Ice Stream Variability and Links to Climate

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Ice Stream Variability and Links to Climate

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Title: Ice Stream Variability and Links to Climate
Author: Robel, Alexander Abram
Citation: Robel, Alexander Abram. 2015. Ice Stream Variability and Links to Climate. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Abstract: This dissertation explores the variability and climatic importance of ice streams, regions of fast flow in ice sheets. Observations indicate that ice stream variability plays an important role in the current mass balance of the West Antarctic Ice Sheet and may be related to periods of rapid climatic change in the past, such as Heinrich Events and glacial-interglacial transitions.

We first explore a thermal-regulation mechanism to explain centennial- to millennial-scale ice stream temporal variability based on a simple model which couples ice stream dynamics to subglacial meltwater production. High geothermal heat flux or warm ice surface temperature lead to steady streaming flow, while low geothermal heat flux or cold ice surface temperature lead to thermally-regulated oscillations in ice stream flow. There is a hysteretic transition between these two regimes associated with a subcritical Hopf bifurcation. This simple model can reproduce the time scale and amplitude of ice stream variability associated with Heinrich Events, as well as modern Siple Coast ice streams which appear to be in an oscillatory parameter regime near the transition to a steady-streaming mode.

To understand how this thermally-regulated ice stream variability is manifested at the grounding line, we use a purpose-built flowline model with lateral shear stresses and freely-varying bed properties. Unforced internal variability in ice streams causes rapid migrations in grounding line position with amplitude over 100 km at rates that can exceed 1 km/yr. An ice stream with net positive mass balance may still undergo retreat over a prograde slope due to thinning near the grounding line as part of unforced oscillatory behavior.

Ice streams exhibiting unforced internal variability are far from a steady-state and their behavior cannot be explained with conventional theories of grounding line stability. The grounding line of a stagnant ice stream may persist on a retrograde slope for hundreds to thousands of years before reversing direction on the same slope. This behavior indicates that identifying whether an ice stream is in an oscillatory regime is critical for evaluating whether a grounding line that is observed to retreat onto a retrograde slope is likely to undergo irreversible retreat.

Ice streams may have played a role in the rapid deglaciations which ended past glacial cycles. We use an idealized configuration of a 3D thermomechanical ice sheet model, which explicitly resolves ice streams, to simulate deglaciation in response to a change in climate motivated by Milankovitch forcing. We show that a large ice sheet, which is able to develop ice streams, is more sensitive to a change in climate forcing. This explains why ice sheets experience several precession and obliquity cycles before responding with a full deglaciation only when they reach a sufficiently large size. The rapid deglaciation of large ice sheets is primarily caused by accelerated calving at ice stream marine margins, which is, in turn, caused by enhanced driving stresses in ice stream onset zones. We therefore conclude that accurately resolving ice streams is critical for simulating deglaciations.
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