A climatological characterization of North Atlantic winter jet streaks and their extremes

Abstract. The jet stream is a hemisphere-wide midlatitude band of westerly wind. Jet streaks, which are regions of enhanced wind speed within the jet stream, characterize it locally. Jet streaks are frequent upper-tropospheric flow features that accompany troughs and ridges and form in tandem with surface cyclones. Upper-level divergence in their equatorward entrance and poleward exit regions couples them to surface weather via vertical motion, and these are regions prone to precipitation formation, which feeds back into the strength of upper level divergence and wind speed via diabatic heat release. This reanalysis-based study presents a systematic characterization of the life cycle of jet streaks and extreme jet streaks over the North Atlantic during winter, their occurrence during three different regimes of the eddy-driven jet, and their relation to Rossby wave breaking (RWB) from a potential vorticity (PV) gradient perspective. Extreme jet streaks are most frequent when the North Atlantic jet is in a zonal regime, while they are least common when the jet is in a poleward-shifted regime. Maximum wind speed on average occurs on the 330 K isentrope, and the peak intensity of jet streaks, defined as the maximum wind speed throughout their evolution, scales with the strength of the PV gradient, with mean values of 1.7 PVU (100 km)−1 for wind speeds exceeding 100 m s−1. The peak intensity of jet streaks also increases with their lifetime, and extreme jet streaks exhibit a prolonged intensification period as well as increased acceleration rates. A positive trend in jet streak intensity seems to have been emerging since 1979, but decadal variability still dominates the 43-year time series. Clustering jet streak events identifies typical Rossby wave patterns in which jet streaks reach peak intensity and their preferred location and orientation within the large-scale environment. In case of anticyclonic RWB, the jet streak sits upstream of the ridge axis, while in case of no RWB, the jet streak is zonally oriented and is located slightly downstream of the ridge axis. In some cases, the jet streak is found farther downstream of the ridge axis, but no case of well-marked cyclonic RWB is found at maximum jet streak intensity. As expected, the presence of an extreme jet streak is associated with a meridionally aligned pair of surface cyclones and anticyclones. More specifically, a cyclone is located poleward of an anticyclone and, in some cases, a mesoscale cyclone upstream of both, which is associated with intense precipitation. This motivates a detailed follow-up study on the relative roles of diabatic and adiabatic processes in the formation of extreme jet streaks.