Observation of Rossby Waves Off the West African Coast Contributing to Offshore Extent of Phytoplankton.

In December 2002 and January 2003 satellite observations of Chlorophyll showed

a strong coastal signal off the west african coast between 10◦ and 22◦ N. A wavelike

pattern with a wavelength of about 750 kms was observed from December 20th 2002 up to mid January 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, was applied during 5 days. A Kelvin wave quickly developed along the

northern edge of the cape, then propagated and left the area in a few days. Simultaneously, a

Rossby wave whose characterisics are 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. Such a Rossby wave can contribute to the offshore extent of dissolved nutrients, and phytoplankton.