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Publication Title | Cooling and Ventilating the Abyssal Ocean

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Cooling and Ventilating the Abyssal Ocean

Alejandro H. Orsi

Department of Oceanography, Texas A&M University, College Station, Texas

Stanley S. Jacobs, Arnold L. Gordon, and Martin Visbeck

Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York

Abstract. The abyssal ocean is filled with cold, dense waters that sink along the Antarctic continental slope and overflow sills that lie south of the Nordic Seas. Recent inte- grations of chlorofluorocarbon-11 (CFC) measurements are similar in Antarctic Bottom Water (AABW) and in lower North Atlantic Deep Water (NADW), but Antarctic inputs are ≈ 2◦C colder than their northern counterparts. This indicates comparable ventilation rates from both polar re- gions, and accounts for the Southern Ocean dominance over abyssal cooling. The decadal CFC-based estimates of recent ventilation are consistent with other hydrographic observa- tions and with longer-term radiocarbon data, but not with hypotheses of a 20th-century slowdown in the rate of AABW formation. Significant variability is not precluded by the available ocean measurements, however, and interannual to decadal changes are increasingly evident at high latitudes.

1. Introduction

It has long been known that the deepest waters of the abyssal ocean originate in the polar regions. The first deep ocean measurement, made in 1751 at 1630 m in the subtropical North Atlantic [Ellis, 1751], was explained as a manifestation of ocean convection, with cold polar sur- face waters sinking to the ocean floor and spreading equa- torward, forcing warm surface waters poleward [Rumford, 1800]. Sparse temperature observations and the distribu- tion of ocean basins and bathymetry led to the contention that the southern source of bottom currents was stronger and more influencial in spatial extent than the northern source [Carpenter et al., 1869; Buchan, 1895]. Since that time, many thousands of measurements have produced a more complete picture of the deep ocean thermal structure [Levitus and Boyer, 1994] (Fig. 1). But does this figure in- dicate that southern inflows are colder, with readier access to the abyss [Mantyla and Reid, 1983] or that overturning is more rapid there? Can it be reconciled with the recent hypothesis that a 20th-century slowdown may have occurred in deep water production in the Southern Ocean [Broecker et al, 1999]? To address these questions we have analyzed complementary observations from well-sited long-term cur- rent meter arrays, and from the time-based tracers chlo- rofluorocarbon (CFC) and radiocarbon (14C). These results provide estimates of the renewal rates for that 40% of the global ocean deeper than 2500 m, the average temperature of

Copyright 2001 by the American Geophysical Union.

Paper number 2001GL012830. 0094-8276/01/2001GL012830$05.00

which is 1.36◦C. This constitutes our ‘abyssal ocean’, defined to exclude the more variable and shallower Labrador Sea and Antarctic Intermediate Waters.

2. Hydrographic and Geochemical Observations

Estimation of the contribution of northern overflows to the abyssal ocean has been facilitated by a series of direct current measurements made between 1973 and 1993 [Saun- ders, 1990; Ross, 1984; Meincke, 1983]. The total transport from the Nordic Seas across 400-900 m sills in the Greenland- Scotland Ridge is 5.8 Sv (1 Sv = 106 m3 s−1) at θ ≈ -0.5◦C to +0.4◦C. These overflows entrain warmer waters and enter the abyssal ocean at a combined rate of 8.6 Sv and with tem- peratures between +0.5◦ C and +1.0◦ C [Dickson and Brown, 1994]. Southern injections into the abyssal ocean have been more difficult to measure directly because the sites are re- mote, frequently covered by sea ice, and distributed along a Slope Front that extends more than 18,000 km around Antarctica [Whitworth et al., 1998]. In addition, spatial variability of thermohaline properties and a variety of bot- tom water definitions have led to different ideas regarding the relative strengths of source regions [e.g., Carmack, 1977; Rintoul, 1998]. Most process and monitoring studies have been carried out in the Weddell Sea, where recent work has confirmed earlier calculations of 3-5 Sv of ‘Weddell Sea Bot- tom Water’ production [Gordon, 1998]. Hydrographic es- timates for generation of the more inclusive AABW have typically ranged from 5-15 Sv [e.g., Carmack, 1977; Jacobs et al., 1985], but are not well constrained.

Geochemical measurements of transient tracers provide important temporal information from which ventilation rates can be derived. For the northern hemisphere, CFC-based analyses are broadly consistent with the physical measure- ments, showing an average overflow of 4.8 Sv from the Nordic Seas between 1970 and 1990, subsequently rising by entrainment to 7.6 Sv of lower NADW entering the abyssal North Atlantic [Smethie and Fine, 2001] (Fig. 2). In the south, integrated CFC measurements below the upper boundary (28.27 kg m3) of AABW indicate 8.1 Sv sinking across the 2500-m isobath on the Antarctic continental rise [Orsi et al., 1999]. About 60% of this volume enters the At- lantic sector of the Southern Ocean at θ ≈ -1.2◦C, and the remainder in the Indian and Pacific sectors at θ ≈ -0.8◦C.

The comparable volume and colder temperatures of the southern sources means that overturning in the Southern Ocean must dominate abyssal cooling (Fig. 1). To mantain the abyssal ocean at a temperature near 1.36◦C, injection of cold deep and bottom waters constitutes an important


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