Composite Analysis of the Processes that Govern the Structure and Evolution of Persistent North Pacific Wintertime Ridge Regimes

Abstract:

Along the west coast of North America, 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. Persistent upper-level ridge regimes can prevent the occurrence of cool season precipitation events, especially when these ridge regimes last for several weeks. Missing out on several of these precipitation events can mean extended drought, significant water shortages, and adverse economy-wide impacts for major population centers in California. 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 persistent West Coast upper-level ridge regimes are presented to illustrate the dynamical processes that govern ridge regime life cycles. Persistent upper-level ridge regimes will be defined by positive 500-hPa height anomalies 1.0 standard deviation that last for longer than 7 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 to uncover dynamical and physical linkages between different groups of persistent upper-level ridge regimes. An analysis of all ridge regimes suggests that ridges in the domains over the central Pacific last longer than those near or over western North America. Deep upstream cyclones with associated poleward-directed ARs on the western flank of the developing ridge are common among the longest lasting ridge regimes in each domain. Understanding this cyclone-AR-ridge relationship throughout the ridge lifecycle will help to further understand how select ridges can lead to persistent weather regimes.