SST/Wind stress mesoscale coupling in the Peru-Chile region : what drives its temporal variations ?

Recent air-sea interaction studies showed that mesoscale (~10-100 km) oceanic structures, e.g. fronts and eddies, induce an atmospheric response which also affect in return the near-surface ocean dynamics. In this study we focus on the oceanic feedback on the surface wind stress (SWS) in the Peru-Chile region.

We use a regional coupled model (WRF-NEMO) at ∼ 9 km horizontal resolution to characterize the interaction between sea surface temperature (SST) and SWS. We compare this coupling to observed values. Spatial and temporal variations are evidenced in the mesoscale SST-SWS relation. In particular, the coupling characteristics are more intense during winter than during summer. We examine this seasonal variation by an in-depth analysis of the underlying mechanisms. A downwind momentum balance shows that SST mesoscale anomalies are associated to wind speed and wind shear anomalies generated by downward mixing of momentum. Near-surface air pressure gradient anomalies have a negligible contribution because of the back-pressure effect related to the air temperature inversion . Turbulent stress anomalies are created by mixing coefficient anomalies and partially compensated by the wind shear anomalies. The response of the stress to the mesoscale SST is mainly modulated by the seasonal variations of the large-scale wind vertical shear.