Subseasonal-to-Seasonal Prediction of the State and Evolution of the North Pacific Jet Stream

Abstract:

The state and evolution of the North Pacific jet (NPJ) stream exhibits considerable influence on downstream temperature and precipitation patterns over North America during the cool season. The state and evolution of the NPJ can be described using an NPJ phase diagram that is constructed from the two leading EOFs of 250-hPa zonal wind during September–May 1979–2019. The first EOF corresponds to a zonal extension or retraction of the climatological NPJ, while the second EOF corresponds to a poleward or equatorward shift of the climatological exit region of the NPJ. The projection of 250-hPa zonal wind anomalies at one or multiple times onto the NPJ phase diagram subsequently provides an objective characterization of the state or evolution of the NPJ, respectively. Prior work utilizing the NPJ phase diagram reveals that the GEFS exhibits considerable forecast skill at capturing the state and evolution of the NPJ at lead times of 8–10 days. This result motivates examining the utility of the NPJ phase diagram at subseasonal-to-seasonal (S2S) time scales and across a larger number of ensemble prediction systems.

This study employs the S2S database available from ECMWF to construct a long-term climatology of ensemble forecasts in the context of the NPJ phase diagram for each of the 11 ensemble prediction systems included within the S2S database. NPJ phase diagram forecasts will be classified based on the NPJ regime at the time of forecast initialization and at the time of forecast verification in order to determine whether the forecast skill over the Pacific–North American sector varies based on whether forecasts are initialized or verified during a particular NPJ regime. NPJ phase diagram forecasts derived from the same ensemble prediction system will be examined to identify model biases with respect to the prediction of each NPJ regime, and forecasts will be ranked based on their ensemble mean error in the context of the NPJ phase diagram to identify the top 10% best and worst NPJ phase diagram forecasts at S2S time scales. The best and worst forecasts will be investigated further to identify the NPJ evolutions and sensible weather impacts over North America that are most frequently associated with the best and worst forecasts for each ensemble prediction system.