Links between African easterly waves, midlatitude circulation and intraseasonal pulsations of the West African heat low

During summer 2006, the intensity of the thermal depression over the Sahara, derived from European Centre for Medium-range Weather Forecast analyses, exhibited a strong decrease during the first couple of weeks of September. Simultaneously, widespread convective activity over the Sahel was detected. The aims of this study are to identify the reasons for this decrease of the West African Heat Low (WAHL) activity, to discuss the possible relationship with convection and to assess the representativity of such an event at the climatological time-scale. From spectral analysis of the daily WAHL thickness during summer 2006, two period bands of significant intraseasonal pulsations were identified, and confirmed using the ERA-40 reanalysis, namely 3–10 d and 10–30 d. In both the 2006 case-study and the climatological composite study, we find that detrimental conditions in the 3–10 d period band are associated with moist and cool advections in the lower troposphere linked to the southerly sector of African easterly waves (AEWs) which increase convective activity over the Sahel. These humid and cold advections from the south are more pronounced when the AEW interacts with a midlatitude depression. In the 10–30 d period band, the impact of midlatitude circulation is demonstrated. During the collapsing period of the 10–30 d pulsations of the WAHL thickness, an upper-level trough is seen over northern West Africa. This situation generates a surge of 700 hPa cold air from Libya into the Sahara, an increase of the 925 hPa anticyclonic circulation and a significant increase of convective activity over the Sahel.