Representing bio-optical interactions in a coupled modelling framework for the Southern European Seas

Chlorophyll and related pigments of phytoplankton modify seawater turbidity and light absorption, with consequences for the upper ocean heat uptake and dynamics. In this work, we investigate how different representations of light-chlorophyll feedback impact physical processes in the Mediterranean and Black Seas using a fully coupled ocean-biogeochemistry-atmosphere modelling system (NEMO-PISCES-WRF). We compare an interactive simulation, in which three-dimensional chlorophyll fields evolve dynamically within the PISCES model, with a non-interactive run forced with satellite-derived surface chlorophyll and assuming a uniform vertical profile, as commonly done in ocean circulation models. In both cases, chlorophyll concentration affects the ocean heat budget by modulating light absorption and local heating rates. Results show that explicitly representing vertical chlorophyll structure strongly modifies subsurface heating, particularly during stratified summer conditions. In productive regions such as the western Mediterranean and the Black Sea, high surface chlorophyll concentration creates a shading effect in both simulations, cooling subsurface waters that are later brought to the surface through winter mixing. Conversely, in the oligotrophic eastern Mediterranean, the presence of a Deep Chlorophyll Maximum enhances shortwave absorption below the mixed layer, resulting in warmer upper-ocean temperatures the following winter and an overall increase in ocean heat content. These results highlight the need to improve bio-optical representations in regional climate modelling of the Southern European seas, especially in low-productivity regions where chlorophyll maxima occur at depth and are not captured by satellite observations.