Seasonal impact of optically significant water constituents on radiative heat transfer in the Western Baltic Sea

Heating rates induced by optically significant water constituents (OACs), e.g. phytoplankton and coloured dissolved organic matter (CDOM), contribute to the seasonal modulation of thermal energy fluxes across the ocean-atmosphere interface in coastal and regional shelf seas. This is investigated in the Western Baltic Sea, a region characterised by considerable inputs of nutrients, CDOM and freshwater, and complex bio-optical and hydrodynamic processes. Using a coupled bio-optical-ocean model (ROMS-BioOptic), the underwater light field is spectrally-resolved in a dynamic ocean and the inherent optical properties of different water constituents are modelled under varying environmental conditions. We estimate the relative contribution of these water constituents to the divergence of the heat flux and heating rates and find that phytoplankton dominates absorption in spring, while CDOM dominates absorption in summer and autumn. In the Pomeranian Bight, water constituent-induced heating rates in surface waters are estimated to be up to 0.1^oC d^-1 in spring and summer, predominantly as a result of increased absorption by phytoplankton and CDOM, respectively during these periods. Warmer surface waters are balanced by cooler subsurface waters. Surface heat fluxes (latent, sensible and net longwave) all increase in response to warmer sea surface temperatures. We find good agreement between our modelled water constituent absorption, and in situ and satellite observations. More rigorous co-located heating rate calculations using an atmosphere-ocean radiative transfer model provide further evidence of the suitability of ROMS-BioOptic model for this purpose. The study shows that seasonal and spatial changes in optically significant water constituents in the Western Baltic Sea have a small but noticeable impact on radiative heating in surface waters and consequences for the exchange of energy fluxes across the air-sea interface and the distribution of heat within the water column. The importance of the light attenuation coefficient, K_d, in shelf seas as a bio-optical driver which provides a pathway to estimating heating rates and connects biological activity with energy fluxes is highlighted.