Sinuosity as a Metric for Quantifying Tropospheric Polar Vortex Modification Associated with Arctic Cyclones

Arctic cyclones (ACs) are synoptic-scale features that are often responsible for incursions of warm, moist air from middle latitudes into the Arctic. AC-related warm-air incursions can result in longitudinally varying horizontal temperature gradients, which are associated with the amplification of the tropospheric polar vortex. This study will focus on how ACs modify the tropospheric polar vortex by quantifying the waviness of its equatorward edge in terms of sinuosity using a methodology adopted by Martin et al. (2017). The methodology is used to calculate circumpolar sinuosity for the circumpolar vortex, and sectorial sinuosity for a longitudinal sector specified according to location of a particular set of AC tracks. Variations in circumpolar and sectorial sinuosity associated with ACs will be grouped by season and location.

Sinuosity climatologies are constructed using ERA-Interim analysis data for the 1979–2018 period. AC data are extracted from the AC climatologies constructed by Sprenger et al. (2017) for the same period and are grouped by season and location of the ACs. At lower-tropospheric levels, sinuosity climatologies are constructed for the equatorward extent of the −5°C isotherm on the 850-hPa surface. At upper-tropospheric levels, sinuosity climatologies are constructed for the equatorward extent of the 920-dam geopotential height contour on the 300-hPa surface following Frauenfeld and Davis (2003), who define the equatorward boundary of the tropospheric polar vortex in terms of the equatorward extent of the geopotential height contour that coincides with the core of the westerlies.

The upper-tropospheric circumpolar and sectorial sinuosity climatologies both show a gradual increase to a maximum in the warm season (June, July, and August) and then a gradual decrease to a minimum in the cold season (December, January, and February), suggesting a more-amplified tropospheric polar vortex in the warm season. A preliminary finding shows that a large portion of ACs originating in any given longitudinal sector is associated with a maximum sectorial sinuosity attained during the life cycle of an AC that is above the 90^th percentile of the sectorial sinuosity climatology.