The Structure and Evolution of Persistent West Coast Cool Season Ridge Regimes

Along the west coast of the United States, a majority of the precipitation for the entire year falls during the winter months as a result of a dozen or so cyclone and atmospheric river-related precipitation events. Upper-level ridge regimes can prevent the occurrence of cool season precipitation events, especially when persistent ridge regimes last for several weeks, in conjunction with a poleward-shifted North Pacific jet stream. Missing out on just a few of these precipitation events can mean extended drought, significant water shortages, and adverse economy-wide impacts for these major population centers. This situation occurred in the now infamous 2011-2016 drought centered on California, leading the state to enact emergency water conservation protocols. An increased understanding of the dynamical and physical processes that enable upper-level rides to persist for extended periods of time during the West Coast rainy season allow decision makers on the ground to better manage water resources in the area.

Given the need to learn more about the causes for persistent West Coast upper-level ridge regimes, climatological analyses of these persistent ridge regime events will be presented. These events will be defined by positive 500-hPa anomalies of greater than 1.5 standard deviations for longer than 10 days. A climatological analysis of persistent ridge regimes will be constructed from the NCEP-NCAR 2.5 gridded dataset for an extended time period to sample enough events for a proper climatology. This resulting dataset will serve as a basis for mapping the frequency of persistent upper-level ridge regimes, for constructing composite analyses of these ridge regimes, and for analyzing representative case studies from inception to persistence to decay.