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

Abstract. The jet stream is a hemispheric-wide mid-latitude 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 right entrance and left exit regions couples them to surface weather via vertical motion and are regions prone to precipitation formation, which feeds back on the strength of upper level divergence and wind speed via diabatic heat release. This reanalysis-based study presents a systematic characterisation 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 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 329 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 2.4 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 rather than increased deepening rates. A positive trend in jet streak intensity appears to be emerging since 1979, but decadal variability still dominates the 43-year time series. A self-organising map technique 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 locates slightly downstream of the ridge axis. In some cases the jet streak is found farther downstream of the ridge axis but surprisingly no well-marked cyclonic RWB is identified at maximum jet streak intensity. As expected, the presence of an extreme jet streak is associated with a meridionally aligned pair of surface cyclones. More specifically, a cyclone is located poleward of an anticyclone plus, 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.