The impact of water constituents on radiative heat transfer in the open ocean and shelf seas
Final Report Abstract
Surface heating rates induced by absorption of light through optically significant water constituents (OSCs), e.g., phytoplankton, detritus 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 has been investigated in the Western Baltic Sea, a marginal sea characterised by considerable inputs of freshwater carrying nutrients and CDOM, and complex bio-optical and hydrodynamic processes. A novel implementation of a coupled ocean, bio-optical, ecosystem model has been developed wherein the inherent and apparent optical properties of different water constituents are explicitly modelled under varying environmental conditions and the resulting underwater light field is spectrally resolved in a dynamic sea. This approach provided a means to estimating the relative contribution of different water constituents to heating rates in surface waters and their impact on air-sea exchange of thermal energy. It also enabled an investigation into the 3-dimensional impact of two significant marine heat wave (MHW) events which took place in the Arkona Sea in May and July 2018 on phytoplankton bloom dynamics. Specifically, we investigated how, and under which circumstances, do MHWs contribute to the initiation and development of phytoplankton blooms, what role do mesoscale and sub-mesoscale dynamics play in sustaining the bloom, how deep is the impact of the MHW felt and whether phytoplankton blooms have an enhancing effect on MHWs by creating a positive feedback from water constituent-induced surface heating. The work highlights the importance of the downward attenuation coefficient, Kd as a link between physics and biology. It provides a pathway to estimating heating rates and connects biological activity with energy fluxes. The project produced a rich modelled data set of bio-optical, bio-physical and hydrodynamic variables covering the year 2018 in the Western Baltic Sea, enabling analysis beyond the present study. Information from the data set has already been used to inform a study on phytoplankton sinking rates and it is currently being used to improve K d modelling in the Western Baltic Sea. The full modelled data set will be made available to the wider research community as a data paper. In the longer term, we anticipate the data set will be of interest to both modellers and observers of physical and biological coupling of bio-optical properties in coastal and marginal seas.
Publications
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Estimating the seasonal impact of optically significant water constituents on surface heating rates in the western Baltic Sea. Biogeosciences, 20(13), 2743-2768.
Cahill, Bronwyn E.; Kowalczuk, Piotr; Kritten, Lena; Gräwe, Ulf; Wilkin, John & Fischer, Jürgen
