7 Abyssal Circulation
7.1 Introduction
The base of the thermocline at a depth of roughly 1 km represents the lower
boundary of the domain of the vigorous ocean circulation driven by the windstress and the large-scale distribution of heating and cooling. Below this region
lies a vast volume of fluid extending to depths typically of 4--5 km although the
depth diminishes to about 2 km in the vicinity of the great midocean ridge
systems. This region is everywhere very cold. In the world ocean the potential
temperature is less than 4 °C virtually everywhere below 2 km and in most
areas is closer to 2 °C at this depth (e.g. Levitus 1982) and the temperature falls
typically to 0--2 °C at 4 km. This abyssal region of the ocean is so vast that
Worthington (1981) attributes an average temperature of only 3.51 °C to the
entire world ocean, so great is the domain of the cold water - in spite of the
widespread areas of surface temperatures of the order of 20 °C .
As Warren (1981) remarks in his excellent review article, the implication of
the widespread presence of cold water in the region below the thermocline,
even in the tropical ocean, was appreciated nearly 200 years ago by Count
Rumford who inferred from this fact the existence of a global, deep circulation.
Waters of such low temperatures can be formed by cooling of the ocean by the
atmosphere only in polar regions. The presence at other latitudes of such cold
water implies a large-scale deep circulation, the abyssal circulation, which
carries the water formed in polar regions to the rest of the ocean. Cold water
flows from the polar regions to fill the deep ocean basins, from which the water
must eventually rise to the surface and, heated to the observed surface
temperatures, must then flow poleward to replace the water which has sunk to
the bottom, forming an endless global cell of motion.
The sources of the cold water are found in both polar regions. In the
Weddell Sea and the Ross Sea of the Antarctic region cold, saline water sinks
to the bottom along the continental shelf to begin its journey northward. At the
same time, water from the high-latitude North Atlantic spills through a
complex series of passages past Greenland, Iceland and the British Isles and
produces the North Atlantic Deep Water that travels southward in the region
beneath the thermocline. Estimates of the transport of deep water from each
source vary, but each polar region is thought to be responsible for an input into
7.1 Introduction
The base of the thermocline at a depth of roughly 1 km represents the lower
boundary of the domain of the vigorous ocean circulation driven by the windstress and the large-scale distribution of heating and cooling. Below this region
lies a vast volume of fluid extending to depths typically of 4--5 km although the
depth diminishes to about 2 km in the vicinity of the great midocean ridge
systems. This region is everywhere very cold. In the world ocean the potential
temperature is less than 4 °C virtually everywhere below 2 km and in most
areas is closer to 2 °C at this depth (e.g. Levitus 1982) and the temperature falls
typically to 0--2 °C at 4 km. This abyssal region of the ocean is so vast that
Worthington (1981) attributes an average temperature of only 3.51 °C to the
entire world ocean, so great is the domain of the cold water - in spite of the
widespread areas of surface temperatures of the order of 20 °C .
As Warren (1981) remarks in his excellent review article, the implication of
the widespread presence of cold water in the region below the thermocline,
even in the tropical ocean, was appreciated nearly 200 years ago by Count
Rumford who inferred from this fact the existence of a global, deep circulation.
Waters of such low temperatures can be formed by cooling of the ocean by the
atmosphere only in polar regions. The presence at other latitudes of such cold
water implies a large-scale deep circulation, the abyssal circulation, which
carries the water formed in polar regions to the rest of the ocean. Cold water
flows from the polar regions to fill the deep ocean basins, from which the water
must eventually rise to the surface and, heated to the observed surface
temperatures, must then flow poleward to replace the water which has sunk to
the bottom, forming an endless global cell of motion.
The sources of the cold water are found in both polar regions. In the
Weddell Sea and the Ross Sea of the Antarctic region cold, saline water sinks
to the bottom along the continental shelf to begin its journey northward. At the
same time, water from the high-latitude North Atlantic spills through a
complex series of passages past Greenland, Iceland and the British Isles and
produces the North Atlantic Deep Water that travels southward in the region
beneath the thermocline. Estimates of the transport of deep water from each
source vary, but each polar region is thought to be responsible for an input into