separate and unrelated: for the zonally averaged
circulation, the northward-flowing bottom water
is balanced by southward-flowing, slightly warmer
deep water; and the northward, wind-driven
Ekman transport is balanced by upper thermocline
flow southward at an average temperature about
9°C less than the Ekman flow. The northward
heat transport of 0.73 PW is due principally to this
shallow Ekman-upper thermocline cell and is sensitive to the size of the Ekman transport. Because
of the 16 000 km width of the 10°N section, small
changes in wind stress lead to large changes in
Ekman transport and the overall meridional heat
transport is sensitive (proportional) to the size of
the Ekman transport. Wijffels et al. estimated that
a 6 Sv change in Ekman transport, comparable
to the differences among existing climatologies,
would cause a 0.4 PW change in meridional heat
transport.
In the South Pacific, the circulation and heat
transport is complicated by the net northward mass
transport associated with the Indonesian Throughflow from the Pacific Ocean to the Indian Ocean,
which is of poorly determined size. Tsimplis et al.’s
(1998) inverse model for the combined 17°S and
32°S trans-Pacific sections could not identify the
size of the Throughflow. Their meridional circulation again implied separate deep water and upper
level circulation cells, with a similar deep circulation of northward-flowing bottom water balanced
by southward-flowing deep water and a similar
upper level circulation with the poleward, winddriven Ekman transport compensated by equatorward flow of slightly colder upper thermocline
waters. In an attempt to determine ocean heat
transport independent of the size of the Throughflow, they separated the heat transport into contributions from the horizontal gyre and from the
vertical meridional overturning, each with mass
conservation imposed, leaving the undetermined
net Throughflow transport to occur at the sectionaveraged temperature. The resulting meridional
heat fluxes due to the horizontal gyre and vertical
meridional overturning are small, only 90.17 PW
and 90.04 PW (southward) at 17°S and 32°S,
respectively. These heat transports appear to be
sensitive not only to the size of the Ekman transport, but also to the strength of the East
Australian Current and to the distribution of the
reference level velocities through the different subbasins of the South Pacific.
In the Tsimplis et al. (1998) analysis procedure,
the mass transport associated with the Indonesian
Throughflow, whatever its magnitude, flows
northward across the 32°S and 17°S sections at the
section-averaged temperatures of about 4°C.
Because the Throughflow from the Pacific Ocean
to the Indian Ocean through the Indonesian archipelago is generally assumed to have a temperature
much greater than 4°C, the Throughflow leads to
a heat transport divergence such that the Pacific
Ocean must gain heat from the atmosphere. The
magnitude of the heat transport divergence in the
Pacific is equal to the product of the Throughflow
transport times the difference in temperature
between the Throughflow (generally about 20°C)
and the section-averaged temperature across the
southern boundary of the Pacific at 32°S. For a
typical value for the Throughflow of 10 Sv, the
heat transport divergence over the Pacific of 0.7
PW associated with the Throughflow is much
larger than the meridional heat fluxes due to
the horizontal or vertical-meridional circulations
across 32°S.
Wijffels et al. (2000) revisited the WOCE 32°S
section using an inverse analysis including added
constraints on the circulation provided by silica
conservation and by both moored current meter
measurements in the deep western boundary
current near the Tonga–Kermadec Ridge and
neutrally buoyant float trajectories deployed
across the section at a depth of about 900 m.
These direct velocity measurements significantly
reduced the uncertainty in the derived flows from
the inverse solution and provided details of the
horizontal structure of the circulation, particularly
in the deep waters, which appeared to match the
direction of the flow implied by tracer distributions. The heat transport estimates are still dominated by the assumed size of the Throughflow,
with a southward heat transport of 0.2 PW for no
Throughflow but with a heat transport divergence
for the Pacific north of 32°S of 0.75 PW for a 10 Sv
Throughflow. Thus, while the Wijffels et al. circulation across 32°S is better resolved by the addition of direct velocity measurements, their heat
transport across 32°S is similar to that derived by
Tsimplis et al. and both estimates are very sensitive
to the size of the Indonesian Throughflow.
For the small ocean area of the South Pacific/
Southern Ocean south of 67°S, Koshlyakov and
Sazhina (1995) found a southward heat transport
6.1 Ocean Heat Transport
463
Bryden and Imawaki
circulation, the northward-flowing bottom water
is balanced by southward-flowing, slightly warmer
deep water; and the northward, wind-driven
Ekman transport is balanced by upper thermocline
flow southward at an average temperature about
9°C less than the Ekman flow. The northward
heat transport of 0.73 PW is due principally to this
shallow Ekman-upper thermocline cell and is sensitive to the size of the Ekman transport. Because
of the 16 000 km width of the 10°N section, small
changes in wind stress lead to large changes in
Ekman transport and the overall meridional heat
transport is sensitive (proportional) to the size of
the Ekman transport. Wijffels et al. estimated that
a 6 Sv change in Ekman transport, comparable
to the differences among existing climatologies,
would cause a 0.4 PW change in meridional heat
transport.
In the South Pacific, the circulation and heat
transport is complicated by the net northward mass
transport associated with the Indonesian Throughflow from the Pacific Ocean to the Indian Ocean,
which is of poorly determined size. Tsimplis et al.’s
(1998) inverse model for the combined 17°S and
32°S trans-Pacific sections could not identify the
size of the Throughflow. Their meridional circulation again implied separate deep water and upper
level circulation cells, with a similar deep circulation of northward-flowing bottom water balanced
by southward-flowing deep water and a similar
upper level circulation with the poleward, winddriven Ekman transport compensated by equatorward flow of slightly colder upper thermocline
waters. In an attempt to determine ocean heat
transport independent of the size of the Throughflow, they separated the heat transport into contributions from the horizontal gyre and from the
vertical meridional overturning, each with mass
conservation imposed, leaving the undetermined
net Throughflow transport to occur at the sectionaveraged temperature. The resulting meridional
heat fluxes due to the horizontal gyre and vertical
meridional overturning are small, only 90.17 PW
and 90.04 PW (southward) at 17°S and 32°S,
respectively. These heat transports appear to be
sensitive not only to the size of the Ekman transport, but also to the strength of the East
Australian Current and to the distribution of the
reference level velocities through the different subbasins of the South Pacific.
In the Tsimplis et al. (1998) analysis procedure,
the mass transport associated with the Indonesian
Throughflow, whatever its magnitude, flows
northward across the 32°S and 17°S sections at the
section-averaged temperatures of about 4°C.
Because the Throughflow from the Pacific Ocean
to the Indian Ocean through the Indonesian archipelago is generally assumed to have a temperature
much greater than 4°C, the Throughflow leads to
a heat transport divergence such that the Pacific
Ocean must gain heat from the atmosphere. The
magnitude of the heat transport divergence in the
Pacific is equal to the product of the Throughflow
transport times the difference in temperature
between the Throughflow (generally about 20°C)
and the section-averaged temperature across the
southern boundary of the Pacific at 32°S. For a
typical value for the Throughflow of 10 Sv, the
heat transport divergence over the Pacific of 0.7
PW associated with the Throughflow is much
larger than the meridional heat fluxes due to
the horizontal or vertical-meridional circulations
across 32°S.
Wijffels et al. (2000) revisited the WOCE 32°S
section using an inverse analysis including added
constraints on the circulation provided by silica
conservation and by both moored current meter
measurements in the deep western boundary
current near the Tonga–Kermadec Ridge and
neutrally buoyant float trajectories deployed
across the section at a depth of about 900 m.
These direct velocity measurements significantly
reduced the uncertainty in the derived flows from
the inverse solution and provided details of the
horizontal structure of the circulation, particularly
in the deep waters, which appeared to match the
direction of the flow implied by tracer distributions. The heat transport estimates are still dominated by the assumed size of the Throughflow,
with a southward heat transport of 0.2 PW for no
Throughflow but with a heat transport divergence
for the Pacific north of 32°S of 0.75 PW for a 10 Sv
Throughflow. Thus, while the Wijffels et al. circulation across 32°S is better resolved by the addition of direct velocity measurements, their heat
transport across 32°S is similar to that derived by
Tsimplis et al. and both estimates are very sensitive
to the size of the Indonesian Throughflow.
For the small ocean area of the South Pacific/
Southern Ocean south of 67°S, Koshlyakov and
Sazhina (1995) found a southward heat transport
6.1 Ocean Heat Transport
463
Bryden and Imawaki