Seasonal cycle in the upper equatorial Atlantic Ocean

The dynamics of the seasonal cycle in the upper equatorial Atlantic ocean are studied using observations and a hierarchy of ocean models. Distinctive features of the seasonal cycle are strong annual and semiannual components: eastward (westward) propagating sea surface height (SSH) and thermocline depth at the equator (off the equator) and westward propagating surface zonal currents at the equator. Modelling results show that linear theory can explain the seasonal cycle in thermocline depth and SSH. While first-order linear theory can also explain the structure of the seasonal cycle of surface zonal currents at the equator, nonlinear terms are required; they weaken the variability and improve its phase and zonal extent. The important terms are meridional and vertical advection and vertical diffusion of zonal momentum. The linear solution is essentially determined by the four gravest baroclinic modes and Kelvin and first meridional mode Rossby waves. The eastward propagation in thermocline depth at the equator results from the Kelvin wave contribution, while the westward propagation in thermocline depth off the equator and surface zonal currents at the equator result from the first meridional mode Rossby wave. The contribution of Kelvin and Rossby waves generated by boundary reflections equals that of the directly forced waves. The semiannual cycle in zonal winds although much weaker than the annual component forces a strong semiannual component in SSH and surface zonal currents, because it excites the basin mode of the second baroclinc mode. This explains the observed feature in the seasonal cycle from March to August.