Diagnosing the Structure of Stratosphere-Troposphere Coupling Using Finite Amplitude Wave Activity

Planetary waves play a central role in our understanding of stratosphere-troposphere coupling. For a surface or stratospheric perturbation, the response in wave activity flux depends on the vertical structure of planetary waves from the surface to the stratosphere (e.g. standing waves versus vertically propagating waves). We investigate the role of planetary waves using a finite amplitude wave activity diagnostic, which quantifies the wave amplitude in spite of the extent the polar vortex is disturbed. The variability in wave activity is also related to the zonal wind acceleration/deceleration through the non-acceleration theorem.

Using the ERA-40 reanalysis, we analyze the annual cycle and transient variability of finite amplitude wave activity in total and decomposed into contributions from wavenumbers one, two, and three. Throughout the annual cycle, the finite-amplitude planetary wave activity in the stratosphere exhibits a maximum in the North Hemisphere midwinter and in the Southern Hemisphere spring, as expected from stratospheric eddy-zonal flow interaction. Furthermore, we study how wave activity perturbations associated with sudden stratospheric warmings (SSWs) propagate into the troposphere, exploring whether split and displacement SSWs can be distinguished by the relative roles of wavenumbers one and two, and whether signals from these different types of warmings propagate differently into the troposphere.