Comparing trends in the Southern Annular Mode and surface westerly jet
We examine trends in the Southern Annular Mode (SAM), and the strength, position and width of the Southern Hemisphere surface westerly wind jet in observations, reanalyses and the Coupled Model Intercomparison Phase 5 (CMIP5) models. First we consider the period over 1951 to 2011, and show that there are differences in the SAM and jet trends between the CMIP5 models, the HadSLP2r gridded sea-level pressure (SLP) dataset, and the Twentieth Century Reanalysis. The relationships between these trends demonstrates that the SAM index cannot be used to directly infer changes in any one kinematic property of the jet. The spatial structure of the observed trends in SLP and zonal winds is shown to be largest, but also most uncertain, in the southeast Pacific. To constrain this uncertainty we include six reanalyses and compare with station based observations of SLP. We find the CMIP5 mean SLP trends generally agree well with the direct observations, despite some climatological biases, while some reanalyses exhibit spuriously large SLP trends. Similarly, over the more reliable satellite era the spatial pattern of CMIP5 SLP trends is in excellent agreement with HadSLP2r, while several reanalyses are not. Then we compare surface winds with a satellite based product, and show that the CMIP5 mean trend is similar to observed in the core region of the westerlies, but that several reanalyses overestimate recent trends. We caution that studies examining the impact of wind changes on the Southern Ocean could be biased by these spuriously large trends in reanalysis products.
Authors: Swart, Neil C., Fyfe, John C., Gillett, Nathan, Marshall, Gareth J.