Energetics of easterly wave disturbances over West Africa and the tropical Atlantic: a climatology from the 1979-95 NCEP/NCAR reanalyses

Abstract

Our purpose is to compute the main terms of the energy budget, following Reed et al. to characterize the maintenance of the easterly wave regimes. Although previous studies have been done on some well-observed cases, here, we use NCEP/NCAR reanalyses over the period 1979-1995 to produce a 17-year climatology of both 3–5-day and 6–9-day easterly waves. Conversions of energy are analyzed through the contrasts between land and ocean (at 10°E, 10°W and 30°W), and through seasonality. Similarities and differences in the characteristics of these two wave regimes are identified. The results relating to the 3–5-day easterly waves confirm the previous results based on smaller samples. Over land, the waves travelling in the Sahel-Sahara band develop most strongly at 900 hPa, through barotropic and baroclinic energy conversions located below the jet level. We have found some indication of a possible contribution of the zonal wind core located at 5°S along the Central Africa coast to the 3–5-day wave development along the southern trajectory. Both the dry convection north of 15°N in the heat low (mainly in June-July) and deep convection in the ITCZ (in August-September) are modulated by the 3–5-day easterly waves over the land. The overall variance and covariance patterns are rather similar for the 6–9-day and the 3–5-day wave regimes. However, over the ocean the wind variance maxima are displaced in mid-levels, as for the 3–5-day waves, and remain greater for the 6–9-day regime, consistent with greater barotropic energy conversion and the more tilted waves at 700 hPa. This 6–9-day wave regime could be connected with variance maxima in the high troposphere at the location of the subtropical westerly jet.