The Changing Impacts of El Niño on US Winter Climate

We conducted statistical analyses with observational data, numerical experiments with a forced atmospheric general circulation model (AGCM), and case studies with major El Niño events since 1950 to show that the impacts produced by the Central-Pacific (CP) and Eastern-Pacific (EP) types of El Niño on US winter climate are very different. During EP El Niño events, positive winter temperature anomalies are concentrated mostly over the northeastern part of the US (particularly over the Great Lakes region) and negative anomalies are most obvious over the southwestern states. During CP El Niño events, the warm anomalies are located in northwestern US and the cold anomalies are centered in the southeastern US. The US temperature impact patterns are rotated by about 90 degrees between these two types of El Niño. As for the winter precipitation, the CP El Niño increases the dry anomalies and decreases the wet anomalies caused by the EP El Niño over the US. Based on these findings, the increasing occurrence of the CP El Niño in the recent decades may have contributed to the US drought conditions. These impact patterns are reproduced in the forced experiments performed with Version 4 of the Community Atmosphere Model (CAM4) from NCAR, in which the SST anomalies of the EP and CP El Niño were separately used to force the model in two different sets of ensemble experiments. These different impact patterns can also be identified from the strongest EP and CP El Niño events. Further analyses were conducted to show that the different impacts result from differing wave train responses in the atmosphere to the sea surface temperature anomalies associated with the two types of El Niño.