Using Principal Oscillation Patterns to Characterize Troposphere–Stratosphere Interactions

Stratosphere-troposphere interactions are conventionally characterized using the first Empirical Orthogonal Function (EOF) of fields such as zonal mean zonal wind. Perpetual winter integrations of an idealized model are used to contrast the vertical structures of EOFs with those of Principal Oscillation Patterns (POPs, the modes of a linearized system governing the evolution of zonal flow anomalies). POP structures are shown to be insensitive to pressure weighting of the time series of interest, a factor that is particularly important for a deep system such as the stratosphere and troposphere. In contrast, EOFs change from being dominated by tropospheric variability with pressure weighting to being dominated by stratospheric variability without it. The analysis reveals separate tropospheric and stratospheric modes in model integrations that are set up to resemble midwinter variability of the troposphere and stratosphere in both hemispheres. Movies illustrating the time evolution of POP structures show the existence of a fast, propagating, tropospheric mode in both integrations, and a pulsing stratospheric mode with a tropospheric extension in the northern hemisphere-like integration.

(Image: Leading stratospheric mode in an NH-like winter integration, phase running from 0 to 8pi, with EP flux anomalies associated with each phase of the mode superimposed. The mode resembles stratospheric vacillation cycles, with both anomalously weak and strong polar vortex events being followed by similarly-signed tropospheric anomalies all the way to the surface.)