The impact of moist processes on the African easterly jet–African easterly wave system



The African Easterly Jet–Easterly Wave (AEJ–AEW) system was explored in an idealised model. Prescribed zonally symmetric surface temperature and moisture profiles determine the AEJ which becomes established through meridional contrasts in dry and moist convection.

As in previous studies, a realistic AEJ developed with only dry convection. Including moist processes, increased its development rate, but reduced its speed and meridional extent. AEWs grew through barotropic–baroclinic conversions. Negative meridional potential vorticity (PV) gradients arose in the zonally symmetric state through the intrusion of the low-PV Saharan boundary layer. Since moist processes strengthened this significantly through diabatically generated PV in the Intertropical Convergence Zone, moist AEWs were three times stronger. Larger barotropic conversions and faster AEJ development increased the moist wave growth-rate. Jet-level and northerly low-level amplitudes grew, but in the moist case the low-level amplitudes weakened as the AEW interacted with convection, consistent with their absence from observations during the peak monsoon. Striking dependencies between the AEJ, AEW and rainfall existed. Two time-scales governed their evolution, depending on the transfer coefficients: (1) the AEJ's replenishment rate influenced by heat fluxes, and (2) the wave growth-rate, by damping, and the slower jet development rate.

Moist AEWs were characterized by intermittent growth/decay, with growth preceded by increased mean rainfall and later, weakening AEJs. These dependencies established an internal 8–10-day variability, consistent with intra-seasonal observations of 9-day rainy sequences. This internal variability offers an alternative explanation to the previously proposed external forcing and a new view of the moist AEW life cycle. Copyright © 2009 Royal Meteorological Society