Environmental Engineering Reference
In-Depth Information
2.3 Hydrosphere and Atmosphere: Thermal and
Mass Fluxes
Oceans cover just over 70% of the Earth (to an average
depth of 3.8 km), store 96.5% of the Earth's water, are
the source of about 86% of all evaporation, and receive
78% of all precipitation. But water's importance for the
planet's energy budget rests more on its extraordinary
properties than on its area and mass. A mere 0.0009%
of the Earth's water is in the atmosphere (where it
accounts for just 0.3% of mass and 0.5% of volume), but
this relatively tiny amount suffices to cover always about
60% of the planet by clouds, and it is the decisive
absorber and radiator of incoming and outgoing radia-
tion. In soils (which contain 0.001% of the Earth's water)
and plants (0.0001% of the Earth's water), it is the carrier
of latent heat; in the ocean it is the planet's largest reser-
voir of warmth; and everywhere it is an unsurpassable
thermal ''flywheel,'' an ideal absorber and a gradual
emitter of large quanta of energy. All of this is possible
only because of water's peculiar properties (M. W.
Denny 1993).
Because of its intermolecular hydrogen bonds, water
has an unusually high boiling point (100
C, whereas all
other similar H
2
X compounds boil at less than 0
C).
Water's specific heat (4.18 J/g
C) is 2.5-3.3 times
that of common land surfaces (soils, rocks). Water's heat
of fusion, 334 J/g, is larger than among similarly struc-
tured compounds. Its anomalously high heat of vaporiza-
tion (2.45 kJ/g) makes it an ideal transporter of energy
as latent heat, and it helps to retain plant and soil mois-
ture in hot environments. Finally, its relatively low viscos-
ity makes it an excellent medium for swimming and an
outstanding carrier of heat
Oceans dominate the planetary energy balance. About
80% of all radiation intercepted by the Earth (total of
173.5 PW) enters the atmosphere above oceans. With
50% reaching the surface and with average oceanic
albedo of 0.06, oceans thus receive about 65 PW, nearly
twice as much energy as is absorbed by the whole atmo-
sphere and four times as much as the continents. Oceans
also absorb about two-thirds of the downward LW radia-
tion (@110 PW), so their global annual heating rate is
about 175 PW. Because of oceans' great average depth,
the air-sea interactions cannot directly affect the entire
water column. Water is densest at about 4
C, and hence
the deep ocean, with temperature stable near that point,
is isolated from the atmosphere by a relatively thin mixed
layer that is agitated by winds and that experiences both
daily and seasonal temperature fluctuations. Solar energy
absorption takes place in the top 100 m, and water's high
specific heat restricts the range of temperature ampli-
tudes. Equatorial waters and the southern seas between
30
S-50
S are the main zones of heat storage.
Principal vertical transport to the surface is upwel-
ling: molecular conduction, convection, and turbulence
are less important. Meridional overturning circulation
(MOC)—a downward flux followed by lateral flow—is
driven by a severe heat loss to the atmosphere, water
cooling, and ice formation in high latitudes (Wunsch
2002). Colder (higher-density) water sinks in high-
latitude ventilation areas, and Macdonald and Wunsch
(1996) concluded that there are two nearly independent
overturning cells, one connecting the Atlantic Ocean to
other basins through the Southern Ocean, and the other
connecting the Indian and Pacific basins through the In-
donesian archipelago. Complex pathways of the Pacific
water flowing into the Indian Ocean in the Indonesian
in countless eddies and
currents.
