El Niño Southern Oscillation controls snow cover on Nevado Coropuna: measurements using Landsat satellites

El Niño Southern Oscillation (ENSO) is generally considered to be the main cause of year-to-year variability in the Andean climate system. But, because the connection between a warming world and ENSO is not yet fully understood, we measured snow cover in 258 Landsat scenes from 1986 to 2014 using the normalized difference snow index at Nevado Coropuna to see if changes in snow extent show an ENSO signal. The glacier body atop Nevado Coropuna Peru is the largest body of ice in the tropics (44.1 km2 in 2014) and is a water supply for 100,000 people in southern Peru. The results of this study are consistent with an inverse relationship between ENSO and snow cover: snow cover is significantly lower for strong El Niños (MEI < 0.5) than for strong La Niñas (MEI > 0.5). During the six strongest El Niño events, snow cover only once exceeded 80 km2, while during the six strongest La Niña events, snow cover was always above 80 km2. The mean maximum snow area when the MEI is less than 0.5 (95.3 km2) is significantly higher than that for mean snow area when MEI is greater than 0.5 (74.7 km2) (t-Test, P = 0.0486). Our results show that the biggest La Niña event, which occurred in 1999 based on annual averages, brought with it the third highest maximum annual snow cover recorded in this dataset. Meanwhile the largest El Niño event, during 1997 to 1998, coincides with the smallest peak annual snow cover on Coropuna. Although the connections between ENSO events and climate change remain unclear, at Coropuna the La Niña events appear to be producing smaller maximum snow cover towards present. The Coropuna ice has experienced an average of 0.42 km2yr-1 of ice loss from 1980 to 2014, presumably due to global warming. This either means that La Niña events are weaker than in previous decades due to climate change or ENSO cycling, or that the snowline has risen and the elevation at which precipitation freezes is increasing faster than in the past due to global or regional warming.