the annual average heat transport across each
transoceanic section for comparison among all such
heat transport estimates to quantify the ocean’s role
in maintaining the global heat balance. Thus, we
are actively investigating how best to join time
series western boundary current measurements with
a one-time transoceanic section.
6.1.7 Summary
A fundamental issue in the global climate system is
the relative amounts of heat carried by the oceans
and the atmosphere to balance the global heat
budget in which radiative heating from the sun
occurs predominantly in tropical regions while
radiative cooling to space occurs more uniformly
over the globe. Progress in estimating ocean heat
flux by three methods was reviewed with emphasis
on the direct method using transoceanic hydrographic sections completed during the WOCE field
programme.
The residual method subtracts atmospheric
energy transport from the radiation budget requirement for total ocean plus atmosphere energy transport to determine ocean heat transport as a
residual. Previously this method was applied to
zonal averages. New studies have derived spatial
patterns of atmosphere–surface heat flux by subtracting atmospheric energy transport divergence
from the net radiation at the top of the atmosphere. Such patterns exhibit substantial and unrealistic annually averaged heat fluxes over the land
but the patterns of heat flux over the oceans
appear realistic. Masking the land to consider only
these air–sea heat fluxes yields ocean heat transports in broad agreement with existing direct estimates of ocean heat transport. Dipole patterns of
heat gain and heat loss over land topography,
however, suggest that refinements in atmospheric
energy transport are still needed.
The bulk formula method for determining air–
sea heat exchange from marine observations has
been used to produce several new global air–sea
flux climatologies. Depressingly, each exhibits an
overall global imbalance such that on average the
ocean gains heat at a rate of 30–50 W m
92
. There
is evidence that spatially uniform corrections are
not appropriate. Refinement of the air–sea fluxes
will require accurate area-averaged estimates of
ocean heat transport divergence over a variety of
oceanic regions and additional buoy measurements
of air–sea heat exchange in inhospitable subpolar
and polar regions.
Making direct estimates of ocean heat transport
from transoceanic sections in each ocean basin
was a primary objective in designing the WOCE
field programme. While many zonal hydrographic
sections were taken during WOCE, there are relatively few published estimates of heat transport
from WOCE sections to date. Most of the recent
estimates are for the Atlantic Ocean, for which
there is general agreement that there is northward
ocean heat transport at all latitudes. There is also
general understanding that this northward heat
transport is associated with the thermohaline circulation in which North Atlantic Deep Water is
formed in the polar and subpolar North Atlantic
and subsequently flows southward throughout the
Atlantic. For the Pacific and Indian Oceans, analyses of pre-WOCE transoceanic sections have concentrated on estimating the heat transports across
the southern boundary at approximately 30°S.
There is general confusion in that there appears to
be only a small poleward heat transport across
30°S out of the Pacific Ocean and the estimates of
poleward heat transport across 30°S in the Indian
Ocean range from 0.1 to 1.3 PW. Uncertainty in
the size of the Indonesian Throughflow clouds the
determination of ocean heat transports across 30°S
in the Indian and Pacific Oceans.
A variety of new measurements were made during WOCE with the aim of providing accurate reference velocities for geostrophic calculations. Such
new measurements include satellite altimeter measurements of sea surface height (Fu, Chapter 3.3),
direct current measurements by underway and
lowered ADCPs (King et al., Chapter 3.1) and by
surface drifters (Niiler, Chapter 4.1) and subsurface floats (Davis and Zenk, Chapter 3.2), and
a variety of geochemical tracer measurements
(Schlosser et al., Chapter 5.8). Because experience
suggests that measurements of western boundary
currents provide the strongest constraints to estimates of ocean heat transport, careful analysis of
WOCE observations of shallow and deep boundary currents in the Pacific, Indian and Atlantic
Oceans, along with velocities from subsurface floats, likely holds the key to direct determinations of ocean heat transport in each ocean
basin and the subsequent understanding of the
role of the oceans in maintaining the global heat
balance.
6.1 Ocean Heat Transport
473
Bryden and Imawaki
transoceanic section for comparison among all such
heat transport estimates to quantify the ocean’s role
in maintaining the global heat balance. Thus, we
are actively investigating how best to join time
series western boundary current measurements with
a one-time transoceanic section.
6.1.7 Summary
A fundamental issue in the global climate system is
the relative amounts of heat carried by the oceans
and the atmosphere to balance the global heat
budget in which radiative heating from the sun
occurs predominantly in tropical regions while
radiative cooling to space occurs more uniformly
over the globe. Progress in estimating ocean heat
flux by three methods was reviewed with emphasis
on the direct method using transoceanic hydrographic sections completed during the WOCE field
programme.
The residual method subtracts atmospheric
energy transport from the radiation budget requirement for total ocean plus atmosphere energy transport to determine ocean heat transport as a
residual. Previously this method was applied to
zonal averages. New studies have derived spatial
patterns of atmosphere–surface heat flux by subtracting atmospheric energy transport divergence
from the net radiation at the top of the atmosphere. Such patterns exhibit substantial and unrealistic annually averaged heat fluxes over the land
but the patterns of heat flux over the oceans
appear realistic. Masking the land to consider only
these air–sea heat fluxes yields ocean heat transports in broad agreement with existing direct estimates of ocean heat transport. Dipole patterns of
heat gain and heat loss over land topography,
however, suggest that refinements in atmospheric
energy transport are still needed.
The bulk formula method for determining air–
sea heat exchange from marine observations has
been used to produce several new global air–sea
flux climatologies. Depressingly, each exhibits an
overall global imbalance such that on average the
ocean gains heat at a rate of 30–50 W m
92
. There
is evidence that spatially uniform corrections are
not appropriate. Refinement of the air–sea fluxes
will require accurate area-averaged estimates of
ocean heat transport divergence over a variety of
oceanic regions and additional buoy measurements
of air–sea heat exchange in inhospitable subpolar
and polar regions.
Making direct estimates of ocean heat transport
from transoceanic sections in each ocean basin
was a primary objective in designing the WOCE
field programme. While many zonal hydrographic
sections were taken during WOCE, there are relatively few published estimates of heat transport
from WOCE sections to date. Most of the recent
estimates are for the Atlantic Ocean, for which
there is general agreement that there is northward
ocean heat transport at all latitudes. There is also
general understanding that this northward heat
transport is associated with the thermohaline circulation in which North Atlantic Deep Water is
formed in the polar and subpolar North Atlantic
and subsequently flows southward throughout the
Atlantic. For the Pacific and Indian Oceans, analyses of pre-WOCE transoceanic sections have concentrated on estimating the heat transports across
the southern boundary at approximately 30°S.
There is general confusion in that there appears to
be only a small poleward heat transport across
30°S out of the Pacific Ocean and the estimates of
poleward heat transport across 30°S in the Indian
Ocean range from 0.1 to 1.3 PW. Uncertainty in
the size of the Indonesian Throughflow clouds the
determination of ocean heat transports across 30°S
in the Indian and Pacific Oceans.
A variety of new measurements were made during WOCE with the aim of providing accurate reference velocities for geostrophic calculations. Such
new measurements include satellite altimeter measurements of sea surface height (Fu, Chapter 3.3),
direct current measurements by underway and
lowered ADCPs (King et al., Chapter 3.1) and by
surface drifters (Niiler, Chapter 4.1) and subsurface floats (Davis and Zenk, Chapter 3.2), and
a variety of geochemical tracer measurements
(Schlosser et al., Chapter 5.8). Because experience
suggests that measurements of western boundary
currents provide the strongest constraints to estimates of ocean heat transport, careful analysis of
WOCE observations of shallow and deep boundary currents in the Pacific, Indian and Atlantic
Oceans, along with velocities from subsurface floats, likely holds the key to direct determinations of ocean heat transport in each ocean
basin and the subsequent understanding of the
role of the oceans in maintaining the global heat
balance.
6.1 Ocean Heat Transport
473
Bryden and Imawaki