On the sensitivities of the stratosphere / troposphere coupling to changes in the lower stratospheric winds

Numerical modeling studies have shown externally-forced stratospheric perturbations being capable of affecting the tropospheric climate (e.g. Song and Robinson 2004; Kushner and Polvani 2004). Although these studies have implicated planetary-scale waves in the vertical coupling, they did not examine how the tropospheric impacts depend on the stratospheric wave climatology. The focus here is to examine the role of planetary waves of realistic amplitude in this context.

Using a simple AGCM, we investigate how topographically-forced stationary waves can influence this stratosphere-troposphere coupling. Our results reveal modest changes to the spatial structure of the topography can provide fundamentally different coupling behavior. In one case, changes in the stratospheric annular mode index (AMI) are precursors to the same-signed changes in the troposphere, as seen in observations. In the other extreme, the tropospheric AMI does not follow the changes in the stratosphere.

In this talk, we will examine the two distinct dynamical behaviors of these two cases and provide a more detailed mechanism in understanding on how the coupling is manifested. More specifically, we will discuss how the tropospheric response to stratospheric perturbations is largely dependent on the eddy, mean flow, feedback in the troposphere, the stratospheric wave climatology, and the variability of the lower stratospheric winds.