New insights into the decadal variability in glacier volume of a tropical ice cap, Antisana (0°29′ S, 78°09′ W), explained by the morpho-topographic and climatic context

We present a comprehensive study of the evolution of the glaciers on the Antisana ice cap (tropical Andes) over the period 1956-2016. Based on geodetic observations of aerial photographs and high-resolution satellite images, we explore the effects of morpho-topographic and climate variables on glacier volumes. Contrasting behaviour was observed over the whole period, with two periods of strong mass loss, 1956-1964 (-0.72 m w.e. yr-1) and 1979-1997 (-0.82 m w.e. yr-1), and two periods with slight mass loss, 1965-1978 (0.10 m w.e. yr-1) and 1998-2016 (-0.26 m w.e. yr-1). There was a 42 % reduction in the total surface area of the ice cap. Individually, glacier responses were modulated by morpho-topographic variables (e.g. maximum and median altitude and surface area), particularly in the case of the small tongues located at low elevations (Glacier 1, 5 and 16) which have been undergoing accelerated disintegration since the 1990s and will likely disappear in the coming years. Moreover, thanks to the availability of aerial data, a surging event was detected on the Antisana Glacier 8 (G8) in the 2009-2011 period; such an event is extremely rare in this region and deserves a dedicated study. Despite the effect of the complex topography, glaciers have reacted in agreement with changes in climate forcing, with a stepwise transition towards warmer and alternating wet-dry conditions since the mid-1970s. Long-term decadal variability is consistent with the warm-cold conditions observed in the Pacific Ocean represented by the Southern Oscillation index.