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(Received March 3, 1999; revised July 6, 1999; accepted July 20, 1999.)
AGU Index Terms: 4267 Paleoceanography; 3022 Marine sediments-processes
and transport; 4255 Numerical modeling; 4532 General circulation
Keywords/Free Terms: Paleoceanography, Sediment transport, Ocean circulation
modeling
It is thought that a global interconnection exists between northern North Atlantic (NNA) and northern North Pacific (NNP), with probable interruptions during some glacial-to-interglacial climate transitions [Boyle and Weaver, 1994; Broecker, 1991; Broecker and Denton, 1989; Gordon et al., 1992]. Yet, the recent study of Mikolajewicz et al. [1997] indicates that teleconnections between these remote northern hemispheric oceans are most probably linked through the atmosphere rather than through the conveyor itself. Seidov and Haupt [1997a] (hereafter SH97) show that although the model NNA meltwater impact can be seen globally, the NNP region is far less affected by this impact than many other parts of the Indian and Pacific Oceans.
The objective of this study is to determine the character, extent, and origins of glacial-to-interglacial changes in the dynamics of the global ocean thermohaline conveyor. We accomplish this objective by modeling the global ocean circulation using an ocean general circulation model to compute the steady state velocity, temperature, and salinity, which then drive a sediment transport model described by Haupt et al. [1994, 1995, 1999] and Seidov and Haupt [1997b, 1999]. This sediment transport model for sedimentation in large ocean basins (SEDLOB) predicts sediment distributions for particular circulation patterns and can be used as an instrument for validating the ocean paleocirculation modeling.
In our study we used a highly idealized and reduced surface source of eolian
sediment to trace the circulation signal in the transported, resuspended and
redeposited sediment that illustrates the ocean current patterns. What are shown
as sediment deposition rates are not the real world rates but imaginary sedimentation
patterns that demonstrate the impact of the deep ocean currents on the sediment
drifts and show the links between the changes of the sedimentation rates in
key areas and changes in water transports and interbasin water exchanges during
glacial-to-interglacial cycles.
Three basic numerical runs represent (1) the present-day sea surface climatology that is referred to as the modern time slice (MOD), (2) the Last Glacial Maximum (LGM), and (3) the meltwater event (MWE) at 13.5 ka, when the first strong deglaciation of the Barents shelf occurred. In order to facilitate multiple millennium-scale runs, a coarse resolution (6° longitude by 4° latitude) with 12 vertical levels was used. It has been shown by many authors [e.g., Cox, 1989; Toggweiler et al., 1989] that MOM is capable of reproducing the rates of deep-water production and thermohaline overturning fairly well in the framework of a rather coarse horizontal resolution.
Modern annual mean SST and SSS are specified from new ocean climatological data sets [Levitus and Boyer, 1994; Levitus et al., 1994]. The LGM and MWE SST and SSS were specified as by Seidov et al. [1996]. Table 1 summarizes the three sets of SST and SSS used as boundary conditions in the numerical runs.
There might be significant uncertainties in the SSS reconstructions. To test the effect of these uncertainties on the global circulation, a number of additional runs were performed in which the MWE SSS in the meltwater pool was altered by as much as 1 practical salinity unit (psu). In none of these runs did the SSS modification prevent the capping of convection and the depression of the conveyor [see also Seidov and Maslin, 1999]. Thus, despite the uncertainties the SSS reconstructions by Duplessy et al. [1991] and Sarnthein et al. [1994] give a very solid foundation for numerical simulations on the basis of these data [Seidov et al., 1996]. The thermohaline circulation collapse due to freshwater impact found by Seidov et al. [1996] compares well with the paleoreconstructions of Sarnthein et al. [1995]. It has been confirmed that the results of the regional NA model by Seidov et al. [1996] are still valid in global circulation experiments [Seidov and Haupt, 1997a, 1999; Seidov and Maslin, 1999]. Moreover, it has been shown in these studies that the meltwater signal in the Nordic Seas during the MWE is so strong that the circulation response is a robust feature in all numerical experiments.
Wind stress fields were extracted from the output of the Hamburg atmospheric circulation model, which was forced by present-day and glacial sea surface climatologies [Lorenz et al., 1996]. Although it has been shown that the alteration of wind stress in the Southern Ocean can by itself cause changes of NADW production [e.g., Rahmstorf and England, 1997; Toggweiler and Samuels, 1995], we focus on the meltwater control, which is the most apparent signature of the glacial-to-interglacial changes. Therefore we use the same glacial wind stress in both LGM and MWE runs.
The MOD and LGM experiments were run for ~10,000 and 8000 years from rest, respectively, to achieve a near steady state. After the replacement of LGM boundary conditions with MWE conditions the model was run for 1000 years to mimic the change of the LGM circulation under a meltwater impact most probably caused by melting of the iceberg flotilla from the Barents Shelf [Sarnthein et al., 1994]. This duration of the chosen meltwater event is estimated as several hundred to 1000 years [Sarnthein et al., 1995].
Figure 1 depicts the total transport of water in a vertical plane; the direction of water flow is clockwise if the values of stream function are positive and counterclockwise if the values are negative (shaded in Figure 1). The modern overturning in Figure 1 looks quite reasonable: NADW penetrates deep enough to set forth the deep-ocean conveyor and gives reasonable overturning intensity. The overturning in the NA, which comprises the NADW production, is as high as 16-18 Sv, which agrees well with the water transport estimations of Schmitz [1995], and the Antarctic Bottom Water (AABW) inflow into the Atlantic Ocean is ~8 Sv. The NADW outflow across 30°S is more than 10 Sv, which fits within the estimates given by different models. These estimates vary in different studies and are known to be sensitive to the geometry of the model continents, bottom topography in the Drake Passage area (see discussion by Toggweiler and Samuels [1995] and by Rahmstorf and England [1997]), and vertical diffusivity [Bryan, 1987; Cummins et al., 1990].
Although the LGM overturning (Figure 1b) is shallower and more than twice as weak as the MOD overturning, the structure of the MOD and the LGM modes do not differ dramatically. However, there is a substantial difference between these two modes (MOD and LGM; Figures Figure 1a and 1b) and the MWE mode with a collapsed conveyor (Figure 1c). The depth and intensity of the NADW outflow, convection patterns (see below) and overall circulation configurations in the present-day and LGM runs, especially in the Southern Ocean, match those by Winguth et al. [1999] (who provide a comparison of modeled circulation with reconstructed biogeochemical tracers).
To save space, we do not show velocities at different horizons here but base our analyses on vector maps of layered transport and deep-ocean velocity fields provided by SH97. These maps illustrate differences in the THC between the MOD and MWE cases because the difference between the MOD and LGM THC patterns is not dramatic. In agreement with the Stommel and Arons [1960b] theory, the deep coherent western boundary current emerges in both MOD and LGM runs because of the deep-ocean water source in the NNA and NGS. Yet, this boundary current is weaker and shallower during the LGM, and the circulation structure looks somewhat different between MOD and LGM scenarios [see also Seidov and Haupt, 1997b; Seidov et al., 1996; Seidov and Maslin, 1999].
In comparison to the MOD and LGM circulation the MWE circulation is considerably different, as shown by the vector maps in SH97. A coherent deep-ocean flow originating in the Atlantic Ocean does not exist in the MWE experiment because there is no NADW production in the MWE experiment. Although the collapse of the Atlantic branch of the conveyor in a numerical study of a freshwater impact is not a new finding, the reversal of the northern flank of the Antarctic Circumpolar Current (ACC) in the Indian Ocean is a novel feature. The Indian branch of this reversed deep-ocean current (shown by SH97) appears to be a robust feature in all meltwater runs, including those described by Seidov and Maslin [1999] and those with disturbed SSS input in the NGS (sensitivity tests with varied low SSS signal; see above).
In order to assess the variation of interbasin water exchange the water transports across different sections are calculated and compared for the three time slices. Figure 3 shows the transport at shallow, deep, and in some instances, intermediate depths for all three runs. The major changes are in the Atlantic Ocean, though all regions are affected. While the conveyor changed dramatically in the Atlantic and noticeably in the Indian and southern Pacific Oceans from the LGM to MWE mode, corresponding changes are absent in the NNP.
The NADW flow structure in the western part of the South Atlantic (at 10°S) can be seen in Figure 3. In the present-day experiment (Figure 3a), there is a strong southward flow of almost 19 Sv between 1160 and 3700 m, which is compensated by northward flow of 12.5 Sv in the upper ocean (0-1160 m) and 9.3 Sv of northward flow of AABW (3700-5000 m). (Note that only one half of the coast-to-coast section is shown; the total transport of the NADW across 10°S is ~12 Sv). The difference between northward and southward transports is balanced by the total northward flow of 2.7 Sv in the eastern South Atlantic). Moreover, most of the northward compensating flow is through the Falkland Current-Benguela Current complex because there is not a northward, but a southward residual flow of ~4.2 Sv to the south of 38°S in the eastern part of the ocean. This type of surface water returning to the Atlantic in the model favors the "cold water path" NADW return via the Drake Passage (see below). In the LGM experiment (Figure 3b), there is only 8.7 Sv of intermediate-to-deep ocean outflow between 1160 and 3700 m, with a NADW compensation flow of 5.6 in the upper ocean (0-1160 m) and AABW inflow of 6.8 Sv (3700-5000 m). Again, a substantial surface flow in the eastern part of the ocean is absent.
In the MWE experiment, there is a southward flow of 2.6 Sv in the upper ocean (0-1160 m) and a northward flow of 6.6 Sv of southern source water across the same section at 38°S. A small (<1 Sv) northward flow replaces the NADW southward transport in the intermediate-to-deep ocean. A considerable structural change in the water transport occurs in many places distant from the meltwater impact. Three regions are most affected, namely, the western and southeastern South Atlantic Ocean, the western Indian Ocean, and the southwestern Pacific Ocean east of New Zealand.
Our ocean-only modeling cannot fully determine what kind of teleconnections exist between remote ocean basins. However, these experiments assess whether the deep-ocean currents, the atmosphere, or the coupled ocean-atmosphere system are responsible for the teleconnections between different basins. For instance, can surface water that travels westward across the Indian Ocean progress around the tip of Africa into the Atlantic and travel northward to replace NADW outflow? The upper ocean velocity maps by SH97 do not support this scheme of surface water return. Although NNP water travels through the Indonesia Throughflow into the Indian Ocean, it failed to advance northward in the Benguela Current in the MOD experiment.
The outflow from the North and central Pacific Ocean into the Indian Ocean through the Indonesian Throughflow is not small, reaching 20 Sv (Figure 3a) in the present-day scenario. However, as Figure 3 and maps by SH97 suggest, little NNP water escapes into the South Atlantic via the western Indian Ocean. It is thought that mesoscale eddies facilitate the Agulhas leakage of warm and salty Indonesian water into the Atlantic over the tip of Africa [e.g., Lutjeharms, 1996]. However, even if the eddies (e.g., in an eddy-resolving model) managed to transport a considerable amount of water into the Atlantic, this transport, estimated as only 6.2 Sv for water warmer than 10°C and 7.2 Sv for water warmer than 8°C [van Ballegooyen et al., 1994], is far too low to be a dominant mechanism for maintaining the necessary volume of NADW production.
Another possible way to maintain the compensating surface flow is by replacing the NADW production by the cold water path of the conveyor [Rintoul, 1991]. The cold water path describes a flow from the Drake Passage into the South Atlantic Ocean as an alternative to the "warm water path," which implies that the Agulhas eddy-induced leakage plays the leading role in maintaining the NADW production. Both coupled ocean-atmosphere models and stand-alone ocean models usually reveal the cold water path scheme [e.g., Drijhout et al. 1996; Manabe and Stouffer,1988, 1997; Seidov and Haupt, 1997a; Washington et al., 1994]. Schmitz [1995] presents a schematic diagram of the circulation system in which the 15 Sv needed to replace the NADW are compensated for by 10 Sv from the flow through the Drake Passage and only 5 Sv from the Pacific-Indian conveyor branch. Macdonald and Wunsch [1996] also support the substantial contribution of the cold water path scenario.
In our model we have only 12 Sv NADW outflow at 10°S (Figure 1a), which means that up to 6 Sv recirculate in the present-day North Atlantic because of the leakage from the deep-sea western boundary layer with subsequent upwelling at the base of the thermocline as predicted by the Stommel and Arons [1960a] theory. Therefore only this portion (12 Sv) of the total NADW production must be compensated from outside the North Atlantic, which anyway favors the cold water path scenario, as can be seen in Figure 3. Our study concludes that this cold water path connection between the NNA and NNP probably existed during Pleistocene glacial cycles as well. No NADW compensation is needed in the meltwater scenario.
The surface source of the sediment is assumed to be the same for all three slices. Therefore the difference between modern and past eolian sedimentation patterns indicates only changes of the ocean circulation. This approach enhances the circulation study but limits our ability to address sedimentological problems that necessitate a far more complicated sediment source. Therefore a word of warning must be issued here concerning the relevance of the simulations. In reality the eolian dust supply to the ocean is not spatially uniform [e.g., Rea, 1994], and its distribution and the atmospheric load are likely to have varied during the glacial-interglacial cycles [Ram and Koenig, 1997]. Also, the production and transport of any biogenic material, which is an important sediment component over much of the ocean, is not yet incorporated in SEDLOB. However, our current goal is not to simulate a totally realistic distribution of eolian sediment but a better understanding of how the glacial-to-interglacial changes in the THC affected sediment accumulation and erosion.
Future advances of SEDLOB will incorporate a surface sediment source that varies temporally and spatially and adding biogenic components. Here it is emphasized that we do not yet model a real world sedimentation pattern. Therefore our sediment maps do not yet look like any real map of surface sediment accumulation rates. However, one specific feature of terrigenous sediment distribution can easily be seen in our maps, namely, the areas of strong impact of the circulation on the sediment drifts. The analysis of such areas helps to link sedimentation and water transports in the deep ocean.
The important aspect of this study is that despite the same small and uniform surface sediment source for all three runs the computed sediment patterns in all three scenarios are substantially different and show substantial spatial variability. In Plate 1 the distribution of sediment deposition determined by the preset-day currents in MOD runs (Plate 1a) is complemented by the maps of the differences between the sediment deposition rates in the LGM and MOD (Plate 1b) and MWE and MOD (Plate 1c).
The most obvious changes in sediment accumulation and distribution are found in the NNA. Changes in the inflow/outflow and the convection in the NNA and Nordic Seas substantially reduce the sedimentation rate in the NNA and NGS in the LGM and MWE scenarios in comparison to the MOD experiment. The lower sedimentation rates are mainly due to the absence of deep convection in the NNA and NGS. Similar results were obtained in a regional North Atlantic model [Seidov and Haupt, 1998].
Variations of the sedimentation rates due to changes in the deep-sea currents occur not only in the convective site areas but also in many regions that do not experience convection. For example, the central Atlantic and the southwestern and northeastern Indian Ocean are convection-free. Nonetheless, there are noticeable changes of sediment deposition rates in these regions among the three simulations.
The largest sedimentation change is in the key conveyor areas, like the vicinity of Iceland, the Drake Passage, the Mozambique Channel, the Indonesian Throughflow, the Kerguelen Plateau, east of the Japan Trench, and south of the Kermadec Trench. The sediment accumulation rate changes in Plates 1b and 1c in these and several other areas are linked to changes in the interbasin water exchange (Figure 3). For example, the Pacific-Indian Ocean exchange is substantially reduced from MOD to LGM to MWE experiments.
Although the Indonesian Throughflow is an important element in the Indian-Pacific oceans water budget and thermohaline property exchange [Godfrey, 1996; Gordon and Fine, 1996], it is unlikely, as indicated by Macdonald and Wunsch [1996], that this throughflow is a key element in changing the global deep-ocean conveyor. As Plate 1 shows, the areas most affected by the glacial and meltwater impacts (other than the NNA) are the South Atlantic and the Atlantic sector of the Southern Ocean, the southwestern Indian Ocean, and the southwestern Pacific northeast of Australia and south and southeast of New Zealand, i.e., in the areas associated with the key elements of the deep-ocean conveyor in the South Hemisphere. The Indonesian Throughflow appears less affected than these areas.
Despite the fact that the terrigenous sediment fluxes in the NNA did not change much during the Holocene a distribution of the LGM sediment is found to be different from the present-day pattern [Cremer et al., 1993]. The difference in sedimentation patterns similar to those between the LGM and MOD simulations is also supported by some authors. For instance, the noticeable decrease in sediment accumulation in the western NNA, with a concurrent increase in the eastern NNA of the area during the Last Glacial Maximum, is strongly supported by reconstructions that indicate stronger glacial sedimentation along the eastern flank of the Mid-Atlantic Ridge [e.g., Dowling and McCave, 1993; Robinson and McCave, 1994]. McCave and Tucholke [1986] show that areas of high sediment deposition coincide with the region of the western boundary current with the maximum of kinetic energy. In our experiments, sediment accumulation along the northern and central American east coast is lower during the LGM than today as a result of a weakened western boundary flow.
In the South Atlantic, SEDLOB predicts high sedimentation rates in the southern part of the Argentine Basin in the area of Falkland Escarpment (~50ºS). Thus the model simulates features of the Zapiola Drift linked to the deep-sea circulation [Flood and Shor, 1988]. In the Indian Ocean our model gives high accumulation rates in the Cape and Agulhas Basins indicated in several studies [e.g., Faugeres et al., 1993; Hollister and McCave, 1984]. The map of suspended material load by Hollister and McCave [1984] corresponds to the high kinetic energy of surface currents and the spread of deep-ocean cold water. Plate 1a exhibits a similar distribution that also corresponds well with the intensity of circulation depicted by SH97. The areas of the highest suspended sediment load in the Southern Hemisphere are the Argentine Basin (see above), the Cape and Agulhas Basins, and the ocean floor southeast of New Zealand (Plate 1a).
The model does not reveal a strong conveyor-like connection between the NNA and NNP Oceans. This result supports the idea [e.g., Mikolajewicz, 1996] that the atmosphere plays the leading role in teleconnecting these two remote oceans in response to the collapse of the conveyor in the Atlantic.
Despite exciting new features facilitated by a combination of a circulation and a sedimentation model, there are inherent limitations to the use of simulated sedimentation patterns to verify circulation models. For example, sediment distribution is most sensitive to near-bottom currents. It is far less indicative of intermediate-depth currents and may not reveal changes of intensity or even the reversal of intermediate-depth currents unless the deeper currents are substantially affected by this reversal.
Our results agree with the findings of Manabe and Stouffer [1997], who showed that the Indo-Pacific ACC responds strongly to an idealized meltwater episode in the NNA. However, it is also possible that even in the Southern Ocean, the change induced by the conveyor collapse could be transferred, at least partially, by the atmosphere rather than by ocean currents (some indication may be found in the modeling by Manabe and Stouffer [1997]). It is difficult to address this issue on the basis of the paleoreconstructions because of the lack of good spatial and temporal coverage across the ocean basins. For instance, the conveyor flow is known to develop in the western Atlantic, and therefore the conveyor dynamics cannot be elucidated using data from only the eastern part of the basin, as in Sarnthein et al. [1994].
Acknowledgments. We appreciate the comments and suggestions by
Margaret Delaney, an anonymous reviewer, Arne Winguth, and Chris Poulsen that
helped substantially to improve the manuscript. We finalized our study at the
Earth System Science Center of Penn State University, and we are most grateful
to Eric Barron for his support of this work and useful suggestions. The study
was partially started at Kiel University, and it could not have been carried
out without the efforts invested by the GPI/SFB313 group at Kiel University
into paleoreconstructions of the North Atlantic. We are grateful to them for
making these data available to us and for their help and support. We are thankful
to Karl Stattegger, Michael Sarnthein, Ralf Prien, and Mara Weinelt for their
help at the earlier stage of this work in Kiel.
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Copyright 1999 by the American Geophysical Union.
Paper number 1999PA900043.
0883-8305/99/1999PA900043$12.00
| Experiments |
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| MOD | SST from present-day sea surface climatology [Levitus and Boyer, 1994]. | SST from present-day sea surface climatology [Levitus et al., 1994]. |
| LGM | CLIMAP [1981] SST is used everywhere except for the NA to the north of 50°N and east of 40°W, where the data from Schulz [1994], summarized by Sarnthein et al. [1995] and processed by Seidov et al. [1996], replace the CLIMAP data | The present day SSS was increased by 1 psu according to Duplessy et al. [1991]; in the NA, to the north of 10°N, the data set is from Duplessy et al. [1991] and Weinelt [1993], summarized by Sarnthein et al. [1995] and processed by Seidov et al. [1996] |
| MWE | as for the LGM except for the NA to the north of 50°N and east of 40°W, where SST from Weinelt [1993], summarized in Sarnthein et al. [1995] and processed in Seidov et al. [1996] replace the LGM SST | As in LGM, except for the NA north of 50°N and east of 40°W where SSS from Weinelt [1993], summarized in Sarnthein et al. [1995] and processed in Seidov et al. [1996] replace the LGM surface salinity. |
Figure 1. Meridional overturning in the Atlantic Ocean (in sverdrups; 1 Sv=106 m3 s-1): (a) modern (MOD), (b) Last Glacial Maximum (LGM), and (c) meltwater event (MWE). The negative values depict counterclockwise motion and are shaded. The Atlantic overturning is valid only within this ocean's geographical boundaries (with meridional walls at both sides; therefore the area to the south of 30°S is masked).
Figure 2. Diagrams of convection: (a) the MOD; (b) the LGM, and (c) MWE. The heights of the bars are equal to the depth of convection. The intensity of convection is emphasized by shading: the darker shades represent deeper convection.
Figure 3. The water transports across the sections in different oceans (in sverdrups; 1 Sv=106 m3 s-1): (a) MOD, (b) LGM, and (c) MWE. Mostly the transport in the upper and deep ocean are shown; in some cases the transports in three layers are shown to differentiate between the flows in the upper, intermediate-to-deep, and deep-to-abyssal flows in cases when the upper and intermediate waters move essentially different.
Plate 1. (a) Sedimentation rates in MOD experiment and (b) differences of sedimentation rates between MOD and LGM and (c) between MOD and MWE experiments. The color bar gives the scale of the thickness (in centimeters) of sediment accumulated during 1000 years, or sedimentation rates in cm kyr-1. The shown sedimentation rates are not realistic because of an idealized character of the sea surface eolian sediment source. The goal is to link the changes of sediment drifts to changes of the ocean circulation patterns and interbasin water exchange (see text).
