6.1.1 The global heat balance
The earth is heated by solar short-wave radiation,
which falls predominantly in tropical regions, and
is cooled by outgoing long-wave (black-body)
radiation, which is reasonably uniform over the
globe (Fig. 6.1.1). Satellite measurements at the
top of the atmosphere show that the net incoming
short-wave radiation from the sun exceeds the outgoing long-wave radiation back to space for latitudes equatorward of about 35°, so that tropical
and subtropical regions gain heat from space.
Poleward of 35°, outgoing radiation exceeds
incoming radiation, so subpolar and polar regions
lose heat to space (Fig. 6.1.2). To maintain the
heat balance for the earth, the atmosphere and
oceans must transport energy poleward and the
maximum combined atmosphere plus ocean
energy transport at 35° latitude is about 5.5 PW
(1 PW:10
15 W). Whether the atmosphere or the
ocean is dominant in transporting heat poleward is
a question of long-standing interest (Bryden,
1993). One recent separation of the total atmosphere plus ocean energy transport as a function of
latitude into three components (ocean heat transport, dry static atmospheric energy transport, and
latent heat transport associated with the meridional freshwater transport, which is a combined
atmosphere–ocean process) indicates that the
oceans and atmosphere may contribute about
equally in maintaining the global heat balance
(Fig. 6.1.3). The freshwater transport is considered
by Wijffels (Chapter 6.2).
To assess the role of the oceans in maintaining
this global heat balance, three methods have been
used in recent years:
1 the traditional method, in which bulk formula
estimates of air–sea heat exchange based on
surface observations are integrated over a given
surface area to infer the regional ocean heat
transport divergence required to balance the
air–sea heat flux;
2 the residual method, in which the atmospheric
energy transport is subtracted from the radiation budget requirements to derive the ocean
heat transport; and
3 the direct method, in which the product of ocean
velocity and temperature measured over the
boundaries of a closed volume are integrated to
determine the ocean heat transport divergence
for the volume.
The advantage of the traditional method is that it
can be applied over arbitrary areas of the ocean,
while its disadvantage has been an inability to identify biases in the surface observations or flux parameterizations that make large-area estimates suspect.
The primary advantage of the residual method is
the availability of satellite radiation measurements
of the total ocean plus atmosphere energy transport
requirement, while the disadvantage has been that
any uncertainty in either the radiation measurements or the atmospheric transport feeds directly
into ocean heat transport estimates. The advantage
of the direct method is that it deals with ocean circulation and the mechanisms of ocean heat transport, while its disadvantage has been that direct
estimates could only be made at a few locations
where high-quality observations were available.
6.1
Ocean Heat Transport
Harry L. Bryden and Shiro Imawaki
455
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
The earth is heated by solar short-wave radiation,
which falls predominantly in tropical regions, and
is cooled by outgoing long-wave (black-body)
radiation, which is reasonably uniform over the
globe (Fig. 6.1.1). Satellite measurements at the
top of the atmosphere show that the net incoming
short-wave radiation from the sun exceeds the outgoing long-wave radiation back to space for latitudes equatorward of about 35°, so that tropical
and subtropical regions gain heat from space.
Poleward of 35°, outgoing radiation exceeds
incoming radiation, so subpolar and polar regions
lose heat to space (Fig. 6.1.2). To maintain the
heat balance for the earth, the atmosphere and
oceans must transport energy poleward and the
maximum combined atmosphere plus ocean
energy transport at 35° latitude is about 5.5 PW
(1 PW:10
15 W). Whether the atmosphere or the
ocean is dominant in transporting heat poleward is
a question of long-standing interest (Bryden,
1993). One recent separation of the total atmosphere plus ocean energy transport as a function of
latitude into three components (ocean heat transport, dry static atmospheric energy transport, and
latent heat transport associated with the meridional freshwater transport, which is a combined
atmosphere–ocean process) indicates that the
oceans and atmosphere may contribute about
equally in maintaining the global heat balance
(Fig. 6.1.3). The freshwater transport is considered
by Wijffels (Chapter 6.2).
To assess the role of the oceans in maintaining
this global heat balance, three methods have been
used in recent years:
1 the traditional method, in which bulk formula
estimates of air–sea heat exchange based on
surface observations are integrated over a given
surface area to infer the regional ocean heat
transport divergence required to balance the
air–sea heat flux;
2 the residual method, in which the atmospheric
energy transport is subtracted from the radiation budget requirements to derive the ocean
heat transport; and
3 the direct method, in which the product of ocean
velocity and temperature measured over the
boundaries of a closed volume are integrated to
determine the ocean heat transport divergence
for the volume.
The advantage of the traditional method is that it
can be applied over arbitrary areas of the ocean,
while its disadvantage has been an inability to identify biases in the surface observations or flux parameterizations that make large-area estimates suspect.
The primary advantage of the residual method is
the availability of satellite radiation measurements
of the total ocean plus atmosphere energy transport
requirement, while the disadvantage has been that
any uncertainty in either the radiation measurements or the atmospheric transport feeds directly
into ocean heat transport estimates. The advantage
of the direct method is that it deals with ocean circulation and the mechanisms of ocean heat transport, while its disadvantage has been that direct
estimates could only be made at a few locations
where high-quality observations were available.
6.1
Ocean Heat Transport
Harry L. Bryden and Shiro Imawaki
455
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER