Elsevier

Progress in Oceanography

Volume 43, Issue 1, January 1999, Pages 55-109
Progress in Oceanography

Circulation, mixing, and production of Antarctic Bottom Water

https://doi.org/10.1016/S0079-6611(99)00004-XGet rights and content

Abstract

The Antarctic source of bottom water to the abyssal layer of the World Ocean is examined, as well as its large-scale flow pattern and ultimate entrainment rate into the deep water above. We make use of the available high-quality station data in the Southern Ocean to construct bottom maps of neutral density and mean property maps, including Chlorofluorocarbon (CFC), for the abyssal layer underneath a selected neutral density surface. The maximum density at the sill depth of Drake Passage is used to distinguish between the voluminous deep water mass that is a continuous component of the Antarctic Circumpolar Current from the relatively denser bottom water originated along the Antarctic continental margins. Based on water density, Antarctic Bottom Water (AABW) is defined here generically to include all volumes of non-circumpolar water of Antarctic origin. Over the shelf regime multiple localized sources of specific AABW types contribute to the abyssal layer of the adjacent Antarctic basins. Characteristics of these dense bottom waters reflect closely those observed on the parent Shelf Water mass. Spreading paths of newly-formed deep and bottom waters over the slope regime, and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the AABW layer. Interior mixing and interbasin exchanges of AABW are deduced from mean characteristic curves following the southern streamline of the Antarctic Circumpolar Current. Outflow and mixing of AABW from the Weddell Sea to the Argentine Basin is depicted using density and CFC distributions of two zonal hydrographic lines. Recirculation and mixing of deep and bottom waters within the Weddell Gyre are also detailed using a meridional section along the Greenwich Meridian. The strength of all localized sources of AABW combined is estimated by two independent approaches. An estimate of the total production rate of AABW is calculated based on the oceanic CFC budget for the AABW layer offshore of the 2500-m isobath. The sum of all downslope inputs of well-ventilated bottom water types underneath the top isopycnal must account for the measured CFC content in the bottom layer. The resulting total AABW production rate is about 8 Sv, which is a conservative figure that neglects the loss of CFC-bearing waters across the top isopycnal in recent years, whereas about 9.5 Sv is calculated assuming a well-mixed bottom layer. Making use of their transient nature, CFC distributions at the top of the AABW layer indicate that more direct and rapid entrainment of CFC-rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing. A second, more ad-hoc but independent oceanic mass budget of the bottom layer is also constructed. A typical basin-wide rate of deep upwelling of 3×10−7 m s−1 requires 10 Sv (1 Sv=106 m3 s−1) of newly-formed AABW to sink down the slope around Antarctica. We have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the AABW. It is expressed as a combination of wind, topographic, and turbulent components, which in turn are functions of the isopycnal depth, bottom depth, and bottom layer thickness. This non-uniform upwelling field yields about 12 Sv of AABW exported across the top isopycnal. Fortuitously, the overall average of upward speed at the top isopycnal (3.7×10−7 m s−1) compares well with previous estimates of deep upwelling in the northern basins. A series of likely sites for strong vertical entrainment of AABW are clearly identified in the modeled distribution of deep upwelling, consonant with the observed CFC distributions on the top isopycnal. Altogether, regions with relatively high upwelling rates (>5×10−7 m s−1) occupy only a quarter of the total areal extent of the top isopycnal, but they account for as much as 45% of the total vertical transport.

Introduction

At a few locations around the Antarctic continental margins and in the northern North Atlantic, upper layer waters lose sufficient buoyancy from air–sea and sea–ice exchanges to sink to abyssal depths. These bottom waters move away from the sinking regions in narrow western boundary currents in all three oceans (Warren, 1981a). Thought to be supplied by abyssal boundary current systems are weaker poleward recirculations within the interior of the basins, where deep waters slowly upwell into the upper levels to balance the downward heat flux from the low latitude atmosphere. Relatively warm waters in the upper layer are advected poleward and begin the cycle anew (Stommel, 1958; Stommel & Arons, 1960aStommel & Arons, 1960b; Schmitz, 1995). Such a pattern of buoyancy forcing and meridional flow is known as the Global Thermohaline Circulation, a key component in the regulation of the Earth's climate.

The major role played by the sinking and spreading of southern bottom water was clear in the early observations. Wüst (1933) noticed that most of the abyssal layer in the Atlantic Ocean is filled from the south, by a water mass with origin in the high southern latitudes and with characteristics significantly different (lower temperature and salinity) from the northern bottom water. The two bottom waters first meet at low latitudes, where the less dense northern water is displaced off the bottom by the relatively colder southern bottom water flowing to the north. The northern water mass continues southward at mid depths, where it is called the North Atlantic Deep Water (NADW), eventually to enter the Antarctic Circumpolar Current (ACC) in the southwestern Argentine Basin. From there NADW is carried eastward around Antarctica, mixing along its path with less dense deep waters imported from the Indian and Pacific oceans and also with the colder southern bottom water beneath it. Sverdrup (1940) differentiated this uniform, voluminous mixing product, which is recirculating within the ACC, from its parent water masses calling it the Circumpolar Deep Water (CDW). (Uniformity non withstanding, the NADW influence to the CDW is recognizable everywhere in the ACC by its characteristic deep salinity maximum; Reid & Lynn, 1971.) In turn, the subpolar circulations located between Antarctica and the ACC gain CDW from that current and carry it toward the continental margins. At a few sites these diluted derivatives of North Atlantic Deep Water mix with near-freezing Shelf Water to form the coldest southern bottom water (Reid, 1979).

Bottom waters with a southern origin that are colder than the northern source bottom water, say with temperatures below 2°C, have been referred to generically as `Antarctic Bottom Water' for quite a long time (Deacon, 1933; Speer & Zenk, 1993). However, as Mantyla and Reid (1983) point out, the bulk of bottom water filling the basins north of the Southern Ocean is not drawn directly from the Antarctic margins. Indeed most of the volume carried equatorward by all of the abyssal Western Boundary Currents in the Southern Hemisphere is exported from deep levels of the ACC, water lying above denser southern bottom water and below the salinity maximum. The last represents a vast source distributed on a circumpolar band located, in most places, far from the Antarctica margins. Thus, at least in the Atlantic, there are two distinct components in the `southern bottom water': the Circumpolar Deep Water exported from the ACC, and the relatively denser deep waters exported from the subpolar circulations located in the Antarctic basins to the south of the ACC (Mantyla & Reid, 1983, Gordon, 1972; Schmitz, 1995). Gordon (1971), Gordon (1972) reserved the term Antarctic Bottom Water for the latter component, to only include deep waters in the Southern Ocean with potential temperatures less than 0°C. In the Argentine Basin, Reid, Nowlin and Patzert (1977) further distinguished between deep waters entering from the Southeast Pacific Ocean and those from the Weddell Sea by means of their density.

This is a global study aimed at the characterization of Antarctic Bottom Water, describing its general circulation in the Southern Ocean, and assessing its replenishment rate over the Antarctic continental margins. It is based on classical hydrographic data, derived primarily from the National Oceanic Data Center (NODC, 1994). Additional hydrographic data, and all of the Chlorofluorocarbon (CFC) data, were obtained directly from the originators. Addition of data from a series of cruises of the World Ocean Circulation Experiment (WOCE) listed in the Acknowledgments were specially useful. CFC measurements in the Indian and Pacific correspond to that Experiment (1990–1996), whereas those in the Atlantic are mostly pre-WOCE (1984–1989) data. Transient tracer data were individually checked for errors, whereas bottle and low-resolution CTD data were more objectively screened for anomalous profiles based on their temperature–salinity relationship.

The first objective of this work is to present a robust, global definition of Antarctic Bottom Water to differentiate unambiguously dense water produced around Antarctica from Circumpolar Deep Water of the Antarctic Circumpolar Current. Similarly, the densest water of the ACC is distinguished from the northern bottom water. Those tasks are addressed in Section 2, based on the observed densities at the bottom of the World Ocean using a new practical method to approximate isentropic surfaces in the ocean (Jackett & MacDougall, 1997). We also present evidence of the intense mixing between deep and bottom waters taking place over the sill of Drake Passage.

With Antarctic Bottom Water clearly defined, we proceed to describe its regional characteristics and circulation in all three sectors of the Southern Ocean. This work is accomplished in Section 3, by inspection of lateral property distributions on the bottom layer occupied by the Antarctic Bottom Water. In constructing these maps we have made use of all available high quality hydrographic data from south of 30°S (Fig. 1(a)). Changes in the mean temperature–salinity relationship of deep and bottom waters circulating through the southern Indian sector of the ACC are examined to describe exchanges of Antarctic Bottom Water produced in different Antarctic Basins.

In Section 4, Chlorofluorocarbon distributions on selected hydrographic lines are used to describe the ventilation of deep and bottom waters in the South Atlantic by Antarctic Bottom Water exported from the Weddell Sea. One section is used to trace the northward outflow of Weddell Sea waters along the southern Scotia Sea and South Sandwich Trench system, and another to depict its circulation in the southern Argentine Basin. The eastward export of Weddell Sea waters to the Enderby Basin and the westward inflow of new Antarctic Bottom Water originating in the Amery Basin, are investigated along the Greenwich Meridian.

The final objective of this study, covered in Section 5, is to provide an estimate for the strength of the southern sources of bottom water in the global meridional circulation. One independent estimate of the global production rate of Antarctic Bottom Water is obtained from the CFC inventory for that layer. Previous estimates of the total production rate of Antarctic Bottom Water are compared with those required to balance the volume flux for the oceanic bottom layer from an ad-hoc mixing model. We have organized the presentation so that quantitative results (Section 5) follow the qualitative descriptions (2 World ocean bottom waters, 3 Southern Ocean circulation, 4 South Atlantic ventilation) of Antarctic Bottom Water.

Section snippets

World ocean bottom waters

Paths followed by the densest waters involved in the Global Thermohaline Circulation are clearly identified on property maps near the bottom of the World Ocean (Wüst, 1935; Lynn & Reid, 1968; Mantyla & Reid, 1983, Mantyla & Reid, 1995). The bottom distribution shown in Fig. 2 is of neutral density (γn), which is particularly useful while tracing water masses that undergo extreme pressure changes. In contrast to the isopycnal approach based on matching various potential density surfaces with a

Southern Ocean circulation

Next is the first global description of the AABW circulation based on the topography of γn=28.27 kg m−3 (Fig. 6), the bottom layer depth-averaged CFC-11 distribution (Fig. 7(a)), and similar maps of AABW potential temperature, salinity and thickness (Fig. 7(b–d)). A schematic of the inferred flow pattern of different density constituents within the AABW layer is presented in Fig. 7(e). Fields shown in Fig. 6, Fig. 7(a–d) were used in the computation of the AABW volumetric characteristics of

South Atlantic ventilation

All of the measured CFC concentrations in the ocean interior are ultimately derived from the atmosphere; the bulk of it via sea-surface gas exchange. The atmospheric signal is imprinted where isopycnals outcrop and thence carried into the ocean interior, where it can be used to trace the pathways followed by water masses and to estimate their ventilation rates (Warner & Weiss, 1992; Doney & Bullister, 1992). CFC concentrations are usually highest in near-surface waters so vertical diffusion can

Antarctic Bottom Water production

Previous estimates of the rates at which new bottom waters are formed around Antarctica resulted from various indirect approaches, or a combination of them; but they all strongly depended on the spatial scales analyzed and on the assumed mixing recipes. One approach relied on property distributions and computed volume transports at the western boundary current of the Weddell Gyre, usually partitioned into newly-formed bottom water sinking toward the bottom of the slope and older, less-dense

Summary

By exporting bottom waters the Southern Ocean ventilates the vast majority of the deep waters in the rest of the World Ocean. Hence, the strength of the southern source of cold bottom waters and their subsequent equatorward flow are key elements of the Global Thermohaline Circulation. It is clear that deep waters branching off the Antarctic Circumpolar Current supply all of the observed Abyssal Boundary Currents, filling the bottom layers over most of the Southern Hemisphere.

The Drake Passage

Acknowledgements

Support was provided by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA37RJ0198, and by grants to the Pacific Marine Environmental Laboratory (PMEL) from the Climate and Global Change Program through the NOAA Office of Global Programs. This paper is JISAO Contribution Number 474 and PMEL Contribution Number 1869. Construction of the global distributions presented in this work was only possible due to the hard work and generous

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    Department of Oceanography, Texas A&M University, College Station, TX 77843-3146, USA. E-mail: [email protected]

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