Abstract
Since 1990, Yahtse Glacier in southern Alaska has advanced at an average rate of ∼100 m/yr despite of a negative mass balance, widespread thinning in its accumulation area, and a low accumulation-area ratio. To better understand the interannual and seasonal changes at Yahtse and the processes driving these changes, we construct velocity and ice surface elevation time series spanning the years 1985-2014 and 2000-2014, respectively, using satellite optical and synthetic aperture radar (SAR) observations. In terms of seasonal changes, we find contrasting dynamics above and below a steep (up to 18% slope) icefall located approximately 6 km from the terminus. Above the icefall, speeds peak in May and reach minima in October synchronous with the development of a calving embayment at the terminus. This may be caused by an efficient, channelized subglacial drainage system that focuses subglacial discharge into a plume, resulting in a local increase in calving and submarine melting. However, velocities near the terminus are fastest in the winter, following terminus retreat, possibly off of a terminal moraine resulting in decreased backstress. Between 1996-2014 the terminus decelerated by ∼40% at an average rate of ∼0.4 m/day/yr , transitioned from tensile to compressive longitudinal strain rates, and dynamically thickened at rates of 1-6 m/yr , which we hypothesize is in response to the development and advance of a terminal moraine. The described interannual changes decay significantly upstream of the icefall, indicating that the icefall may inhibit the upstream transmission of stress perturbations. We suggest that diminished stress transmission across the icefall could allow Yahtse’s upper basin to remain in a state of mass drawdown despite of moraine-enabled terminus advance. Our work highlights the importance of glacier geometry in controlling tidewater glacier re-advance, particularly in a climate favoring increasing equilibrium line altitudes.
Highlights
The rapid retreat and thinning of tidewater glaciers is governed by processes that can be substantially decoupled from climate (e.g., Post et al, 2011)
Ice thickness did not change significantly between the years 2000 (277 ± 32 m) and 2014 (294 ± 66 m), surface slopes near the terminus decreased by ∼70% (Figure 4)
C is positioned ∼8 km upstream of the March 2014 terminus and 2 km upstream of the icefall, the seasonality of velocities at C is similar to that reported near the termini of other Alaskan tidewater glaciers (e.g., Mcnabb et al, 2015; Stearns et al, 2015) — velocities are highest in late spring/early summer, at a minimum in the late fall, and intermediate during the winter
Summary
The rapid retreat and thinning of tidewater glaciers is governed by processes that can be substantially decoupled from climate (e.g., Post et al, 2011). The contributions to sea level rise from tidewater glaciers are highly variable and contribute to large uncertainties in sea level rise projections (Pachauri et al, 2014). Tidewater glaciers lose mass through a combination of surface ablation and frontal ablation (itself the sum of losses from submarine melt and iceberg calving). Mass loss is enhanced during tidewater glacier retreat due to dynamic thinning, in which accelerated flow leads to thinning of upstream ice followed by further acceleration (Meier and Post, 1987; Pfeffer, 2007). Dynamic thinning ends once a tidewater glacier terminus restabilizes, greatly reducing mass loss through frontal and surface ablation. While the prevalence and urgency of tidewater glacier retreat has resulted in comparatively well-studied retreat processes, the processes by which tidewater glaciers transition from retreat into a stable or advance phase are poorly understood (Post et al, 2011) despite their importance for developing longterm projections of sea level rise
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