Intraseasonal variability of the ocean - atmosphere coupling in the Gulf of Guinea during boreal spring and summer
Résumé
Statistical analyses of the satellite TMI sea-surface temperature (SST) and QuikSCAT surface winds in boreal spring and summer are performed to investigate the intraseasonal variability of air-sea interactions in the Gulf of Guinea. There, empirical orthogonal function decomposition shows the existence of peaks around 15 days, and their lagged cross-correlation the signature of an expected 5-day lag wind forcing and 3-day lag strong negative SST feedback. Lagged linear regressions are performed onto a reference SST index of the cold tongue northern front in the Gulf of Guinea. A cold SST anomaly covering the equatorial and coastal upwelling is forced after about one week by stronger-than-usual south-easterlies linked to the St Helena anticyclone, suggesting that intraseasonal variability in the Gulf of Guinea is connected to large-scale fluctuations in the South Atlantic. Within about 5°S and 5°N, two retroactions between SST and surface wind appear to dominate near-surface atmosphere conditions. When the wind leads the SST, stronger monsoonal winds north of 2°N are partly sustained by the developing SST anomaly and bring more humidity and rainfall toward the continent. When the SST leads the wind, a reversal of anomalous winds is observed mainly south of 2°N, closing a negative feedback loop with a biweekly periodicity. Eventually, further investigation with an ocean model emphasizes the contribution of the horizontal advection in shaping these intraseasonal SST signals. The contribution of vertical processes may also be important but was more difficult to estimate.