Science http://doi.org/kfp (2013)

In recent decades, the westerly winds of the Southern Hemisphere have shifted polewards. This change is strongest in the austral summer and affects weather and climate. Modelling studies have found the shift to be caused by either increasing concentrations of greenhouse gases, or decreasing stratospheric ozone, with ozone predicted to have a greater impact

Sukyoung Lee and Steven Feldstein, of Pennsylvania State University, USA, used observational data to separate the two forcings and quantify their contribution to the shifting jetstream. They use a reanalysis data product — daily ERA-Interim data — and applied a cluster analysis. Greenhouse-gas warming was represented by an 11-day cluster, and ozone depletion by a 7-day cluster. The trends in the occurrence frequency indicate that ozone forcing had a greater effect on the jetstream — by almost 50% — supporting the model results previously reported.