The Southern Ocean meridional overturning is
induced by the large-scale wind and by buoyancy
forcing of the very cold conditions along the margins of Antarctica. Deep water (potential temperature 1–2°C, salinity 34.7–34.9) with characteristics
developed in more northern latitudes, upwells
around Antarctica to be converted into Antarctic
Intermediate Water (AAIW; 3–5°C, 34.2–34.4) and
Antarctic Bottom Water (AABW; :91°C, 34.65–
34.75). Southern Ocean overturning induces large
upward heat flux, limiting sea ice thickness
(Gordon and Huber, 1990). Southern Ocean overturning is most effective in projecting Southern
Ocean water mass properties to the global scale
(see Sloyan and Rintoul, 2000b).
An interesting aspect of the ACC relevant to
interocean heat and freshwater fluxes, the theme
of this chapter, which may contribute to the saltiness of the Atlantic Ocean, was pointed out by
Gordon and Piola (1983), who inspected the salinity change of the subpolar surface water within the
ACC en route across the South Atlantic. They
found that the observed freshening equals the net
evaporation minus precipitation of the subtropical
South Atlantic. The water vapour derived from the
subtropics is carried southward, to balance excess
precipitation over the ACC (see Fig. 12.10 of
Peixoto and Oort, 1992). The ACC exports the
fresh water across the Indian Ocean into the
Pacific Ocean. This situation arises because
the poleward extension of South America and the
Andes limits freshwater transfer from the Pacific
to the Atlantic by both ocean and atmosphere.
There is no such block provided by Africa, which
does not reach the position of the maximum westerlies. The same may be true of the Indian sector,
as southern Australia is also well north of the
maximum westerlies. Extraction of fresh water
from the South Atlantic increases the salinity of
the South Atlantic subtropical water, which has an
extensive region of surface salinity of greater than
36.4, an attribute lacking in the other southern
hemisphere subtropical gyres (Fig. 4.7.1b, see Plate
4.7.1b, p. 300). Saline South Atlantic surface
water eventually spreads northward within the
North Brazil current and may be a factor in the
saltiness of the North Atlantic Ocean.
4.7.1.2 Northern oceans
The great ocean embayments of the northern hemisphere vary markedly from each other. Surplus
precipitation and runoff over evaporation in the
North Pacific induces low surface salinity with a
highly stable halocline inhibiting deep-reaching convection. The lack of deep convection in the subpolar
North Pacific and associated meridional overturning circulation limits poleward spreading of warm
low-latitude waters, which further suppresses evaporation (Warren, 1983). The North Atlantic Ocean,
with excess evaporation over precipitation and
runoff, forms a saline surface layer prone to deep
convection and the formation of North Atlantic
Deep Water (NADW). Sinking of surface water
associated with NADW formation draws compensatory warmer surface water poleward. Warmer
SST in the western subtropical North Atlantic (relative to that of the North Pacific, Fig. 4.7.1a, see
Plate 4.7.1a, p. 300) may encourage further evaporation, invigorating Atlantic meridional overturning
circulation. Evaporation of the subtropical North
Atlantic and net precipitation over the equatorial
Pacific are coupled by westward water vapour flux
(about 0.3 Sv; Zaucker and Broecker, 1992) across
the Isthmus of Panama, thought to be a major force
behind NADW formation (Zaucker et al., 1994).
The Pacific halocline abruptly developed, along
with intensification of northern hemisphere glaciation, about 2.73 million years ago (Haug et al.,
1999). It is likely that this coincided with severing
of the direct oceanic link between the North
Pacific and North Atlantic Oceans with the rise of
the Isthmus of Panama and, by inference, the
development of a saline North Atlantic, conductive to NADW formation. Thus a change in interocean exchange 2.73 million years ago forced a
different pattern of horizontal and vertical circulation that remains active in today’s ocean.
The Indian Ocean north of 10°S is exposed to a
strongly monsoonal climate. Convection into the
thermocline and intermediate levels occurs within
the evaporative, saline Arabian and Red Seas.
Enormous influx of fresh water produces stratification reminiscent of an estuarine environment
within the Bay of Bengal. Exchange of fresh water
between the Arabian Sea and the Bay of Bengal
may be viewed as a regional analogue to the
Atlantic–Pacific global-scale system.
4.7.1.3 The global chain of interocean
thermohaline links
Interocean exchange is suspected as being an important part of the present-day global thermohaline
SECTION 4 THE GLOBAL FLOW FIELD
304
induced by the large-scale wind and by buoyancy
forcing of the very cold conditions along the margins of Antarctica. Deep water (potential temperature 1–2°C, salinity 34.7–34.9) with characteristics
developed in more northern latitudes, upwells
around Antarctica to be converted into Antarctic
Intermediate Water (AAIW; 3–5°C, 34.2–34.4) and
Antarctic Bottom Water (AABW; :91°C, 34.65–
34.75). Southern Ocean overturning induces large
upward heat flux, limiting sea ice thickness
(Gordon and Huber, 1990). Southern Ocean overturning is most effective in projecting Southern
Ocean water mass properties to the global scale
(see Sloyan and Rintoul, 2000b).
An interesting aspect of the ACC relevant to
interocean heat and freshwater fluxes, the theme
of this chapter, which may contribute to the saltiness of the Atlantic Ocean, was pointed out by
Gordon and Piola (1983), who inspected the salinity change of the subpolar surface water within the
ACC en route across the South Atlantic. They
found that the observed freshening equals the net
evaporation minus precipitation of the subtropical
South Atlantic. The water vapour derived from the
subtropics is carried southward, to balance excess
precipitation over the ACC (see Fig. 12.10 of
Peixoto and Oort, 1992). The ACC exports the
fresh water across the Indian Ocean into the
Pacific Ocean. This situation arises because
the poleward extension of South America and the
Andes limits freshwater transfer from the Pacific
to the Atlantic by both ocean and atmosphere.
There is no such block provided by Africa, which
does not reach the position of the maximum westerlies. The same may be true of the Indian sector,
as southern Australia is also well north of the
maximum westerlies. Extraction of fresh water
from the South Atlantic increases the salinity of
the South Atlantic subtropical water, which has an
extensive region of surface salinity of greater than
36.4, an attribute lacking in the other southern
hemisphere subtropical gyres (Fig. 4.7.1b, see Plate
4.7.1b, p. 300). Saline South Atlantic surface
water eventually spreads northward within the
North Brazil current and may be a factor in the
saltiness of the North Atlantic Ocean.
4.7.1.2 Northern oceans
The great ocean embayments of the northern hemisphere vary markedly from each other. Surplus
precipitation and runoff over evaporation in the
North Pacific induces low surface salinity with a
highly stable halocline inhibiting deep-reaching convection. The lack of deep convection in the subpolar
North Pacific and associated meridional overturning circulation limits poleward spreading of warm
low-latitude waters, which further suppresses evaporation (Warren, 1983). The North Atlantic Ocean,
with excess evaporation over precipitation and
runoff, forms a saline surface layer prone to deep
convection and the formation of North Atlantic
Deep Water (NADW). Sinking of surface water
associated with NADW formation draws compensatory warmer surface water poleward. Warmer
SST in the western subtropical North Atlantic (relative to that of the North Pacific, Fig. 4.7.1a, see
Plate 4.7.1a, p. 300) may encourage further evaporation, invigorating Atlantic meridional overturning
circulation. Evaporation of the subtropical North
Atlantic and net precipitation over the equatorial
Pacific are coupled by westward water vapour flux
(about 0.3 Sv; Zaucker and Broecker, 1992) across
the Isthmus of Panama, thought to be a major force
behind NADW formation (Zaucker et al., 1994).
The Pacific halocline abruptly developed, along
with intensification of northern hemisphere glaciation, about 2.73 million years ago (Haug et al.,
1999). It is likely that this coincided with severing
of the direct oceanic link between the North
Pacific and North Atlantic Oceans with the rise of
the Isthmus of Panama and, by inference, the
development of a saline North Atlantic, conductive to NADW formation. Thus a change in interocean exchange 2.73 million years ago forced a
different pattern of horizontal and vertical circulation that remains active in today’s ocean.
The Indian Ocean north of 10°S is exposed to a
strongly monsoonal climate. Convection into the
thermocline and intermediate levels occurs within
the evaporative, saline Arabian and Red Seas.
Enormous influx of fresh water produces stratification reminiscent of an estuarine environment
within the Bay of Bengal. Exchange of fresh water
between the Arabian Sea and the Bay of Bengal
may be viewed as a regional analogue to the
Atlantic–Pacific global-scale system.
4.7.1.3 The global chain of interocean
thermohaline links
Interocean exchange is suspected as being an important part of the present-day global thermohaline
SECTION 4 THE GLOBAL FLOW FIELD
304