380
Abyssal Circulation
the major basins of between 10--20 10 6 x m 3 /s (i.e., 10--20 sverdrups). Not all of
this water has been produced at the surface by cooling. A fraction (see below) is
estimated to be water entrained during the sinking as well as entrained laterally
as the source water enters the deep basins. In the North Atlantic, for example,
McCartney and Talley (1984) have estimated a formation rate of North
Atlantic Deep Water of 13-14 sverdrups. In both polar regions the zones of
sinking are quite narrow, and the convective sinking itself is difficult to observe
directly. The sinking instead is largely inferred from the distribution of ocean
properties such as temperature, salinity, and especially oxygen.
The midocean velocities expected in response to this ocean-filling advent of
cold water are very low, about an order of magnitude smaller than the winddriven circulation. At the lowest order these small velocities allow us to
calculate the wind-driven circulation without paying much attention to the
abyssal circulation which in the context of the thermocline theory is negligible.
Nevertheless, the abyssal circulation is of interest in its own right because
(a) it occupies such a large proportion of the ocean by volume where, small as
is the velocity, it is the general circulation, and (b) due to the large volumes
involved, its transport is commensurate with the thermocline transport. It is
also of considerable importance because of its role in determining the deep
temperature, which in turn is responsible in conjunction with the surface
heating for determining the background stratification of the ocean. Although
considerations of our planet's climate do not enter into the discussion of this
book, it should also be intuitively apparent that the massive flux of cold water
from the poles to equatorial regions, balanced as it must be by warmer surface
water flowing poleward to replace it, must play a key role in the earth's heat
balance.
How the abyssal circulation takes place was a subject of considerable early
controversy, and the reader is referred to Warren's essay (1981) for a
fascinating historical review. Because of the difficulty in making the required
long-term direct measurements of currents in the abyss, the deep circulation is
still much less well constrained by observations than that of the upper ocean.
Therefore, although it occupies the major part of the ocean by volume, the
limited theoretical and observational understanding of the deep circulation is
reflected here by the nearly inverse proportion of its representation in this
book.
In spite of the difficulty in making direct velocity measurements, the
measurement of tracer fields and standard hydrographic measurements of the
temperature and salinity reveal a circulation which is anything but simple.
Figure 7 .1.1 from Pickard and Emery ( 1982) shows meridional cross sections of
temperature, salinity, density, and oxygen in the Atlantic Ocean. Especially in
the salinity field the layered structure of the circulation is evident. Cold
Antarctic Bottom Water extends northward in a giant wedge overlain by North
Atlantic Deep Water whose southward motion is inferred from the property
fields. Above the North Atlantic water is another tongue of Antarctic water,
the Antarctic Intermediate Water, which extends northward into the northern
Abyssal Circulation
the major basins of between 10--20 10 6 x m 3 /s (i.e., 10--20 sverdrups). Not all of
this water has been produced at the surface by cooling. A fraction (see below) is
estimated to be water entrained during the sinking as well as entrained laterally
as the source water enters the deep basins. In the North Atlantic, for example,
McCartney and Talley (1984) have estimated a formation rate of North
Atlantic Deep Water of 13-14 sverdrups. In both polar regions the zones of
sinking are quite narrow, and the convective sinking itself is difficult to observe
directly. The sinking instead is largely inferred from the distribution of ocean
properties such as temperature, salinity, and especially oxygen.
The midocean velocities expected in response to this ocean-filling advent of
cold water are very low, about an order of magnitude smaller than the winddriven circulation. At the lowest order these small velocities allow us to
calculate the wind-driven circulation without paying much attention to the
abyssal circulation which in the context of the thermocline theory is negligible.
Nevertheless, the abyssal circulation is of interest in its own right because
(a) it occupies such a large proportion of the ocean by volume where, small as
is the velocity, it is the general circulation, and (b) due to the large volumes
involved, its transport is commensurate with the thermocline transport. It is
also of considerable importance because of its role in determining the deep
temperature, which in turn is responsible in conjunction with the surface
heating for determining the background stratification of the ocean. Although
considerations of our planet's climate do not enter into the discussion of this
book, it should also be intuitively apparent that the massive flux of cold water
from the poles to equatorial regions, balanced as it must be by warmer surface
water flowing poleward to replace it, must play a key role in the earth's heat
balance.
How the abyssal circulation takes place was a subject of considerable early
controversy, and the reader is referred to Warren's essay (1981) for a
fascinating historical review. Because of the difficulty in making the required
long-term direct measurements of currents in the abyss, the deep circulation is
still much less well constrained by observations than that of the upper ocean.
Therefore, although it occupies the major part of the ocean by volume, the
limited theoretical and observational understanding of the deep circulation is
reflected here by the nearly inverse proportion of its representation in this
book.
In spite of the difficulty in making direct velocity measurements, the
measurement of tracer fields and standard hydrographic measurements of the
temperature and salinity reveal a circulation which is anything but simple.
Figure 7 .1.1 from Pickard and Emery ( 1982) shows meridional cross sections of
temperature, salinity, density, and oxygen in the Atlantic Ocean. Especially in
the salinity field the layered structure of the circulation is evident. Cold
Antarctic Bottom Water extends northward in a giant wedge overlain by North
Atlantic Deep Water whose southward motion is inferred from the property
fields. Above the North Atlantic water is another tongue of Antarctic water,
the Antarctic Intermediate Water, which extends northward into the northern