Nonlinear precipitation response to El Nino and global warming in the Indo-Pacific

Precipitation changes over the Indo-Pacific during El Nio events are studied using an Atmospheric General Circulation Model forced with sea-surface temperature (SST) anomalies and changes in atmospheric CO2 concentrations. Linear increases in the amplitude of the El Nio SST anomaly pattern trigger nonlinear changes in precipitation amounts, resulting in shifts in the location and orientation of the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). In particular, the maximum precipitation anomaly along the ITCZ and SPCZ shifts eastwards, the ITCZ shifts south towards the equator, and the SPCZ becomes more zonal. Precipitation in the equatorial Pacific also increases nonlinearly. The effect of increasing CO2 levels and warming SSTs is also investigated. Global warming generally enhances the tropical Pacific precipitation response to El Nio. The precipitation response to El Nio is found to be dominated by changes in the atmospheric mean circulation dynamics, whereas the response to global warming is a balance between dynamic and thermodynamic changes. While the dependence of projected climate change impacts on seasonal variability is well-established, this study reveals that the impact of global warming on Pacific precipitation also depends strongly on the magnitude of the El Nio event. The magnitude and structure of the precipitation changes are also sensitive to the spatial structure of the global warming SST pattern.