Subseasonal Winter Weather Predictability Associated with Single versus Multiple Wave Pulse Events and Their Impacts on the Arctic Stratospheric Polar Vortex

Extreme winter weather is often dominated by severe cold-air outbreaks and strong extratropical cyclones which have large socioeconomic impacts. Among other teleconnection patterns, these extreme weather events are tied to variability in the stratospheric polar vortex. Dynamically, increased vertical wave activity from the troposphere into the polar stratosphere fluxes anomalous momentum and heat towards the pole, resulting in the warming and weakening of the stratospheric polar vortex. Analyzing these wave pulse events, including their fundamental characteristics and statistics, may shed light on the different impacts these events have on the stratospheric polar vortex and subsequently improve subseasonal winter weather forecasts.

This study compares and contrasts the impacts of single versus multiple (i.e., a series of anomalously strong vertically propagating waves entering the polar stratosphere) events in both ERA-Interim and hindcasts of operational subseasonal models in the S2S Prediction Project Database. First, we construct a catalogue of single and multiple wave pulse events in the recent past (1979-2018) from a specific set of criteria based on the vertical structure and duration of vertically propagating waves. These event dates are analyzed via lag compositing to investigate the difference between the two events in both the extratropical stratosphere and troposphere. Precursors found to multiple pulse events include anomalous ridging over Northern Eurasia and the North Atlantic, co-located with vertical wave activity flux anomalies leading up to the events. Single pulse events, by contrast, are found to be relatively short-lived and weaker compared to multiple pulse events. As a result, the vortex is much weaker following multiple pulse events than single pulse events. The 3-D structure of these wave pulses along with their primary forcing mechanisms (e.g., synoptic-scale vs. planetary scale) are also studied to resolve why some events are single or multiple pulse events. Results from reanalysis are then contrasted with the same criteria applied to the S2S models to identify potential biases. Applications of these wave pulse events to predicting surface weather patterns on the subseasonal timescale are also discussed.