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.

The salience of nonlinearities in the boreal winter response to ENSO: North Pacific and North America

“…As discussed above, the increase in precipitation over the Western United States during extreme EN events is only expected to become statistically significant if more than 10 extreme EN events are compared to the composited response for moderate EP EN events. That being said, other studies have provided evidence that the dryness in Southern California during the 2015/2016 event specifically arose partially from SST variability in the Pacific Ocean (Siler et al 2017;Jong et al 2018;Lim et al 2018). …”

“…8a, b that a few individual ensemble members simulate a drier Western United States during the 97/98 and 82/83 events, even though the ensemble mean and most individual integrations clearly indicate a wetter winter as indeed occurred. Hence variability in precipitation in this region has both a forced and unforced component (Zhang et al 2018;Lim et al 2018;Jong et al 2018), and it is premature to conclude anything about the forced response to the SST anomalies present in 2015/2016 from the observed lack of increased precipitation in California. As discussed above, the increase in precipitation over the Western United States during extreme EN events is only expected to become statistically significant if more than 10 extreme EN events are compared to the composited response for moderate EP EN events.…”

Ongoing Increase in Eastern Tropical North Pacific Denitrification as Interpreted Through the Santa Barbara Basin Sedimentary δ15N Record

“…No evidence was found for coherent and significant variations in bulk sedimentary δ 15 N due to ENSO variability, with differences in δ 15 N records between the 1997 and 2015 El Niño event potentially attributable to variations in rainfall and terrestrial δ 15 N inputs (Figure ; Jong et al, ; Lee et al, ; Sweeney & Kaplan, ). A decrease in sediment trap δ 13 C org is associated with both El Niño events.…”

Climate Forcing and Response to Greenhouse Gases, Aerosols, and Ozone in CESM1

“…features of the late-20 th -century climate evolution (Acosta Navarro et al, 2017;Bollasina et al, 2011;Hwang et al, 2013;Jong et al, 2018;Liu et al, 2013;Ropelewski & Halpert, 1987). The global precipitation pattern displays a striking resemblance between the equilibrium (Figure 3e) and transient (Figure 3f) responses, although the former features anomalies of larger magnitude.…”