The role of large-scale conditions on African easterly waves' variability and associated rainfall is investigated in simulations with the variable resolution version of the Arpege-Climat General Circulation Model (GCM). Easterly waves are identified from the 850 hPa meridional winds. The simulated waves' characteristics and their frequency response are compared with that in the reanalyses of the European Center for Medium-range Weather Forecasts (ECMWF) during summer 1992. The zonal wavelength of the simulated waves increases toward the equator. This increase, found also in previous works, is explained by the increase in 850-hPa mean zonal winds toward the equator as a result of low-level monsoon flow. A pronounced seasonal variability indicating a late summer enhancement of wave activity and related precipitation is found in both simulations and reanalyses. This feature, which has been found from observation campaigns, is explained by the variability of the large-scale circulation, which gives favorable conditions for the penetration of the easterly waves into the moist layer in the late summer. A shift of the spectra towards low frequencies is found in the simulated waves when compared to the reanalysis. The shift is explained by the relatively weak westward winds within the African Easterly Jet in the model, which tend to generate waves with low phase speed and frequency. We suggest that the weakness of the winds in the jet is caused by the strong eastward monsoon flow in the model, which may be due to surface condition parameterizations.
ASJC Scopus subject areas
- Atmospheric Science