The impact of SST-forced and unforced teleconnections on 2015/16 El Niño winter precipitation over the western United States.

The factors impacting western U.S. winter precipitation during the 2015/16 El Niño 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 Niño was to produce wetter than normal conditions over much of the west coast including California - a result at odds with the negative precipitation anomalies observed over much of the Southwestern U.S. 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 U.S. 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 intra-ensemble 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 U.S. 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 U.S., some differences remain, indicating that model error is also playing a role.

A Modeling Study of the Causes and Predictability of the Spring 2011 Extreme US Weather Activity.

This study examines the causes and predictability of the spring 2011 U.S. extreme weather using the Modern-Era Retrospective analysis for Research and Applications (MERRA) reanalyses and Goddard Earth Observing System, version 5 (GEOS-5) Atmospheric General Circulation Model simulations. The focus is on assessing the impact on precipitation of sea surface temperature (SST) anomalies, land conditions, and large-scale atmospheric modes of variability. A key result is that the April record-breaking precipitation in the Ohio River Valley was primarily the result of the unforced development of a positive North Atlantic Oscillation (NAO)-like mode of variability with unusually large amplitude, limiting the predictability of the precipitation in that region at one month leads. SST forcing (La Nina conditions) contributed to the broader continental scale pattern of precipitation anomalies, producing drying in the southern plains and weak wet anomalies in the northeast, while the impact of realistic initial North American land conditions was to enhance precipitation in the upper Midwest and produce deficits in the southeast. It was further found that: The March 1 atmospheric initial condition was the primary source of the ensemble mean precipitation response over the eastern U.S. in April (well beyond the limit of weather predictability) suggesting an influence on the initial state of the previous SST forcing and/or tropospheric/stratospheric coupling linked to an unusually persistent and cold polar vortex.Stationary wave model experiments suggest that the SST-forced base state for April enhanced the amplitude of the NAO response compared to that of the climatological state, though the impact is modest and can be of either sign.