What can we learn from the SWOT Fast Sampling Phase on coastal ocean circulation: example of the North Current (NW Mediterranean Sea)?

The monitoring of ocean currents is a key component in many coastal applications, ranging from biogeochemical resources to marine pollution or search and rescue. During the last three decades, satellite altimetry has played an essential role in the understanding and monitoring of ocean currents at global scale. But its use is still limited in coastal areas due to a poorer data quality as we approach the coast, and a spatio-temporal data resolution considered as sparse relatively to the scales of coastal dynamical features. However, many recent studies addressing the different issues related to the derivation and exploitation of altimeter-derived coastal current velocities have shown that they efficiently complement coastal velocity fields derived from in-situ data (e.g., hydrographic observations, surface drifter and moored or ship-based acoustic Doppler velocities) or from shore‐based HF radars. Indeed, one of the major advantages of this measurement technique is to provide long time series (i.e. > 30 years) of spatially and temporally homogeneous information about the circulation and to be available at near-global scale. The data quality problem can be partially overcome thanks to dedicated processing with adequate corrections. Additionally, merging data from multiple missions has been shown to improve the spatial and temporal resolution. But few data sets including coastal processing and several altimetry missions still exist. The SWOT mission represents the beginning of a new class of altimeters. Associated to substantial improvements in terms of spatial resolution and altimetry data accuracy, it could considerably change the situation in terms of coastal applications. In this study, we study and quantify the ability of SWOT to observe coastal currents compared with conventional nadir missions on a case study: the Northern Current (NW Mediterranean Sea). In particular, we take advantage of the 1-day repeat orbit during the Fast Sampling Phase as a prototype to explore what future altimetry based on such temporal resolution could bring in coastal oceanography.