Information on Chlorophyll Treatment in NorESM2

[oyvindseland]

Siyu Meng, a PhD student at the Climatic Research Unit, University of East Anglia, UK has a question on chlorophyll treatment in NorESM2 in particular over ocean that I hope can be answered. Questions added below.

My PhD project focuses on the radiant heating by phytoplankton in the upper ocean, specifically on understanding how this process is represented in ESMs. After reaching out to the UKESM team, we realized that the representations of this bio-physical feedback are quite inconsistent among models. I am gathering information on how each CMIP6 ESM treats chlorophyll as background information for a manuscript that I am putting together.

I have read the model description paper (Seland et al., 2020), but I may have missed some information and could not find detailed information on how chlorophyll-induced solar absorption and heating are treated. I am particularly interested in if (1) chlorophyll is actively simulated in a biogeochemical module (as opposed to e.g. being set by a climatology), and (2) modelled/set/constant chlorophyll affects the absorption of light in the physical ocean model.

This information will be crucial for evaluating the differences in oceanic responses to chlorophyll-induced heating under different configurations in the next stage of my PhD work. I would be happy to share the collated information (likely in the form of a table, say) when it’s completed if you wish.

Your assistance in this matter would be greatly appreciated. If this research is published, I will ensure that your input is acknowledged.

[JorgSchwinger]

Hi Siyu

(1) In NorESM2 iHAMOCC actively simulates phytoplankton biomass, which can be converted to chlorophyll assuming some C/Chl ratio.
(2) This is NOT used in the physical ocean model to calculate absorption/heating. Instead BLOM uses (at least in our CMIP6 version) a parameterization based on Jerlov water types (constant in space and time).

Hope this helps, let me know if you have further questions.

[TomasTorsvik]

@oyvindseland , @JorgSchwinger - I'm moving this from "issues" to "discussions", since the question does not relate to any current model development.