The Impact of SST-Forced and Unforced Teleconnections on 2015/16 El Nino Winter Precipitation over the Western United States

The factors impacting western U.S. winter precipitation during the 2015/16 El Nino are investigated using the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), data, and simulations with the Goddard Earth Observing System, version 5 (GEOS-5), atmospheric general circulation model forced with specified sea surface temperatures (SSTs). Results reveal that the simulated response to the tropical Pacific SST associated with the 2015/16 El Nino was to produce wetter than normal conditions over much of the North American west coast including Californiaa result at odds with the negative precipitation anomalies observed over much of the southwestern United States. It is shown that two factors acted to partly counter the canonical ENSO response in that region. First, a potentially predictable but modest response to the unusually strong and persistent warm SST in the northeastern Pacific decreased precipitation in the southwestern United States by increasing sea level pressure, driving anticyclonic circulation and atmospheric descent, and reducing moisture transport into that region. Second, large-scale unforced (by SST) components of atmospheric variability (consisting of the leading modes of unpredictable intraensemble variability) resembling the positive phase of the North Atlantic Oscillation and Arctic Oscillation are found to be an important contributor to the drying over the western United States. While a statistical reconstruction of the precipitation from our simulations that account for internal atmospheric variability does much to close the gap between the ensemble-mean and observed precipitation in the southwestern United States, some differences remain, indicating that model error is also playing a role.