GPS-derived estimates of surface mass balance and ocean-induced basal melt for Pine Island Glacier ice shelf, Antarctica

David E Shean,Mitchell Bushuk,Knut Christianson,C Max Stevens,Kristine M Larson,David M Holland,Ian R Joughin,Stefan R M Ligtenberg,Ben E Smith

doi:10.5194/tc-11-2655-2017

Abstract

Abstract. In the last 2 decades, Pine Island Glacier (PIG) experienced marked speedup, thinning, and grounding-line retreat, likely due to marine ice-sheet instability and ice-shelf basal melt. To better understand these processes, we combined 2008–2010 and 2012–2014 GPS records with dynamic firn model output to constrain local surface and basal mass balance for PIG. We used GPS interferometric reflectometry to precisely measure absolute surface elevation (zsurf) and Lagrangian surface elevation change (Dzsurf∕ Dt). Observed surface elevation relative to a firn layer tracer for the initial surface (zsurf − zsurf0′) is consistent with model estimates of surface mass balance (SMB, primarily snow accumulation). A relatively abrupt ∼ 0.2–0.3 m surface elevation decrease, likely due to surface melt and increased compaction rates, is observed during a period of warm atmospheric temperatures from December 2012 to January 2013. Observed Dzsurf∕ Dt trends (−1 to −4 m yr−1) for the PIG shelf sites are all highly linear. Corresponding basal melt rate estimates range from ∼ 10 to 40 m yr−1, in good agreement with those derived from ice-bottom acoustic ranging, phase-sensitive ice-penetrating radar, and high-resolution stereo digital elevation model (DEM) records. The GPS and DEM records document higher melt rates within and near features associated with longitudinal extension (i.e., transverse surface depressions, rifts). Basal melt rates for the 2012–2014 period show limited temporal variability despite large changes in ocean temperature recorded by moorings in Pine Island Bay. Our results demonstrate the value of long-term GPS records for ice-shelf mass balance studies, with implications for the sensitivity of ice–ocean interaction at PIG.

Highlights

The widespread availability of precise Global Positioning System (GPS) measurements has revolutionized the study of ice dynamics and glacier mass balance (e.g., Gao and Liu, 2001)
Our results demonstrate the value of long-term GPS records for ice-shelf mass balance studies, with implications for the sensitivity of ice–ocean interaction at Pine Island Glacier (PIG)
For the observed ice thickness, magnitude, and length scale of surface variations, as well as the relatively long timescales involved, we argue that the hydrostatic assumption is reasonable, and any vertical elevation change due to evolving bridging stresses should be negligible compared to the magnitude of observed Dzsurf/Dt and our conservative error estimates

Summary

Introduction

The widespread availability of precise Global Positioning System (GPS) measurements has revolutionized the study of ice dynamics and glacier mass balance (e.g., Gao and Liu, 2001). Operating dual-frequency GPS receivers provide high-frequency (1 Hz or less), highly accurate (< 1– 3 cm) measurements of position, which can be used to derive surface velocity and elevation change. For applications involving ice dynamics, these measurements offer important constraints for the mass continuity equation, which equates surface elevation change with ice flux divergence, surface mass balance (SMB), and basal mass balance (BMB). We explore a methodology to constrain each of these components directly from GPS observables. SMB processes include precipitation, sublimation, wind redistribution of surface snow, and meltwater runoff. Regional climate models forced by reanalysis output pro-

Results

Discussion

Conclusion