A Robust Intensification of Wintertime Jet Stream Extremes in Future Climates

The east coast of North America experienced a record-breaking jet stream event on 20 Feb 2019, with peak wind speeds exceeding 110 m/s observed by weather balloons over Nova Scotia. At the time this was the strongest wind speed ever recorded over North America. The extreme `jet streak' propagated out over the North Atlantic where it played a key role in the subsequent development of a large and rapidly deepening cyclone on 22 Feb 2019. The cyclone had little societal impact because it did not make landfall. It did however act to amplify a large scale Rossby wave, producing a strong poleward advection of warm air towards western Europe, and leading to record-breaking February warmth in several European countries on 27 Feb 2019. The whole sequence of events took just over a week to complete.

This case provides an illustration of how climate extremes (here the record warmth in western Europe) are often the result of complex and chaotic nonlinear interactions of the atmosphere on weather timescales. The particular sequence of events is not uncommon, but both the strength of the initial jet streak over North America and the resulting temperatures in Europe were. Given the observed trend in surface temperatures, it seems likely that the temperatures were at least partly enhanced in a passive way by the warming climate. A more difficult question to answer is whether climate change is also impacting the frequency or amplitude of the preceding sequence of weather events. As a first step to answering this question, this study asks the question: do we expect extreme jet streak events to intensify in future?

Based on an analysis of CMIP simulations over the North Atlantic, we find a robust intensification of wintertime jet extremes in future climates, with the strongest instantaneous wind speeds increasing in every model. This contrasts with the strength of the time mean jet streams, which do not exhibit a robust change across the ensemble. Possible reasons for the differing behaviour of the mean winds and the extreme winds are discussed and a hypothesis is suggested to explain the robust increase in the latter.