CMIP5 model simulations of the impacts of the two types of El Nino on the US winter temperature

Thirty Coupled Model Intercomparison Project phase 5 (CMIP5) preindustrial simulations are examined to contrast impacts of the two types of El Nino on the U. S. winter temperatures. The CMIP5 models are found more capable of simulating the observed eastern Pacific (EP) El Nino impacts (a warm northeast, cold southwest pattern over the U. S.) but less capable of simulating the observed central Pacific (CP) El Nino impacts (a warm northwest, cold southeast pattern). During EP El Nino, sea surface temperature (SST) anomalies influence the Walker circulation giving rise to a basin-wide pattern of outgoing longwave radiation (OLR) anomalies. The modeled atmospheric responses to the EP El Nino are thus less sensitive to the detailed structure of the simulated SST anomalies and can be well simulated by most of the CMIP5 models. In contrast, the SST anomalies during the CP El Nino affect the strength of the Walker circulation less effectively than the EP El Nino. OLR anomalies are local, rather than basin wide. The modeled atmospheric responses to the CP El Nino therefore depend more on how realistically the CP El Nino SST anomalies are simulated in the models. As a result, the CP El Nino's impact on the U. S. winter temperature, controlled by the atmospheric wave train response to the OLR forcing, is less well simulated by the CMIP5 models. This conclusion is supported by an examination of the Pacific North American and tropical/Northern Hemisphere patterns produced by the CMIP5 models in response to the two types of El Nino.