Onset and variability of the South American Monsoon

Although the seasonal evolution of high cloudiness and associated circulation features are well documented for South America, it has proven difficult to elucidate the precise mechanisms for the onset of the monsoon season in this part of the world. The main objectives of this project are to improve our understanding of 1) the interannual variability and predictability of monsoon season onset and precipitation in South America, and 2) the mechanisms for monsoon onset.

As a first step toward achieving these goals, we will present a study of the interannual variability of monsoon season onset in South America using data from the Global Precipitation Climatology Project (GPCP) dataset from 1979-2004. The NCEP reanalysis data will be used to determine the atmospheric circulation and thermodynamic characteristics (850 mb and 200 mb winds, vertical profiles of equivalent potential temperature) during the pentads prior to, during and following SAMS onset for 1979-2004. The NCEP reanalysis fields will help to determine how the large-scale dynamic and thermodynamic conditions evolve as onset progresses, and to assess regional differences in large-scale conditions at the time of onset. The focus will be on variations in the occurrence and strength of the SALLJ, changes in extent and strength of tropical easterly winds, circulations associated with propagating baroclinic systems, and the strength and location of the Bolivia High and midlatitude jet.