A methodology for the comparison of blocking climatologies across indices, models and climate scenarios

There is urgent need for a consistent blocking identification method that can be used and compared across reanalyses, models and climate scenarios. We present such a method and diagnose daily blocking frequency in 43 years (1958-2000) of ERA-40 Reanalysis for indices defined on both the commonly used geopotential height and potential temperature fields as well as a zonal wind index. Applications of various blocking indices to the same data highlights the importance of a consistent methodology for comparison and a method that identifies blocks along a path that varies with the latitude of the storm track. Since the method accommodates blocking detection using 500 mb zonal-wind which is readily available in climate model output, we diagnose blocking in 14 CMIP3 models under two different greenhouse gas scenarios. Blocking duration remains nearly constant among the scenarios, but a robust reduction in blocking frequency with global warming is demonstrated.