Downward influence of stratospheric final warming events in an idealized model.

摘要

The stratospheric final warming is the final 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 significant 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 influence from the stratosphere.;We first explore the hypothesis that much of the observed tropospheric signal of the final warming is initiated from the stratosphere. Large ensembles of final 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 final warming are induced from the stratosphere.;We further investigate the mechanisms of the downward influence of the final warmings on the tropospheric circulation. Results from our zonally symmetric model suggest that stratospheric wave driving can induce a residual circulation and affect the tropospheric circulation. The tropospheric signals due to this mechanism are, however, very weak and are mostly confined to the upper troposphere. On the other hand, the stratosphere can affect the propagation of planetary waves from the troposphere, resulting in a burst of wave activity and the zonal wind deceleration prior to the final warming in the troposphere.;We also perform a series of perturbation experiments for the sudden and final warmings to test the roles of the troposphere and the stratosphere in determining the predictability of stratospheric warmings. For a late final warming, almost all of the predictability comes from the troposphere. For the rest of the final warmings and for sudden warmings, however, the troposphere determines the predictability until very close to the time of warming onset. This finding, consistent with the conventional view of the warming, reflects 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 final 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 influence of the stratospheric final warming on the tropospheric circulation is real and substantial, especially in the Northern Hemisphere.