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.

Northern Hemisphere extratropical cyclone biases in ECMWF subseasonal forecasts

“…The underestimated cyclone frequency in the eastern North Pacific (Figure 3a), which is the largest bias in winter, is likely linked to the known bias in the variability of the North Pacific jet stream: Winters (2021) showed that subseasonal forecasts of the ECMWF tend to overpredict the zonally retracted phase of the jet but underpredict its zonally extended phase. This is in line with Vitart et al (2022), who found the eastern edge of the North Pacific jet to be too far to the west in the same model.…”

“…The latter bias was shown to be linked to an underestimated blocking frequency over Europe. Focusing on specific storm-track regions, Winters (2021) identified significant subseasonal forecast biases in the occurrence frequency of the main North Pacific jet stream regimes in winter, which have a strong impact in North America and further downstream. Some of these biases are consistent with the aforementioned study by Vitart et al (2022).…”

Synergistic Effects of Upstream Disturbances and Oceanic Fronts on the Subseasonal Evolution of Western Pacific Jet Stream in Winter

“…But these works are incomplete due to the limited data and research content. However, the temporal‐spatial features of WPJS are closely associated with the data resolution (Manney et al., 2017; Rikus, 2018; Winters, 2021; Yu et al., 2021). To sum up, the subseasonal feature and mechanism of winter WPJS are still unclear at present.…”