Evidence for Increased Waviness of the Northern Hemisphere Wintertime Polar and Subtropical Jets

A feature-based metric of the waviness of the wintertime [December-February (DJF)], Northern Hemisphere, tropopause-level polar and subtropical jets is developed and applied to the NCEP-NCAR reanalysis data. The analysis first identifies a “core isertel” along which the circulation per unit length is maximized in the separate polar (315:330 K) and subtropical (340:355 K) jet isentropic layers. Since the core isertel is, by design, an analytical proxy for the respective jet cores, calculation of its sinuosity is a robust measure of the waviness of the jets.

Analysis of the seasonal average waviness over a 66-year time series reveals that both jets have become systematically wavier while exhibiting no trends in their average speeds. Correlations of the daily sinuosities of the two jets suggest that the waviness of each evolves fairly independently of the other in most cold seasons. Finally, comparison of the composites of the waviest and least wavy seasons for each species reveals that interannual variability of the subtropical (polar) jet preferentially impacts Pacific (Atlantic) basin circulation anomalies in the troposphere. Meanwhile, in the lower stratosphere, wavy polar (subtropical) jet years are associated with an intensified (weakened) polar vortex.