El Niño and Greenhouse Warming: Results from Ensemble Simulations with the NCAR CCSM

The changes in model ENSO behavior due to an increase in greenhouse gases (according to the IPCC Business-As-Usual scenario) are investigated using a 62 member ensemble 140-year simulation (1940–2080) with the National Center for Atmospheric Research Community Climate System Model (CCSM, version 1.4). Although the global mean surface temperature increases by about 1.2 K over the period 2000–2080, there are no significant changes in the ENSO period, amplitude and spatial patterns. To explain this behavior, an analysis of the simulation results is combined with results from intermediate complexity coupled ocean-atmosphere models. It is shown that this version of the CCSM is incapable of simulating a correct meridional extension of the equatorial wind stress response to equatorial SST anomalies. The wind response pattern is too narrow and its strength is insensitive to background SST. This leads to a more stable Pacific climate system, a shorter ENSO period and a reduced sensitivity of ENSO to global warming.