What is the rate of ocean heat dispersion and the mechanism(s)?

Question

Let's assume that energy enters the surface and surface layers of the ocean (for this question the method of heating is irrelevant).

If we consider only the vertical movement of heat energy in the various layers of the oceans, how long does it take for this energy to be dispersed or transported to depth (and vice versa)?

For the second part, what is the mechanism for heat exchange between (or within) ocean layers: radiation, conduction or convection? If a combination, which is dominant? And what is the heat exchange rate for each?

Answer 1

Water is a poor conductor (570 mW/m-K), http://en.wikipedia.org/wiki/Water_%28data_page%29#Physical_and_thermodynamic_tables

but since it is a fluid, it can conduct heat upward using convection. This does not help so much if the water on top is less dense than the water below either because it is warmer or less salty. While there are reasonably strong horizontal currents on the surface, http://en.wikipedia.org/wiki/File:Corrientes-oceanicas.gif

the difference in salinity between the layers of ocean speaks volumes about the lack of vertical currents beyond the eddies caused by the horizontal surface currents. http://www.windows2universe.org/earth/Water/salinity_depth.html



Edit: Regardless of whether we are talking about sea water or fresh water, there is a temperature at which maximum density is reached. Since water expands as it freezes, that temperature is near, but a few degrees above freezing. As the latitude increases, and thus, the surface temperature decreases, we get to a point where the surface temperature gets near to the maximum density temperature of the water. Water chilled by the surface at this point will sink to the bottom. Thus, we have thermohaline circulation. http://en.wikipedia.org/wiki/File:Thermohaline_Circulation_2.png



Edit2:

The thermohaline circulation takes about 17 centuries. If it is responsible for Bond events, that may be more like 15 centuries. http://en.wikipedia.org/wiki/Bond_event

If we assume that the water travels a distance equivalent of 1 earth diameter along the bottom of the ocean in 147 decades, that would be about (4000 m/(147 years) or 27 m/yr.



27 m/yr = 74 mm/day

That clearly puts the current theoretically in the laminar flow region where there is virtually nothing in the way of eddies, and no mixing between vertical layers of water. http://hyperphysics.phy-astr.gsu.edu/hbase/pfric.html

In bodies of water where there is little in the way of currents at the bottom, it appears that the mixing due to motion at the surface loses its power between 600 and 800 meters depth. http://www.noc.soton.ac.uk/o4s/euroargo/argoeu_3a.php

As for the time it takes to move temperature from the bottom of the ocean to the top. consider still water 3 Km deep (ignoring the top 800 meter which would make our hypothertical 3800 meters deep). Assume a temperature difference of 5 kelvins. A cubic meter of water has a mass of 1060 Kg, and a heat capacity of 3.75 KJ/Kg-K assuming a salinity of 8% at 273 K. http://web.mit.edu/lienhard/www/Thermophysical_properties_of_seawater-DWT-16-354-2010.pdf

Using an area of one square meter, the 1060 Kg will be 1 meter deep.



(570 mW/m-K)(5 K) = 2.85 W/m

(2.85 W/m)(1 m^2/3000 m) = 950 microW

(3750 J/Kg-K)(1060 Kg)(3000 m/2)/950 uW = 6276 Tsec/Kelvin

= 73 Gdays/K = 199 Kmillennia to change the temperature one kelvin.

Thus, the temperature adjusts about 7.5 millikelvins from the time the water sinks to the bottom to the time it upwells to the surface about 15 centuries later. http://en.wikipedia.org/wiki/Upwelling

Answer 2

You must be deliberately trying to be stupid if you don't think that saltier water at 5 C can sink and transport heat to fresher water at 4 C.



Also, seawater doesn't have a density maximum at 4 C, that's for freshwater only (see figure below). This is probably causing the confusion for you since density of seawater is a non-intuitive function of salinity, temperature, and pressure. This means you can things going on like warmer fresher water getting compressed and being more dense than colder saltier water around it. This can drive very deep convective events, and also lead to even odder things like cabeling (which is mixing induced by density currents).



Anyway, the reference below talks about the vertical transport of heat in the ocean (you can find more information about these shallow overturning events in the Pacific by googling something called "mode water" which is sort of the same thing, I think). I apologize beforehand that it's not simple. Oceanography is like that.



It is really puzzling to me where you are going with this. Do you really not understand how cold things can transport heat to even colder things? That something at 0.0001 K has some heat in it and can warm something at 0.0000001 K?

Answer 3

In the "oceanic conveyor belt", the process is dense water sinking, not cold water sinking. Water masses near the poles get denser primarily from extraction of fresh water through surface freezing, leaving denser brine behind. The sinking water is actually warmer than the ice above it.

In ocean-wide mixing across the main thermocline, the main process is turbulent mixing. The mecahanisms I'm familiar with are coriolis forces (Eckman transport), breaking internal waves, and biological mixing due to vertical migration (primarily daily motion of the deep scattering layer).

The "more explanation" your mind requires is available in just about any physical oceanography text.



Edit :



"Ekman transport and Coriolis apply to surface or near surface currents " - No, there's surface forcing, but the effects are not near-surface only. All of the major upwelling areas so important to world fisheries are driven by Coriolis effects. Eddies alongside all major ocean currents result in warm-core and cold-core rings, 10s of kilometers across, with vertical transport of 1000 meters and more. This is evinced not only by the physical characteristics of the water mass but by the species present in the rings. Remnants of mixing events like this are also responsible for the thermal inversions so important to submariners.



Your disappointment in finding satisfactory answers is likely due to reading for quote-mining rather than comprehension.

Answer 4

I think the so called lag in the heating and CO2 increase/decrease of several hundred years might be as good a clue as any. The mathematics of the physics of the convection currents and the unknowns (we don't have a trillion thermometers or current detectors in the ocean), so it isn't something that could easily guessed at IMO. I think the heat exchange is all of the above but mostly convection but being a geologist, I might be biased toward that mechanism. Obviously convection isn't only vertical.



Dook's answer is instructive as a caricature of all alarmists. Notice how any attempt to get at facts is seen as threatening his cherished belief system.

Answer 5

The answer is whatever science factoid can be best spun to support your paranoid campaign of anti-science deceptions here. The question is most likely fake, however, because you could very likely find an "answer" more quickly on Wattsup or one of your other favorite fossil fuel serving anti-science blogs if that was what you cared about.