Wind and thermal conditions along the equatorial Pacific

Variability in heat storage of the equatorial Pacific Ocean during El Nino Southern Oscillation episodes is analyzed from subsurface temperature observations in order to show how the patterns of heat content are related to changes in the Walker Circulation. Large-scale zonal gradients in depth of the thermocline (and dynamic height) are largely in equilibrium with local zonal wind stress. Depth of the thermocline is positively correlated with mixed layer heat content and sea-surface temperature in the central Pacific during a minor El Nino/Southern Oscillation episode in 1969, indicating that the oceanic dynamic response to wind forcing as well as thermodynamic response to surface heat fluxes influence local heat storage. The distribution of heat in the mixed layer along the equator has distinctive patterns associated with pre-El Nino and mature El Nino stages. Heat is accumulated in the western Pacific during the first stage and in the central Pacific during the second. The shift in pattern is associated with a change in direction of zonal wind stress in the western Pacific. Rainfall observations show that the ascending branch of the Walker cell is always located over the region where heat is accumulated in the ocean. These observations are incorporated into a hypothetical model of the mechanism by which the equatorial ocean and atmosphere become locked into an anomalous pattern during El Nino periods.