understand ocean circulation. It is not sufficient
just to calculate the heat transport. The mechanisms of ocean heat transport must be understood
and the above separation represents a quantitative
method to present the mechanisms of ocean heat
transport for comparisons between ocean basins
and between observed and simulated basin-scale
ocean circulations.
6.1.6 Challenges
Using the WOCE data sets to best advantage for
the determination of the circulation and heat
transport in each ocean basin is clearly the principal challenge over the next few years. While the
Atlantic circulation and heat transport will
undoubtedly be refined from continued analysis,
the Pacific and Indian Ocean circulations represent
the principal challenge for developing our understanding of the global ocean circulation and ocean
heat transport. We know that about 15 Sv of cold
deep waters exit the South Atlantic into the Southern Ocean; somewhere in the Indian, Pacific or
Southern Ocean these waters must rise and warm
before they return to flow northward through the
Atlantic. There are two common conjectures for
the required upwelling and warming of deep
waters: the Warm Water Pathway argues that deep
waters upwell in the tropical and equatorial Pacific
and Indian oceans and are then carried back into
the Atlantic by the Agulhas Current around southern Africa (Gordon, 1986); the Cold Water Pathway argues that deep waters upwell around the
Antarctic and then warm as they flow eastward
in the Antarctic Circumpolar Current, through
Drake Passage, and then northward into the
Atlantic (Rintoul, 1991). Clearly the upwelling of
deep waters and their warming need not occur
simultaneously.
The Warm Water Pathway attributes a crucial
role to warming of the cold waters in the Pacific
and Indian tropical regions and hence implies a
sizeable heat gain by the ocean from the atmosphere and a sizeable southward ocean heat transport across the combined southern boundary of
the Pacific–Indian Ocean at about 32°S. Determining the circulation and associated meridional heat
transport across 32°S in the Pacific and Indian
oceans would critically test the Warm Water
Pathway conjecture. As shown above, there is little
evidence for a deep upwelling in the equatorial
Pacific as the deep and shallow water circulations
appear to be separate and self-contained and the
present estimate of 12 Sv upwelling in the equatorial Indian Ocean remains controversial because the
large mixing processes that must accompany such
upwelling have not been measured and are not
understood.
The Cold Water Pathway attributes a crucial
role for the warming of upwelled waters to the
Southern Ocean and attributes a small role for the
Indian and Pacific equatorial regions. While it is
hard to believe that the Antarctic circumpolar
region of the Southern Ocean could gain much
heat from the atmosphere (Trenberth and Solomon,
1994), da Silva et al.’s (1994) adjusted air–sea flux
climatology does include sizeable ocean heat gain
over latitudes south of 45°S. Validation of the
Cold Water Pathway with such polar and subpolar
heat gain may require a concerted programme of
buoy measurements of air–sea fluxes in the inhospitable Southern Ocean region. Indirectly, however, determining the circulation and heat
transport across 32°S in Pacific and Indian Oceans
could help to validate the Cold Water Pathway if
their deep and shallow circulations were found to
be separate and self-contained so there were effectively no poleward heat transports out of the
Pacific and Indian Oceans.
Thus, the principal challenge is to quantify the
circulation and heat transports of the Pacific and
Indian Oceans, particularly across their southern
boundaries. The WOCE measurements included
sections across southern boundaries of the Pacific
and Indian Oceans, time series measurements of
deep western boundary current at 32°S in the
Pacific and at 20°S in the Indian, and time series
measurements of western boundary at 30°S in the
East Australian Current and at 31°S in the Agulhas Current. Along each section, upper ocean currents were monitored by the shipboard underway
ADCP and nutrients and biogeochemical tracers
were measured throughout the water column. In
addition, there are long time-series of sea surface
height from the TOPEX/POSEIDON altimetric
satellite, subsurface float measurements at about
1000 m depth, and surface drifter measurements.
Classical analysis of open-ocean hydrographic
measurements has concentrated on identifying a
zero velocity surface for geostrophic velocity and
transport estimates based on the distribution of
temperature, salinity, oxygen and nutrients. During
SECTION 6 LARGE-SCALE OCEAN TRANSPORTS
470
just to calculate the heat transport. The mechanisms of ocean heat transport must be understood
and the above separation represents a quantitative
method to present the mechanisms of ocean heat
transport for comparisons between ocean basins
and between observed and simulated basin-scale
ocean circulations.
6.1.6 Challenges
Using the WOCE data sets to best advantage for
the determination of the circulation and heat
transport in each ocean basin is clearly the principal challenge over the next few years. While the
Atlantic circulation and heat transport will
undoubtedly be refined from continued analysis,
the Pacific and Indian Ocean circulations represent
the principal challenge for developing our understanding of the global ocean circulation and ocean
heat transport. We know that about 15 Sv of cold
deep waters exit the South Atlantic into the Southern Ocean; somewhere in the Indian, Pacific or
Southern Ocean these waters must rise and warm
before they return to flow northward through the
Atlantic. There are two common conjectures for
the required upwelling and warming of deep
waters: the Warm Water Pathway argues that deep
waters upwell in the tropical and equatorial Pacific
and Indian oceans and are then carried back into
the Atlantic by the Agulhas Current around southern Africa (Gordon, 1986); the Cold Water Pathway argues that deep waters upwell around the
Antarctic and then warm as they flow eastward
in the Antarctic Circumpolar Current, through
Drake Passage, and then northward into the
Atlantic (Rintoul, 1991). Clearly the upwelling of
deep waters and their warming need not occur
simultaneously.
The Warm Water Pathway attributes a crucial
role to warming of the cold waters in the Pacific
and Indian tropical regions and hence implies a
sizeable heat gain by the ocean from the atmosphere and a sizeable southward ocean heat transport across the combined southern boundary of
the Pacific–Indian Ocean at about 32°S. Determining the circulation and associated meridional heat
transport across 32°S in the Pacific and Indian
oceans would critically test the Warm Water
Pathway conjecture. As shown above, there is little
evidence for a deep upwelling in the equatorial
Pacific as the deep and shallow water circulations
appear to be separate and self-contained and the
present estimate of 12 Sv upwelling in the equatorial Indian Ocean remains controversial because the
large mixing processes that must accompany such
upwelling have not been measured and are not
understood.
The Cold Water Pathway attributes a crucial
role for the warming of upwelled waters to the
Southern Ocean and attributes a small role for the
Indian and Pacific equatorial regions. While it is
hard to believe that the Antarctic circumpolar
region of the Southern Ocean could gain much
heat from the atmosphere (Trenberth and Solomon,
1994), da Silva et al.’s (1994) adjusted air–sea flux
climatology does include sizeable ocean heat gain
over latitudes south of 45°S. Validation of the
Cold Water Pathway with such polar and subpolar
heat gain may require a concerted programme of
buoy measurements of air–sea fluxes in the inhospitable Southern Ocean region. Indirectly, however, determining the circulation and heat
transport across 32°S in Pacific and Indian Oceans
could help to validate the Cold Water Pathway if
their deep and shallow circulations were found to
be separate and self-contained so there were effectively no poleward heat transports out of the
Pacific and Indian Oceans.
Thus, the principal challenge is to quantify the
circulation and heat transports of the Pacific and
Indian Oceans, particularly across their southern
boundaries. The WOCE measurements included
sections across southern boundaries of the Pacific
and Indian Oceans, time series measurements of
deep western boundary current at 32°S in the
Pacific and at 20°S in the Indian, and time series
measurements of western boundary at 30°S in the
East Australian Current and at 31°S in the Agulhas Current. Along each section, upper ocean currents were monitored by the shipboard underway
ADCP and nutrients and biogeochemical tracers
were measured throughout the water column. In
addition, there are long time-series of sea surface
height from the TOPEX/POSEIDON altimetric
satellite, subsurface float measurements at about
1000 m depth, and surface drifter measurements.
Classical analysis of open-ocean hydrographic
measurements has concentrated on identifying a
zero velocity surface for geostrophic velocity and
transport estimates based on the distribution of
temperature, salinity, oxygen and nutrients. During
SECTION 6 LARGE-SCALE OCEAN TRANSPORTS
470