Annual cycle and interannual variability of the mid-depth tropical Atlantic Ocean

Several shipboard sections acquired in the western equatorial Atlantic along 35 ∘ W allow for the first time to analyze the annual cycle of the density and velocity field in the upper 2000 m within this region. The amplitude of the annual harmonic of the velocity field shows several distinct maxima at the equator. Strong amplitudes up to 15 cm s - 1 are found in the depth range of the equatorial intermediate current (EIC) between 400 and 1000 m depth that are slightly shifted northward with respect to the equator. The meridional structure of the annual harmonics as well as upward phase propagation is consistent with downward propagating odd meridional mode Rossby waves. The observations are compared with a regional numerical model with very high vertical resolution. Good agreement is found between the simulated and observed structure and the amplitude of the annual harmonics. The model results suggest the presence of equatorial beams composed of wind-generated Kelvin and Rossby beams (causing seasonality in the near surface layer) as well as Rossby beams generated by the reflection of Kelvin beams at the eastern boundary (causing seasonality in the depth range of the EIC at the 35 ∘ W section). The annual cycle of the sea surface height (SSH) observed by TOPEX/Poseidon altimetry indicates an east–west seesaw pattern that corresponds to the fast response (about 30 days lag) of the zonal SSH gradient to the annual cycle of the zonal wind field at the equator due to the propagation of lowest baroclinic mode equatorial waves. The increase of the easterly winds in the western equatorial Atlantic associated with El Nino in the beginning of 1997 led almost instantaneously to an adjustment of the zonal SSH gradient with elevated (depressed) SSH in the western (eastern) basin. In contrast, the velocity field at intermediate depths lags the zonal wind forcing by several months up to a year as the model simulation reveals.