The stratospheric ???nal warming is the ???nal transition of the zonal winds from wintertime westerlies to summertime easterlies as the solar heating of the high latitude stratosphere increases in the springtime. Recent observational analyses suggested that stratospheric final warming makes a signi???cant contribution to the spring transitions in the lower troposphere, especially in the Northern Hemisphere. It is still not clear, however, whether these transitions are due to the downward in???uence from the stratosphere.
We ???rst explore the hypothesis that much of the observed tropospheric signal of the ???nal warming is initiated from the stratosphere. Large ensembles of ???nal warmings are simulated in an idealized dynamical core model, by imposing a radiative equilibrium temperature
transition from winter to summer only in the stratosphere. Our results suggest that a substantial fraction of the observed tropospheric changes that occur in conjunct with the ???nal warming are induced from the stratosphere.
We further investigate the mechanisms of the downward in???uence of the ???nal warmings on the tropospheric circulation. Results from our zonally symmetric model suggest that stratospheric wave driving can induce a residual circulation and a???ect the tropospheric circulation. The tropospheric signals due to this mechanism are, however, very weak and are mostly con???ned to the upper troposphere. On the other hand, the stratosphere can a???ect the propagation of planetary waves from the troposphere, resulting in a burst of wave activity and the zonal wind deceleration prior to the ???nal warming in the troposphere.
We also perform a series of perturbation experiments for the sudden and ???nal warmings to test the roles of the troposphere and the stratosphere in determining the predictability of stratospheric warmings. For a late ???nal warming, almost all of the predictability comes from the troposphere. For the rest of the ???nal warmings and for sudden warmings, however, the troposphere determines the predictability until very close to the time of warming onset. This ???nding, consistent with the conventional view of the warming, re???ects that center role of the troposphere in determining the stratospheric warmings.
Results from a comprehensive global climate model, the Whole-Atmosphere Community Climate Model (WACCM), are used to analyze the ???nal warmings in both hemispheres. Although WACCM has zonal winds that are too strong in the spring, which causes the mean onset dates to be delayed at least one month with respect to the observations, the
zonal wind evolutions resemble the observations. The similarity between the models and observations suggests that the downward in???uence of the stratospheric ???nal warming on the tropospheric circulation is real and substantial, especially in the Northern Hemisphere
