Major sudden stratospheric warming and polar vortex breakdown in early March

The polar vortex, which is a robust stratospheric circulation always above the pole in the winter half of the year, has shown a weaker intensity this season. In addition, according to current predictions, a significant disruption of this polar vortex over the Arctic is expected in early March. Due to the penetration of warmer air into the polar stratosphere, a so-called major sudden stratospheric warming (SSW) will occur, which will cause the polar vortex to split. In the middle stratosphere (around 10 hPa, ~30 km), the zonal mean zonal winds at 60°N are expected to reverse from their westerly direction to easterly, thus fulfilling the formal criterion for a major SSW, as illustrated by the forecast showing a clear transition from positive to negative winds in early March (Fig. 1). This places the upcoming event among the more dynamically intense stratospheric disturbances, as it involves not only a weakening, but also a complete breakup of the vortex into two separate vortices.

The mechanism of this development is associated with increased propagation of planetary (Rossby) waves from the troposphere to the stratosphere. These waves slow down the westerly circulation over the pole and disrupt the polar vortex. As wave activity increases, the vortex increasingly shifts and elongates, and eventually may break up. Part of this process is also the reversal of stratospheric winds from westerly to easterly, as noted above. Planetary waves transport warmer air from the troposphere to the stratosphere, which is reflected in strong positive temperature anomalies over the Arctic at 10 hPa (~30 km) during early March, together with a disturbed vortex structure evident in geopotential height (Fig. 2). The current warming is also evident in Fig. 3.

The breakdown of the polar vortex can influence tropospheric weather with a delay of several weeks, although this response does not occur in every case. Downward coupling following a major SSW can be understood as a gradual descent of circulation anomalies from the stratosphere into the troposphere. After a wind reversal in the upper stratosphere, easterly anomalies and associated increases in geopotential height propagate downward over days to weeks, modulating the tropospheric jet stream. If this signal remains coherent, it typically projects onto a negative AO/NAO pattern, favouring higher pressure over the Arctic and lower pressure in mid-latitudes. The resulting meridional flow weakens the zonal circulation, promotes high-latitude blocking (often over Greenland or Scandinavia), and increases the likelihood of cold Arctic or continental air advection into mid-latitudes.