Observations of Planetary Boundary-Layer Structure in the Eastern Equatorial Pacific

Abstract

Atmospheric soundings were collected from a research ship in the eastern equatorial Pacific to determine the kinematic and thermodynamic structure of the planetary boundary layer (PBL) in the vicinity of a strong front in sea surface temperature (SST). These observations have been used to test the hypothesis proposed by Wallace et al. and Hayes et al. that the low-level wind shear was enhanced on the cold side of the front because of greater static stability. The PBL structure was analyzed from a transect along 110°W from 10°N to 5°S, with a focus on the ?500-m-deep atmospheric mixed layer, as defined with respect to virtual potential temperature (?v). Significant mixed-layer variability was found due to diurnal and other effects, but there were systematic differences in mean structure as a function of latitude. The mixed layer was neutrally stratified over warm water between 6.5°N and 1.5°N and was slightly stably stratified over the cooler water between 1°N and 2°S. The mean total vertical wind shear within the mixed layer (mostly in the form of speed shear) was ?2?10?1 s?1 over the warm water and ?8?10?3 s?1 over the cooler water. These observations are consistent with the hypothesis proposed by Wallace et al. and Hayes et al. and can account for the weakness in southerly winds at the surface on the cool side of the oceanic frontal zone.