Annales Geophysicae, 14, 976-985
On Long-Term Evolution of Seasonal Precipitation in
Southwestern Europe
Francisco Valero, Francisco J. Doblas-Reyes, and Jesús F. González
Dpto. Física de la Tierra, Astronomía y Astrofísica
II,
Universidad Complutense, Ciudad Universitaria, 28040 Madrid, Spain
Annual cycles in long time series of precipitation from sixteen southwest
European observatories have been analysed using complex demodulation. The
stations have been clustered into two distinct regions and a hybrid one.
They are referred to as the southwestern Europe precipitation Atlantic
regime (SEPAR) and the southwestern Europe precipitation Mediterranean
regime (SEPMER), with a hybrid regime referred to in terms of the mean
amplitude ratios between semiannual and annual rainfall components. Some
evidence of linking between seasonal cycle harmonic amplitudes and the
zonal circulation has been found for SEPAR stations and a more obscured
relationship for the SEPMER region. Within the SEPAR region the strength
of the relationship is diminished towards the north. A trend analysis of
the amplitudes against time since 1920 has also been carried out and the
results reveal a divergent pattern in trends between annual and semiannual
component amplitudes for the SEPAR region. In fact, both an increasing
annual-amplitude trend and a decreasing semiannual-amplitude trend are
observed, in each case statistically significant. The fact that the seasonal
cycle variability of rainfall in southwestern Europe becomes more sensitive
southwards to changes in atmospheric zonal circulation over the North Atlantic
might, in our opinion, be related to the swing of the circumpolar vortex.