Role of Equatorial Pacific SST Forecast Error in the Late Winter California Precipitation Forecast for the 2015/16 El Niño

Abstract

During the strong 2015/16 El Niño, only normal to below-average precipitation fell across California in the late winter. This disagrees with both predictions by the ensemble mean of forecast models and expectations for strong El Niños. The authors examine one of the possible reasons why this event did not bring expected precipitation to California in the late winter. The maximum equatorial Pacific sea surface temperature anomalies (SSTAs) were located, compared to the 1982/83 and 1997/98 strong El Niños, farther to the west in the 2015/16 winter, which possibly caused less convection in the eastern tropical Pacific and shifted the teleconnection patterns westward in the North Pacific, thus weakening the influences on California. The SSTA and precipitation forecast for February–April 2016, based on the North American Multimodel Ensemble, showed large discrepancies from observations, with the ensemble mean of most of the models overestimating SSTAs in the eastern tropical Pacific and California precipitation. Atmospheric general circulation model (AGCM) experiments were conducted to test the hypothesis that the warmer eastern tropical Pacific SSTA forecast may have caused the wetter forecast in California in 2015/16 compared to observations. The AGCM experiments suggest it is difficult to assert that the eastern tropical Pacific SSTAs caused the too-wet California precipitation forecast, especially in Southern California, given that the models disagree. Results indicate forecast error can be influenced by atmosphere-model sensitivity to forecast SSTs, but they also indicate atmospheric internal variability may have been responsible for the combination of a strong El Niño and near-normal California precipitation.