The Influence of the North Atlantic Oscillation on Rainfall Triggering of Landslides near Lisbon |
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Authors: | Email author" target="_blank">Ricardo?M?TrigoEmail author José L?Zêzere Maria?L?Rodrigues Isabel?F?Trigo |
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Institution: | (1) Centro de Geofísica da Universidade de Lisboa, Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Ed C8, Piso 6, 1749-016 Lisboa, Portugal;(2) Departamento de Engenharias, Universidade Lusófona, Lisboa, Portugal;(3) Centro de Estudos Geográficos, Universidade de Lisboa, Portugal;(4) Instituto de Meteorologia, Lisboa, Portugal |
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Abstract: | The majority of landsliding episodes in the area north of Lisbon are associated with rainfall events of short (less than 5
days) medium (5–20 days) or long duration (more than 20 days). The precipitation regime in Portugal is highly irregular, with
large differences between wet and dry years. We have assessed the impact of the North Atlantic Oscillation (NAO) on both the
winter precipitation and the timing and magnitude of associated landslide events. Results show that the large inter-annual
variability of winter precipitation is largely modulated by the NAO mode. The precipitation composite corresponding to high
NAO index presents a considerable lower median value (47 mm/month) than the corresponding low NAO index class (134 mm/month).
The entire precipitation distribution associated with the low NAO index composite encompasses a wider range of values than
the corresponding high NAO index composite. This non-linear behavior is reflected in the probability of occurrence of a very
wet month (precipitation above the 90% percentile) that is just 1% for the positive NAO class and 23% for low NAO index months.
Results for the low NAO class are crucial because these months are more likely associated with long-lasting rainfall episodes
responsible for large landslide events. This is confirmed by the application of a 3-month moving average to both NAO index
and precipitation time series. This procedure allowed the identification of many months with landslide activity as being characterized
by negative average values of the NAO index and high values of average precipitation (above 100 mm/month). Finally, using
daily data we have computed the return periods associated with the entire set of landslide episodes and, based on these results,
obtained a strong linear relationship between critical cumulative rainfall and the corresponding critical rainfall event duration. |
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Keywords: | NAO landslides extreme precipitation Portugal |
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