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. However, recent obser million cu m/sec. By way of salinity than NADW, the result vations indicate a surprising comparison, the Amazon River ant thermohaline cell, as viewed phenomenon - the Drake Pas outflow is only 0.18 million cu in the meridional vertical plane, sage AAIW water, rather than m/sec. The NADW production strongly influences the Atlantic's flowing directly to the north rate would replace all of the meridional heat and salinity along the western margins of the deep water of the global ocean in fluxes. It is estimated that if Atlantic, may cross the South about 2000 years. NADW formation were to cease, Atlantic, entering the southwest NADW has a variety of com the compensating flow of warm Indian Ocean, where its mixes ponents; each differs in its water into the North Atlantic with saltier water before flowing thermohaline characteristics, would diminish, and the surface hack to the southeast Atlantic though they all share a common water of the northern North At- Ocean. This injection of AAIW
46 Thermohaline Circulation & Climate Change into the Atlantic, spiced with ex NADW formation may be influ Pacific drainage basins at a rate tra salt from the Indian Ocean, enced by the variability in the of about 0.3 million cu mlsec, may be the chief means of bal Indian Ocean factor, a concept mostly across the Isthmus of ancing the Atlantic export of that is being researched. Panama. This atmospheric flux NADW and of transferring Indi Tracking of the Indian and supports a salty Atlantic and sus an Ocean salt into the Atlantic. Pacific components from the tains NADW formation while The salinity enhancement of the South Atlantic to the northern prohibiting deep reaching con AAIW may boost North Atlantic North Atlantic cannot be done vection in the North Pacific. upper-layer salinity by 0.2 parts by simple water mass indicators. However, NADW production per thousand, enough to encour Mixing of thermocline and inter draws warm water to high north age more NADW formation. mediate water, particularly in the ern latitudes where it enhances Thermocline water from the tropical regions, and the altera evaporation. In other words, the Indian Ocean also enters the tions by ocean-atmosphere inter process of NADW formation Atlantic. Most of this water action, obscures these traits. strongly influences the fresh seems to return to the Indian Tracking may be possible by water balance of the Atlantic, Ocean within the upper layer of understanding the sequence of providing excess water vapor for the ocean (Fig. 3), rather than regional oceanographic condi the atmosphere to carry into flow into the North Atlantic to tions encountered en route. the Pacific. As the atmosphere feed NADW formation. How Eventually, with the help of a ever, due to mixing processes it global-scale numerical model Fig. 2: Potential temperature vs sa may leave behind in the Atlantic that faithfully portrays the ther linity relationship of the three major some of its excess salt, which mohaline circulation, it might water masses, or "end-members, " eventually spreads to the north, be possible to trace the full that compete for dominance of the also acting to boost Atlantic salin structure and vigor of the deep ocean interior. The North ity. Recent work indicates varia thermohaline circulation. Atlantic Deep Water (NADW) is de bility in the Indian Ocean inflow rived from three sources: there is the - an energetic influx of Indian Stopping the NADW war.mlsalty outflow from the Medi Ocean water into the Atlantic terranean Sea, the cold lower salin occurred during the 1980s. This Conveyor Belt ity for.med within the Labrador Sea, suggests that the Atlantic's The atmosphere transfers water with the bulk of the North Atlantic salinity and susceptibility to vapor from the Atlantic to the Deep Water derived from the over flow of dense water across the ridge Fig. 2 12° C from Greenland to Scotland. The 4 36,6 0'0 overflow from east of Iceland is LABRADOR SEA /' somewhat war.mer and saltier than MEDITERRANEAN the overflow through the Denmark OVERFLOW o Straits west of Iceland. o 3 • The North Pacific Deep Water w (NPDW) is about the same tempera c: '
Thermohaline Circulation & Climate Change 47 process may be a response to However, the presence of thc Fig. 3: Mop view of the two major NADW it is difficult to sort out "right" (warm and salty) surface thermohaline circulation cells or cause and effect. Might the water makes the NADW forma "conveyor belts," driven by the ex ocean circulation in the South tion difficult to stop; it is in change of heat and water between Atlantic with its links to the effect a positive feedback. But ocean and atmosphere. A relatively Pacific and Indian Oceans one possible mechanism that worm and saltier cell is induced by "condition" the Atlantic for might stop NADW formation formation of North Atlantic Deep NADW formation and perhaps would be the capping of the sur water (NADWj at a number of sites initiate variability of NADW face ocean with low-salinity wa within the North Atlantic. As production? Might the distri ter. A large source of such water N AD W spreads into the rest of the bution of low salinity surface is the Arctic Ocean. River ocean, slow upwelling and return water ftom the Arctic govern runoff, excess precipitations, and flow to the North Atlantic completes NADW production rates a flow of low salinity surface the circulation cel/. and recipes? water from the Pacific (via the Formation of Antarctic Bottom For example, the Atlantic Bering Strait), maintains a stable water (AABWj at various sites with meridional heat and freshwater cap of very low salinity water. in the Southern Ocean drives a cold fluxes are very sensitive to the The intense stratification re er and fresher thermohaline cel/. As ratio of these two routes. The duces heat flux from the warmer AABW spreadr along the seafloor to Atlantic would become saltier if deep water of the Arctic into the the north, it displaces resident bot the Agulhas route is favored, surface layer, thus allowing a per tom water, which returns to the fresher if the Drake Passage in sistent lid of sea ice to reside in llABW formation sites. A combina put is favored. The more salt the Arctic Ocean. tion of N ADW and AABW flows that can be obtained from the Yet during the last century into the North Pacific, where a rela evaporative Indian Ocean the there have been at least two tively warm but low salinity water less is the burden on the at episodes of low-salinity water mass, North Pacific Deep Water mosphere over the Atlantic to outbreaks (referred to as the (NPDWj forms and spreadr south remove the water vapor as Great Salinity Anomaly). These ward to dose both the N ADW and required to keep the northern large pools, also associated with the AABW thermohaline ceiLr. Atlantic salty - and thus the anomalous expansion of sea-ice thermohaline conveyor belt in cover in the regions of the Ice motion. land and Labrador seas, migrated
~ NADW (North Atlantic Deep Water) ~ AAIW (Antarctic Intermediate Water) -..;...~ AABW (Antarctic Botlom Water) NPDW (North Pacific Deep Water) ~THERM~ Antarctic Circumpolar Current
48 Thermohaline Circulation & Climate Challge about the Northern Atlantic, and Pacific, too low in salinity to be uppcr-layer characteristics. As reduced NADW formation rates. a simple mix of NADW and Ant such, it does not "ventilate" the Thus the NADW recipe may be arctic Bottom Water (AABW). deep ocean in terms of atmos quite variable as the production (AABW forms at the Antarctic pheric gases, but rather it alters rate of each component changes, continental margin as the very the heat and freshwater storage at a variety of time scales, and cold Antarctic air mass spreads of the deep ocean; it also allows for a variety of reasons. out over the adjacent ocean. accumulation of carbon, and so The stability of the forma The OCean surface freezes, re has the potential to influence tion rates of the varied forms of sulting in a cold dense surface global climate. NADW is one of the major con water that sinks to the seafloor, While most of the deep cerns in evaluating future climate and slips below ther ACC into water entering the North Pacific change. Changes in the 20th the world ocean, well into the eventually exits as NPDW, some century related to the Great Northern hemisphere.) With its deep water of the Pacific must Salinity Anomaly are small com low oxygen and high nutrient reach the surface layer. This pared to the suspected changes concentrations, it is the anti water, diluted by the excess pre in NADW formation rates during thesis of NADW. This North cipitation of the North Pacific, the swings between glacial to in Pacific Deep water (NPDW) enters the Arctic through the ter-glacial periods. During the spreads southward and even Bering Strait (at about 1 million disintegration of the ice sheets tually enters the Circumpolar cu m/sec), and the Indian Ocean from the last glacial epoch, a Deep Water (COW) and can through the Indonesian Seas series of events of sudden in clearly be observed spreading (at approximately 5 million cu jections of melt water into the well into the Atlantic Ocean. m/sec). These flows are an im North Atlantic induced low salin Presumably it leaves the Atlantic portant part of the interocean ity surface water and dramatic after mixing with NADW. water budgets, and changes in reduction of NADW formation. How does NPDW form? It their strength would influence These events indicate that the may be a consequence of vertical the global climate system, ocean can change from a mixing, which carries into the though the extent and even condition comparable to the deep ocean the properties of the direction of such change is not modern vigorous NADW for low salinity North Pacific ther known. mation state to a low NADW mocline and intermediate water While the impact on the production state characteristic of masses. Deep and bottom water, complex global climate system of the last glacial maximum in just a blend of NADW and AABW a change in fresher water outflow 300 years or less. Might we ex (mostly the latter), flows into the from the Pacific is not known, a pect rapid changes in NADW North Pacific, mainly through speculation is offered by the fol during a Greenhouse-induced the Samoan Passage, where it lowing thought experiment. As global warming? If so, will slowly upwells and mixes with freshwater outflow from the NADW production go up or low salinity water brought down Pacific via the Bering Straits and down? Answers to these ques by a mixing process. Because Indonesian Seas decreases, the tions are crucial if we are to gain of the immensity of the North Arctic and Indian Ocean would a predictive ability of the global Pacific, just a small degree of become saltier. As these waters climate system. vertical mixing can produce great spread into the Atlantic Ocean, quantities of NPDW - reason it too will eventually become North Pacific Deep Water able estimates of the intensity of saltier and more susceptible to vertical mixing suggest that NADW formation. The Pacific It is often said that no deep wa NPDW formation can easily be would become fresher as it accu ter forms in the North Pacific. maintained at a level of 10 mil mulates fresh water. Lowering of This is true if we refer to deep lion cu m/sec. the Pacific's surface layer salinity water formation by convection, NPDW does not represent increases the stability of the wa like NADW. The North Pacific injection into the deep ocean of ter column and should lower surface water is too low in salini 10 million cu mlsec of surface vertical mixing, inducing less ty to permit deep convection. water, but rather the modification NPDW formation. End result: Nonetheless, a unique deep wa of existing deep water by slow the deep waters of the world ter mass is found in the North process of downward diffusion of ocean become enriched in
Thermohaline Circulation & Climate Change 49 NAOW, saltier and warmer. world ocean, it slowly upwells layer under the winter ice cover Of course, the climate is in an and is modified by mixing with induces melting even before the interlocked series of forces, so less dense water, including Spring atmospberic radiation changing the Pacific outflow NAOW and NPOW, to flow warms sufficiently to melt the would be associated with other back to the Southern Ocean as sea ice directly from above. changes that may induce other COw. Upwelling of relatively Ocean heat flux also limits sea responses. In general, we like to warm deep water (about 1° C) is ice thickness during the winter, think of our system as difficult to converted into the cold (near the to less than a meter) in contrast change - that somehow the net freezing point of sea water- to the three-meter ice of the work of negative feedbacks is 1.85° C) Southern Ocean surface more stable Arctic Ocean. difficult to overcome. However, water, where it resides for only As the exchange of mixed if a system is kicked hard one to two years, before even layer water and deep water with enough, it might suddenly shift tually contributing to the associated vertical heat and salin into another stable system. formation of AABW. ity flux is responsible for the The sea ice cover of the spring melt and limited winter Southern Ocean acts to decouple sea ice thickness, we may con The Southern Ocean the ocean from the atmosphere; sider that the vigor of Southern Another thermohaline cell is ini the insulating blanket of sea ice Ocean ventilation potential is tiated along the polar edge of the protects the ocean from the cold directly related to sea ice sea Southern Ocean (i.e. the body of atmosphere, limiting cooling. sonality, e. g. year-round con water approximately south of 30° The extreme seasonality and the stant sea ice cover is indicative of S that connects the three major rapid spring melting of the a strongly stratified ocean with oceans). Antarctic Bottom Water Southern Ocean sea ice cover small vertical heat flux, whereas (AABW) forms at a vigorous rate suggest that the heat carried into a strongly seasonal ice cover is of 20 to 30 million cu m/sec; this the surface layer by rapid deep linked to substantial vertical is compensated by the upwelling water upwelling is the key to oceanic flux, which melts the ice of deep water known as Circum understanding the Southern cover because the Spring atmos polar Oeep Water (COW). As Ocean sea ice budget. Thus the phere heat budget cannot AABW spreads into the buildup of heat within the mixed remove the oceanic heat flux.
Fig. 4
SAMW Subantarctic Mode Water AAIW Antarctic Intermediate Water RSDW Red Sea Deep Water NADW North Atlantic Deep Water " Fig. 4: Map view oj the major AABW Antarctic Bottom Water ~ pathwaysfor links within NADW NPDW North Pacific Deep Water ~ L-____~~ ______a_n_d_j_1A_B_ll_1_'v_n_v_e_ye_r_b_e_k_s_ . _____A_C_c __ A_n_t_ar_c_tiC_C_ir_cu_m_po_l_a_rc_ur_re_n_t ___ 1 so Thermohaline CirCliiatioll & Climate Change Variations in vertical heat flux are NADW tends to make the deep from the NADW cell is most expected to yield interannual ocean saltier, the NPDW fresher. likely instrumental in maintain changes in ice cover extent and The chilling effect of the South ing the vigor of the AABW cell, seasonality, and in isolated re ern Ocean developed as Antarc which would otherwise become gions, ice-free conditions tica became tectonically isolated fresher, eventually unable to (polynyas) even in the dead of and grew a persistent glacial ice force convection to the seafloor. winter. sheet about 14 million years ago. In terms of global warming, There are indications that Before that time the deep oceans the capacity of water to absorb during the last glacial stage the were significantly warmer. heat is high relative to the sea ice around Antarctica was a What if all of the two major atmosphere; this means that as bit more extensive and did not convective deep water-mass the ocean warms less heat is avail exhibit the large amount of sea sources were turned off? The. able to warm the atmosphere. If sonality characteristic today. only communication the deep global warming causes the deep Following the arguments stated ocean would then have with the ocean to warm, say by a decrease above, this suggests that during atmosphere would be through of cold AABW production or an the glacial period the overturn the slow process of downward increase in the production rates ing of the ocean below the sea diffusion (as is presently the sit of NADW (particularly of warmer ice cover was less vigorous than uation for the North Pacific) and components-from the it is today. This implies reduced the rain of organic particles. The Mediterranean and Labrador production of AABW. Coupled ocean could either freshen or be Seas or NPDW - though the lat to the indications that NADW come saltier, depending on the ter, being driven by vertical mix production was also diminished dominant surface salinity. But ing processes, may be subject to during the glacial period, one since the warm thermocline less variability), this would fore concludes that deep ocean ven dominates the ocean's areal ex stall global warming. Of course, tilation was less efficient during tent, the net result would be a warming of the ocean increases the glacial periods. downward flux of heat, and the its volume due to thermal expan ocean would warm up. The lack sion. For example, a 1° C warm of ventilation of oxygenated wa ing of the 4000 m water column Conclusions ter by convection would lead to increases sea level by 0.6 m. The North Atlantic, North Pacif an ocean with lower deep-water Changes in the ratio of ic and Southern Ocean each oxygen and higher carbon stor importance of the three water forms, by its own distinctive pro age. The net effect of the lack of masses could also affect the car cess, a unique deep and bottom convective deep-water formation bon dioxide budget. Of the water mass. NADW forming at a may be to forestall global green estimated anthropogenic input of rate of 15 to 20 million cu mlsec house warming, though because carbon dioxide only 60% remains and AABW forming at a rate of of the difference in the time in the atmosphere, the rest is be 20 to 30 million cu mlsec, would scales these processes may not lieved to reside in the ocean, in replace the water of the deep neatly compensate each other. which case the ocean takes up ocean in somewhat less than Eventually the warming and excess carbon dioxide at a rate of 1000 years. These water masses density decrease of the deep rwo billion tonnes per year. The differ in terms of temperature, ocean would enable dense sur ocean carbon storage is about 50 salinity, and nutrients. The trio face water somewhere in the times that of the atmosphere. of water masses battle for domi world ocean to fe-activate con Small changes in the ocean car nance of the deep ocean; the vective ventilation. bon storage lead to large changes ratio of their importance changes One should not think of the in the carbon burden of the with time and with it so does the rwo primary thermohaline cells atmosphere. atmosphere, temperature and as separate entities. There is ex chemistry. The Southern Ocean change of water and ocean proper is a cold ocean, and tends to cool ties across the convoluted inter the deep and bottom waters of face between the cells. The vigor the modern world ocean, of this exchange may determine whereas NADW and NPDW the relative size of the cells. For tend to warm the deep ocean. example, the injection of salt
Thermohaline Circu/atioll & Climate Chanf{e .51