Stefan Hastenrath

Interannual Variability and Annual Cycle: Mechanisms of Circulation and Climate in the Tropical Atlantic Sector

Abstract General circulation mechanisms instrumental in both annual cycle and interannual variability of rainfall are studied with reference to key regions of the tropical Americas and Africa, including the Central American–Caribbean area, northern Northeast Brazil, Subsaharan Africa, the Angola coast and the zaire (Congo) and Amazon basins. For most of these regions, rainfall anomalies tend to be associated with departures in the large-scale atmospheric and oceanic fields that correspond to the pattern changes in the annual alternation of dry and rainy seasons. The interannual variability of climate and circulation thus appears largely as enhancement and reduction of the annual cycle.

Atmospheric‐hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean

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.

Circulation mechanisms related to northeast Brazil rainfall 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, 1971–1988), 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.

Forcing of anomalous sea surface temperature evolution in the tropical Atlantic during Pacific warm events

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.

Variations in Low-Latitude Circulation and Extreme Climatic Events in the Tropical Americas

Abstract Departure characteristics of the general circulation in the tropical Atlantic and eastern Pacific are studied in relation to extreme climatic events identified from collectives of long-term rainfall stations and other hydrometeorological parameters, with emphasis to the Central American-Caribbean region (CARIB). Ship observations during 1911–72 compiled with a 1° square latitude-longitude resolution and extending between 30°N–30°S from the African coast to the eastern Pacific form a major observational basis. The quality of the rainy season in CARIB has a large negative correlation with annual precipitation in the U.S. Central Great Plains, and with rainfall and sea surface temperature along the Ecuadorian/Peruvian littoral; in addition there are weak negative correlations with northeastern Brazil rainfall and with discharge and water level in northern tributaries of the Amazon, and a strong positive correlation with hydrometeorological events in Subsaharan Africa. Northeastern Brazil rainfall is...

Further Work on the Prediction of Northeast Brazil Rainfall Anomalies

Abstract This study expands our earlier climate prediction work for Brazils Nordeste to develop methods of forecasting the March–June precipitation with differing lead times by exploring the potential of various data sources and options of information extraction. Observations include indices of Nordeste rainfall, an index of sea surface temperature (SST) in the equatorial Pacific, and the fields of meridional wind component and SST in the tropical Atlantic. Empirical orthogonal function (EOF) analysis was applied to construct indices of the meridional wind component and SST. These series formed the input to stepwise multiple regression models, an experimental neural network model, as well as to linear discriminant analysis. The dependent dataset 1921–57 (excluding 1943–47) was used for the method development, while the independent dataset 1958–89 was reserved for prediction. Of primary interest is the prediction of March–June rainfall from information through January. A new SST dataset with improved qual...

Estimates of holocene precipitation for Rajasthan, India, based on pollen and lake-level data

Abstract A pollen profile obtained from lake sediments at Lunkaransar, Rajasthan, in northwest India was used along with a pollen-climate calibration function to estimate past precipitation. Between 10,500 and 3500 yr B.P., the estimated precipitation was about 500 mm/yr, or about 200 mm/yr above the modern value. A model was used for the energy and hydrologic budget of a lake basin and lake at Sambhar (located 240 km SE of Lunkaransar) to calculate that a 200 mm/yr increase in rainfall above the modern amount would have raised the lake level about 20 m above the modern level. Topographic charts and satellite imagery provided some evidence in support of an enlarged paleolake of that elevation, but field exploration would be required to confirm the size and date of a former lake. After about 3500 yr B.P., the Lunkaransar profile indicated a desiccated lake bed; because no pollen was preserved, the pollen-climate calibration function was of no use for estimating the amount of the precipitation decline. A reduction of precipitation of about 200 mm/yr below the modern amount was estimated from the energy and hydrologic budget for paleolake Sambhar by assuming that the lake was one-tenth of its present size during the dry interval.

Late Pleistocene climate and water budget of the south American Altiplano

Abstract Geomorphic evidence from the Peruvian-Bolivian Altiplano indicates that during episodes prior to 28,000 and around 12,500-11,000 yr B.P., lakes covered an area about six and four times as large as at present, respectively. Within the constraints of the heat and water budget, model calculations are used to estimate the precipitation rate that would allow hydrologic equilibrium. On this basis it is suggested that rainfall on the Altiplano during the episodes of enlarged lakes was, respectively, some 300 and 200 mm annum−1 larger than at present, representing increases of about 75 and 50%, respectively. Field evidence suggests that the episodes of enlarged lakes on the Altiplano may have preceded or coincided with periods of maximum glaciation in the neigh-boring Andes. In this region with high elevation of the ice equilibrium line, increased precipitation is particularly conducive to glaciation.

Case studies of extreme climatic events in the Amazon basin

Abstract The mechanisms of climate anomalies in the Amazon basin were explored from surface climatological and hydrological series, upper-air, and satellite observations. The paper is focused on the March–April rainy season peak in the northern portion of Amazonia. Case studies for the moderately wet year 1986 (WET), showed a relatively far-southerly location of the Atlantic near-equatorial trough, and embedded intertropical convergence zone (ITCZ); strong ascending motion and vigorous convection over the Amazon basin, contrasting with pronounced subsidence off the west coast of South America,, and weak subtropical westerly jets (STWJ) in both hemispheres. In contrast, the extremely dry El Nino year 1983 (DRY), featured a more northward located ITCZ; subsidence over the Amazon basin; ascending motion and convective rainfall to the west of the Andes; and strong STWJ. In synthesis from these analyses, some major mechanisms of extreme rainfall events in northern Amazonia stand out, but only for the late aust...

Recent Advances in Tropical Climate Prediction

Abstract A review is given of the current status of long-range forecasting in the low latitudes. Promising leads have developed over the past five years in the seasonal forecasting for certain target regions of the Tropics. Various approaches are of interest: (a) empirical methods based on the combination of general circulation diagnostics and statistical techniques; (b) numerical modeling, itself requiring also a diagnostic understanding from empirical analyses; and (c) purely statistical techniques. Regional targets include Indian monsoon, Nile and Ethiopia, eastern Africa, southern Africa, Sahel, Nordeste, North Atlantic storms, northwest Pacific storms, Australia, and El Nino and Southern Oscillation. Forecasts in real time are now being issued for most of these targets, and mostly from methods combining general circulation diagnostics with statistics. El Nino and Southern Oscillation are the target of real-time forecasting from purely statistical techniques, as well as the subject of numerical modeli...