Understanding climate model biases in Southern Hemisphere mid-latitude variability.

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• Dr Isla Simpson, Department of Physics, University of Toronto
• Wednesday 23 May 2012, 13:00-14:00
• Centre for Mathematical Sciences, meeting room MR14.

If you have a question about this talk, please contact Dr Alex Archibald.

NOTE - unusual time and venue

The Southern Annular Mode (SAM) represents a latitudinal shifting of the Southern Hemisphere (SH) jet stream and is the dominant mode of variability in the SH mid-latitude circulation. A common bias among global climate models is that they tend to exhibit SAM variability that is much too persistent, particularly in the summer season. Many climate forcings such as ozone depletion/recovery and increasing greenhouse gas concentrations result in tropospheric circulation changes that project strongly onto the SAM and therefore the inability of models to simulate natural SAM variability correctly is of concern for the ability of such models to accurately predict future circulation changes.

Here, specifically targetted experiments with one global climate model, the Canadian Middle Atmosphere Model (CMAM), will be used to investigate the cause of this bias in SAM variability. The role of biases in stratospheric variability and climatological tropospheric jet structure will be assessed. It is found that even in the absence of stratospheric variability the SAM timescales are biased long relative to the observations, suggesting a role for a bias in internal tropospheric dynamics. Furthermore, it is found that this bias is not alleviated when the climatological tropospheric jet structure is improved in an artificial way. An analysis of tropospheric eddy feedbacks reveals a difference in the strength of planetary scale feedbacks on the SAM between the model and observations.

This talk is part of the Cambridge Centre for Climate Science series.

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