Recent trends in the waviness of the Northern Hemisphere wintertime polar and subtropical jets

Among the most ubiquitous structural features of the Earth's atmosphere are the narrow, tropopause-level wind speed maxima known as jet streams or jets. These jets, often found nearly girdling the globe while exhibiting large meridional meanders, are the primary phenomena at the interface between synoptic-scale weather systems and the large-scale circulation. Consequently, they play a substantial role in the production of sensible weather in the midlatitudes while serving as particularly influential governors of regional climate. Decades of observational work has identified two main varieties of jets, distinguished by their underlying dynamical origins. The polar jet (POLJ) forms as a result of eddy momentum flux convergence associated with the development of midlatitude baroclinic waves (e.g., Held, 1975; Panetta, 1993; Rhines, 1975) and is connected, via the thermal wind relationship, to the troposphere-deep baroclinicity of the middle latitudes. The subtropical jet (STJ) forms in response to angular momentum transport by the thermally direct Hadley circulation (Held & Hou, 1980) and is, therefore, tied to the poleward edge of the tropical Hadley Cell. As a consequence of their different origins, the POLJ and STJ are often widely separated by latitude as well as elevation. The Northern Hemisphere (NH) jet stream has centers of maximum intensity located over the western Atlantic and western Pacific Oceans with the wintertime Pacific jet extending from East Asia to the date line. Unlike the Atlantic jet, the wintertime Pacific jet is regularly characterized by a collocation (or vertical superposition) of POLJ and STJ components and thus is often a hybrid feature (e.g., Christenson et al., 2017). 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.