Wintertime Trends in Directional Variability and Intensity of Westerly Air Flow in the Polar Jet Region

Abstract

In the current scientific debate, considerable attention is given to identifying changes in the midlatitude upper-tropospheric wind field that may be associated with Arctic warming. The air flow patterns, such as waviness, blocking and splitting, are among the most frequently studied. In this paper, we analyse changes in both the geometry and intensity of the westerly flow within the polar front jet (PFJ) stream sector. For this purpose, we introduce zonally defined circular statistics. Significant changes in the wind field were detected in the northern flank of the PFJ, close to the Arctic border. In this region, the directional variability of the wind vector calculated in the latitudinal domain increases over a multi-year period, and the latitudinal series of wind vectors becomes increasingly deflected from the purely zonal West-to-East direction. The most significant deflection trend is observed for northerly winds. Wind rose diagrams constructed for the Arctic boundary latitude band complement the picture of these changes, indicating an increased frequency of wind vectors from the N-E-S semicircle during the period 2001–2022 relative to 1980–2000. Alongside changes in flow geometry, the entire sector also experiences PFJ dissipation. The sign of the trends reverses in the southern flank of the PFJ; however, wind directional changes do not reach statistical significance. Within this flank, the PFJ and the subtropical jet (STJ) occur more frequently, which is accompanied by the stabilisation of westerly flow directions. These results support the hypothesis that, in the era of Arctic Amplification, non-zonal or irregular flow patterns are intensifying in the upper-tropospheric polar latitudes.