The Andean Glacier and Water Atlas

Precipitation Precipitation trends in the Andes are difficult to identify, due to the lack of reliable long-term observational records and the generally high variability in annual precipitation (Vuille et al., 2018). A number of studies reveal an intensification of rainfall, as opposed to an increase in frequency or duration of wet days, basically translating in a change in seasonality and an increase in extreme rainfall events (e.g. de los Milagros Skansi et al., 2013; Castino et al., 2017; Vuille et al., 2018). However, yearly precipitation can vary markedly with both severe reduction or increase in annual volumes, depending on location and influenced by ENSO events (e.g. Heidinger et al., 2018; Ruiz et al., 2017; Lenaerts et al., 2014; Garreaud, 2009). Studies looking at snow cover indicate an overall decreasing trend in the last two decades in correlation with rising temperatures. Snow loss has been especially significant in the Central Andes and on the east flanks (Saavedra et al. 2018). In the Southern Andes, the snow line is also moving up. There exist quite strong fluctuations from year to year, as well as a convincing link to ENSO, however long-term projections are difficult (Malmros et al., 2018).

transports moisture from the Amazon Basin towards southern Brazil and northern Argentina (Marengo, Douglas & Silva Dias, 2002). The high rainfall supports agriculturally productive areas and wetlands (Garreaud, 2009). On the other side of the range, the coasts of northern Chile and southern Peru are extremely dry, evidenced by the development of barren areas, such as the Atacama desert, the driest desert on earth (Garreaud, 2009; Schulz, Boisier & Aceituno, 2012). Further south, in the Wet Andes, rain is primarily the result of strong moisture laden westerly surface winds from the Pacific (Garreaud, 2009). The winds are strongest during winter months, during which they can reach further north into central Chile. Cyclonic depressions, driven by the westerlies, rise over the Andes, producing high precipitation on the Pacific-facing slopes, while the eastern slopes receive dramatically less rainfall (Aravena & Luckman, 2009). Annual mean precipitation south of 40° is more than 5 m on the western flanks, decreasing to less than 1 m on the eastern slopes and dropping to less than 500 mm on the low-lying Argentinian steppes (Lenaerts et al., 2014). Consequently, the western side of the mountain range supports flourishing vegetation at lower altitudes and massive glaciers at higher altitudes, while the eastern slopes have less vegetation (Ruiz et al., 2017).

The projections displayed in the following maps (Precipitation in the Andes; Precipitation seasonality in the Andes; and Mean

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