Chlorophyll and related pigments in phytoplankton influence the vertical distribution of solar radiation and temperatures in the upper ocean by altering shortwave penetration. Previous studies have highlighted the role of chlorophyll-induced solar absorption in regulating the thermal structure of the upper ocean, with broader implications for climate. However, in more than half of the Earth System Models (ESMs) used in the CMIP6, chlorophyll-induced solar absorption is poorly represented, typically relying on globally uniform or time-invariant chlorophyll concentrations for solar radiation calculations. This approach leaves the feedbacks between chlorophyll and ocean physics, as well as broader climate processes, unclear.

In this study, we use a global-scale coupled ocean-biogeochemical model (NEMO-MEDUSA) to investigate the role of chlorophyll in determining upper ocean physics and the uncertainties this process introduces into climate simulations in ESMs. Our findings indicate that chlorophyll-induced solar absorption causes surface ocean warming of up to 1.72°C day^-1 and a warming rate of 0.14°C day^-1 m^-1 in the mixed layer. Notably, in regions with pronounced dynamic processes like coastal upwelling regions, surface chlorophyll has a shading effect on subsurface waters, where the colder water is then upwelled, resulting in a cooling effect in coastal upwelling zones.

Our results also show that the unrealistic representation of chlorophyll-induced solar absorption in ESMs leads to a 21% uncertainty in the upper ocean heat budget when projecting future climate change. Therefore, incorporating this biophysical feedback into future versions of ESMs is crucial for improving the accuracy of ocean physics simulations and enhancing our understanding of climate dynamics.
 

Earth system modeling, bio-physical feedback, climate change, chlorophyll-a, coastal upwelling