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Deep Ocean Science
Source Definition For Deep Ocean Water in
Hawaii (Cont. 2) ...
Deep Ocean Water Circulation
The long term (centuries and millennia) and large scale (Global) circulation of the world ocean deep layer is driven by gravitational forces and is named thermohaline circulation. When sea ice forms in the Arctic Ocean and around Antarctica in the Southern Ocean, the water just below the ice is both colder and more saline and hence denser. This denser water sinks and moves toward the equator. This sinking cold water is the “source” of thermohaline circulation. The coldest water (as low as -2?c) tends to form around Antarctica and move Northward to form the deepest layer of all oceans. In the Pacific this Antarctic water can be found as far North as Alaska which it reached by traveling Northward in the Eastern Pacific because of the barrier formed in the Mid to Western Pacific by the Hawaiian Ridge.
The intermediate and upper portions of the Deep Ocean Water layer originate from both the Arctic Ocean and the Southern Ocean around Antarctica. The Arctic water moves Southward along the axis of the Atlantic Ocean – then Eastward around Africa and into the Indian and Pacific Oceans. This water them joins with the sinking water originating from the Southern Ocean around Antarctica and moves Northward throughout both the Indian and Pacific Oceans.
The “sink” for the thermohaline circulation is net evaporation in the tropical zone of all the oceans. This net evaporation rate is an average of about 1 cm per day. The loss of this water from the tropical ocean surface is made up by an upward flow from the Deep Ocean Water layer. Since Hawaii is in the Northern portion of the tropical zone the net flow direction of thermohaline circulation here is from North to South. The speed of thermohaline circulation in Hawaii is too slow for direct measurement.
The intermediate term (months to years) and intermediate scale (hundreds to thousands of km) circulation of the deep ocean is affected by a number of principally climate and season related phenomena. During regular trade wind conditions as well as during La Nin/a events the net flow direction of the Deep Ocean Water layer around Hawaii is from West to East (to counter the net flow of the surface layer around Hawaii in the opposite direction). During intense El Nin/o events the deep layer net flow direction would tend to be from East to West.
Short term (hours and days) and small scale (tens of km) motions of the deep layer around Hawaii are primarily tide related. Measurements off Kona and off Kahe on Oahu show that different tidal components dominate the current structure at different depths. For example, the principal factor in the current ellipse at a depth of 350m off Kahe is the S2 tidal component. In any case, these tide related currents within a few kilometers of shore tend to be parallel to bathymetric contours and tend to trace out elliptical paths.
A passing hurricane is another event that can
affect the short term and small scale transport and water quality conditions
of the deep ocean layer around Hawaii. The principal mechanism in such a case is
the triggering of a turbidity current along a susceptible coastal area. The
mound of water in the low pressure area of the center of the hurricane
can cause a sudden downward flow in the deep layer which in turn can
cause the turbidity current. Such a condition was directly observed
off Kahe in 1982 during Hurricane Iwa. However, because the sediments
found on the steep slopes susceptible to turbidity currents are relatively
coarse, they do not get suspended high into the water column and a separation
of 100m from the bottom is generally sufficient to avoid water quality
problems.
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