The Andean Glacier and Water Atlas

annual temperature in the Andes) are based on high-resolution spatial interpolation. These have their limitations in areas with complex topography like in the Andes. These projections do not show any elevation dependent warming (altitude amplification) nor other topographic effects because the climate models do not have that information to start with (meteorological stations are sparse, and data from these is needed for calibrating and validating these climate models). For short-term projections, they differ a lot from actual observations. However, they can provide an indicator for trends in the long-term. There is generally a slightly higher degree of agreement for temperature projections than for precipitation projections. Future rainfall predictions are difficult to estimate. However, most models predict an increase in precipitation during the wet season and a decrease during the dry season in the tropical Andes (Vera et al., 2006). This also appears to be the case over the Altiplano region (Seth et al., 2010; Minvielle & Garreaud, 2011, Neukom et al., 2015). Under the IPCC high emission scenario (Jiang et al. 2000) precipitation is projected to increase by 2100 along the coastal regions of Colombia and Ecuador and in some places along the eastern Andes, south of the equator. However, precipitation is projected to decrease in the southern (tropical) Andes, including the Altiplano regions by 2100, leading to increased drought.

The effect of El Niño on the weather in the Andes The Andean climate is influenced by El Niño-Southern Oscillation (ENSO) events, which are associated with a band of warm water, which develops in the equatorial Pacific. El Niño events generally produce intense rainfall at the low altitudes along the Pacific side of the Andes, while areas above 2,000 m receive less rain and experience higher temperatures than normal (Garreaud, 2009). The strongest El Niño rainfall anomalies occur during the austral summer (December–January–February) and are associated with heavy rainfall and flooding along the coast of northern Peru and southern Ecuador (Sulca et al., 2017). During La Niña years (when the sea surface temperature across the equatorial Pacific is lower than normal) the opposite generally occurs (Garreaud, 2009). In the central and southern Andes ENSO has a less marked influence, but El Niño has been associated, for example, with increased rainfall in Central Chile (Verbist et al., 2010; Robertson et al., 2013) and increased streamflow in Patagonia (Rivera et al., 2018). In the Tropical Andes, though, variations of glacier mass balance are subject to ENSO (Veettil et al., 2017).

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