The impact of the state of the troposphere on the response to stratospheric heating in a simplified GCM

A meridional shift of the tropospheric mid-latitude jets is the dominant response of the mid-latitude circulation to many climate forcings such as increasing greenhouse gases, Antarctic ozone depletion and Solar variability. It has recently become apparent that the magnitude of such annular mode-like responses to forcings as well as the timescale of natural annular mode variability can be highly dependent on model specification. There is a tendency for models with lower latitude climatological jets to exhibit larger mid-latitude jet shifts and longer timescale annular mode variability. This is true of comprehensive GCMs, chemistry climate models and simplified GCMs.

Here we make use of simulations with a simplified GCM to understand why the annular mode-like response to a forcing depends on the structure of the climatological jet. It is found that in response to heating of the equatorial stratosphere, the mid-latitude jet shifts poleward and that the magnitude of this response is highly dependent on jet structure, with lower latitude jets having a much larger response to the stratospheric heating. Recently, many studies have investigated the mechanisms for production of such a response and several of the proposed mechanisms depend on a feedback between the eddies and the mean flow. In these stratospheric heating experiments it is found that lower latitude jets exhibit a larger response due to a stronger feedback between the eddies and the mean flow. The reason for the dependence of the strength of the feedback on the climatological jet structure will be discussed.