Sensitivity of the Northern Hemisphere blocking frequency to the detection index

[1] The sensitivity of Northern Hemisphere blocking features to the use of two different blocking indices is investigated using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses (ERA) and data from a climate simulation carried out with the atmospheric general circulation model Action de Recherche, Petite Echelle, Grande Echelle ARPEGE. The seasonal cycle of blocking frequency shows a good agreement between both indices, for both analyses and model, when the seasonal cycle of the 500-hPa geopotential height is previously removed from the data. Furthermore, the blocking frequency seasonal cycle agrees well with the 5- to 45-day band-pass-filtered standard deviation (a proxy indicator of blocking activity) seasonal cycle. In contrast to previous results, this demonstrates that a consensus estimate of blocking features may be obtained with two different blocking indices. Nevertheless, some differences in the results from the two indices have been detected, particularly for the Euro-Atlantic blocking. Regional differences suggest that on some occasions, an index may detect blocking events that the other index misses, and vice versa. An important consequence is that uncertainty in blocking interannual variability estimates, which depend on the index used, is expected to be high. The study shows that the model underestimates the blocking frequency regardless of the index used. However, the model proves to be able to simulate realistic blocks when removing an estimate of the geopotential time-mean bias. Blocking drawbacks in the model are interpreted as a consequence of a specific zonal wind forcing associated with a decrease in the frequency of large-scale ridges over the main blocking regions. These results strengthen the interest of analyzing model blocking as a complex phenomenon with regional dynamical differences and tight links with other atmospheric phenomena.