Environmental Engineering Reference
In-Depth Information
flux, close to 200 W/m
2
, is associated with the warm
waters of the Kuroshio (off Japan's Pacific coast) and
Gulf streams (along the U.S. seaboard). The global
mean of the flux is close to 90 W/m
2
, which means that
the Earth's water cycle is driven at a rate of about 45 PW,
corresponding to daily evaporation of about 3 mm, or an
annual total of 1.1 m.
Evaporation exceeds precipitation in the Atlantic and
Indian oceans; the reverse is true in the Arctic Ocean;
and over the Pacific the two processes are nearly bal-
anced. Irregular patterns of oceanic evaporation (maxima
up to 3 m/a) and precipitation (maxima up to 5 m/a)
require substantial compensating flows from the regions
with excess rainfall in order to maintain sea level (Schmitt
1999). The North Pacific, particularly its eastern tropics,
is the largest surplus region (and hence its water is
less salty), whereas evaporation dominates the Atlantic
waters. Cyclones, low-pressure systems ranging from
thunderstorms to monsoons, are characterized by strong,
often destructive winds, but they carry much more en-
ergy in latent heat.
Thunderstorms that leave behind just 1 cm of rain in
20-40 min over 100-200 Mm
2
release 2.5-5 PJ (1-4
TW) of heat, 10-100 times the kinetic energy total.
Most of this energy goes into heating the atmosphere.
An average year sees about 80 tropical cyclones that de-
velop by heat transfer from ocean surfaces warmer than
26
C (Emanuel 2003). When maximum winds reach 33
m/s, cyclones are classed as hurricanes (in the western
North Atlantic) or typhoons (in the eastern North Pa-
cific). Mature tropical cyclones have axisymmetric flows
whose energy cycle (with nearly isothermal expansion
and compression) resembles that of an ideal Carnot
engine (Bister and Emanuel 1998). Large cyclones dis-
charge less than 0.5% of the planetary latent heat flux,
southward heat flows throughout the Southern Hemi-
sphere (as much as
1.6 PW at 18
S) and appreciable
northward positive transport at about 20
N. Globally
these patterns result in a considerable asymmetry across
the equator: the vigorous northward transport in the
South Atlantic nearly cancels the net southward flow in
the Indian and Pacific oceans. Estimated heating totals
2.3 PW in the tropics, and 70% of the corresponding
cooling (
1.7 PW) takes place north of 24
N.
Warm seawater can be employed to generate electricity
by using thermal gradients greater than 20
C between
the surface and deep cold layers (
<
1 km). The warm
water can be used either to evaporate a fluid with a low
boiling point, or the seawater can be evaporated in a vac-
uum to generate low-pressure steam. Actual efficiencies
of these conversions would not most likely surpass 2%.
The theoretical limit of ocean thermal energy conver-
sions is given by cooling the surface waters because a
change greater than 10
C would have widespread cli-
matic consequences. The limit of about 10 TW would
prorate to 200 mW/m
2
over the world's warmest tropi-
cal seas. Only the sites with the highest thermal gradients
very close to the shore can be seriously considered
for OTEC (ocean thermal energy conversion), but the
commercial viability of these conversions is yet to be
demonstrated.
Energy release from the ocean to the atmosphere is
dominated by thermal radiation (@80%) and by evapora-
tion as latent heat (@16%), and its global pattern shows
the highest fluxes generated by strong northward ocean
currents. With water's extraordinarily high heat of vapor-
ization it takes 28-29 W/m
2
in order to evaporate 1
mm/day. This much energy is available as the annual
mean even at 70
N in the Atlantic and up to about
60
S in the Antarctic waters. The highest latent heat
