An Attribution Analysis of Winter 2015/16 Atmospheric and Terrestrial Anomalies over North America

In this study, attribution of the possible reasons for the observed winter 2015/16 precipitation anomalies that were opposite to the mean El Niño signal over the southwest North America were analyzed. The analysis focuses on the role of SST forcing and the contributions of atmospheric internal variability based on the ensemble forecasts of hindcasts and real-time forecasts from the NCEP Climate Forecasts System version 2 (CFSv2).

The results show that the pattern of 2015/16 winter North America atmospheric anomalies as an ensemble mean atmospheric response to SSTs compared favorably with the El Niño composite from the historical period and were much the same as the previous El Niño events of 1982/83 and 1997/98 with similar amplitude of ENSO SSTs. Therefore, the tele-connection pattern and precipitation response pattern in winter 2015/16 had not changed from the mean El Niño signal. The observed negative precipitation anomalies over the southwest NA in DJF2015/16 was not the result of SST forced signal.

However, there are considerable variations in the outcome of 2015/16 winter precipitation over the North America from one forecast to another even though the SSTs are the same. The observed 2015/16 winter precipitation anomalies were well within the envelope of possible outcomes. In conclusion, the reason why the observed 2015/16 winter precipitation anomalies over the southwest North America differed greatly from the El Niño composite was mainly due to the large contribution of the atmospheric internal variability (noise) which overwrote the atmospheric response (signal) to the ENSO SSTs. This unpredictable inherently nature of atmospheric variability leads to low predictability and prediction skill for the North America seasonal precipitation.