Flux Modification due to Couple Wind and SST Impacts Oceanic Ekman Upwelling

Satellite observations have revealed a small-scale (< 1000 km) air–sea coupling in regions of strong sea surface temperature (SST) fronts (e.g., currents, eddies, and tropical instability waves), where the surface wind and wind stress are altered by the underlying SST. Surface winds and wind stresses are persistently higher over the warm side of the SST front compared to the cool side, causing perturbations in the dynamically and thermodynamically curl and divergence fields. Capturing this small-scale SST–wind variability is important because it can significantly impact both local and remote (i.e., large scale) oceanic and atmospheric processes. The SST–wind relationship is not well represented in numerical weather prediction (NWP) and climate models, and the relative importance of the physical processes that are proposed to be responsible for this relationship is actively and vehemently debated. This study focuses on the physical mechanisms that are primarily responsible for the SST-induced changes in surface wind and wind stress, and on the physical implication on ocean forcing through Ekman pumping. This study shows that the baroclinic-related changes in Ekman pumping are significant (first-order) over a seasonal (2003 winter season) time scale and that the meso-scale impacts are quite important over larger spatial scales. These findings highlight the need to consider the small-scale SST–wind relationship even in coarser resolution model simulation, which may be feasible to parameterize because of the linear nature of the baroclinic-related effect.