Subseasonal prediction of the state and evolution of the North Pacific
jet stream
Abstract
The state and evolution of the North Pacific jet (NPJ) stream strongly
influences the character of the downstream synoptic-scale flow pattern
over North America. This study employs data from nine models within the
Subseasonal-to-Seasonal Reforecast Database hosted by the European
Centre for Medium-Range Weather Forecasts to examine the subseasonal (2
weeks–1 month) predictability of the NPJ through the lens of an NPJ
phase diagram. The NPJ phase diagram provides a visual representation of
the state and evolution of the NPJ with respect to the two leading modes
of NPJ variability. The first mode of NPJ variability corresponds to a
zonal extension or retraction of the climatological jet-exit region,
whereas the second mode corresponds to a poleward or equatorward shift
of the climatological jet-exit region. The analysis reveals that
ensemble forecasts of the prevailing NPJ regime, as determined from the
NPJ phase diagram, are skillful into week 3 of the forecast period.
Forecasts initialized during a jet retraction, or verifying during a jet
retraction and equatorward shift, generally feature the largest errors
during the forecast period. Examination of the worst-performing 21-day
forecasts from each model demonstrates that the worst-performing
forecasts are uniformly associated with development, maintenance, and
decay of upper-tropospheric ridges over the high-latitude North Pacific.
These results demonstrate that bias-corrected NPJ phase diagram
forecasts have the potential to identify periods that may exhibit
enhanced forecast skill at subseasonal lead times based on the
anticipated NPJ evolution.