Climate Simulations of African Easterly Waves

Abstract

The three-dimensional structure and propagation characteristics of African easterly waves for the 1986‐94 period are studied with June‐August European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, National Centers for Environmental Prediction (NCEP) reanalyses, and Canadian Centre for Climate Modelling and Analysis GCM output. Specific consideration is given to the differences in the timemean circulation, synoptic-scale variance, covariance, and principal oscillation patterns. The African easterly waves derived from the ECMWF and NCEP analyses are very similar, with both providing a reasonably realistic depiction of African easterly waves—given the agreement with one another, earlier station data studies, and theory. Where significant differences exist between the results from the two data assimilation systems they expectably do so over the tropical Atlantic, and in fields that are not directly observed (such as vertical velocity). The situation with the GCM is not as favorable but there are some encouraging areas of agreement—despite the GCM’s relatively coarse resolution and absence of observed data to constrain it. Selected points of agreement and disagreement between the GCM and the analyses include the following. (i) The GCM African easterly wave energetics are comparable with the analyses in terms of the sign and magnitude of the energy transfers from the time-mean state to the waves. (ii) The northern track of easterly waves seen in the analyses terminates prematurely in the GCM at the African coast. (iii) The southerly track of moist easterly waves seen in the analyses near the seasonal rainband is absent in the GCM. Possible reasons for the deficiencies are discussed. The sensitivity of the GCM-simulated African easterly waves to CO2 doubling is investigated. Together with significant mean warming and moistening over the northern Sahara, the level of simulated African easterly wave activity increases with CO 2 doubling.