drift

jump to intro, climate, distrib, formation, types, drift, satellite, glossary, references SEA ICE DRIFT

Polar sea ice is under almost constant drift and deformation. Although it is possible for sea ice t o remain motionless, it is an exception to the rule unless it is very near shore [Hibler, 1989]. Winds, tides, and ocean currents keep ice in motion, causing it to break and its fragments to collide with one another. Ice drift creates gaps in the ice cover and therefore increases rates ice formation. Ice drift has important influence upon regional freshwater and heat budgets because ice transport redistributes buoyant freshwater and latent heat energy.

Ice drifts tend to meander and reversals in direction are common [Hibler, 1989]. This is because on short time scales of hous to days to weeks, sea ice drift is influenced by local winds that vary significantly in direction and magnitude over time. On longer time scales of weeks to months to years, sea ice motion is influenced primarily by surface currents in the ocean. Surface currents are influenced by large scale atmospheric phenomena (specifically, the persistence of high and low pressure air masses over regions of the high latitudes).

The redistribution of sea ice from its location of formation to other areas by ocean and wind currents causes freshwater to be discharged from the higher latitudes to lower ones. Ice formation processes effectively expel salt from seawater and reintroduce relatively fresh water to the ocean where sea ice melts. The drift of sea ice is therefore important in the heat and freshwater budgets of the regional climate.

ANTARCTIC REGION
The Antarctic ice pack is kept in perpetual motion by winds and ocean currents. Speeds of Antarctic ice averages 10-20 km/day and is divergent northward with divergences of up to 10% per day possible under extreme conditions [Antarctic CRC, 1996]. Leads and ridges commonly develop along the Antarctic coast.

ARCTIC OCEAN
The Arctic Ocean ice cover circulation can be characterized by two major large scale motions. There is a strong transpolar drift current which transports sea ice primarily from the Bering Strait and the ice forming shelves of the Laptev and East Siberian Seas (see the following figure) over the North Pole and down the East coast of Greenland. The other large-scale motion is the Beaufort Gyre. Much of the Arctic winter is characterized by a persistent high pressure center over the western Arctic region, stimulating clockwise rotation of surface winds and waters which cause large accumulations of drifting sea ice to float around the Beaufort Gyre for several years.

Sea ice doesn't form exclusively within the Arctic Ocean circulation for transport out into the North Atlantic. A small amount of ice is formed in the Bering Sea and transported into the Arctic Ocean through the Bering Strait. It enters at a mean speed that is smaller than the ocean currents beneath due to the variability of the winds [Pease and Turet, 1989].
The following figure displays selected drift data from ships and drift stations collected since and including the 1894-6 "voyage" of the Fram icebreaker, Fritjof Nansen's famous first drift across the Arctic which established the great depths of the Arctic Ocean as well as its importance in the global climate.

The drift data confirm the patterns and slow speeds of theTranspolar Drift Stream and the Beaufort Gyre. Speeds are very slow except for along the North Alaskan coast and especially through the constriction of Fram Strait. The following figure shows average drift vectors for the Arctic region during the colder months. Data was collected during an 8-year interval from drifting buoys of the Arctic Ocean Buoy Program (AOBP). Data for other times of the year are similar in direction but slightly smaller in magnitude. The influence of high pressure over the western Arctic and low pressure over the eastern Arctic is quite apparent in the drift data.

from [Gloersen, et al, 1992]

Most of the Arctic region is covered by thick perennial ice (leaving only ~3% open water), but the peripheral seas maintain 15 - 40% of open water [Gloersen, et al, 1992]. The thickest ice accumulations occur off of the Canadian Archipelago (refer to the following figure) where ice is forced southward off of the ice pack by winds. Substantial ridging caused by the interaction of those winds with strong winds through the complex channels of the Canadian Archipelago compresses drift ice to thicknesses in excess of 6 meters [Hibler, 1989] .

from [Hibler, 1989]

Tides also have important influences upon Arctic sea ice. The largest tide in the Arctic region is generated in the Atlantic Ocean and enters through Fram Strait. It maintains a general turbulence on the ice shelves and maintains stresses within the ice cover. Tidal current drifts are strongest through Fram Strait and along the continental slopes of the East Siberian and Laptev Seas [Kowalik and Proshutinsky, 1994]. As the undersurface of ice roughens in the winter winds, more momentum is transferred from the ocean to the ice [Kowalik and Proshutinsky, 1994].

SEA ICE DRIFT AND THE GLOBAL CONVEYOR BELT

Sea ice drift can have very important consequences for global oceanic circulation. The redistribution of heat and freshwater not only impact local and regional phenomena, but can also impact the rest of the global ocean by altering the strength of the convection of North Atlantic deep water formation. It is believed, for example, that an increase in freshwater discharge into the North Atlantic Ocean from the Arctic Ocean was responsible for the weakening of the global convection cell. It resulted in a weakening of the Gulf Stream and associated currents that transport heat to the European high latitudes, causing the little ice age known as the Younger Dryas [Aagard and Carmack, 1994].

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