Opposite spectral properties of Rossby waves during weak and strong stratospheric polar vortex events

This study characterizes and contrasts Rossby wave activity during the 25 sudden stratospheric warming (SSW) and 31 strong polar vortex (SPV) events that occurred in the period 1979–2021. While the events are tied to a decrease or increase, respectively, in background flow, it is less clear how the associated properties of Rossby waves change, e.g., how their phase speed is affected. The goal is to identify the specific tropospheric and stratospheric waves exhibiting anomalous behavior during these events. Applying space-time spectral analysis to ERA5 reanalysis data allows us to assess both the wavenumber and the zonal phase speed of the waves.

We find that SSW events are associated with a reduced phase speed of Rossby waves, first in the stratosphere and then in the troposphere, while SPV events are tied to a concomitant increase of phase speed across vertical levels. These phase speed anomalies become significant around the event and persist for 2-3 weeks thereafter. In the stratosphere, both SSW and SPV events are dominated by the change in the background flow, manifested as a systematic reduction or increase, respectively, in eastward propagation of Rossby waves across most wavenumber.

In the troposphere, the effect of the background flow is complemented by changes in wave properties, with a shift towards higher wavenumbers during SSW events and towards lower wavenumbers during SPV events. This opposite response between SSW and SPV events is also visible in the meridional heat and momentum flux co-spectra, which highlight from a novel perspective the connection between stratospheric Rossby waves and upward propagation of waves.