The boreal summer upwelling along the northern coast of the Gulf of Guinea (GG) is characterized using new in situ sea surface temperature (SST) from onset sensor and satellite TRMM Microwave Imager (TMI) datasets. This study aims to encourage intensive in situ SST measurements at the northern coast of the Gulf of Guinea. It shows good agreement between daily in situ SST and TMI SST and similar coastal upwelling onset date, end date, and durations calculated using both datasets. Interannual evolution of the onset date at four stations along the northern coast of GG indicates that the upwelling can be initiated at one cape or simultaneously at both the cape of palms and the cape of three points. It can be also initiated eastward towards Cotonou or globally off all the northern coasts of GG. Nonsignificant trend is found on upwelling onset date and end date variability. Moreover, this study shows that SST is significantly warm or cold some years. Ocean conditions during these years are related to known physical processes. 1. Introduction Coastal upwellings are characterized by seasonally low sea surface temperature (SST). They generally result from the response of the coastal ocean to alongshore winds, leading to the production of a relatively intense current with a small offshore and a large alongshore component . This causes the pumping of cooler and nutrient-rich waters from the subsurface to the ocean surface. Upwelling areas are economically important even though the global area constituted by these regions is less than 1% of the global ocean . Moreover, coastal upwellings have a great impact on local climate. Particularly, the coastal ocean surface conditions in the Gulf of Guinea situated in the northeastern equatorial Atlantic influence the West African climate . Understanding the ocean dynamic of this region is then of great interest, (i) firstly because the Gulf of Guinea is the principal source of the water vapour which constitutes most of the precipitation on the continent. For example, Gu and Adler  linked the rainfall peak in May along the coastal area of the Gulf of Guinea to the seasonal forcing of the ocean. Eltahir and Gong  observed that the intensity of the West African monsoon depends on the meridional gradient of the static humid energy in the boundary layer between the ocean and the continent. (ii) Secondly, this tropical Atlantic area has the largest SST seasonal amplitude of about 5–8°C . A coastal upwelling is observed each year along the northern coast of the Gulf of Guinea during the boreal winter and summer
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