Abstract. In December 2002 and January 2003 satellite observations of Chlorophyll showed a strong coastal signal along the west african coast between 10° and 22° N. In addition, a wavelike pattern with a wavelength of about 750 kms was observed from December 20th 2002 and was detectable for one month in the open sea, south west to the Cape Verde peninsula. Such a pattern suggests the existence of a locally generated Rossby wave which slowly propagated westward during this period. To verify this hypothesis a numerical study based on a reduced gravity shallow water model has been conducted. A wind burst, broadly extending over the region where the offshore oceanic signal is observed, is applied during 5 days. A Kelvin wave quickly develops along the northern edge of the cape, then propagates and leaves the area in a few days. Simultaneoulsly, a Rossby wave whose characterisics seem similar to the observed pattern forms and slowly propagates westward. The existence of the peninsula limits the extent of the wave to the north. The spatial extent of the wind burst determines the extent of the response and correspondingly the time scale of the phenomenon (about 100 days in the present case). When the wind burst has a large zonal and small meridional extent, the behaviour of a wave to the north of the peninsula differs from that to the south. These results are corroborated and completed by an analytical study of a linear reduced gravity model using a non-Cartesian coordinate system. This system is introduced to evaluate the potential impact of the coastline shape. The analytical computations confirm that, considering the value of the wavelength, a time scale around 100 days can be associated with the observed wave. They also show that the role of the coastline remains moderate at such time scales. On the contrary, when the period becomes shorter (smaller than 20–30 days), the behaviour of the waves is modified because of the shape of the coast. South of the peninsula, a narrow band of sea isolated from the rest of the ocean by two critical lines appears. Its meridional extent is about 100 km and Rossby waves could propagate there towards the coast.