| The aim of this study is to assess the impact of the North Atlantic
Oscillation (NAO) on both the winter precipitation and the temporal
occurrence of different landslide types in Portugal. The analysis is applied
to five sample areas located just north of Lisbon, the capital of Portugal.
These sites are particularly relevant because actual dates of most of the
recent landslide events are known but also because the landslides occurred
in a suburban area with growing urbanization pressure.
Results show that the large inter-annual variability of winter precipitation
observed in western Iberia, i.e. Portugal and parts of Spain, is largely
modulated by the NAO mode. In particular, precipitation falling in Portugal
between November and March presents a correlation coefficient of R=–0.66
with the NAO index. Precipitation distribution for the reference rain gauge
in the study area reveals that the probability of a wet month to occur is
much higher for low NAO index composites than for the corresponding high NAO
index composite. It is shown that this control, exerted by NAO on the
precipitation regime, is related to corresponding changes in the associated
activity of North-Atlantic storm tracks that affect the western Iberia.
Landslide activity in the study area is related to both intense, short
duration precipitation events (1–15 days) and long-lasting rainfall episodes
(1–3 months). The former events trigger shallow translational slides while
the later episodes are usually associated with deeper and larger slope
movements. This second group of landslides is shown to be statistically
associated with the 3-month average of the NAO index. |