Biological processes and wind contributions to 3-D fine-scale dynamics in a coastal area

Ocean fine-scale dynamics such as submesoscale processes constitute one of the key points in understanding current velocities with a tridimensional approach. Their in situ observation remains challenging due to their short space and time extent and duration. The Northern Current in the Western Mediterranean Sea, corresponding to the Northern branch of the basin cyclonic circulation, can be influenced by wind events, inducing intrusions on the continental shelf and associated fine-scale dynamics. In order to detect these phenomena, the JULIO mooring (JUdicious Location for Intrusion Observation) located on the Eastern side of the Gulf of Lion’s shelf on the 100m isobath, measures tridimensional current velocities since 2012 using an acoustic Doppler current profiler (ADCP) at 300kHz. In addition, vertical velocities have been episodically measured with other methods such as the FreeFall-ADCP (FF-ADCP), and an autonomous Vertical Velocity Profiler (VVP) both developed at MIO. First, the JULIO time-series has shown a significant contribution of biology to vertical motions, with systematic negative vertical velocities measured at night time. This effect was particularly strong in the subsurface layer at 15 to 25 meters depth and enhanced during spring. Second, strong observed 3D-currents, coinciding with wind events, induce current shears and intrusions as well as vertical velocities in this dynamical coastal region with complex bathymetric constraints. Furthermore, the SWOT satellite mission launched in 2022 constitutes a powerful ally by providing a new tool to detect fine-scale features from the surface dynamics near JULIO, and especially during the daily fast sampling phase from April to July 2023. Given the impact of wind forcing on current dynamics in this region, the question arises to what extent the Northern Current and its intrusions on the continental shelf might be affected by climate change.