Incompatibility of Alpine Glaciers and Early Summer Heat Waves: Examples from North Cascade Range, USA and Central Andes, Chile

This century the frequency and intensity of heat waves affecting alpine glacier regions globally has increased. Heat waves leave a greater portion of a glacier snow free, which increases melt rates and mass balance loss. This is most pronounced when a heat wave occurs early in the melt season after a winter of poor snowfall exposing bare glacier ice or older firn for the bulk of the melt season. The result is a higher ablation rates from the darker firn and ice surfaces, short term runoff increase followed by long term decrease. We provide observations of the specific impact of early ablation season heat waves during 2015 and 2021 in the North Cascade Range, WA and summer 2019/20 and 2021/22 in the Central Andes of Chile on accumualtion area ratio (AAR) and mass balance (Ba).

During the 2015 season Sholes Glacier had an AAR of 97% on June 15, declining to 55% by July 9, 20% on Aug. 8 and 0% by the end of the melt season. In 2021 the regions record heat wave began on June 25 and Sholes Glacier had an AAR of 100%, by Aug. 6 the AAR was 40% and by the end of the melt season was 24%. The key difference was that in 2015 accumulation season snowfall was of 71% ofaverage and in 2021 it was of 114% of average. All 10 glaciers in the North Cascades with a Ba record of more than 30 years experienced their most negative Ba in 2015, with nine having an AAR of 0%. The loss in this single year is 5-10% of the glacier volume.

On Olivares Beta and Olivares Gamma Glacier on Dec. 2, 2019 the glaciers had an AAR of ~90%, declining by Dec. 31, 2019 to an AAR of 25%. On Dec. 10, 2021 the AAR was ~90% and by December 31, had declined to 35% and on Jan. 13, 2022, the AAR was less than 10%, at the mid-point of the melt season. Both years were during an extended period of drought featuring early season heat waves with December temperature averaging +2oC at the Olivares Alfa AWS at 4240 m. The essentially snow free conditions persisted from mid-Jan. until the end of March 2022. This leads to an accelerated spring/early summer glacier wide albedo decrease, that combined with surface impurities, might be responsible for the above-average mass losses in the catchment. The snowpack loss in 2021/22 summer affected glaciers across Central Andes of Chile and Argentina with an AAR below 10% observed in mid-January on Bajor Del Plomo Glacier, Cortaderal Glacier, Palomo Glacier, Volcan Overo Glaciers and Volcan San Jose Glaciers.