Evaluating Indices of Blocking Anticyclones in Terms of Their Linear Relations With Surface Extremes

Changes in frequencies of blocking anticyclones are sometimes used to explain changes in surface hot extremes. However, there is no consensus on the definition of blocking anticyclones, and several indices have been developed to measure them. Here we linearly regress interannual variations of hemispheric continental summer surface hot extreme area on the corresponding variations of blocking anticyclones in the ERA-Interim reanalysis data and use cross-validation test error to measure the blocking-extreme link. Relative mean square validation error is at least 0.91 (relative to no knowledge of blocking) for existing indices defined on 500-hPa geopotential height, when summed over land and ocean. This can be reduced to 0.55, largely by excluding blocks over ocean and optimizing parameters/thresholds and partly by using anomaly-based or anomaly-reversal hybrid indices. This framework helps to quantify the association of hot extremes and blocks with an uncertainty estimate and can be used for other extremes as well. Plain Language Summary Blocking anticyclones refer to a type of high pressure system that blocks the atmospheric jet stream. They often cause weather extremes such as heat waves, but there is no consensus on how to measure them. Here we present a framework to evaluate different measures of blocking anticyclones by examining their linear relation with heat waves. This framework helps to quantify the association of heat waves and blocking anticyclones along with an uncertainty estimate and can be used for other extremes as well.