Abstract
The ice flux divergence of a glacier is an important quantity to examine because it determines the rate of temporal change of its thickness. Here, we combine high‐resolution ice surface velocity observations of Nioghalvfjerdsfjorden (79north) Glacier, a major outlet glacier in north Greenland, with a dense grid of ice thickness data collected with an airborne radar sounder in 1998, to examine its ice flux divergence. We detect large variations, up to 100 m/yr, in flux divergence on grounded ice that are incompatible with what we know of the glacier surface mass balance, basal mass balance and thinning rate. We examine the hypothesis that these anomalies are due to the three‐dimensional flow of ice around and atop bumps and hollows in basal topography by comparing the flux divergence of three‐dimensional numerical models with its surface equivalent. We find that three‐dimensional effects have only a small contribution to the observed anomalies. On the other hand, if we degrade the spatial resolution of the data to 10 km the anomalies disappear. Further analysis shows that the source of the anomalies is not the ice velocity data but the interpolation of multiple tracks of ice thickness data onto a regular grid using a scheme (here block kriging) that does not conserve mass or ice flux. This problem is not unique to 79north Glacier but is common to all conventional ice thickness surveys of glaciers and ice sheets; and fundamentally limits the application of ice thickness grids to high‐resolution numerical modeling of glacier flow.
Highlights
[1] The ice flux divergence of a glacier is an important quantity to examine because it determines the rate of temporal change of its thickness
We examine the hypothesis that these anomalies are due to the three‐dimensional flow of ice around and atop bumps and hollows in basal topography by comparing the flux divergence of three‐dimensional numerical models with its surface equivalent
To understand and model these dynamic changes, advanced, high‐resolution numerical models are needed because standard model simplifications, such as the Shallow Ice Approximation (SIA), cannot explain these observations
Summary
[2] Significant changes in ice sheet mass balance have been observed in the past decades that are mainly caused by the rapid evolution of outlet glaciers at the periphery of ice sheets [Rignot and Kanagaratnam, 2006]. To understand and model these dynamic changes, advanced, high‐resolution numerical models are needed because standard model simplifications, such as the Shallow Ice Approximation (SIA), cannot explain these observations. These advanced models must operate at a higher spatial resolution compatible with the size and thickness (less than 1 km) of these glaciers to capture critical dynamical processes that drive their temporal evolution. These models must rely on data assimilation techniques to constrain unknown model parameters such as basal friction under grounded ice and ice viscosity of floating shelves [e.g., Joughin et al, 2009; Morlighem et al, 2010]. We make recommendations on the gridding of ice thickness maps for ice sheet modeling studies
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