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Keywords:

  • African rainfall;
  • atmospheric circulation;
  • composite analysis;
  • ENSO;
  • interannual variability;
  • NCEP/NCAR reanalysis;
  • seasonal patterns;
  • tropical SST

Abstract

A 47-year record (1951–1997) of gridded data covering Africa south of the Sahara was used to document the spatial and seasonal patterns of the correlation between precipitation and sea-surface temperatures (SST) in key tropical areas, as depicted by the NIÑO3, South Atlantic and North Atlantic indices. El Niño–Southern Oscillation (ENSO) is confirmed as playing a dominant part in northeastern, eastern and southern Africa. However, its impact is also found over the Sahel during the northern summer, and other parts of the Gulf of Guinea region outside this season, a hitherto poorly documented feature. Over these two areas, ENSO and Atlantic SST (predominantly South Atlantic) contribute to different parts of the rainfall variance. The correlation with South Atlantic SST appears as a south–north dipole (positive/negative correlation) which shifts northward following the Inter-tropical Convergence Zone (ITCZ) translation between the northern low-sun and high-sun periods. A typing of the seasonal correlation patterns and a mapping of the multiple correlation coefficients are carried out in order to synthesize the space–time impacts of the three SST indices. Decadal-scale changes affect the strength of the teleconnections with both Atlantic and East Pacific SST, as reflected for instance by a small rise of the correlation with the NIÑO3 index since 1970–1975 in the Sahel and southern Africa, and additional shifts for the Atlantic Ocean, but the main patterns remain generally apparent over the whole period.

The circulation anomalies associated with the teleconnections were assessed using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. A study of the dataset accuracy in depicting long-term climatic variations revealed that a major shift, mainly artificial, is found in 1967–1968 in the time-series of most of the variables. The rest of the work thus concentrated on the 1968–1997 period. A number of changes in east–west circulation patterns have been found to be associated to ENSO variations. Over West Africa, El Niño events tend to result in enhanced northeasterlies/reduced monsoon flow, coupled to weakened upper easterlies, and hence dry conditions over West Africa close to the surface position of the ITCZ, in July–September, as well as January–March. Over the southwestern Indian Ocean, the positive equatorial temperature/geopotential height anomalies, which at 200 hPa accompany El Niño events, are conducive to an eastward shift of the mid-latitude upper troughs, thus being detrimental to summer rainfall over South Africa. Abnormally wet ‘short rains’ in East Africa can be accounted for by an ENSO-forced weakening of the equatorial Walker-type (east–west) cell which is found over the Indian Ocean during that season. By contrast, the impact of South Atlantic warmings is mostly shown in low-level dynamics, as exemplified by the weakened trades and monsoon flow which directly result in a southward shift of the ITCZ. The combination of ENSO and Atlantic SST anomalies are found to give rise to complex wind flow changes in the near-equatorial Atlantic. In addition to large-scale SST-forced atmospheric dynamics, a few regional atmospheric signals are found to explain residual parts of rainfall variance. For instance, a strengthening of the African Easterly Jet, or northerly wind anomalies across the Sahara, are shown to be related to drought conditions in the Sahel (July–September) and the Gulf of Guinea area (January–March), once the remote effect of SST anomalies is removed. Copyright © 2001 Royal Meteorological Society