Advancing the retrieval of accurate upper-layer ocean circulation products responding to the needs of the different key players engaged in the transition toward a clean, safe, sustainable and productive environment is the driving element of the World Ocean Circulation project. To achieve the objective, the common driver of the project to enhance the understanding, the monitoring and the phasing forecast of upper ocean currents, with a particular focus on the most intense ocean small-scale structures. In other word, our challenge is to retrieve the ocean velocities at the right place at the right time. An overview of the main results obtained during the first phase of the project will be presented. The project is organised around 4 themes. The theme 1 is focused Sea-state current interactions for Safe Navigation whereas theme 2 deals with 3D currents and vertical motion for Sustainable Fisheries. In theme 3 we address the Surface Lagrangian drift for a Clean Ocean. Theme 4 is dedicated to high resolution wave and current model assessment for a Productive Ocean. The main outcome of the first year will be presented. This first phase 1 was dedicated to the development of new algorithms where one year of products have been generated and validated for each theme. We will also present preliminary results of the impact assessment studies done by the WOC users.
Visit https://www.worldoceancirculation.org/ to follow the main steps of the project.

The Agulhas current is one of the strongest western boundary currents (WBCs) in the world’s oceans [Lutjeharms, 2006]. In a recent study, Le Goff et al [Le Goff al 2021] estimate this current by processing the messages sent by the commercial vessels through the Automatic Identification System (AIS). These estimations show a remarkable improvement in the spatio-temporal resolution compared to altimetric-derived geostrophic currents. One result of this study was the fact that the main strength (weakness) of the AIS-derived currents is the very high (low) density of estimates in the absence (presence) of southwestern storm-swell wave conditions. To prevent the extreme sparsity of observation during these storm events and to take advantage of all available observation sources: we use AIS derived surface current in synergy with along-track altimetric data from DUACS DT2021. The merging is done through the Multiscale Inversion for Ocean Surface Topography (MIOST) [Ubelmann et al., 2020] variational tool to retrieve both geostrophic and ageostrophic current. This tool allows the decomposition of the signal into representors representing different time and space scales (mesoscale to large-scale) and different physical signals (geostrophy, internal waves, near-inertial oscillations, Ekman current…). Once the oceanic current is estimated, we study the behavior of the ships and especially their anormal behavior during the storms.

Ocean waves Interacting with ocean currents is a frequent cause of sea-state variability [Ardhuin et al 2017, Quilfen et al 2018, Quilfen and Chapron 2019]. Such situations can lead to sea-state hazards, crucial for shipping security. The Great Agulhas current system is an area of very intensive maritime traffic, where dangerous localized sea-state amplificated by the current are regularly reported. 

In absence of wind and wave-induced motions, the heading and drift of every ship along its trajectory can be estimated from oceanic surface current map. This first guess can then be compared with real ship parameters obtained from satellite-collected ship Automatic Identification System (AIS) messages. During Southwestern storm-swell wave conditions, with wind and waves aligned against the current, some ships experience pronounced navigation difficulties, slowing down up to 2 m/s, and frequently maneuvering to keep their heading perpendicular to dominant waves. Superposed multiple individual ship trajectories can then help map anomalous areas, and to relate them to localized strong wave-current effects such as large refraction of waves by the oceanic current.

[Lutjeharms, 2006] : LUTJEHARMS, J. R. E. The Agulhas Current Springer-Verlag. Berlin, Germany, 2006.

[Le Goff et al 2021] : Le Goff, C., Boussidi, B., Mironov, A., Guichoux, Y., Zhen, Y., Tandeo, P., et al. (2021). Monitoring the greater Agulhas Current with AIS data information. Journal of Geophysical Research: Oceans, 126, e2021JC017228. https://doi.org/10.1029/2021JC017228

[Ardhuin et al 2017] : Ardhuin, F., S. T. Gille, D. Menemenlis,C. B. Rocha, N. Rascle, B. Chapron, J. Gula, and J. Molemaker (2017), Small-scale open ocean
currents have large effects on wind wave heights, J. Geophys. Res. Oceans, 122, 4500–4517, doi:10.1002/2016JC012413.

[Quilfen et al 2018] :Quilfen Yves, Yurovskaya M., Chapron Bertrand, Ardhuin Fabrice (2018). Storm waves focusing and steepening in the Agulhas current: Satellite observations and modeling. Remote Sensing Of Environment, 216, 561-571. Publisher's official version :

[Quilfen and Chapron, 2019] : Quilfen, Y., & Chapron, B. (2019). Ocean surface wave-current signatures from satellite altimeter measurements.
Geophysical Research Letters, 46.

[Ubelmann et al., 2020] : Ubelmann C., G. Dibarboure, L. Gaultier, A. Ponte, F. Ardhuin, M. Ballarota, and Y. Faugère (2020): Reconstructing Ocean Surface Current Combining Altimetry and Future Spaceborne Doppler Data. Submitted to Journal of Geophysical Research.

The altimetry constellation, exceeding four to five simultaneous satellites in the recent years, allowed a precise mapping of the Ocean turbulence at mesoscale  (e.g. Aviso/CMEMS maps) and also a mapping of coherent Internal Tides (IT) (e.g. Zaron 2019, Zhao 2021) that actively participate in the dynamical interactions of the Ocean circulation. With the perspective of higher resolution Altimetry in the next few years (e.g. SWOT mission) the study of IT and its interactions should be promising. 

However, disentangling the signals between short mesoscale structures and IT is a challenge, so it is important to prepare with efficient mapping algorithms. Most of the approaches developed so far consider the mapping of both dynamics separately, potentially leading to some leakage :  In particular, the mesoscale solution subtracted before computing the IT  may remove a non-negligible part of the original IT signal leading to underestimated IT solutions.

To mitigate this problem, we are currently investigating a simultaneous mapping of the mesoscale and IT in a single massive inversion covering a multi-decadal period. If the standard optimal interpolation approach presently used for mesoscale mapping could not invert such a volume of observations, it is possible to perform an inversion using a variational approach. Following a wavelet basis in time and space for the mesoscale and a plane-wave basis for IT, we can solve for more than O(108)  parameters  iteratively with a conjugate-gradient descent. 

After presenting the method, we will show the results of a global implementation with the main tidal components. The IT solutions will be compared with existing solutions, by looking at a variance reduction diagnostic with an independent altimetry dataset. The results may suggest a significant gain of performing the simultaneous mesoscale-IT inversion. Finally, the possibility of considering  non-stationary ITW in the same inversion will be presented.

Direct sensing of total ocean surface currents with microwave Doppler signals is a growing topic of interest for oceanography, with relevance to several new ocean mission concepts proposed in recent years. Among those, WaCM (Rodriguez et al., 2019), SKIM (Ardhuin et al., 2019) and the ESA Earth Explorer candidate Harmony (ESA, 2020) and SEASTAR (Gommenginger et al., 2019).
Since 2014, the spaceborne C-band SAR instruments of the Copernicus Sentinel-1 (S1) mission routinely ac-quire microwave Doppler data, distributed to users through operational S1 Level-2 ocean radial velocity (L2 OCN RVL) products. S1 L2 RVL data could produce high-resolution maps of ocean surface currents that would benefit ocean observing and modelling, particularly in coastal regions. However, uncorrected platform effects and instrument anomalies continue to impact S1 RVL data and prevent direct exploitation.
In this paper, a simple empirical method is proposed to calibrate and correct operational S1 L2 RVL products and retrieve two-dimensional maps of surface currents in the radar line-of-sight. The study focuses on the Ger-man Bight where wind, wave and current data from marine stations and an HF radar instrumented site provide comprehensive means to evaluate S1 retrieved currents. Analyses are deliberately limited to Sentinel-1A (S1A) ascending passes to focus on one single instrument and fixed SAR viewing geometry. The final dataset compris-es 78 separate S1A acquisitions over 2.5 years, of which 56 are matched with collocated HF radar data. The em-pirical corrections bring significant improvements to S1A RVL data, producing higher quality estimates and much better agreement with HF radar radial currents.
Comparative evaluation of S1A against HF radar currents for different WASV corrections reveal that best results are obtained in this region when computing the WASV with sea state rather than wind vector input. Accounting for sea state produces S1 radial currents with a precision (std of the difference) around 0.3 m/s at ~1 km resolu-tion. Precision improves to ~0.24 m/s when averaging over 21 × 27 km2, with correlations with HF radar data reaching up to 0.93. Evidence of wind-current interactions when tides and wind align and short fetch conditions call for further research with more satellite data and other sites to better understand and correct the WASV in coastal regions.
Finally, 1 km resolution maps of climatological S1A radial currents obtained over 2.5 years reveal strong coastal jets and fine scale details of the coastal circulation that closely match the known bathymetry and deep- water coastal channels in this region. The wealth of oceanographic information in corrected S1 RVL data is encourag-ing for Doppler oceanography from space and its application to observing small scale ocean dynamics, atmos-phere and ocean vertical exchanges and marine ecosystem response to environmental change.
Ardhuin, F., et al., 2019. SKIM, a candidate satellite mission exploring global ocean currents and waves. Front. Mar. Sci. 6, 209. https://doi.org/10.3389/ fmars.2019.00209.
ESA, 2020. Report for Assessment: Earth Explorer 10 Candidate Mission Harmony. ESA- EOPSM-HARM-RP-3784 European Space Agency Noordwijk, The Netherlands.
Gommenginger, et al., 2019. SEASTAR: a mission to study ocean submesoscale dynamics and small-scale atmosphere-ocean processes in coastal, shelf and polar seas. Front. Mar. Sci. 6, 457. https://doi.org/ 10.3389/fmars.2019.00457.
Rodríguez, E., Bourassa, M., Chelton, D., Farrar, J.T., Long, D., Perkovic-Martin, D., Samelson, R., 2019. The winds and currents mission concept. Front. Mar. Sci. 6, 438. https://doi.org/10.3389/fmars.2019.00438.

For more than 25 years, altimetry has allowed the study of near-global sea surface height (SSH) at scales longer than 150 km and drastically transformed our understanding of mesoscale processes in the oceans. But the scales resolved by nadir (along-track) altimetry are limited by the 100-300 km spacing between one dimensional satellite ground tracks. Even by merging several nadir measurements, the space time resolution of the resulting 2D SSH maps does not allow to conveniently characterize and study small mesoscale ( < 150km) motions. Still, recent studies, based on numerical models, have highlighted the impacts of short-mesoscale and submesoscale processes on ocean dynamics.

The Surface Water and Ocean Topography (SWOT) mission is expected to provide Sea Surface Height (SSH) measurements resolving scales of a few tens of kilometers. Over a large fraction of the globe, the SSH signal at scales < 100km is essentially a superposition of a component due to balanced motions (BMs) and another component due to internal tides (ITs). These two classes of motions are associated with different dynamical processes and therefore impact the ocean kinetic energy budget differently. To make the best use of the future SWOT SSH observations, BMs and ITs will need to be separated. However, the main difficulty to separate the signals lies in the strong interactions between BMs and ITs, generating non-stationary ITs that cannot be estimated by conventional statistical methods.

In that context, we introduce a dynamical method that process altimetric observations to simultaneously estimate the SSH signatures of BMs and ITs on two-dimensional regular grids. The method, based on original data assimilation techniques, uses simple dynamical model, each specific to the mapping of one component: a quasi-geostrophic model for BMs, and a linear shallow-water model for ITs. A particular effort is made to perform the inversions in well-chosen reduced-order basis.

The algorithm is tested with Observation System Simulation Experiments (OSSE), where the true state of the ocean is provided by the MITgcm global LLC4320 simulation. We focus on a realistic observational scenario in the California Current System during the SWOT’s fast sampling phase. The proposed algorithm is able to map and separate a large amount of the variance of BMs and ITs. Importantly, in addition to the reconstruction of stationary ITs, the algorithm estimates the amplitude and phase of nonstationary ITs, which is very encouraging for the process of future SWOT data.