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September 1, 2009

Tracking Ocean Currents

Do ocean currents inside Greenland’s fjords regulate the speed of mass loss from the ice sheet? What controls the onset and strength of low oxygen events in Puget Sound, WA? Though seemingly unrelated, questions like these underline the importance of understanding how coastal ocean processes can interact with estuaries prone to sensitive environmental issues, such as Greenland’s fjords or Puget Sound.

Over the last decade, observations in Greenland have shown the ice sheet to be losing an increasing amount of mass, with evidence pointing towards an ocean source as the cause. However, oceanographers’ understanding of how the fjord circulation works, which connects the outlet glaciers to the continental shelf, is extremely limited. Dr. Sutherland’sresearch is aimed at describing the processes that control the currents inside these fjords, such as Sermilik Fjord in southeast Greenland (left), by measuring water velocities, temperature and salinity (or “saltiness”) within the fjord. Dr. Sutherland and colleagues have found warm, Atlantic-origin waters penetrating the deep parts of Sermilik Fjord, which is close to 3000 feet deep (>1/2 mile!). The presence of this relatively warm water in close contact with the ice may play a role in driving glacial melt (read more).

In Puget Sound, a large fjord in Washington, coastal ocean processes also significantly impact the estuary. Puget Sound is home to millions of people in the greater Seattle area, and this population stress has been implicated in an increasing trend of hypoxic, or low-oxygen, events in some areas of the Sound. However, isolating the human effects on the Sound versus what is brought in from the coastal ocean naturally is difficult. Dr. Sutherland has developed a numerical computer model to simulate the ocean currents in Puget Sound (right) and to improve our understanding of the estuary functioning. With this tool, we aim to make predictions of what variables, such as tidal strength, winds, and/or river discharge, and what areas, such as Admiralty Inlet, affect Puget Sound the most. These predictions can help coastal managers set policy for fisheries and aquaculture, as well as aid in site identification for future tidal energy projects (read more).

March 7, 2008

What holds glaciers up?

If the glaciers and ice sheets that rest on top of Earth’s continents could slide away into the ocean at once, they would cause sea level to rise by several hundred feet. The terminations of ice ages have witnessed sea level changes of similar magnitudes in the past, but most current rates of flow are reassuringly glacial. Important exceptions occur on the margins of Greenland and Antarctica, where ice streams and outlet glaciers slide along at breakneck speeds of up to several miles per year past less dynamic ice ridges that stick to more normal velocities that are 100 times slower. This curious behavior is of more than simple academic interest. The most recent IPCC report emphasizes that projections for future sea level rise have no representation of ice stream behavior beyond the naive expectation that they will simply continue to behave in the same way as is seen today, no matter how the conditions that surround them change.

The ice streams sit on top of porous sediments that contain water at pressures that are almost high enough to support the entire glacier weight – almost, but thankfully not quite. A small, but important part of the load is supported instead by intermolecular forces that act between the ice and the sediment grains themselves so that friction can help to resist even more rapid sliding. In a recent paper published in the Journal of Geophysical Research – Earth Surface (Paper), Alan Rempel outlines a mathematical model that accounts for the microphysics of these ice–particle interactions.

The theory demonstrates how the same forces that cause needles of ice to grow and lift up bark mulch and small clumps of dirt at the Earth’s surface (see photo above and to the right) are instrumental in determining the fraction of ice stream weight that keeps them from flotation and firmly grounded on a layer of till instead. Further work is underway to examine how variations in the overall level of till support produce thick fringes of partially frozen till (see graph to the left) that might mark the boundaries between ice streaming regions and the more stagnant ridges alongside.

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