Spatiotemporal Variability of West African Rainfall. Part II: Associated Surface and Airmass Characteristics

Serge Janicot METEO-France, Service Central d'Exploitation Météorologique, Paris, France

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Abstract

In the second part of this study, the variability of surface and airmass characteristics associated with West African rainfall fields is investigated. Ship observations over the tropical Atlantic and rawinsonde records over West Africa are used for the months of July, August, and September from 1948 to 1978.

A composite analysis based on the sign of monthly rainfall departures north of 10°N indicates that the Sahelian drought may result from a more southern than normal position of the intertropical convergence zone (ITCZ) over the tropical Atlantic.

A composite analysis into four monthly rainfall types is performed by using rainfall information north and south of 10°N. It shows two types of Sahelian drought. The first is linked to an enhanced surface atmospheric circulation over the northern tropical Atlantic, higher sea surface temperatures in the Gulf of Guinea, a convergent moisture flux anomaly south of 10°N along the western African coast, and a lower (larger) water content in the monsoon flux north (south) of 10°N. It may be associated with a southward location of the ITCZ and of the Hadley cells. The second is related to positive sea level pressure and surface easterly circulation anomalies over the northern tropical Atlantic and weaker convection inside the ITCZ over West Africa. It may be associated with an enhancement and/or an eastward and equatorward displacement of the subsiding branch of the Atlantic Walker cell.

Abstract

In the second part of this study, the variability of surface and airmass characteristics associated with West African rainfall fields is investigated. Ship observations over the tropical Atlantic and rawinsonde records over West Africa are used for the months of July, August, and September from 1948 to 1978.

A composite analysis based on the sign of monthly rainfall departures north of 10°N indicates that the Sahelian drought may result from a more southern than normal position of the intertropical convergence zone (ITCZ) over the tropical Atlantic.

A composite analysis into four monthly rainfall types is performed by using rainfall information north and south of 10°N. It shows two types of Sahelian drought. The first is linked to an enhanced surface atmospheric circulation over the northern tropical Atlantic, higher sea surface temperatures in the Gulf of Guinea, a convergent moisture flux anomaly south of 10°N along the western African coast, and a lower (larger) water content in the monsoon flux north (south) of 10°N. It may be associated with a southward location of the ITCZ and of the Hadley cells. The second is related to positive sea level pressure and surface easterly circulation anomalies over the northern tropical Atlantic and weaker convection inside the ITCZ over West Africa. It may be associated with an enhancement and/or an eastward and equatorward displacement of the subsiding branch of the Atlantic Walker cell.

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