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334 BOTTOM WATER FORMATION

Sayles FL and Dickinson WH (1991) The ROLAI2D Lander: a benthic lander for the study of exchange across the sediment}water interface. Deep-Sea Research 38: 505}529.

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

oxygen consumption to abyssal depths. Limnological

Oceanography 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 solutions and 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 salinity through the PaciRc and Atlantic Oceans (Figure 1) reveal that in the PaciRc below 2000m, more than half of the ocean depth, the water is colder than 23C. The Atlantic is somewhat warmer, but there too the lower 1000m of the ocean is well below 23C. Only within the surface layer, generally less than the upper 500m of the ocean is the water warmer than 103C, amounting to only 10% of the total ocean volume. The coldness of the deep ocean is due to interaction of the ocean with the polar atmosphere. There, surface water reaches the freezing point of sea water. Streams of very cold water can be traced spreading primarily from the Antarctic along the sea Soor, warming en route by mixing with overlying water, into the world’s oceans (Figure 2).

The coldest bottom water, Antarctic Bottom Water (AABW), is derived from the shores of Ant- arctica. There, freezing point, high oxygen concen- tration, water is produced during the winter over the continental shelf. At a few sites the shelf water salinity is sufRciently high, greater than 34.61P, that, on cooling to the freezing point, the surface water density is sufRciently high to allow it to sink to great depths of the ocean. As the shelf water descends over the continental slope into the deep ocean it mixes with adjacent deep water, but this water is also quite cold so the Rnal product arriving at 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 potential temperature (the temperature corrected for adiabatic heating due to hydrostatic pressure) less than 03C.

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

In the Atlantic Ocean the 23C isotherm marks the base of a wedge of relatively salty water, associated with high dissolved oxygen and low silicate concen- trations (see Figure 1). This water mass is called North Atlantic Deep Water (NADW). The densest component of NADW is formed as cold surface waters during the winter in the Greenland and Nor- wegian Seas. This water sinks to Rll the basin north of a ridge spanning the distance from Greenland to Scotland. Excess cold water overSows the ridge crest, mixing on descent with warmer more saline water, producing a bottom water product of about #1.03C. The overSow water stays in contact with the sea Soor to near 403N in the Atlantic Ocean, where on spreading southward it is lifted over the remnants of denser AABW.

Export of Greenland and Norwegian Sea bottom water has been estimated from a series of current measurements. Transports of about 2Y106 m3 s’1 of near 0.43C water occur between the Faroe Bank and Scotland, 1Y106m3s’1 of similar water passes through notches between Iceland and Faroe Bank, and 3Y106 m3 s’1 of near 03C is exported through the Denmark Strait, between Greenland and Ice- land. The overSow plumes rapidly entrain warmer waters, producing bottom water of near #1.03C. With entrainment of other deep water, a production rate of about 8Y106 m3 s’1 of overSow water is likely. Less dense components of NADW, that do not contact the seaSoor are formed in the Labrador Sea and Mediterranean Sea. The total production of NADW is estimated as 15Y106 m3 s’1.

As the Antarctic is the primary source of the cold bottom waters of the world ocean, Antarctic Bot- tom Water is discussed in this article. See North Atlantic Deep Water for further information on that Northern Hemisphere deep water mass.

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