Evaluating Indices of Blocking Anticyclones in Terms of Their Relations with Surface Hot Extremes

Blocking anticyclones can cause subsidence, clear-sky radiative forcing and anomalous advection, which can lead to surface hot extremes. There is no consensus on the definition of blocking anticyclones, and a number of indices have been developed in the past 60 years to identify or measure them. These indices detect different features of blocking patterns and sometimes even produce conflicting blocking statistics. However, there is no framework to evaluate these indices and there has not been much work on comparing multiple blocking indices and assessing their relationship to weather extremes, which is what ultimately most care about.

In this study, we present a framework to assess the relationship between blocking anticyclones and surface hot extremes, on both hemispheric scale and local scale, in ERA-interim reanalysis data. Five 2-D indices from literature defined on Z500 are used with some modifications. As a single number summary, hemispheric Pearson correlation between interannual variation of summer-averaged area of blocking anticyclones and summer-averaged land area experiencing surface hot extremes are calculated. We find correlations in a wide range from 0.1 to 0.7 using different indices, unveiling the importance of the choice of index. This result can potentially help and guide the research on blocking anticyclones, the resulting surface hot extremes, and their changes with climate change.