Sayles FL and Dickinson WH (1991) The ROLAI2DLander: a benthic lander for the study of exchangeacross the sediment}water interface. Deep-SeaResearch 38: 505}529.

Smith KL, Jr, Glatts RC, Baldwin RJ et al. (1997) Anautonomous bottom-transecting vehicle for makinglong time-series measurements of sediment community

oxygen consumption to abyssal depths. LimnologicalOceanography 42: 1601}1612.

Tenberg A, De Bovee F, Hall P et al. (1995)Benthic chamber and proRling landers in oceano-graphy. A review of design, technical solutionsand functioning. Progress in Oceanography 35: 253}294.

BOTTOM WATER FORMATION

A. L. Gordon, Columbia University, Palisades,NY, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0006

Introduction

Meridional sections of temperature and salinitythrough the PaciRc and Atlantic Oceans (Figure 1)reveal that in the PaciRc below 2000m, more thanhalf of the ocean depth, the water is colder than23C. The Atlantic is somewhat warmer, but theretoo the lower 1000 m of the ocean is well below23C. Only within the surface layer, generally lessthan the upper 500 m of the ocean is the waterwarmer than 103C, amounting to only 10% of thetotal ocean volume. The coldness of the deep oceanis due to interaction of the ocean with the polaratmosphere. There, surface water reaches thefreezing point of sea water. Streams of very coldwater can be traced spreading primarily from theAntarctic along the sea Soor, warming en route bymixing with overlying water, into the world’soceans (Figure 2).

The coldest bottom water, Antarctic BottomWater (AABW), is derived from the shores of Ant-arctica. There, freezing point, high oxygen concen-tration, water is produced during the winter overthe continental shelf. At a few sites the shelf watersalinity is sufRciently high, greater than 34.61�,that, on cooling to the freezing point, the surfacewater density is sufRciently high to allow it to sinkto great depths of the ocean. As the shelf waterdescends over the continental slope into the deepocean it mixes with adjacent deep water, but thiswater is also quite cold so the Rnal product arrivingat the seaSoor at the foot of Antarctica is about!1.03C. DeRnitions used by different authors vary,but generally AABW is deRned as having a potentialtemperature (the temperature corrected for adiabaticheating due to hydrostatic pressure) less than 03C.

AABW spreads into the lower 1000 m of the worldocean, where it cools and renews oxygen concentra-tions drawn down by oxidation of organic materialwithin the deep ocean. AABW is said to ventilatethe deep ocean.

In the Atlantic Ocean the 23C isotherm marks thebase of a wedge of relatively salty water, associatedwith high dissolved oxygen and low silicate concen-trations (see Figure 1). This water mass is calledNorth Atlantic Deep Water (NADW). The densestcomponent of NADW is formed as cold surfacewaters during the winter in the Greenland and Nor-wegian Seas. This water sinks to Rll the basin northof a ridge spanning the distance from Greenland toScotland. Excess cold water overSows the ridgecrest, mixing on descent with warmer more salinewater, producing a bottom water product of about#1.03C. The overSow water stays in contact withthe sea Soor to near 403N in the Atlantic Ocean,where on spreading southward it is lifted over theremnants of denser AABW.

Export of Greenland and Norwegian Sea bottomwater has been estimated from a series of currentmeasurements. Transports of about 2�106 m3 s�1 ofnear 0.43C water occur between the Faroe Bank andScotland, 1�106 m3 s�1 of similar water passesthrough notches between Iceland and Faroe Bank,and 3�106 m3 s�1 of near 03C is exported throughthe Denmark Strait, between Greenland and Ice-land. The overSow plumes rapidly entrain warmerwaters, producing bottom water of near #1.03C.With entrainment of other deep water, a productionrate of about 8�106 m3 s�1 of overSow water islikely. Less dense components of NADW, that donot contact the seaSoor are formed in the LabradorSea and Mediterranean Sea. The total production ofNADW is estimated as 15�106 m3 s�1.

As the Antarctic is the primary source of the coldbottom waters of the world ocean, Antarctic Bot-tom Water is discussed in this article. See NorthAtlantic Deep Water for further information on thatNorthern Hemisphere deep water mass.

334 BOTTOM WATER FORMATION

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Figure 1 Meridional sections of temperature and salinity through the Pacific and Atlantic Oceans. Antarctica is at the center of thefigure.

Formation of Antarctic Bottom Water

Antarctic Bottom Water is formed at a few sitesalong the continental margin of Antarctica duringthe winter months (Figure 2). The shelf dense waterslips across the shelf break to descend to the deepocean, perhaps within the conRnes of incised can-yons on the continental slope. The shelf water ismade dense (Figure 3) by the very cold winds fromAntarctica, which spur the formation of sea ice.Normally sea ice acts to insulate the ocean fromfurther heat loss and thus attenuates the continuedformation of sea ice. But along the shores of Antarc-tica sea ice is blown northward by the strong windsdescending over the cold glacial ice sheet of Antarc-tica. The removal of the sea ice exposes the oceanwater to the full blast of the cold air, formingcoastal polynyas (persistent bands of ice-free ocean

adjacent to Antarctica; Figure 4). Production andremoval of yet more sea ice continues within thecoastal polynyas, which act as ‘sea ice factories’. Assea ice has a lower salinity than the sea water fromwhich it formed, approximately 5� versus 34.5�of the sea water, salt rejected during ice formation,concentrates in the remaining freezing point seawater making it saltier, and hence denser. The expo-sure of the ocean to the atmosphere also raises thedissolved oxygen concentration within the shelfwater. As the ability of sea water to hold oxygenincreases with lowering temperature, the oxygenconcentration of the shelf water is very high, about8 ml l�l.

The shelf water at some sites, such as the Weddelland Ross seas, is made even colder on contactwith Soating ice shelves that are composed of glacial(freshwater) ice. Ocean contact with glacial ice

BOTTOM WATER FORMATION 335

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Figure 2 Bottom potential temperature along the seafloor, for oceanic areas deeper than 4000m. The four symbols along thecoast of Antarctica mark the places where Antarctic Bottom water forms.

occurs not only at the northern face of the ice sheet,but also at hundreds of meters depth along the basesof Soating ice shelves. As the freezing point of seawater is lowered with increasing pressure (!0.073Cper 100 m of depth), the shelf water in contact withthe base of the ice shelves often at a depth of manyhundreds of meters, attains temperatures well below!2.03C. The cooling of shelf water in contact withthe glacial ice is linked to melting of the glacial ice,hence this very cold water is slightly less saline thanthe remaining shelf water. Ocean}glacial ice interac-tion is believed to be a major factor in controllingAntarctica’s glacial ice mass balance and stability.The resultant water, called ice shelf water, can beidentiRed as streams within the main mass of shelfwater by its low potential temperatures. Ice shelfwater with potential temperatures as low as!2.23C have been measured. This very cold watermay act to encourage formation of AABW, as theseawater compressibility increases with loweredtemperature. Thus as the shelf water begins its de-scent to the deep ocean, compressibility of the iceshelf water induces water of greater density, whichaccelerates the descent, limiting mixing with adjac-ent water.

As shelf water escapes the shelf environs, offshorewater must Sow onto the shelf to compensate forthe shelf water loss. The offshore water is warmerthan the dense shelf water, with temperatures closerto #1.03C. Once on the shelf this water cools torenew the reservoir of freezing point shelf water.The onshore Sow is drawn from deep water of theworld ocean that slowly Sows southward and up-

ward, crossing the Antarctic Circumpolar Current.It may be viewed as ‘old’ AABW returning from itsnorthern sojourn. In this way the overturning ther-mohaline circulation cell forced by AABW forma-tion is closed; the escape of very cold shelf water,spreading northward, mixing en route with warmeroverlying water, eventually results in upwelling toreturn to the Antarctic. The whole process takessome hundreds of years. Only now can the chloro-Suorocarbons (CFC) added to the ocean surfacelayer in the last 70 or so years, be detected reachingalong the seaSoor into the midlatitudes of theSouthern Hemisphere.

Formation Rate of Antarctic BottomWater

Measurement of the formation and escape of Ant-arctic shelf water and the subsequent formation ofAntarctic Bottom Water is difRcult because theformation regions are geographically remote,covered by sea ice year-round, and distributed alonga shelf break frontal region that extends more than18 000 km around Antarctica. In addition, the ther-mohaline properties of source waters vary spatially,which can lead to quite different ideas regarding themixing ‘recipes’ and speciRc processes leading to theultimate cold products. It can be argued that forevery 1�106 m3 of shelf water escaping 2�106 m3

of AABW is formed.The best observed bottom water formation is in

the Weddell Sea (the extreme southern Atlantic

336 BOTTOM WATER FORMATION

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Figure 3 Schematic of circulation and water masses in the Antarctic continental shelf.

Ocean). There, deep-reaching convective plumes ofvery cold surface water descend over the continentalslope into the deep ocean (Figure 5), producingWeddell Sea Bottom Water. This is a particularlycold form of AABW, having initial potential temper-atures at the seaSoor of !1.53C. The transport ofbottom water of less than !0.73C emanating fromthe Weddell Sea is estimated to be 2}5�106 m3 s�1,presumably drawing from a shelf water Sux of1}3�106 m3 s�1. Because of its coldness WeddellBottom Water has a major effect on the bottomwater properties of the World Ocean, particularlywithin the Atlantic Ocean which has the coldestbottom water.

Circumpolar estimates for the formation rateof AABW, generally deRned as having a potentialtemperature of less than 03C, are in the range of10}15�106 m3 s�1, but such values are notwell constrained by the sparse observations. Basedon CFC measurements a Rrm estimate of9.4�106 m3 s�1 has been made for the circum-polar production of AABW colder than !1.03C

descending to depths greater than 2500m.This value is similar to estimates of descendingNADW from the Greenland and Norwegian SeasoverSow.

It is not clear what controls the rate of shelf waterexport from the continental shelf. At the edge of theshelf is a strong ocean front (Figure 5). Movementof this front may be associated with escape of denseshelf water. The presence of canyons incised into thecontinental slope may also act as paths for descentto the deep ocean.

Deep Convection Within theSouthern OceanIn addition to descending dense water plumes overthe continental slope of Antarctica, deep reachingconvection over the deep ocean may occur. In win-ter, a thin veneer of sea ice stretches from Antarcticanorthward, reaching half the distance to the Ant-arctic Circumpolar Current. A delicate balance isachieved between the cold atmosphere and upward

BOTTOM WATER FORMATION 337

Figure 4 Satellite image of a coastal polynya shown as the dark region extending from the ice-covered shore and sea ice on theleft. (See also Polynyas.)

Sux of oceanic heat into the atmosphere, resultingin the formation of approximately 0.6 m of sea ice.In this balance cold, low-salinity water sits stablyover a warmer, more saline deep water layer.

During the austral winters of 1974 to 1976, nearthe Greenwich Meridian and 663S in the vicinity ofa seamount called Maud Rise, the ice displayedstrange behavior, which has not been repeated since.A large region normally covered by sea ice in winterremained ice free throughout the winter, though itwas surrounded by sea ice. This remarkable anom-aly, is referred to as the Weddell Polynya. Thougha full Weddell Polynya has not been observed sincethe mid-1970s, short lived polynyas (lasting roughlyone week) are frequently observed by satelliteimaging in the Maud Rise region. During theWeddell Polynya episode the normal stratiRcationwas disturbed as cold surface water convected todepths of 3000m. It is estimated that during thethree winters of the persistent Weddell Polynya,1}3�106 m3 s�1 of surface water entered thedeep ocean, as a form of AABW. The WeddellPolynya convection may represent another mode ofoperation of Southern Ocean processes, one inwhich surface water sinks into the deep ocean atsites away from the continental margins, in whatoceanographers refer to as open ocean convection,

in contrast to the continental slope plume convec-tion discussed above.

It is not clear what triggered the Weddell Polynyaof the mid-1970s. It is clear that if enough deepwater can be brought to the sea surface to melt allof the ice cover, then further upwelled deep water,not to be diluted by ice melt, would produce coldsaline water that can then sink back into the deepocean. What would cause enhanced upwelling ofdeep water? This is not clear, though interaction ofocean circulation with the Maud rise is suspected toplay a key role.

Conclusion

Bottom water of the western Weddell Sea in theearly 1990s is colder and fresher than observed inprevious decades. The same is true for the SouthernOcean south of Australia. These observations sug-gest an increased role of the low salinity, ice shelfwater in recent decades. However, the period overwhich observations have been taken within thehostile Southern Ocean, is not long enough to placemuch importance on this trend.

The presence of sea ice and the rather small spa-tial and temporal scales associated with the convec-tive plumes, makes AABW formation processes very

338 BOTTOM WATER FORMATION

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Figure 5 Potential temperature (A,C) and salinity (B,D) along a section at 67340�S in the western Weddell Sea, showing the variedstratification over the continental shelf and slope. The sea floor is stippled. The values inserted along the seafloor are the bottomtemperature and salinity within the descending plume of dense water. The sharp change in water properties on the upper 500 m at563W marks the shelf/slope front. (Reproduced from Gordon AL (1998) Western Weddell Sea thermohaline stratification. In: JacobsSS and Weiss R (eds) Ocean, Ice and Atmosphere, Antarctic Research Series, vol. 75, pp. 215}240. Washington, DC: AmericanGeophysical Union.)

BOTTOM WATER FORMATION 339

1 Clockwise in the northern hemisphere and counter-clockwisein the southern hemisphere.

difRcult to observe and model. Advancement ofAABW research represents a signiRcant technolo-gical challenge to Reld and computer oceanogra-phers.

Summary

The ocean is cold. Its average temperature of 3.53Cis far colder than the warm veneer capping much ofthe ocean. Waters warmer than 103C amount toonly 10% of the total ocean volume; about 75% ofthe ocean is colder than 43C. Along the seaSoor theocean temperature is near 03C. The cold bottomwater is derived from Southern Ocean, the oceanbelt surrounding Antarctica. Sea water at itsfreezing point of !1.93C is formed in winter overthe continental shelf of Antarctica. Where the saltcontent of shelf water is high enough, roughly34.61� (parts per thousand) the water is sufR-ciently dense to descend as convective plumes overthe continental slope into the adjacent deep ocean.In so doing Antarctic Bottom Water is formed. Itis estimated that on average between 10 and15�106 m3 of Antarctic bottom water forms everysecond! Antarctic Bottom Water spreads away fromAntarctica into the world oceans, chilling the deepocean to temperatures near 03C. Bottom waterwarms en route on mixing with warmer overlyingwaters. Cold winter water also forms in the Green-land and Norwegian Seas of the northern NorthAtlantic. This water ponds up behind a submarineridge spanning the distance from Greenland to Scot-land. This water overSows the ridge crest into theocean to the south. As the overSow water mixeswith warmer saltier water during descent into thedeep ocean, it results in a warmer, more salinewater mass than Antarctic Bottom water. TheGreenland and Norwegian Sea overSow waterforms the densest component of the water masscalled North Atlantic Deep Water and is estimatedto form at a rate of around 8�106 m3 s�1. The

overSow water stays in contact with the seaSooruntil the northern fringes of Antarctic Bottom Waterencountered in the North Atlantic near 403N, lifts itto shallower levels.

See also

Antarctic Circumpolar Current. Polynyas. RotatingGravity Currents. Sub Ice-shelf Circulation andProcesses. Weddell Sea Circulation.

Further ReadingFahrbach E, Rohardt G, Scheele N et al. (1995) Forma-

tion and discharge of deep and bottom water in thenorthwestern Weddell Sea. Journal of Marine Research53(4): 515}538.

Foster TD and Carmack EC (1976) Frontal zone mixingand Antarctic Bottom Water formation in the southernWeddell Sea. Deep-Sea Research 23: 301}317.

Gordon AL and Tchernia P (1972) Waters offAdelie Coast. Antarctic Research Series, vol. 19,pp. 59}69, Washington, DC: American GeophysicalUnion.

Jacobs SS, Fairbanks R and Horibe Y (1985) Origin andevolution of water masses near the Antarctic continen-tal margin: Evidence from H2

18O/H216O ratio in sea-

water. In: Jacobs SS (ed.) Oceanography of AntarcticContinental Margin, Antarctic Research Series vol. 43,pp. 59}85. Washington, DC: American GeophysicalUnion.

Jacobs SS and Weiss R (eds) (1998) Ocean. Ice andAtmosphere: Interactions at the Antarctic ContinentalMargin, Antarctic Research Series, vol. 75. Washing-ton DC: American Geophysical Union.

Nunes RA and Lennon GW (1996) Physical oceanographyof the Prydz Bay region of Antarctic waters. Deep-SeaResearch 43(5): 603}641.

Orsi AH, Johnson GC and Bullister JL (1999) Circulation,mixing, and production of Antarctic Bottom Water.Progress in Oceanography 43: 55}109.

Tomczak M and Godfrey JS (1994) RegionalOceanography: An Introduction. London: PergamonPress.

BRAZIL AND FALKLANDS (MALVINAS) CURRENTS

A. R. Piola, Universidad de Buenos Aires,Buenos Aires, ArgentinaR. P. Matano, Oregon State University, Corvallis,OR, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0358

Introduction

The zonal component of the mean prevailing winds,low latitude easterlies and mid-latitude westerliesinduce anticyclonic1 upper ocean circulation pat-

340 BRAZIL AND FALKLANDS (MALVINAS) CURRENTS