Does the troposphere care about the stratosphere? (Invited Presentation)

The troposphere influences the stratosphere through the vertical propagation of planetary waves. This is readily observed and well understood. Downward influences of the middle atmosphere on the troposphere are, by contrast, controversial and difficult to detect. This asymmetry results from the upward decrease of density in the atmosphere, and the fact that the generation and release of available potential energy are concentrated in the lower atmosphere. Here, possible dynamical mechanisms by which the stratosphere could affect the troposphere are reviewed.

The propagation of planetary waves out of the troposphere is sensitive to the mean flow in the lower stratosphere. Changes in stratospheric mean flows can thus modify tropospheric quasi-stationary eddies. Further, changes in the meridional propagation of stationary eddies can lead to changes in the tropospheric zonal flow. This is the most promising mechanism for downward influence, though it should be noted that, according to linear wave models, tropospheric stationary planetary waves are sensitive to the zonal flow only in the lowest regions of the stratosphere.

The irreversible rearrangement of stratospheric potential vorticity by breaking planetary waves can influence the tropospheric flow. Tropospheric flows change almost immediately to maintain hydrostatic and geostrophic balance with altered distributions of stratospheric potential vorticity. For example the footprint of the February 1979 sudden warming, the splitting of the polar vortex, was evident even at 1000 mb. On longer timescales, through the stratospheric balance between wave driving and the secondary circulation, changes in stratospheric wave driving influence tropospheric flows - the so-called “downward control”. The resulting changes in the troposphere could modify the sources of planetary waves. The density effect is, however, a formidable obstacle to any robust downward influence mediated by secondary circulations.

The likely weakness, compared with internally generated variability, of downward influences on the troposphere suggests that such effects may be important only to the extent that they stimulate the troposphere’s internal modes of variability. In this way, downward influences from the stratosphere may behave similarly to the upward influences of anomalous sea-surface temperatures.