Analysis of Wet Season Rainfall Over the Nordeste of Brazil, South America.

Rainfall variability in the Nordeste of Brazil, South America is extreme. This study investigates short -period (one to five day) changes in the tropical atmosphere surrounding the Nordeste during the rainy season of 1979. Data include numerous rainfall stations in the Nordeste and FGGE level lllb grid-point values of wind, temperature and humidity from the European Centre for Medium-range Weather Forecasts (ECMWF). Regions of the Nordeste over which daily rainfall is highly correlated were isolated, allowing for a general spatial description of short-period rainfall over the entire Nordeste. Frontal intrusions from the middle latitudes and upper-level cyclonic vortices appear to be important in regulating precipitation over much of the Nordeste. In order to describe temporal changes in the atmosphere associated with precipitation over the Nordeste, a single wet and dry episode is isolated based on an average rainfall index. Comparisons are made between the wet and dry episode by considering spatial analyses and cross-sections of trade -wind strength, vertical motion, moist static energy and moisture advection. During the wet episode, outstanding differences from the dry episode include weaker trade winds particularly over the tropical South Atlantic, greater moist static energy immediately over the Nordeste, and stronger and more organized vertical motions over the tropical Atlantic. These are evidence of a weaker trade-wind inversion over the Nordeste during wet days. By using a Monte-Carlo simulation procedure, weaker trades during wet days were found to be statistically significant.