Characteristics of long-track tropopause polar vortices

Matthew T Bray,Steven M Cavallo

doi:10.5194/wcd-3-251-2022

Abstract

Abstract. Tropopause polar vortices (TPVs) are closed circulations centered on the tropopause that form and predominately reside in high latitudes. Due to their attendant flow, TPVs have been shown to influence surface weather features, and thus, a greater understanding of the dynamics of these features may improve our ability to forecast impactful weather events. In this study, we focus on the subset of TPVs that have lifetimes of longer than 2 weeks (the 95th percentile of all TPV cases between 1979 and 2018); these long-lived vortices offer a unique opportunity to study the conditions under which TPVs strengthen and analyze patterns of vortex formation and movement. Using ERA-Interim data, along with TPV tracks derived from the same reanalysis, we investigate the formation, motion, and development of these long-lived vortices. We find that these TPVs are significantly stronger, occur more often in the summer, and tend to remain more poleward than an average TPV. Similarly, these TPVs are shown to form at higher latitudes than average. Long-lived TPVs form predominately by splitting from existing vortices, but a notable minority seem to generate via dynamic processes in the absence of pre-existing TPVs. These non-likely split genesis events are found to occur in select geographic regions, driven by Rossby wave growth and breaking. Seasonal variations emerge in the life cycles of long-lived vortices; notably, winter TPVs progress more equatorward and generally grow to stronger amplitudes. These long-lived TPVs also appear as likely as any TPV to exit the Arctic and move into the mid-latitudes, doing so via two primary pathways: through Canada or Siberia.

Highlights

Tropopause polar vortices (TPVs) are a well-studied feature of the upper troposphere and lower stratosphere in high-latitude regions (Hakim and Canavan, 2005; Cavallo and Hakim, 2009, 2010, 2013)
This trend is expected as summers will tend to have lower wind shear across the Arctic and fewer features propagating from the mid-latitudes that may disrupt radiative TPV strengthening
Though, the tails of each distribution extending into the mid-latitudes match up almost exactly, indicating that while long-track TPVs may spend a greater amount of their lifetime in the high Arctic, they are about as likely as any TPV to exit the Arctic eventually

Summary

Introduction

Tropopause polar vortices (TPVs) are a well-studied feature of the upper troposphere and lower stratosphere in high-latitude regions (Hakim and Canavan, 2005; Cavallo and Hakim, 2009, 2010, 2013). Based on established characteristics of TPVs, these long-track vortices are expected to form via dynamic processes and occur in especially low-shear environments (e.g., in the summer and in the high Arctic away from mid-latitude jets). These long-track TPVs are expected to possess behaviors somewhat different from an average TPV, they provide a more manageable case set through which to study the processes of vortex formation and movement. Long-lived anomalies are especially likely to impact surface weather, either by interacting with multiple Arctic cyclones or by entering the midlatitudes

Data and methods

Long-track TPV selection and statistical methods

Characteristics and spatial distributions of long-track TPVs

Long-track TPV genesis

Long-track TPV development and motion

Conclusions