Feedbacks of biotically induced radiative heating on upper-ocean heat budget, circulation, and biological production in a coupled ecosystem-circulation model

A coupled ecosystem-circulation model of the North Atlantic Ocean is used to investigate the impact of radiative heating by biotically induced absorption of solar radiation on the ocean's heat budget, on water column stability and circulation, and on biological production itself. For fixed atmospheric conditions, the local sensitivity of the nonsolar heat flux to changes in sea surface temperature leads to a net cooling of the ocean by the biota at a rate of about 1 W m^-2. As a result, simulated winter mixed-layer depths are deeper by more than 100 m in parts of the subpolar gyre, whereas upper-ocean stratification is enhanced in the tropics and subtropics, and coastal upwelling and associated nutrient supply are reduced by about 10% compared to a model run with optical properties of clear seawater. Simulated chlorophyll concentrations increase, indicating a positive feedback, only in subpolar regions that exhibit a pronounced phytoplankton spring bloom. Here biotically induced trapping of heat closer to the sea surface leads to a faster shoaling of the mixed layer and a more intense spring bloom in the model. On the basin average, simulated surface chlorophyll concentrations, however, decrease by 3%, constituting a weak negative feedback of 0.03 W m^-2, when heating by biotic absorption of solar radiation is accounted for. These findings are based on the approximation of the atmosphere as a passive heat buffer and will have to be tested against results from fully coupled atmosphere-ocean models with interactive marine biology.