Although coastal downwelling processes have traditionally received less attention in scientific literature, largely because most studies have focused on upwelling due to its well‑known role in enhancing coastal productivity, downwelling also plays an essential role in coastal ocean dynamics. Among others, they influence physical transport patterns and biogeochemical functioning in ways that are increasingly recognized as ecologically and climatically relevant. Both up- and downwelling events play a crucial role in shaping the local hydrodynamics, hydrography and marine ecosystems, also at submesoscales. In the southeastern Bay of Biscay, high-frequency (HF) radar surface current measurements have been used for investigating current patterns in response to upwelling and downwelling wind events. However, while HF radar provides valuable insights into surface currents, it does not capture subsurface ocean dynamics. To address this limitation, data collected from a glider mission have been combined with HF radar observations, mooring data, and numerical model outputs. This combination provides data at an unprecedented spatiotemporal resolution in this area, enabling a detailed examination of these events and their influence on the transport of passive particles, such as marine litter, within coastal and shelf waters.

From 23 September to 13 October 2022, a glider equipped with a Conductivity-Temperature-Depth (CTD) crosscut the continental shelf during and after a 5-day storm that generated onshore surface currents and a subsequent downwelling event. After the storm, the hydrographic profiles depicted a gradual relaxation of this event, with conditions returning to their initial state after 5 days, while current conditions showed an intensified eastward along-coast transport. The CUI coastal upwelling index based on HF radar surface current data showed good agreement with the glider observations. Ongoing work focuses on expanding this index to also account for the intensity of the downwelling in terms of subsurface density gradients.