Estimating oceanic physics-driven vertical velocities in a wind-influenced coastal environment

Abstract

Despite the challenge of measuring them due to their small intensities, oceanic vertical velocities (W) constitute essential key variables in understanding ocean dynamics, and ocean-atmosphere interactions and biogeochemical processes. Coastal events and fine-scale processes (1–100 km and days to weeks) can lead to high-intensity vertical velocities. Such processes can be observed in the Northwestern Mediterranean Sea. In particular, the Gulf of Lion is a region prone to intense north-westerly and easterly wind episodes that strongly impact the oceanic circulation. This work presents mooring ADCPs as reliable tools for physics-driven W measurements, with an adaptive algorithm which is applicable anywhere offshore in the ocean to detect W in fine-scale processes. The JULIO mooring (JUdicious Location for Intrusion Observation) is located on the boundary of the eastern side of the Gulf of Lion's shelf at the 100 m isobath. JULIO provides Eulerian measurements of three-dimensional current velocities over two main time-periods: 2012–2015, and since 2020. Vertical velocity measurements from JULIO show a good agreement with two independent methods: a Free-Fall Acoustic Doppler Current Profiler and an innovative Vertical Velocity Profiler. To measure physics-driven vertical velocities, we developed a method to identify and filter out biology-induced vertical velocities. Combining satellite and in situ observations with wind model outputs, we identify wind-induced downwelling and upwelling events at JULIO associated with physics-driven vertical velocities with maximum amplitudes of −465/127 m d−1. Hence, this analysis underlines the need for long term multimethod observations in such coastal areas forced by intense wind episodes.

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