Exceptionally High 2018 Equilibrium Line Altitude on Taku Glacier, Alaska

Mauri Pelto

doi:10.3390/rs11202378

Abstract

The Juneau Icefield Research Program (JIRP) has been examining the glaciers of the Juneau Icefield since 1946. The height of the transient snowline (TSL) at the end of the summer represents the annual equilibrium line altitude (ELA) for the glacier, where ablation equals accumulation. On Taku Glacier the ELA has been observed annually from 1946 to 2018. Since 1998 multiple annual observations of the TSL in satellite imagery identify both the migration rate of the TSL and ELA. The mean ELA has risen 85 ± 10 m from the 1946–1985 period to the 1986–2018 period. In 2018 the TSL was observed at: 900 m on 5 July; 975 m on 21 July; 1075 m on 30 July; 1400 m on 16 September; and 1425 m on 1 October. This is the first time since 1946 that the TSL has reached or exceeded 1250 m on Taku Glacier. The 500 m TSL rise from 5 July to 30 July, 8.0. md−1, is the fastest rate of rise observed. This combined with the observed balance gradient in this region yields an ablation rate of 40–43 mmd−1, nearly double the average ablation rate. On 22 July a snow pit was completed at 1405 m with 0.93 m w.e. (water equivalent), that subsequently lost all snow cover, prior to 16 September. This is one of eight snow pits completed in July providing field data to verify the ablation rate. The result of the record ELA and rapid ablation is the largest negative annual balance of Taku Glacier since records began in 1946.

Highlights

The transient snowline (TSL) is the location of the transition from snow cover to bare glacier ice or older firn on a glacier at any time during a hydrologic year [1]
We report the equilibrium line altitude (ELA) or TSL to the nearest 5 m as it is an average elevation of a number of points on the glacier surface
On Taku Glacier, the largest glacier of the Juneau Icefield, the 1946–2018 record of TSL and ELA observations indicate that 2018 had the highest ELA of the 73-year period, which at 1425 m, exceeded the previous highest observation by 200 m (1225 m in 1997)

Summary

Introduction

The transient snowline (TSL) is the location of the transition from snow cover to bare glacier ice or older firn on a glacier at any time during a hydrologic year [1]. In standard glaciological mass balance field assessment, ablation has been measured by drilling stakes into a glacier and observing the change in the height of the stake above the glacier surface [7]. Ablation is the product of the change in the height of the stake above the glacier surface and the density of snow or ice. Migration of the TSL can provide an additional measure of ablation if the balance gradient in the elevation range that the TSL traverses is known [3,4,8]. Hulth et al [8] used repeat snowline mapping to determine accumulation, based on the observed and modeled amount of melt from TSL movement; this is the inverse of determining snowpack ablation rate

Methods

Discussion

Conclusion