CMIP5 model simulations of the impacts of the two types of El Niño on the U.S. winter temperature

Abstract

Thirty Coupled Model Intercomparison Project phase 5 (CMIP5) preindustrial simulations are examined to contrast impacts of the two types of El Niño on the U.S. winter temperatures. The CMIP5 models are found more capable of simulating the observed eastern Pacific (EP) El Niño impacts (a warm northeast, cold southwest pattern over the U.S.) but less capable of simulating the observed central Pacific (CP) El Niño impacts (a warm northwest, cold southeast pattern). During EP El Niño, 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 Niño 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 Niño affect the strength of the Walker circulation less effectively than the EP El Niño. OLR anomalies are local, rather than basin wide. The modeled atmospheric responses to the CP El Niño therefore depend more on how realistically the CP El Niño SST anomalies are simulated in the models. As a result, the CP El Niño'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 Niño.