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Abstract

Departure characteristics of the large-scale circulation are studied in relation to extreme drought and flood years in northeastern Brazil identified from a collective of long-term rainfall stations and series of river discharge. Ship observations during 1911–72 compiled with a one degree square resolution and extending between 30°N and 30°S from the African coast to the eastern Pacific form a major observational basis.

The rainy season of northeastern Brazil is narrowly centred around March/April and is related to the southernmost seasonal migration of a lower-tropospheric confluence axis over the adjacent eastern tropical Atlantic. the ‘Sěcas’ of Ceará province are characterized by an equatorward expansion of the South Atlantic, and a poleward retraction of the North Atlantic, high - associated with a northward displacement of the enclosed near-equatorial trough of low pressure. Concurrently, the zonally oriented bands of maximum cloudiness and precipitation frequency stay farther north, the North Atlantic trades weaken, and the South Atlantic trades become stronger than in the long-term mean. the sea surface temperature pattern during a deficient rainy season in northeast Brazil is characterized by positive departures in a broad band across the North Atlantic and in the eastern Pacific, and anomalously cold waters in most of the South and equatorial Atlantic. During abundant rainy seasons in northeast Brazil, departure patterns are approximately inverse to those typical of drought years.

A strong negative linkage exists between northeast Brazil rainfall and sea surface temperature along the Ecuador/Peru coast. This seems to be caused by inverse long-term pressure variations over the eastern South Pacific and South Atlantic oceans, which in turn may be part of large-scale mass adjustments of the Southern Oscillation type.

Development of the equatorward expansions of the North and South Atlantic highs, pre-season precipitation in northeast Brazil and the Guayanas, and location of the cloudiness and precipitation belts over the adjacent western tropical Atlantic, are among the more promising predictors for seasonal foreshadowing of extreme rainy season behaviour in northeast Brazil.