Potential links between stratospheric and tropospheric extremes in the winter of 2019/20

Abstract:

The winter period of 2019/20 was one of the most extreme ever recorded both in terms tropospheric and stratospheric dynamics. In particular the arctic oscillation (AO) index, a measure of the tropospheric large-scale circulation, and the stratospheric polar vortex strength showed anomalous persistence and reached extreme positive anomalies of several standard deviations.

In the present study we use a set of comprehensive forecast model simulations with realistic initial conditions to investigate the role of stratosphere-troposphere coupling in creating the extreme conditions observed in early 2020. We find that February was mainly dominated by upward coupling, i.e., a strengthening of the polar vortex due to reduced wave forcing from the troposphere. In contrast, early and mid March are found to be mostly characterised by downward coupling, i.e., a strengthening of the tropospheric jet by persistent positive zonal wind anomalies in the (lower) stratosphere. We investigate whether anomalous planetary wave fluxes can reveal possible mechanisms of the up- and downward coupling. For example, we find that over the course of the winter, episodes of planetary wave reflection contributed to stratospheric vortex strengthening.

Overall, this study aims to quantify the role of coupled stratosphere-troposphere dynamics for the particular case of the winter of 2020. Thereby, it expands on the results on stratosphere-troposphere coupling based on statistics over many events. Further, this case study aims to improve our general understanding of potential feedbacks between circulation anomalies in the troposphere and stratosphere, which may cause extreme and more persistent anomalies over the course of several weeks.