Circulation Regimes of Rainfall Anomalies in the African-South Asian Monsoon Belt

Abstract

This study explores the spatial differentiation of climate anomalies and associated circulation mechanisms across the African-South Asian monsoon belt through empirical analyses mainly for the period 1948?83. Observations include surface ship observations in the tropical Atlantic, eastern Pacific, and Indian oceans, and various hydrometeorological and circulation index series, representing the water discharge of the Senegal River (SENEGAL), the rainfall in the West African Sahel (SAHEL), the discharge of rivers in the Nile basin (ROSEIRES, ATBARA), India monsoon rainfall (NIR), and the Southern Oscillation (SO). The field significance of correlation patterns is ascertained through Monte Carlo experiments. There is a strong correlation of hydrometeorological conditions from Senegal to the Sahel, a decrease from each of these domains eastward to the Nile catchment, and even more so to India. Conversely, correlations are remarkably high between the water discharge of the Nile basin (ROSEIRES, ATBARA) and Indian rainfall (NIR). An SO index is correlated positively with the hydrometeorological conditions throughout the monsoon belt, but most strongly in the East (NIR) and least in the West (SENEGAL). Correlation analyses for the July-August height of the boreal summer monsoon indicate that abundant rainfall in the western Sahel is, in the Atlantic sector, associated with weak northeast trades and in the western Indian Ocean with low pressure overlying cool surface waters. By comparison, copious river discharge in the eastern portion of the Subsaharan zone coincides with weakened northeast trades over the Atlantic but more pronounced circulation departures in the Indian Ocean sector, consisting of anomalously low pressure, cold surface waters, and abundant cloudiness in the northern Indian Ocean. This ensemble of atmosphere-ocean anomalies is also characteristic of abundant Indian monsoon rainfall. During the positive SO phase, rainfall tends to be relatively abundant throughout the monsoon belt, but most markedly so in the eastern portion of Subsaharan Africa and in India. Overall, the western Sahel shows strongest associations with circulation departures in the tropical Atlantic, whereas the eastern portion of Subsaharan Africa and India are more closely related to the, circulation of the Indian Ocean sector.