Air-sea interaction along the California coast and offshore waters

The present study demonstrates COAMPS ability to replicate observed air-sea interaction over the CA current in summertime. We examine 4-yr summertime statistics of COAMPS forecast wind stress and analyzed SST along the U.S. west coast and offshore waters. In a tightly coupled system, the atmospheric response to a change in SST is characterized by the wind stress curl (WSC) and wind stress divergence (WSD). These fields are averaged into overlapping 29-day means at seven-day intervals so as to minimize weather variability and emphasize time scales appropriate for oceanic forcing. Evaluation of individual 29-day averages as well as the entire 4-yr summertime mean shows fine-scale features in the derivative wind stress fields that closely correspond to spatial variations in the SST gradients. Correlation maps indicate broad regions where there is a tight interaction between the ocean and atmosphere.

A robust atmospheric response to oceanic forcing is evident within 100 km of the coast despite the influence of the topographic barrier, which imparts abrupt variation in the low-level wind field independent of the underlying SST. The U.S. west coast region in summertime is demonstrated to be a fully coupled system at the air-sea interface by the linkage of topographically generated wind stress curl patterns to variations SST and the SST feedback on the atmosphere via perturbations in the wind stress derivatives. Away from the influence of the coast, air-sea interaction is enhanced by 30%. COAMPS offshore coupling coefficient, the slope between anomalies of crosswind SST gradient magnitude and wind stress curl, are about 20% less than for QuikSCAT observations. However, it is considerably greater than those reported by other mesoscale, regional and climate models, which utilize more coarsely resolved SST fields for the atmosphere's lower boundary condition over the ocean.