Distinct Outlet-Glacier Responses to Surface-Mass-Balance and Ocean Variability

Marine-terminating outlet glaciers exist in settings with vigorous natural climate variability. The response to variability is thus a fundamental part of outlet glacier dynamics, and is also important context for understanding responses to anthropogenic forcing. Here, we conduct idealized model experiments to focus on the general dynamics governing outlet-glacier responses to different sources of climate variability.First, we find that outlet glaciers have a fundamentally different transient response to surface-mass-balance forcing applied over the interior than to oceanic forcing applied at the grounding line. We use a recently developed reduced model which represents outlet glacier dynamics on two distinct timescales: a multi-decadal response of ice near the grounding zone, and a millennial response of interior ice. The reduced model emulates the behavior of a more complex numerical model of ice flow. Together, these models demonstrate that ocean forcing first engages a fast, local response, followed by the slow adjustment of interior ice, whereas the response to surface-mass-balance forcing is dominated by the slow interior adjustment. The statistics of natural terminus variability thus depend on the source of climate variability, because of geometric differences in the glacier responses.

The timescales of natural variability also affect natural glacier fluctuations, and we focus here on decadal ocean variability. We show that decadal variability drives much larger glacier fluctuations compared to interannual variability, because of its increased effect on slow interior ice dynamics. Synthetic case studies illustrate how this affects thresholds for detecting a glacier's response to anthropogenic forcing. In particular, decadal ocean variability makes it more difficult to detect the forced response on decade-to-century timeframes. We conclude that the combination of fast and slow glacier dynamics, and the full spectrum of ocean variability, should be considered when interpreting recent changes in outlet glacier termini. Finally, we discuss ongoing work to incorporate these results into the context of unstable bed geometries.