Influence of the Rossby waves on the seasonal cycle in the tropical Atlantic

The importance of the long "midlatitude" Rossby waves to the seasonal cycle in the Tropical Atlantic is investigated. An analytical model of the baroc linic Rossby waves is forced with the annual and semiannual components of t he climatological wind stress in the reopica (20 degrees S to 20 degrees N) , excluding the Equator. The Rossby waves are fast enough in the tropics to influence the ocean on the seasonal timescale. The seasonal migrations of the sea surface troughs (lows) and ridges (highs) are linked to the forced Rossby waves and to their latitudinal speed dependency. The sea surface hei ght of the analytical Rossby wave solutions are compared to data from the T OPEX/Poseidon satellite, the Levitus et al. [1994] climatology, a high-reso lution ocean general circulation model, a baroclinic primitive equation mod el, and a numerical solution of the Rossby wave equation. The regions where the Rossby waves seem to play an important role are those with the stronge st observed seasonal variability, in particular the region just north of Br azil. The forced Rossby wave is composed of one free Rossby wave part and o ne part due to the local forcing; both of equal importance in the whole reg ion. The observed phase propagation is westward and decreases with latitude but not as much and as clearly as the theoretical Rossby waves do.