Winter 2015/16 Atmospheric and Precipitation Anomalies over North America: El Nino Response and the Role of Noise

The possible causes for the observed winter 2015/16 precipitation anomalies, which were opposite to the mean El Nino signal over the U.S. Southwest, are analyzed based on the ensemble of forecasts from the NCEP Climate Forecast System, version 2 (CFSv2). The analysis focuses on the role of anomalous sea surface temperature (SST) forcing and the contributions of atmospheric internal variability. The model-predicted ensemble mean forecast for December-January-February 2015/16 (DJF 2015/16) North American atmospheric anomalies compared favorably with the El Nino composite, although some difference existed. The predicted pattern was also like that in the previous strong El Nino events of 1982/83 and 1997/98. Therefore, the model largely predicted the teleconnection and precipitation response pattern in DJF 2015/16 like the mean El Nino signal. The observed negative precipitation anomalies over the U.S. Southwest in DJF 2015/16 were not consistent either with the observed or with the model-predicted El Nino composite. Analysis of the member-to-member variability in the ensemble of forecast anomalies allowed quantification of the contribution of atmospheric internal variability in shaping seasonal mean anomalies. There were considerable variations in the outcome of DJF 2015/16 precipitation over North America from one forecast to another even though the predicted SSTs were nearly identical. The observed DJF 2015/16 precipitation anomalies were well within the envelope of possible forecast outcomes. Therefore, the atmospheric internal variability could have played a considerable role in determining the observed DJF 2015/16 negative precipitation anomalies over the U.S. Southwest, and its role is discussed in the context of differences in response.