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Landslides as a delayed signal of warm phase of ENSO in the Aconcagua Park (32 SL).
Encuentro; Meeting of the Americas, AGU; 2013
The region of Aconcagua mount (6,929 m asl), the top of the Andes and second in altitude only to the peaks in the Himalayas, is characterised by high elevations and abrupt topography. These extreme conditions seem to have favoured the occurrence of pre-historic and historic landslides. According to spatial analyses this kind of processes has affected about 25% of the Aconcagua Park extending 71,000 ha (32º 36´- 32º 48’ S, 69º 50´-70º 04´ W). Temporal reconstruction of landslides with historical sources from the late 19th century, official reports and occasional observers let to know the occurrence of 72 damaging events. Concerning to triggering mechanism, rainfall are mentioned as the main cause of landslides in historical sources; but this assertion could not be confirmed on the basis of available meteorological data beginning after 1940. Hence, establishing a certain relation between precipitation (mm) and landslide occurrence had not been possible for the Aconcagua Park. The most reliable rainfall threshold value corresponds to a 19 mm (daily precipitation) representing 8.7% of the annual precipitation and 8% of the mean annual precipitation for this region. However, rainfall does not seem to be the main cause of landslides in the Aconcagua Park. Firstly, because main precipitations occur as snowfall in elevated areas, favoring snow avalanche occurrence rather than landslides; secondly, infrequent rainfall has been related to historical rock fall reported by sources in lower parts of the study area. Consequently, slope instability in this high elevated region of the Central Andes seems to be mostly related to terrain saturation by snow melting and ice thawing of ice-core moraines or rock glaciers during the warm season (November - February). Several debris flows have been generated from moraine deposits or from the toe of covered glaciers in this year period even though no correlation was found for temperature records. A delayed link between warm phases of ENSO and slope instability could be established by a relationship found between landslides and local river stream flow. The typical warm phase of ENSO begins in November of one year, increasing during July—August, and ends in February of the following year (Caviedes and Waylen, 1991). Greater snowfall and positive glacier balance has been linked to the ENSO-warm phase. Consequently, increased stream flows of Andean rivers will be measured the following summer. In fact, ENSO-related features in the tropical Pacific play a major role in regulating the hydrological variability in the region with increased (decreased) summer and annual river discharges following El Niño (La Niña) events. Summering, after winters with above-average snow accumulation, an increase of slope instability has been documented in the Aconcagua Park.