Understanding Climate Model Biases in Southern Hemisphere Mid-latitude variability

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, a combination of analysis of the CMIP-5 models and specifically targetted experiments with one global climate model, the Canadian Middle Atmosphere Model (CMAM), are used to investigate the cause of this bias in SAM variability. The role of biases in stratospheric variability and climatological tropospheric jet structure are assessed. This reveals a bias in internal tropospheric dynamics that it not caused by either the climatological jet latitude bias or biases in stratospheric variability. An analysis of eddy-mean flow feedbacks reveals that the strength of the planetary wave feedbacks onto the SAM differ between models and the reanalysis, leading to the enhanced SAM persistence.