The Atmospheric Water Vapor Cycle in South America and theTropospheric Circulation

Abstract

The main characteristics of the atmospheric water vapor cycle over the South American continent and the adjacent oceans are investigated using the 22-yr period, from 1976 to 1997, of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) 40-Year Reanalysis Project database. Precipitation rate and water vapor content fields obtained from this dataset are compared over the region with newly available observed datasets, which combine ground-based and satellite-derived observations. The temporal variation and spatial distribution of the atmospheric water vapor balance equation terms (precipitation rate, evaporation rate, and water vapor flux convergence) are examined with regard to their consistency and relative importance. The net effect of the atmospheric water vapor transport, represented in the last term of the balance equation, is decomposed into the horizontal and vertical convergence terms. The analysis of the latter highlights those regions where the topographic uplift makes a substantial contribution to the total precipitation rate. The former term is further decomposed into the stationary and transient water vapor flux contributions. The comparison of these terms with relevant characteristics of the large-scale tropospheric circulation provides a better understanding of the different precipitation regimes in South America. The mean annual balance satisfactorily closes over most of the oceanic regions. However, important imbalances found in the vicinity of high topographic features, such as in the central Andes, are attributed to large errors in the local computation of the atmospheric water vapor flux. The current results corroborate previous findings on the role of the stationary water vapor flux convergence in the spatial distribution and seasonal variation of the rainfall rate in tropical and subtropical latitudes and extend over the less-investigated continental midlatitudes. The magnitude of the transient water vapor flux convergence is, in general, lower than that of the stationary flux. Nonetheless, in some oceanic and continental regions, they are comparable and seem to be dynamically linked. This interaction, which can be explained by means of a simple transport-gradient model of the transient water vapor flux, could help to clarify the observed seasonal and interannual variability of the rainfall rate in the humid-to-dry transition zone in the southern part of the continent.