Future changes of El Nino in two global coupled climate models

Idealized forcing experiments with 1% per year CO2 increase to stabilized doubled and quadrupled CO2, twenty-first century transient scenario experiments (SRES scenarios A1B and B1), and stabilized twenty-second century A1B and B1 experiments with two global coupled climate models (PCM and CCSM3) are analyzed for possible future changes of El Ni (n) over tildeo events. With increased CO2 in the models, there is a reduction of amplitude of El Ni (n) over tildeo events. This is particularly apparent with larger forcing in the stabilized 4xCO(2) experiment in PCM and the stabilized greenhouse gas A1B experiment in CCSM3, where the reduction of amplitude is outside the range of the inherent multi-century variability of El Ni (n) over tildeo in the control runs of the models and is statistically significant. With moderately increased forcing (stabilized 2xCO(2) in PCM and the stabilized B1 experiment in CCSM3), the reduction in amplitude is evident, but it is not significant. The change in El Ni (n) over tildeo behavior with larger forcing is attributed to the change in base state temperature in the equatorial Pacific, which is similar with increased greenhouse gases (GHGs) in both models. Positive temperature anomalies in and below the thermocline, associated with a reduction of the trade winds, and weakened Pacific Ocean subtropical cells, produce a less intense thermocline, and consequently lower amplitude El Ni (n) over tildeo events. The previously noted intensification of El Ni (n) over tildeo tropical precipitation anomalies in a warmer mean base state that applied when there was no appreciable change in El Ni (n) over tildeo amplitude does not hold in the present study where the El Ni (n) over tildeo events decrease in magnitude in a future warmer climate. North American surface temperature anomalies associated with El Ni (n) over tildeo are reduced and become less significant in the future events, with the anomalously deepened Aleutian low in the North Pacific weakened and moved eastward with greater radiative forcing. Part of this is attributed to the smaller amplitude events and thus lower amplitude teleconnections as indicated by contrasting composites of medium and high amplitude El Ni (n) over tildeo events from the control runs. The change in midlatitude base state circulation also contributes to the change in El Ni (n) over tildeo teleconnections. The effects of this change in base state on the weakened El Ni (n) over tildeo teleconnections over North America are confirmed in sensitivity experiments with a version of the atmospheric model in which heating anomalies are specified to mimic El Ni (n) over tildeo events in a base state changed due to increased GHGs.