Evolution of Marine Atmospheric Boundary Layer Structure across the Cold Tongue–ITCZ Complex

Abstract

The structure of the marine atmospheric boundary layer (MABL) over the tropical eastern Pacific Ocean is influenced by spatial variations of sea surface temperature (SST) in the region. As the MABL air is advected across a strong SST gradient associated with the cold tongue?ITCZ complex (CTIC), substantial changes occur in the thermodynamic structure, surface fluxes, and cloud properties. This study attempts to define and explain the variability in the MABL structure and clouds over the CTIC. Using data collected on research cruises from the fall seasons of 1999?2001, composite soundings were created for both the cold and warm sides of the SST front to describe the mean atmospheric boundary layer (ABL) structure and its evolution across this front. The average difference in SST across this front was ?6°C; much of this difference was concentrated in a band only ?50 km wide. During the fall seasons, on the cold side of the gradient, a well-defined inversion exists in all years. Below this inversion, both fair-weather cumulus and stratiform clouds are observed. As the MABL air moves over the SST front to warmer waters, the inversion weakens and increases in height. The MABL also moistens and eventually supports deeper convection over the ITCZ. Both the latent and sensible heat fluxes increase dramatically across the SST front because of both an increase in SST and surface wind speed. Cloudiness is variable on the cold side of the SST front ranging from 0.2 to 0.9 coverage. On the warm side, cloud fraction was quite constant in time, with values generally greater than 0.8. The highest cloud-top heights (>3 km) are found well north of the SST front, indicating areas of deeper convection. An analysis using energy and moisture budgets identifies the roles of various physical processes in the MABL evolution.