Rainfall patterns and critical values associated with landslides in Povoacao County (Sao Miguel Island, Azores):: relationships with the North Atlantic Oscillation

Sao Miguel Island (Azores) has been affected by hundreds of destructive landslide episodes in the last five centuries, triggered either by earthquakes, volcanic eruptions or rainfall episodes, which were responsible for many deaths and very important economic losses. Among the instability causes, meteorological factors are of primary importance on Povoacao County, namely the high recurrence rate of calamitous rainfall triggering landslides. The most recent catastrophic episode took place on the 31st October 1997 when almost 1000 soil slips and debris flows were triggered, and 29 people died in the Ribeira Quente village. The role of rainfall on regional landslide activity was analysed applying cumulative rainfall methods. The method comprises the reconstruction of both absolute and calibrated antecedent rainfalls associated with each major landslide event. The critical rainfall combination (amount-duration) responsible for each landslide event was assessed and a rainfall critical threshold for landslide occurrence was calculated. Rainfall-triggered landslides in the study area are ruled by the function I=144.06 D (- 0.5551), and they are related both to short duration precipitation events (1 - 3 days) with high average intensity (between 78 and 144 mm/day) and long-lasting rainfall episodes (1 - 5 months) with a lower intensity (between 9 and 22 mm/day). The impact of the North Atlantic Oscillation (NAO) on the regional precipitation regime was evaluated. It is shown that the monthly precipitation of Sao Miguel is largely modulated by the NAO mode presenting a significant negative correlation with the NAO index. This result arises from the NAO control on the travelling latitude of most storm tracks that cross the Northern Atlantic Ocean. Copyright (c) 2007 John Wiley & Sons, Ltd.