[1] The warming is preceded by a slowing then reversal of the westerly winds in the stratospheric polar vortex, commonly measured at 60 ° latitude at the 10 hPa level.
Most of the SH SSWs fall into this category as their onsets most commonly occur sometime in austral spring months, and the stratospheric wind and temperature anomalies tend to persist until early summer.
One reason for major stratospheric warmings in the NH is that orography and land-sea temperature contrasts are responsible for the generation of long (wavenumber 1 or 2)[clarification needed] Rossby waves in the troposphere.
These planetary-scale waves travel upward to the stratosphere and dissipate there, decelerating the westerly winds and warming the Arctic.
This blocking pattern causes[clarification needed] Rossby waves with zonal wavenumber 1 and/or 2[23] to grow to unusually large amplitudes.
[clarification needed] Thus the polar night jet weakens and simultaneously becomes distorted by the growing planetary waves.
Because the wave amplitude increases with decreasing density, this easterly acceleration process is not effective at fairly high levels.[why?]
[8][25] The upward propagation of planetary waves and their interaction with the stratospheric mean flow is traditionally diagnosed via so-called Eliassen-Palm fluxes.
Thus, there exists a statistically significant imbalance between the frequency of sudden stratospheric warmings if these events are grouped according to the QBO phase (easterly or westerly).
Following a sudden stratospheric warming, the high altitude westerly winds reverse and are replaced by easterlies.
[1] Similar downward processes are found in the SH in the austral late spring to early summer seasons.
SH SSWs in austral spring tend to cause the Antarctic ozone concentration to be higher than normal from spring to early summer,[15][29][30][31] and both weaker vortex and higher Antarctic ozone act to cause the tropospheric jet to shift equatorward,[32] which is expressed as a negative phase of the Southern Annular Mode (SAM)[33] in the SH extratropical geopotential height and surface pressure fields in the subsequent late spring to early summer seasons.