Atmospheric response to sea-surface temperature in the eastern equatorial Atlantic at quasi-biweekly time-scales

The surface-wind response to sea-surface temperature (SST) and SST meridional gradient is investigated in the Gulf of Guinea by using daily observations and re-analyses in the 2000–2009 decade, with a focus on boreal spring and summer months (May to August), where quasi-biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi-biweekly equatorial SST anomalies. Following a large-scale wind acceleration (deceleration), an equatorial SST cold (warm) anomaly is created within a few days. In order to explain the local atmospheric response to this SST anomaly, the two following mechanisms are invoked: first, a colder (warmer) ocean decreases (increases) the vertical stability in the marine atmospheric boundary layer, which favours a weaker (stronger) surface wind; and second, a negative (positive) anomaly of SST meridional gradient induces a positive (negative) anomaly of the sea-level-pressure meridional gradient, which decelerates (accelerates) the surface wind. The first mechanism has an immediate effect in the equatorial belt between 1°S and 1°N (and to a lesser extent between 3°S and 1°S), whereas the second takes 1 or 2 days to adjust and damps anomalous southeasterlies up to 800 hPa in the low troposphere between 7°S and 1°N, through reversed anomalies of meridional SST and pressure gradient. This negative feedback leads to weaker (stronger) winds in the southeastern tropical Atlantic, which forces the opposite phase of the oscillation within about 1 week. Around the Equator, where the amplitude of the oscillation is found to be maximal, both mechanisms combine to maximize the wind response to the front fluctuations. Between the Equator and the coast, a low-level secondary atmospheric circulation takes control of the surface-wind acceleration or deceleration around 3°N, which reduces the influence of the SST-front fluctuations.