High Latitude Deep Water Sources During the Last Glacial Maximum and the Intensity of the Global Oceanic Circulation
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Last Glacial Maximum
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The connection between water vapor transport and the thermohaline circulation is examined with a simple global coupled ocean‐atmosphere energy‐salinity balance model (ESBM). Both latitudinal intrabasin and interbasin water vapor fluxes are considered. It is demonstrated that interbasin and intrabasin water vapor fluxes play interdependent and competitive roles in affecting the state of the thermohaline circulation. Increasing intrabasin water vapor flux in the North Atlantic, by decreasing water density in the high latitudes, decreases the North Atlantic deep water production and hence the thermohaline circulation, while increasing interbasin water vapor flux from the Atlantic to the Pacific, by increasing the mean density of the Atlantic and decreasing that of the Pacific, increases the strength of the global thermohaline circulation. The global thermohaline circulation and its asymmetry are sensitive to the latitudinal hydrological cycle in the North Atlantic because of the large water vapor flux from the Atlantic to Pacific Ocean. Global thermohaline circulation exhibits bimodal equilibria as a consequence of imbalances in rates of change of advective and eddy freshwater fluxes in the high‐latitude North Atlantic. One equilibrium mode resembles the modern ocean circulation with a strong global asymmetric thermohaline circulation associated with dominant deep water production in the North Atlantic and an effective “heat pump” operating in the Atlantic Ocean. In the other equilibrium, deep water is produced primarily in the Southern Ocean; in particular, North Atlantic deep water is replaced by Southern Ocean deep water. Deep water is produced in the North Pacific in this mode, but is, for reasonably large interbasin water vapor transport, insufficient to reverse the direction of deep water flow into the South Pacific. Based on estimated water vapor fluxes for the present climate, our study suggests that the present thermohaline circulation is dynamically stable, i.e., far from the critical regions of rapid transition between two modes.
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Circumpolar deep water
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Stratification (seeds)
Temperature salinity diagrams
Circumpolar deep water
Outflow
Entrainment (biomusicology)
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In the Indian Ocean sector of the “great ocean conveyor” scheme the North Atlantic Deep Water (NADW) replacement is realized by the transformation of the sinking Circumpolar Deep Water (CDW) from the Southern Ocean to a prevailingly upward flow in the northern Indian Ocean. This water‐mass transformation scenario has been studied by using hydrographie data including potential temperature, salinity, dissolved oxygen, and nutrients in a water‐mass mixing model and by calculation of dianeutral velocity. The model comprises a mixing system of three major deep water masses, NADW, CDW, and North Indian Deep Water. This third water mass is introduced as a virtual water mass to represent the aged CDW. The assumption is feasible as the artificial water mass can be eliminated on the deepest σ N = 28.12 neutral surface, characterized by isopycnal mixing. The experiment is fulfilled when a conservative variable, initial phosphate PO 4 0 , is introduced and all conservative parameters are used in the mixing scheme. The σ N = 28.12 surface provides a major isopycnal transition path of CDW to the northern Indian Ocean, and thus CDW is transformed to an upward flow. Dominant dianeutral up welling is detected on all four neutral surfaces north of 30°S, showing increasingly strong dianeutral velocity toward shallower surfaces. With the water‐mass mixing pattern and spreading path, dianeutral circulation, and dynamical information of the acceleration potential (10 m 2 s −2 ) mapped on neutral surfaces a schematic of deep water circulation and ventilation of the Indian Ocean emerges from this study. The contribution of NADW is largely limited to the southwest Indian Ocean. The northward CDW is found mainly in two paths: one in the western Indian Ocean (apparent between 2000 and 3000 m) through the western Crozet and Madagascar Basins, and farther north into the Somali Basin and Arabian Basin and another in the Central Indian Basin (apparent below 3000 m where ridge‐blocking becomes effective in the west and NADW salinity reaches a maximum). While the major path switches from west to the central basin in the deeper layer (3000–3500 m), the flow in the western Indian Ocean reverses southward. North of the Southeast Indian Ridge, the deep path bifurcates, from which point the eastern branch feeds the deep western boundary flow in the West Australian Basin. Except for part of the transformed CDW, which is contributed to the upward flow in the north, the return flow of CDW is found on the eastern side of these basins.
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Abstract The thermohaline circulation transports heat, salt and chemical elements through the deep ocean. At high latitudes, cooling and salting of surface water produce dense waters that sink in the deep ocean. North Atlantic Deep Water is formed in the Arctic Seas and Antarctic Bottom water is produced around Antarctica. It triggers the thermohaline circulation that flushes the deep ocean on a time scale of ∼900 y as indicated by the radioactive decay of 14C. Anthropogenic tracers mark the inflow of newly (< 60 y) formed deep waters. 14C-transient tracer comparison highlights water recirculation in the deep basins. The closure of the thermohaline circulation requires the upwelling of deep waters to the ocean surface. In the deep basins upwelling is enhanced by current flowing over rough topographies. The bottleneck of the thermocline is passed in the Southern Ocean where steep isopycnal surfaces connect the deep and the surface ocean.
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Abstract. The distribution of the main water masses in the Atlantic Ocean are investigated with the Optimal Multi-Parameter (OMP) method. The properties of the main water masses in the Atlantic Ocean are described in a companion article; here these definitions are used to map out the general distribution of those water masses. Six key properties, including conservative (potential temperature and salinity) and non-conservative (oxygen, silicate, phosphate and nitrate), are incorporated into the OMP analysis to determine the contribution of the water masses in the Atlantic Ocean based on the GLODAP v2 observational data. To facilitate the analysis the Atlantic Ocean is divided into four vertical layers based on potential density. Due to the high seasonal variability in the mixed layer, this layer is excluded from the analysis. Central waters are the main water masses in the upper/central layer, generally featuring high potential temperature and salinity and low nutrient concentrations and are easily distinguished from the intermediate water masses. In the intermediate layer, the Antarctic Intermediate Water (AAIW) from the south can be detected to ~30 °N, whereas the Subarctic Intermediate Water (SAIW), having similarly low salinity to the AAIW flows from the north. Mediterranean Overflow Water (MOW) flows from the Strait of Gibraltar as a high salinity water. NADW dominates the deep and overflow layer both in the North and South Atlantic. In the bottom layer, AABW is the only natural water mass with high silicate signature spreading from the Antarctic to the North Atlantic. Due to the change of water mass properties, in this work we renamed to North East Antarctic Bottom Water NEABW north of the equator. Similarly, the distributions of Labrador Sea Water (LSW), Iceland Scotland Overflow Water (ISOW), and Denmark Strait Overflow Water (DSOW) forms upper and lower portion of NADW, respectively roughly south of the Grand Banks between ~50 and 66 °N. In the far south the distributions of Circumpolar Deep Water (CDW) and Weddell Sea Bottom Water (WSBW) are of significance to understand the formation of the AABW.
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The global‐scale circulation has long been one of oceanography's most challenging and exciting research topics. A few features of the abyssal (near bottom) and deep circulation of the Atlantic Ocean have been known for over 50 years, and in the past decade or so there has been a developing focus on the world oceans' thermohaline circulation. The term thermohaline circulation as used here applies not only to a direct response to atmospheric buoyancy fluxes but also in the general sense of water mass modification or conversion, where mechanisms may be associated with internal mixing processes and even wind forcing (i.e., wind‐induced upwelling or wind‐driven mixing). The thermohaline circulation components reviewed and summarized in the following are associated with water mass conversion processes that are involved with interbasin exchange. Updated summary maps of the volume transports (in sverdrups; 1Sv = 10 6 m³ s −1 ) for the interbasin‐scale pathways of the abyssal and deep thermohaline circulation and associated upper level compensating flows are developed for two to four vertical layers or potential density intervals, based primarily on a synthesis of published observational results. The cell(s) involving the largest worldwide exchange transport‐wise (53 Sv) are associated with an interaction between various deep and bottom water components via Circumpolar Deep Water (CDW). The first major conversion step in the replacement path for the renewal (14 Sv) of North Atlantic Deep Water (NADW) is taken to be primarily to CDW. Bottom water in the Indian Ocean originates as lower CDW which recirculates while also moving equatorward in deep western boundary currents with eventual conversion to both deep and intermediate layer flows. Some of the intermediate water so formed in the Indian Ocean moves through the Agulhas Current system (ACS) and may “leak” into the Benguela Current regime (BCR), although probably primarily flowing through the ACS into the Subantarctic Frontal Zone (SFZ). It is modified throughout its transit in the SFZ south of the Indian Ocean, south of Australia, and across the South Pacific. Up to 10 Sv of the least dense brand of intermediate water flows through the northern sector of Drake Passage, becomes involved in a Malvinas Current‐Brazil Current‐Subtropical Gyre interaction, and then joins the BCR after perhaps also interacting with the ACS again. This compensating flow is warmed and becomes more saline in the South Atlantic and is later further modified and upwelled in the equatorial Atlantic, crossing the equator and moving through the Gulf Stream system to replace NADW. There is also an NADW replacement path of secondary importance westward around the tip of Africa (∼4 out of 14 Sv) associated with an interbasin circulation pattern throughout the southern hemisphere oceans involving an O (10 Sv) Indonesian Throughflow.
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Inverse methods are applied to historical hydrographic data to address two aspects of the general circulation of the Atlantic Ocean. The method allows conservation statements for mass and other properties, along with a variety of other constraints, to be combined in a dynamically consistent way to estimate the absolute velocity field and associated property transports. The method is first used to examine the exchange of mass and heat between the South Atlantic and the neighboring ocean basins. The Antarctic Circumpolar Current (ACC) carries a surplus of intermediate water into the South Atlantic through Drake Passage which is compensated by a surplus of deep and bottom water leaving the basin south of Africa. As a result, the ACC loses .25±.18x1015 W of heat in crossing the Atlantic. At 32°S the meridional flux of heat is .25±.19x1015 W equatorward, consistent in sign but smaller in magnitude than other recent estimates. This heat flux is carried primarily by a meridional overturning cell in which the export of 17 Sv of North Atlantic Deep Water (NADW) is balanced by an equatorward return flow equally split between the surface layers, and the intermediate and bottom water. No "leak" of warm Indian Ocean thermocline water is necessary to account for the equatorward heat flux across 32°S; in fact, a large transfer of warm water from the Indian Ocean to the Atlantic is found to be inconsistent with the present data set. Together these results demonstrate that the Atlantic as a whole acts to convert intermediate water to deep and bottom water, and thus that the global thermohaline cell associated with the formation and export of NADW is closed primarily by a "cold water path," in which deep water leaving the Atlantic ultimately returns as intermediate water entering the basin through Drake Passage. The second problem addressed concerns the circulation and property fluxes across 24°and 36°N in the subtropical North Atlantic. Conservation statements are considered for the nutrients as well as mass, and the nutrients are found to contribute significant information independent of temperature and salinity. Silicate is particularly effective in reducing the indeterminacy of circulation estimates based on mass conservation alone. In turn, the results demonstrate that accurate estimates of the chemical fluxes depend on relatively detailed knowledge of the circulation. The zonal-integral of the circulation consists of an overturning cell at both latitudes, with a net export of 19 Sv of NADW. This cell results in a poleward heat flux of 1.3±.2x1015 Wand an equatorward oxygen flux of 2900±180 kmol S-l across each latitude. The net flux of silicate is also equatorward: 138±38 kmol s-1 and 152±56 kmol s -1 across 36°and 24° N, respectively. However, in contrast to heat and oxygen, the overturning cell is not the only important mechanism responsible for the net silicate transport. A horizontal recirculation consisting of northward flow of silica-rich deep water in the eastern basin balanced by southward flow of low silica water in the western basin results in a significant silicate flux to the north. The net equatorward flux is thus smaller than indicated by the overturning cell alone. The net flux of nitrate across 36°N is n9±35 kmol 8- 1 to the north and is indistinguishable from zero at 24°N (-8±39 kmol 8-1 ), leading to a net divergence of nitrate between these two latitudes. Forcing the system to conserve nitrate leads to an unreasonable circulation. The dominant contribution to the nitrate flux at 36°N results from the correlation of strong northward flow and relatively high nitrate concentrations in the sub-surface waters of the Gulf Stream. The observed nitrate divergence between 24°and 36°N, and convergence north of 36°N, can be accounted for by a shallow cell in which the northward flow of inorganic nitrogen (nitrate) in the Gulf Stream is balanced by a southward flux of dissolved organic nitrogen in the recirculation gyre. Oxidation of the dissolved organic matter during its transit of the subtropical gyre supplies the required source of regenerated nitrate to the Gulf Stream and consumes oxygen, consistent with recent observations of oxygen utilization in the Sargasso Sea.
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The basin‐averaged, latitude‐depth ocean model of Wright and Stocker (1992) is used to simulate the deep circulation of the world ocean. Under present‐day surface forcing, sinking occurs in the North Atlantic and the southern ocean, and realistic temperature and salinity structures are obtained in the Atlantic, Pacific, and Indian oceans. “Color” tracers and radiocarbon are used to identify the composition of the deepwater masses and the associated renewal time scales. While broad agreement with observations is found in all basins, the water masses in the southern ocean are too young. The global thermohaline circulation and the composition of the deepwater masses are sensitive to the buoyancy contrast between the southern ocean and the North Atlantic. This contrast can be modified by changing relaxation values of temperature and salinity at the northern and southern high latitudes. If the model is forced with the zonal averages of the observed surface salinity, North Atlantic Deep Water is the dominant deep ocean water mass, and hardly any Antarctic Bottom Water flows into the Atlantic. Choosing instead the observed salinities of the newly formed deep water as the restoring values, the model realistically simulates the penetration of Antarctic Bottom Water into the different ocean basins. This has a global effect through reducing both strength and depth of North Atlantic Deep Water formation. If higher surface salinity values are applied in the southern ocean, a steady state is obtained whose tracer distributions and overturning are consistent with reconstructions of the deep circulation during the last glacial maximum. The two states are stable also under mixed boundary conditions and transitions are possible by smoothly varying the surface freshwater flux of one state to that of the other. These experiments suggest the importance of modified high‐latitude forcing in glacial‐to‐interglacial transitions.
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Abstract. The change of the thermohaline circulation (THC) between the Last Glacial Maximum (LGM, &approx; 21 kyr ago) and the present day climate are explored using an Ocean General Circulation Model and stream functions projected in various coordinates. Compared to the present day period, the LGM circulation is reorganised in the Atlantic Ocean, in the Southern Ocean and particularly in the abyssal ocean, mainly due to the different haline stratification. Due to stronger wind stress, the LGM tropical circulation is more vigorous than under modern conditions. Consequently, the maximum tropical transport of heat is slightly larger during the LGM. In the North Atlantic basin, the large sea-ice extent during the LGM constrains the Gulf Stream to propagate in a more zonal direction, reducing the transport of heat towards high latitudes and reorganising the freshwater transport. The LGM circulation is represented as a large intrusion of saline Antarctic Bottom Water into the Northern Hemisphere basins. As a result, the North Atlantic Deep Water is shallower in the LGM simulation. The stream functions in latitude-salinity coordinates and thermohaline coordinates point out the different haline regimes between the glacial and interglacial period, as well as a LGM Conveyor Belt circulation largely driven by enhanced salinity contrast between the Atlantic and the Pacific basin. The thermohaline structure in the LGM simulation is the result of an abyssal circulation that lifts and deviates the Conveyor Belt cell from the area of maximum volumetric distribution, resulting in a ventilated upper layer above a deep stagnant layer, and an Atlantic circulation more isolated from the Pacific. An estimation of the turnover times reveal a deep circulation almost sluggish during the LGM, and a Conveyor Belt cell more vigorous due to the combination of stronger wind stress and shortened circulation route.
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Abstract Despite the renewed interest in the Southern Ocean, there are yet many unknowns because of the scarcity of measurements and the complexity of the thermohaline circulation. Hence the authors present here the analysis of the thermohaline circulation of the Southern Ocean of a steady-state simulation of a coupled ice–ocean model. The study aims to clarify the roles of surface fluxes and internal mixing, with focus on the mechanisms of the upper branch of the overturning. A quantitative dynamical analysis of the water-mass transformation has been performed using a new method. Surface fluxes, including the effect of the penetrative solar radiation, produce almost 40 Sv (1 Sv ≡ 106 m3 s−1) of Subantarctic Mode Water while about 5 Sv of the densest water masses (γ > 28.2) are formed by brine rejection on the shelves of Antarctica and in the Weddell Sea. Mixing transforms one-half of the Subantarctic Mode Water into intermediate water and Upper Circumpolar Deep Water while bottom water is produced by Lower Circumpolar Deep Water and North Atlantic Deep Water mixing with shelf water. The upwelling of part of the North Atlantic Deep Water inflow is due to internal processes, mainly downward propagation of the surface freshwater excess via vertical mixing at the base of the mixed layer. A complementary Lagrangian analysis of the thermohaline circulation will be presented in a companion paper.
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Deep ocean water
Antarctic Bottom Water
Mode water
Physical oceanography
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