Seasonal changes in the Somali Current system are studied using a four-layer model where the lowest layer is at rest. The results suggest that barotropic instability is likely to cause the generation of the Great Whirl in early June. We find a very good agreement between the observed undercurrents and the simulations in the model. Equatorial onshore flow below the thermocline in June is associated with the disappearance of the undercurrent below the Somali Current. The early return of the southward undercurrent in the fall is caused by baroclinic instability of the Great Whirl followed by a reversal to eastward undercurrents along the equator. Remote winds are suggested to control the intermediate depth flow along the Somali coast acting through equatorial waves and Rossby waves generated along the west coast of India. Experiments where the duration of the summer monsoon is extended show that the initial decrease in the magnitude of the Great Whirl is due to eastward and downward energy transfer rather than due to relaxation of the wind. The model solutions suggest that baroclinic instability plays an important role in the decay of the Great Whirl. ¿ American Geophysical Union 1991