Sudden Stratospheric Warming - Dynamics

Dynamics

In a usual northern-hemisphere winter, several minor warming events occur, with a major event occurring roughly every two years. One reason for major stratospheric warmings to occur in the Northern hemisphere is because orography and land-sea temperature contrasts are responsible for the generation of long (wavenumber 1 or 2) Rossby waves in the troposphere. These waves travel upward to the stratosphere and are dissipated there, producing the warming by decelerating the mean flow. This is the reason that major warmings are only observed in the northern-hemisphere, with one exception. In 2002 a southern-hemisphere major warming was observed. This event to date is not fully understood.

At an initial time a blocking-type circulation pattern establishes in the troposphere. This blocking pattern causes planetary zonal wave number 1 and/or 2 to grow to unusually large amplitudes. The growing wave propagates into the stratosphere and decelerates the mean zonal winds. 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. If the waves are sufficiently strong the mean zonal flow may decelerate sufficiently so that the winter westerlies turn easterly. At this point planetary waves may no longer penetrate into the stratosphere (due to the Charney-Drazin criteria ). Hence further upward transfer of energy is completely blocked and a very rapid easterly deceleration and the polar warming occur at this critical level, which must then move downward until eventually the warming and zonal wind reversal affect the entire polar stratosphere.

There exists a link between sudden stratospheric warmings and the quasi-biennial oscillation: If the QBO is in its easterly (westward) phase, the atmospheric waveguide is modified in such a way that upward-propagating Rossby waves are focused on the polar vortex, intensifying their interaction with the mean flow. 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).