Dynamics and impacts of eddy-driven air-sea interaction in a regional air-sea coupled model for the US West Coast

Abstract:

The US West Coast coastal oceans feature energetic mesoscale eddies. The associated sea surface temperature (SST) and surface current modify the wind stress, leading to significant dynamic feedback on to the air-sea coupled system. Dynamics of the interaction and impacts on the regional coastal climate are however not well understood; this is an important research question for regional modeling studies for the coastal climate. A high-resolution (7km) SCOAR regional air-sea coupled climate model is used to investigate this question by implementing a novel model coupling technique that separates spatial scale of air-sea interaction. It allows the large-scale coupling effect to be preserved while suppressing the eddy-driven coupling via interactive spatial smoothing of SST and surface current. When the eddy-induced surface current is allowed to modify the wind stress, the eddy kinetic energy (EKE) is reduced by 42%, and this is primarily due to enhanced surface eddy drag. In contrast, the eddy-induced SST-wind coupling has little impact on the EKE. Eddies also modify the Ekman pumping; the resultant Ekman pumping velocity due to surface current attenuates the amplitude of eddies while the SST-induced Ekman pumping affects the propagation of eddies. Rectified change in time-mean SST is determined by the altered offshore temperature advection by the mean and eddy currents, but the magnitude of the mean SST change is greater with the eddy-induced current effect. The subsequent influence on the downstream winter rainfall variability on the US West Coast is stronger with the eddy-induced SST effect because of the proximity of SST anomalies to the coasts. The strong dynamical response in the coastal climate system to the eddy-driven air-sea interaction suggests that the fine-scale air-sea coupling should be better represented in the regional climate modeling studies for the coastal environments and the marine weather.