Using Empirical Orthogonal Function/Principal Component Analysis to Explore Wintertime Pacific Jet Variability

The wintertime Pacific jet stream exerts a powerful influence on the evolution of both the large- and synoptic-scale circulations within and beyond the Pacific basin. The jet is characterized by two leading modes of variability (Athanasiadis et al. 2010). The leading mode involves a zonal extension or retraction of the jet exit region between 160°E to 120°W (Jaffe et al. 2011). The next leading mode is characterized by a meridional shift of the jet exit region in which the latitude of the jet axis can vary by 20°. While recent research has investigated transitions between the poles of each individual mode, especially those of the leading mode, little is known regarding the structural evolution and dynamics associated with transitions from one mode to the other. This study aims to increase understanding of the variability of the wintertime Pacific jet by first identifying preferred transitions between modes and subsequently investigating their characteristic synoptic evolutions.

We employ the daily 250 hPa zonal wind from the NCEP/NCAR Reanalysis over 68 cold seasons (1948/49-2015/16) to portray Pacific jet variability in an empirical orthogonal function (EOF)/principle component (PC) phase space. The analysis represents the daily Pacific jet as a point in a two dimensional phase space where axes represent the separate extension/retraction and meridional shift modes of the variability. Preferred pathways from one mode to the other, along with the basin-scale structural evolutions that characterize them, are presented and discussed.