A7 Stratospheric Circulation Disruptions

Topic

Sudden stratospheric warming is the most dramatic of all stratospheric meteorological events: within just a few days, the temperature of the polar stratosphere—at altitudes of around thirty-two kilometers—can rise by thirty to fifty degrees. When this warming is sufficiently intense, the stratospheric polar vortex weakens to the point where the wind normally circling the pole reverses completely—a phenomenon known as "major sudden stratospheric warming"—or even splits into two separate circulation centers. Sudden stratospheric warming almost always occurs in the Northern Hemisphere during the boreal winter, particularly between January and February, and is recorded, on average, slightly less than once a year.

The trigger for sudden stratospheric warming is the vertical propagation of amplified Rossby waves rising from the troposphere; when these Rossby waves reach the stratospheric polar vortex, they deposit the angular momentum they carry into the stratosphere, and this transfer of angular momentum decelerates the vortex. This deceleration reduces the centrifugal effect that normally holds the vortex together, and it is precisely this loss of centrifugal force that allows polar air to descend and warm through adiabatic compression, producing the temperature rise that initiates the sudden stratospheric warming event. Between two and six weeks after the sudden stratospheric warming occurs, the troposphere responds: the Arctic Oscillation and the North Atlantic Oscillation shift to a negative phase, and this negative phase increases the likelihood of Arctic air surging into Europe and North America.

The temperature of the stratospheric polar vortex not only determines the intensity of the sudden stratospheric warming event; it also independently controls the depletion of stratospheric ozone that occurs during the polar winter and spring. When the temperature of the stratospheric polar vortex drops to around minus eighty degrees Celsius, polar stratospheric clouds form; on the surfaces of these clouds, chemical reactions—which occur only on solid surfaces—activate the chlorine released by chlorofluorocarbons as well as the bromine present in the stratosphere. When polar spring sunlight arrives, this activated chlorine and bromine catalyze massive ozone destruction, giving rise to the Antarctic ozone hole.

Sudden stratospheric warming itself can interrupt this ozone destruction process: by warming the polar stratosphere, the warming event dissipates polar stratospheric clouds before the surface chemical reactions can activate all the available chlorine and bromine, resulting in less ozone destruction for that season. However, this interaction rarely occurs in the Southern Hemisphere because sudden stratospheric warming is an extremely rare event there, unlike in the Northern Hemisphere.

Thus, a single event—sudden stratospheric warming, triggered by Rossby waves that decelerate the stratospheric polar vortex and heat polar air through adiabatic compression—leads to two very different consequences: weeks later, it drives the Arctic Oscillation and the North Atlantic Oscillation into their negative phases, favoring outbreaks of Arctic air over Europe and North America; and, within the stratosphere where it originated, it can curb ozone destruction by dissipating the clouds that enabled it. Sudden stratospheric warming thus links—via a single mechanism—surface weather patterns and ozone chemistry in the high polar stratosphere.

ID:596