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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]

This study focuses on the determination and validation of a new global mean sea surface (MSS) model, named the Shandong University of Science and Technology 2020 (SDUST2020), with a grid size of 1′×1′. This new model was established with a 19-year moving average method and fused multi-satellite altimetry data over a 27-year period (from January 1993 to December 2019). The data of HaiYang-2A, Jason-3, and Sentinel-3A were first ingested in the SDUST2020 MSS but not in any other global MSS model, such as the CLS15 and DTU18 MSS models. Validations, including comparisons with the CLS15 and DTU18 MSS models, GPS-leveled tide gauges, and altimeter data, were performed to evaluate the quality of the SDUST2020 MSS model, all of which showed that the SDUST2020 MSS model is accurate and reliable. The SDUST2020 MSS dataset is freely available at the site (data DOI: 10.5281/zenodo.6555990, Yuan et al., 2022). [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]

Global ocean surface currents estimated from satellite derived data based on a regular global grid are affected by the grid's shape and placement. Due to different neighbourhood relationships, the rectangular lat/lon grids lose accuracy when interpolating and fitting elevation data. Hexagonal grids have shown to be advantageous due to their isotropic, uniform neighbourhood. Considering these merits, this paper aims to estimate global ocean surface current using a global isotropic hexagonal grid from satellite remote sensing data. First, gridded satellite altimeter data and wind data with different resolutions are interpolated into the centre of the global isotropic hexagonal grid. Then, geostrophic and Ekman currents components are estimated according to the Lagerlof Ocean currents theory. Finally, the inversion results are verified. By analyzing the results, we conclude that the ocean surface currents estimated based on the global isotropic hexagonal grid have considerable accuracy, with improvement over rectangular lat/lon grids. [ABSTRACT FROM AUTHOR]

• Sea surface height and SSH derived geostrophic current from S3A are evaluated. • Measurable accuracy improvements are observed in DD/SAR compared to LRM. • DD/SAR altimetry can resolve small scale current features in the NW Atlantic shelf. In this study, we evaluate Sentinel-3A satellite synthetic aperture radar (SAR) altimeter observations along the Northwest Atlantic coast, spanning the Nova Scotian Shelf, Gulf of Maine, and Mid-Atlantic Bight. Comparisons are made of altimeter sea surface height (SSH) measurements from three different altimeter data processing approaches: fully-focused synthetic aperture radar (FFSAR), un-focused SAR (UFSAR), and conventional low-resolution mode (LRM). Results show that fully-focused SAR data always outperform LRM data and are comparable or slightly better than the nominal un-focused SAR product. SSH measurement noise in both SAR-mode datasets is significantly reduced compared to LRM. FFSAR SSH 20-Hz noise levels, derived from 80-Hz FFSAR data, are lower than 20-Hz UFSAR SSH with 25% noise reduction offshore of 5 km, and 55–70% within 5 km of the coast. The offshore noise improvement is most likely due to the higher native along-track data posting rate (80 Hz for FFSAR, and 20 Hz for UFSAR), while the large coastal improvement indicates an apparent FFSAR data processing advantage approaching the coastlines. FFSAR-derived geostrophic ocean current estimates exhibit the lowest bias and noise when compared to in situ buoy-measured currents. Assessment at short spatial scales of 5–20 km reveals that Sentinel-3A SAR data provide sharper and more realistic measurement of small-scale sea surface slopes associated with expected nearshore coastal currents and small-scale gyre features that are much less well resolved in conventional altimetric LRM data. [ABSTRACT FROM AUTHOR]

Canada ratified the United Nations Convention on the Law of the Sea (UNCLOS) in 2003. With that ratification is an obligation to submit data and information to the U.N. pertaining to the limits of the country's extended continental shelf (ECS); the portion of the juridical continental shelf that extends beyond 200 nautical miles. A team of Canadian scientists, managers, and legal experts that included representation from three Federal Departments (Natural Resources Canada, Fisheries and Oceans Canada, and Global Affairs Canada) with additional support from other departments, spent 13 years compiling and acquiring data to provide the scientific evidence to support delineation of Canada's seaward most maritime limit. The submission has the potential to provide Canada with 2.4 million km2 of additional submarine landmass in the Atlantic and the Arctic oceans over which Canada exercises sovereign rights for the purpose of exploring and exploiting its natural resources. Specific information such as the tectonic framework of the continental margin, the geomorphology of the margin and in particular the continental slope, the geologic nature of adjoined ridges, rises, and plateaux, and sediment thickness within adjacent basins are examples of fundamental pieces of geoscientific information needed to substantiate Canada's outermost maritime limits. This paper highlights a number of segments of Canada's continental margins to showcase this scientific evidence and how it is applied in the UNCLOS context. In doing so, the paper demonstrates the geologic complexity of Canada's margins as illustrated in scientific publications that have resulted from these new data collections, while at the same time presenting new scientific evidence and interpretations. This collection of data and information provides a wealth of new knowledge in Canada's offshore regions. The massive data compilation in the Atlantic led to conception of continental margins, in a source-to-sink scenario, as having an equilibrium base level or graded form, comparable to river systems. Departures from this shape relate to the interplay of sedimentary processes and in particular to those processes that do not fit the source-to-sink paradigm. For example, a significant part of the Atlantic margin is shown to be heavily influenced by along-slope geostrophic currents that generated massive contourite drift deposits. These deposits reflect lateral transport of sediment that had a significant impact on the morphology of the margin. The role of mass transport processes in shaping continental margins is also highlighted, and in particular the collapses of entire segments of the margin were observed. The prominent role mass failure processes play in delivering sediment to the adjacent abyssal plain is also critical in the ECS context. These observations challenge the entrenched notion of a continental margin comprising a shelf, slope, and rise and in particular the concept of the "continental rise". Prior to 2006, regions of the Arctic Ocean seaward of the Canadian landmass had fewer than 5000 km of seismic reflection data. The massive efforts of Arctic coastal States to map their margins for ECS purposes have led to a leap in technological advances to acquire data in ice-covered seas and have led to a wealth of new geoscientific knowledge. Perhaps foremost amongst this knowledge is demonstration that Canada Basin is indeed a fully developed ocean basin, albeit significantly infilled with sediment. Based on this knowledge and identification of related structures, new realistic tectonic scenarios for opening of the Amerasia Basin are proposed that include a significant component of transform or strike-slip motions. With seismic velocity and rock sample information, the continental nature of Alpha and Mendeleev ridges has been substantiated. Even bathymetric data were lacking in the Arctic and new editions of seafloor maps now support grids of 500 m spacing; although some regions remain sparse. Once thought to be relatively stagnant, sedimentary processes such as found in many ocean basins were discovered in the Arctic Ocean. Evidence of geostrophic currents, sediment mass failures, and deep-sea turbidity current channels were found to be ubiquitous, even in the deepest parts of the Arctic's basins. [ABSTRACT FROM AUTHOR]

Purpose. The aim of the work is to clarify the spatial structure of the climatic dynamic fronts (geostrophic current jets) and to estimate the relationship between their position and that of the large-scale temperature fronts on the surface of the Scotia Sea and the adjacent water areas in the southwestern part of the Atlantic sector of Antarctica. Methods and Results. The daily averaged data arrays of the CMEMS (1993–2017) and NOAA OI SST (1982–2017) reanalysis at the regular 0.25° grid were used. The CMEMS reanalysis contains the sea surface geostrophic velocity values, the NOAA OI SST reanalysis – the sea surface temperature ones which were reduced to the climatic form through their averaging for each month of the corresponding periods. Position of the current jets and the temperature fronts was determined using the maximums of the geostrophic velocity components and the extremes of the temperature horizontal gradients. The updated scheme of the average long-term position of dynamic fronts was constructed. It shows that in the areas of the most pronounced bottom topography inhomogeneities (the northern boundary of the Falkland Plateau and the Tierra del Fuego shelf, the boundaries of the Falkland Islands shelf and the Birdwood Bank, the Shackleton Ridge and the South Shetland Islands shelf), the fronts do not change their latitudinal position during a year. It is revealed that in most of the water area, the temperature horizontal gradient extremes (temperature fronts) correspond to the geostrophic velocity maximums (dynamic fronts). The Northern and Central Branches of the Antarctic Circumpolar Current are most clearly manifested in the temperature field. In general, in the water area under study, the average annual latitudinal position of the Subantarctic and Antarctic Polar Fronts is displaced to the south relative to the position of the Northern and Central branches jets of the Antarctic Circumpolar Current by 0.25–0.5° and 0.25–1°, respectively. Conclusions. It is shown that, being influenced by the bottom topography, the large-scale jets of geostrophic currents form intense topographic meanders and recirculation branches which are stably manifested on the climatic scale. The Antarctic Circumpolar Current branches being affected by the bottom topography, can merge forming the joint flows, and then diverge forming a system of separate jets again. It is found that the main spatial features of frontal structure in the geostrophic velocities field persist throughout the whole year and are conditioned mainly by the bottom topography. The most of the dynamic fronts are shown to be clearly pronounced in the temperature field on the surface during a year. A high level of linear correlation between the positions of temperature fronts and current jets was revealed; the correlation coefficient values are 0.6–0.97.

Determining an accurate picture of ocean currents is an important societal challenge for oceanographers, aiding our understanding of the vital role currents play in regulating Earth’s climate, and in the dispersal of marine species and pollutants, including microplastics. The geodetic approach, which combines satellite observations of sea level and Earth’s gravity, offers the only means to estimate the dominant geostrophic component of these currents globally. Unfortunately, however, geodetically-determined geostrophic currents suffer from high levels of contamination in the form of geodetic noise. Conventional approaches use isotropic spatial filters to improve the signal-to-noise ratio, though this results in high levels of attenuation. Hence, the use of deep learning to improve the geodetic determination of the ocean currents is investigated. Supervised machine learning typically requires clean targets from which to learn. However, such targets do not exist in this case. Therefore, a training dataset is generated by substituting clean targets with naturally smooth climate model data and generative machine learning networks are employed to replicate geodetic noise, providing noisy input and clean target pairs. Prior knowledge of the geodetic noise is exploited to develop a more realistic training dataset. A convolutional denoising autoencoder (CDAE) is then trained on these pairs. The trained CDAE model is then applied to unseen real geodetic ocean currents. It is demonstrated that our method outperforms conventional isotropic filtering in a case study of four key regions: the Gulf Stream, the Kuroshio Current, the Agulhas Current, and the Brazil-Malvinas Confluence Zone.

The variation of boundary layer circulation caused by the influence of complex underlying surface is one of the reasons why it is difficult to forecast hourly heavy rainfall (HHR) in the middle Yangtze River Valley (YRV). Based on the statistics of high-resolution observation data, it is found that the low resolution data underestimate the frequency of HHR in the mountain that are between the twain-lake basins in the middle YRV (TLB-YRV). The HHR frequency of mountainous area in the TLB-YRV is much higher than that of Dongting Lake on its left and is equivalent to the HHR frequency of Poyang Lake on its right. The hourly reanalysis data of ERA5 were used to study the variation of boundary layer circulation when HHR occurred. It can be found that the boundary layer circulation corresponding to different underlying surfaces changed under the influence of the weather system. Firstly, the strengthening of the weather system in the early morning resulted in the strengthening of the southwest low-level air flow, which intensified the uplift of the windward slope air flow on the west and south slopes of the mountainous areas in the TLB-YRV. As a result, the sunrise HHR gradually increases from the foot of the mountain. The high-frequency HHR period of sunrise occurs when the supergeostrophic effect is weakened, the low-level vorticity and frontal forcing are strengthened, and the water vapor flux convergence begins to weaken. Secondly, the high-frequency HHR period of the sunset is caused by stronger local uplift and more unstable atmospheric stratification, but the enhanced local uplift is caused by the coupling of the terrain forcing of the underlying surface and the enhanced northern subgeostrophic flow, which causes the HHR to start closer to the mountain top at sunset than at sunrise. [ABSTRACT FROM AUTHOR]

We describe an idealized primitive-equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable. The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography. The model is perfectly adiabatic and spans the Equator and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally include diabatic processes and buoyancy forcing. We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence. [ABSTRACT FROM AUTHOR]

The newly developed fast Earth system model CLIMBER-X is presented. The climate component of CLIMBER-X consists of a 2.5-D semi-empirical statistical–dynamical atmosphere model, a 3-D frictional–geostrophic ocean model, a dynamic–thermodynamic sea ice model and a land surface model. All the model components are discretized on a regular lat–long grid with a horizontal resolution of 5∘×5∘. The model has a throughput of ∼ 10 000 simulation years per day on a single node with 16 CPUs on a high-performance computer and is designed to simulate the evolution of the Earth system on temporal scales ranging from decades to >100000 years. A comprehensive evaluation of the model performance for the present day and the historical period shows that CLIMBER-X is capable of realistically reproducing many observed climate characteristics, with results that generally lie within the range of state-of-the-art general circulation models. The analysis of model performance is complemented by a thorough assessment of climate feedbacks and model sensitivities to changes in external forcings and boundary conditions. Limitations and applicability of the model are critically discussed. CLIMBER-X also includes a detailed representation of the global carbon cycle and is coupled to an ice sheet model, which will be described in separate papers. CLIMBER-X is available as open-source code and is expected to be a useful tool for studying past climate changes and for the investigation of the long-term future evolution of the climate. [ABSTRACT FROM AUTHOR]

Filtering methods are usually used to combine the mean sea surface (MSS) and geoid (computable by global geopotential model (GGM)) into a common subspace, to model mean dynamic topography (MDT), which may lead to signal leakage and distortion problems. The use of the rigorous least squares (LS) method and multivariate objective analysis (MOA) alleviates these problems, and the derived MDTs from these two methods show better performance than MDTs derived from filtering methods. However, the advantages and disadvantages of these two methods have not been evaluated, and no direct comparison has yet been conducted between these two approaches regarding the performances in MDT recovery. In this study, we compare the performances of the MOA method with the LS method, providing information with respect to the usability of different methods in MDT modeling over regions with heterogeneous ocean states and hydrological conditions. We combined a recently published mean sea surface called DTU21MSS, and a satellite-only GGM named GO_CONS_GCF_2_DIR_R6, for MDT computation over four typical study areas. The results showed that the MDTs derived from the LS method outperformed the MOA method, especially over coastal regions and ocean current areas. The root mean square (RMS) of the discrepancies between the LS-derived MDT and the ocean reanalysis data was lower than the RMS of the discrepancies computed from the MOA method, by a magnitude of 1–2 cm. The formal error of the MDT estimated by the LS method was more reasonable than that derived from the MOA method. Moreover, the geostrophic velocities calculated by the LS-derived MDT were more consistent with buoy data than those calculated by the MOA-derived solution, by a magnitude of approximately 1 cm/s. The reason can be attributed to the fact that the LS method forms the design matrix segmentally, based on the error characteristics of the GGM, and suppresses high-frequency noise by applying constraints in different frequency bands, which improves the quality of the computed MDT. Our studies highlight the superiority of the LS-derived method versus the MOA method in MDT modeling. [ABSTRACT FROM AUTHOR]

The article presents the first evidence of a dual kinetic energy (KE) cascade in the ocean by analyzing in situ velocity measurements from surface drifters. Topics discussed include seasonal modulation of surface KE, seasonality of interscale KE transfers, and statistical metrics and error estimates.

In the Arabian Sea, southeast of the Arabian peninsula, an oceanic dipole, named the Ras Al Hadd (RAH) dipole, is formed each year, lying near the Ras Al Hadd cape. The RAH dipole is the association of a cyclonic eddy (CE) to the northeast, with an anticyclonic eddy (AE) to the southwest. This dipole intensifies in the summer monsoon and disappears during the winter monsoon. This dipole has been described previously, but mostly for its surface expression, and for short time intervals. Here, we describe the 3D structure of this dipole over the 2000–2015 period, by combining colocalized ARGO float profiler data (a total of 7552 profiles inside and outside the RAH dipole) with angular momentum eddy detection and tracking algorithm (AMEDA) surface data. We show first the different water masses in and near the RAH dipole. The presence of the Persian Gulf water (PGW) below 200 m depth is confirmed in both eddies. Arabian Sea high salinity water (ASHSW) is found exclusively in the AE; a layer of fresh and cold water is observed above 100 m depth in both eddies. By analyzing the potential density structures, we show that the CE has a surface-intensified structure while the AE is subsurface-intensified. The sea level anomaly shows a 0.04 m elevation above the AE and a 0.2 m depression over the CE. The CE has a faster geostrophic velocity, (vertical velocity, respectively) 0.6 m s−1 than the AE, 0.15 m s−1 (respectively, 3 m day−1 for the CE and 0.6 m day−1 for the AE). After presenting the vertical structure of the dipole, we show the dominance of the nonlinear Ekman pumping in the CE over the linear pumping affecting the dipole. As a consequence, we explain the CE's longer lifetime by its intensity and shallowness, and by its sensitivity to the interaction with the atmosphere (in particular the wind stress) and with neighboring eddies. We examined the possible (co)existence of symmetric, barotropic, and baroclinic instabilities in both eddies. These instabilities coexist near the surface in both eddies. They are intensified for the CE, which suggests that the CE is unstable and the AE is rather stable or may need a long time to be unstable. [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]

Six oceanographic cruises in a NE-SW transect were made nearshore of southern Sinaloa and Nayarit from March 2006 through May 2008, where no in situ hydrographic data are available. Applying the Thermodynamic Equation of Seawater 2010 (TEOS-10) to the observations, the hydrography and geostrophic currents of the region were characterized. Results indicate that surface variability (0-50 m) emerged mainly from seasonal atmospheric forcing. A relative salinity maximum was present during all cruises below this surface layer, which is attributed to a water mass intrusion of Subtropical Subsurface Water that could be associated with the Mexican Coastal Current. Another water mass intrusion is from the California Current. Samples from the 2007-2008 La Niña produced an uncommon circulation, where water flowing from the Gulf of California along the coast of Sinaloa was observed, opposite to what is commonly known as a mean circulation. This uncommon circulation matches the generation of anticyclonic eddies around the Islas Marias archipelago. [ABSTRACT FROM AUTHOR]

Based on the observational data of wind, current, and sea surface temperature in the Jiazi sea area of the northeastern South China Sea in 2018 and the satellite remote sensing data of sea surface temperature in the northern South China Sea, this paper explores the formation reasons and evolution law of winter counter-wind currents in the Jiazi sea area of the northeastern South China Sea. The results show that: (1) The counter-wind current in the Jiazi sea occurs only in certain time periods instead of the entire winter; (2) When the eastern component of wind stress weakens, the eastern component of seafloor friction also weakens to some extent. A high-frequency northeast current often occurs in the bottom layer of the sea area, indicating that the formation of winter counter-wind current in the Jiazi sea area is a result of the concerted action of wind stress, the baroclinic effect, and geostrophic effect; (3) When the counter-wind current is formed, there is a low temperature water mass in the northwest of Jiazi and a high-temperature water mass in the southeast. The baroclinic effect causes the sea water to flow to the shore and produce a westward flow on the shore, and northeastward counter-wind current occurs on convergent sea water on the shore due to the baroclinic effect and geostrophic effect (Ekman effect). Therefore, two different current systems are formed in the northeastern South China Sea in winter with 116° E as the boundary. The appearance of cold water masses in the northwest of 116° E sea area and warm water masses in the southeast of the South China Sea is the key to the formation of both the two different current systems with 116° E as the boundary and the winter counter-wind flow; (4) The formation and disappearance of the counter-wind current can be divided into four stages: in the first stage, the northeast monsoon gradually relaxes to become the southeast wind, forming the northwest current; in the second stage, the warm water masses on the west side of the Luzon Strait flow to the coastal waters due to the northwest current, forming a significant onshore pressure gradient force; in the third stage, a high-temperature seawater convergence zone is formed in the Jiazi sea area, forming southwest and northeast pressure gradient forces, and the northwest coastal current forms a counter-wind current under the combined action of pressure gradient force and geostrophic effect; in the fourth stage, the northeast monsoon intensifies and the counter-wind current weakens gradually until it disappears, and the sea water flows to the southwest again. [ABSTRACT FROM AUTHOR]

This article focuses on the determination and the validation of a new global mean sea surface (MSS) model which is named as SDUST2020 (Shandong University of Science and Technology 2020) with a grid of 1'×1'. This new model is established with 19-year moving average method and merges multi-satellite altimetry data over 27-year (from January 1993 to December 2019). The data of HY-2A, Jason-3 and Sentinel-3A are first ingested in the SDUST2020 MSS, but not in any other global MSS model, such as CLS15 and DTU18 MSS models. Validations, including the comparison with CLS15 and DTU18 MSS models, the comparison with GPS-leveled tide gauges and the comparison with altimeter data, are performed to estimate the quality of the SDUST2020 MSS model, and all the results show that the SDUST2020 MSS model is accurate and reliable. The SDUST2020 MSS dataset are freely available at the site (data DOI: https://doi.org/10.5281/zenodo.6555990, Yuan et al., 2022). [ABSTRACT FROM AUTHOR]

Ocean spiciness is referred to as density-compensated temperature and salinity variations with warm (cool) and salty (fresh) waters having high (low) spiciness, respectively. The structure and evolution of density-compensated (warm/salty or cool/fresh) spiciness anomalies are investigated in the North Pacific thermocline using Argo observations during the period 2004–20. Two well-organized decadal spiciness events are identified through isopycnal surface analyses. One warm/salty spiciness anomaly of about 0.15°C and 0.05 g kg−1 temperature and salinity perturbations on the 25 kg m−3 isopycnal surface appeared in the eastern subtropical North Pacific at (18°–30°N, 120°–150°W) in 2007, which then migrated southwestward along the mean circulation and arrived in the western tropics at (∼15°N, 145°E–175°W) in 2012–13, with the reduced salinity perturbation of about 0.043 g kg−1. Another cool/fresh spiciness anomaly of about −0.2°C and −0.07 g kg−1 temperature and salinity perturbations originated from the eastern subtropics at (120°–150°W, ∼30°N) in 2014 and followed a similar advective pathway during the period from 2014–15 to 2019–20. The subduction pathway can be adequately determined by the mean Montgomery stream function on the isopycnal surface; the propagation direction and speed are in good agreement with the expectation for the role played by advection due to the mean geostrophic current. Moreover, the subducted decadal spiciness anomalies can have negative feedback on sea surface temperature (SST) variability in the western tropical Pacific through the diapycnal processes. The identifications of these density-compensated spiciness anomalies and their propagation pathways provide a clear illustration of the oceanic extratropics-tropics interactions in the North Pacific Ocean. [ABSTRACT FROM AUTHOR]

This data set was established from two open sources: (a) geoid undulation (N) from EIGEN‐6C4, which is a static global combined gravity field model up to degree and order 2,190, and (b) 'World ocean geostrophic velocity inverted from World Ocean Atlas 2013 with the P‐vector method' (NCEI accession 0,121,576). With the given non‐positive values of N in the oceans, absolute geostrophic currents (u, v) are easily obtained on N with 1°×1° resolution from the second data set except the equatorial zone (5oS – 5oN) due to the non‐existence of the geostrophic balance. Altogether, the data set contains 15,868 (u, v) data pairs. The data set shows that the hypothetical situation that there would be no flow on the geoid does not exist. [ABSTRACT FROM AUTHOR]

This study aims to advance our knowledge in the genesis of extreme climatic events with the dual aim of improving forecasting methods while clarifying the role played by anthropogenic warming. Wavelet analysis is used to highlight the role of coherent Sea Surface Temperature (SST) anomalies produced from short-period oceanic Rossby waves resonantly forced, with two case studies: a Marine Heatwave (MHW) that occurred in the northwestern Pacific with a strong climatic impact in Japan, and an extreme flood event that occurred in Germany. Ocean–atmosphere interactions are evidenced by decomposing state variables into period bands within the cross-wavelet power spectra, namely SST, Sea Surface Height (SSH), and the zonal and meridional modulated geostrophic currents as well as precipitation height, i.e., the thickness of the layer of water produced during a day, with regard to subtropical cyclones. The bands are chosen according to the different harmonic modes of the oceanic Rossby waves. In each period band, the joint analysis of the amplitude and the phase of the state variables allow the estimation of the regionalized intensity of anomalies versus their time lag in relation to the date of occurrence of the extreme event. Regarding MHWs in the northwestern Pacific, it is shown how a warm SST anomaly associated with the northward component of the wind resulting from the low-pression system induces an SST response to latent and sensible heat transfer where the latitudinal SST gradient is steep. The SST anomaly is then shifted to the north as the phase becomes homogenized. As for subtropical cyclones, extreme events are the culmination of exceptional circumstances, some of which are foreseeable due to their relatively long maturation time. This is particularly the case of ocean–atmosphere interactions leading to the homogenization of the phase of SST anomalies that can potentially contribute to the supply of low-pressure systems. The same goes for the coalescence of distinct low-pressure systems during cyclogenesis. Some avenues are developed with the aim of better understanding how anthropogenic warming can modify certain key mechanisms in the evolution of those dynamic systems leading to extreme events. [ABSTRACT FROM AUTHOR]

At the sub-Antarctic Prince Edward Islands (PEIs) in the Southern Ocean, the Island Mass Effect (IME) plays an important role in maintaining an ecosystem able to support diverse biological communities; however, limited in situ sampling has severely constrained our understanding of it. As such, our study used satellite chlorophyll a (chla) to provide the first detailed characterisation of the spatial extent and seasonal variability of the IME at the PEIs. Seasonal surface chla variations were remarkable, with localised increases observed from mid-austral spring to the end of autumn (October to May). In contrast, during June to September, there were no distinguishable differences between chla at the PEIs and that further afield. Seasonal chla changes were significantly correlated with higher light levels, warmer waters, and shallow upper mixed layer depths reflecting enhanced water column stability during summer and autumn, with the opposite pattern in winter and spring. The IME extended northeast of the islands and remained spatially distinct from elevated chla around the northern branch of the sub-Antarctic Front and the southern branch of the Antarctic Polar Front. From December to February, the IME was spatially connected to the island shelf. In contrast, during March–May and in October, higher chla was observed only to the northeast, some distance away from the islands, suggesting a delayed IME, which has not previously been observed at the PEIs. The clear association of this higher chla with the weak mean geostrophic circulation northeast of the islands suggested retention and accumulation of nutrients and phytoplankton biomass, which was likely aided by wind-driven northeastward transport of water from the shelf. Climatological mean chla to the northeast was generally higher than that on the PEI shelf, and further research is required to determine the importance of this region to ecosystem functioning at the islands. [ABSTRACT FROM AUTHOR]

Circulation plays an essential role in the creation of physical and biogeochemical fluxes in the Baltic Sea. The main aim of the work was to study the quasi-steady circulation patterns under prevailing forcing conditions. A total of 6 months of continuous vertical profiling and fixed-point measurements of currents, two month-long underwater glider surveys, and numerical modeling were applied in the central Baltic Sea. The vertical structure of currents was strongly linked to the location of the two pycnoclines: the seasonal thermocline and the halocline. The vertical movements of pycnoclines and velocity shear maxima were synchronous. The quasi-steady circulation patterns were in geostrophic balance and highly persistent. The persistent patterns included circulation features such as upwelling, downwelling, and boundary currents, as well as a sub-halocline gravity current. The patterns had a prevailing zonal scale of 5–60 km as well as considerably higher magnitude and different direction than the long-term mean circulation pattern. A northward (southward) geostrophic boundary current in the upper layer was observed along the eastern coast of the central Baltic in the case of southwesterly (northerly) wind. The geostrophic current at the boundary was often a consequence of wind-driven, across-shore advection. The sub-halocline quasi-permanent gravity current with a width of 10–30 km from the Gotland Deep to the north over the narrow sill separating the Fårö Deep and Nothern Deep was detected in the simulation, and it was confirmed by an Argo float trajectory. According to the simulation, a strong flow, mostly to the north, with a zonal scale of 5 km occurred at the sill. This current is an important deeper limb of the overturning circulation of the Baltic Sea. The current was stronger with northerly winds and restricted by the southwesterly winds. The circulation regime had an annual cycle due to seasonality in the forcing. The boundary current was stronger and more frequent northward during the winter period. The sub-halocline current towards the north was strongest in March–May and weakest in November–December. [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.

Satellite altimetry missions flying over the ice-covered Arctic opened the possibility to further understand changes in the ocean beneath the sea ice. This requires complex processing of satellite signals emerging from leads within the sea ice, with efforts to generate consistent Arctic-wide datasets 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 one month 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 and in-situ data. We find that our dataset is well correlated with independent satellite data at monthly time scales and agrees with in-situ observed variability at seasonal to interannual time scales. Our geostrophic velocity fields can resolve the variability of boundary currents wider than about 50 km. Large scale seasonal features emerge: a wintertime Arctic-wide maximum in sea surface height, with the highest amplitude over the shelves, and 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. [ABSTRACT FROM AUTHOR]

The large discrepancy between Eulerian and Lagrangian work motivates us to examine the leakage of Eulerian eddies and quantify the contribution of coherent eddy transport in the South China Sea (SCS). In this study, Lagrangian particles with a resolution of 1/32° are advected by surface geostrophic currents derived from satellite observations spanning 23 years, and two types of methods are employed to identify sea surface height (SSH) eddies and Lagrangian coherent structures. SSH eddies are proven to be highly leaky during their lifetimes, with more than 80% of the original water leaking out of the eddy interior. As a result of zonal and meridional eddy propagation, the leaked water exhibits a spatial pattern of asymmetry relative to the eddy center. The degree of eddy leakage is found to be independent of several eddy parameters including the nonlinearity parameter U/c, which has been commonly used to assess eddy coherency. Finally, the Lagrangian coherent structures in the SCS are diagnosed and the associated coherent eddy diffusivity is calculated. It is found that coherent eddies contribute to less than 5% of the total eddy material transport in both zonal and meridional directions. These findings suggest that previous studies based on the Eulerian framework significantly overestimate the contribution of coherent eddy transport in the SCS. [ABSTRACT FROM AUTHOR]

The mechanism of convection initiation (CI) over Ili River Valley (IRV) in Xinjiang, Northwest China, was investigated based on both weather stations and radar observations and Weather Research and Forecasting (WRF) model simulation. Observations showed that many CIs occurred repeatedly, and most of them underwent significant intensification both in size and intensity, and eventually formed an extreme rainfall-producing mesoscale convective system (MCS) in the IRV. Besides, there was a relatively strong boundary layer westerly jet (BLWJJ, horizontal wind speed exceeding 12 m s−1) along with significant cold advection. The overall features of the CIs and associated MCS are generally well reproduced by the WRF simulation. Simulation results showed that there were some horizontal convective rolls (HCRs) developed ahead of the BLWJ and generated significant convergence (up to ~2 × 10−3 s−1) in the low level that further induced intense updraft aloft (vertical velocity exceeding 3.5 m s−1) which finally resulted in the CIs. Further investigations indicated that, as the main contributor to the HCRs, the BLWJ was generated due to the funneling effect when the descending westerlies entered the middle reaches of the IRV. A qualitative analysis based on the quasi-geostrophic omega equation revealed that the descending flow in mid- to lower troposphere in the middle reaches of the IRV was mainly contributed by the vorticity advection, while the descending in the lower troposphere was dominated by both vorticity advection and cold advection. [ABSTRACT FROM AUTHOR]

A previous study proposed an adaptive observation error inflation (AOEI) method for an ensemble Kalman filter-based atmospheric data assimilation system to assimilate all-sky infrared brightness temperatures. Brightness temperature differences between clear- and cloudy-sky radiances are large, and observation-minus-forecast differences or innovations are therefore likely to be large around boundaries between clear- and cloudy-sky regions. The AOEI method mitigates these discrepancies by adaptively inflating observation errors. Ocean frontal regions have similar characteristics to the borders between clear- and cloudy-sky regions with large innovations. Consequently, we have implemented the AOEI with an EnKF-based regional ocean data assimilation system, in which the assimilation interval is set to 1 day to utilize frequent satellite observations. We conducted sensitivity experiments to investigate the impacts of the AOEI on salinity structure, geostrophic balance, and accuracy. A control run, in which the AOEI is not applied, shows the degradation of low-salinity North Pacific Intermediate Water around the Kuroshio Extension region, where the innovation amplitude and forecast ensemble spread are large in association with the fronts and eddies. The resulting large temperature and salinity increments weaken the density stratification, leading to large vertical diffusivity. As a result, the low salinity water in the intermediate layer is lost through strong vertical diffusion. When the AOEI is used, the salinity structure in the ocean interior is preserved because the AOEI suppresses the salinity degradation by reducing the temperature and salinity increments. We also demonstrate that the AOEI provides significant improvement of the geostrophic balance and the accuracy of temperature, salinity, and surface flow fields. [ABSTRACT FROM AUTHOR]

The shelf-edge and continental slope current off the west coast of Canada has been monitored at a site off West Vancouver Island since 1985. However, observations at this site may not represent the characteristics of the shelf-edge and slope current off the entire west coast of Canada. Here, we use along-track satellite altimetry data over six transects to investigate the characteristics of the surface geostrophic currents over the shelf edge and continental slope off the west coast of Canada from 1992 to 2020. It is shown that along-track satellite altimetry is well suited for monitoring longshore and climatic variations of the near-surface shelf-edge and slope currents off the west coast of Canada. It is found that the surface current over the shelf edge and slope has different features from the south to the north. While the surface current is poleward in winter and equatorward in summer off South Vancouver Island, it is poleward year-round off the rest of the west coast of Canada. The seasonal current anomalies show longshore correlation significant at the 95% confidence level, except at the North Haida Gwaii transect. The first empirical orthogonal function mode of the seasonal current anomalies is correlated with the longshore wind anomalies both off South Vancouver Island and off Oregon. However, this first mode is not correlated with either the Niño 3.4 index or the Pacific Decadal Oscillation index, though they often show large episodic events during strong El Niño and La Niña years. Consistent with previous findings, the present study indicates that the surface currents over the shelf edge and continental slope off the west coast of Canada are related to regional and remote longshore wind forcing. [ABSTRACT FROM AUTHOR]

In the Southern Ocean, the sub-Antarctic Prince Edward Islands (PEIs) play a significant ecological role by hosting large populations of seasonally breeding marine mammals and seabirds, which are particularly sensitive to changes in the surrounding ocean environment. In order to better understand climate variability at the PEIs, this study used satellite and reanalysis data to examine the interannual variability and longer-term trends of Sea Surface Temperature (SST), wind forcing, and surface circulation. Long-term trends were mostly weak and statistically insignificant, possibly due to the restricted length of the data products. While seasonal fluctuations accounted for a substantial portion (50–70%) of SST variability, the strongest variance in wind speed, wind stress curl (WSC), and currents occurred at intra-annual time scales. At a period of about 1 year, SST and geostrophic current variability suggested some influence of the Southern Annular Mode, but correlations were weak and insignificant. Similarly, correlations with El Niño Southern Oscillation variability were also weak and mostly insignificant, probably due to strong local and regional modification of SST, wind, and current anomalies. Significant interannual and decadal-scale variability in SST, WSC, and geostrophic currents, strongest at periods of 3–4 and 7–8 years, corresponded with the variability of the Antarctic Circumpolar Wave. At decadal time scales, there was a strong inverse relationship between SST and geostrophic currents and between SST and wind speed. Warmer-than-usual SST between 1990–2001 and 2009–2020 was related to weaker currents and wind, while cooler-than-usual periods during 1982–1990 and 2001–2009 were associated with relatively stronger winds and currents. Positioned directly in the path of passing atmospheric low-pressure systems and the Antarctic Circumpolar Current, the PEIs experience substantial local and regional atmospheric and oceanic variability at shorter temporal scales, which likely mutes longer-term variations that have been observed elsewhere in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Oceanic influences on shelf seas are mediated by flow along and across continental slopes, with consequences for regional hydrography and ecosystems. Here we present evidence for the variable North Atlantic influence on European shelf seas over the last 4 decades using ocean analysis and reanalysis products, as well as an eddy-resolving ocean model hindcast. To first order, flows oriented along isobaths at the continental slope are related to the poleward increase in density in the adjacent deep ocean that supports a geostrophic inflow towards the slope. In the North Atlantic, this density gradient and associated inflow have undergone substantial, sometimes abrupt, changes in recent decades. Inflow in the range 10–15 Sv is identified with eastward transport in temperature classes at 30 ∘ W in the latitude range 45–60 ∘ N. Associated with major subpolar warming around 1997, a cool and fresh branch of the Atlantic inflow was substantially reduced, while a warm and more saline inflow branch strengthened, with respective changes of the order of 5 Sv. Total inflow fell from ∼ 15 Sv pre-1997 to ∼ 10 Sv post-1997. In the model hindcast, particle tracking is used to trace the origins of poleward flows along the continental slope to the west of Ireland and Scotland before and after 1997. Backtracking particles up to 4 years, a range of subtropical and subpolar pathways is identified from a statistical perspective. In broad terms, cold, fresh waters of subpolar provenance were replaced by warm, saline waters of subtropical provenance. These changes have major implications for the downstream shelf regions that are strongly influenced by Atlantic inflow, in particular the northern North Sea, where "subtropicalization" of ecosystems has already been observed since the late 1990s. [ABSTRACT FROM AUTHOR]

This study proposes an approach to estimate the ocean sea surface height signature of coherent internal tides from a 25-year along-track altimetry record, with a single inversion over time, resolving both internal tide contributions and mesoscale eddy variability. The inversion is performed on a reduced-order basis of topography and practically achieved with a conjugate gradient. The particularity of this approach is to mitigate the potential aliasing effects between mesoscales and internal tide estimation from the uneven altimetry sampling (observing the sum of these components) by accounting for their statistics simultaneously, while other methods generally use a prior mesoscale. The four major tidal components are considered (M2, K1, S2, O1) over the period 1992–2017 on a global configuration. From the solution, we use altimetry data after 2017 for independent validation in order to evaluate the performance of the simultaneous inversion and compare it with an existing model. [ABSTRACT FROM AUTHOR]

To investigate the South Equatorial Current (SEC) in the Indian Ocean, 18 drifters were deployed off Sumatra‐Java during boreal spring 2019. These drifter observations revealed a large SEC meander that swung southward about 1,100 km during 40 days along ∼100°E. The southward motion occurred on the eastern edge of the Seychelles‐Chagos thermocline ridge, accompanied by an anticyclonic eddy/Rossby wave from the east, indicated by the geostrophic velocity induced by the zonal pressure gradient. Meanwhile, the ageostrophic velocity induced by the southeasterly wind also contributes to this southward motion. This is verified by a Lagrangian Trajectory Model in which drifter trajectories are well reproduced using Chinese‐French Oceanic SATellite winds products. Further analysis of the historical drifters with similar trajectories reveals that the geostrophic and ageostrophic currents jointly drive drifters to move southward. Ageostrophic component could contribute up to 45% of the southward movement of the SEC meander. Plain Language Summary: The South Equatorial Current (SEC) is westward in the tropical Indian Ocean. During boreal spring 2019, 18 drifters were deployed off Sumatra‐Java, and their trajectories show a large SEC meander, with a typical drifter moved southward over 1,100 km during 40 days along ∼100°E. The geostrophic component, associated with the Seychelles‐Chagos thermocline ridge and an anticyclonic eddy/Rossby wave, and ageostrophic component, induced by southeasterly wind, jointly drive drifters to move southward. Further diagnoses of historical drifters show that the ageostrophic component could take up to 45% contribution in the southward movement in the SEC meander. Key Points: South Equatorial Current in the Indian Ocean advected drifters southward more than 1,100 km within 40 days (∼32 cm s−1) in 2019Combined effects of the zonal pressure gradient and the southeasterly wind contribute to the southward movementSouthward movement usually occurs in spring and summer, contributing from ageostrophic current up to ∼45% [ABSTRACT FROM AUTHOR]

Satellite and in‐situ measurements are used in this study to investigate a possible link between the Florida Current transport (FCT) and temperature changes in the equatorial Pacific. Consistent with the geostrophic nature of the current, the FCT variability shows good correspondence with the changes in sea surface height differences (∆SSH) between the eastern (SSHeast) and western (SSHwest) sides of the Florida Straits. While the variability of SSHwest is mostly associated with seasonal and shorter timescale fluctuations of ∆SSH and FCT, changes in SSHeast are strongly related to the interannual variability of ∆SSH and FCT. A significant correlation is found between the FCT and the Oceanic Niño Index (ONI) on interannual timescales, which explains 21% of the interannual FCT variance. The connection of ONI with FCT is through its impact on SSHeast, associated with the anomalous convergence/divergence in the Caribbean region and the Bahamas forced by ONI‐induced wind stress curl changes. Plain Language Summary: Variations in the Florida Current transport (FCT) have been linked to widespread weather and climate phenomena. Various driving mechanisms for the FCT variability have been proposed, but none can fully explain its wide spectrum variability. In this study, we analyzed in‐situ and satellite observations and found a linear link between the FCT and temperature changes in the equatorial Pacific on interannual timescales. A warming condition in the equatorial Pacific results in low pressure anomalies in the Gulf of Mexico and high pressure anomalies extended into the Caribbean Sea from the tropical Atlantic. This atmospheric pressure pattern is associated with anticyclonic winds over the Caribbean Sea and the Bahamas that drive anomalous oceanic convergence and, therefore, cause higher sea levels in those regions. A higher sea level near the Bahamas often steepens the sea level slope across the Florida Straits, which corresponds to a stronger FCT. A cooling condition in the equatorial Pacific will have the opposite effect, resulting in a reduced FCT. Key Points: Interannual variations in Florida Current transport are highly correlated with sea level at eastern side of the Florida Straits (SSHeast)Changes in SSHeast exhibit a linear link to warming and cooling conditions in the equatorial PacificThis connection is through the wind stress curl changes remotely forced from the equatorial Pacific [ABSTRACT FROM AUTHOR]

Free-drift estimates of sea ice motion are necessary to produce a seamless observational record combining buoy and satellite-derived sea ice motion vectors. We develop a new parameterization for the free drift of sea ice based on wind forcing, wind turning angle, sea ice state variables (thickness and concentration), and estimates of the ocean currents. Given the fact that the spatial distribution of the wind–ice–ocean transfer coefficient has a similar structure to that of the spatial distribution of sea ice thickness, we take the standard free-drift equation and introduce a wind–ice–ocean transfer coefficient that scales linearly with ice thickness. Results show a mean bias error of -0.5 cms-1 (low-speed bias) and a root-mean-square error of 5.1 cms-1 , considering daily buoy drift data as truth. This represents a 35 % reduction of the error on drift speed compared to the free-drift estimates used in the Polar Pathfinder dataset. The thickness-dependent transfer coefficient provides an improved seasonality and long-term trend of the sea ice drift speed, with a minimum (maximum) drift speed in May (October), compared to July (January) for the constant transfer coefficient parameterizations which simply follow the peak in mean surface wind stresses. Over the 1979–2019 period, the trend in sea ice drift in this new model is +0.45 cms-1 per decade compared with +0.39 cms-1 per decade from the buoy observations, whereas there is essentially no trend in a free-drift parameterization with a constant transfer coefficient (-0.09 cms-1 per decade) or the Polar Pathfinder free-drift input data (-0.01 cms-1 per decade). The optimal wind turning angle obtained from a least-squares fitting is 25 ∘ , resulting in a mean error and a root-mean-square error of +3 and 42 ∘ on the direction of the drift, respectively. The ocean current estimates obtained from the minimization procedure resolve key large-scale features such as the Beaufort Gyre and Transpolar Drift Stream and are in good agreement with ocean state estimates from the ECCO, GLORYS, and PIOMAS ice–ocean reanalyses, as well as geostrophic currents from dynamical ocean topography, with a root-mean-square difference of 2.4, 2.9, 2.6, and 3.8 cms-1 , respectively. Finally, a repeat of the analysis on two sub-sections of the time series (pre- and post-2000) clearly shows the acceleration of the Beaufort Gyre (particularly along the Alaskan coastline) and an expansion of the gyre in the post-2000s, concurrent with a thinning of the sea ice cover and the observed acceleration of the ice drift speed and ocean currents. This new dataset is publicly available for complementing merged observation-based sea ice drift datasets that include satellite and buoy drift records. [ABSTRACT FROM AUTHOR]

During the winters (December–February) between 1985 and 2015, the North China Plain (NCP, 30–40.5 ∘ N, 112–121.5 ∘ E) suffered many periods of heavy haze, and these episodes were contemporaneous with extreme rainfall over southern China; i.e., south rainfall–north haze events. The formation of such haze events depends on meteorological conditions which are related to the atmospheric circulation associated with rainfall over southern China, but the underlying physical mechanism remains unclear. This study uses observations and model simulations to demonstrate that haze over the NCP is modulated by anomalous anticyclonic circulation caused by the two Rossby wave trains, in conjunction with the north–south circulation system, which ascends over southern China, moves north into northern China near 200–250 hPa, and then descends in the study area. Moreover, in response to rainfall heating, southern China is an obvious Rossby wave source, supporting waves along the subtropical westerly jet waveguide and finally strengthening anticyclonic circulation over the NCP. Composite analysis indicates that these changes lead to a stronger descending motion, higher relative humidity, and a weaker northerly wind, which favors the production and accumulation of haze over the NCP. A linear baroclinic model simulation reproduced the observed north–south circulation system reasonably well and supports the diagnostic analysis. Quasi-geostrophic vertical pressure velocity diagnostics were used to quantify the contributions to the north–south circulation system made by large-scale adiabatic forcing and diabatic heating (Q). The results indicated that the north–south circulation system is induced mainly by diabatic heating related to precipitation over southern China, and the effect of large-scale circulation is negligible. These results provide the basis for a more comprehensive understanding of the mechanisms that drive the formation of haze over the NCP. [ABSTRACT FROM AUTHOR]

A series of numerical simulations with different forcing conditions are carried out, to investigate the roles played by buoyancy and wind forcing on the upper ocean gyres, and to contrast the laminar and eddying regimes. Model experiments show that the buoyancy-driven eastward geostrophic flow tends to suppress the formation of the wind-driven subpolar gyre, but the northward eddy heat transport can homogenize the subpolar water and reduce the meridional temperature gradient by about two-third, thus counteracting the buoyancy effect and saving the subpolar gyre. For the subtropical gyre, its transport is enhanced by eddy mixing, and the role of buoyancy forcing is very sensitive to the choice of diapycnal diffusivity. Our results suggest that eddy effects must be considered in the dynamics of the subpolar gyre, and vertical diffusivity should be selected carefully in simulating the basin-wide circulations. [ABSTRACT FROM AUTHOR]

Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (∼ 54 ∘ N) and northern Norway (∼ 69 ∘ N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLT-SL) using SMR and PRR winds and (2) the mesosphere summer length (M-SL) using the PRR and MLS. Under both definitions, the summer begins around April and ends around middle September. The largest year-to-year variability is found in the summer beginning in both definitions, particularly at high latitudes, possibly due to the influence of the polar vortex. At high latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity as well as large-scale atmospheric influences (e.g., quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO. [ABSTRACT FROM AUTHOR]

Off‐equatorial wind stress curl anomalies that mainly drive geostrophic transport are essential for the phase transition of the El Niño‐Southern Oscillation (ENSO) induced by a recharge‐discharge process. The ENSO‐induced zonal wind stress anomaly also drives surface Ekman currents, which may counteract the geostrophic transport, but its effect on ENSO periodicity remains unclear in the recharge oscillator theory. Here, we extended an ENSO diagnostic framework, called the Bjerknes‐Wyrtki‐Jin index, to evaluate Ekman transport. The extended method applied to reanalysis data sets shows that the ENSO period lengthens by approximately 30%, reconciling the discrepancy between the observed ENSO frequency and estimates from the conventional method. This result demonstrates that the self‐inhibitory process by Ekman transport is an inherent feature of the observed ENSO periodicity. In addition, Ekman transport efficiently works with a wide meridional structure of ENSO, suggesting that the meridional width of individual ENSO events explains the difference in their duration. Plain Language Summary: The dominant interannual climate variability, El Niño‐Southern Oscillation (ENSO), is an oscillatory phenomenon accompanying anomalous sea surface temperature in the eastern equatorial Pacific. ENSO drives extreme weather all over the world; therefore, understanding its periodic nature is important. Previous studies have developed a powerful tool, called the Wyrtki period index, to diagnose ENSO periodicity based on the known ENSO dynamics; however, it has a systematic bias that leads to a shorter ENSO period than those observed. Here, we extended the Wyrtki period index to include the contribution of the surface Ekman transport explicitly, which retards the ENSO phase reversal. Our diagnostics show that the ENSO period is approximately 30% longer than that of the conventional method and reproduces the observed ENSO spectrum well. Key Points: Equatorial wind stress pattern associated with El Niño‐Southern Oscillation (ENSO) drives the Ekman transport, which acts to slow down the ENSO recharge‐discharge processWe extended a theoretical ENSO period index to demonstrate the role of meridional Ekman transport that lengthens the ENSO periodicityEkman transport contributes to lengthening the ENSO period by 30% relative to the estimation by geostrophic transport alone [ABSTRACT FROM AUTHOR]

We present a test case of river plume spreading to evaluate numerical methods used in coastal ocean modeling. It includes an estuary–shelf system whose dynamics combine nonlinear flow regimes with sharp frontal boundaries and linear regimes with cross-shore geostrophic balance. This system is highly sensitive to physical or numerical dissipation and mixing. The main characteristics of the plume dynamics are predicted analytically but are difficult to reproduce numerically because of numerical mixing present in the models. Our test case reveals the level of numerical mixing as well as the ability of models to reproduce nonlinear processes and frontal zone dynamics. We document numerical solutions for the Thetis and FESOM-C models on an unstructured triangular mesh, as well as ones for the GETM and FESOM-C models on a quadrilateral mesh. We propose an analysis of simulated plume spreading which may be useful in more general studies of plume dynamics. The major result of our comparative study is that accuracy in reproducing the analytical solution depends less on the type of model discretization or computational grid than it does on the type of advection scheme. [ABSTRACT FROM AUTHOR]

Observations of changes in terrestrial water storage (TWS) obtained from the satellite mission GRACE (Gravity Recovery and Climate Experiment) have frequently been used for water cycle studies and for the improvement of hydrological models by means of calibration and data assimilation. However, due to a low spatial resolution of the gravity field models, spatially localized water storage changes, such as those occurring in lakes and reservoirs, cannot properly be represented in the GRACE estimates. As surface storage changes can represent a large part of total water storage, this leads to leakage effects and results in surface water signals becoming erroneously assimilated into other water storage compartments of neighbouring model grid cells. As a consequence, a simple mass balance at grid/regional scale is not sufficient to deconvolve the impact of surface water on TWS. Furthermore, non-hydrology-related phenomena contained in the GRACE time series, such as the mass redistribution caused by major earthquakes, hamper the use of GRACE for hydrological studies in affected regions. In this paper, we present the first release (RL01) of the global correction product RECOG (REgional COrrections for GRACE), which accounts for both the surface water (lakes and reservoirs, RECOG-LR) and earthquake effects (RECOG-EQ). RECOG-LR is computed from forward-modelling surface water volume estimates derived from satellite altimetry and (optical) remote sensing and allows both a removal of these signals from GRACE and a relocation of the mass change to its origin within the outline of the lakes/reservoirs. The earthquake correction, RECOG-EQ, includes both the co-seismic and post-seismic signals of two major earthquakes with magnitudes above Mw9. We discuss that applying the correction dataset (1) reduces the GRACE signal variability by up to 75 % around major lakes and explains a large part of GRACE seasonal variations and trends, (2) avoids the introduction of spurious trends caused by leakage signals of nearby lakes when calibrating/assimilating hydrological models with GRACE, and (3) enables a clearer detection of hydrological droughts in areas affected by earthquakes. A first validation of the corrected GRACE time series using GPS-derived vertical station displacements shows a consistent improvement of the fit between GRACE and GNSS after applying the correction. Data are made available on an open-access basis via the Pangaea database (RECOG-LR: Deggim et al., 2020a, 10.1594/PANGAEA.921851; RECOG-EQ: Gerdener et al., 2020b, 10.1594/PANGAEA.921923). [ABSTRACT FROM AUTHOR]

In this paper we studied the impact of winds on Arctic sea ice through the ocean's memory by using numerical simulations. We found that the changes in halosteric height induced by wind perturbations can significantly affect the Arctic sea ice drift, thickness, concentration and deformation rates regionally even years after the wind perturbations. Changes in the Arctic liquid freshwater content and thus in halosteric height can cause changes in the sea surface height and surface geostrophic currents, which further enforce a lasting and strong impact on sea ice. The changes in both sea surface height gradient force (due to changes in sea surface height) and ice–ocean stress (due to changes in surface geostrophic currents) are found to be important in determining the overall ocean effects. The revealed ocean effects are mainly associated with changes in sea ice dynamics, not thermodynamics. Depending on the preceding atmospheric mode driving the ocean, the ocean's memory of the wind forcing can lead to changes in Arctic sea ice characteristics with very different spatial patterns. We obtained these spatial patterns associated with Arctic Oscillation, Arctic Dipole Anomaly and Beaufort High modes through dedicated numerical simulations. The dynamical impact of the ocean has strong seasonal variations, stronger in summer and weaker in winter and spring. This implies that declining trends of Arctic sea ice will very possibly allow a stronger ocean impact on the sea ice in a warming climate. [ABSTRACT FROM AUTHOR]

On the basis of satellite and field atmospheric and marine observations, the water circulation processes of the north-western shelf (the NWS) of the Black Sea in the summer of 2017 were studied. The study indicated high stability of summer offshore winds of northern and north-northwest directions and 12 cases of coastal upwelling. Three cases of upwelling were instrumentally detected on the across-the-shore oceanographic sections during seasonal field works performed by the oceanographic unit of the branch “Odesa Area of State Hydrographic Service” of the state institution "State Hydrographic Service of Ukraine (SHSU)". The increase of coastal water density led to an abnormally active transfer along the coast of the Danube-Dniester interfluve area to the northernmost parts of the NWS. In 2017 a visual manifestation of anticyclonic character of summer circulation of the NWS's water could be observed. The obtained data confirm the previous conclusions on frequent cases of change in summer periods of traditional cyclonic water circulation to the anticyclonic one. The abnormal development of the summer circulation regime allowed us to record for the first time the transfer of coccolithophores phytoplankton from the open sea to the northern regions of the NWS and to reveal its intraseasonal spatial transformation and development process duration. In the seaward part of the Gulf of Odessa a frequently repeated vortex formation of cyclonic vorticity with spatial dimensions of up to 7-8 miles and orbital velocities, according to the presented data, of 0.12–0.18 m.c-1, and according to the latest field work, of over 0.30 m.c-1, was found. When analyzing the considered situations associated with transfer and vorticity of sea water a significant role was played by high-resolution visual images obtained from Sentinel-2 and Landsat-8 satellites having a spatial resolution of 10 and 30 meters respectively, as well as by similar satellites of earlier modifications.

A submesoscale‐permitting global ocean simulation is used to study the upper ocean turbulence in high kinetic energy (KE) regions. Submesoscale processes peak in winter so that the geostrophic KE spectra tend to be relatively shallow in winter (∼k−2) with steeper spectra in summer (∼k−3). This transition in KE spectral scaling has two phases. In the first phase (late autumn), KE spectra show the presence of two spectral regimes: ∼k−3 power‐law in mesoscales and ∼k−2 power‐law in submesoscales. The first phase appears with the onset of mixed‐layer instabilities, which convert available potential energy into KE, and this process results in a flattening of KE spectra at submesoscales. However, KE spectra at longer wavelengths follow ∼k−3 scaling associated with a forward enstrophy transfer. In the second phase (late winter), KE produced through mixed‐layer instabilities is transferred to larger scales, and k−2 power‐law also develops in mesoscales. Plain Language Summary: Mesoscale and submesoscale motions are important for heat and material transport in the oceans. In the upper ocean, submesoscale turbulence shows seasonal variability and is pronounced in winter. The same distinction is reflected in horizontal wavenumber spectra of kinetic energy (KE) and spectral energy fluxes. In this study, geostrophic KE spectra are analyzed in a submesoscale‐permitting global ocean model to study the seasonal variability in the upper ocean turbulence. We interpret the results in terms of different physical mechanisms and their effects on the evolution of KE spectra over an annual cycle. We find that both mesoscale and submesoscale processes contribute to their characterization. Key Points: Seasonality in the upper ocean turbulence is interpreted in terms of interior baroclinic instabilities and mixed‐layer instabilitiesTemporal variability in the geostrophic kinetic energy spectrum can be subdivided into distinct seasonal phasesBoth a forward enstrophy transfer and an inverse kinetic energy transfer control the spectral slope in the kinetic energy spectrum [ABSTRACT FROM AUTHOR]

Determining the long‐term nature of the Atlantic Meridional Overturning Circulation (AMOC) presents a major step in understanding ocean temperature forcing, and is crucial both for placing the recently observed AMOC slowdown in the context of climate change and for predicting future climate. We present a time series for AMOC strength over the last 120 years which is derived solely from observations. Application of the Bernoulli inverse to hydrography yields the general geostrophic circulation in the North Atlantic without requiring additional sea surface height information, allowing AMOC evaluation in the pre‐satellite era. AMOC varies on a multidecadal timescale, leading the Atlantic Multidecadal Variability (AMV) in sea surface temperature by 2.5 years. We consider the dynamics of the implied AMOC/AMV coupling, and hypothesize that the low‐frequency variability in both parameters is driven by large‐scale density anomalies circulating in the AMOC. Plain Language Summary: The large‐scale ocean currents which influence the Earth's climate system are driven in large part by gradients in the sea surface. However, sea surface height measurements are scarce before 1993, when satellite observations began, limiting our knowledge of ocean currents over climate‐relevant time scales. Here, we use a method for determining sea surface height which does not require satellite data. We can therefore estimate the current strength in the North Atlantic over the last 120 years. We find that changes in the current strength are closely tied to changes in sea surface temperature. Key Points: Atlantic Meridional Overturning Circulation (AMOC) variability leads Atlantic Multidecadal Variability (AMV) by 2.5 yearsAMV is strongly coupled to ocean dynamicsAMOC variability associated with circulation of large‐scale density anomalies [ABSTRACT FROM AUTHOR]

Tropical cyclones (TC), which are among the most destructive natural phenomena on Earth, have significant impacts on the western boundary current (WBC). We quantify the direct impact of TCs on the surface geostrophic velocity of the Kuroshio, the WBC in the western North Pacific, by analyzing satellite‐derived geostrophic current data and using results from theoretical and numerical models. The study reveals that TCs decrease the surface geostrophic velocity by 14 cm s−1 which is 16% of the mean velocity of the Kuroshio (85 cm s−1), with the reduced velocity maintained for a month. The decrease in velocity of the Kuroshio is mainly (87%) due to the effect of TC‐driven upwelling ("Cooling effect"), the effect of ocean heat uptake after the TC ("Warming effect") being minor (13%); these are basically enabled by the strong thermal gradient around the Kuroshio. Plain Language Summary: The tropical cyclone (TC), also known as hurricane or typhoon, has a strong impact on the ocean as well as the land. In this study, we investigate how TCs affect ocean currents, with special focus on the Kuroshio, the western boundary current (WBC) in the North Pacific region. By analyzing satellite‐derived data, and building theoretical and numerical models, we deduce that when a TC hits the Kuroshio, its surface velocity decreases by 16%. This work makes a significant contribution by providing a quantitative understanding of the influence of TCs on the WBC. Further, given that the WBC is the strongest ocean current system on Earth and plays a key role in maintaining global heat balance, this study emphasizes the potential importance of the TC‐induced change on the current for the global distribution of heat energy. Key Points: Tropical cyclones (TC) decrease the surface geostrophic velocity of the Kuroshio by 14 cm s−1 (16% of the mean velocity)"Cooling Effect" of the TC‐driven upwelling explains 87% of the geostrophic velocity decrease immediately after a TC"Warming Effect" of the ocean heat uptake explains 13% of the geostrophic velocity decrease, which is maintained for ∼30 days after a TC [ABSTRACT FROM AUTHOR]

The large-scale system of ocean currents that transport warm waters in the upper 1000 m northward and return deeper cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system; hence, there is a need for long-term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme and other mooring programmes have revolutionised our understanding of large-scale circulation; however, by design they are constrained to measurements at a single latitude and cannot tell us anything pre-2004. Nearly global coverage of surface ocean data from satellite altimetry has been available since the launch of the TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual timescales including previous studies that have investigated empirical correlations between sea surface height variability and the overturning circulation. Here we show a direct calculation of ocean circulation from satellite altimetry of the upper mid-ocean transport (UMO), the Gulf Stream transport through the Florida Straits (GS), and the AMOC using a dynamically based method that combines geostrophy with a time mean of the vertical structure of the flow from the 26 ∘ N RAPID moorings. The satellite-based transport captures 56 %, 49 %, and 69 % of the UMO, GS, and AMOC transport variability, respectively, from the 26 ∘ N RAPID array on interannual (18-month) timescales. Further investigation into the vertical structure of the horizontal transport shows that the first baroclinic mode accounts for 83 % of the interior geostrophic variability, and the combined barotropic and first baroclinic mode representation of dynamic height accounts for 98 % of the variability. Finally, the methods developed here are used to reconstruct the UMO and the AMOC for the time period pre-dating RAPID, 1993 to 2003. The effective implementation of satellite-based method for monitoring the AMOC at 26 ∘ N lays down the starting point for monitoring large-scale circulation at all latitudes. [ABSTRACT FROM AUTHOR]

Large-eddy simulations are used to investigate the influence of a horizontal frontal zone, represented by a stationary uniform background horizontal temperature gradient, on the wind- and wave-driven ocean surface boundary layers. In a frontal zone, the temperature structure, the ageostrophic mean horizontal current, and the turbulence in the ocean surface boundary layer all change with the relative angle among the wind and the front. The net heating and cooling of the boundary layer could be explained by the depth-integrated horizontal advective buoyancy flux, called the Ekman buoyancy flux (or the Ekman–Stokes buoyancy flux if wave effects are included). However, the detailed temperature profiles are also modulated by the depth-dependent advective buoyancy flux and submesoscale eddies. The surface current is deflected less (more) to the right of the wind and wave when the depth-integrated advective buoyancy flux cools (warms) the ocean surface boundary layer. Horizontal mixing is greatly enhanced by submesoscale eddies. The eddy-induced horizontal mixing is anisotropic and is stronger to the right of the wind direction. Vertical turbulent mixing depends on the superposition of the geostrophic and ageostrophic current, the depth-dependent advective buoyancy flux, and submesoscale eddies. [ABSTRACT FROM AUTHOR]