On the freshwater forcing and transport of the Atlantic thermohaline circulation
TL;DR: In this article, it is argued that the freshwater loss to the atmosphere arises mainly in the subtropical South Atlantic and is balanced by northward freshwater transport in the wind-driven sub-tropical gyre, while the thermohaline circulation transports freshwater southward.
Abstract: The 'conveyor belt' circulation of the Atlantic Ocean transports large amounts of heat northward, acting as a heating system for the northern North Atlantic region. It is widely thought that this circulation is driven by atmospheric freshwater export from the Atlantic catchment region, and that it transports freshwater northward to balance the loss to the atmosphere. Using results from a simple conceptual model and a global circulation model, it is argued here that the freshwater loss to the atmosphere arises mainly in the subtropical South Atlantic and is balanced by northward freshwater transport in the wind-driven subtropical gyre, while the thermohaline circulation transports freshwater southward. It is further argued that the direction of freshwater transport is closely linked to the dynamical regime and stability of the 'conveyor belt': if its freshwater transport is indeed southward, then its flow is purely thermally driven and inhibited by the freshwater forcing. In this case the circulation is not far from Stommel's saddle-node bifurcation, and a circulation state without NADW formation would also be stable.
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TL;DR: It will be substantially harder to quantify the range of possible changes in the hydrologic cycle than in global-mean temperature, both because the observations are less complete and because the physical constraints are weaker.
Abstract: What can we say about changes in the hydrologic cycle on 50-year timescales when we cannot predict rainfall next week? Eventually, perhaps, a great deal: the overall climate response to increasing atmospheric concentrations of greenhouse gases may prove much simpler and more predictable than the chaos of short-term weather. Quantifying the diversity of possible responses is essential for any objective, probability-based climate forecast, and this task will require a new generation of climate modelling experiments, systematically exploring the range of model behaviour that is consistent with observations. It will be substantially harder to quantify the range of possible changes in the hydrologic cycle than in global-mean temperature, both because the observations are less complete and because the physical constraints are weaker.
2,267 citations
TL;DR: Synthesis of river-monitoring data reveals that the average annual discharge of fresh water from the six largest Eurasian rivers to the Arctic Ocean increased by 7% from 1936 to 1999, a large-scale change in freshwater flux.
Abstract: Synthesis of river-monitoring data reveals that the average annual discharge of fresh water from the six largest Eurasian rivers to the Arctic Ocean increased by 7% from 1936 to 1999. The average annual rate of increase was 2.0 ± 0.7 cubic kilometers per year. Consequently, average annual discharge from the six rivers is now about 128 cubic kilometers per year greater than it was when routine measurements of discharge began. Discharge was correlated with changes in both the North Atlantic Oscillation and global mean surface air temperature. The observed large-scale change in freshwater flux has potentially important implications for ocean circulation and climate.
1,442 citations
TL;DR: In this article, the authors argue that this cooling event was forced by a massive outflow of fresh water from the Hudson Strait, based on the estimates of the marine 14C reservoir for Hudson Bay which, in combination with other regional data, indicate that the glacial lakes Agassiz and Ojibway (originally dammed by a remnant of the Laurentide ice sheet) drained catastrophically ∼8,470 calendar years ago; this would have released >1014 m3 of freshwater into the Labrador Sea.
Abstract: The sensitivity of oceanic thermohaline circulation to freshwater perturbations is a critical issue for understanding abrupt climate change1 Abrupt climate fluctuations that occurred during both Holocene and Late Pleistocene times have been linked to changes in ocean circulation2,3,4,5,6, but their causes remain uncertain One of the largest such events in the Holocene occurred between 8,400 and 8,000 calendar years ago2,7,8 (7,650–7,200 14C years ago), when the temperature dropped by 4–8 °C in central Greenland2 and 15–3 °C at marine4,7 and terrestrial7,8 sites around the northeastern North Atlantic Ocean The pattern of cooling implies that heat transfer from the ocean to the atmosphere was reduced in the North Atlantic Here we argue that this cooling event was forced by a massive outflow of fresh water from the Hudson Strait This conclusion is based on our estimates of the marine 14C reservoir for Hudson Bay which, in combination with other regional data, indicate that the glacial lakes Agassiz and Ojibway9,10,11,12, (originally dammed by a remnant of the Laurentide ice sheet) drained catastrophically ∼8,470 calendar years ago; this would have released >1014 m3 of fresh water into the Labrador Sea This finding supports the hypothesis2,7,8 that a sudden increase in freshwater flux from the waning Laurentide ice sheet reduced sea surface salinity and altered ocean circulation, thereby initiating the most abrupt and widespread cold event to have occurred in the past 10,000 years
1,144 citations
TL;DR: It is found that only one mode of Atlantic Ocean circulation is stable: a cold mode with deep water formation in the Atlantic Ocean south of Iceland; this provides an explanation why glacial climate is much more variable than Holocene climate.
Abstract: Abrupt changes in climate, termed Dansgaard-Oeschger and Heinrich events, have punctuated the last glacial period (approximately 100-10 kyr ago) but not the Holocene (the past 10 kyr). Here we use an intermediate-complexity climate model to investigate the stability of glacial climate, and we find that only one mode of Atlantic Ocean circulation is stable: a cold mode with deep water formation in the Atlantic Ocean south of Iceland. However, a 'warm' circulation mode similar to the present-day Atlantic Ocean is only marginally unstable, and temporary transitions to this warm mode can easily be triggered. This leads to abrupt warm events in the model which share many characteristics of the observed Dansgaard-Oeschger events. For a large freshwater input (such as a large release of icebergs), the model's deep water formation is temporarily switched off, causing no strong cooling in Greenland but warming in Antarctica, as is observed for Heinrich events. Our stability analysis provides an explanation why glacial climate is much more variable than Holocene climate.
982 citations
TL;DR: Evidence implicates ocean circulation in abrupt and dramatic climate shifts, such as sudden temperature changes in Greenland on the order of 5–10 °C and massive surges of icebergs into the North Atlantic Ocean.
Abstract: Oceans cover more than two-thirds of our blue planet. The waters move in a global circulation system, driven by subtle density differences and transporting huge amounts of heat. Ocean circulation is thus an active and highly nonlinear player in the global climate game. Increasingly clear evidence implicates ocean circulation in abrupt and dramatic climate shifts, such as sudden temperature changes in Greenland on the order of 5-10 degrees C and massive surges of icebergs into the North Atlantic Ocean --events that have occurred repeatedly during the last glacial cycle.
928 citations
References
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01 Jan 1982
TL;DR: A project to objectively analyze historical ocean temperature, salinity, oxygen, and percent oxygen saturation data for the world ocean has recently been completed at the National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey.
Abstract: A project to objectively analyze historical ocean temperature, salinity, oxygen, and percent oxygen saturation data for the world ocean has recently been completed at the National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey. The results of the project are being made available through distribution of the Climatological Atlas of the World Ocean (NOAA Professional Paper No. 13), and through distribution of magnetic tapes containing the objective analyses.
The sources of data used in the project were the Station Data, Mechanical Bathythermograph, and Expendable Bathythermograph files of the National Oceanographic Data Center (NODC) in Washington, D.C., updated through 1977–1978. The raw data were subjected to quality control procedures, averaged by one-degree squares, and then used as input to an objective analysis procedure that fills in one-degree squares containing no data and smooths the results. Due to the lack of synoptic observations for the world ocean, the historical data are composited by annual, seasonal, and (for temperature) monthly periods.
3,029 citations
"On the freshwater forcing and trans..." refers background in this paper
...Schiller’s inverse model of the Atlantic, in contrast, is tied to the observed salinity and temperature distribution at 30 7S (Schiller restores temper- ature and salinity to the climatology of Levitus 1982 at time scales of 30–250 days, depending on depth, south of 30 7S)....
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...As a consequence, the northern North Atlantic is about 4 7C warmer than comparable latitudes in the Pacific (Levitus 1982)....
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...Global mean salinity was conserved by an opposite perturbation in the equatorial Pacific Fig....
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...This overturning circulation, with a volume transport of about 17 Sv (Roemmich and Wunsch 1985; 1 Sverdrupp106 m3/s), leads to the ‘anomalous’ heat transport of the Atlantic: unlike the Pacific and Indian oceans, which move heat from the tropics to the highlatitudes of both hemispheres, the Atlantic transports heat northward at all latitudes, even south of the equator....
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...It is often stated that the saline outflow of NADW from the Atlantic enhances the salinity of Circumpolar Deep Water (CDW) and even the deep waters of the Pacific and Indian Oceans....
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Book•
01 Jun 1982TL;DR: A project to objectively analyze historical ocean temperature, salinity, oxygen, and percent oxygen saturation data for the world ocean has recently been completed at the National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey.
Abstract: A project to objectively analyze historical ocean temperature, salinity, oxygen, and percent oxygen saturation data for the world ocean has recently been completed at the National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey. The results of the project are being made available through distribution of the Climatological Atlas of the World Ocean (NOAA Professional Paper No. 13), and through distribution of magnetic tapes containing the objective analyses.
The sources of data used in the project were the Station Data, Mechanical Bathythermograph, and Expendable Bathythermograph files of the National Oceanographic Data Center (NODC) in Washington, D.C., updated through 1977–1978. The raw data were subjected to quality control procedures, averaged by one-degree squares, and then used as input to an objective analysis procedure that fills in one-degree squares containing no data and smooths the results. Due to the lack of synoptic observations for the world ocean, the historical data are composited by annual, seasonal, and (for temperature) monthly periods.
2,929 citations
TL;DR: In this paper, wind and air-minus-sea temperatures are calculated in a form suitable for determining stress by any bulk aerodynamics model in which the drag coefficient can be represented by six or less coefficients of a second-degree polynomial in wind speed and stability.
Abstract: Over 35 million surface observations covering the world ocean from 1870–1976 have been processed for the purpose of calculating monthly normals and standard errors of the eastward and northward components of the wind stress and work done by the winds in the lower 10 m of the atmosphere. The fields are intended to serve as boundary conditions for models of the ocean circulation. Wind and air-minus-sea temperatures are calculated in a form suitable for determining stress by any bulk aerodynamics model in which the drag coefficient can be represented by six or less coefficients of a second-degree polynomial in wind speed and stability. The particular case of the wind speed and stability dependent drag coefficient discussed by Bunker is selected for analysis. January and July charts of wind stress, curl of the wind stress, mass transport stream-function, divergence of the Ekman transport and the rate of mechanical energy transfer are illustrated and discussed.
1,872 citations
"On the freshwater forcing and trans..." refers background in this paper
...The experiments started from a steady equilibrium state corresponding to the present-day circulation, driven by observed winds (Hellerman and Rosenstein 1983) and a prescribed freshwater flux field derived from a spinup integration (Rahmstorf 1995b)....
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TL;DR: In this paper, it is proposed that this return flow is accomplished primarily within the ocean's warm water thermocline layer, where the main thermoclines of the ocean are linked as they participate in a thermohaline-driven global scale circulation cell associated with NADW formation.
Abstract: Formation of North Atlantic Deep Water (NADW) represents a transfer of upper layer water to abyssal depths at a rate of 15 to 20 x 10 6 m3/s. NADW spreads throughout the Atlantic Ocean and is exported to the Indian and Pacific Oceans by the Antarctic Circumpolar Current and deep western boundary currents. Naturally, there must be a compensating flow of upper layer water toward the northern North Atlantic to feed NADW production. It is proposed that this return flow is accomplished primarily within the ocean's warm water thermocline layer. In this way the main thermoclines of the ocean are linked as they participate in a thermohaline-driven global scale circulation cell associated with NADW formation. The path of the return flow of warm water is as follows: Pacific to Indian flow within the Indonesian Seas, advection across the Indian Ocean in the 10o-15oS latitude belt, southward transfer in the Mozambique Channel, entry into the South Atlantic by a branch of the Agulhas Current that does not complete the retroflection pattern, northward advection within the subtropical gyre of the South Atlantic (which on balance with the southward flux of colder North Atlantic Deep Water supports the northward oceanic heat flux characteristic of the South Atlantic), and cross-equatorial flow into the western North Atlantic. The magnitude of the return flow increases along its path as more NADW is incorporated into the upper layer of the ocean. Additionally, the water mass characteristics of the return flow are gradually altered by regional ocean-atmosphere interaction and mixing processes. Within the Indonesian seas there is evidence of strong vertical mixing across the thermocline. The cold water route, Pacific to Atlantic transport of Subantarctic water within the Drake Passage, is of secondary importance, amounting to perhaps 25% of the warm water route transport. The continuity or vigor of the warm water route is vulnerable to change not only as the thermohaline forcing in the northern North Atlantic varies but also as the larger-scale wind-driven criculation factors vary. The interocean links within the Indonesian seas and at the Agulhas retroflection may be particularly responsive to such variability. Changes in the warn: water route continuity may in turn influence formation characteristics of NADW.
1,236 citations
"On the freshwater forcing and trans..." refers background in this paper
...The relative contribution of surface and intermediate water to the northward inflow is thus still an open question; this issue is closely linked to the debate over the relative importance of the ‘cold water route’ and the ‘warm water route’ for the conveyor belt (Gordon 1986; Rintoul 1991)....
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...Stommel (1980) and Piola and Gordon (1986) have attempted to balance the observed vapour loss north of 30 7S in the Atlantic by a vertical overturning circulation consisting of a northward flow of thermocline and intermediate waters and a southward flow of deep water....
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...…the Atlantic basin and the conveyor’s freshwater export in the global GCM. Repeating the budget calculation with the same water masses as Piola and Gordon (1986), but balancing only the overturning transport components found in Schiller’s (1995) inverse model (0.36 PW northward heat transport,…...
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...Balancing the Atlantic in this way required an overturning of 92 Sv; a refined version of this model (Piola and Gordon 1986) still required a value of 54 Sv NADW outflow at 30 7S, rather than the observed ca. 12 Sv....
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TL;DR: The ocean's conveyor appears to be driven by the salt left behind as the result of water-vapor transport through the atmosphere from the Atlantic to the Pacific basin this paper.
Abstract: A DIAGRAM DEPICTING the ocean's \"conveyor belt\" has been widely adopted as a logo for the Global Change Research Initiative. This diagram (Fig. 1) first appeared as an illustration in an article about the Younger Dryas event that was published in the November 1987 issue of Natural History. It was designed as a cartoon to help the largely lay readership of this magazine to comprehend one of the elements of the deep sea's circulation system. Had I suspected that it would be widely adopted as a logo, I would have tried to \"improve\" its accuracy. In hindsight such repairs would likely have ruined the diagram both for the readers of Natural History and for use as a logo. The lure of this logo is that it symbolizes the importance of linkages between realms of the Earth's climate system. The ocean's conveyor appears to be driven by the salt left behind as the result of water-vapor transport through the atmosphere from the Atlantic to the Pacific basin. A byproduct of its operation is the heat that maintains the anomolously warm winter air temperatures enjoyed by northern Europe. A millennium of very cold conditions known as the Young Dryas appears to have been the result of a temporary shutdown of the conveyor. Thus the conveyor logo portrays the concern that led to the launching of the Global Change Research Initiatives: that complex interconnections among the elements of our Earth's climate system will greatly complicate our task of predicting the consequences of global pollution. Most of the concepts involved in this story have roots that extend well back in time. The most important feature of the conveyor is the production of deep water in the northern Atlantic. This aspect of the ocean's thermohaline circulation was thoroughly described by Wrist (1935) and Wrist and Defant (1936) more than 50 years ago. In 1906 Chamberlain explored the importance of freshwater transport to ocean circulation. He raised the question as to whether changes in the pattern of deep circulation could be responsible for the climate changes of glacial time. My contribution
1,148 citations
"On the freshwater forcing and trans..." refers background in this paper
...The strong form of the concept goes further (Broecker 1991; Zaucker and Broecker 1992): it proposes that the overall effect of the freshwater forcing felt by the ‘conveyor belt’ in the Atlantic is a net freshwater loss (i....
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...On the other hand, a number of authors have attributed NADW formation to the high evaporation rate in the Atlantic (e.g. Weyl 1968; Reid 1979; Warren 1983; Broecker and Denton 1989; Schmitt et al. 1989; Broecker et al. 1990b; Broecker 1991; Zaucker and Broecker 1992); in particular the papers (co-)authored by W....
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...In our GCM there is thus no salt accumulation in the Atlantic during times of weak NADW flow as required for the simple ‘salt oscillator’ mechanism originally proposed by Broecker et al. (1990a). Such an oscillator could only work if other feedbacks lead to salt accumu-...
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