Cross-equatorial structure and temporal modulation of intraseasonal variability at the surface of the Tropical Atlantic Ocean

Intraseasonal variability (10-50 days) in the equatorial Atlantic Ocean is analyzed from multiyear (1999-2005) satellite gridded products of sea-level anomalies (SLA) and sea-surface temperature (SST). Two regions with distinct intraseasonal variability have been identified. The first one, west of 10 degrees W, is dominated by westward-propagating anomalies, with maximum values in SLA along 5 degrees N and in SST along 2 degrees N: They occur in boreal summer with periods of 25-50 days and are known to correspond to tropical instability waves (TIWs). We show that TIWs have also a signature, though weaker, south of the equator, especially along 5 degrees S, in SLA. Northern and southern anomalies propagate together westward, being mostly out of phase, suggesting that equatorial wave dynamics is involved in TIWs variability. An SST signature of TIWs is also observed near 2 degrees S, in quadrature with SST anomalies detected in the Northern Hemisphere. The interannual modulations of the TIW signature in SLA and SST are compared and discussed. The second dominant intraseasonal signal is only seen east of 10 degrees W in SST and corresponds to an equatorially trapped variability, confined to the Gulf of Guinea with periods between 10 and 20 days. This signal is present in boreal summer when an intense SST front is observed just north of the equator. Intraseasonal variability with comparable periods is also observed in the meridional wind stress throughout the year. Comparison of SST and meridional wind stress anomalies suggests that the 10- to 20-day variability in SST is forced by the wind stress but seasonally modulated by the presence of the SST front.