Southern Ocean sea‐ice control of the glacial North Atlantic thermohaline circulation
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A coupled model simulates a shallower and weaker North Atlantic Deep Water circulation at the Last Glacial Maximum, compared to the modern, with an enhanced intrusion of Antarctic Bottom Water into the North Atlantic. These circulation changes are caused by the enhanced Antarctic Bottom Water formation, which is triggered by the enhanced equatorward sea‐ice transport, ultimately by increased westerlies, in the Southern Ocean at the Last Glacial Maximum.Keywords:
Antarctic Bottom Water
Westerlies
Atlantic Equatorial mode
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Abstract Recent studies have proposed the Southern Ocean as the site of large water-mass transformations; other studies propose that this basin is among the main drivers for North Atlantic Deep Water (NADW) circulation. A modeling contribution toward understanding the role of this basin in the global thermohaline circulation can thus be of interest. In particular, key pathways and transformations associated with the thermohaline circulation in the Southern Ocean of an ice–ocean coupled model have been identified here through the extensive use of quantitative Lagrangian diagnostics. The model Southern Ocean is characterized by a shallow overturning circulation transforming 20 Sv (1 Sv ≡ 106 m3 s−1) of thermocline waters into mode waters and a deep overturning related to the formation of Antarctic Bottom Water. Mode and intermediate waters contribute to 80% of the upper branch of the overturning in the Atlantic Ocean north of 30°S. A net upwelling of 11.5 Sv of Circumpolar Deep Waters is simulated in the Southern Ocean. Antarctic Bottom Water upwells into deep layers in the Pacific basin, forming Circumpolar Deep Water and subsurface thermocline water. The Southern Ocean is a powerful consumer of NADW: about 40% of NADW net export was found to upwell in the Southern Ocean, and 40% is transformed into Antarctic Bottom Water. The upwelling occurs south of the Polar Front and mainly in the Indian and Pacific Ocean sectors. The transformation of NADW to lighter water occurs in two steps: vertical mixing at the base of the mixed layer first decreases the salinity of the deep water upwelling south of the Antarctic Circumpolar Current, followed by heat input by air–sea and diffusive fluxes to complete the transformation to mode and intermediate waters.
Circumpolar deep water
Antarctic Bottom Water
Physical oceanography
Mode water
Deep ocean water
Boundary current
Antarctic Intermediate Water
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A coupled model simulates a shallower and weaker North Atlantic Deep Water circulation at the Last Glacial Maximum, compared to the modern, with an enhanced intrusion of Antarctic Bottom Water into the North Atlantic. These circulation changes are caused by the enhanced Antarctic Bottom Water formation, which is triggered by the enhanced equatorward sea‐ice transport, ultimately by increased westerlies, in the Southern Ocean at the Last Glacial Maximum.
Antarctic Bottom Water
Westerlies
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Atlantic Equatorial mode
Tropical Atlantic
Intertropical Convergence Zone
Westerlies
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Deep ocean water
Physical oceanography
Circulation (fluid dynamics)
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A coupled climate model study indicates the paleoclimate record of glacial thermohaline circulation (THC) and reversed deep‐sea temperature‐salinity (T‐S) distribution in the Atlantic can be explained largely by lower glacial atmospheric CO 2 alone. The reduced CO 2 leads to increased Southern Ocean wintertime sea‐ice cover and salinity, increased production of dense Antarctic Bottom Water (AABW), enhanced cold and saline AABW penetration into the deep North Atlantic, increased oceanic vertical stability, and reduced North Atlantic Deep Water (NADW) circulation. The dominant role of CO 2 forcing during the glacial implies a positive feedback between the Southern Ocean regulated THC and the glacial global carbon cycle.
Antarctic Bottom Water
Paleoclimatology
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Antarctic Bottom Water
Last Glacial Maximum
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Atlantic Equatorial mode
Gulf Stream
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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.
Circumpolar deep water
Abyssal zone
Deep ocean water
Physical oceanography
Antarctic Bottom Water
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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.
Circumpolar deep water
Deep ocean water
Antarctic Bottom Water
Mode water
Physical oceanography
Mixed layer
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