| A spectral analysis of
the time series corresponding to the main monthly precipitation regimes of the
Iberian Peninsula was performed using two methods, the Multi-Taper Method and
Monte Carlo Singular Spectrum Analysis. The Multi-Taper Method gave a
preliminary view of the presence of signals in some of the time series. Monte
Carlo Singular Spectrum Analysis discriminated between potential oscillations
and noise.
From the results of the two methods it is concluded that
there exist three significant quasi-oscillations at the 95% level of
confidence: a 5.0 year quasi-oscillation and a long-term trend in the Atlantic
pattern of March, a 3.2 year quasi-oscillation in the Cantabrian pattern of
January, and a 4.0 year quasi-oscillation in the Catalonian pattern of
February. These quasi-oscillations might be related to climatic variations with
similar periodicities over the North Atlantic Ocean.
The possible simultaneity of high values of precipitation
generated by the significant quasi-oscillations and high sea–level pressures
was studied by means of composite maps. It was found that high values of
precipitation generated by the oscillations of the Atlantic patterns of January
and March exist simultaneously with a specific high pressure structure over the
North Atlantic Ocean, that allow cyclonic perturbations to cross the Iberian
Peninsula. During the non-wet years, this high pressure structure moves
northwards, keeping the track of the low pressure centers to the north, far
from the Iberian Peninsula.
On the other hand, high values of precipitation generated by
the oscillation of the Cantabrian pattern of January exist simultaneously with
a high pressure structure over the Galicia region and the Cantabrian Sea, that
allow a northerly flow over the region. Also, a positive trend in the NAO index
for March has been found, starting in the sixties, which is not evident for
other winter months. This trend agrees with the decreasing trend found in the
March Atlantic pattern.
Key words. Meteorology and
atmospheric dynamics (climatology; precipitation) Oceanography: general
(climate and interannual variability) |