Dynamic Coupling of the Stratosphere With the Troposphere and Sudden Stratospheric Warmings

Abstract

Numerical time integrations of a nine-layer, quasi-geostrophic, highly truncated spectral model of the atmosphere are used to study tropospheric-stratospheric interaction with particular regard to sudden stratospheric warmings. The model is global, extends to 0.05 mb (71 km) with roughly 10-km resolution in the stratosphere, and includes an annual heating cycle. Model integrations simulating the months of December and January were made (1) without nonzonal forcing and (2) with nonzonal heating and orography included, to represent Southern Hemisphere and Northern Hemisphere winters, respectively. The presence of nonzonal heating in the winter hemisphere brought about an increase in circulation intensity and produced a stationary perturbation having a strong westward slope with height extending high into the stratosphere. This feature is similar to the Aleutian system. It was accompanied by considerably warmer temperatures in the polar night stratosphere and a weaker stratospheric westerly jet. Sudden stratospheric warmings occurred as a result of large increases in the intensity of planetary scale waves in the troposphere, which in turn produced surges of upward propagating energy. The energetics of the warming occurred in two phases. A change from a baroclinically direct to a driven circulation occurred as the stratospheric temperature gradient reversed. This coincided with a change from enhancement to absorption of the vertical energy flux. The mechanism of the warming was similar to that described by Matsuno. Nonlinear interactions between the progressive long wave and the nonzonal heating were primarily responsible for the tropospheric events that produced the transient upward flux of energy and thus the warmings. A seasonally coupled index cycle in the long waves was also of significance, while interactions with other waves and orographic forcing were of secondary importance in the long-wave energetics of sudden warmings.