More ocean mixing
August 3, 2009, 2:35 pm
Filed under: Earth history, Geoengineering

A sperm whale, off to stir up the depths

A sperm whale, off to stir up the depths

Following on from the post about artificial ocean mixing, last week’s Nature has a new development in a story I’ve always thought rather fun: the role of animals in stirring up the ocean. The problem that this addresses is what can be called the “missing mixing”. For the oceans to circulate from top to bottom, water has to sink — as it does in the North Atlantic and round Antarctica — but it also has to be lifted up; deeper, denser waters have to return to the surface. (If sinking was the whole story the ocean would simply remain stratified.) Considering how vast the ocean is, and what a huge and climate-crucial amount of heat it pumps around, the magnitude  of the mixing required is remarkably small: two or three terawatts, or as William Dewar strikingly puts it in a News and Views piece accompanying the Nature article,

Roughly speaking, all the energy needed to mix a cubic kilometre of subsurface ocean could be provided by a single hand-held kitchen mixer.

The problem is that, small as it is on a planetary scale, no one is sure how that couple of terawatts of mixing comes about. Astronomy offers a figure for the dissipation of energy by tides of almost 4TW, but a lot of the work done there is done in shallow seas not deep oceans. Carl Wunsch and others have estimated the amount of  work attributable to wind at about 1TW. But there’s no guarantee that all that work actually cashes out as mixing — you can do work on water that doesn’t mix it up much. So winds and tides alone might be enough — but its quite possible that they are not.

Dewar has been arguing for a while that some of the missing mixing might be attributable to the movement of animals. In an endearing order of magnitude calculation a few years ago he suggested that the world’s 360,000 sperm whales, which spend 80% of their time at depth expending about 5kW on their swimming, contribute more than a gigawatt of mixing. Dewar argues on various grounds that there could be a terawatt of biogenic mechanical energy in the oceans, about half of it due to the biosphere to fish and other swimmers and the other half to wigglers and splashers and squelchers of the prawny/jellyfishy/whatever persuasion.

But how much does that matter? Viscosity damps down turbulence,  so the fact that something little is thrashing about doesn’t necessarily mean it’s contributing to large scale mixing. This is where the new paper from John Dabiri’s lab at Caltech comes in. Dabiri and his student Kakani Katija have looked at a mechanism of mixing first discussed by Charles Galton Darwin (grandson of the more famous CD, head of Britain’s National Physical Lab for a while, and a gloomy eugenicist with whom I believe Robert Heinlein was much taken) fifty years ago, which probably shouldn’t be called entrainment, but sort of looks like it. This is the mixing due to water carried along with the mixer, and depends on the shape and volume of the creature involved. The paper concentrates on jellyfish, which seem to be a Dabiri specialty, and backs up its theoretical analysis with some nice videos demonstrating what goes on in practice which I don’t currently seem to be able to embed.

The paper shows  that the Darwin entrainment effect is distinctly different from the wake turbulence previously considered; it also seems to operate at larger scales than the turbulence created by say the flapping legs of a shrip. Katija and Dabiri estimate that taking it into account could increase estimates of the animals’ possible contribution from 1TW to at least 2TW, though as Dewar cautions “translation of Katija and Dabiri’s results from anecdotes to assessments of possible global impacts remains to be carried out.”

In an accompanying news piece, Roberta Kwok quotes Carl Wunsch on the “forbidding challenge” the work might pose to climate modellers, since it could require them to revisit estimates of the ocean’s diffusivity, a key modelling parameter, and quite possibly vary it from place to place and time to time. An earlier bit of Nature journalism went into this, and the whole missing mixing story, in further detail (and both point out that some people, such as Andre Visser, aren’t at all convinced there’s anything of interest going on here, fish-stirring-wise).

The scientific debate looks set to run on for some time, and to be quite a lot of fun: if climate and people influence fish (jelly and otherwise) and fish influence mixing and mixing influences climate and people there could be all sorts of fun and important stuff to learn there. But it struck me that it was worth seeing whether there was an engineering issue, too. A couple of terawatts is, in geoengineering terms, pretty small potatoes (the effects of something like a stratospheric aerosol veil would be in the hundreds to thousands of terawatts) and if an intervention on such a small scale could have an effect on the rate at which the oceans transport heat, or replenish their nutrients, or transport cold carbon-dioxide-saturated waters into the deep, that could be interesting (I am *so* not advocating rearranging the ocean currents here: I’m just thinking about what sort of leverage there might be).

A rough estimate shows that this isn’t an issue for the purportedly hurricane-thwarting pumps I wrote about last week. Though in terms of heat transfer such systems might clock in at 10GW or 20GW, that’s because water has a large heat capacity. In terms of the mechanical work they would do my schoolboy physics suggests that it’s on the order of a megawatt. But then those pumps aren’t designed to do mechanical work: who knows what the inventive mind of Steve Salter would come up with if that was the primary objective. If we wanted to mix up the oceans as much as the fish or the winds do, it might not be all that hard.

[In an experiment with a new posting format, here are the main references to papers and aricles, corralled together]

The new paper: A viscosity-enhanced mechanism for biogenic ocean mixing, Katija, K and Dabiri, J, Nature 460, 624-626 (30 July 2009) | doi:10.1038/nature08207

Dewar’s News and Views article

Roberta Kwok’s news story

Quirin Schiermeier’s earlier News Feature on the missing mixing: Churn Churn Churn Nature 447, 522-524 (31 May 2007) | doi:10.1038/447522a

Dewar et al’s paper setting out the terms of the debate:  Does the Marine Biosphere Mix the Ocean? Journal of Marine Research, 64, 541-561 (July 2006)

Carl Wunsch’s homepage: Munk W. and Wunsch, C., 1998, Abyssal Recipes-II: Energetics of Tidal and Wind Mixing (pdf) is of particular relevance

Sperm whale image from flickr user doublebug, under a creative commons licence: this version of the researchers’ video is from Wired Science


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