Gravity waves disturbing the stratospheric polar vortex

What is the stratospheric polar vortex?
The stratospheric polar vortex is a winter season westerly (west-to-east) jet at about 30 km altitude, far above the tropospheric jet stream. As the stratospheric winds circle the cold winter pole, they are often disturbed by global scale waves, known as planetary waves, that propagate up from the troposphere and distort the flow of the stratospheric vortex. If the planetary wave amplitudes are large enough, they can even dramatically break down the stratospheric vortex causing polar temperatures to rise in what is known as a stratospheric sudden warming event.

Aside from planetary waves what else can alter the persistence and stability of the polar vortex?
There is another type of wave that plays an important role in the stratosphere: gravity waves (GWs). Unlike planetary waves that depend on the Earth's rotation for their restoring force and have a horizontal scale of thousands of kilometers, GWs are restored by gravity, analogous to water surface waves, and have horizontal scales from ten to hundreds of kilometers. GWs differ from water surface waves in that they can easily propagate vertically where the decreasing atmospheric density creates strongly increasing winds and temperatures associated with the GWs. When the vertical motion associated with the GWs becomes great enough, the waves can "break", much like water waves, and disturb the larger flow.

What is special about the polar vortex in January 2022?
During January of 2022, the typically strong planetary waves were weak, providing a unique opportunity to isolate the GW effects on the stratospheric polar vortex. The strong January 2022 GWs were generated by mountains in Europe, as well as Greenland and Iceland. The winds from the troposphere through the stratosphere were westerly, an alignment enabling these orographically generated waves to travel to the middle of the stratosphere before breaking.

How can we visualize the wave breaking process and evolution?
We can use two variables:

(a) We visualize the wave breaking process on a surface near the middle of the stratosphere, at a level where the "potential" temperature is 850 K. Potential temperature means that if the low density air at this level were compressed to surface density, it would heat up to 850 K. Since potential temperature is independent of changes in air density, it can remain nearly constant as air changes density through vertical motion. Thus a potential temperature surface is close to being a material surface, similar to a water surface. As the potential temperature surface rises, the actual air temperature cools and the air expands. Conversely, when the potential temperature surface lowers, the actual air temperature warms and the air compresses. In Animation 1, we visualize the temperature surface on the 850 K potential temperature and raise the surface by the changes in height. The polar vortex appears as a cold patch due to lack of solar radiation in polar winter. There are also large cold and warm areas created by the up and down motion of a planetary wave. More strikingly, we can see the gravity waves rippling/breaking over Europe's mountains, where the cold air in that region becomes even colder in the strong upward velocity regions of the gravity waves. We can also see strong cold air at the Greenland edge due to the gravity wave generated, fast rising motion of air.

(b) Another nearly conserved quantity is Ertel's Potential Vorticity (PV). Vorticity is a measure of the spin in the Earth’s atmosphere. The vorticity itself is not conserved because as a column of air is stretched in the vertical it spins faster, increasing its vorticity. Conversely, when the column of air is compressed in the vertical, it spins slower, decreasing its vorticity. However, PV is a ratio of the vorticity to the stretch or compression of the air column, and therefore that ratio tends to be conserved. We also visualize PV on the 850 K temperature in Animation 2. Here the ripples represent the gravity waves, as in Animation 1, while the PV tracks the air motion. The breaking gravity waves create speckled regions of very high and very low PV, right at the limit of the global model’s resolution. These identify the regions where the gravity waves are breaking and significantly disrupting the structure of the stratospheric polar vortex. At first (10 January 2022) the high PV region (brown area) is highly symmetric with strong flow along the edge of the high PV region, however, the gravity wave breaking disturbs the polar vortex flow leading to the evolution of localized very high PV regions (red). The flow spins very tightly around these regions creating the high PV values. At this time (27 January 2022), the polar vortex flow has become more convoluted, enhancing the potential for mixing air outside of the polar vortex into the polar vortex. Mixing into the stratospheric polar vortex at this level can potentially transport mid-latitude, high ozone air into the polar region.

Thus armed with these two conserved quantities: potential temperature and PV, a value of PV followed on a potential temperature surface can be expected to be conserved, somewhat analogous to an actual atmospheric tracer.

References
Coy, L., P. A. Newman, W. M. Putman, S. Pawson, and M. J. Alexander, 2024: Gravity Wave–Induced Instability of the Stratospheric Polar Vortex Edge. J. Atmos. Sci., 81, 2001–2015, https://doi.org/10.1175/JAS-D-24-0005.1