Heat and moisture budgets of the Walker circulation and associated rainfall anomalies during El Nino events

摘要

Large-scale aspects of the heat and moisture budgets of the Walker circulation (WC) are analysed for the austral summer during the period from 1970 to 2000. Since several earlier studies discuss theoretical and modelling aspects of the heat and moisture budgets of the WC, our main objective is to make an observational analysis of such budgets, including a composite of five strong El Nino-southern oscillation events covered by the period of study. For this purpose, data from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis and Global Precipitation Climatology Project are used. Analysis of the moisture budget in the near-equatorial regions of the Southern Hemisphere showed that the three continents (South America, Africa and maritime continents) are characterized by high precipitation and that low precipitation occurs in the intermediate oceans. Regions of high (low) precipitation are associated with the vertically integrated moisture flux convergence (divergence). Analysis of the heat budget for the WC showed, as deduced in some theoretical studies, that the diabatic heating (cooling) is balanced by adiabatic cooling (heating) over regions with a convective (non-convective) regime. The loss of radiation to space over non-convective areas is balanced by adiabatic heating due to the sinking motion. There is heating over the continents, probably associated with the ascending motion in the WC, and cooling over the intermediate oceans, which is related to the loss of radiation in the descending branch of the WC. Analysis of the moisture balance for the entire period and a composite of El Ni?o periods shows that the WC is associated with precipitation variations in the near-equatorial regions of the continents. The low-level changes in the WC are associated with the moisture flux changes and have the main role in the maintenance of the rainfall anomalies during the El Nino events.