Will there be a significant change to El Niño in the 21 st century ?

The El Niño/Southern Oscillation (ENSO) response to anthropogenic climate change is assessed in the following 1◦ nominal resolution CCSM4 CMIP5 simulations: 20th century ensemble, pre-industrial control, 21st century projections and stabilized 2100-2300 ‘extension runs’. ENSO variability weakens slightly with CO2; however, various significance tests reveal that changes are insignificant at all but the highest CO2 levels. Comparison with the 1850 control simulation suggests that ENSO changes may become significant on centennial timescales; the lack of signal in the 20th vs. 21st century ensembles is due to their limited duration. Changes to the mean state are consistent with previous studies: a weakening of the subtropical wind stress curl, an eastward shift of the tropical convective cells, a reduction in the zonal SST gradient and an increase in vertical thermal stratification take place as CO2 increases. The extratropical thermocline deepens throughout the 21st century, with the tropical thermocline changing slowly in response. The adjustment timescale is set by the relevant ocean dynamics, and the delay in its effect on ENSO variability is not diminished by increasing ensemble size. The CCSM4 results imply that 21st century simulations may simply be too short for identification of significant tropical variability response to climate change. An examination of atmospheric teleconnections, in contrast, shows that the remote influences of ENSO do respond rapidly to climate change in some regions, particularly during boreal winter. This suggests that changes to ENSO impacts may take place well before changes to oceanic tropical variability itself becomes significant.