Air-sea interaction in the eastern Equatorial Atlantic at intraseasonal timescales in boreal spring and summer: a quasi-biweekly equatorial oscillation

A quasi-biweekly oscillation in the northern front of the eastern equatorial Atlantic's cold tongue is investigated by using observations and reanalyses in the 2000-2009 decade, with a focus on boreal spring and summer months (May to August). Fluctuations of the front latitude induce quasi-biweekly equatorial sea surface temperature (SST) anomalies that are strongly coupled with surface wind anomalies. Linear regressions performed onto a Northern Cold Tongue Index emphasize the two following mechanisms to explain the atmospheric adjustment to SST anomalies: first, a colder (warmer) ocean decreases (increases) the vertical mixing in the marine atmospheric boundary layer, which favors a weaker (stronger) surface wind; and second, a positive (negative) anomaly of SST meridional gradient induces a negative (positive) anomaly in sea level pressure meridional gradient, which accelerate (decelerate) the surface wind. The first mechanism has an immediate effect, while the second takes one or two days to adjust. Following a large-scale wind acceleration (deceleration), the second mechanism weakens (strengthens) southeasterlies in the low troposphere south of 1°N, through reversed SST and meridional pressure gradient anomalies, thereby forcing the opposite phase of the oscillation within about 6 to 8 days. Between the equator and the coast, both mechanisms possibly explain the observed intensification of the recirculation loop between the equator and the Guinean coast in the low troposphere, which sustains and accelerates surface wind around 3°N