Atmospheric Circulation Changes Associated with Rainfall Anomalies over Tropical Brazil

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  • 1 Climate Analysis Center, National Meteorological Center, NOAA/NWS, Washington. DC 20233
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Abstract

During February–March 1981, striking contrasts existed in the rainfall distribution over most of Brazil and in the atmospheric circulation features over South America and the North and South Atlantic Oceans. Drier than normal conditions prevailed during February in the eastern and northeastern sections of Brazil. This was followed by an excessively wet March. The onset of the rainfall was associated with a low level equatorward propagating convergence zone that originated as a cold front at higher latitudes in the Southern Hemisphere. During the period of strong convection, when observed rainfall rates in many areas exceeded 20 mm d−1, a strong anticyclonic circulation developed in the upper troposphere to the west-southwest of the maximum rainfall rates. Another anticyclonic center developed north of the equator forming a couplet which is strikingly similar to the theoretical upper level flow pattern associated with a tropical heat source. The Northern Hemisphere midlatitude circulation changes over the Atlantic are quite similar to those associated with the recently studied 30–60 day oscillation, and to the North Atlantic Oscillation.

Abstract

During February–March 1981, striking contrasts existed in the rainfall distribution over most of Brazil and in the atmospheric circulation features over South America and the North and South Atlantic Oceans. Drier than normal conditions prevailed during February in the eastern and northeastern sections of Brazil. This was followed by an excessively wet March. The onset of the rainfall was associated with a low level equatorward propagating convergence zone that originated as a cold front at higher latitudes in the Southern Hemisphere. During the period of strong convection, when observed rainfall rates in many areas exceeded 20 mm d−1, a strong anticyclonic circulation developed in the upper troposphere to the west-southwest of the maximum rainfall rates. Another anticyclonic center developed north of the equator forming a couplet which is strikingly similar to the theoretical upper level flow pattern associated with a tropical heat source. The Northern Hemisphere midlatitude circulation changes over the Atlantic are quite similar to those associated with the recently studied 30–60 day oscillation, and to the North Atlantic Oscillation.

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