Closure of the Global Overturning Circulation Through the Indian, Pacific, and Southern Oceans: Schematics and Transports
TL;DR: The overturning pathways for the surface-ventilated North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) and the diffusively-formed Indian Deep Water and Pacific Deep Waters (IDW and PDW) are intertwined.
Abstract: The overturning pathways for the surface-ventilated North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) and the diffusively-formed Indian Deep Water and Pacific Deep Water (IDW and PDW) are intertwined. The global overturning circulation (GOC) includes both large wind-driven upwelling in the Southern Ocean and important internal diapycnal transformation in the deep Indian and Pacific Oceans. All three northern-source Deep Waters (NADW, IDW, PDW) move southward and upwell in the Southern Ocean. AABW is produced from the denser, salty NADW and a portion of the lighter, low oxygen IDW/PDW, which upwells above and north of NADW. The remaining IDW/PDW stays at the surface, moving into the subtropical thermoclines, and ultimately sources about 1/3 of the NADW. Another third of the NADW comes from AABW in the Atlantic. The remaining third comes from AABW upwelling to the thermocline in the Indian-Pacific. Atlantic cooling associated with NADW formation (0.3 PW north of 32°S) (1 PW = 10 W) and Southern Ocean cooling associated with AABW formation (0.4 PW south of 32°S) are balanced mostly by 0.6 PW of deep diffusive heating in the Indian and Pacific Oceans; only 0.1 PW is gained at the surface in the Southern Ocean. Thus, while an adiabatic model of NADW global overturning driven by winds in the Southern Ocean, with buoyancy addition only at the surface in the Southern Ocean, is a useful dynamical idealization, the associated heat changes require full participation of the diffusive Indian and Pacific Oceans, with a basin-averaged diffusivity on the order of the Munk value of 10 m/sec.
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TL;DR: The authors used numerical climate simulations, paleoclimate data, and modern observations to study the effect of growing ice melt from Antarctica and Greenland and found that ice mass loss from the most vulnerable ice, sufficient to raise sea level several meters, is better approximated as exponential than by a more linear response.
Abstract: . We use numerical climate simulations, paleoclimate data, and modern observations to study the effect of growing ice melt from Antarctica and Greenland. Meltwater tends to stabilize the ocean column, inducing amplifying feedbacks that increase subsurface ocean warming and ice shelf melting. Cold meltwater and induced dynamical effects cause ocean surface cooling in the Southern Ocean and North Atlantic, thus increasing Earth's energy imbalance and heat flux into most of the global ocean's surface. Southern Ocean surface cooling, while lower latitudes are warming, increases precipitation on the Southern Ocean, increasing ocean stratification, slowing deepwater formation, and increasing ice sheet mass loss. These feedbacks make ice sheets in contact with the ocean vulnerable to accelerating disintegration. We hypothesize that ice mass loss from the most vulnerable ice, sufficient to raise sea level several meters, is better approximated as exponential than by a more linear response. Doubling times of 10, 20 or 40 years yield multi-meter sea level rise in about 50, 100 or 200 years. Recent ice melt doubling times are near the lower end of the 10–40-year range, but the record is too short to confirm the nature of the response. The feedbacks, including subsurface ocean warming, help explain paleoclimate data and point to a dominant Southern Ocean role in controlling atmospheric CO2, which in turn exercised tight control on global temperature and sea level. The millennial (500–2000-year) timescale of deep-ocean ventilation affects the timescale for natural CO2 change and thus the timescale for paleo-global climate, ice sheet, and sea level changes, but this paleo-millennial timescale should not be misinterpreted as the timescale for ice sheet response to a rapid, large, human-made climate forcing. These climate feedbacks aid interpretation of events late in the prior interglacial, when sea level rose to +6–9 m with evidence of extreme storms while Earth was less than 1 °C warmer than today. Ice melt cooling of the North Atlantic and Southern oceans increases atmospheric temperature gradients, eddy kinetic energy and baroclinicity, thus driving more powerful storms. The modeling, paleoclimate evidence, and ongoing observations together imply that 2 °C global warming above the preindustrial level could be dangerous. Continued high fossil fuel emissions this century are predicted to yield (1) cooling of the Southern Ocean, especially in the Western Hemisphere; (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss; (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region; (4) increasingly powerful storms; and (5) nonlinearly growing sea level rise, reaching several meters over a timescale of 50–150 years. These predictions, especially the cooling in the Southern Ocean and North Atlantic with markedly reduced warming or even cooling in Europe, differ fundamentally from existing climate change assessments. We discuss observations and modeling studies needed to refute or clarify these assertions.
471 citations
TL;DR: In this paper, the main advancements of the Beijing Climate Center (BCC) climate system model from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to phase 6 (CMP6) are presented, in terms of physical parameterizations and model performance.
Abstract: . The main advancements of the Beijing Climate Center (BCC) climate system model from
phase 5 of the Coupled Model Intercomparison Project (CMIP5) to phase
6 (CMIP6) are presented, in terms of physical parameterizations and model
performance. BCC-CSM1.1 and BCC-CSM1.1m are the two models involved in CMIP5,
whereas BCC-CSM2-MR, BCC-CSM2-HR, and BCC-ESM1.0 are the three models configured for
CMIP6. Historical simulations from 1851 to 2014 from BCC-CSM2-MR (CMIP6) and
from 1851 to 2005 from BCC-CSM1.1m (CMIP5) are used for models assessment.
The evaluation matrices include the following: (a) the energy budget at
top-of-atmosphere; (b) surface air temperature, precipitation, and atmospheric circulation for
the global and East Asia regions; (c) the sea surface temperature (SST) in the
tropical Pacific; (d) sea-ice extent and thickness and Atlantic Meridional
Overturning Circulation (AMOC); and (e) climate variations at different timescales, such as the global warming trend in the 20th century, the stratospheric
quasi-biennial oscillation (QBO), the Madden–Julian Oscillation (MJO), and the diurnal
cycle of precipitation. Compared with BCC-CSM1.1m, BCC-CSM2-MR shows
significant improvements in many aspects including the tropospheric air
temperature and circulation at global and regional scales in East Asia and
climate variability at different timescales, such as the QBO, the MJO, the diurnal cycle
of precipitation, interannual variations of SST in the equatorial Pacific,
and the long-term trend of surface air temperature.
424 citations
01 Apr 2003
TL;DR: In this article, the authors applied residual mean theory to the streamwise-averaged Antarctic Circumpolar Current to obtain a concise description of the processes that set up its stratification and meridional overturning circulation on an f plane.
Abstract: Residual-mean theory is applied to the streamwise-averaged Antarctic Circumpolar Current to arrive at a concise description of the processes that set up its stratification and meridional overturning circulation on an f plane. Simple solutions are found in which transfer by geostrophic eddies colludes with applied winds and buoyancy fluxes to determine the depth and stratification of the thermocline and the pattern of associated (residual) meridional overturning circulation.
362 citations
TL;DR: In this article, the authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles, supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from mooring profiles, (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth loweredacoustic Doppler currentprofilers (LADCP) and CTD profiles.
Abstract: The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixingobtainedfrom(i)Thorpe-scaleoverturnsfrommooredprofilers,afinescaleparameterizationappliedto (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strainfromfull-depthloweredacousticDoppler currentprofilers (LADCP)andCTDprofiles. Verticalprofiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10 24 )m 2 s 21 and above 1000-m depth is O(10 25 )m 2 s 21 . The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variabilityin theratiobetweenlocal internalwavegeneration and local dissipation.Insomeregions,the depthintegrateddissipationrateiscomparabletotheestimatedpowerinputintothelocalinternalwavefield.Inafew cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However,atmostlocationsthetotalpowerlostthroughturbulentdissipationislessthantheinputintothelocal internal wave field. This suggests dissipation elsewhere, such as continental margins.
350 citations
Cites methods from "Closure of the Global Overturning C..."
...These values are roughly consistent with a variety of inverse models summarized in Wunsch and Ferrari (2004), and the meridional overturning circulation as a whole (Talley 2013)....
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TL;DR: This study shows that this rearrangement of deep water masses is dynamically linked to the expansion of summer sea ice around Antarctica, and may help quantify the ocean’s role in regulating atmospheric carbon dioxide on glacial–interglacial timescales.
Abstract: In the modern climate, the ocean below 2 km is mainly filled by waters sinking into the abyss around Antarctica and in the North Atlantic. Paleoproxies indicate that waters of North Atlantic origin were instead absent below 2 km at the Last Glacial Maximum, resulting in an expansion of the volume occupied by Antarctic origin waters. In this study we show that this rearrangement of deep water masses is dynamically linked to the expansion of summer sea ice around Antarctica. A simple theory further suggests that these deep waters only came to the surface under sea ice, which insulated them from atmospheric forcing, and were weakly mixed with overlying waters, thus being able to store carbon for long times. This unappreciated link between the expansion of sea ice and the appearance of a voluminous and insulated water mass may help quantify the ocean’s role in regulating atmospheric carbon dioxide on glacial–interglacial timescales. Previous studies pointed to many independent changes in ocean physics to account for the observed swings in atmospheric carbon dioxide. Here it is shown that many of these changes are dynamically linked and therefore must co-occur.
315 citations
Cites background from "Closure of the Global Overturning C..."
...Talley (30) gives an excellent review of our current understanding of the pathways of the overturning circulation in the global ocean based on estimates of the heat, freshwater, and nutrient transports....
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References
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TL;DR: Using the Levitus climatology, the authors showed that 2.1 TW (terawatts) is required to maintain the global abyssal density distribution against 30 Sverdrups of deep water formation.
1,958 citations
"Closure of the Global Overturning C..." refers result in this paper
...These diffusivity estimates based on basinscale transport estimates are remarkably similar to the Munk (1966) inferred value for the deep Pacific Ocean, which has held up with more modern budget studies (Munk and Wunsch, 1998; Wunsch and Ferrari, 2004)....
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TL;DR: The air-sea fluxes of momentum, heat, freshwater and their components have been computed globally from 1948 to 2006 at frequencies ranging from 6-hourly to monthly as mentioned in this paper.
Abstract: The air–sea fluxes of momentum, heat, freshwater and their components have been computed globally from 1948 at frequencies ranging from 6-hourly to monthly. All fluxes are computed over the 23 years from 1984 to 2006, but radiation prior to 1984 and precipitation before 1979 are given only as climatological mean annual cycles. The input data are based on NCEP reanalysis only for the near surface vector wind, temperature, specific humidity and density, and on a variety of satellite based radiation, sea surface temperature, sea-ice concentration and precipitation products. Some of these data are adjusted to agree in the mean with a variety of more reliable satellite and in situ measurements, that themselves are either too short a duration, or too regional in coverage. The major adjustments are a general increase in wind speed, decrease in humidity and reduction in tropical solar radiation. The climatological global mean air–sea heat and freshwater fluxes (1984–2006) then become 2 W/m2 and −0.1 mg/m2 per second, respectively, down from 30 W/m2 and 3.4 mg/m2 per second for the unaltered data. However, decadal means vary from 7.3 W/m2 (1977–1986) to −0.3 W/m2 (1997–2006). The spatial distributions of climatological fluxes display all the expected features. A comparison of zonally averaged wind stress components across ocean sub-basins reveals large differences between available products due both to winds and to the stress calculation. Regional comparisons of the heat and freshwater fluxes reveal an alarming range among alternatives; typically 40 W/m2 and 10 mg/m2 per second, respectively. The implied ocean heat transports are within the uncertainty of estimates from ocean observations in both the Atlantic and Indo-Pacific basins. They show about 2.4 PW of tropical heating, of which 80% is transported to the north, mostly in the Atlantic. There is similar good agreement in freshwater transport at many latitudes in both basins, but neither in the South Atlantic, nor at 35°N.
1,424 citations
"Closure of the Global Overturning C..." refers background in this paper
...(However, the heat fluxes discussed next suggest that the Agulhas water does not directly flow into the South Atlantic; it is first cooled by a large amount, most likely along the Agulhas Return Current, becomes SAMW/AAIW and then enters the South Atlantic via the “cold water” route of Rintoul (1991).)...
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...The large cooling of the surface waters occurs along the north side of the Agulhas Return Current (Large and Yeager, 2009; Cerovecki et al., 2011), which advect much of the Agulhas water southeastward towards Kerguelen....
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...Most of the Agulhas waters turn southeastward rather than entering the Atlantic, losing heat on the southeastward path along the Agulhas Return Current, before joining the SAMW and ultimately entering the Atlantic as cooler SAMW and AAIW rather than warm Agulhas waters....
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...circulation are not explored here, including the potential for an export from the South Atlantic of low density NADW directly to the upper overturning cell in the Southern Ocean, which emerges from Lumpkin and Speer’s (2007) transport analysis and is featured in Marshall and Speer (2012); this light NADW export is much weaker in the transport analysis herein; this likely reflects uncertainty due to differences in approaches to the initial velocity analysis and possibly differences in the results using two different sections (32°S herein, vs....
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...Air-sea heat gain in the Southern Ocean, invoked in Lumpkin and Speer (2007) and Marshall and Speer (2012), while important for return of upwelled deep waters to the subtropical thermocline, is only part the required heating that must begin with warming of bottom waters, based on heat budgets shown in Section 5 below, and consistent with the best estimates of Southern Ocean air-sea heat flux (Large and Yeager, 2009; Cerovecki et al., 2011)....
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TL;DR: In particular, small-scale mixing processes are necessary to resupply the potential energy removed in the interior by the overturning and eddy-generating process as discussed by the authors, and it is shown that over most of the ocean significant vertical mixing is confined to topographically complex boundary areas implies a potentially radically different interior circulation than is possible with uniform mixing.
Abstract: ▪ AbstractThe coexistence in the deep ocean of a finite, stable stratification, a strong meridional overturning circulation, and mesoscale eddies raises complex questions concerning the circulation energetics. In particular, small-scale mixing processes are necessary to resupply the potential energy removed in the interior by the overturning and eddy-generating process. A number of lines of evidence, none complete, suggest that the oceanic general circulation, far from being a heat engine, is almost wholly governed by the forcing of the wind field and secondarily by deep water tides. In detail however, the budget of mechanical energy input into the ocean is poorly constrained. The now inescapable conclusion that over most of the ocean significant “vertical” mixing is confined to topographically complex boundary areas implies a potentially radically different interior circulation than is possible with uniform mixing. Whether ocean circulation models, either simple box or full numerical ones, neither explic...
1,356 citations
"Closure of the Global Overturning C..." refers result in this paper
...These diffusivity estimates based on basinscale transport estimates are remarkably similar to the Munk (1966) inferred value for the deep Pacific Ocean, which has held up with more modern budget studies (Munk and Wunsch, 1998; Wunsch and Ferrari, 2004)....
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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
"Closure of the Global Overturning C..." refers background or result in this paper
..., 2011) and owe a great deal to previous work, particularly Gordon (1991), Schmitz (1995, 1996), and Lumpkin and Speer (2007). The GOC pathways in the global map of Figure 1, based on Talley et al....
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...For many decades the dominant paradigm of the global overturning circulation was of two nearly independent cells: the popularized North Atlantic Deep Water (NADW) “great ocean conveyor”, with the formation of NADW in the northern North Atlantic returned by upwelling in the Indian and Pacific Oceans (Gordon, 1986a; Broecker, 1987); and a second cell associated with Antarctic Bottom Water (AABW) formation in the south (Gordon, 1986b, 1991; Broecker, 1991; Schmitz, 1995)....
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... 12 we would draw these pole-to-pole cells (Gordon, 1986b)....
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..., 2011) and owe a great deal to previous work, particularly Gordon (1991), Schmitz (1995, 1996), and Lumpkin and Speer (2007). The GOC pathways in the global map of Figure 1, based on Talley et al. (2011), are similar to those of Marshall and Speer (2012), illustrating convergence in thinking about the GOC. The pathways are associated in Section 5 with quantitative transports and energy balances from Talley (2008)....
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...[Both authors also separately described the AABW global cell at about the same time but did not clarify the connection between the NADW and AABW global cells (Gordon, 1986b; Broecker, 1991; Gordon, 1991 as reproduced in Richardson, 2008).]...
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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