The breakup of stratospheric polar vortices.

摘要

The temperature in the Earth's stratosphere (between 12 to 50 km) increases with altitude due to ultraviolet heating of ozone. A strong circumpolar flow, called the stratospheric polar vortex, forms during polar winter when ultraviolet heating ceases in this region. These polar vortices last until spring and then break up as ultraviolet heating returns. The structure and evolution of the polar vortices have significant implications on ozone depletion and on global and regional climates.; In this thesis 40 years of meteorological analyses are used to examine the evolution of, and stirring around, the Arctic vortex, with focus on the breakup stage. Two extreme regimes of vortex breakup are found, late and early breakups, with very different decay characteristics. The occurrence of these different breakups is closely related to the interannual variability of the forcing due to the upward propagating planetary waves.; A shallow water model is used to examine the basic processes responsible for the different observed regimes. This model includes topographic forcing, to represent the upward propagating waves, and relaxation to an equilibrium state, to represent radiative processes. The roles of, and competition between, topographic wave forcing and radiative relaxation are examined by systematically varying the different parameters. Both steady and vacillation states are produced, which may be related to cold and warm polar stratospheres, respectively. Calculations with an annual cycle for the radiative equilibrium state and different timing and seasonality of the bottom forcing are also performed to produce the observed breakup regimes.; A new hybrid numerical method which includes diabatic forcing and can resolve steep gradients is also tested and used to further examine the competition between the external forcing and radiative relaxation. Calculations of idealized planar flows show that the relaxation damps out the oscillations in the elongation as well as orientation of a forced vortex, but steep vorticity gradients are preserved for at least 10 vortex overturning periods.