The Eastern Pacific Investigation of Climate Processes (EPIC) experiment was designed to improve understanding of the intertropical convergence zone (ITCZ), its interactions with the cold tongue of water that extends along the equator, and the physics of the cloud deck that forms over the cool waters off South America. In this presentation, we analyze the structure and evolution of the solar and longwave cloud forcing, its relation to water vapor, rainfall, and surface temperature, and its effect on the ocean temperature and salinity fields. The primary data used in the analysis are from the EPIC enhanced monitoring array that includes an IMET mooring at 20S, 85W in the stratus deck region, 10 enhanced Tropical Atmosphere and Ocean (TAO) moorings along 95W, and sounding, CTD, surface flux and boundary layer data collected from the TAO tender ship which visited the 95W moorings at 6 monthly intervals.

During the three 1999-2001 fall transects, a strong northern hemisphere ITCZ and well-developed cold tongue on the equator was observed. In contrast, during the three 2000-2002 Spring transects, a variety of ITCZ structures were observed: a double ITCZ during Spring 2000, a single equatorial ITCZ during Spring 2001, and a single southern ITCZ during the Spring 2002 transect. In all cases, the ITCZ deep convection required sea surface temperatures in excess of ~28C. Rainfall associated with both the northern hemisphere and southern hemisphere ITCZs caused freshening in the ocean surface salinity. At all sites, solar cloud forcing was anti-correlated with longwave cloud forcing: Clouds caused both a reduction in solar radiation and an increase in downwelling longwave radiation. However, while solar cloud forcing magnitudes were largest in the northern hemisphere ITCZ region with values of roughly -175 W/m2, longwave cloud forcing was largest in the southern hemisphere stratus region, with values of roughly 50 W/m2. In all regions, the solar cloud forcing magnitude was larger than the longwave cloud forcing. Implications for understanding and modeling eastern tropical Pacific ocean-atmosphere interactions will be discussed.