Sporadic wind-driven upwelling/downwelling and associated cooling/warming along Northwestern Mediterranean coastlines

Abstract

Intermittent wind-driven coastal upwelling and downwelling are ubiquitous processes that drive a large part of the high frequency variability of coastal hydrography, with potential implications for ecosystems and socio-economic activities. Little synoptic information exists however on these processes, especially in regions characterized by rapidly changing atmospheric forcing and complex shorelines. Combining multi-annual hourly observations of nearshore temperatures with a long-term archive of Sea Surface Winds (SSW from ERA5 reanalysis), we investigate the statistical occurrence of wind-driven upwelling and downwelling events and their associated thermal responses along the northwestern Mediterranean coastlines. A Wind-based Upwelling and Downwelling Index (WUDI) is calculated at 20 km spatial resolution and validated against time-series of surface and subsurface in-situ temperatures at 11 coastal locations.We find that the WUDI index allows monitoring robustly all year round both up- and downwelling intermittent events that effectively cause coastal cooling/warming. On average, significant thermal responses to favorable winds appear after short delays (spanning 6–54 h for upwelling, 12–66 h for downwelling, depending on the site considered) with intensities 5 to 10 times stronger in stratified as compared to non-stratified conditions. Maximum near-surface cooling (subsurface warming, respectively) recorded after the most extreme events can reach up to -12.5 °C (+11 °C, respectively) during the period of seasonal stratification.A climatological database of wind-driven events that can be associated with typical thermal responses is constructed for the Northwestern Mediterranean shorelines over the last four decades. It shows that up/downwelling events are favored along certain portions of coastline, called “cells”, and are characterized by specific magnitudes, frequencies of occurrence and durations with respect to seasonality. We demonstrate that shorelines ranging from 4.0°E to 6.2°E are dominated by wind-driven upwelling, while shorelines spanning 3.0-3.5°E are dominated by wind-driven downwelling. Furthermore, it reveals previously overlooked cells, such as around Fréjus/Cannes and Livorno/Piombino for upwelling and near Albengua for downwelling, which are however activated about 2–3 times less frequently than the prominent cells in the Gulf of Lion.Despite differential responses, these wind-driven events are more frequent during winter–spring than during summer–autumn: for both upwelling and downwelling, the mean occurrences at the most active cells are 11 days per month in winter–spring compared to 8 days per month in summer–autumn. While the main upwelling (resp. downwelling) events are generated by the prevailing northwesterlies (resp. easterlies), both winds also force the opposite process depending on the shoreline orientation and small changes in wind direction.