Intraseasonal modulation of African Easterly Wave activity

In summer, African Easterly Waves (AEWs) are the dominant mode of variability over West Africa on synoptic scales, and they contribute to the modulation and spatial organization of convection and precipitation within the monsoon. They are also considered as precursors for tropical depressions and cyclones developing in the Atlantic. Recent work by Leroux et al. (2010) has highlighted that AEW activity varies significantly on intraseasonal timescales, and they pointed out the influence of the background environment. Ventrice et al (2011) have also suggested the role of the Madden Julian Oscillation. Our work aims to explore further the intraseasonal modulation of AEW activity in two other directions: (1) How does variation in AEW activity relate to the leading modes in convection on shorter intraseasonal time scales than the MJO (10-25 day, “Quasi-Biweekly-Zonal-Dipole” and “Sahel” mode, e.g. Janicot et al 2011)? (2) How does variation in AEW activity relate to the occurrence of extratropical troughs penetrating over northern Africa? The influence of the latter on AEW activity has been suggested in recent work by Chauvin et al (2011) and Leroux et al (2011). Our analysis is based on MERRA and ERA-interim reanalysis for the dynamical fields, and on OLR and CLAUS brightness temperature as convection proxies. AEW activity is diagnosed through Eddy Kinetic Energy (EKE) of the 2-10 day pre-filtered wind at 850 hPa, as well as through spatial EOF analysis of the 2-10 day pre-filtered convective signal in the West African region. On the other hand, the first two leading modes in convection on intraseasonal timescales are isolated through spatial EOF analysis of the 10-25 pre-filtered data. Extra-tropical troughs penetrating over northern-eastern Africa are detected in the Potential Vorticity (PV) field at 700 hPa. The statistical relationship between AEW activity and these different intraseasonal modes is analyzed at different time-lags with regression/composite techniques, and compared to some selected case studies.