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DIVAnd (Data-Interpolating Variational Analysis, in n-dimensions) is a tool to interpolate observations on a regular grid using the variational inverse method. We have extended DIVAnd to include additional dynamic constraints relevant to surface currents, including imposing a zero normal velocity at the coastline, imposing a low horizontal divergence of the surface currents, temporal coherence and simplified dynamics based on the Coriolis force, and the possibility of including a surface pressure gradient. The impact of these constraints is evaluated by cross-validation using the HF (high-frequency) radar surface current observations in the Ibiza Channel from the Balearic Islands Coastal Ocean Observing and Forecasting System (SOCIB). A small fraction of the radial current observations are set aside to validate the velocity reconstruction. The remaining radial currents from the two radar sites are combined to derive total surface currents using DIVAnd and then compared to the cross-validation dataset and to drifter observations. The benefit of the dynamic constraints is shown relative to a variational interpolation without these dynamical constraints. The best results were obtained using the Coriolis force and the surface pressure gradient as a constraint which are able to improve the reconstruction from the Open-boundary Modal Analysis, a quite commonly used method to interpolate HF radar observations, once multiple time instances are considered together. [ABSTRACT FROM AUTHOR]

To study the water dynamics in the western Bering Sea, the time series of geostrophic velocities derived from satellite altimetry, Global total current (Copernicus Globcurrent) velocities, Argo buoys, and ship, borne data are analyzed. We demonstrate that the surface currents in the study area are characterized by a significant seasonal and interannual variability. In January, the southwestern currents are prevailing in the western Bering Sea, the geostrophic currents are connected to the continental slope, and mesoscale anticyclonic movement of waters is observed on the shelf. In July, the surface flow is oriented to the northeast along the continental slope, and cyclonic water movement prevails on the shelf. The formation of mesoscale anticyclones in late winter – early spring – is related to the supply of low-temperature and low-salinity Bering Sea shelf water. The temporal variability of the direction and current velocity in the western part of the Bering Sea during the summer period is associated with the wind and coastal upwelling. The presence of coastal upwelling and eddies should be considered as one of the factors leading to the high concentration of chlorophyll a on a shelf and continental slope in the summer. [ABSTRACT FROM AUTHOR]

Surface current variability is investigated using 2.5 years of continuous velocity measurements from an high frequency radar (HFR) located in the Ibiza Channel (Western Mediterranean Sea). The Ibiza Channel is identified as a key geographical feature for the exchange of water masses but still poorly documented. Operational, quality controlled, HFR derived velocities are provided by the Balearic Islands Coastal Observing and Forecasting System (SOCIB). They are assessed by performing statistical comparisons with current-meter, ADCP, and surface lagrangian drifters. HFR system does not show significant bias, and its accuracy is in accordance with previous studies performed in other areas. The main surface circulation patterns are deduced from an EOF analysis. The first three modes represent almost 70 % of the total variability. A cross-correlation analysis between zonal and meridional wind components and the temporal amplitudes of the first three modes reveal that the first two modes are mainly driven by local winds, with immediate effects of wind forcing and veering following Ekman effect. The first mode (37 % of total variability) is the response of meridional wind while the second mode (24 % of total variability) is linked primarily with zonal winds. The third and higher order modes are related to mesoscale circulation features. HFR derived surface transport presents a markedly seasonal variability being mostly southwards. Its comparison with Ekman-induced transport shows that wind contribution to the total surface transport is on average around 65 %. [ABSTRACT FROM AUTHOR]

The original method is presented for parameterization of surface wind-driven currents in the Black Sea using drifters, altimetry, and wind data. Buoys velocities for the cases with small time variability of the altimetry-derived geostrophic currents were parameterized as a function of the amplitude coefficient b and rotation angle a between wind and residual current direction at drifter drogue depth ∼ 15 m. The results show that the variability of a and b for different cases is very high: angle a changes from 10 to 90° to the right, amplitude coefficient b varies from 0.01 to 0.03. At the same time, it is shown that amplitude coefficient b and rotation angle a on 15 m depth are not independent parameters and there is a robust dependence between them. Using quasi-Ekman theory and the revealed dependence, we can determine parameterization for wind-driven currents exactly on the surface. As a result, we found that wind-driven currents at z = 0 m are directed on b = 13° to the right of wind direction with amplitudes equal to k = 2.8 % of wind velocity. It is important that these parameters did not depend on Ekman layer depth and can be used under different ocean conditions. This parameterization is used to compute the total surface currents and determine the trajectories of oil slick propagation observed on satellite images. Comparison shows that the presented method can be successfully used to predict floating objects trajectories basing on the satellite data. [ABSTRACT FROM AUTHOR]

As shown in the literature, ocean surface circulation can be estimated from sequential satellite imagery by using the maximum cross-correlation (MCC) technique. This approach is very promising since it offers the potential to acquire synoptic-scale coverage of the surface currents on a quasi-continuous temporal basis. However, MCC has also many limits due, for example, to cloud cover or the assumption that Sea Surface Temperature (SST) or other surface parameters from satellite imagery are considered as conservative passive tracers. Also, since MCC can detect only advective flows, it might not work properly in shallow water, where local heating and cooling, upwelling and other small-scale processes have a strong influence. Another limitation of the MCC technique is the impossibility of detecting currents moving along surface temperature fronts. The accuracy and reliability of MCC can be analysed by comparing the estimated velocities with those measured by in situ instrumentation, but the low number of experimental measurements does not allow a systematic statistical study of the potentials and limitations of the method. Instead, an extensive analysis of these features can be done by applying the MCC to synthetic imagery obtained from a realistic numerical ocean model that takes into account most physical phenomena. In this paper a multi-window (MW-) MCC technique is proposed, and its application to synthetic imagery obtained by a regional high-resolution implementation of the Regional Ocean Modeling System (ROMS) is discussed. An application of the MW-MCC algorithm to a real case and a comparison with experimental measurements are then shown. [ABSTRACT FROM AUTHOR]

Surface winds are crucial for accurately modeling the surface circulation in the coastal ocean. In the present work, high-frequency radar surface currents are assimilated using an ensemble scheme which aims to obtain improved surface winds taking into account European Centre for Medium-Range Weather Forecasts winds as a first guess and surface current measurements. The objective of this study is to show that wind forcing can be improved using an approach similar to parameter estimation in ensemble data assimilation. Like variational assimilation schemes, the method provides an improved wind field based on surface current measurements. However, the technique does not require an adjoint, and it is thus easier to implement. In addition, it does not rely on a linearization of the model dynamics. The method is validated directly by comparing the analyzed wind speed to independent in situ measurements and indirectly by assessing the impact of the corrected winds on model sea surface temperature (SST) relative to satellite SST. [ABSTRACT FROM AUTHOR]