What’s behind the extreme atmospheric ridges causing California’s droughts?

New NOAA-funded research in the Journal of Climate uncovers the processes behind the persistent atmospheric ridge of high pressure off the west coast of North America, associated with California’s damaging droughts. California is no stranger to dry weather and has been experiencing one of the worst droughts in its historical record over the past few years. During the winters of 2013-2014 and 2014-2015, very persistent and extreme ridges of high pressure dominated the atmospheric circulation off the west coast, without any apparent connection to La Niña. La Niña is one phase of a natural climate pattern, the El Nino-Southern Oscillation (ENSO), with unusually cool water in the tropical Pacific, typically associated with dry winters in California; El Niño, the abnormally warm water phase of ENSO, is associated with wet California winters. The ridge disrupted storm tracks, blocking moisture-laden storms from delivering much needed rain to California, and keeping the state warm and dry. Understanding what produced these ridges could help scientists better predict future droughts and help communities better prepare for future impacts.
Previous research has attributed the ridges to natural variability of sea surface temperatures or tropical circulation, but scientists have yet to determine what in particular played a key role in causing the ridges. Haiyan Teng and Grant Branstator, with the National Center for Atmospheric Research, aimed to investigate whether these west coast high-pressure ridges were a result of natural atmospheric variability that is sometimes triggered by warming in the tropics. Using model simulations, the authors found that the west coast ridges are one phase of a natural long-wave circulation pattern in the upper troposphere (the lowest layer of the atmosphere). In addition, they found that warming in tropical regions that does not depend on ENSO can spark responses such as these extreme west coast ridges. Importantly, the scientists’ results provide an additional source of predictability that could help improve drought forecasts.
This research was funded, in part, by the NOAA Office of Oceanic and Atmospheric Research Climate Program Office’s Modeling, Analysis, Predictions, and Projections Program as part of its Drought Task Force research activities.
View the paper: http://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-16-0524.1