Rainfall Anomaly Patterns and Wind Field Signals over West Africa in August (1958–1989)

Abstract

Relationships between monthly West African rainfall anomaly patterns and monthly tropospheric wind changes are investigated for the 32 months of August (1958?1989) at an interannual time step. Regarding the Sahelian belt alone, results show that wet (dry) months are significantly linked to an increase (decrease) in both upper easterlies and lower southwesterlies, along with weaker (stronger) than usual midtropospheric easterlies south and under the main axis of the African Easterly Jet (AEJ). However, the most interesting signals are obtained with consideration of the West African rainfall anomaly patterns. Large droughts over Sahelian and Guinean areas are significantly associated with a less southward extension of upper easterlies and a decrease of the upper-meridional diffluence in the Hadley circulation. Contrasted rainfall anomaly patterns exhibiting both Sahelian droughts and Guinean floods are mainly linked to easterly anomalies in the equatorial stratosphere and enhanced easterlies south and under the AFJ, associated with a reduction of westerlies in the monsoon flow and midtropospheric southerly anomalies north of 15°N. The opposite-contrasted rainfall patterns (Sahelian floods and Guinean droughts) are characterized by a southward extension of upper easterlies, a decrease of the easterlies extending south of the AEJ, an increase of wind components in the monsoon flow, and an increased confluence of the meridional components in the midtroposphere above 5°N. Two indexes taking into account the vertical (200 hPa/850 hPa) and the 850 hPa latitudinal (10°N/20°N) relationships in the zonal wind component discriminate the two contrasted rainfall anomaly patterns. A third index, using the meridional components in high levels for documenting diffluence variability in the Hadley circulation, characterizes large-scale West African droughts.