Air-sea coupling over the California Current from 4 years of summertime COAMPS forecasts

Air-sea coupling has been explored recently in several observational analyses utilizing satellite measurements of wind stress and SST over the world's oceans. Among these studies is that of Chelton et al. (2007) over the California current for which a robust linear relationship is found between wind stress curl and divergence to cross- and downwind SST gradient. From these findings and the well-know impact of wind stress and wind stress curl at driving coastal upwelling through Ekman transport and pumping, this region should be considered a fully coupled system. Unfortunately, many climate and mesoscale models generally contain insufficient horizontal resolution to capture the observed air-sea coupling signatures that occur on such fine scales or utilize spatially averaged SST fields in which these scales are damped or severely smoothed.

In this study, the predictive capability of air-sea coupling is examined within the Navy's mesoscale predictive system COAMPS, forced at the lower boundary by an analyzed sea surface temperature (SST) updated at a 12-h frequency. We find that the COAMPS model contains robust air-sea coupling features congruent with satellite observations over the California current. Statistics from a 9-km horizontal resolution reanalysis over U.S. West coastal and offshore waters encompass 4 summer time periods between 2002-2005. Summertime means, monthly/daily/hourly standard deviations of SST, wind stress and their gradient or derivative fields are computed to ascertain variability and coupling between the ocean and overlying atmosphere. Evaluation of correlation maps and grid point time series of overlapping monthly averages suggests regions of strongest coupling reside upwind of persistent, localized ‘expansion fan' features in the marine atmospheric boundary layer wind field. These locations have strong wind forcing that drive coastal upwelling and transport SST filaments offshore, tightening SST fronts that then feedback to the atmosphere through the wind stress curl and divergence. In comparison with observed coupling coefficients documented by Chelton et al. (2007), COAMPS yields a weaker atmospheric response to SST gradients by 25%, about half of which is related to COAMPS SST which has 4 times the resolution of the satellite SST retrievals.