Ocean–Atmosphere Covariability in the Western Arabian Sea*

Abstract

The western Arabian Sea exhibits strong spatial variability in sea surface temperature (SST) during the southwest monsoon, with changes in SST that can exceed 5°C over 200 km. Exploration of satellite-based and in situ data shows a strong connection between mesoscale SST features and changes in the atmospheric boundary layer. The fundamental relationship is that of weak (strong) wind velocities overlying cold (warm) SST features. There are also coherent changes in other near-surface meteorological parameters, such as the air–sea temperature difference and relative humidity—indicating changes in the stability of the planetary boundary layer over the mesoscale SST features. These relationships are similar to those recently reported over the equatorial Pacific tropical instability wave region. This observed covariability of atmospheric boundary layer structure and SST results in variations of the surface heat and moisture fluxes; latent heat flux is modified by changes in relative humidity (principally through the temperature dependence of saturation specific humidity), wind speed, and boundary layer stability over the cold filaments. The nonlinear dependence of latent heat flux on the three parameters leads to a net enhancement of latent heat flux from the mesoscale features, as compared to that computed using spatially averaged parameters. Additionally, the spatial structure of the heat-flux variability will tend to dampen the mesoscale SST features. The mesoscale wind variability results in strong wind stress curl patterns on the same spatial scales as the oceanic features. The resulting Ekman pumping variations may play an important role in the evolution of the ocean eddy fields in this region. Further examination of the processes controlling the observed covariability, and the oceanic and atmospheric response to the coupling should therefore be undertaken.