Heating the Ocean

Peter Minnett has what I consider to be a rather confusing RealClimate post. Based on the comments, a lot of other people felt the same confusion.
Observations of ocean temperatures have revealed that the ocean heat content has been increasing significantly over recent decades (Willis et al, 2004; Levitus et al, 2005; Lyman et al, 2006). This is something that has been predicted by climate models (and confirmed notably by Hansen et al, 2005), and has therefore been described as a 'smoking gun' for human-caused greenhouse gases.
However, some have insisted that there is a paradox here - how can a forcing driven by longwave absorption and emission impact the ocean below since the infrared radiation does not penetrate more than a few micrometers into the ocean? Resolution of this conundrum is to be found in the recognition that the skin layer temperature gradient not only exists as a result of the ocean-atmosphere temperature difference, but also helps to control the ocean-atmosphere heat flux. (The 'skin layer' is the very thin - up to 1 mm - layer at the top of ocean that is in direct contact with the atmosphere). Reducing the size of the temperature gradient through the skin layer reduces the flux. Thus, if the absorption of the infrared emission from atmospheric greenhouse gases reduces the gradient through the skin layer, the flow of heat from the ocean beneath will be reduced, leaving more of the heat introduced into the bulk of the upper oceanic layer by the absorption of sunlight to remain there to increase water temperature.

The post is confusing because it seems to imply that heating the skin layer decreases the ocean skin layer –> atmosphere sensible and latent heat flux. It doesn’t – it increases it. What is decreased is the net flux from the ocean below to the skin layer. The basic principle is very simple – more heat incident on the ocean makes it warmer.

The supposed paradox arises in the fact that there is no obvious way for the heat absorbed in the skin layer to be transported to the warmer ocean below. This idea demonstrates that a little microphysics can be a dangerous thing. What the warmer skin layer does is result in less net conduction to the surface (on account of the smaller gradient) and suppression of convection which would otherwise transport more of the colder skin layer water into the depths (likewise).

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