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FIGURE 11.28. Top: Estimate of global ocean circulation patterns based on Ganachaud and Wunsch (2000) modified from Alley et al. (2002). The circulation is separated into 3 layers: shallow (red, < 2 km), deep (blue, 2—4 km), and bottom (green, > 4 km). Horizontal arrows across the marked sections represent the volume transport in Sv. Circles (© for upwelling, ® for downwelling) represent the vertical transport out of the layer in question in Sv. Bottom: A cartoon, based on the quantitative estimates shown above, of the ocean's 'shallow to deep' overturning circulation illustrating the asymmetry between the Atlantic and Pacific basins and between northern and southern hemispheres. Blue represents deep flow (2—4 km, red shallow flow (< 2 km). Transitions between shallow and deep are also indicated. This global overturning pattern has become known as the 'conveyer belt'. It is a schematic representation of a highly complex, turbulent flow.

FIGURE 11.28. Top: Estimate of global ocean circulation patterns based on Ganachaud and Wunsch (2000) modified from Alley et al. (2002). The circulation is separated into 3 layers: shallow (red, < 2 km), deep (blue, 2—4 km), and bottom (green, > 4 km). Horizontal arrows across the marked sections represent the volume transport in Sv. Circles (© for upwelling, ® for downwelling) represent the vertical transport out of the layer in question in Sv. Bottom: A cartoon, based on the quantitative estimates shown above, of the ocean's 'shallow to deep' overturning circulation illustrating the asymmetry between the Atlantic and Pacific basins and between northern and southern hemispheres. Blue represents deep flow (2—4 km, red shallow flow (< 2 km). Transitions between shallow and deep are also indicated. This global overturning pattern has become known as the 'conveyer belt'. It is a schematic representation of a highly complex, turbulent flow.

In deriving the above we have written Prefv = -d^o /dz in which YO is the stream-function for the mass transport in the meridional plane and made use of the fact that (i) YO = 0 at the top and bottom of the ocean and (ii) the mass transport can be expressed as the product of the density of water multiplied by the volume transport of the meridional overturning circulation (MOC), wmoc thus:

FIGURE 11.29. A schematic diagram of the ocean's meridional overturning circulation, in which warm waters flow poleward at the surface, are cooled by loss of heat to the atmosphere, sink to depth and return equatorward. Such a circulation achieves a poleward transport of heat.

™ bottom that the heat transport can be expressed in terms of the mass transport in temperature layers, or, more generally, noting the multiplication by cw, the mass transport in energy layers. It is then useful to write down an approximate form thus:

where Aê is the difference in potential temperature between the poleward and equatorward branches, and Y0max is the strength of the overturning mass transport.

Figure 11.30 shows yM0C for the global ocean plotted in the (1,z) and (1,d) plane6: we see that it has a magnitude of order 20 Sv, implying a meridional mass transport of Y0max = 20 x 109 kgs-1. If the temperature difference across the MOC is Aê = 15 K, typical of the temperature drop across the main thermocline (see Fig. 9.5), then Eq. 11-15 yields a heat transport of 1.2 PW, of the order presented in Fig. 11.27. Thus Eq. 11-15 is a useful vantage point from which to discuss mechanisms of ocean heat transport and—see below—the partition of heat transport between the atmosphere and the ocean.

Both wind-driven and thermohaline circulations play an important role in setting the magnitude and pattern of Yo. Fluid which is pumped down by the wind in middle latitudes is compensated in part by poleward transport of warm surface waters from the tropics in surface Ekman layers. The subsurface equatorial return flow occurs at a colder temperature, resulting in poleward heat transport. Indeed the wind is responsible for the two shallow 'wheels' of overturning circulation symmetrically disposed about the equator in Fig. 11.30 (right), reaching up to ±40°. This is very evident in the Pacific with a heat transport which is symmetric about the equator. In polar latitudes we see cells driven by convective processes feeding the abyssal ocean associated with NADW in the north and AABW in the south. As already mentioned, Atlantic heat transport is northward at all latitudes, consistent with idea that a giant interhemispheric meridional overturning cell associated with polar sinking is a dominant heat transport mechanism, as sketched in Fig. 11.28 (bottom). As we shall discuss in more detail in Section 12.3.5, variability in the MOC in the Atlantic Ocean is often invoked as a player of climate change because of the likely sensitivity of arctic processes to meridional heat transport mediated by the MOC and vice versa.

The framework provided by Eq. 11-15 can also be used to come to some understanding of the processes that set the partition of heat transport between the atmosphere and ocean. We can express the atmospheric heat transport in the same form:

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