Propagation of a 14.7-Day Wave along the Northern Coast of the Guinea Gulf

Long time series of sea level and sea surface temperatures measured at different coastal stations along the northern coast of the Gulf of Guinea are analyzed statistically. The results indicate that pronounced fortnightly oscillations in sea level are composed of two waves: one is the lunar fortnightly tide Mf (13.661day period) which has a constant phase all along the coast. The other wave has a period of 14.765 days which is the period of the luni-solar fortnightly tide Msf; this wave propagates westward along the eastwest oriented coastline with a mean phase speed of 53 cm s-' and a wavelength of 675 km. These waves have important effects on the thermal structure and give rise to strong vertical oscillations of th$ subsurface isotherms throughout the year. The sea surface temperature, however, has pronounced oscillations around the Msf frequency during the upwelling season (June-September) only. The 14.765-day wave is of tidal origin and is due to a nonlinear interaction of the Mz and Sz (barotropic or baroclinic) tides but the generation mechanism is obscure.