Tropical monsoons around Africa:: stability of El Nino-Southern Oscillation associations and links with continental climate -: art. no. 3151

Interannual fluctuations of monsoons around Africa and the stability of associations with the El Nino-Southern Oscillation (ENSO) and African rainfall are studied. The statistical analysis employs sea surface temperature (SST), surface and upper winds, and surface pressure averaged over key monsoon areas of the tropical Atlantic and Indian Oceans. The time series span the period 1958-1998, and wavelet analysis is applied to localize relationships in time as well as in frequency and enable us to examine how the amplitude and time delay at interannual scales varies through the record. Comparisons are made with Nino3 SST and other known ENSO signals in the African hemisphere. It is found that upper zonal winds over the tropical Atlantic are an integral part of the global ENSO. Zonal winds are associated with SST changes in the equatorial east Atlantic, which are antiphase to those in the west-central Indian Ocean. A composite analysis of warm and cool events in the Indian Ocean reveals that evaporation, radiative fluxes, and wind curl interact constructively. Anticyclonic curl (depression of isotherms) leads warm events, while cool events may initiate from oceanic advection and are sustained by evaporative fluxes. Rainfall fluctuations across Africa are analyzed, and three coherent areas are identified: West (Sahel-Guinea), Southern (Kalahari-Zambezi), and East (Kenya-Tanzania). Multivariate regression algorithms are fitted to the continuous filtered rainfall series over the period 1958-1988. Using three monsoon indices in a multivariate model, about 40% of the variance is explained at zero lag. An influential variable for most African rainfall areas is the zonal wind over the tropical Atlantic. The north-south SST gradient in the tropical Atlantic modulates rainfall in West Africa as expected. At 6 month lead, surface pressure in the north Indian Ocean is a key determinant for West African climate. For southern African rainfall, SST in the southwest Indian Ocean and monsoon indices in the west-central Indian Ocean play significant roles. East African rainfall fluctuations are linked with zonal winds in the east Indian Ocean. The findings address current Climate Variability and Predictability program (CLIVAR) priorities for understanding how continental climate interacts with ENSO and other regional modes of variability.