7.4 Thermohaline Circulation
221
7.4 Thermohaline Circulation
The wind-induced currents are limited to the uppermost levels of the ocean.
Ocean waters lying beneath, although not directly affected by the wind, are
in motion as well. The important causes for this motion are the differences
in temperature and salinity, resulting from differential heating, evaporation
and precipitation at the sea surface. All of these factors eventually affect sea
water density. Vertical convection, circulation and mixing provide the mechanisms for distribution of sea water density differences from the sea surface
into a deeper ocean. Without deep mixing, the ocean would turn, within a
few thousand years, into a stagnant pool of cold salt water with equilibrium
maintained locally by near-surface mixing. The horizontal density differences,
and consequently horizontal pressure differences, generate subsurface currents
known as thermohaline circulation. This circulation is a very slow process,
and is difficult to study. On the other hand, it should be remembered that the
deep ocean, where the thermohaline circulation is dominant, contains about 90
percent of the total volume of ocean water.
From the Equator to approximately 50 degrees latitude there is a surface
layer of water, less than 1 km deep, known as the 'thermocline', which has
temperatures ranging from 5°C up to 28°C. The salinity in the thermocline
also varies, and is highest in the tropics due to high rates of evaporation, and is
much lower in temperate zones. In the rest of the ocean, temperature decreases
with latitude and depth.
The deep and bottom water in the oceans apparently sinks and spreads away
with a time scale of years to centuries, and eventually attains an equilibrium in
terms of mass distribution. However, small volumes of sea water maintain their
temperature and salinity identity even after they have travelled thousands of
kilometres from their point of origin on the sea surface. This property of the
deep-ocean water allows oceanographers to trace its lengthy path and determine
its area of origin.
In general, sea water is very homogeneous in terms of temperature and salinity; over 75% of all ocean water has a temperature in the range of 0°-5°C and a
salinity of 34-35 ppm. This means that deep oceans are filled with cold water
originating from the polar latitudes. Usually in the ocean, four classes of water are distinguished: central waters which extend below the ocean surface
down to the underside of the main thermocline, intermediate waters extending down to '" 2 km, deep waters lying further down, and bottom waters
which are very cold and very dense. At present, thermocline circulation is
modelled as being a part of the ocean general circulation (Whitehead, 1995;
McWilliams, 1996). Descriptions of water structure in particular ocean basins
can be found elsewhere (for example, Pinet, 1992), and the energetics of tidal
and wind mixing needed to maintain the global abyssal density distribution
was recently discussed by Munk and Wunsch (1998).
221
7.4 Thermohaline Circulation
The wind-induced currents are limited to the uppermost levels of the ocean.
Ocean waters lying beneath, although not directly affected by the wind, are
in motion as well. The important causes for this motion are the differences
in temperature and salinity, resulting from differential heating, evaporation
and precipitation at the sea surface. All of these factors eventually affect sea
water density. Vertical convection, circulation and mixing provide the mechanisms for distribution of sea water density differences from the sea surface
into a deeper ocean. Without deep mixing, the ocean would turn, within a
few thousand years, into a stagnant pool of cold salt water with equilibrium
maintained locally by near-surface mixing. The horizontal density differences,
and consequently horizontal pressure differences, generate subsurface currents
known as thermohaline circulation. This circulation is a very slow process,
and is difficult to study. On the other hand, it should be remembered that the
deep ocean, where the thermohaline circulation is dominant, contains about 90
percent of the total volume of ocean water.
From the Equator to approximately 50 degrees latitude there is a surface
layer of water, less than 1 km deep, known as the 'thermocline', which has
temperatures ranging from 5°C up to 28°C. The salinity in the thermocline
also varies, and is highest in the tropics due to high rates of evaporation, and is
much lower in temperate zones. In the rest of the ocean, temperature decreases
with latitude and depth.
The deep and bottom water in the oceans apparently sinks and spreads away
with a time scale of years to centuries, and eventually attains an equilibrium in
terms of mass distribution. However, small volumes of sea water maintain their
temperature and salinity identity even after they have travelled thousands of
kilometres from their point of origin on the sea surface. This property of the
deep-ocean water allows oceanographers to trace its lengthy path and determine
its area of origin.
In general, sea water is very homogeneous in terms of temperature and salinity; over 75% of all ocean water has a temperature in the range of 0°-5°C and a
salinity of 34-35 ppm. This means that deep oceans are filled with cold water
originating from the polar latitudes. Usually in the ocean, four classes of water are distinguished: central waters which extend below the ocean surface
down to the underside of the main thermocline, intermediate waters extending down to '" 2 km, deep waters lying further down, and bottom waters
which are very cold and very dense. At present, thermocline circulation is
modelled as being a part of the ocean general circulation (Whitehead, 1995;
McWilliams, 1996). Descriptions of water structure in particular ocean basins
can be found elsewhere (for example, Pinet, 1992), and the energetics of tidal
and wind mixing needed to maintain the global abyssal density distribution
was recently discussed by Munk and Wunsch (1998).