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Atmosphere-ocean mechanisms of rainfall anomalies at the coast of eastern Africa are studied using long-term ship observations in the Indian Ocean, surface current measurements, subsurface casts, upper air analyses by the European Centre for Medium Range Weather Forecasts, rain gauge series in eastern Africa and India, and an index of the Southern Oscillation (SO), the high-SO phase being defined by anomalously high/low pressure at Tahiti/Darwin. The causalities of precipitation anomalies at the coast of eastern Africa differ for the two rainy seasons centered on April-May and October-November, and only the latter is strongly related to the SO. In the high-SO phase, April-May pressure is low over the entire Indian Ocean domain, whereas in October-November, pressure is high in the west and low in the east. Concomitantly, surface waters are anomalously cold in the west, and strong westerlies sweep the equatorial zone of the Indian Ocean. Eastern African rainfall anomalies are related to the SO through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern Africa. (1) Equatorial westerly winds are conducive to lower tropospheric divergence over equatorial East Africa, and in the high-SO phase these are accelerated, especially in October-November, owing to the anomalous eastward pressure gradient. (2) The equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere, which entails cold-water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds. (3) In addition, cold-water anomalies in the western Indian Ocean, most pronounced in October-November during the high-SO phase, also suppress convection. (4) In the high-SO phase, the Indian summer monsoon tends to be strong, leaving behind an anomalously cold western Indian Ocean, which in turn feeds into mechanisms 1 to 3. The eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern Africa. 1.reversal of the wind field between winter and summer. It is, then, not surprising that meteorological and oceanographic research has traditionally focused on the extremes of the planetary-scale annual cycle and particularly the boreal summer monsoon, with its powerful ocean current systems, copious rainfall, and latent heat release over southern Asia. By comparison, little attention has been given to the monsoon transitions, which feature a short-lived and intense upper oceanic jet in the equatorial zone of the Indian Ocean and are in fact the main rainy seasons in most of eastern Africa. The interannual variability in this region has by statistical analyses [Ogallo, 1988;Ogallo et al., 1988; Farmer, 1988;Hutchinson, 1992;Beltrando and Camberlin, 1993] been associated with the Southern Oscillation (SO). Likewise, on purely statistical grounds, Farmer [1988] and Hutchinson...

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 191 1--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 'S&cas' of CearA province are characterized by an equatorward expansion of the South Atlantic, and a poleward retraction of the North Atlantic, highassociated 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.

The seasonal evolution of anomalous interhemispheric sea surface temperature (SST) gradients in the tropical Atlantic from January to April is studied by composites of the 10 warmest (warm) and 10 coldest (cold) Januaries during 1948–1993 in the equatorial Pacific using Comprehensive Ocean‐Atmosphere Data Set ship observations. In the warm as compared with the cold years, an anomalous weakening of the northward SST gradient develops, mainly due to anomalous warming in the tropical North Atlantic. This stems from the combination of three forcings all related to the weakened North Atlantic trade winds during Pacific warm events. Most important are the reduced latent heat flux in much of the tropical North Atlantic and anomalous downwelling equatorward of 20°N, with a further contribution from increased net radiation resulting from the reduced cloudiness due to the diminished convergence in the downstream portion of the North Atlantic trades. In response to the development of warm anomalies in the tropical North Atlantic during January–March, the cross‐equatorial northward winds accelerate to April, and this leads south of the equator not only to Ekman downwelling, enhanced divergence, reduced cloudiness, and increased net radiation, but also to enhanced wind speed and evaporation. The result is a modest anomalous warming in the western tropical South Atlantic. Increased air temperature over the entire basin, presumably due to atmospheric advection from the Pacific, has little effect on the Atlantic SST pattern. The anomalous interhemispheric SST gradient, controlled primarily by the warm anomaly in the North Atlantic, has a pivotal role to play in steering the late boreal winter atmospheric circulation in the tropical Atlantic sector and thus regional climate anomalies.

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.

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