Francisco J. Doblas-Reyes
Dpto. Física de la Tierra, Astronomía
y Astrofísica II,
Universidad Complutense, Ciudad Universitaria, 28040
Madrid, Spain
María Jesús Casado and María Asunción Pastor
Instituto Nacional de Meteorología
Pº de las Moreras s/n
28040 Madrid, Spain
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 bandpass-
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.