California’s recent weather extremes

During California’s drought-stricken winters of 2013–14 and 2014–15, an extremely strong ridge of high pressure off the coast dominated atmospheric circulation patterns. Storms that would normally reach the Golden State were diverted northward. In early 2017 a stubborn low-pressure trough settled in near the coast, which led to a conveyor belt of storms that made landfall and caused major flooding.

Despite widespread attention to California’s recent weather extremes, scientists have had trouble determining what regulates the offshore ridges and troughs. Grant Branstator and Haiyan Teng from the National Center for Atmospheric Research in Boulder, Colorado, now pinpoint a possible answer: an atmospheric phenomenon called wavenumber-5 that develops at midlatitudes in the Northern Hemisphere. Computer simulations suggest that wavenumber-5 occurs when strong jet streams trap meandering planetary-scale atmospheric undulations called Rossby waves and channel the waves’ energy into a stationary belt of five pairs of pressure ridges (red in the maps below) and troughs (blue) that circle the globe. One ridge got stuck off California during the drought years. In 2017 wavenumber-5 reemerged and brought the flood-inducing trough to the coast.

The researchers found an association between tropical heating anomalies in the western Pacific Ocean that are not necessarily associated with El Niño and the formation of wavenumber-5. Branstator, Teng, and other scientists are investigating whether it’s possible to predict those anomalies and other risk factors and thus provide advanced warnings for Californians. (H. Teng, G. Branstator, J. Clim. 30, 1477, 2017 ; G. Branstator, H. Teng, J. Atmos. Sci., in press , doi:10.1175/JAS-D-16-0305.1.)