Multidecadal variability of moisture and heat budgets of the South American monsoon system

Abstract

The variability of sources or sinks of moisture and heat for the South American monsoon system (SAMS) region is investigated for the 1958–1995 period. So, moisture and heat budget equations are applied to data from the National Centers for Environmental Prediction (NCEP) reanalysis project. Sources or sinks of moisture and heat are the equation residues and are referred to as residue and diabatic terms, respectively. Using empirical orthogonal function (EOF) analysis, the dominant variability modes of these terms are obtained for the study period, with the monthly anomalies of the residue and diabatic terms employed. For residue EOF01, negative (positive) principal component (PC01) values correspond to positive (negative) residue anomalies over tropical South America (TSAM), which indicate a moisture source (sink) before (after) 1976, cold (warm) phase of the Pacific Decadal Oscillation (PDO). For the EOF01 of the diabatic term, negative (positive) PC01 values correspond to negative (positive) diabatic anomalies over TSAM, indicating a heat sink (source) before (after) 1976. Thus, a moisture source (sink) and a heat sink (source) occur over TSAM before (after) 1976. These findings are corroborated by composite analysis of the anomalies of precipitable water, 850 hPa air temperature, 500 hPa vertical pressure velocity, vertically integrated moisture flux (VIMF), VIMF divergence, and precipitation. In terms of system thermodynamics, these composites are indicative of SAMS weakening (strengthening) before (after) 1976, cold (warm) PDO phase.