Recent Trends in the Waviness of the Northern Hemisphere Wintertime Polar and Subtropical Jets

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.

Changing features of the Northern Hemisphere 500-hPa circumpolar vortex

“…It could also be related to the fact that, in contrast to the cold pool analysis of Martin (2015) which only considers DJF, the analysis here considers the entire year. But the trend identified here could be counteracted by a tendency for increased wave amplitudes amid a decreasing equator-topole temperature gradient, as observed by Martin (2021), as the forces for geostrophic flow weaken, in a manner analogous to the increased meandering of a stream amid a gentler gradient near its mouth. The net result would be a relationship between area and circularity but with no overall linear temporal trend in R c despite a temporally decreasing NHCPV area.…”

A New Atmospheric Background State to Diagnose Local Waveguidability

“…The vertical structure shows the distinct identities of the North Atlantic and Asian/Pacific waveguides on the Northern Hemisphere more clearly than 330 K alone. The midlatitude waveguides over the North and South Atlantic oceans are primarily found on lower isentropic levels than 330 K, while the subtropical waveguides are found at higher levels (Martin, 2021; Martius et al., 2010).…”

Aircraft observations and reanalysis depictions of trends in the North Atlantic winter jet stream wind speeds and turbulence

“…There is also a striking correlation with the North Atlantic Oscillation (NAO), with Pearson correlation coefficients of 0.88–0.89. Martin (2021) has pointed out that changes in polar jet stream waviness also bear some relation to the NAO. In some sense the wind speeds in our three eastern North Atlantic boxes are a rough proxy for the NAO index.…”

“…In a separate study that emphasizes changes in the waviness of the winter polar jet stream, Martin (2021) also addresses its wind speed changes. He finds no secular changes for three reanalyses (his figure 9) and little agreement on the presence or absence of such changes in the literature.…”