On the Atmospheric Boundary Layer over the Equatorial Front

Abstract

This paper presents surface meteorological data and boundary layer profiles from a single meridional transect across the equator along 125°W collected during the Pan American Climate Study mooring cruise in September of 1998. These observations are unique because they occurred at the start of a cold event in the eastern tropical Pacific when the SST gradients along the equatorial front were near their climatological maximum. In addition to the anomalous cold conditions encountered near the equator, an added benefit of this limited study is the relatively high station resolution across the front. The meridional minimum in SST, and center of the cold tongue, was 20.9°C and occurred at 0.4°N. An equatorial front was located to the north where SST increased linearly to 25.5°C at 2.0°N. The atmospheric boundary layer (ABL) was stable over the cold tongue. An unstable ABL developed over the warm side of the front in less than 55 km. The unstable ABL was found at two more stations over the warm side of the front and had a height of 350?450 m. Collocated with the formation of the unstable ABL was a rapid acceleration of the surface wind field. These observations are consistent with the hypothesis that the surface wind field is modulated by stability-dependent boundary layer effects. These observations also suggest that the spatial scale of the surface wind acceleration is less than the spatial scale of the SST front.