African Wave Disturbances in the GISS GCM and their Relationship to Climate Variability

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ATM-9407018 Druyan, Leonard Columbia University Title: African Wave Disturbances in the GISS GCM and their Relationship to Climate Variability African wave disturbances are wave-like perturbations in the mid- tropospheric easterly circulation of boreal summer over northern Africa which organize distinctive spatial patterns of convergence and often precipitation. Although Sahel seasonal precipitation is associated with these waves, GCM studies of the interannual variability of sub-Saharan rainfall have not explicitly considered their influence. The research is based on preliminary work which found a reasonably realistic representation of African waves in the GISS GCM. The project will study wave behavior in the model at 4x5 degree and 2x2.5 degree horizontal resolution, and the relationship between this modeled behavior and the principal circulation features of the African summer monsoon, namely the tropical and African Easterly Jets. An archive of GCM simulations of some 10-20 summer seasons will be created from model runs forced with observed global SST for selected years during the 1950's through the 1980's (including years of anomalously sparse or abundant Sahel rainfall). In addition, wave characteristics will be analyzed from an ensemble of three equilibrium simulations with doubled CO2 concentration. For each run, model winds and precipitation will be saved as six- hour averages for grid boxes over northern Africa. These data will be used to construct wave composites of winds, divergence and precipitation rates for different locations, months and years. Power spectra of meridional wind and precipitation time series will show the spectral peaks that are related to African wave periodicities. Cross-spectra will document the mutual correlations of circulation and precipitation and spatial mapping of selected spectral amplitudes will highlight the geographical areas of preferred activity. Interannual variations in the composites, spectra, cross-spec tra and spatial and spatial distributions of spectral amplitudes will reflect variability of wave behavior for contrasting scenarios of drought and heavy precipitation and for the warmer, double-CO2 climate. Model results will be compared with analyses of observations in the literature and with analyses of time series of corresponding twice-daily observed ECMWF winds. This research is import because it seeks to increase our ability to simulate and predict climate variability.