Circulation mechanisms related to northeast Brazil rainfall anomalies

Abstract

This study explores the role of the upper ocean for circulation anomalies in the overlying atmosphere, with focus on the late boreal winter and extreme climatic events in northeast Brazil. Data sources comprise rainfall records in the Nordeste, surface ship observations of the sea surface temperature (SST), pressure, and wind fields (COADS, period 1948-1990), satellite observations of tropical convection (HRC, 1971(HRC, -1988, and upper air analyses of the European Centre for Medium Range Weather Forecasts (ECMWF, 1980(ECMWF, -1991, all for the tropical Atlantic sector. Interhemispheric SST gradients are most strongly associated with north-south contrasts in pressure and the meridional wind component, whereas the inverse SST-pressure relations in situ are less close. Enhanced northward temperature increase in the tropical Atlantic is accompanied by steeper meridional pressure gradient and accelerated southerly wind component, which is representative of a northward displaced Intertropical Convergence Zone (ITCZ), with the latter in turn leading to drought in northeast Brazil. Interhemispheric contrasts in the 850/1000 mbar layer mean temperature are similar to those of SST and account for most of the meridional surface pressure gradient. During wet as compared to dry years in northeast Brazil, the tropical North Atlantic is cooler and the South Atlantic warmer, and accordingly the lower tropospheric thickness is reduced to the north but inflated to the south of the equator, resulting in increased/reduced surface pressure over the North/South Atlantic, as well as reduced southerly surface wind, stronger subsidence over the outer tropics of the northern hemisphere and intensified ascending motion and convective activity over the Nordeste, related to a southward displaced near-equatorial convection belt. The surface and upper air evidence thus indicates the ways in which the interhemispheric SST gradients exert a hydrostatic control on the lower tropospheric thickness pattern, and thus force the south-north surface pressure gradients and the surface meridional wind component, and hence modulate the latitude position of the ITCZ and Nordeste rainfall.1.

Interannual relations between South American rainfall and tropical sea surface temperature anomalies before and after 1976

“…Moreover, a SST variability mode in the tropical Atlantic displaying an anti-symmetric SST anomaly pattern in relation to the equator has long been related to interannual climate variations over northeastern Brazil (e.g. Hastenrath and Heller, 1977;Weare, 1977;Hastenrath, 1978Hastenrath, , 1990Moura and Shukla, 1981;Hastenrath and Greischar, 1993;Nobre and Shukla, 1996;Souza et al, 2000). Furthermore, positive simultaneous correlations between the SST anomalies in the southwestern subtropical Atlantic and rainfall anomalies over Uruguay and southern Brazil during austral summer have been documented (Diaz et al, 1998).…”

Interannual and longer‐term variability of upper air circulation in the northeast Brazil‐tropical Atlantic sector

“…Over the past four decades the interhemispheric SST gradient in late boreal winter has been shown [Wagner, 1996] In the long-term mean a near-equatorial band of warmest surface waters is known [Hastenrath and Greischar, 1993] to induce hydrostatically a low-pressure trough, into which the lower tropospheric airstreams from the two hemispheres converge, thus forming the notorious ITCZ with its intense convective activity. The present analysis documented the associated vigorous upward motion feeding the upper tropospheric divergence and showed that the divergent outflow is directed mainly into a band of upper tropospheric convergence and subsidence over the tropical North Atlantic.…”

“…An index of March-April rainfall in Brazil's Nordeste (Figures 1 and 2e) was compiled from a network of 27 precipitation gages, as described by Hastenrath [1990] and Hastenrath and Greischar [1993].…”

Atmospheric centres of action associated with the Atlantic ITCZ position

“…The most persistent feature related to the rainy season in this region mainly in the northern sector, is the Intertropical Convergence Zone (ITCZ) (e.g. Hastenrath and Heller, 1977;Moura and Shukla, 1981;Hastenrath and Greischar, 1993). Located in the upper branch of the Hadley Cell, the ITCZ is characterized by low surface pressure (the equatorial trough), maximum sea surface temperature (SST), confluence of northern and southern trade winds, maximum convergence and maximum convective cloud cover.…”