Abstract. Climate models show a wide range of Southern Hemispheric jet responses to greenhouse gas forcing. One approach
to constrain future jet response is by utilising the fluctuation-dissipation theorem (FDT) that links forced response to internal
variability timescales, with the Southern Annular Mode (SAM) the most dominant mode of variability of the Southern Hemispheric jet. We show that stratospheric variability approximately doubles the SAM timescale during austral summer in both re-analysis data and models from the Coupled Model Intercomparison Project, phase 5 (CMIP5). Using a simple barotropic
model, we demonstrate how the enhanced SAM timescale subsequently leads to an overestimate of the forced jet response
based on FDT, and introduce a method to correct for the stratospheric influence. Even after accounting for this influence, the
SAM timescale cannot explain inter-model differences in the forced jet shift across CMIP5 models during austral summer,
owing to other confounding factors.