Northern Hemisphere Atmospheric Blocking in CMIP6 Climate Projections using a Hybrid Index

Abstract Atmospheric blocking is quantified by a variety of different blocking indices. Here, the index by Davini et al. (DAV12) is modified to a hybrid index by additionally considering the extent of the blocking anticyclone. Applying both indices, the DAV12 and the hybrid index, to the 20th century reanalyses ERA-20C and the 20CRv3 (hereafter 20CR) ensemble reveals large differences between the reanalyses. The annual blocking frequency increases widespread and significantly in ERA-20C, but only regionally and non-significantly in the 20CR ensemble-mean. Trend analysis reveals a higher reliability of the 20CR ensemble-mean than ERA-20C blocking results. Seasonally, the changes in summer blocking are most pronounced and significantly positive around Greenland. The CMIP6-mean historical climate simulation agrees well in magnitude with 20CR during the period 1900–2014, with some underestimation of blocking frequency in the last decades related to inconsistent trends. For a future climate, the average of the CMIP6 models projects a decrease in blocking frequency in major parts of the Northern Hemisphere in summer and in winter. A CMIP6 sub-set of models which agree best to reanalyses shows a less negative future trend. Because of the mismatch of historical trends, especially in summer, the future projections are uncertain.