Sources of Annular Mode timescales

Timescales derived from Annular Mode (AM) variability provide dynamical insight into stratosphere-troposphere coupling and are linked to the strength of AM responses to climate forcings. AM timescales reflect decorrelation times of geopotential height in the stratosphere and troposphere. But geopotential height involves a vertical integral via the hypsometric equation, and this makes ambiguous some aspects of the dependence of the timescales on vertical level. In this study, a methodology for decomposing AM variability into contributions from surface pressure and from temperature is presented that is based on a linearization of the hypsometric equation. The decomposition is then used to interpret stratosphere-troposphere coupling events and the seasonal variation of Annular Mode timescales in reanalysis products and in two versions of a general circulation model that have distinctly different stratospheric representation. It is shown that surface pressure variations best account for tropospheric AM variability and timescales, while stratospheric temperature variations best account for stratospheric AM variability and timescales. The rapid phase speed of descent of AM anomalies into the troposphere during stratosphere-troposphere coupling events is related to the relatively weak variations of zonal mean temperature there. The analysis makes explicit the point that surface pressure and stratospheric temperature yield dynamically separable, but nevertheless coupled, contributions to AM variability. The time scales due to these separate contributions as well as their coupled effect may be isolated. The decomposition might serve as the basis for further theoretical analysis on the origins of stratosphere-troposphere coupling. For example, it is shown that the surface pressure contribution reflects the timescale of upper tropospheric eddy momentum fluxes while the stratospheric temperature contribution reflects the timescale of stratospheric eddy heat fluxes.