Understanding the Dynamical and Thermodynamical Processes That Govern the Structure and Evolution of Persistent West Coast Cool Season Ridge Regimes

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 along the west coast of the United States. Persistent upper-level ridge regimes can prevent the occurrence of cool season precipitation events, especially when these ridge regimes last for several weeks, in conjunction with a poleward-shifted North Pacific jet stream. Missing out on several of these precipitation events can mean extended drought, significant water shortages, and adverse economy-wide impacts for these major population centers in California. This situation occurred during the now infamous 2011-2016 drought centered on California, leading the state to enact emergency water conservation protocols. However, not all upper-level ridges will impact the west coast of North America the same way. Factors such as geographic proximity, latitudinal extent, and ridge orientation will all determine how persistent upper-level ridges impact sensible weather across western North America. An increased understanding of the dynamical and physical processes that govern upper-level ridge persistence during the West Coast rainy season would allow decision makers to better manage water resources and motivates this presentation.

Climatological analyses of these persistent ridge regime events will be presented in order to learn more about the causes for persistence West Coast upper-level ridge regimes. Persistent upper-level ridge regimes will be defined by positive 500-hPa anomalies 1.5 standard deviations that last for longer than 10 days. A climatological analysis of persistent ridge regimes will be constructed from the NCEP-NCAR 2.5°gridded dataset (available from 1948 to present) for an extended time period to sample enough events to construct a proper climatology. This climatology will be used to construct composite analyses using simple geographic grouping as well as machine learning techniques to uncover dynamical and physical linkages between different groups of persistent upper-level ridge regimes. Representative individual cases within these groups will be examined in detail using higher-resolution datasets (e.g., the ERA-5) to illustrate the variability in the evolution of distinct ridge types and to increase our overall understanding about the dynamical and thermodynamical processes that govern persistent upper-level ridge regime formation, duration, and destruction. We anticipate that recurving and transitioning tropical cyclones, polar disturbances on synoptic time scales, and the phase and amplitude of the Madden-Julien Oscillation on sub-seasonal time scales will play a role in upper-level ridge formation, duration, and destruction.