Future changes of El Nino

With future increases of greenhouse gases (GHGs) in two global coupled climate models (PCM and CCSM3), there is a reduction of amplitude of El Nino events. This is particularly apparent with larger forcing in the stabilized 4XCO2 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 Nino in the control runs of the models and is statistically significant. With moderately increased forcing (stabilized 2XCO2 in PCM and the stabilized B1 experiment in CCSM3), the reduction in amplitude is evident, but it is not significant. The change in El Nino behavior with larger forcing is attributed to the change in base state temperature in the equatorial Pacific, which is similar with increased GHGs in both models. Positive temperature anomalies in and below the thermocline, associated with a weakening of the Pacific Ocean subtropical cells, produce a less intense thermocline, and consequently lower amplitude El Nino events. North American surface temperature anomalies associated with El Nino 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 increased GHGs. 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 Nino events from the control runs. The change in midlatitude base state circulation also contributes to the change in El Nino teleconnections. The effects of this change in base state on the weakened El Nino 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 Nino events in a base state changed due to increased GHGs. Though the results for these two models are relatively unambiguous, other global coupled climate models show a wide variety of changes, with both increases and decreases of future El Nino events, thus making it difficult to make definitive conclusions about future El Nino behavior.