Air-sea interaction in the Gulf of Guinea at intraseasonal time-scales: wind bursts and coastal precipitation in boreal spring

The role of airsea interaction in the boreal spring precipitation of the West African monsoon is explored through the wind variability in the Gulf of Guinea. Linear regressions are performed in MayJune during the decade 20002009 to investigate the origin and effect of the surface wind strengthening north of the Equator. It appears that the equatorial sea-surface temperature cooling intensifies a surface-wind equatorial divergence/coastal convergence circulation, and generates a cross-equatorial pressure gradient, which both strengthen the southerlies north of the Equator. This increases subsidence above the ocean and convection in the northern Gulf of Guinea. In addition, an abrupt change is observed in the surface wind pattern in the eastern equatorial Atlantic (EEA) between April and July. To investigate the transition mechanisms, a reference date is defined as the date when the surface wind north of the Equator becomes and remains stronger than south of the Equator. Thus the maintenance of strong southerlies north of the Equator is linked to a coincident installation of a deep circulation on the whole troposphere and a northward shift of the low atmospheric local circulation. The resulting sharp seasonal transition coincides each year with a southeasterly wind burst, suggesting that the equatorial SST cooling plays a role in the precipitation along the African coast during boreal spring. Copyright (c) 2012 Royal Meteorological Society