Multi-decadal trends in SST, chlorophyll- a, NPP and atmospheric forcing across oligotrophic and upwelling regions of the Northeast Atlantic

Since the post-industrial era, sea surface temperature (SST) has shown a consistent warming trend at a global scale, while chlorophyll-α (Chl-α) concentrations have generally exhibited declining trends in the open ocean. Some recent studies suggest that intensified coastal upwelling, driven by increased alongshore winds, may locally counteract these negative trends by enhancing biological productivity. This study assesses the evolution of long-term trends in SST, Chl-α, meridional wind stress and Saharan dust over the Northeast Atlantic, focusing on both open-ocean oligotrophic regions and coastal upwelling systems. We applied the methodology developed by Siemer et al. (2021), using the same satellite and in situ datasets, updated to include the most recent years, and defining additional subregions to better resolve smaller open-ocean areas of interest. Our results reveal a significant acceleration of SST warming across the entire study area during the last six years. In open-ocean regions, this acceleration is accompanied by a strengthening of negative Chl-α trends, indicating a continued decline in phytoplankton biomass. In contrast, coastal upwelling regions, particularly the Northwest African upwelling system, exhibit a slowdown in the decline of Chl-α and productive area. However, trends in upwelling-favourable wind stress over the African coast are predominantly negative, suggesting a weakening of the atmospheric forcing traditionally associated with enhanced coastal productivity. The inclusion of Saharan dust variability allows us to assess the combined role of atmospheric forcing and aerosol deposition in modulating recent biophysical trends in the region.