This is the second of two papers which provide a comparison between the state of the atmosphere observed during the GATE Phase 3 period (30 August - 19 September 1974) and a recent 15-year (1979-1993) climatology for the same three-week period that is based on the ECMWF Re-Analyses (ERA). While the first paper by Vincent et al. focused on the background climatological flow in which the African Easterly Waves (AEWs) were embedded, the present paper extends this analyses to some characteristics of the AEWs. Several ERA variables (e.g., low-level zonal and meridional wind components, relative vorticity) were subjected to a 2-6 day bandpass filter to represent the AEW activity. Band- pass-filtered Outgoing Longwave Radiation (OLR) will also be used as an independent indicator for the convective activity associated with AEWs.
First, we examine how anomalous the AEW activity was during the Phase 3 period. To approach this goal, we will examine the long-term mean AEW activity for this three-week period and how this period is embedded in the mean seasonal cycle of the wave distur- bances. Next, we will address the question if, and up to what vertical level, in the bound- ary layer a diurnal cycle is discernible in the wave activity. Such a diurnal cycle was noted by Reed et al. (1977) to occur only at the surface, both over West Africa and the eastern Atlantic Ocean. The gridded ERA data set contains six pressure levels below 500 hPa which enables us to investigate (a) if the vertical range of the diurnal cycle in the band- pass filtered parameters is related to the north-south decline in the mixed layer height over land, and (b) if such a diurnal cycle also exists in the ERA data off the coast of West Africa over the open Atlantic Ocean. If time permits, we will elaborate on some further land-sea contrasts in the AEW activity, as they were noted by Reed at al. for the Phase 3 period, and provide some comments on potential effects of data sparsity, changes in the data base, and internal dynamics of the ECWMF forecast model on the ERA climatology of AEWs.