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Stratospheric variability and tropospheric annular mode timescales

The annular modes are the dominant modes of variability in each hemisphere. In the troposphere they represent latitudinal migrations of the eddy driven mid-latitude jets. Many climate forcings such as ozone depletion and increasing greenhouse gas concentrations result in circulation changes that project strongly onto these modes. It is therefore important that the dynamics behind annular mode variability be understood and that such variability is captured correctly in climate models.

Observational evidence suggests that stratospheric variability enhances annular mode timescales, although it is difficult to prove this unambiguously as other factors, such as tropospheric jet structure, vary along side stratospheric variability. Here, model experiments will be presented that unambiguously identify a role for stratospheric variability in enhancing tropospheric annular mode persistence. These experiments also reveal a bias in the internal dynamics of tropospheric variability in the Southern Hemisphere summer which likely inhibits our ability to quantitatively accurately predict future SH climate change in that season.