The structure of the marine atmospheric boundary layer (MABL) over the tropical eastern Pacific Ocean is influenced by the spatial variations of sea surface temperature (SST) in the region. As the MABL is advected across a strong SST gradient associated with the cold tongue-ITCZ complex (CTIC), changes occur in the thermodynamic structure, surface fluxes, and cloud properties. The purpose of this study is to define and explain the variability in the MABL structure and clouds over the CTIC. Using data collected on cruises from the fall seasons of 1999-2001 and spring seasons of 2000- 2001, composite atmospheric soundings were created for both the cold and warm sides of the SST front to describe the mean ABL structure and its evolution across this front. Observations from the cruises and the TAO buoy array provide a consistent description of the surface sensible and latent heat fluxes along 95 and 105 °W transects. Remote sensing observations from the ship and satellite analyses define the evolution of cloud properties across the CTIC.
The average SST gradient observed during the fall cruises was about 5°C over 5° latitude, while for spring observations the cold tongue virtually disappears and SST gradients are weak. 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 can occur. As the MABL air moves over the equatorial front to warmer waters, the inversion weakens and increases in height while the entire MABL moistens and warms. This rapid evolution of the boundary layer structure develops conditions with the potential for supporting deeper convection over the ITCZ. Cloudiness is variable on the cold side of the SST front ranging from 0.2 - 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 equatorial front, indicating areas of deeper convection. Both the latent and sensible heat fluxes increase dramatically across the SST front due to both an increase in SST and wind speed. An analysis using energy and moisture budgets defines the the roles of the surface fluxes and entrainment on the observed MABL evolution.