An investigation of the Annual Cycle of Convective Activity over the Tropical Americas

Abstract

Outgoing longwave radiation (OLR) is used to describe the annual cycle of convection that resides over the Amazon Basin during austral summer and over Central America and the adjacent waters of the Pacific during austral winter. The preferred locations of the convective activity during the wet season in the respective hemispheres are determined, and the beginning and ending of these seasons is specified. The onset of the wet season over Amazonia usually occurs within a single month, while the onset of the wet season over Central America typically requires from one to three months. The annual cycle of convective activity in this regime is shown to exhibit a seasonal regularity and degree of symmetry with respect to the equator which exceeds those characterizing the other two annually varying regimes in the tropical belt. Analyses produced by the European Centre for Medium-range Weather Forecasts (ECMWF) are superimposed upon OLR fields to illustrate features of the atmospheric circulation in the vicinity of the tropical Americas that are associated with the annual cycle of convection. The onset and demise of the wet season in the Amazon Basin are further described by means of composites of these data. It is found that the Bolivian high inferred from the ECMWF data develops rapidly during the onset transition in a manner that is temporally and spatially consistent with the distribution of OLR.