Like the atmosphere, the ocean is constantly
in motion. Ocean currents are defined by the movement
paths of water in regular courses, driven by the wind
and thermohaline forces across the ocean basins. Shallow
currents are driven primarily by the wind but are systematically
deflected by the Coriolis force to the right of the atmospheric
wind directions in the Northern Hemisphere, and to
the left of the prevailing winds in the Southern Hemisphere.
Therefore, shallow water currents tend to be oriented about
45° from the predominant wind directions.
Deepwater currents, however, are driven primarily by
thermohaline effects, that is the movement of water driven
by differences in temperature and salinity. The Atlantic and
Pacific Ocean basins both show a general clockwise rotation
in the Northern Hemisphere, and a counterclockwise spin in
the Southern Hemisphere, with the strongest currents in the
midlatitude sectors. The pattern in the Indian Ocean is
broadly similar but seasonally different and more complex
because of the effects of the monsoon. Antarctica is bound
on all sides by deep water and has a major clockwise current
surrounding it known as the Antarctic Circumpolar Current,
lying between 40° and 60° south. This is a strong current,
moving at 1.6–5 feet per second (0.5–1.5 m/s), and has a
couple of major gyres in it at the Ross Ice Shelf and near the
Antarctic Peninsula. The Arctic Ocean has a complex pattern,
because it is sometimes ice covered and is nearly completely
surrounded by land with only one major entry and
escape route east of Greenland, called Fram Strait. Circulation
patterns in the Arctic Ocean are dominated by a slow,
0.4–1.6-inch per second (1–4 cm/s) transpolar drift from
Siberia to the Fram Strait, and by a thermohaline-induced
anticyclonic spin known as the Beaufort Gyre that causes ice
to pile up on the Greenland and Canadian coasts. Together
the two effects in the Arctic Ocean bring numerous icebergs
into North Atlantic shipping lanes and send much of the
cold deep water around Greenland into the North Atlantic
ocean basin.
See also EKMAN SPIRALS; GEOSTROPHIC CURRENTS; THERMOHALINE
CIRCULATION.
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