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Changes in the El Niño-Southern Oscillation under climate regime shift and increased greenhouse gases Ye, Zhengqing


The observed El Niño/Southern Oscillation (ENSO) has shown eastward displacements of the zonal wind stress anomalies and surface heat flux anomalies in the equatorial Pacific during El Niño episodes in the 1981-1995 regime relative to the 1961-1975 regime. Numerical experiments with a modified Zebiak-Cane intermediate complexity coupled model generally reproduced such displacements when the model climatological fields were replaced by the observed and simulated climatologies for the 1981-1995 regime. The later regime climatology resulted in eastward shifts in the ENSO system during El Niño but not La Niña, through the eastward shift of the atmosphere convergence heating rate in the coupled model. The ENSO period and ENSO predictability were also enhanced in the coupled model under the later regime climatology. A similar behavior was found in the Lorenz (1963) 3-component chaos system, i.e. in both the ENSO and Lorenz systems, stronger nonlinearity appears to lengthen the period thereby enhancing the predictability. Using nonlinear principal component analysis, we demonstrated that the leading ENSO mode had changed on a physical basis since the late 1970s. The ENSO modes resembled the destabilized 'ocean basin mode' and 'the recharge-mode' in the pre- and post-1980s regimes, respectively. The surface zonal current acted as an intensifier of ENSO in the earlier regime and played a role in the transition of the ENSO cycle in the later regime. With data from an ensemble of coupled general circulation models in the IPCC-AR4 project, climate under year 2000 greenhouse gas (GHG) level was compared with climate under pre-industrial conditions. In the tropical Pacific, the warming in the mean sea surface temperatures (SST) was found to have an El Niño-like pattern, in agreement with the observed SST data (1900-1999). The models showed that both the equatorial zonal overturning circulation and the shallow meridional overturning circulation weakened under increased GHG forcing. For ENSO, the asymmetry in the SST anomalies between El Niño and La Niña was found to be enhanced under increased GHG, for both the ensemble model data and the observed data. The enhanced asymmetry was mainly caused by the intensified vertical nonlinear dynamic heating. Furthermore, the enhanced GHG simulations showed that the asymmetry between El Niño and La Niña increased in the zonal wind stress anomalies, the equatorial undercurrent anomalies and the shallow meridional overturning circulation anomalies.

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