The Structure, Evolution, and Life Cycles of Intraseasonal Fluctuations of the North Pacific Jet Stream

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The work aims to substantially increase our knowledge of wintertime Pacific jet variability through a combination of statistical, synoptic-climatological and case study analyses. As the primary phenomenon at the interface between synoptic-scale weather systems and the large-scale circulation, upper tropospheric jets are particularly strong governors of regional climate. Despite the substantial influence that the wintertime Pacific jet has on both sensible weather and large-scale circulation within and beyond the Pacific basin, current understanding of its intraseasonal variability is far from complete. In prior work the team has identified two leading modes of this variability. In this project the team will identify preferred transitions between the leading modes of variability of the Pacific jet; the results of this will inform construction of composites of the basin-wide structure, evolution and weather features associated with each species of preferred transition. Using the phase space analysis to expand the jet retraction climatology, the relationship between the areal extent of lower tropospheric cold air over the Siberia/Eurasia sector and the zonal extent of the Pacific jet will be examined. Since the Pacific jet is a hybrid of the polar and subtropical jets, reanalysis data will also be used to examine the behavior of these two components in identified cases of jet retraction as a first step toward segregating the population of retraction events into those forced primarily by extratropical and tropical variability, respectively. Finally, in order to better understand their underlying large- and synoptic-scale dynamics, detailed case study analyses of two representative cases of jet retraction employing output from fine-scale numerical simulations of the selected cases will be undertaken using the NCAR Weather Research and Forecasting (WRF) model. This project will shed insights into wintertime Pacific jet variability; this has implications for improved prediction for the state of Hawaii as well as the west coast. Thus the broader impacts of this project are substantial.