María Jesús Casado and Asunción Pastor
Instituto Nacional de Meteorología
Ciudad Universitaria, 28040 Madrid, Spain
Francisco J. Doblas-Reyes
ECMWF, Shinfield Park
RG2 9AX, Reading, UK
Extra-tropical atmospheric circulation variability is addressed in this study using two
complementary approaches: circulation types and modes of variability. Principal component
analysis (PCA) in T- and S-modes has been used to estimate the features. An objective
synoptic classification of Euro-Atlantic atmospheric circulation is described. Eight
circulation types have been identified and described in terms of their spatial features,
mean frequency and duration, transitions and trends. The most persistent type is connected
with the presence of a ridge over the British Isles and Iceland, while the less persistent
type is related to a tilted ridge west of the continent. An increase in the persistence of
some circulation types such as the Scandinavian and the Euro-Atlantic blockings and a ridge
with axis over Western Europe have been found in the 1990s. In a second step, the main modes
of variability have been identified using daily data. An analysis of the relationship between
these modes and the circulation types points out that certain circulation types increase
the probability of finding specific phases of certain modes of variability. Data from two
independent re-analyses are used for testing the robustness of the results. The main
differences have been found in the distribution of events as a function of their duration.