Edge of the Arctic Shelf
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by Bob Pickart

The Arctic Ocean is unique among the world’s oceans for many reasons. It is largely ice-covered (much of it year-round, the rest seasonally) and it has an abnormally high ratio of shelf to abyssal area. It receives about 10% of the global river runoff, but because of its small size (it contains 2% and 5% of the global ocean volume and area, respectively) and the low temperatures that prevail throughout the Arctic Ocean, salinity plays the dominant role in establishing density. Thus the small volume (area) to large runoff fraction is important to this ocean. Although the interior of the Arctic ocean is small, it contains deep basins which are separated by a series of ridges. The most pronounced is the Lomonosov Ridge, which separates the eastern Arctic (the Eurasian Basin) from the western Arctic (the Canada Basin).
aqua berg
Arctic Ocean circulation. Arrows indicate currents. Blue arrow is Pacific inflow, red arrow is warm Atlantic inflow, magenta arrow is cold Arctic outflow.
Click to enlarge.

The Arctic Ocean is largely isolated from the rest of the world ocean by land. The two major openings are Fram Strait, between Greenland and the Norwegian Island of Svalbard, and Bering Strait, between Russia and Alaska. (The former is the only deep passage into the Arctic.) Warm “Atlantic Water” (red arrow on map) flows through Fram Strait and the neighboring Barents Sea; this is water which originally came from the Gulf Stream. Once in the Arctic, this water is cooled as it travels cyclonically around the perimeter as a boundary current, finally exiting Fram Strait as a colder, fresher water mass (magenta arrow on map). This warm-to-cold conversion is a crucial component of the global ocean’s overturning circulation that helps maintain the earth’s climate.

As a result of the influx of Atlantic Water, the mid-depth layer of the Arctic ocean is filled with warm water. In fact, there is more than enough heat stored in this water to melt the polar ice cover from below. The reason this does not happen is because of a relatively thin layer of cold, salty water overlying the Atlantic Water. This layer is called the halocline, and it acts as a shield protecting the ice. What is the origin of the halocline? While there is no definitive answer, oceanographers believe that the water maintaining the halocline most likely originates from the shelf, somehow injected laterally into the deep basins. Part of the halocline water comes from the “Pacific Water” entering through Bering Strait (blue arrow on map). This water, which starts out fresh, is transformed by a process known as brine rejection. The cold arctic air freezes the water, thereby removing freshwater (contained in the ice), leaving behind heavier, saltier water which sinks and enters the halocline. The winds strongly affect this process. Winds also drive a system of currents throughout the Arctic.

This complex system in the Arctic---which impacts the entire food web---is in a delicate balance that will likely be upset by climate change. As such, the Arctic Ocean is the focus of intense study by oceanographers of all disciplines.

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