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As pointed out in previous lectures, deep-water masses form by sinking (because of high density) from the surface and spreading laterally at depth. Moderately dense water masses also sink, but spread out at intermediate depths.
The densest water masses of the world ocean are formed at high
latitudes in the Atlantic because
(1) The Atlantic and adjacent seas extend to highest latitudes
and thus have
coldest surface temperatures.
(2) North Atlantic surface waters are relatively saline. Water
vapor that
evaporates in the North Atlantic is transported by atmospheric
circulation
(mostly the Trade Winds) to the Pacific. In addition, saline waters
that
flow out of the Mediterranean and Caribbean increase the salinity
of the
North Atlantic.
There are three major bottom- and deep-water masses that form
at high latitudes in the Atlantic:
1. Antarctic Bottom Water (AABW)
2. North Atlantic Deep Water (NADW)
3. Antarctic Circumpolar Water (ACW)
Antarctic Bottom Water (AABW) forms during seasonal cooling and sea-ice formation in the Weddell Sea (adjacent to Antarctica). AABW forms with a temperature of -0.5 deg. C and salinity of 34.8 g/kg. It flows northward along the sea floor in the Western Atlantic well into the Northern Hemisphere. (It is restricted from the east side of the Atlantic by the Mid-Atlantic Ridge system.)
North Atlantic Deep Water (NADW) is a mixture of cold surface currents flowing out of the Arctic Ocean with saline surface waters of the North Atlantic. It is the largest deep-water mass. During winter cooling, sea-ice formation, and evaporation NADW sinks south of Greenland with T = 2-4 deg. C and S = 34.9 g/kg. It flows south over AABW in the Western Atlantic.
Antarctic Circumpolar Water (ACW) forms as NADW upwells off Antarctica, cools further, and mixes with Antarctic waters, including AABW. It flows eastward (as the Antarctic Circumpolar Current) around Antarctica then northward into Indian Ocean and into Pacific Ocean. ACW forms the deep and bottom waters of those oceans.
Intermediate (shallower) water masses in the world ocean
Mediterranean Intermediate Water (MIW) occurs in the Atlantic at a depth of ~ 1 km. It is the outflow of saline (~35.5 ppt) but warm (~10°C) water from the Mediterranean Sea.
Antarctic Intermediate Water (AAIW) [in all oceans] and Arctic Intermediate Water (AIW) [in the Pacific] both form by the convergence of surface waters at 40 - 50 deg. lat. These intermediate waters are cold (~5°C) but not very saline (~34 g/kg). They sink and spread laterally at depths of about 1 km.
Deep circulation and water masses in the Pacific and Indian Oceans. The bottom and deep waters in both of these major basins is Antarctic Common Water. In the Pacific, intermediate waters (both AIW and AAIW) are formed by sinking of cold surface waters in sub-polar latitudes. In the Indian, there is obviously no intermediate waters from the Arctic! But AAIW is observed. In addition, there is an outflow of warm and saline waters from the Red Sea to form an intermediate water mass in the northern Indian Ocean.
Global Conveyor Belt. This is the term that a prominent oceanographer recently applied to the coupling of thermohaline (deep) and wind-driven (surface) circulation. It describes the global transport of sea water (and the heat and salt that sea water contains) by the overall circulation of the oceans.
The conveyor belt begins with the formation of North Atlantic Deep Water (NADW), the largest water mass in the ocean. This is where saline North Atlantic surface waters sink into the abyss to begin their thousand-year journey in the deep sea. NADW flows southward to the coastal seas of Antarctic, where it upwells, cools again, and mixes with Antarctic waters to form Antarctic Common Water (ACW). ACW flows eastward around Antarctic and northward to fill the Indian and Pacific basins.
Obviously, water that sinks into the abyss must eventually come back to the surface. This occurs as upwelling of deep water in Equatorial and high-latitude oceans due to slight warming by heat from Earth's interior escaping at the sea floor, especially in the Eastern Pacific. Water that upwells in the Pacific returns to the North Atlantic by surface currents. Equatorial currents in the Pacific carry water through the islands of Indonesia. Circulation in the Indian Ocean carry water around the southern tip of Africa. Once into the South Atlantic, surface currents move water to the Equator. Because of the bulge of South America , Equatorial currents in the Atlantic are largely deflected toward the north to form the Gulf Stream and the North Atlantic Drift. The journey is complete!
The Global Conveyor Belt has been operating pretty much as described above for the past 10,000 years. Vigorous thermohaline circulation has sucked warm surface waters into the high-latitude North Atlantic, thus warming northern Europe. But it hasn't always operated that way! Evidence from deep-sea sediments indicate that the conveyor actually shut-down 10-15,000 years ago. As a consequence, northern Europe was cold (and partly glaciated). What caused the conveyor to stop? The most likely explanation is that low-salinity (and hence low-density) waters covered the North Atlantic. This would have inhibited sinking and slowed (or stopped) the formation of NADW. Where did the low-salinity water come from? Probably the partial melting of glaciers that covered Canada at that time.
28-1. Identify the three major deep-water masses that form
in the Atlantic. Which is the densest, and therefore the deepest?
28-2. Where and how does Antarctic Bottom Water (AABW) form? Why
does it only flow northward along the western side of the Atlantic?
28-3. Where and how does North Atlantic Deep Water (NADW) form?
What happens to NADW as it upwells to the surface in the Southern
Ocean off Antarctica?
28-4. What is Antarctic Circumpolar Water (ACW)? How is it related
to the deep and bottom waters of the Indian and Pacific Oceans?
28-5. What is the origin of "Intermediate" water masses
(at depths of about 1 km)? Why are they less saline than underlying
deep-water masses?
28-6. What is the origin of Mediterranean Intermediate Water (MIW)
in the Atlantic? Why is it warmer but more saline than the underlying
NADW?
28-7. What is the origin of the deep and bottom waters in the
Indian and Pacific Oceans? (See question 27-9.)
28-8. Study Text Figs. 7.5 and 7.6, which show the salinity and
temperature profiles in the major ocean basins. You should be
able to relate the temperature and salinity profiles to thermohaline
circulation and to identify the principal bottom, deep, and intermediate
water masses from the profiles. For example, what is the origin
of the relatively saline water mass at about 1 km depth in the
North Atlantic?
28-9. What is the "global conveyor belt?" In general
terms, describe its formation and pathway.
28-10. What is the effect of the global conveyor belt on the climate
of northwestern Europe? What would happen if that circulation
shut down? Did the global conveyor belt ever shut down?
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