An Observed Trend in Central South American Precipitation


Brant Liebmann

NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado

Carolina S. Vera

Centro de Investigaciones del Mar y la Atmósfera/CONICET, Departamento de Ciencias de la Atmósfera y los Océanos, Universidad de Buenos Aires, Buenos Aires, Argentina

Leila M.V. Carvalho

Department of Atmospheric Sciences, Institute of Astronomy and Geophysics, University of São Paulo, São Paulo, Brazil

Inés A. Camilloni
Centro de Investigaciones del Mar y la Atmósfera/CONICET, Departamento de Ciencias de la Atmósfera y los Océanos, Universidad de Buenos Aires, Buenos Aires, Argentina

Martin P. Hoerling, Dave Allured
NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado

Vicente R. Barros
Departamento de Ciencias de la Atmósfera y los Océanos, Universidad de Buenos Aires, Buenos Aires, Argentina

Julián Báez
Dirección de Meteorologia e Hidrologia, y Dirección Nacional de Aeronautica Civil (DINAC), Luque, Paraguay

Mario Bidegain
Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay


(Manuscript received 15 January 2004, in final form 26 May 2004)

Abstract

Seasonal linear trends of precipitation from South American station data, which have been averaged onto grids, are examined, with emphasis on the central continent. In the period 1976-1999, the largest trend south of 20°S occurs during the January-March season, is positive, and is centered over Southern Brazil. From 1948-1975 the trend is also positive, but with less than half the slope. The trend is not due to a systematic change in the timing of the rainy season, which almost always starts before January and usually ends after March, but rather results from an increase in the percent of rainy days, and an increase in the rainy day average. The dynamic causes of the trend are not obvious. It does not appear to be accounted for by an increase in synoptic wave activity in the region. The precipitation trend is related to a positive sea-surface temperature trend in the nearby Atlantic Ocean, but apparently not causally. The trend in the Atlantic seems to result from a decrease in mechanical stirring and coastal upwelling associated with a decrease in the strength of the western edge of the circulation associated with the South Atlantic High.