Physical drivers of seasonal sea surface temperature variability off Northwest Africa: upwelling and beyond

The upwelling region off Northwest Africa exhibits pronounced seasonal variability and high productivity, playing a critical role in supporting regional fisheries. However, the physical processes driving its seasonal sea surface temperature (SST) variability remain insufficiently understood. Here, we combine daily satellite observational estimates and ocean model simulations to analyze the seasonal cycle of SST from 1993 to 2022 and apply a mixed-layer heat budget analysis to identify the underlying physical drivers. We distinguish between a permanent upwelling region (21–30°N) and a seasonal upwelling region (12–19°N), where upwelling peaks in boreal winter and spring and weakens during summer. Both satellite and model data reveal distinct air–sea interactions between the two regions. In the permanent zone, the coastal–offshore SST difference lags wind-driven upwelling by 75–85 days, whereas in the seasonal zone, it leads by 15–20 days. In the permanent region, wind-driven upwelling, vertical diffusion, and zonal advection associated with the North Equatorial Current jointly maintain cooler coastal SST relative to offshore waters throughout the year. In contrast, in the seasonal region, meridional advection associated with the poleward Mauritania Current drives coastal warming in summer, while wind-driven upwelling leads to coastal cooling in winter and spring. These findings highlight the combined roles of upwelling and large-scale ocean circulation in regulating the Northwest African upwelling system, and provide a foundation for improving predictions of upwelling variability and its impacts on regional marine ecosystems.