Abstract

An ocean model is used to study the effects of remote annual forcing in the eastern tropical Atlantic. The model is linear, viscid and continuously stratified. The ocean basin is an idealized version of that of the tropical Atlantic, and the wind stress forcing the model is an idealized representation of the annual variation of the equatorial trades in the western Atlantic. Solutions are represented as expansions of the baroclinic modes of the system. The response of each mode is found numerically, not by integrating the equations of motion forward in time, but at a fixed frequency (2π year–1) using techniques that are typically used in models of the tides. Prominent features of the solution are the following. When the remote trades strengthen, sea level drops and the pycnocline rises markedly throughout the Gulf of Guinea. At 4W the annual response is tightly trapped to the equator and to the coast of Africa near 5N. In contrast, the response propagates offshore along the southern coast of Africa near 10E. Events propagate upward everywhere in the Gulf of Guinea and poleward (nearly) everywhere along the coast of Africa. These features compare favorably with observations. A single baroclinic mode does not dominate the response. Instead, waves associated with several modes superpose to form beams that propagate energy vertically as well as horizontally (McCreary, 1984). Along the equator the response is predominantly a combination of a beam of equatorial Kelvin waves and a lowest order (l – 1) Rossby beam. Along the coast of Africa at 5N it is primarily a beam of coastal Kelvin waves.

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