Posts Tagged ‘OHC’

Ocean Heat Content: Can we monitor the transfer of heat through the top 700 metres?

October 12, 2011

At RC, Gavin Schmidt and Roger Pielke Sr are discussing how Ocean Heat Content (OHC) has changed in the recent past. The disagreement seems to be on how the apparent slowdown in heat uptake of the top 700 metres can be reconciled with apparent warming of the deeper ocean. It’s very informative to witness two experts debate this in public (even though the discussion is hampered by frequent misunderstandings and other derailing issues). I’ve found this a puzzling issue for a while and am still not sure if I fully grasp it, but here goes: 

On his blog, RPSr writes:

If heat is being sequested in the deeper ocean, it must transfer through the upper ocean. In the real world, this has not been seen that I am aware of. In the models, this heat clearly must be transferred (upwards and downwards) through this layer. The Argo network is spatially dense enough that this should have been seen.

Gavin responds to this statement at RC:

Obviously heat going below 700 m must have passed through the upper ocean. However, the notion that Argo could see this is odd. Argo measures temperature, not flux. The net flux into a layer is calculated by looking at the change in temperature. It cannot tell you how much came in at the top and left at the bottom, only how much remained.

Both arguments make intuitive sense. Whether indeed temperature measurements should have seen a transfer of heat depends on the precision and on the (spatial and temporal) density of the Argo network. Besides that, it also depends on the mode of heat transfer (episodic or continuous). Or perhaps better put: the extent to which the influx and outflux of heat balance each other.

Gavin does not agree with Roger’s last statement (that it should have been see), but argues instead that the signal would not likely be observable amidst the variability (response to 140): 

I have no confidence that the observations will be sufficient to distinguish the anomalous heat flux from the climatological mean with sufficient precision to be helpful.

Roger concedes that the observation network’s precision is an important precondition for heat transfer to have been observed, when he writes in response to my little summary over there: (Roger)

(…) we should still see a slight elevation in the temperature anomalies IF the Argo data precision is good enough. I do not know the precision of the temperature data measurements, and hope someone else can answer that.

And in 193 Roger writes:

First, I stated that the Argo data density was fine enough to see the movement of the heat downward, but am now unclear on this, and look forward to an Argo specialist to give us an overview of capability in this regards.

There is however a second if-statement to make, about the mode of transfer. In response to 2, Gavin wrote:

Most heat transport into the deep ocean will occur in the down-welling branches of the overturning circulation, centered in theNorth Atlanticand the Southern Oceans. Diffusive fluxes in the rest of the ocean will be much smaller.

Roger (140) says more or less the same, but arrives at a conclusion that is not shared by Gavin:

if this transfer occurs in globs associated with mesoscale and larger ocean circulation features (as suggested in the ECMWF data), we should clearly see this movement of heat.

About the mode of heat transfer Gavin writes in response to 155:

Heat transfer will be mainly continuous, not episodic.

I.e. the heat transfer is strongest in specific locations (agreed on by both), but continuous in time (which prompted a question from Roger “how do you know?”).

In a continuous case, for a while the same amount of heat may enter the top 700 m from above, as leaves it from below. As a result, no warming signal in this layer will be observed, whereas heat is being transferred through it. In an episodic case, it would in principle be observable (though still dependent on the precision and signal to noise ratio of the measurements).

In contrast to what I wrote in my little summary at RC (175), the disagreement is not so much on whether the heat transfer is concentrated in space (both seem to agree that it is), but rather on whether the heat transfer is continuous or episodic in time (Gavin thinks it’s the former; Roger doesn’t say) and on whether the data precision is sufficient (Gavin thinks it isn’t; Roger doesn’t say).

Figure 9b from Hansen et al., ACPD 2011, “Earth’s Energy Balance and Implications”. Note that this Fig gives the heat uptake, which is the slope of a figure of heat content (in Joules): A positive heat uptake means that the heat content is increasing.

Figure caption: Six year trends of ocean heat uptake estimated by Levitus et al. (2009) and Lyman et al. (2010) for upper 700 m of the ocean, and estimates based on Argo float data for the upper 2000 m for 2003–2008 and 2005–2010.

I’ll probably update this post as the discussion progresses. Over at SkS, there have also been informative discussions between Roger and the regulars over there.

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