Inter-annual variability of physical and biogeochemical dynamics in the South-East Atlantic Ocean, focusing on the Benguela upwelling System: Remote versus local forcing

The respective contribution of equatorial remote (Equatorial Kelvin Waves) and local atmospheric (wind, heat fluxes) forcing on the South-Eastern Atlantic near-shore coastal physical and biogeochemical dynamics is investigated using a set of 4 sensitivity experiments based on a regional physical-biogeochemical coupled model. Model results show that over the 2000-2008 period, at sub-seasonal time-scales (10-100 day), the coastal oceanic variability (currents, temperature, density, thermocline and sea level) is mainly driven by the local forcing, while at inter-annual timescales (13-20 months), it is dominated by the remote forced Coastal Trapped Waves (CTW) that propagate poleward along the African West coast up to the Northern part of the Benguela Upwelling System at 24°S. During their propagation, CTW induce significant temperature (±2°C), vertical (±5m.day^-1) and alongshore currents (±5cm.s^-1) anomalies with maximum values in sub-surface. Temperature anomalies are primarily driven by alongshore and vertical advection processes. Then, evidence is presented that most of the inter-annual variability of surface and sub-surface nutrient, chlorophyll and oxygen concentrations along the African West coast are also mainly forced by CTW. The currents associated with upwelling CTW induce an input of nutrients that trigger a substantial increase of primary production and phytoplankton in the Benguela Upwelling System while downwelling CTW induce a decrease of primary production and phytoplankton. Finally, we put in evidence that two triggering mechanisms limit the southward propagation of the CTW : the inter-annual variability of the equatorward Benguela current prescribed at the model Southern boundary (30°S) and the variability of the local forcing that modulates the magnitude of the observed coastal inter-annual events. When the local wind stress forcing is (out) in phase, the magnitude of the inter-annual event (decreases) increases.