From Watts Up With That?

By Andy May

My previous post was a discussion about an important paper by Elizabeth Wong and Peter Minnett. The paper discusses the interaction between the thermal (or electromagnetic) skin layer (TSL) on the ocean and the bulk ocean. The TSL is only about 10 microns thick on average, although the thickness and temperature profile through it and under it changes throughout the day and night. Virtually all greenhouse gas (GHG) infrared radiation (IR) is absorbed in the TSL, whereas over 99% of solar shortwave radiation (SW) passes right through it and is absorbed deeper in the ocean (the “bulk ocean”) in the tropics under clear skies (Wong & Minnett, 2018).

As Nick Stokes explained in a comment, in his view (also the mainstream position) is that both GHG IR and solar SW “simply add” in passing energy to the ocean. The ocean temperature then adjusts to the radiation by changing its temperature overall to balance the total IR and SW incoming radiation to match the outgoing energy at an infinitesimally small ocean surface. He claims that the TSL is so thin, it must act the same as this infinitesimally small surface; and the fact that it absorbs all the GHG IR and only 0.049% of the solar SW makes no difference.

The TSL, at least according to Wong and Minnett, reacts to absorbed IR by changing its temperature and its shape, thus also its size and volume. Thus, it has mass and it changes both the amount of its mass, temperature, and shape to balance the incoming IR with the energy leaving (through evaporation, outgoing IR, and sensible heat). We don’t really know how it reacts to SW, Wong and Minnett did not try and measure that since the SW absorbed in the TSL is very small. There will be some changes in the TSL through changing SW, since their measurements were based on the incoming radiation on cloudy periods versus clear periods. More clouds, less SW to the ocean and more IR. However, I can accept that the amount of absorbed SW in the TSL is very small, so the changes probably don’t matter much.

Wong and Minnett explicitly say that IR does not directly affect the bulk ocean and the interaction between the TSL and the bulk ocean is minimal. The key quote is:

“incident IR radiation does not directly heat the upper few meters of the ocean.”

SW does directly heat the bulk (penetration to ~1-10 m or more). IR is confined to the TSL.

The TSL directly interacts with the atmosphere only, this is also the layer that releases water vapor through evaporation. Changes to the overall upper ocean temperature gradient, caused by the changing temperature and mass within the TSL, can affect bulk ocean temperature and ocean heat content according to Wong and Minnett.

As noted above, the mainstream view emphasizes total flux balance at the ocean surface (the “boundary”) without splitting hairs on the depth of penetration of IR and SW. This is a very common assumption in general circulation climate models (GCMs). But Wong & Minnett refute this idea: The TSL-atmosphere interaction dominates, with minimal direct TSL-bulk mixing. IR’s bulk ocean effect is via gradient changes, not direct heating.

SW and IR both affect the TSL but IR is almost entirely absorbed in the TSL, so IR dominates the absorption and SW adds only a tiny fraction (~0.05% perhaps in the tropics under clear skies). The two forms of radiation do not affect the bulk ocean in the same way (Watt per Watt). SW affects the bulk ocean directly and any influence that IR has on the bulk ocean is indirect and small.

The TSL is not a massless surface or interface. Further its volume changes with changes in radiation, as well as its temperature and total mass. It adjusts itself to keep surface energy exchanges stable. Part of why I wrote the post was to do a very thorough analysis of Wong and Minnett, a very important paper. Nick Stokes comment helps in that regard by bringing up the boundary condition idea. It took me a while to figure out what he was saying, but once I did it was very thought provoking.

Conclusions

This is a classic clash between macro-scale modeling (Nick: fluxes add, boundary balances everything) and micro-scale observations (Wong & Minnett: penetration depths matter, IR and SW aren’t equivalent). Nick’s explanation is consistent with basic thermodynamics but underplays the TSL’s unique role in “gating” IR effects—it’s not a passive boundary or infinitesimally small interface; it actively reshapes to help buffer the bulk ocean. Separating direct/indirect effects is valid, especially in climate contexts where we can question if IR back-radiation (e.g., from GHGs) heats oceans like SW does. Per the paper, IR does contribute to ocean heat content indirectly, but the mechanisms differ, potentially affecting evaporation rates and stratification in ways climate models probably miss.

Works Cited

Wong, E. W., & Minnett, P. J. (2018). The Response of the Ocean Thermal Skin Layer to Variations in Incident Infrared Radiation. Journal of Geophysical Research: Oceans, 123(4). https://doi.org/10.1002/2017JC013351