observed state (Wood and Bryan, Chapter 2.3).
Under flux adjustment, an amount of heat (or
fresh water) is taken away from the ocean but not
put into the atmosphere. The size of such flux
adjustment can be used to represent the model
error in ocean heat transport; in early coupled
model runs it was quite large (Manabe et al.,
1992). Recently, however, coupled models have
been run both by the National Center for Atmospheric Research and by the Hadley Centre for
hundreds of years without flux adjustment (Boville
and Gent, 1998; Gordon et al., 1998b) and the
model ocean and atmosphere have not drifted too
far from their initial states. Boville and Gent
showed that the reason that such a coupled model
can run without substantial drift is that the ocean
heat transport in the ocean component and the
ocean heat transport required by the atmospheric
component are very similar in magnitude and latitudinal structure. Ocean heat transports from the
Hadley Centre coupled model are in general agreement with Macdonald’s (1998) global synthesis
estimates (Table 6.1.2) and such a coupled model
may ultimately have the ability to help refine our
estimates of ocean heat transport. Again, however,
we suggest careful comparison between the air–sea
heat exchanges in the model and global flux climatologies, comparison between model ocean heat
transports and direct estimates of ocean heat
transport, and assessment of the (regional) drifts in
model heat content to evaluate critically such a
coupled model.
6.1.5.3 Mechanisms of ocean heat transport
For oceanographers, it is traditional to discuss
ocean circulation in terms of the transport of various water masses, as we did above for the deep
Atlantic circulation in terms of AABW, NADW
and AAIW. In fact, the precise definitions of these
water masses change from section to section and
the spatial variations in water mass characteristics
and transports provide information on the
amounts of mixing and advection in the interior
ocean. Applying a consistent definition of the density boundaries between water masses for Atlantic
sections at 45°S, 11°S, 11°N, 14°N and 24°N,
Speer et al. (1996) were surprised by the substantial exchange and mixing across the water mass
boundaries required for a consistent circulation of
AAIW, NADW and AABW from 45°S to 24°N in
the Atlantic. Thus, argued from the perspective of
water mass analysis, the advection and mixing of
each water mass must be sorted before the circulation can be considered finalized.
For ocean heat transport, however, the net
meridional advection of water masses is generally
the most important factor because small changes
in water mass characteristics due to mixing contribute little to heat transport on basin scales.
Analysis of the circulation and heat transport can
then be focused on the net flow in a set of layers
separated by isopycnals (e.g. Robbins and Toole,
1997). Such analyses naturally emphasize the
vertical-meridional overturning aspects of the
basin-scale circulation because eddies or gyre-scale
circulation within any single density layer can contribute little to heat transport, as the temperature
along each density surface exhibits only small variations (which are compensated by salinity). Thus,
analysis in density coordinates effectively sees
only the meridional overturning defined in density
classes.
Yet ocean circulation is commonly discussed in
terms of wind-driven gyre circulation and thermohaline-driven vertical-meridional overturning. As
discussed above, the Atlantic circulation and heat
transport are dominated by the meridional overturning. The North Pacific provides a contrast in
that its subtropical gyre exhibits a circulation in
which warm water flows northward in the
Kuroshio along the western boundary, loses heat
to the atmosphere and returns southward in the
central and eastern Pacific at colder temperature
(Bryden et al., 1991). One naturally describes such
a process of northward heat transport as being due
to the wind-driven horizontal circulation. In a
density analysis, however, the warm, less dense
water is transformed into colder, denser water, so
this same process also becomes an overturning in
density.
To preserve the distinction between the mechanisms of horizontal circulation and verticalmeridional overturning, we prefer to break up
the calculation of ocean heat transport into horizontal and vertical components, as follows. For a
zonal transoceanic section, separate meridional
velocity, v, and potential temperature, ⌰, into three
components:
¥ section-averaged values :v9 and :⌰9Ј
¥ zonally averaged baroclinic values :v9(z) and
:⌰9(z)
SECTION 6 LARGE-SCALE OCEAN TRANSPORTS
468
Under flux adjustment, an amount of heat (or
fresh water) is taken away from the ocean but not
put into the atmosphere. The size of such flux
adjustment can be used to represent the model
error in ocean heat transport; in early coupled
model runs it was quite large (Manabe et al.,
1992). Recently, however, coupled models have
been run both by the National Center for Atmospheric Research and by the Hadley Centre for
hundreds of years without flux adjustment (Boville
and Gent, 1998; Gordon et al., 1998b) and the
model ocean and atmosphere have not drifted too
far from their initial states. Boville and Gent
showed that the reason that such a coupled model
can run without substantial drift is that the ocean
heat transport in the ocean component and the
ocean heat transport required by the atmospheric
component are very similar in magnitude and latitudinal structure. Ocean heat transports from the
Hadley Centre coupled model are in general agreement with Macdonald’s (1998) global synthesis
estimates (Table 6.1.2) and such a coupled model
may ultimately have the ability to help refine our
estimates of ocean heat transport. Again, however,
we suggest careful comparison between the air–sea
heat exchanges in the model and global flux climatologies, comparison between model ocean heat
transports and direct estimates of ocean heat
transport, and assessment of the (regional) drifts in
model heat content to evaluate critically such a
coupled model.
6.1.5.3 Mechanisms of ocean heat transport
For oceanographers, it is traditional to discuss
ocean circulation in terms of the transport of various water masses, as we did above for the deep
Atlantic circulation in terms of AABW, NADW
and AAIW. In fact, the precise definitions of these
water masses change from section to section and
the spatial variations in water mass characteristics
and transports provide information on the
amounts of mixing and advection in the interior
ocean. Applying a consistent definition of the density boundaries between water masses for Atlantic
sections at 45°S, 11°S, 11°N, 14°N and 24°N,
Speer et al. (1996) were surprised by the substantial exchange and mixing across the water mass
boundaries required for a consistent circulation of
AAIW, NADW and AABW from 45°S to 24°N in
the Atlantic. Thus, argued from the perspective of
water mass analysis, the advection and mixing of
each water mass must be sorted before the circulation can be considered finalized.
For ocean heat transport, however, the net
meridional advection of water masses is generally
the most important factor because small changes
in water mass characteristics due to mixing contribute little to heat transport on basin scales.
Analysis of the circulation and heat transport can
then be focused on the net flow in a set of layers
separated by isopycnals (e.g. Robbins and Toole,
1997). Such analyses naturally emphasize the
vertical-meridional overturning aspects of the
basin-scale circulation because eddies or gyre-scale
circulation within any single density layer can contribute little to heat transport, as the temperature
along each density surface exhibits only small variations (which are compensated by salinity). Thus,
analysis in density coordinates effectively sees
only the meridional overturning defined in density
classes.
Yet ocean circulation is commonly discussed in
terms of wind-driven gyre circulation and thermohaline-driven vertical-meridional overturning. As
discussed above, the Atlantic circulation and heat
transport are dominated by the meridional overturning. The North Pacific provides a contrast in
that its subtropical gyre exhibits a circulation in
which warm water flows northward in the
Kuroshio along the western boundary, loses heat
to the atmosphere and returns southward in the
central and eastern Pacific at colder temperature
(Bryden et al., 1991). One naturally describes such
a process of northward heat transport as being due
to the wind-driven horizontal circulation. In a
density analysis, however, the warm, less dense
water is transformed into colder, denser water, so
this same process also becomes an overturning in
density.
To preserve the distinction between the mechanisms of horizontal circulation and verticalmeridional overturning, we prefer to break up
the calculation of ocean heat transport into horizontal and vertical components, as follows. For a
zonal transoceanic section, separate meridional
velocity, v, and potential temperature, ⌰, into three
components:
¥ section-averaged values :v9 and :⌰9Ј
¥ zonally averaged baroclinic values :v9(z) and
:⌰9(z)
SECTION 6 LARGE-SCALE OCEAN TRANSPORTS
468