The role of tropical heating and internal variability in the California response to the 2015/16 ENSO event

Abstract

Ensemble reforecast experiments are performed for the boreal winters of 2015/16 (an El Niño year) and 2016/17 in order to understand the origin of the highly unexpected low precipitation over Southern California (SoCal) during 2015/16. The specific hypothesis tested is that the December–March anomalies in (i) SoCal precipitation (highly negative) and (ii) the 200-hPa height field in the Pacific–North American (PNA) region (unusual for an El Niño year) were forced by tropical diabatic heating, as estimated from reanalysis fields. Ten-member ensemble reforecasts for November–March for each year were carried out using the Community Earth System Model and the Climate Forecast System, version 2, models. They were repeated with diabatic heating fields added to the temperature tendencies produced by the models over the tropical Indo-Pacific region throughout the reforecasts. These added fields were obtained from an iterative process that greatly reduces the error in the seasonal mean, trend, and parabolic fit of the model heating (compared to reanalysis estimates). The SoCal precipitation and PNA height field ensemble-mean differences between 2015/16 and 2016/17 for the control and added-heating experiments showed a strong El Niño–like response in both models, including high SoCal precipitation anomalies. Using partial least squares regression to relate variations in SoCal precipitation to the PNA height field within the reforecasts, we further confirm that the observed anomalies are due to noise, and do not constitute a response forced by the seasonal-mean tropical diabatic heating in the Indo-Pacific region.