All three ocean basins exchange large amounts of
water through linking passages: the Southern Ocean,
the Indonesian Archipelago and the Bering Strait.
As these interbasin fluxes are much larger than
the exchange through the atmosphere, we choose
instead to present only the divergent part of the
oceanic freshwater flux (in contrast to Wijffels et al.,
1992). This is equivalent to removing an unknown
constant equal to the Pacific–Indian Throughflow
for the Indian and South Pacific Oceans, and the
Bering Strait flow in the North Pacific and Atlantic.
Hence in the plots to follow, the direct transport
estimates shown will be the divergence relative to
the entrances of the Bering and Throughflow straits
(South of Mindanao, Philippines).
While the size and salinity of the Bering Strait
flow is relatively well known (Coachman and
Aagaard, 1988), that of the Pacific–Indian
Throughflow is not. Hence, investigators have had
to make assumptions about both its size and salinity in order to generate an estimate of the basin
divergence – that is to calculate the ‘leakage’ term
in equation (6.2.4). Direct estimates for the longterm average Throughflow range between 5 and
10 Sv (Meyers, 1996; Molcard et al., 1996; Gordon
et al., 1999a), though synoptically, higher variability (0–20 Sv) is observed (Fieux et al., 1994; Bray
et al., 1997). For a 5–10 Sv transport range the
resulting freshwater estimates between the
Throughflow entrance channels and 30°S WOCE
section in the Pacific is only about 0.2 Sv (Wijffels
et al., 2000), consonant with our estimates of the
general uncertainty. Clearly, however, increased
knowledge of the freshwater flux associated with
the Throughflow is needed.
The majority of direct flux estimates derive from
single-section or regional analyses of long hydrographic lines, many of which were accomplished
during WOCE (Table 6.2.2). To date few truly
global syntheses have been completed, with the
exception of MacDonald (1995). The latter study,
however, specified freshwater fluxes using Baumgartner and Reichel (1975) and so is not included
here. The flux divergences produced from the Southern Ocean analysis of Sloyan and Rintoul (2000b)
are included in basin integral plots (Figs 6.2.6–6.2.8,
see Plates 6.2.6–6.2.8, p. 492) by assuming zero
freshwater flux from the Antarctic continent.
Figures 6.2.6–6.2.8 (see Plates 6.2.6–6.2.8,
p. 492) present the divergent part of the freshwater transport for each ocean basin found from
accumulating the surface and runoff fluxes southwards from basin reference points. In the Indian
Ocean transports are relative to a zero flux condition at the Through- flow channels (Pacific side);
for the Pacific, zero flux is applied at Bering Strait
and the Throughflow channels (Pacific side); while
the Atlantic curve is relative to a zero flux at
Bering Strait, and thus includes the freshwater
fluxes into the Arctic Oceans.
In the Indian Ocean, only two latitudes are
currently constrained by direct flux estimates: 32
and 18°S (Fig. 6.2.6, see Plate 6.2.6, p. 492). Over
the evaporative zone between latitudes 15 and 40°S,
the indirect flux estimates are fairly consistent (similar slopes) and are also in reasonable agreement
with the direct estimates. It is in the regions of high
precipitation north of 10°S and south of 40°S that
the curves diverge, confirming again the large scatter among available estimates of precipitation over
the ocean. Note in particular the 0.5 Sv variability
in the net freshwater divergence north of 18°S.
The Atlantic Ocean is best covered by direct
flux estimates (Fig. 6.2.7, see Plate 6.2.7, p. 492),
which are remarkably consistent, except for the
estimates of Dobroliubov (1997) at 24°N. Nearly
all of the major transport maxima are delineated
by the direct estimates. Again, the indirect estimates diverge most strongly over regions of high
precipitation. When integrated over the Atlantic
between Bering Strait and 40°S, the indirect transport divergences range between 1.0 and 90.1 Sv,
while the direct estimates indicate very little net
fresh-water divergence over the basin.
Problems with biases in the indirect transport
estimates are even more pronounced in the Pacific
Ocean due to its huge size (Fig. 6.2.8, see Plate
6.2.8, p. 492). Not surprisingly the indirect estimates vary wildly over the South Pacific where little
in-situ atmospheric data is available. Here again,
despite the variations in assumptions made to close
the ocean mass balance, Throughflow sizes and
different data sets, the direct estimates are quite
consistent, and show much less scatter than the indirect estimates. It is remarkable that despite 25 years
between sections, the estimates of Wunsch et al.
(1983) and Wijffels et al. (2000) are very similar.
Though a more consistent and larger set of direct
ocean transports estimates can be anticipated from
the WOCE synthesis, the ensemble available to
date can already be used to reassess ideas about
the interocean exchange of fresh water. Using
SECTION 6 LARGE-SCALE OCEAN TRANSPORTS
484
water through linking passages: the Southern Ocean,
the Indonesian Archipelago and the Bering Strait.
As these interbasin fluxes are much larger than
the exchange through the atmosphere, we choose
instead to present only the divergent part of the
oceanic freshwater flux (in contrast to Wijffels et al.,
1992). This is equivalent to removing an unknown
constant equal to the Pacific–Indian Throughflow
for the Indian and South Pacific Oceans, and the
Bering Strait flow in the North Pacific and Atlantic.
Hence in the plots to follow, the direct transport
estimates shown will be the divergence relative to
the entrances of the Bering and Throughflow straits
(South of Mindanao, Philippines).
While the size and salinity of the Bering Strait
flow is relatively well known (Coachman and
Aagaard, 1988), that of the Pacific–Indian
Throughflow is not. Hence, investigators have had
to make assumptions about both its size and salinity in order to generate an estimate of the basin
divergence – that is to calculate the ‘leakage’ term
in equation (6.2.4). Direct estimates for the longterm average Throughflow range between 5 and
10 Sv (Meyers, 1996; Molcard et al., 1996; Gordon
et al., 1999a), though synoptically, higher variability (0–20 Sv) is observed (Fieux et al., 1994; Bray
et al., 1997). For a 5–10 Sv transport range the
resulting freshwater estimates between the
Throughflow entrance channels and 30°S WOCE
section in the Pacific is only about 0.2 Sv (Wijffels
et al., 2000), consonant with our estimates of the
general uncertainty. Clearly, however, increased
knowledge of the freshwater flux associated with
the Throughflow is needed.
The majority of direct flux estimates derive from
single-section or regional analyses of long hydrographic lines, many of which were accomplished
during WOCE (Table 6.2.2). To date few truly
global syntheses have been completed, with the
exception of MacDonald (1995). The latter study,
however, specified freshwater fluxes using Baumgartner and Reichel (1975) and so is not included
here. The flux divergences produced from the Southern Ocean analysis of Sloyan and Rintoul (2000b)
are included in basin integral plots (Figs 6.2.6–6.2.8,
see Plates 6.2.6–6.2.8, p. 492) by assuming zero
freshwater flux from the Antarctic continent.
Figures 6.2.6–6.2.8 (see Plates 6.2.6–6.2.8,
p. 492) present the divergent part of the freshwater transport for each ocean basin found from
accumulating the surface and runoff fluxes southwards from basin reference points. In the Indian
Ocean transports are relative to a zero flux condition at the Through- flow channels (Pacific side);
for the Pacific, zero flux is applied at Bering Strait
and the Throughflow channels (Pacific side); while
the Atlantic curve is relative to a zero flux at
Bering Strait, and thus includes the freshwater
fluxes into the Arctic Oceans.
In the Indian Ocean, only two latitudes are
currently constrained by direct flux estimates: 32
and 18°S (Fig. 6.2.6, see Plate 6.2.6, p. 492). Over
the evaporative zone between latitudes 15 and 40°S,
the indirect flux estimates are fairly consistent (similar slopes) and are also in reasonable agreement
with the direct estimates. It is in the regions of high
precipitation north of 10°S and south of 40°S that
the curves diverge, confirming again the large scatter among available estimates of precipitation over
the ocean. Note in particular the 0.5 Sv variability
in the net freshwater divergence north of 18°S.
The Atlantic Ocean is best covered by direct
flux estimates (Fig. 6.2.7, see Plate 6.2.7, p. 492),
which are remarkably consistent, except for the
estimates of Dobroliubov (1997) at 24°N. Nearly
all of the major transport maxima are delineated
by the direct estimates. Again, the indirect estimates diverge most strongly over regions of high
precipitation. When integrated over the Atlantic
between Bering Strait and 40°S, the indirect transport divergences range between 1.0 and 90.1 Sv,
while the direct estimates indicate very little net
fresh-water divergence over the basin.
Problems with biases in the indirect transport
estimates are even more pronounced in the Pacific
Ocean due to its huge size (Fig. 6.2.8, see Plate
6.2.8, p. 492). Not surprisingly the indirect estimates vary wildly over the South Pacific where little
in-situ atmospheric data is available. Here again,
despite the variations in assumptions made to close
the ocean mass balance, Throughflow sizes and
different data sets, the direct estimates are quite
consistent, and show much less scatter than the indirect estimates. It is remarkable that despite 25 years
between sections, the estimates of Wunsch et al.
(1983) and Wijffels et al. (2000) are very similar.
Though a more consistent and larger set of direct
ocean transports estimates can be anticipated from
the WOCE synthesis, the ensemble available to
date can already be used to reassess ideas about
the interocean exchange of fresh water. Using
SECTION 6 LARGE-SCALE OCEAN TRANSPORTS
484