A modelling framework for EBUS: from seasonal to decadal time scales

Abstract

Along major Eastern Boundary Upwelling Systems (EBUS) local and seasonal changes in the equatorward wind stress (e.g. Ekman Upwelling) and cross-shore gradients of the wind (e.g. Ekman Pumping) are the main drivers of the three dimensional ocean circulation that controls coastal sea surface temperature (SST) and the upwelling of ocean tracers (e.g. nutrients). Through modulation of these local conditions, also climate modes seem to have an impact on upwelling, regulating the EBUS variability at interannual and decadal timescales. In light of the importance of the forcing, in a ocean modelling framework, the aims of this thesis are to understand the impact of different wind structures on the dynamics of SST and upwelling and, to detect and compare low-frequency variability and changes in EBUS. To achieve these objectives we performed long-term ocean hindcasts with the global ORCA025 configuration (25 km of horizontal resolution) of NEMO general circulation model forced by three atmospheric products, that differ for spatial resolution. The numerical results suggest that (1) coastal upwelling and Ekman pumping are locally forced by differences in alongshore wind stress and wind stress curl, respectively; (2) equatorward currents are intensified under enhanced wind stress condition, while poleward undercurrents respond to wind stress curl changes; (3) SST differences are mostly related to a stronger nearshore wind stress rather than incoming heat fluxes (4) global covariability across EBUS is not detected, at basin scale the El Niño–Southern Oscillation and its low frequency modulation exert influence on Pacific systems while the Atlantic Meridional Oscillation is the predominant mode over Atlantic domains.