Modelling of the sea surface microlayer

The sea surface microlayer (SML) seems to act like oceans’ skin and control the rate of exchange of energy and matter between oceans and the atmosphere. Since around 71% of Earth’s surface is covered in oceans (see Wikipedia), understanding the behaviour and the role of the SML seems key to model climate processes and understand potential feedback loops.

The behaviour of the SML seems to be affected by its composition. More surfactants in the SML can significantly reduce CO2 uptake by the oceans (see The Ocean’s Vital Skin: Toward an Integrated Understanding of the Sea Surface Microlayer), and according to this there seem to be a negative feedback loop in which the warmer oceans get, the less CO2 they absorb and thus the more GHG is kept in the atmosphere.

In light of the apparent importance of the SML acting as sea-atmosphere interface (please let me know if I miss something), have existing climate models and forward simulations (especially those used in the latest IPCC report - AR6) accounted for the potential huge negative feedback loops coming from change in composition of the SML? (e.g. less ocean CO2 uptake than expected? More H2O vapor from ocean evaporation, producing more GHG?)

Like, have IPCC/recent models actually modelled the SML as a stateful air/sea interface? (to be able to simulate different scenarios with different oceanic CO2 uptake, water evaporation and air/sea energy transfers more generally)

If not:

  • Are there models taking the SML effect into consideration?
  • What are the best approaches to date to model the SML?

I’ve looked a bit more into this and have exchanged with a couple of scientists working on the ocean<>atmosphere interface. Here are some of my takeaways (any error is mine here and may reflect my misunderstanding of the discussions).

It is my understanding that the SML is not yet really integrated in models. The reason for this is that there are still many open questions about the SML (e.g. its chemical and microbial composition, its net effect on gas and aerosol exchange, its seasonality and world-wide relevance etc.). While there are several studies on the SML (some of which show how the SML impacts CO2 exchange, study the influence of plastics on the SML and its surfactants etc.), the general picture is still missing.
Now, this does not answer the question of how to include the SML into models. Modellers may be able to chime in here to discuss how they could integrate the SML into their existing models (to do so, we probably need to discuss the current approach to modelling the ocean<>atmosphere interface first, to see how we can refine it to account for the SML).
My understanding is also that is would be very useful for scientists doing such measurements and studying the SML to know which kind of information/measurements modellers would need to have to account for the SML in their models (for backtesting purposes I presume…).
As far as I can tell, couplers (e.g. OASIS, OpenPalm) are generally used to act as “bridge” between different climate models (e.g. ocean models < > coupler < > atmospheric model, etc.). So, if my understanding is correct, this is in the couplers that the SML effect should be reflected/modelled. I would very much welcome perspectives and clarifications if any modeller passes by (especially if I miss something here)!

Note: The fact that IPCC predictions are not accounting for the SML effect has also been echoed by this Linkedin post by Howard Dryden.
If you read this and have more information to provide on this topic, please do chime in and share your views/references! :slight_smile:

Thanks to Luisa Galgani for letting me know about Marine plastics alter the organic matter composition of the air-sea boundary layer, with influences on CO2 exchange: a large-scale analysis method to explore future ocean scenarios and sharing a link to it. Thanks for Manuela van Pinxteren for sharing comments on the first posts of this thread.

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