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Abstract

Blocking is associated with outbreaks of easterlies induced by a continuum of features including anticyclones, cyclones or both. Blocking identification methods disagree on the levels of high-latitude blocking (HLB) activity. We investigate the cause of the disagreement in HLB activity over the Northern Hemisphere obtained by two 2D methods: the PV– (Formula presented.) index and the Absolute Geopotential Height (AGH) reversal method. Although both classify as absolute field methods, the former yields nearly twice the winter HLB activity of the latter method. We show that this discrepancy is caused by the addition of a poleward criterion in the AGH method that requires strong poleward westerlies. The additional criterion in the AGH method shifts the focus on the detection of blocking ridges and thus other blocking circulation patterns are under-represented. Both methods agree on the climatology of midlatitude blocking because the poleward criterion has been tuned to capture the strong midlatitude blocking, but the discrepancy grows in high latitudes. HLBs are different because they occur on the northern flank of the westerlies and are associated with the equatorward displacement of the midlatitude jet. HLB anticyclones are weaker and do not induce strong poleward westerlies compared to their midlatitude counterparts. The implementation of a strict poleward criterion designed to identify midlatitude blocks rejects many HLBs. The use of the less strict cut-off threshold (CT) of 0 m (°lat)−1 in the poleward criterion for latitudes higher than 60°N results in the convergence of climatology, interannual variability and trends of HLB between the two methods, especially during winter. The additional HLBs identified by the modified AGH algorithm develop from cyclonic wave breaking that is typical for oceanic blocking. The modified AGH method can be useful in detecting more robust HLB trends in climate model projections. © 2020 The Authors.