Ong

atmos

where now YAmax is the magnitude of the mass transport overturning circulation of

6The overturning circulation shown in Fig. 11.30 is derived from a model constrained by obervations, rather than inferred directly from observations, because it is all but impossible to observe Vmoc directly.

FIGURE 11.30. The meridional overturning circulation, wmoc, in a model of the global ocean plotted in the (i, z) plane on the left and the (i, 6) plane on the right. Note that on the left the scale over the top km of the ocean is greatly expanded.

the atmosphere in "energy" layers, appropriately chosen based on moist static energy, such that cpAñA = cp AT + gAz + LAq is the vertical change in the moist static energy (see Eq. 8-16 and the discussion of meridional transport of heat in Section 8.4) across the overturning circulation.

In Fig. 11.31 we plot annual-mean mass transport stream functions for the atmosphere and ocean, and TO, in energy layers, cpAñA for the atmosphere and cwAñO for the ocean. The horizontal axis is latitude, the vertical axis is an energy coordinate with units of cAñ, i.e., J kg-1. To facilitate comparison of the strength of the overturning circulation in the two fluids, we have (temporarily) redefined the Sverdrup as an equivalent unit of mass transport: in Fig. 11.31, 1 Sv is the mass transport associated with a volume flux of 106 m3 s-1 of water, which is 106 m3 s-1 x 103kgm-3 = 109kgs-1. The first striking feature of Fig. 11.31 is that, in contrast to Fig. 5.21, the atmospheric overturning circulation comprises one giant cell from equator to pole. This is because, unlike the overturning circulation plotted in Fig. 5.21, Fig. 11.31 includes a large eddy contribution. In middle-to-high latitudes, mass (like heat) is transferred by weather systems rather than by the mean flow. Secondly, note that the intensity of the oceanic cell is much weaker than its atmospheric counterpart. Even at 20°, where To reaches its maximum, the atmospheric mass transport is roughly four times that of the ocean. It is only within the deep tropics that the two transports are comparable. The third important feature is that the 'thickness' of the overturning cells in the two fluids are comparable in energy space. In midlatitudes, cwAôo/cpAôA is of order unity, the differences in heat capacity (cw/cp ~ 4) being compensated by a larger temperature difference across the atmospheric cell (Aûa ~ 40 K compared to Aêo ~ 10 K). The dominance of atmospheric over ocean heat transport in middle-to-high latitudes can thus be rationalized as being a consequence of TA greatly exceeding To (see Fig. 11.31).7 Finally, note that To is

7It is remarkable that despite the density of air being typically one thousand times less than that of water, the meridional mass transport in the atmosphere (~100 Sv = 1011kgs-1) exceeds that of the ocean by a factor of 4. This is because meridional wind speeds greatly exceed those in the ocean.

FIGURE 11.31. Annual mean atmospheric (black) and oceanic (green) mass streamfunction within constant energy layers. The contour interval is horizontal axis is latitude; the vertical axis is an energy coordinate (cAS) in units of 104Jkg-1. From Czaja and Marshall (2006).

FIGURE 11.31. Annual mean atmospheric (black) and oceanic (green) mass streamfunction within constant energy layers. The contour interval is horizontal axis is latitude; the vertical axis is an energy coordinate (cAS) in units of 104Jkg-1. From Czaja and Marshall (2006).

is dominated by two large symmetrically disposed cells around the equator. As mentioned previously, this, in the main, is the overturning cell associated with wind-driven subtropical gyres, carrying heat up to 40° N, S or so. The weaker cells at lower temperatures in polar latitudes are the signature of overturning cells directly associated with convectively-induced, polar thermohaline circulations.

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