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The offshore transport of Greenland coastal waters influenced by freshwater input from ice sheet melting during summer plays an important role in ocean circulation and biological processes in the Labrador Sea. Many previous studies over the last decade have investigated shelfbreak transport processes in the region, primarily using ocean model simulations. Here, we use 27 years of surface geostrophic velocity observations from satellite altimetry, modified to include Ekman dynamics based on atmospheric reanalysis, and virtual particle releases to investigate seasonal and interannual variability in transport of coastal water in the Labrador Sea. Two sets of tracking experiments were pursued, one using geostrophic velocities only, and another using total velocities including the wind effect. Our analysis revealed substantial seasonal variability, even when only geostrophic velocities were considered. Water from coastal southwest Greenland is generally transported northward into Baffin Bay, although westward transport off the west Greenland shelf increases in fall and winter due to winds. Westward offshore transport is increased for water from southeast Greenland so that, in some years, water originating near the east Greenland coast during summer can be transported into the central Labrador Sea and the convection region. When wind forcing is considered, long-term trends suggest decreasing transport of Greenland coastal water during the melting season toward Baffin Bay, and increasing transport into the interior of the Labrador Sea for water originating from southeast Greenland during summer, where it could potentially influence water column stability. Future studies using higher-resolution velocity observations are needed to capture the role of submesoscale variability in transport pathways in the Labrador Sea. [ABSTRACT FROM AUTHOR]

Surface currents in oceanic environment are of vital importance from economical, biological and environmental aspects. Modelling ocean currents has generally been performed using numerical ocean circulation models as a solution to initial-boundary value problems in oceanic domain. Due to lack of knowledge about model parameters as well as initial and boundary values, they need to be externally calibrated for accurate local and regional applications. In this study, an alternative approach is proposed to incorporate spaceborne geodetic observations as well as hydrographic data to estimate the total surface current in the Persian Gulf and the Oman Sea. Being the data-driven approach, the method is comparable to numerical ocean models and regionally it is more accurate and simpler in application. The proposed method focuses on the computation of dynamic topography (DT) by least squares variance component estimation combining two different schemes. They are (1) DT estimation via direct observations of sea surface height from satellite altimetry and (2) steric and non-steric modeling of sea level anomaly using temperature and salinity data for the steric component; and Gravity Recovery and Climate Experiment observations for the non-steric component. Ultimately, the total surface current is obtained by computing the horizontal gradient of DT using geostrophic equation and adding the components of the Ekman current. Moreover, the estimated total surface current is further improved by assimilating with in situ current meter data using 3D-Variational data assimilation method and it is validated against two control stations. This assimilation leads to improvement of about 3 to 15 cm/s in total surface current computed using geostrophic equation and Ekman current. Besides, to illustrate the significance of the proposed approach, the estimated total surface current is externally validated and compared with the output of Copernicus Marine Environment Monitoring Service (CMEMS), as a numerical ocean model developed for oceanographic applications. Our comparison reveals that the proposed method is more accurate and reliable than CMEMS products. As for the circulation and current pattern, the estimated surface velocities reveal the existence of eddies in the region of the Persian Gulf and the Oman Sea, indicating the occurrence of cyclonic and anti-cyclonic circulations. Moreover, they elucidate that the velocities are lower in spring and summer and higher in autumn and winter. [ABSTRACT FROM AUTHOR]

This paper reviews the recent progress in our estimation of ocean dynamic topography and the derived surface geostrophic currents, mainly based on multiple nadir radar altimeter missions. These altimetric observations provide the cornerstone of our ocean circulation observing system from space. The largest signal in sea surface topography is from the mean surface dominated by the marine geoid, and we will discuss recent progress in observing the mean ocean circulation from altimetry, once the geoid and other corrections have been estimated and removed. We then address the recent advances in our observations of the large-scale and mesoscale ocean circulation from space, and the particular challenges and opportunities for new observations in the polar regions. The active research in the ocean barotropic tides and internal tidal circulation is also presented. The paper also addresses how our networks of global multi-satellite and in situ observations are being combined and assimilated to characterize the four-dimensional ocean circulation, for climate research and ocean forecasting systems. For the future of ocean circulation from space, the need for continuity of our current observing system is crucial, and we discuss the exciting enhancement to come with global wide-swath altimetry, the extension into the coastal and high-latitude regions, and proposals for direct total surface current satellites in the 2030 period. [ABSTRACT FROM AUTHOR]

Mesoscale eddies play a crucial role in the transport of mass, heat, salt and nutrients, exerting significant influence on ocean circulation patterns, biogeochemical processes and the global climate system. Based on Lagrangian-Averaged Vorticity Deviation (LAVD) method, this study applies 27 years (1993–2019) of geostrophic current velocity data to detect Rotationally Coherent Lagrangian Vortices (RCLVs) in the Northwest Pacific (NWP; 10°N–30°N, 115°E–155°E), with the spatiotemporal characteristics of Eulerian Sea Surface Height Eddies (SSH eddies) and RCLVs being compared. A higher number of SSH eddies and RCLVs can be observed in spring and winter, and their inter-annual variations are similar. SSH eddies show higher generation number and larger radius in the Subtropical Countercurrent region, while RCLVs occur more favorably in the ocean basin. The propagation speed distributions of both eddy types are nearly identical and decrease with increasing latitude. Due to the material coherent transport maintained by RCLVs within a finite time interval, the coherent cores of RCLVs are considerably smaller in scale as compared to those of SSH eddies. The average zonal transports induced by SSH eddies and RCLVs are estimated to be −0.82 Sv and −0.51 Sv (1 Sv = 106 m3/s), respectively. For non-overlapping SSH eddies with RCLVs, approximately 80% of the water within the eddy leaks out during the eddy's lifespan. In the case of overlapping SSH eddies, the ratio of coherent water inside the eddy decreases with increasing radius, and the leakage rate is around 58%. Finally, an examination of 36 shedding RCLVs events from the Kuroshio near the Luzon Strait, which induce an average zonal transport of −0.14 Sv, reveals that 54% of the water within the shedding RCLVs originates from the Kuroshio. [ABSTRACT FROM AUTHOR]

The Antarctic Circumpolar Current plays a key role in the circulation of the Southern Ocean and affects redistribution of heat by the ocean on a global scale. The study of the dynamics and structure of this current becomes especially relevant in a changing climate. The current is well manifested based on the satellite altimetry data, which makes it possible to study the variability of its structure in time and space on different scales. The methods for determining the position of individual fronts of the Antarctic Circumpolar Current based on satellite altimetry data becomes most important in solving these tasks. In this paper, we compare different approaches to the detection of fronts. To do this, the structure of the northernmost branch of the Antarctic Circumpolar Current, the Subantarctic Front, and its spatial and temporal variability have been considered based on the satellite altimetry data from 1993 to 2020 and also on the results of a hydrographic section made onboard the R/V Akademik Mstislav Keldysh across the northern part of the Drake Passage in February 2020. To assess the dynamics of the front, a section of the TOPEX/Poseidon, Jason-1,2,3 satellite altimeter track from Tierra del Fuego to the south, 350 km long, was selected. Criteria are presented for determining the position of the northern and main branches of the Subantarctic Front based on the satellite altimetry data. A long-term trend of shifting the position of the fronts with respect to the previously accepted levels of absolute dynamic topography has been found. It has been established that the accuracy of determining the position of fronts from fixed values of dynamic topography can decrease in time, in particular, due to the variations in the mean sea level. A statistically significant long-term trend in sea level rise in the region of the Subantarctic Front at a rate of 4 mm/year for the northern branch and 2.5 mm/year for the main branch was found. [ABSTRACT FROM AUTHOR]

The ocean circulation dynamics can be represented as a high‐dimensional multi‐scale nonlinear system with inhomogenous meanfields and topography. Numerical simulations of the ocean dynamics resolve the large scales of motion on a computational grid, with the unresolved subgrid interactions parameterized. These simulations are highly dependent upon the grid resolution, unless the subgrid terms are appropriately parameterized. There are five fundamental classes of subgrid interactions: eddy‐eddy; eddy‐topographic; eddy‐meanfield; meanfield‐meanfield; and meanfield‐topographic. Scale dependent parameterizations representing each of these interaction classes are presented here in oceanic flows for the first time. Subgrid parameterizations are calculated and validated in baroclinic quasi‐geostrophic simulations of idealized Antarctic Circumpolar Current flows with representative mean currents and ocean floor topography. The parameterization coefficients are derived from the coarse grained statistics of high resolution reference simulations in spectral space. Stochastic and deterministic parameterizations are developed for the eddy‐eddy interactions, and deterministic forms for the remaining classes. The kinetic energy spectra and meanfield resulting from large eddy simulations (LES) adopting these coefficients accurately replicate those of the reference simulation. The eddy‐eddy interactions are dominant, but all classes need to be parameterized for best results. For LES where baroclinic instability is explicitly resolved the stochastic variants out‐perform the deterministic ones across all scales. When baroclinic instability is not explicitly resolved, the stochastic variants out‐perform the deterministic cases at the large scales, but introduce some distortions at the smallest resolved scales. This study provides an assessment of the relevant strengths of the subgrid interaction classes in an idealized yet representative ocean. Plain Language Summary: Numerical simulation of oceanic circulations is incredibly challenging due in part to the large range of scales. There are oscillations on the size of entire ocean basins, through to fine scale centimeter sized turbulence, and everything in between. Global simulations of the ocean typically capture the large scale features on a discrete grid. To ensure these simulations are physically representative, the influence of the missing smaller scales needs to be captured accurately. Here we present a first calculation of parameterizations governing the interactions between the resolved dynamics and the missing small scale eddies, meanfield and topographic features. Reduced resolution simulations incorporating these parameterizations without tuning have statistics in very good agreement with the high resolution simulations from which they were derived. Key Points: Subgrid models of all interaction classes between eddies, meanfield and topography are calculated from reference global ocean simulationsInter‐eddy interactions are modeled both stochastically and deterministically, with deterministic models used for the other interactionsCoarse simulations using these subgrid coefficients without tuning are in very good agreement with those of the high‐resolution simulation [ABSTRACT FROM AUTHOR]

The Kuroshio Current in the North Pacific displays path changes on an interannual‐to‐decadal time scale. In an idealized barotropic quasi‐geostrophic model of the double‐gyre wind‐driven circulation under stochastic wind‐stress forcing, such variability can occur due to transitions between different equilibrium states. The high‐dimensionality of the problem makes it challenging to determine the probability of these transitions under the influence of stochastic noise. Here we present a new method to estimate these transition probabilities, using a Dynamical Orthogonal (DO) field approach. In the DO approach, the solution of the stochastic partial differential equations system is decomposed using a Karhunen–Loève expansion and separate problems arise for the ensemble mean state and the so‐called time‐dependent DO modes. The original method is first reformulated in a matrix approach which has much broader application potential to various (geophysical) problems. Using this matrix‐DO approach, we are able to determine transition probabilities in the double‐gyre problem and to identify transition paths between the different states. This analysis also leads to the understanding which conditions are most favorable for transition. Plain Language Summary: In an idealized wind‐driven double‐gyre circulation model of the midlatitude ocean circulation, a northward displaced jet (jet‐up state) and a southward displaced jet (jet‐down state) exist under the same forcing conditions. The existence of these states can help explain behavior of, for example, the Kuroshio Current in the North Pacific, which undergoes path variations on an interannual‐to‐decadal time scale. Under the influence of wind‐stress noise, the model circulation may undergo a transition between the jet‐up state to the jet‐down state and vice versa. Determining transition probabilities between the two states is challenging as the model has many degrees of freedom and transition probabilities are very small when the noise is low. In this paper, we use a new formulation of a model order reduction technique, the so‐called Dynamical Orthogonal (DO) field approach, to determine both transition probabilities and transition paths. Key Points: A matrix version of the Dynamical Orthogonal (DO) field method is presentedDO is used to determine transition probabilities in multi‐stable stochastic dynamical systemsThe method is applied to study transition behavior of ocean western boundary currents [ABSTRACT FROM AUTHOR]

We present a new gridded sea surface height and current dataset produced by combining observations from nadir altimeters and drifting buoys. This product is based on a multiscale and multivariate mapping approach that offers the possibility to improve the physical content of gridded products by combining the data from various platforms and resolving a broader spectrum of ocean surface dynamic than in the current operational mapping system. The dataset covers the entire global ocean and spans from 1 July 2016 to 30 June 2020. The multiscale approach decomposes the observed signal into different physical contributions. In the present study, we simultaneously estimate the mesoscale ocean circulations as well as part of the equatorial wave dynamics (e.g. tropical instability and Poincaré waves). The multivariate approach is able to exploit the geostrophic signature resulting from the synergy of altimetry and drifter observations. Sea-level observations in Arctic leads are also used in the merging to improve the surface circulation in this poorly mapped region. A quality assessment of this new product is proposed with regard to an operational product distributed in the Copernicus Marine Service. We show that the multiscale and multivariate mapping approach offers promising perspectives for reconstructing the ocean surface circulation: observations of leads contribute to improvement of the coverage in delivering gap-free maps in the Arctic and observations of drifters help to refine the mapping in regions of intense dynamics where the temporal sampling must be accurate enough to properly map the rapid mesoscale dynamics. Overall, the geostrophic circulation is better mapped in the new product, with mapping errors significantly reduced in regions of high variability and in the equatorial band. The resolved scales of this new product are therefore between 5 % and 10 % finer than the Copernicus product (https://doi.org/10.48670/moi-00148 , Pujol et al., 2022b). [ABSTRACT FROM AUTHOR]

Satellite altimetry missions flying over the ice-covered Arctic Ocean have opened the possibility of further understanding changes in the ocean beneath the sea ice. This requires complex processing of satellite signals emerging from the sea surface in leads within the sea ice, with efforts to generate consistent Arctic-wide datasets of sea surface height ongoing. The aim of this paper is to provide and assess a novel gridded dataset of sea surface height anomaly and geostrophic velocity, which incorporates both the ice-covered and open ocean areas of the Arctic. Data from the CryoSat-2 mission in the period 2011–2020 were gridded at monthly intervals, up to 88 ∘ N, using the Data-Interpolating Variational Analysis (DIVA) method. To examine the robustness of our results, we compare our dataset to independent satellite data, mooring time series and Arctic-wide hydrographic observations. We find that our dataset is well correlated with independent satellite data at monthly timescales. Comparisons to in situ ocean observations show that our dataset provides reliable information on the variability of sea surface height and surface geostrophic currents over geographically diverse regions of the Arctic Ocean and different dynamical regimes and sea ice states. At all comparison sites we find agreement with in situ observed variability at seasonal to interannual timescales. Furthermore, we find that our geostrophic velocity fields can resolve the variability of boundary currents wider than about 50 km, a result relevant for studies of Arctic Ocean circulation. Additionally, large-scale seasonal features emerge. Sea surface height exhibits a wintertime Arctic-wide maximum, with the highest amplitude over the shelves. Also, we find a basin-wide seasonal acceleration of Arctic slope currents in winter. We suggest that this dataset can be used to study not only the large-scale sea surface height and circulation, but also the regionally confined boundary currents. The dataset is available in netCDF format from PANGAEA at 10.1594/PANGAEA.931869. [ABSTRACT FROM AUTHOR]

To assess the contribution of wind drag and Stokes drift on the near-surface circulation, a methodology to isolate the geostrophic surface current from high-frequency radar data is developed. The methodology performs a joint analysis utilizing wind field and in situ surface currents along with an unsupervised neuronal network. The isolation method seems robust in the light of comparisons with satellite altimeter data, presenting a similar time variability and providing more spatial detail of the currents in the coastal region. Results show that the wind-induced current is around 2.1% the wind speed and deflected from the wind direction in the range [18°, 23°], whereas classical literature suggests higher values. The wave-induced currents can represent more than 13% of the ageostrophic current component as function of the wind speed, suggesting that the Stokes drift needs to be analyzed as an independent term when studying surface sea currents in the coastal zones. The methodology and results presented here could be extended worldwide, as complementary information to improve satellite-derived surface currents in the coastal regions by including the local physical processes recorded by high-frequency radar systems. The assessment of the wave and wind-induced currents have important applications on Lagrangian transport studies. [ABSTRACT FROM AUTHOR]

The instability of a surface-trapped jet over rough bottom topography is examined using a linearized quasigeostrophic model. The jet is laterally sheared and thus susceptible to both barotropic and baroclinic instability. The relative magnitude of the two depends on the jet width and on the spectral characteristics and amplitude of the bathymetry. The most unstable eddies in the upper layer are typically smaller over bathymetry than with a flat bottom. Topography also alters momentum flux convergence in the upper layer and causes the perturbations to resemble eddies in a 1.5-layer flow. But as long as the jet is wider than the deformation radius, baroclinic instability is present, yielding deep eddies that are phase-locked to those at the surface. In addition, topography facilitates scattering of energy at depth to other scales. So, instability over rough topography could be an efficient, and largely overlooked, means of transferring mesoscale energy to the dissipative scales. Significance Statement: This study investigates the effects of bottom roughness on large-scale ocean currents and their associated eddies. Roughness affects the eddy size and speed and how they exchange energy with the mean flow. Roughness also facilitates energy transfer to smaller scales where it can be dissipated. Thus, instability over rough topography could be an important part of the oceanic energy balance. [ABSTRACT FROM AUTHOR]

In this study, the upper-ocean absolute geostrophic currents in the southern Indian Ocean are constructed using Argo temperature and salinity data from the middepth (1000 m) zonal velocity derived from the Argo float trajectory. The results reveal alternating quasi-zonal striation-like structures of middepth zonal velocity in the equatorial and southern tropical Indian Ocean. Specifically, the eastward time-mean flows are located at the equator and 2°, 5°, 8°, 13°, 16°, 18°–19°, and 21°–22°S, with a meridional scale of ∼300 km. The generation mechanisms of the striation-like zonal velocity structure differ between the near-equatorial and off-equatorial regions. The triad of baroclinic Rossby wave instability plays a significant role in near-equatorial striations. In the south, the high potential vorticity (PV) of Antarctic intermediate water and low PV of southern Indian Ocean Subantarctic Mode Water lead to strong baroclinic instability, which increases the eddy kinetic energy in the middepth layer, thus contributing to a turbulent PV gradient. The convergence/divergence of the eddy PV flux generates the quasi-zonal striations. The meridional scale of the striations is controlled by the most unstable wavelength of baroclinic instability, which explains the observations. Significance Statement: The middepth zonal velocity resembles a system of eastward/westward jets with a considerably smaller width than the larger-scale ocean surface circulation. Such a phenomenon always occurs in a turbulent ocean that presents eddy or eddy–mean flow interactions. This study used float observations to reveal a robust middepth zonal velocity in the southern tropical Indian Ocean, where the width of the eastward time-mean flows is approximately 300 km. Smaller eddies drive the zonal currents with a smaller width, and the energy of the eddies is released from the unstable vertical structure at middepths. This study provides new insights into the generation mechanism of small-width zonal current structures in the deep ocean. [ABSTRACT FROM AUTHOR]

The tropical Pacific currents from January 2004 to December 2018 are computed based on the gridded Argo temperatures and salinities using the P-vector method on an f plane and the geostrophic approximation on a β plane. Three branches of the South Equatorial Current (SEC) are identified, i.e., SEC(N) (2°S–5°N), SEC(M) (7°–3°S), and SEC(S) (20°–8°S), with the maximum zonal velocity of −55 cm s−1 and total volume transport of −49.8 Sv (1 Sv ≡ 106 m3 s−1) occurring in the central-east Pacific. The seasonal variability of each branch shows a distinct and different westward propagation of zonal current anomalies, which are well mirrored by the SLA differences between 2°S and 5°N, between 3°S and 6°S, and between 8°S and 15°S, respectively. Most of the seasonal variations are successfully simulated by a simple analytical Rossby wave model, highlighting the significance of the first-mode baroclinic, linear Rossby waves, particularly those driven by the wind stress curl in the central-east Pacific. However, the linear theory fails to explain the SEC(M) variations in certain months in the central-east Pacific, where the first baroclinic mode contributes only around 50% of the explained variance to the equatorial surface currents. A nonlinear model involving higher baroclinic modes is suggested for a further diagnosis. Considering the crucial role played by the tropical Pacific in the natural climate variability via the El Niño–Southern Ocean dynamics and the ocean response to anthropogenic forcing via the ocean heat uptake in the eastern tropical Pacific, advancing the process understanding of the SEC from observations is critical. [ABSTRACT FROM AUTHOR]

We present a new gridded sea surface height and current dataset produced by combining observations from nadir altimeters and drifting buoys. This product is based on a multiscale & multivariate mapping approach that offers the possibility to improve the physical content of gridded products by combining the data from various platforms and in resolving a broader spectrum of ocean surface dynamic than in the current operational mapping system. The dataset covers the entire global ocean and spans from 2016-07-01 to 2020-06-30. The multiscale approach decomposes the observed signal into different physical contributions. In the present study, we simultaneously estimate the mesoscale ocean circulations as well as part of the equatorial wave dynamics (e.g., tropical instability and Poincaré waves). The multivariate approach is able to exploit the geostrophic signature resulting from the synergy of altimetry and drifter observations. Sea level observations in Arctic leads are also used in the merging to improve the surface circulation in this poorly mapped region. A quality assessment of this new product is proposed against the DUACS operational product distributed in the Copernicus Marine Service. We show that the multiscale & multivariate mapping approach offers promising perspectives for reconstructing the ocean surface circulation: leads observations contribute to improve the coverage in delivering gap free maps in the Arctic; drifters observations help to refine the mapping in regions of intense dynamics where the temporal sampling must be accurate enough to properly map the rapid mesoscale dynamics; overall, the geostrophic circulation is better mapped in the new product, with mapping errors significantly reduced in regions of high variability and in the equatorial band; the effective resolution of this new product is hence between 5% and 10% finer than the Copernicus product. [ABSTRACT FROM AUTHOR]

Hundreds of full-depth temperature and salinity profiles collected by Deepglider autonomous underwater vehicles (AUVs) in the North Atlantic reveal robust signals in eddy isopycnal vertical displacement and horizontal current throughout the entire water column. In separate glider missions southeast of Bermuda, subsurface-intensified cold, fresh coherent vortices were observed with velocities exceeding 20 cm s−1 at depths greater than 1000 m. With vertical resolution on the order of 20 m or less, these full-depth glider slant profiles newly permit estimation of scaled vertical wavenumber spectra from the barotropic through the 40th baroclinic mode. Geostrophic turbulence theory predictions of spectral slopes associated with the forward enstrophy cascade and proportional to inverse wavenumber cubed generally agree with glider-derived quasi-universal spectra of potential and kinetic energy found at a variety of locations distinguished by a wide range of mean surface eddy kinetic energy. Water-column average spectral estimates merge at high vertical mode number to established descriptions of internal wave spectra. Among glider mission sites, geographic and seasonal variability implicate bottom drag as a mechanism for dissipation, but also the need for more persistent sampling of the deep ocean. Significance Statement: Relative to upper-ocean measurements of temperature, salinity, and velocity, deep ocean measurements (below 2000 m) are fewer in number and more difficult to collect. Deep measurements are needed, however, to explore the nature of deep ocean circulation contributing to the global redistribution of heat and to determine how upper-ocean behavior impacts or drives deep motions. Understanding of geographic and temporal variability in vertical structures of currents and eddies enables improved description of energy pathways in the ocean driven by turbulent interactions. In this study, we use newly developed autonomous underwater vehicles, capable of diving to the seafloor and back on a near daily basis, to collect high-resolution full ocean depth measurements at various locations in the North Atlantic. These measurements reveal connections between surface and deep motions, and importantly show their time evolution. Results of analyzing these vertical structures reveal the deep ocean to regularly "feel" events in the upper ocean and permit new comparisons to deep motions in climate models. [ABSTRACT FROM AUTHOR]

The article presents a study which explores the surface warming–induced global acceleration of upper ocean currents. It mentions that study points a way forward for investigating ocean circulation change and evaluating the uncertainty. It discusses that how the ocean circulation changes in a warming climate.

The article presents a study which explores the surface warming–induced global acceleration of upper ocean currents. It mentions that study points a way forward for investigating ocean circulation change and evaluating the uncertainty. It discusses that how the ocean circulation changes in a warming climate.

As large-scale ocean circulation is a key regulator in the redistribution of oceanic energy, evaluating the multi-decadal trends in the western Pacific Ocean circulation under global warming is essential for not only understanding the basic physical processes but also predicting future climate change in the western Pacific. Employing the hydrological observations of World Ocean Atlas 2018 (WOA18) from 1955 to 2017, this study calculated the geostrophic currents, volume transport and multi-decadal trends for the North Equatorial Current (NEC), the North Equatorial Countercurrent (NECC), the Mindanao Current (MC), the Kuroshio Current (KC) in the origin and the New Guinea Coastal Undercurrent (NGCUC) within tropical western Pacific Ocean over multi-decades. Furthermore, this study examined the contributions of temperature and salinity variations. The results showed significant strengthening trends in NEC, MC and NGCUC over the past six decades, which is mainly contributed by temperature variations and consistent with the tendency in the dynamic height pattern. Zonal wind stress averaged over the western Pacific Ocean in the same latitude of each current represents the decadal variation and multi-decadal trends in corresponding ocean currents, indicating that the trade wind forcing plays an important role in the decadal trend in the tropical western Pacific circulation. Uncertainties in the observed hydrological data and trends in the currents over the tropical western Pacific are also discussed. Given that the WOA18 dataset covers most of the historical hydrological sampling data for the tropical western Pacific, this paper provides important observational information on the multi-decadal trend of the large-scale ocean circulation in the western Pacific. [ABSTRACT FROM AUTHOR]

The mean, interannual variability, decadal variability, and longer-term trends in the across-track geostrophic velocity of the poleward-flowing boundary current along the Fiordland coast (the Fiordland Current, FC) are examined over the altimeter record (September 1992–May 2017). In the mean, the FC is southwestward, with stronger velocities in an inner current near the coast and weaker velocities in an outer current further offshore. Examination of the alongshore momentum balance suggests that the mean flow is driven by a poleward downsloping alongshore pressure gradient. At interannual time-scales, variability in the FC was correlated with the alongshore wind stress, indicating that an equatorward (poleward) wind stress drives an equatorward (poleward) velocity anomaly. At decadal time-scales, variability in the FC was correlated with the South Pacific wind stress curl, indicating that as the wind stress curl increases (decreases) the poleward flow in the FC weakens (strengthens), consistent with the response of a traditional eastern boundary current. Long-term trends show a weakening outer current and strengthening inner current. Increasing flow in the inner current may result in increased transport of subtropical water around southern New Zealand. [ABSTRACT FROM AUTHOR]

Given the importance of coastal upwelling in the California Current System (CCS), there is a considerable interest in predicting its response to global warming. However, upwelling changes are often treated as synonymous with changes in upwelling‐favorable winds, while the role of geostrophic transport is unaccounted for. Here, we examine the respective roles of Ekman and geostrophic transports using the Community Earth System Model Large Ensemble. In some parts of the CCS, the contribution of geostrophic transport to long‐term changes in upwelling is equal or greater than the contribution from Ekman transport. The combination of the two transports nearly close the momentum budget, and thus reproduce the mean state, interannual variability, and long‐term changes in upwelling. These results highlight the importance of accounting for ocean circulation when quantifying upwelling and its variability and change. Plain Language Summary: The California Current System (CCS) is an eastern boundary current system along the west coast of North America. The CCS hosts a diverse marine ecosystem and a high level of production for commercially valuable fish. Upwelling delivers deep, nutrient‐rich water to the sunlit surface, which provides nutrients that stimulate phytoplankton growth, fueling the marine ecosystem. Thus, potential future changes in coastal upwelling are of concern for the health of the CCS ecosystem, the livelihood of fishers, and food security. To date, projections of upwelling change have focused primarily on modifications of the wind‐driven (Ekman) transport. However, a second component of the ocean circulation, the geostrophic transport, can also impact coastal upwelling. In this study, we investigate the roles of both transport components in coastal upwelling change under global warming. We find that changes in the geostrophic currents, which have been neglected in many previous studies, make a substantial contribution to overall changes in the CCS coastal upwelling. Key Points: Changes in alongshore wind stress are insufficient to obtain overall changes in the California Current System (CCS) upwelling in the CESM1 large ensembleChanges in geostrophic transport make a substantial contribution to changes in the CCS upwellingA combination of changes in Ekman and geostrophic transports reproduces overall changes in the CCS upwelling [ABSTRACT FROM AUTHOR]

Eleven climate system models that participate in the Coupled Model Intercomparison Project phase 5 (CMIP5) were evaluated based on an assessment of their simulated meridional transports in comparison with the Sverdrup transports. The analyses show that the simulated North Pacific Ocean circulation is essentially in Sverdrup balance in most of the 11 models while the Argo geostrophic meridional transports indicate significant non-Sverdrup gyre circulation in the tropical North Pacific Ocean. The climate models overestimated the observed tropical and subtropical volume transports significantly. The non-Sverdrup gyre circulation leads to non-Sverdrup heat and salt transports, the absence of which in the CMIP5 simulations suggests deficiencies of the CMIP5 model dynamics in simulating the realistic meridional volume, heat, and salt transports of the ocean. [ABSTRACT FROM AUTHOR]

Unprecedented coastal upwelling off the southern coast of the Korean Peninsula was reported during the summer of 2013. The upwelling continued for more than a month after a plunge in upwelling-favourable winds and had serious impacts on fisheries. This is a rare phenomenon, as most coastal upwelling events relax a few days after the wind weakens. In this study, observational data and numerical modelling results were analysed to investigate the cause of the upwelling and the reason behind it being sustained for such an extended period. Coastal upwelling was induced by an upwelling-favourable wind in July, resulting in the dynamic uplift of deep, cold water. The dynamic uplift decreased the steric sea level in the coastal region. The sea level difference between the coastal and offshore regions produced an intensified cross-shore pressure gradient that enhanced the surface geostrophic current along the coast. The strong surface current maintained the dynamic uplift due to geostrophic equilibrium. This positive feedback between the dynamic uplift and geostrophic adjustment sustained the coastal upwelling for a month following a plunge in the upwelling-favourable wind. [ABSTRACT FROM AUTHOR]

Estimates of 3D ocean circulation are needed to improve our understanding of ocean dynamics and to assess their impact on marine ecosystems and Earth climate. Here we present the OMEGA3D product, an observation-based time series of (quasi-)global 3D ocean currents covering the 1993–2018 period, developed by the Italian Consiglio Nazionale delle Ricerche within the European Copernicus Marine Environment Monitoring Service (CMEMS). This dataset was obtained by applying a diabatic quasi-geostrophic (QG) diagnostic model to the data-driven CMEMS-ARMOR3D weekly reconstruction of temperature and salinity as well as ERA Interim fluxes. Outside the equatorial band, vertical velocities were retrieved in the upper 1500 m at 1/4 ∘ nominal resolution and successively used to compute the horizontal ageostrophic components. Root mean square differences between OMEGA3D total horizontal velocities and totally independent drifter observations at two different depths (15 and 1000 m) decrease with respect to corresponding estimates obtained from zero-order geostrophic balance, meaning that estimated vertical velocities can also be deemed reliable. OMEGA3D horizontal velocities are also closer to drifter observations than velocities provided by a set of reanalyses spanning a comparable time period but based on data assimilation in ocean general circulation numerical models. The full OMEGA3D product (released on 31 March 2020) is available upon free registration at https://doi.org/10.25423/cmcc/multiobs%5fglo%5fphy%5fw%5frep%5f015%5f007 (Buongiorno Nardelli, 2020a). The reduced subset used here for validation and review purposes is openly available at 10.5281/zenodo.3696885 (Buongiorno Nardelli, 2020b). [ABSTRACT FROM AUTHOR]

We investigated the effect of the assimilation of surface velocities from two HF-Radars (HFRs) and satellite sea surface temperature (SST) into an ocean circulation model, implemented on the North-Western Mediterranean Sea, using the ROMS 4D-Var Data Assimilation (DA) System. A three months period from September to November 2020 is analysed. A model free run is used as reference simulation to quantify the impact of the data assimilation procedure on the surface circulation. The DA framework consists of a sequence of 3-day windows during which the observations are assimilated into the model, followed by forecast simulations, starting from the last time step of the analysis run. The comparison of simulated velocities and SST against the assimilated observations shows that both forecast and analysis runs improve the solution as compared to the freerun, with error reduction up to the 47% and doubling of correlation values. If SST is not assimilated, the simulated sea surface temperature slightly degrades as a result of the DA procedure. Furthermore, we used surface velocities from Lagrangian drifters to test the model against independent observations. The results show that both the forecast and the analysis simulate surface circulation better than the free run within the areas covered by HFR observations, with error reduction ranging from 10% to 50%, and increase in correlation from 5% to more than doubled. Nevertheless, farther afield, outside the area covered by HFRs, improvements and degradations of the solution due to the DA procedure balance each other. The impact of assimilated observation on the alongshore transport of the upper 50 m is analysed along three transects adjacent to the two HFR areas and along a section in between. The transport increment is significant where the surface velocities are assimilated whereas it is negligible elsewhere. The effect of SST is to modify the velocity distribution along the transects, while keeping the transport increment unchanged. Modifications to the initial conditions have the most significant effect on the alongshore transport, whereas the corrections to boundary conditions and atmospheric forcing have different importance according to the analysed transects. The assimilation of surface velocities and SST affects more the ageostrophic component of the surface velocity field, rather than the geostrophic one. • 4D-Var DA of HF-Radar surface velocities and SST improve analysis and forecast. • Validation with independent data shows that benefits are limited to the HF-Radar area. • HF-Radar data change transport magnitude and SST modulates the velocity distribution. [ABSTRACT FROM AUTHOR]

Numerical ocean circulation forecasts diverge from reality due to non-deterministic processes. Through data assimilation, ocean forecasting systems routinely make corrections to scalar ocean variables of temperature and salinity derived from ocean observations. Velocity observations, however, are vector quantities that require different processing. To assimilate existing ocean velocity observations, which generally are depth limited, vertical error covariances are needed to relate the velocity errors throughout the water column. In this paper we show a velocity assimilation approach that relies on historical observations and geostrophy to relate velocity, temperature, and salinity. Geopotential calculations transform temperature and salinity covariances into corresponding geopotential covariances. This paper describes the methods and examines details of the solution process for ocean velocity data assimilation. The approach employs two separate three-dimensional variational (3DVAR) analyses combined to create one set of initial conditions during each daily forecast cycle. The first 3DVAR analysis utilizes temperature, salinity, and sea surface height anomaly data and reduces the background error variance based on these observations. The second 3DVAR analysis uses the reduced error variance and performs velocity data assimilation using surface drifter data, and these observations further reduce the error variance of temperature and salinity for the next cycle. The second 3DVAR analysis for velocity data assimilation utilizes the new full depth error covariances. We show diagnostic results from a Gulf of Mexico simulation during extensive drifter deployments in the summer of 2020. Results reveal that the two-step data assimilation cycle applies vector velocity innovations to create increments that correct the background field towards a geostrophic balance in temperature and salinity. • Surface drifter positions create Eulerian innovations for assimilation. • Empirically derived vertical covariances distribute velocity increments. • Velocity forecast error is derived from empirical vertical covariances. • Velocity increments constrain forecasted geostrophic velocities. [ABSTRACT FROM AUTHOR]

Gravitational attraction and loading (GAL) effects associated with ongoing long-term changes in land ice are expected to cause spatially varying trends in absolute sea level ζ, as measured by satellite altimeters. The largest spatial gradients in ζ trends, predicted from solving the sea level equation using GRACE retrievals of mass distribution over land for the period 2005-15, occur near Greenland and West Antarctica, consistent with a strong local land ice loss. Misinterpreting the estimated static GAL trends in ζ as dynamic pressure gradients can lead to substantial errors in large-scale geostrophic transports across the Southern Ocean and the subpolar North Atlantic over the analyzed decade. South of Greenland, where altimeter sea level and hydrography (Argo) data coverage is good, the residual z minus steric height trends are similar in magnitude and sign to the gravitationally based predictions. In addition, estimated GAL-related trends are as large--if not larger than--other factors, such as deep steric height, dynamic bottom pressure, and glacial isostatic rebound. Thus, accounting for static GAL effects on ζ records, which are commonly neglected in oceanographic studies, seems important for a quantitative interpretation of the observed ζ trends. [ABSTRACT FROM AUTHOR]

Previous modeling and observational studies have established that it is possible to accurately monitor the Atlantic Meridional Overturning Circulation (AMOC) at 26.5°N using a coast-to-coast array of instrumented moorings supplemented by direct transport measurements in key boundary regions (the RAPID/MOCHA/WBTS Array). The main sources of observational and structural errors have been identified in a variety of individual studies. Here a unified framework for identifying and quantifying structural errors associated with the RAPID array-based AMOC estimates is established using a high-resolution (eddy resolving at low-mid latitudes, eddy permitting elsewhere) ocean general circulation model, which simulates the ocean state between 1978 and 2010. We define a virtual RAPID array in the model in close analogy to the real RAPID array and compare the AMOC estimate from the virtual array with the true model AMOC. The model analysis suggests that the RAPID method underestimates the mean AMOC by ∼1.5 Sv (1 Sv = 10 6  m 3  s −1 ) at ∼900 m depth, however it captures the variability to high accuracy. We examine three major contributions to the streamfunction bias: (i) due to the assumption of a single fixed reference level for calculation of geostrophic transports, (ii) due to regions not sampled by the array and (iii) due to ageostrophic transport. A key element in (i) and (iii) is use of the model sea surface height to establish the true (or absolute) geostrophic transport. In the upper 2000 m, we find that the reference level bias is strongest and most variable in time, whereas the bias due to unsampled regions is largest below 3000 m. The ageostrophic transport is significant in the upper 1000 m but shows very little variability. The results establish, for the first time, the uncertainty of the AMOC estimate due to the combined structural errors in the measurement design and suggest ways in which the error could be reduced. Our work has applications to basin-wide circulation measurement arrays at other latitudes and in other basins as well as quantifying systematic errors in ocean model estimates of the AMOC at 26.5°N. [ABSTRACT FROM AUTHOR]

The Mascarene Plateau lies in the south-west Indian Ocean between the islands of Mauritius and the Seychelles Bank, and is characterised by a series of shallow banks separated by deep (>1 000 m), narrow channels. The plateau acts as an obstruction to the general ocean circulation in this region, separating the westward-flowing South Equatorial Current (SEC) into two branches downstream of the plateau. In this article, we present the results of a survey conducted along the entire Mascarene Plateau during the Northeast Monsoon, in October–November 2008. In addition, data from Argo floats were used to determine the origin of water masses entering this region. The plateau contains three gaps through which branches of the SEC are channelled. The northern, central and southern gaps receive 14.93 Sv, 14.41 Sv and 6.19 Sv, respectively. Although there are differences in water-mass properties to the west and east of the Mascarene Plateau due to mixing, the SEC acts as a sharp boundary between water masses of southern and northern Indian Ocean origin. Mixing occurs in the central gap between intermediate water masses (Red Sea Water [RSW] and Antarctic Intermediate Water [AAIW]) as well as in the upper waters (Subtropical Surface Water [STSW] and Indonesian Throughflow Water [ITW]). Through the northern gap, mixing occurs between Arabian Sea High-Salinity Water (ASHSW), ITW and Tropical Surface Water (TSW), while through the southern gap, mixing occurs between STSW and ITW. North Indian Deep Water (NIDW) is present in the region but the plateau appears to have no effect on it. [ABSTRACT FROM AUTHOR]

We analyse the hindcast skill for the Atlantic meridional overturning circulation (AMOC) against 10 years of RAPID/MOCHA AMOC observations, which are now long enough to remove the mean seasonal cycle prior to the hindcast skill analysis. We analyse AMOC hindcast skill in two hindcast ensembles generated with two differently initialised decadal prediction systems that are both based on the earth system model MPI-ESM. We evaluate the hindcast skill for the AMOC and its components in both prediction systems against RAPID/MOCHA observations both with and without the mean seasonal cycle removed using anomaly correlation (COR) and root-mean-square error as skill measures. We find significant hindcast skill for most lead years up to 5 for monthly-mean AMOC variations only in the newer of the two prediction systems and only using COR, but with and without the mean seasonal cycle removed. In both systems and for all analysed lead years, the two geostrophic transport components (the upper-mid-ocean transport and Florida Strait combined, that is: AMOC minus Ekman) are the main source of hindcast skill. In the present model setup and with the currently available observational time series, we cannot relate AMOC hindcast skill to the upper-mid-ocean transport alone. Yet, we can show that the seasonal variability of the upper-mid-ocean transport in the free coupled model originates from eastern boundary density variability. Overall, our results indicate modest yet robust AMOC hindcast skill above the uninitialized simulation, independent of the treatment of the seasonal cycle, although we cannot directly link this hindcast skill to the initialisation of the density field with either initialisation method. [ABSTRACT FROM AUTHOR]

A process-oriented numerical modeling study was conducted to investigate the formation and underlying forcing of an anticyclonic eddy train observed in the northern South China Sea. Observations showed that long-lived anticyclonic eddies formed an eddy train along an eastward separated jet across the northern South China Sea in summer. The eddy train plays a critical role in regulating ocean circulation in the region. Forced by the southwesterly monsoon and prevailing dipole wind stress curl in the summer, the northward coastal jet separates from the west boundary of the South China Sea basin and overshoots northeastward into the basin. The anticyclonic recirculation of the separated jet forms the first anticyclonic eddy in the eddy train. The jet meanders downstream with a strong negative shear vorticity that forms a second and a third anticyclonic eddy along the jet's path. These three eddies form the eddy train. These eddies weaken gradually with depth from surface, but they can extend to approximately 500 m deep. The inherent stratification in the region regulates the three-dimensional scale of the anticyclonic eddies and constrains their intensity vertical extension by weakening the geostrophic balance within these eddies. Analyses of the vorticity balance indicate that the eddy train's negative vorticity originates from the beta effect of northward western boundary current and from the subsequent downstream vorticity advection in the jet. The jet separation is a necessary condition for the formation of the eddy train, and the enhanced stratification, increased summer wind stress, and associated negative wind stress curl are favorable conditions for the formation of the anticyclonic eddies. [ABSTRACT FROM AUTHOR]

Hydrographic measurements together with Ship mounted Acoustic Doppler Current Profilers and Lowered Acoustic Doppler Current Profilers (LADCP) obtained in October 2014 are used to describe water masses, geostrophic circulation and mass transport of the Canary Current System, as the Eastern Boundary of the North Atlantic Subtropical Gyre. Geostrophic velocities are adjusted to velocities from LADCP data to estimate an initial velocity at the reference layer. The adjustment results in a northward circulation at the thermocline layers over the African slope from an initial convergent flow. Final reference velocities and consequently absolute circulation are estimated from an inverse box model applied to an ocean divided into 13 neutral density layers. This allows us to evaluate mass fluxes consistent with the thermal wind equation and mass conservation. Ekman transport is estimated from the wind data derived from the Weather Research and Forecasting model. Ekman transport is added to the first layer and adjusted with the inverse model. The Canary Current located west of Lanzarote Island transports to the south a mass of − 1.5 ± 0.7 Sv (1 Sv = 10 6 m 3 s − 1 ≈ 10 9 kg s − 1 ) of North Atlantic Central Water at the surface and thermocline layers (~ 0–700 m). In fall 2014, hydrographic data shows that the Canary Current in the thermocline (below at about 80 m depth to ~ 700 m) recirculates to the north over the African slope and flows through the Lanzarote Passage. At intermediate layers (~ 700–1400 m), the Intermediate Poleward Undercurrent transports northward a relatively fresh Antarctic Intermediate Water in the range of 0.8 ± 0.4 Sv through the Lanzarote Passage and west of Lanzarote Island beneath the recirculation of the Canary Current. [ABSTRACT FROM AUTHOR]

The mean subthermocline and intermediate zonal circulation in the tropical Pacific is investigated using a compilation of shipboard ADCP measurements and absolute geostrophic velocities constructed from a high-resolution 0-2000-m Argo climatology referenced to a 1000-m velocity field derived from Argo float drifts. This reference field is dominated by basinwide alternating zonal jets with a meridional wavelength of about 3°. In regions where the sampling of SADCP data is sufficient, the consistency between the two independent datasets is striking; using the Argo drift reference is crucial to capture the current structures. Two apparently distinct systems of alternating westward and eastward zonal jets are seen in both datasets equatorward of 10°: a series of low-latitude subthermocline currents (LLSCs) below the thermocline, extending from about 200 to 800 m, including the eastward Tsuchiya jets; and a series of low-latitude intermediate currents (LLICs), extending from about 700 to at least 2000 m. These systems seem to merge poleward of 10°. Both series shoal to lighter densities eastward. The subthermocline currents and their associated potential vorticity structures undergo a major shift near 155°W, suggesting some difference in the dynamic regime between the regions west and east of this longitude. Differing behaviors (the LLSCs tend to angle poleward to the east, whereas the LLICs angle slightly equatorward) suggest that these jets may be dynamically distinct, with different forcing mechanisms. [ABSTRACT FROM AUTHOR]

The objection of this paper is to study the asymptotical behavior of solutions for the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise. In this paper, we prove the existence of random attractors for the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise. Furthermore, we prove the random attractorof the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise will be close to the global attractorof the three dimensional planetary geostrophic equations of large-scale ocean circulation for anywhen the parameter of the perturbationgoes to zero. [ABSTRACT FROM PUBLISHER]

The article reports that Wind is the main driver of general ocean circulation, using satellite data and a multiscale process analysis method, Shikhar Rai and colleagues at the University of Rochester that wind deposits kinetic energy in geostrophic ocean flow with wind extracts energy.

This study deals with the physical properties and 3D structures of the wave motions with finite wavelengths of O(100-550) km in the tropical western North Pacific and their variation as propagating from the Pacific to the South China Sea (SCS) using conductivity-temperature-depth observations taken in October and November 2005 and concurrent satellite altimeter data. Three wave components with wavelength bands of O(100), O(200), and O(550) km are derived from the isopycnal undulation signals along 21°, 18°, and 15°N using the ensemble empirical mode decomposition analysis. Their maximum amplitudes are over 100 m in the layer of 1000-2000 m. Phase speeds are derived from cruise-observed vertical profiles of zonal-mean geostrophic flow velocity and the Brunt-Väisälä frequency based on linear quasigeostrophic wave theory with background flow and topography. The speeds are also derived from concurrent sea level anomaly data with the objective Radon transform method. They are close to that of the first baroclinic mode of theoretical solutions, implying that the observed wave motions possess the physical properties of Rossby waves (RWs). The vertical structures of the first generalized modes are derived from cruise observations at three sections. It is shown that the RWs continuously propagate from the Pacific to the SCS, and the available potential energy of RWs 1 and 2 intensify 3-4 times in the Luzon Strait and the SCS compared to that in the Pacific. [ABSTRACT FROM AUTHOR]

Understanding of the three-dimensional circulation in the South China Sea (SCS) is crucial for determining the transports of water masses, energy, and biogeochemical substances in the regional and adjacent larger oceans. The circulation's kinematic and dynamic natures, however, are largely unclear. Results from a three-dimensional numerical ocean circulation model and geostrophic currents, derived from hydrographic data, reveal the existence of a unique, three-layer, cyclonic-anticyclonic-cyclonic (CAC) circulation in the upper (<750 m), middle (750-1500 m), and deep (>1500 m) layers in the SCS with differing seasonality. An inflow-outflow-inflow structure in Luzon Strait largely induces the CAC circulation, which leads to vortex stretching in the SCS basin because of a lateral planetary vorticity flux in each of the respective layers. The formation of joint effects of baroclinicity and relief (JEBAR) is an intrinsic dynamic response to the CAC circulation. The JEBAR arises from the CAC flow-topography interaction in the SCS. [ABSTRACT FROM AUTHOR]

Utilizing the framework of effective surface quasigeostrophic (eSQG) theory, this study explores the potential of reconstructing the 3D upper-ocean circulation structures, including the balanced vertical velocity w field, from high-resolution sea surface height (SSH) data of the planned Surface Water and Ocean Topography (SWOT) satellite mission. Specifically, the authors utilize the 1/30°, submesoscale-resolving, OFES model output and subject it to the SWOT simulator that generates the along-swath SSH data with expected measurement errors. Focusing on the Kuroshio Extension region in the North Pacific where regional Rossby numbers range from 0.22 to 0.32, this study finds that the eSQG dynamics constitute an effective framework for reconstructing the 3D upper-ocean circulation field. Using the modeled SSH data as input, the eSQG-reconstructed relative vorticity ζ and w fields are found to reach a correlation of 0.7-0.9 and 0.6-0.7, respectively, in the 1000-m upper ocean when compared to the original model output. Degradation due to the SWOT sampling and measurement errors in the input SSH data for the ζ and w reconstructions is found to be moderate, 5%-25% for the 3D ζ field and 15%-35% for the 3D w field. There exists a tendency for this degradation ratio to decrease in regions where the regional eddy variability (or Rossby number) increases. [ABSTRACT FROM AUTHOR]

It is typically assumed that the meridional density gradient in the North Atlantic is well and positively correlated with the Atlantic meridional overturning circulation (AMOC). In numerical 'water-hosing' experiments, for example, imposing an anomalous freshwater flux in the Northern Hemisphere leads to a slowdown of the AMOC. However, on planetary scale, the first-order dynamics are linked to the geostrophic balance, relating the north-south pressure gradient to the zonal circulation. In this study, these two approaches are reconciled. At steady state and under geostrophic dynamics, an analytical expression is derived to relate the zonal and meridional pressure gradient. This solution is only valid where the meridional density gradient length scale is shorter than Earth's curvature length scale, that is, north of 35°N. This theoretical expression links the north-south density gradient to the AMOC and can be used as a closure for zonally averaged ocean models. Assumptions and shortcomings of the approach are presented. Implications of these results for paleoclimate problems such as AMOC collapse and asymmetry in the meridional overturning circulation of the Atlantic and of the Pacific are discussed. [ABSTRACT FROM AUTHOR]

With increased geoid resolution provided by the gravity and steady-state ocean circulation explorer (GOCE) mission, the ocean's mean dynamic topography (MDT) can be now estimated with an accuracy not available prior to using geodetic methods. However, an altimetric-derived MDT still needs filtering in order to remove short wavelength noise unless integrated methods are used in which the three quantities are determined simultaneously using appropriate covariance functions. We studied nonlinear anisotropic diffusive filtering applied to the ocean´s MDT and a new approach based on edge-enhancing diffusion (EED) filtering is presented. EED filters enable controlling the direction and magnitude of the filtering, with subsequent enhancement of computations of the associated surface geostrophic currents (SGCs). Applying this method to a smooth MDT and to a noisy MDT, both for a region in the Northwestern Pacific Ocean, we found that EED filtering provides similar estimation of the current velocities in both cases, whereas a non-linear isotropic filter (the Perona and Malik filter) returns results influenced by local residual noise when a difficult case is tested. We found that EED filtering preserves all the advantages that the Perona and Malik filter have over the standard linear isotropic Gaussian filters. Moreover, EED is shown to be more stable and less influenced by outliers. This suggests that the EED filtering strategy would be preferred given its capabilities in controlling/preserving the SGCs. [ABSTRACT FROM AUTHOR]

We have studied, for the first time, variations in absolute surface geostrophic currents (SGC) using satellite data only. The proposed approach combines 18 years' altimetry data, which provide reliable measurements of absolute sea level (ASL), with a gravity field and steady-state ocean circulation explorer geoid model to obtain dynamic topography, and achieves unprecedented precision and accuracy. Our proposal overcomes the main limitations of existing approaches based solely on altimetry data (which suffer from lack of an independent reference for derivation of ASL maps), and approximations based on in-situ data (which are characterized by a sparse and inhomogeneous coverage in time and space). Features of annual variations of SGC are also addressed. As a result of our study we provide new absolute SGC climatology in the form of a 52-week data set of surface current fields, gridded at quarter degree longitude and latitude resolution and resolving spatial scales as short as 140 km. For presentation, this data set is averaged monthly and the results, presented as monthly climatology, are compared with climatology based on in-situ observations from drifter data. [ABSTRACT FROM AUTHOR]

Large-scale baroclinic instability is investigated as a potential source of Rossby waves and large-scale variability in the ocean. This baroclinic instability is first reviewed in a 2.5-layer model. As already noticed by several authors, the instability arises in westward surface mean flow when the phase velocities of the two vertical modes are made equal by mean flow influence. This large-scale instability is stronger at low latitudes and thus is likely to happen in the westward return flow of the subtropical gyres. Further investigations with a continuous stratification quasigeostrophic model show that the instability is stronger where the mean flow projects negatively on the second baroclinic mode (imposing positive vertical modes at the surface). The linear stability calculation is then performed on Argo-derived mean flow along with mean stratification data. The results show that the unstable regions are situated at low latitudes in every oceanic basin, in western boundary currents, and in some part of the Antarctic Circumpolar Current. The location of these unstable regions is well correlated with the region of negative projection of the mean flow on the second baroclinic mode. Given that the unstable mode growth times are generally smaller than 6 months at low latitudes, these unstable modes are likely to be observable in satellite altimetry. Therefore, results of the present article suggest that the large-scale instability is indeed a source of large-scale variability at low latitudes. [ABSTRACT FROM AUTHOR]

In this paper, a stabilized proper orthogonal decomposition (POD) reduced-order model (ROM) framework is developed for the barotropic vorticity equation. Two different closure ideas are utilized in order to model truncated modes in the ROMs. We apply the POD-ROMs to mid-latitude simplified oceanic basins, which are standard prototypes of more realistic large-scale ocean dynamics. Two closure schemes are used to model the effects of the discarded POD modes: a mode dependent eddy viscosity closure model and a Smagorinsky-type model. A sensitivity analysis with respect to the free eddy viscosity stabilization parameter is performed for various POD-ROMs with different numbers of POD modes. The POD-ROM results are validated against the Munk layer resolving direct numerical simulations using a fully conservative fourth-order Arakawa scheme. A comparison with the standard Galerkin POD-ROM without any stabilization or closure is also included in our investigation. For a four-gyre ocean circulation problem, the new POD-ROM closure models show significant improvements in accuracy over the standard Galerkin model. This first step in the numerical assessment of the POD-ROMs shows that they could represent a computationally efficient tool for large scale oceanic simulations over long time intervals. [ABSTRACT FROM AUTHOR]

An ocean circulation model is run using two different idealized equatorial basin configurations under steady wind forcing. Both model versions produce bands of vertically alternating zonal flow at depth, similar to observed Equatorial Deep Jets (EDJs) and with a time scale corresponding to that of the gravest equatorial basin mode for the dominant baroclinic vertical normal mode. Both model runs show evidence for enhanced variability in the surface signature of the North Equatorial Counter Current (NECC) with the same time scale. We also find the same link between the observed NECC and the EDJs in the Atlantic by comparing the signature of the EDJ in moored zonal velocity data at 23° W on the equator with the signature of the NECC in geostrophic velocities from altimeter data. We argue that the presence of a peak in variability in the NECC associated with the EDJ basin mode period is evidence that the influenceatthis time scale is upward, from the EDJ to the NECC. [ABSTRACT FROM AUTHOR]

The interannual variability of the upper-ocean circulation forced by seasonally varying monsoonal wind is investigated in a two-layer quasigeostrophic (QG) model, with the aim to understand the low-frequency variability of the South China Sea (SCS) circulation. It is demonstrated that the seasonally varying monsoonal wind can force the upper-ocean circulation with significant internal variability, which is mainly associated with the intrinsic nonlinear dynamics of the summer double-gyre system. This arises from the fact that the intrinsic variability, characterized by the Rossby wave adjustment in the winter single-gyre system, is much weaker than that in the summer double-gyre system driven by the intergyre eddy potential vorticity flux through barotropic instability. [ABSTRACT FROM AUTHOR]

The mean flow and meso-scale eddy variability in the region around the Grand Banks of Newfoundland (GBN) are quantified by analyzing surface drifter observations, mean dynamic topography (MDT), along-track satellite altimeter observations, and the solutions of two high-resolution ocean models. By increasing the horizontal resolution from 6.5 km (CREG12) to 2.2 km (GBN36), the modeled mean kinetic energy (MKE) decreases and the eddy kinetic energy (EKE) increases in the study area. GBN36 obtains the MKE of surface geostrophic currents and total currents similar to that derived from MDT and drifter data, whereas CREG12 overestimates these quantities by 40-53 %. CREG12 and GBN36 underestimate the EKE of surface geostrophic currents by 46 and 30 %, respectively, with respect to the EKE derived from along-track altimeter data. The models do not reproduce the strong eddy activity in the Gulf Stream and its downstream region, possibly related to a northward shift of the Gulf Stream position by 0.5° in latitude compared with observations. Both models obtain wavenumber spectra of sea level anomaly in close agreement with the spectrum derived from along-track altimeter data, with a slope of −5 at wavelengths near 100 km on logarithmic spectral density scales. [ABSTRACT FROM AUTHOR]

The west-to-east crossover of boundary currents has been seen in mean circulation schemes from several past models of the Red Sea. This study investigates the mechanisms that produce and control the crossover in an idealized, eddy-resolving numerical model of the Red Sea. The authors also review the observational evidence and derive an analytical estimate for the crossover latitude. The surface buoyancy loss increases northward in the idealized model, and the resultant mean circulation consists of an anticyclonic gyre in the south and a cyclonic gyre in the north. In the midbasin, the northward surface flow crosses from the western boundary to the eastern boundary. Numerical experiments with different parameters indicate that the crossover latitude of the boundary currents changes with f0, β, and the meridional gradient of surface buoyancy forcing. In the analytical estimate, which is based on quasigeostrophic, β-plane dynamics, the crossover is predicted to lie at the latitude where the net potential vorticity advection (including an eddy component) is zero. Various terms in the potential vorticity budget can be estimated using a buoyancy budget, a thermal wind balance, and a parameterization of baroclinic instability. [ABSTRACT FROM AUTHOR]

River plumes are generated by the flow of buoyant river water into the coastal ocean, where they significantly influence water properties and circulation. They comprise dynamically distinct regions spanning a large range of spatial and temporal scales, each contributing to the dilution and transport of freshwater as it is carried away from the source. River plume structure varies greatly among different plume systems, depending on the forcing and geometry of each system. Individual systems may also exhibit markedly different characteristics under varied forcing conditions. Research over the past decade, including a series of major observational efforts, has significantly improved our understanding of the dynamics and mixing processes in these regions. Although these studies have clarified many individual processes, a holistic description of the interaction and relative importance of different mixing and transport processes in river plumes has not yet been realized. [ABSTRACT FROM AUTHOR]

The first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s−1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s−1 and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. These vertical variations are not captured by ocean models presently used for climate studies, but they significantly affect the ventilation of different water masses. [ABSTRACT FROM AUTHOR]

Motivated by the potential of oceanic mesoscale eddies to drive intrinsic low-frequency variability, this paper examines geostrophic turbulence in the frequency-wavenumber domain. Frequency-wavenumber spectra, spectral fluxes, and spectral transfers are computed from an idealized two-layer quasigeostrophic (QG) turbulence model, a realistic high-resolution global ocean general circulation model, and gridded satellite altimeter products. In the idealized QG model, energy in low wavenumbers, arising from nonlinear interactions via the well-known inverse cascade, is associated with energy in low frequencies and vice versa, although not in a simple way. The range of frequencies that are highly energized and engaged in nonlinear transfer is much greater than the range of highly energized and engaged wavenumbers. Low-frequency, low-wavenumber energy is maintained primarily by nonlinearities in the QG model, with forcing and friction playing important but secondary roles. In the high-resolution ocean model, nonlinearities also generally drive kinetic energy to low frequencies as well as to low wavenumbers. Implications for the maintenance of low-frequency oceanic variability are discussed. The cascade of surface kinetic energy to low frequencies that predominates in idealized and realistic models is seen in some regions of the gridded altimeter product, but not in others. Exercises conducted with the general circulation model suggest that the spatial and temporal filtering inherent in the construction of gridded satellite altimeter maps may contribute to the discrepancies between the direction of the frequency cascade in models versus gridded altimeter maps seen in some regions. Of course, another potential reason for the discrepancy is missing physics in the models utilized here. [ABSTRACT FROM AUTHOR]

The changes in the field of the Arabian Sea level anomaly and the geostrophic currents are analyzed based on the data of satellite altimetry measurements in 1993-2008 within the framework of the AVISO project. On the intra-annual scale, the current field generally agrees with the circulation schemes published in a number earlier works. Their differences are due to the occurrence of the mesoscale eddies and jet currents revealed by the authors. [ABSTRACT FROM AUTHOR]