny et al., 1994), although no mechanism for such a selection process was proposed. ...
... First, the binning reduces the overall number of observations and, secondly, it reduces noise in the data. The thinning of the input profiles permits the application of the classical optimal interpolation method without the use of a fast multiscale optimal interpolation algorithm proposed by Menemelis et al. (1997). ...
... By combining the data SSH, SST and Chla in determining the potential fishing zones will generate information that has more value. By applying the best fit interpolation method, attempted to produce a spatial resolution of the data in more detail so that SSH can be used or integrated with remote sensing data ( Sandwell, 1987;Fieguth et al., 1998;Wilkin et al., 2002;Kuragano And Kamachi, 2003). The objective of this study is to determine optimum interpolation method for changing the size of the spatial resolution of satellite altimetry data is to be used or integrated in remote sensing data in the determination of potential fishing ground. ...
... The influences of small-scale variability on long-range acoustic propagation were perhaps first highlighted with the observations of the SLICE'89 experiment at 1-Mm range in the central North Pacific ( Duda et al., 1992). Shadow-zone arrivals, which are stable, ray-like arrivals that occur at times and depths outside the arrival pattern predicted using a smooth sound speed section ( Dushaw et al., 1999;Dushaw et al., 2009;Van Uffelen et al., 2010;Dzieciuch et al., 2013), are obvious examples of the influence of small-scale variability. How can such data be exploited for ocean estimation by an inverse when they cannot be represented by a conventional forward problem? ...

## Top co-authors (50)

• California Institute of Technology
• Massachusetts Institute of Technology
• University of California, San Diego
• Massachusetts Institute of Technology
• California Institute of Technology
• University of California, Irvine

## Affiliations

Department
• Jet Propulsion Laboratory
Department
• Department of Atmospheric and Oceanic Sciences (AOS)
Department
• Department of Systems Design Engineering
Department
• Department of Earth Atmospheric and Planetary Sciences
Department
• Applied Physics Laboratory

Citations
3,766