22 Aug 2022
22 Aug 2022
Status: this preprint is currently under review for the journal ESD.

How does the phytoplankton-light feedback affect marine N2O inventory?

Sarah Berthet1, Julien Jouanno2, Roland Séférian1, Marion Gehlen3, and William Llovel4 Sarah Berthet et al.
  • 1CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • 2LEGOS, Université de Toulouse, IRD, CNRS, CNES, UPS, Toulouse, France
  • 3LSCE, Institut Pierre Simon Laplace, Gif-Sur-Yvette, France
  • 4LOPS, CNRS/University of Brest/IFREMER/IRD, Brest, France

Abstract. The phytoplankton-light feedback (PLF) depicts how phytoplankton biomass interacts with the downwelling shortwave radiation entering the ocean. Considering the PLF allows differential heating across the ocean water column as a function of the phytoplankton concentration. Only one third of the CMIP6 Earth system models include a complete representation of the PLF. In other models, the PLF is mimicked either thanks to a prescribed climatology of chlorophyll or not represented at all. Consequences of an incomplete representation of the PLF on the marine biogeochemical content haven’t been assessed yet and remain a source of multi-model uncertainty in future projection. Here, we scrutinize with a single modelling set-up how various representation of the PLF can impact ocean physics and ultimately marine production of a major greenhouse gas, the nitrous oxide (N2O). Global sensitivity experiments considering the ocean, sea ice and marine biogeochemistry have been performed at 1-degree of horizontal resolution over the last two decades (1999–2018). We show that the representation of the PLF has significant consequences on the ocean heat uptake and temperature of the first 300 meters of the tropical ocean. Temperature anomalies due to an incomplete PLF representation drive perturbations of the ocean stratification, dynamics and oxygen concentration. Different projection pathways for N2O production result from the choice of the PLF representation. Considering an incomplete representation of the PLF overestimates the oxygen concentration in the North-Pacific oxygen minimum zone and underestimates the local N2O production. This leads to important regional differences of sea-to-air N2O fluxes: fluxes are enhanced by up to 24 % in the south Pacific and south Atlantic subtropical gyres, but reduced by up to 12 % in oxygen minimum zones of the northern hemisphere. Our results based on a global ocean-biogeochemical model at CMIP6 state-of-the-art thus shine a light on a current uncertainty of the modelled marine nitrous oxide budget in that climate models.

Sarah Berthet et al.

Status: open (until 03 Oct 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esd-2022-28', Rémy Asselot, 25 Aug 2022 reply
  • RC2: 'Comment on esd-2022-28', Anonymous Referee #2, 16 Sep 2022 reply

Sarah Berthet et al.

Sarah Berthet et al.


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Short summary
Phytoplankton absorbs the solar radiation entering the ocean surface, and contributes to keep the associated energy in surface waters. This natural effect is not commonly represented in the oceanic part of climate models, or often suffers simplifications. We show that an incomplete representation of this biophysical interaction affects the way climate models capture ocean warming, what in turn uncertains the forecast of oceanic emissions of an important greenhouse gas called the nitrous oxide.