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The Role of Thermohaline Circulation in Global Climate Change

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The Role of Thermohaline Circulation in Global Climate Change The Role of Thermohaline Circulation in Global Climate Change • by Arnold Gordon ©&prinlfrom lAmonl-Doherl] Ceologicoi Observatory 1990 & /99/ &PorI Lamont-Doherty Geological Observatory of Columbia University Palisades, NY 10964 (914)359-2900 The Role of • The world ocean consists of 1.3 billion cu km of salty water, and covers 70.8% of the Earth's suiface. This enormous body of Thermohaline water exerts a poweiful influence on Earth's climate; indeed, it is an integral part of the global climate system. Therefore, under­ Circulation in standing the climate system requires a knowledge of how the ocean and the atmosphere exchange heat, water and greenhouse gases. If Global Climate we are to be able to gain a capability for predicting our changing climate we must learn, for example, how pools of warm salty Change water move about the ocean, what governs the growth and decay of sea ice, and how rapidly the deep ocean's interior responds to the changes in the atmosphere. The ocean plays a considerable features for the most part only role in the rate of greenhouse move heat and water on horizon­ warming. It does this in two tal planes. It is the slower ther­ ways: it absorbs excess green­ mohaline circulation, driven by house gases from the atmos­ buoyancy forcing at the sea sur­ phere, such as carbon dioxide, face (i.e., exchanges of heat and methane and chlorofluoro­ fresh water between ocean and methane, and it also absorbs atmosphere change the density • some of the greenhouse-induced or buoyancy of the surface water; by Arnold L. Gordon heat from the atmosphere. Both cooling andlor increased salt con­ these processes tend to forestall centration induced by excessive the greenhouse effect, and it is evaporation, form dense water possible that without them the which sinks into the ocean's inte­ global average temperature of the rior), that on the one hand forces atmosphere would now be )'_2' C the ocean's deep interior to inter­ warmer. If this is the case, the act with the atmosphere, and on question is whether the ocean the other can effectively seques­ will continue to retard such ter heat and other properties into warming-and whether the rate the enormous volume of the of this influence will be reduced deep ocean. or accelerated as the process con­ This sinking of dense surface tinues. These very important water occurs in a few restricted questions must be answered be­ regions, and so initiates the fore we can predict the full ex­ thermohaline circulation. The tent of global climate change deepest convection occurs in the with real confidence. For such northern North Atlantic and confidence we need a much around Antarctica. In, the North better understanding of ocean Atlantic, thermocline water (the circulation. upper km of the ocean separating the warm surface layer from the The Thermohaline colder deep water) with a long Circulation history of contact with the at­ While surface circulation cer­ mosphere is cooled and sinks as tainly plays a key role in the relatively salty water into the climate system, its wind-driven deep ocean. In the Southern 44 Thennohaline Circulation & Climate Chaage Fig. 1 POTENTIAL TEMPERATURE ~w ;:~, ~~,~-~-,C_'~\7(:7:-~~-~C7~!~~-::-~:~~=i?_~~ ={t?~ ~L~~~~> ---:i \ 0', 'co C~\ (>,,,~~:~7~~ :-~~~- £/~~,~~ ~~_L : _'_ /,~:~~:_Z/J L?:~-- ~ooo -- --- -2 ,= - _oj \ I ---~ ~ 4000 o '000 6000 -i6;':N - W-;·~,,~, -'-C·--',~=---c:~--;c~=~~=---'C"'-h~60' 70·S 70'S 6~0;";;~~~';;"-,,,~;.: ~~Y':'~O. 20" 30;': -40'- 5O-F ATLANTIC PACIFIC SALINITY ~~~~~:~~~~?-~-:6:_~'~_~~~~:~~ ~OOO ---------- '000 . ,ft\;1lt :: 10- 2.0° 30· 40° J~~J ~~~~ 0' 50' 60°. ,'"70'5 70"5 60'... 50" 40' 30· 20' .... O' 10· 20· 30° 40· 50' N iO--- ATLANTIC PACIFIC ~ 10Cl0- ~ 2000 ~ 3000 I 5 4000 o '000 70°5 60' 50" 40" 30· 20' 00' eo' 30~-- 40' SO'N6000 PACIFIC SILICATE ~O~----~ ~~~c1 ... L'~~~~~~~~~{f1U,ooo 70"N 60· 50' 40· 30° 20' 10· O· 10· 20· 30° 40' 50° 60' 70'S 70·S 60" 50" 40· 30' 20' 10' O' 10· 20' 30· 40' SO·N ATLANTIC PACIFIC Fig, 1: Meridional sections of cumantarctic belt. In the Atlantic lantic; it is lower in salinity, lower temperature. salinity, oxygen and Ocean it spreads well into the North­ in oxygen, and higher in silicate-­ silicate across the Atlantic and Pacif­ ern Hemisphere, and traces can be these latter two characteristics are the ic oceans. The vertical bar at the found as far north as 50° N. In the results of long isolation from the at­ center is Antarctica. These sections North Pacific a salinity minimum mosphere. This North Pacific Deep are constructed from Ceosec data layer at similar depths is derived Water (NPDW). fonned by the slow (1972 in the Atlantic, 1976 in the from the northwestern comer of the process of vertical mixing. spreads to Pacijil). ocean, from the Sea of Okhotsk. the south near 3000 m depth, and Temperatures generally decrease The northern North Atlantic lacks via the Antarctic Circumpolar with increasing depth. This trend is a subsurface salinity minimum; it Current. into the Atlantic Ocean. strongest within the upper km of the is replaced by high salinity water Along the seafloor is the very cold. ocean, in a feature called the ther­ with high oxygen and low silicate high silicate Antarctic Bottom Water mocline. The J00 C isothenn may be concentrations, sinking from the (AABW), derived from the margins taken as the base of the thennocline. sea surface to spread southward of Antarctica, cooled sharply by the Antarctic I ntennediate Water near ;3000 m depth. This is the cold atmosphere. This dense water ( AAIW) is the low salinity layer North Atlantic Deep Water slips down the continental slope to near JOOO m depth in the Southern (NADW). the seafloor, under the Antantic Cir­ Hemisphere. This feature emanates The deep water of the North Pacif­ cumpolar Current (ACC), and into from the surface waters of the cir- ic is opposite to that of the North At- the world ocean, Thermohaline Circulation & Climate Change 45 Hemisphere upwelling deep attribute of being relatively warm Ian tic would he as much as 6° C water, long removed from direct and salty compared to the aver­ cooler. It is the warm water contact with the atmosphere, is age deep waters of the world drawn into this region by quickly converted to cold denser ocean. All are drawn from rough­ NADW formation, not the Gulf water, and re-enters the deep ly the upper kilometer of the Stream alone, that supplies the and bottom layers of the ocean. ocean, including the thermo­ heat and moisture to the at­ These water masses (each haline and intermediate strata. mosphere that moderates the with their own characteristic The largest component of climates of northern Europe. properties) spread throughout NADW, with a formation rate The cooled surface water, still the ocean and force a slow but of 13 million eu mlsec, is de­ relatively salty, sinks to continue steady upwelling of the "resi­ rived from the Greenland and the NADW formation process. dent" deep ocean water. This Norwegian seas. As relatively As NADW spreads across the resident deep water is composed warm-salty water flows with the South Atlantic, it reaches the of older (in terms of time since Norwegian Current into the Antarctic Circumpolar Current exposure to the atmosphere) Greenland and Norwegian seas, (ACC), a strong clockwise cur­ water that has been modified by it cools, becomes denser, and rent that encircles Antarctica. vertical mixing processes, by sinks into the deep basin north The ACC is the primary pathway organic material descending of a submarine ridge that spans where the three major oceans can from the sea surface, and by the distance from Greenland exchange large amounts of water, contact with the seafloor sedi­ to Scotland. This water slips making the thermohaline circula­ ments. Eventually, the upwelled through passages across the ridge tion cells into a global system. water migrates back to the initial and cascades into the deep ocean sinking regions, to complete a to the south. Smaller contribu­ Compensating thermohaline circulation cell tions are made in the Labrador often referred to (for both the Sea, and a particularly warm Return Flow Atlantic-driven cell and for the salty constituent is derived from The export of NADW from the Southern-Ocean-induced cell) as the Mediterranean Sea outflow. Atlantic Ocean via the circum­ a "conveyor belt," Though small in volume flux, polar belt of the Southern Ocean the Mediterranean water infuses requires a compensating import great amounts of salt into the of upper-layer water into the North Atlantic NADW, branding it with its Atlantic. There are two possible Deep Water telltale salinity maximum. sources for this: Pacific inflow Thus one mass, called the North As su rface water sinks to via the Drake Passage of cool Atlantic Deep Water (NADW), form NADW and is exported to low-salinity Antarctic Inter­ forms in the northern North At­ the South Atlantic and to the mediate Water, or AAIW), and a lantic, and flows southward from other oceans, a compensating warm salty inflow from the Indi­ there into the southern Atlantic, amount of thermocline and in­ an Ocean's thermocline, which and eventually spreads via the termediate water masses from makes its way to the Atlantic circumantarctic deep ocean belt the world ocean is drawn towards around the southern rim of Africa. into the Indian and Pacific the North Atlantic. As these wa­ Most of the return flow is in Oceans. Total NADW produc­ ters are of higher temperature the form of AAIW via the Drake tion is estimated as 15 to 20 and (initially) on average lower Passage.
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