GLOBAL ATMOSPHERIC RESPONSE TO SPECIFIC LINEAR-COMBINATIONS OF THE MAIN SST MODES .1. NUMERICAL EXPERIMENTS AND PRELIMINARY-RESULTS

This article investigates through numerical experiments the controvers ial question of the impact of El Nino-Southern Oscillation (ENSO) phen omena on climate according to large-scale and regional-scale interhemi spheric thermal contrast. Eight experiments (two considering only inve rsed Atlantic thermal anomalies and six combining ENSO warm phase with large-scale interhemispheric contrast and Atlantic anomaly patterns) were performed with the Meteo-France atmospheric general circulation m odel. The definition of boundary conditions from observed composites a nd principal components is presented and preliminary results concernin g the month of August, especially over West Africa and the equatorial Atlantic are discussed. Results are coherent with observations and sho w that interhemispheric and regional scale sea-surface-temperature ano maly (SST) patterns could significantly modulate the impact of ENSO ph enomena: the impact of warm-phase ENSO, relative to the atmospheric mo del intercomparison project (AMIP) climatology, seems stronger when em bedded in global and regional SSTA patterns representative of the post -1970 conditions [i.e. with temperatures warmer (colder) than the long -term mean in the southern hemisphere (northern hemisphere)]. Atlantic SSTAs may also play a significant role.