On the Dynamics of Droughts in Northeast Brazil: Observations, Theory and Numerical Experiments with a General Circulation Model

Abstract

It is proposed that a possible mechanism for the occurrence of severe droughts over northeast Brazil is the establishment of a thermally direct local circulation which has its ascending branch at about 10°N and its descending branch over northeast Brazil and the adjoining oceanic region. The driving for this anomalous circulation is provided by warming due to enhanced moist convection associated with warmer sea surface temperature anomalies over the northern tropical Atlantic, and cooling associated with colder sea surface temperature anomalies in the southern tropical Atlantic. The combined effects of thermally forced subsidence and reduced evaporation and moisture flux convergence produces severe drought conditions over northeast Brazil. We have examined the monthly mean sea surface temperature anomalies over the tropical Atlantic and rainfall anomalies over two selected stations (Fortaleza, 3°46?S 38°31?W and Quixeramobim, 5°12?S 39°18?W) for 25 years (1948?72). It is found that the most severe drought events are associated with the simultaneous occurrence of warm sea surface temperature anomalies over north and cold sea surface temperature anomalies over the south tropical Atlantic. Simultaneous occurrences of warm sea surface temperature anomaly at 15°N, 45°W and cold sea surface temperature anomaly at 15°S, 5°W were always associated with negative anomalies of rainfall, and vice versa. A simple primitive equation model is used to calculate the frictionally controlled and thermally driven circulation due to a prescribed beating function in a resting atmosphere. The heating function is designed to simulate a heat source to the north and a heat sink to the south of the equator. This prescribed thermal forcing produces a thermally direct circulation with ascending motion to the north and descending motion to the south. Low-level cyclonic circulation and high-level anticyclonic circulation is generated to the north of the equator and low-level anticylonic circulation and high-level cyclonic circulation is generated to the south of the equator. The analytical solutions agree well with the results of numerical experiments carried out with a multilevel global general circulation model. We also have carried out a series of numerical experiments to test the sensitivity of the GLAS (Goddard Laboratory for Atmospheric Sciences) model to prescribed sea surface temperature anomalies over the tropical Atlantic. It is found that the sea surface temperature anomaly patterns, which resemble the observed ones during drought years, produce an intensified convergence zone, enhanced rainfall and low-level cyclonic circulation to the north, and reduced rainfall and anticyclonic circulation to the south. The reduction of rainfall over continental northeast Brazil is large enough to give further support to the proposed mechanism. Since the sea surface temperature anomalies over the tropical Atlantic persist for several months. the proposed mechanism could provide guidance for predicting droughts over northeast Brazil.