Modelled impact of global warming on ENSO-driven precipitation changes in the tropical Pacific

Abstract

The El Nino Southern Oscillation (ENSO) is the primary source of interannual climate variability over the tropical Pacific. Here we use an ensemble of Atmospheric General Circulation Model (AGCM) experiments to estimate the impact of global warming on ENSO-driven precipitation anomalies over the tropical Pacific. The AGCM is forced using observed time-varying sea surface temperatures (SSTs) from 1951 to 2010, with and without an added warming pattern (the CMIP3 multi-model mean change in SSTs projected for the last 20 years of the twenty-first century under the SRES A2 scenario). In the tropical Pacific, the AGCM’s El Nino rainfall response to the applied warming pattern agrees with rainfall responses in coupled models. With the warming pattern, rainfall is generally greater along the equatorial Pacific throughout the ENSO cycle. The Intertropical Convergence Zone dries over the eastern Pacific and the South Pacific Convergence Zone exhibits increased rainfall along its south-eastern flank and drying along its north-western flank. The magnitude and spatial structure of the changes differ between El Nino and La Nina events, and also depend on the magnitude of the events. Empirical Orthogonal Function analysis shows that the AGCM does not project any significant increase in the frequency of extreme El Nino events (or ‘single zonal convergence zone’ events) in this framework, although the magnitude of such events is increased by approximately 25 %. The modelled zonal wind anomalies show clear spatial and temporal differences between strong and weak El Nino and La Nina events.