Relationships Between TPV Characteristics and the Arctic Oscillation

Tropopause Polar Vortices (TPVs) are long lived, coherent vortices traceable on the dynamic tropopause poleward of the polar jet stream. These features are important to the development of mid-latitude surface cyclones in Type-B cyclogenesis events. The coherent nature of these vortices and the semi-conservative properties of potential vorticity, especially over the Arctic where diabatic effects are relatively small, allow fluid properties within TPVs to be traced for long periods of time. Previous studies have shown that lifetimes of TPVs span from days to months. Given the generally sub-synoptic spatial scales of TPVs, and relatively little knowledge of the physical processes of TPVs and their significance in larger-scale patterns, TPVs provide a unique opportunity to study predictability and connections between Arctic and mid-latitude processes.

The Arctic Oscillation (AO) is an annular mode described by the leading empirical orthogonal function of monthly-mean sea level pressure poleward of 20 degrees N. It is robust, being seen across nearly any reanalysis data set used. The oscillation has two phases, which are linked to changes in the large-scale atmospheric circulation throughout the entire Arctic troposphere and lower stratosphere. The positive (negative) phase corresponds to lower (higher) geopotential heights throughout the Arctic and a more (less) zonal jet stream. Given the much smaller spatial scales of TPVs, we hypothesize that TPVs characteristics depend on AO phase. In particular, Arctic to mid-latitude connections may be preferred during the negative phase of the AO when a weaker zonal winds promote blocked flow and greater interaction between TPVs and the polar jet stream.

To address our hypothesis, we first cross-validate TPV characteristics in three separate data sets: ERA-Interim, NASA MERRA, and CFSR in order to establish their an analysis sensitivity. We then categorized TPVs into positive, negative, and neutral AO regimes. Results show distinct differences in TPV tracks, which are more frequently located over lower latitudes during negative phases and higher latitudes during positive phases. During negative phases, higher overall heights over the Arctic act to block movement of TPVs poleward. Additionally, a more amplified jet stream interacts with TPVs more frequently, leading to impacts on mid-latitude cyclogenesis.