Assessment and Consequences of the Delayed Breakup of the Antarctic Polar Vortex in Two Versions of the GEOS Chemistry-Climate Model

In mid-winter, winds circle the globe at speeds greater than 200 km/hr (approximately 130mph) in the middle atmosphere. This strong jet bounds the region known as the polar vortex. The presence of the Antarctic polar vortex is a key ingredient in the formation of the 'ozone hole', because the air inside the vortex is cold and isolated from lower latitudes, creating ideal conditions for large-scale chemical ozone depletion. Many atmospheric models are not able to reproduce observed winds in the middle atmosphere. Specifically, the polar vortices tend to break down too late and peak wind speeds are higher than observed. Hurwitz et al. find that the delayed break-up of the Antarctic polar vortex is due to weaker-than-observed wave driving from the lower atmosphere during the October-November period. The delayed break-up of the Antarctic polar vortex changes the temperature structure of the middle atmosphere, which biases the amount of chemical ozone depletion that can occur in late winter and spring. Also, the extended lifetime of the polar vortex strengthens the 'overturning' circulation cell in the middle atmosphere, changing the amount of ozone, methane and other chemical species that is transported from low to high latitudes. As greenhouse gas concentrations continue to rise, the atmospheric temperature structure and resulting wind structure are expected to change. Clearly, if models cannot duplicate the observed late 20th century high-latitude winds, their ability to simulate the polar vortices in future must be poor. Understanding model weaknesses and improving the modeled polar vortices will be necessary for accurate predictions of ozone recovery in the coming century.