of 0.03 PW due to 8 Sv of waters above 2000 m
flowing toward Antarctica, compensated by 8 Sv
flowing northward below 2000 m depth. The
uncertainty in the size of the horizontal heat transport, however, makes this heat transport estimate
unreliable because any small difference in temperature between the 131 Sv branch of the Antarctic
Circumpolar Current entering the area and subsequently leaving the area would swamp the overturning heat flux of 0.03 PW.
6.1.4.3 Indian Ocean
As for the Pacific, the unknown size of the Indonesian Throughflow complicates quantitative estimates of heat transport divergence in the Indian
Ocean. Because the Throughflow entering the
Indian Ocean is warm (about 20°C) relative to its
temperature as it exits the Indian Ocean across the
southern boundary at 32°S, the overall effect of
the Throughflow is to lead to an ocean heat transport convergence over the Indian Ocean. Most of
the analyses of Indian Ocean circulation and heat
transport assume a certain size and temperature
for the entering Throughflow, and these assumed
values for the Throughflow have a strong effect on
the resulting estimate of heat transport divergence
for the Indian Ocean.
Macdonald’s (1993) inverse calculations of the
circulation across 32°S were not able to determine
the size of the Throughflow. Her results indicated
that not only the heat transport but also the size
of the meridional overturning circulation varied
substantially with the size of the Throughflow.
With zero Throughflow, Macdonald’s estimate of
the southward heat transport across 30°S was
1.2 PW. While she did not quantify the changes in
heat transport divergence over the Indian Ocean,
increasing the Throughflow resulted in a decreased
meridional circulation, with the amount of
deep water flowing equatorward dropping from
21 Sv for a 0 Sv Throughflow to 5 Sv for a 20 Sv
Throughflow.
Toole and Warren (1993) analysed the circulation and heat transport from a 1987 trans-Indian
hydrographic section across 32°S. They emphasized the role of deep western boundary currents
carrying deep water northward and estimated a
large meridional overturning in the Indian Ocean
whereby 27 Sv of deep and bottom water flowed
across 32°S and returned much warmer and shallower in the southward flow of the Agulhas Current.
Their estimate of the overall heat gain by the
Indian Ocean north of 32°S was 0.98 PW. In a reanalysis of the same measurements, Robbins and
Toole (1997) showed that Toole and Warren’s
overturning circulation was incompatible with silica conservation in the Indian Ocean. Imposing a
silica conservation constraint within <100 kmol s
91
reduced the meridional overturning to about
12 Sv, as now most of the northward-flowing deep
western boundary current transport recirculates
southward across 32°S over the interior ocean.
With silica conservation, the heat gain by the
Indian Ocean north of 32°S is reduced to 0.42
PW. Each of these analyses was done with a
Throughflow of 7 Sv, similar to recent estimates
for its transport (Wijffels et al., 1996b), and with
an Agulhas Current transport of 85 Sv, appreciably larger than recent measurements of the Agulhas Current that feature a newly identified
Undercurrent (Beal and Bryden, 1997).
Ganachaud et al. (2000) have very recently used
the 1987 32°S section along with WOCE sections
across 20°S, 8°S and across the Indonesian
Throughflow at 110°E in an inverse analysis of
Indian Ocean circulation that utilizes biogeochemical flux constraints in addition to mass, heat and
salt constraints. They initially set reference levels
in the Agulhas Current at 32°S to agree with
recent measurements (Beal and Bryden, 1999), set
the Throughflow to be 7 Sv and imposed a silica
flux constraint of <700 kmol s
91 . The resulting
circulation exhibited a Throughflow transport of
15 Sv, a meridional overturning with 11 Sv of deep
water flowing into the Indian Ocean across 32°S
below 2000 m depth, and a small net heat gain of
only 0.1 PW for the Indian Ocean north of 32°S.
Since there are large uncertainties in the size of
the Throughflow transport as well as in what
physical processes over the subtropical and tropical Indian Ocean can convert large amounts of
cold deep water into warm upper layer waters, the
circulation of the Indian Ocean and its meridional
heat transport, particularly across its southern
boundary at 32°S, are ongoing research topics.
Revised estimates are likely when the WOCE measurements are thoroughly analysed.
6.1.4.4 Global synthesis
Macdonald (1998) and Macdonald and Wunsch
(1996) have made an impressive global inverse
analysis of selected high-quality hydrographic
SECTION 6 LARGE-SCALE OCEAN TRANSPORTS
464
flowing toward Antarctica, compensated by 8 Sv
flowing northward below 2000 m depth. The
uncertainty in the size of the horizontal heat transport, however, makes this heat transport estimate
unreliable because any small difference in temperature between the 131 Sv branch of the Antarctic
Circumpolar Current entering the area and subsequently leaving the area would swamp the overturning heat flux of 0.03 PW.
6.1.4.3 Indian Ocean
As for the Pacific, the unknown size of the Indonesian Throughflow complicates quantitative estimates of heat transport divergence in the Indian
Ocean. Because the Throughflow entering the
Indian Ocean is warm (about 20°C) relative to its
temperature as it exits the Indian Ocean across the
southern boundary at 32°S, the overall effect of
the Throughflow is to lead to an ocean heat transport convergence over the Indian Ocean. Most of
the analyses of Indian Ocean circulation and heat
transport assume a certain size and temperature
for the entering Throughflow, and these assumed
values for the Throughflow have a strong effect on
the resulting estimate of heat transport divergence
for the Indian Ocean.
Macdonald’s (1993) inverse calculations of the
circulation across 32°S were not able to determine
the size of the Throughflow. Her results indicated
that not only the heat transport but also the size
of the meridional overturning circulation varied
substantially with the size of the Throughflow.
With zero Throughflow, Macdonald’s estimate of
the southward heat transport across 30°S was
1.2 PW. While she did not quantify the changes in
heat transport divergence over the Indian Ocean,
increasing the Throughflow resulted in a decreased
meridional circulation, with the amount of
deep water flowing equatorward dropping from
21 Sv for a 0 Sv Throughflow to 5 Sv for a 20 Sv
Throughflow.
Toole and Warren (1993) analysed the circulation and heat transport from a 1987 trans-Indian
hydrographic section across 32°S. They emphasized the role of deep western boundary currents
carrying deep water northward and estimated a
large meridional overturning in the Indian Ocean
whereby 27 Sv of deep and bottom water flowed
across 32°S and returned much warmer and shallower in the southward flow of the Agulhas Current.
Their estimate of the overall heat gain by the
Indian Ocean north of 32°S was 0.98 PW. In a reanalysis of the same measurements, Robbins and
Toole (1997) showed that Toole and Warren’s
overturning circulation was incompatible with silica conservation in the Indian Ocean. Imposing a
silica conservation constraint within <100 kmol s
91
reduced the meridional overturning to about
12 Sv, as now most of the northward-flowing deep
western boundary current transport recirculates
southward across 32°S over the interior ocean.
With silica conservation, the heat gain by the
Indian Ocean north of 32°S is reduced to 0.42
PW. Each of these analyses was done with a
Throughflow of 7 Sv, similar to recent estimates
for its transport (Wijffels et al., 1996b), and with
an Agulhas Current transport of 85 Sv, appreciably larger than recent measurements of the Agulhas Current that feature a newly identified
Undercurrent (Beal and Bryden, 1997).
Ganachaud et al. (2000) have very recently used
the 1987 32°S section along with WOCE sections
across 20°S, 8°S and across the Indonesian
Throughflow at 110°E in an inverse analysis of
Indian Ocean circulation that utilizes biogeochemical flux constraints in addition to mass, heat and
salt constraints. They initially set reference levels
in the Agulhas Current at 32°S to agree with
recent measurements (Beal and Bryden, 1999), set
the Throughflow to be 7 Sv and imposed a silica
flux constraint of <700 kmol s
91 . The resulting
circulation exhibited a Throughflow transport of
15 Sv, a meridional overturning with 11 Sv of deep
water flowing into the Indian Ocean across 32°S
below 2000 m depth, and a small net heat gain of
only 0.1 PW for the Indian Ocean north of 32°S.
Since there are large uncertainties in the size of
the Throughflow transport as well as in what
physical processes over the subtropical and tropical Indian Ocean can convert large amounts of
cold deep water into warm upper layer waters, the
circulation of the Indian Ocean and its meridional
heat transport, particularly across its southern
boundary at 32°S, are ongoing research topics.
Revised estimates are likely when the WOCE measurements are thoroughly analysed.
6.1.4.4 Global synthesis
Macdonald (1998) and Macdonald and Wunsch
(1996) have made an impressive global inverse
analysis of selected high-quality hydrographic
SECTION 6 LARGE-SCALE OCEAN TRANSPORTS
464