Interannual variability in the South‐East Atlantic Ocean, focusing on the Benguela Upwelling System: Remote versus local forcing

Abstract

AbstractWe investigate the respective roles of equatorial remote (Equatorial Kelvin Waves) and local atmospheric (wind, heat fluxes) forcing on coastal variability in the South‐East Atlantic Ocean extending up to the Benguela Upwelling System (BUS) over the 2000–2008 period. We carried out a set of six numerical experiments based on a regional ocean model, that differ only by the prescribed forcing (climatological or total) at surface and lateral boundaries. Results show that at subseasonal timescales (<100 days), the coastal oceanic variability (currents, thermocline, and sea level) is mainly driven by local forcing, while at interannual timescales it is dominated by remote equatorial forcing. At interannual timescales (13–20 months), remotely forced Coastal‐Trapped Waves (CTW) propagate poleward along the African southwest coast up to the northern part of the BUS at 24°S, with phase speeds ranging from 0.8 to 1.1 m.s−1. We show that two triggering mechanisms limit the southward propagation of CTW: interannual variability of the equatorward Benguela Current prescribed at the model's southern boundary (30°S) and variability of local atmospheric forcing that modulates the magnitude of observed coastal interannual events. When local wind stress forcing is in (out) of phase, the magnitude of the interannual event increases (decreases). Finally, dynamical processes associated with CTW propagations are further investigated using heat budget for two intense interannual events in 2001 and 2003. Results show that significant temperature anomalies (±2°C), that are mostly found in the subsurface, are primarily driven by alongshore and vertical advection processes.