Characteristics of Long Track Tropopause Polar Vortices

Tropopause polar vortices (TPVs) are closed circulations on the tropopause that form and predominately reside in high latitudes. The majority of TPVs are cold core and feature cyclonic flow, appearing as regions of lowered tropopause potential temperature on a surface of constant potential vorticity. Due to their attendant flow, TPVs have been shown to influence surface weather features, including Arctic cyclones, and create jet streaks upon entering the midlatitudes and interacting with the polar jet. Thus, a greater understanding of the vortex dynamics of these features may improve our ability to forecast significant weather events. In this study, we focus on the subset of TPVs which have lifetimes of longer than two weeks (the ninety-fifth percentile of all TPV cases between 1979 and 2018); these long-lived vortices offer a unique opportunity to study the conditions under which TPVs are able to strengthen and analyze patterns of vortex movement over time. We anticipate that these long track TPVs will share common vortex characteristics, move through predictable pathways, and occur in similar background environments, different from those of shorter-lived vortices.

Utilizing ERA-Interim data, along with TPV tracks derived from the same reanalysis, we investigate the formation, movement, and ambient conditions associated with these long-lived vortices. We find that these long track TPVs are significantly stronger, occur preferentially 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, with most vortices emerging over the central Arctic Ocean. The long-track TPVs also appear as likely as any TPV to exit the Arctic and move into the mid-latitudes, though this occurs late in the vortex lifetime, immediately preceding vortex lysis in most cases. The long-lived TPVs form predominately by splitting from existing vortices, but a notable minority seem to generate via physical processes in the absence of pre-existing circulations. These non-split genesis events are found to occur in select regions and under specific flow conditions. We also find that these long-track vortices exhibit preferred pathways of motion, including pathways out of the Arctic. On the whole, these vortices seem to spend most of their lifetimes in low shear environments (as seen in the preference for high latitudes and the summer season), which likely helps to explain their extended durations.