Calculation of EOFs: The mean and the annual and diurnal cycles were removed from 6-hourly, 250-hPa zonal wind data from the CFSR
during 1979-2014. Data was then restricted to only the cool season (i.e., Sept.-May). An EOF analysis was subsequently performed on the 250-hPa zonal wind anomalies
within the domain shown below over the North Pacific.
EOF Patterns: The first EOF pattern (shading) shown below corresponds to a zonal extension (+EOF1) or zonal retraction (-EOF1) of the
exit region of the North Pacific Jet. The second EOF pattern (shading) shown below corresponds to a poleward (+EOF2) or equatorward (-EOF2) shift of the exit
region of the North Pacific Jet. Together the first two EOFs explain roughly 20% of the variance of 250-hPa zonal wind.
Interpretation of North Pacific Jet Phase Diagram: Employing these two EOF patterns, the instantaneous 250-hPa zonal wind anomalies at any time can be projected
onto EOF1 and EOF2, resulting in a single point on a North Pacific Jet phase diagram.
In this diagram, the x-axis corresponds to the standardized PC for EOF1 and the y-axis corresponds to the standardized PC for EOF2. The distance along either axis corresponds to how strongly the 250-hPa zonal wind anomalies at one time project onto the two
EOF patterns shown below. In all real-time, archived, and verified phase diagram products, a point on the phase diagram is a weighted average of projections onto the North Pacific Jet phase diagram in a 48-h period centered on the time of interest.
Extreme Event Identification: For information on the identification of extreme events, please consult this presentation. Of the events identified, only those that occurred
during the cool season (Sept.-May) were used to construct the composites. Composite results should be considered preliminary.
References: The resultant EOF patterns and analysis techniques
described above are consistent with and motivated by related work on the North Pacific Jet:
Athanasiadis, P. J., J. M. Wallace, and J. J. Wettstein, 2010: Patterns of wintertime jet stream variability and their relation to the storm tracks.
J. Atmos. Sci., 67, 1361-1381.
Griffin, K. S., and J. E. Martin, 2016: Synoptic features associated with temporally coherent modes of variability of the North Pacific jet stream. J. Climate, 29, in press.
Jaffe, S. C., J. E. Martin, D. J. Vimont, and D. L. Lorenz, 2011: A synoptic climatology of episodic, subseasonal retractions of the Pacific jet. J. Climate, 24, 2846-2860.