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Oceanic Transport of Heat: Deep vs. Shallow Circulation
Reference
Boccaletti, G., Ferrari, R., Adcroft, A., Ferreira, D. and Marshall, J.  2005.  The vertical structure of ocean heat transport.  Geophysical Research Letters 32: 10.1029/2005GL022474.

Background
The authors note that "one of the most important contributions the ocean makes to earth's climate is through its poleward heat transport," and they say that concern has arisen over the possibility that global warming could affect this phenomenon by "reducing high latitude convection and triggering a collapse of the deep overturning circulation."  More recently, however, they report that "by identifying water mass ventilation for shallow, intermediate and deep waters," Talley (2003) has concluded that "the contribution of the shallow circulation to the global heat transport is indeed much larger than previously thought, and overall dominates the heat transport."

What was done
Enlarging upon Talley's work, Boccaletti et al. develop the tools for calculating the sizes of the contributions each of the three levels of circulation (shallow, intermediate and deep) make to the total poleward heat transport of the global ocean.

What was learned
In the words of the authors, "virtually all of the heat transport in the southern hemisphere is confined to a shallow circulation in the upper 500m," while "in the northern hemisphere a contribution by a deeper circulation is still present and represents the contribution of North Atlantic Deep Water."  However, they say "it only amounts to 0.4 PW [= 0.4 x 1015 W] out of the total [1.5 PW], the rest being surface intensified."

What it means
Boccaletti et al. conclude that since their work shows "the surface circulation dominates the heat transport in a model of the global ocean," it is "more likely that ocean heat flux will respond to changes in atmospheric winds, which drive the shallow overturning circulations associated with midlatitude subduction and equatorial upwelling (Boccaletti et al., 2004), rather than to changes in abyssal mixing and convection that drive the deep circulations."  As a result, they further conclude that "a decrease in deep overturning circulation alone does not imply a significant reduction of the global oceanic heat transport," in contrast to the views of Rahmstorf (1995) and Alley et al. (2003), which provide much anti-CO2 fodder for the world's climate alarmists.

References
Alley, R.B., Marotzke, J., Nordhaus, W.D., Overpeck, J.T., Peteet, D.M., Pielke Jr., R.A., Pierrehumbert, R.T., Rhines, P.B., Stocker, T.F., Talley, L.D. and Wallace, J.M.  2003.  Abrupt climate change.  Science 299: 2005-2010.

Boccaletti, G., Pacanowski, R.C., Philander, S.G.H. and Fedorov, A.  2004.  The thermal structure of the upper ocean.  Journal of Physical Oceanography 34: 888-902.

Rahmstorf, S.  1995.  Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle.  Nature 388: 825-826.

Talley, L.  2003.  Shallow, intermediate and deep overturning components of the global heat budget.  Journal of Physical Oceanography 33: 530-560.

Reviewed 26 October 2005