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, 1971–1988), and upper air analyses of the European Centre for Medium Range Weather Forecasts (ECMWF, 1980–1991), 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.