Interannual variability of Atlantic hurricane activity and some features of West African climate

Abstract

The aim of this study is to describe over the Atlantic Ocean and West Africa, the large scale differences between an inactive hurricane period (1991 - 1994) and an active hurricane period (1998 - 2001), before and during the peak of the cyclone season (May - June - July; MJJ and August- September - October; ASO). Over West Africa, the monsoon flow at low level extends more northward during the active period. This period is also characterized by the northward shift and the westward extension over the Atlantic Ocean of the African Easterly Jet (AEJ) and by the existence of a strong Tropical Easterly Jet (TEJ). Moreover, intense low-level cyclonic vortices propagate in an unstable and weak sheared environment. These favorable conditions were generally present over Africa and over the North Atlantic Ocean during MJJ. The relationships between Atlantic cyclonic activity and the West African climate are strong during active cyclone years. African Easterly Waves (AEWs) are more intense and more frequent during the active period, both in their Sahelian and Gulf of Guinea tracks, while over the ocean, their activity and number decrease. During MJJ, the AEWs are more active over West Africa during active than inactive years. Key words: African easterly waves, tropical cyclones, African monsoon.