Comparing changes in the SAM, westerly jet and Southern Ocean fronts

We compare changes in the Southern Annular Mode and the surface westerly jet in reanalyses, the CMIP5 models, and the best available in situ and satellite data products. We demonstrate that the changes in the SAM and surface jet simulated by the CMIP5 models are generally in agreement with the observations while many reanalysis products show larger, and demonstrably spurious trends. For example, many reanalyses show significant strengthening near the peak of the jet, but this is not seen in the satellite wind products. All models, reanalyses and observations generally agree that the SAM has increased, and that the jet has shifted poleward during the Austral summer (DJF). However, due to opposing trends in the Austral winter, the annual mean position of the jet has not changed. This raises the question of whether the mean positions of the Southern Ocean fronts have indeed shifted poleward, as previously suggested. We use satellite altimetry to show that annual mean sea-surface height changes in the Southern Ocean largely relate to global sea-level rise, rather than corresponding to changes in the local dynamics, in accord with other recent work. We then investigate whether a seasonal shift can be detected in the fronts (as may be expected from the wind forcing), and consider implications for previously identified Southern Ocean warming, and other climate relevant processes.