Recent Trends in the Waviness of the Northern Hemisphere Wintertime Polar and Subtropical Jets
Jonathan E. Martin
Abstract
Abstract A feature‐based metric of the waviness of the wintertime, Northern Hemisphere polar, and subtropical jets is developed and applied to three different reanalysis data sets. The analysis first identifies a “core isertel” along which the circulation per unit length is maximized in the separate polar (315:330K) and subtropical (340:355K) jet isentropic layers. Since the core isertel is, by design, an analytical proxy for the respective jet cores, the waviness of each jet is derived by calculating a hemispheric average of the meridional displacements of the core isertel from its equivalent latitude—the southern extent of a polar cap whose area is equal to the area enclosed by the core isertel. Analysis of the seasonal average waviness over the time series of the various data sets reveals that both jets have become systematically wavier while exhibiting no trends in their average speeds. The waviness of each jet evolves fairly independently of the other in most cold seasons and the slow northward creep of the polar jet is statistically significant. Finally, comparison of the composites of the waviest and least wavy seasons for each species reveals that such interannual variability is manifest in familiar large‐scale circulation anomalies.