Kinematic processes contributing to the intensification of anomalously-strong North Atlantic jets

An intense extratropical cyclone impacted western Europe on 9 February 2020 with wind gusts in excess of 35 m s^–1, heavy rainfall, and considerable inland and coastal flooding. This extratropical cyclone developed beneath the poleward-exit region of an anomalously strong North Atlantic jet streak that featured wind speeds in excess of 100 m s^–1 and enabled a British Airways 747 to break the record for a subsonic commercial flight between New York City and London at the time. Anomalously strong jet streaks, such as that which accompanied the aforementioned extratropical cyclone, also feature strong lower-tropospheric baroclinicity and strong horizontal and vertical wind shear favorable for the development of clear-air turbulence. Consequently, improved understanding of the processes that influence the development anomalously strong jet streaks is necessary not only because of their influence on the production of high-impact weather, but also their influence on aviation.

This presentation focuses on the kinematic processes that contribute to the development of anomalously strong jet streaks over the North Atlantic with wind speeds in excess of 100 m s^–1 during September-May 1979–2018, and how those processes vary across a large sample of cases identified within the NCEP Climate Forecast System Reanalysis. A composite analysis demonstrates that anomalously strong North Atlantic jets are most frequent during the winter compared with the fall and spring, and that their development is preceded by low-level warm-air advection, poleward moisture advection, and moist ascent within the warm conveyor belt of a surface cyclone beneath the equatorward jet-entrance region. A diagnosis of the irrotational and nondivergent components of the ageostrophic wind within the near-jet environment reveals that both wind components facilitate jet intensification via their nonnegligible contributions to negative potential vorticity (PV) advection and PV frontogenesis in the vicinity of the dynamic tropopause. The foregoing results indicate that a diagnosis of jet intensification during anomalously strong jet events is strongly dependent on an accurate representation of the cumulative effects of latent heating within the near-jet environment.