Preprints
https://doi.org/10.5194/wcd-2021-78
https://doi.org/10.5194/wcd-2021-78

  30 Nov 2021

30 Nov 2021

Review status: this preprint is currently under review for the journal WCD.

Relationship between Southern Hemispheric jet variability and forced response: the role of the stratosphere

Philipp Breul1, Paulo Ceppi1,2, and Theodore Gordon Shepherd3 Philipp Breul et al.
  • 1Department of Physics, Imperial College London, London, United Kingdom
  • 2Grantham Institute, Imperial College London, London, United Kingdom
  • 3Department of Meteorology, University of Reading, Reading, United Kingdom

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.

Philipp Breul et al.

Status: open (until 06 Feb 2022)

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Philipp Breul et al.

Philipp Breul et al.

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Short summary
Understanding how the mid-latitude jet stream will respond to a changing climate is highly important. Unfortunately, climate models predict a wide variety of possible responses. Theoretical frameworks can link an internal jet variability timescale to its response. However, we show that stratospheric influence approximately doubles the internal timescale, inflating predicted responses. We demonstrate an approach to account for the stratospheric influence and recover correct response predictions.