Notes on ocean “warming”

I don’t have much time for research or writing these days, more’s the pity. So I must make do with snippets when they’re available. My favourite oceanographer made a few comments the other day on the ocean “heating” being discussed in the blogosphere. I’d like to pass them on.

He made some interesting and helpful remarks for the benefit of those of us not intimately acquainted with oceanography. However, to quieten the discussion which was threatening to get out of control he said pointedly, “I don’t have time to waste on Skeptical Science distortions.” We must hope that doesn’t make John Cook feel too inadequate.

Anyway, this is what he had to say about warming between 700–2000 m in the oceans.

1. The data below 700 m is very sparse — even with the increase in ocean drifters such as Argo. Therefore, the extremely small change in temperature associated with the very small heat content change is not reliable.

2. The main thermocline in the oceans lies between 500 and 1500 m on average depending on latitude. This is where the steepest gradient in temperature occurs in the bulk of the ocean. Note that I said on average. The position of the thermocline does vary over time. Best known example is the changes associated with ENSO — La Nina, thermocline depressed; El Nino, thermocline rises across the equatorial Pacific. There is evidence of shifts associated with the PDO. Doesn’t take much of a shift to account for an apparent global change in the 700–2000 m depth as low latitudes have higher sampling density than high latitudes — a change in the frequency of El Nino and La Nina events could do it.

3. Warm water doesn’t need to sink vertically to reach depth when dealing with intermediate waters (those around the thermocline). It is easier to move sideways along a density boundary than down through it. The main density boundary (pycnocline) is associated with the steepest temperature gradient (thermocline). It is not a flat surface — it is bowl-shaped; shallower at high latitudes and equator, and deepest around the tropics. Water at high latitudes sinks (subducts) by sliding sideways along the pycnocline. So warming of the Southern Ocean and Bering Sea eventually appears at 700–2000 m depth in the Central Pacific. Two little things though (1) the warming is solar driven and (2) it takes time to move (we think the shallowest parts may be of the order 50–70 years = PDO, and the deeper parts 100–500 y). So it is most likely a reflection of solar heating decades to centuries ago.

This is all basic stuff we teach in the introductory oceanography course — we don’t tend to get many atmospheric chemists attending.

So it’s no surprise they don’t understand this stuff.

I would emphasise the general unreliability of ocean temperatures. We don’t know enough about them to make a case for anything much. Also the amazingly long periods required for energy to move through the oceans.

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