Variations in intensity of the westerly monsoon-like flow from the tropical Atlantic and summer rainfall over equatorial and tropical southern Africa
Résumé
An empirical orthogonal function (EOF) of the onshore flow of moisture along the west coast of southern Africa using NCEP-DOE AMIP-2 re-analyses suggests two dominant modes of variability that are linked to: (a) variations within the circulation linked to the mid-latitude westerlies and the South Atlantic anticyclone, (b) the intensity of the westerly flow from the tropical Atlantic. The second mode, referred to as the Equatorial Westerly mode, contributes the most to moisture input from the Atlantic onto the subconti-nent at tropical latitudes. This mode appears to be associated with large-scale rainfall anomalies over the upper lands surrounding the Congo basin in January–February, with potential impacts on land hydrology persisting until April–May to the east of the Great Rift valley. It is preceded in November–December by a strengthening/weakening of the South Atlantic anticyclone. Enhanced (reduced) advection of moisture over the Congo basin is accompanied by increased (inhibited) convection processes. In the positive phase of this mode, the excess water vapour is channelled from the Congo basin to the east and southeast at surface, while the southern extension of the African Easterly Jet (AEJ) could play a role in transporting more moisture southwards at mid-tropospheric levels, leading to above-normal rainfall. During its negative phase, often related to ENSO, an eastward shift of the ascending branch of the Walker-type circulation is found to reduce convection and thus rainfall over the upper lands surrounding the Congo basin. Further research into water vapour transport is required to better understand southern African rainfall variability.
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