22 Nov 2022
22 Nov 2022
Status: this preprint is open for discussion.

Estimating the seasonal impact of optically significant water constituents on surface heating rates in the Western Baltic Sea

Bronwyn E. Cahill1,2, Piotr Kowalczuk3, Lena Kritten2, Ulf Gräwe1, John Wilkin4, and Jürgen Fischer2 Bronwyn E. Cahill et al.
  • 1Physical Oceanography and Instrumentation, Leibniz Institute for Baltic Sea Research, Warnemünde 18119, Germany
  • 2Institute of Meteorology, Free University Berlin, Berlin 12165, Germany
  • 3Institute of Oceanology PAS, Powstańców Warszawy 55, 81-712 Sopot, Poland
  • 4Department of Marine and Coastal Sciences, Rutgers University, New Brunswick, 08901 NJ, USA

Abstract. Heating rates induced by optically significant water constituents (OSCs), 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 marginal sea characterised by considerable inputs of freshwater carrying nutrients and CDOM, and complex bio-optical and hydrodynamic processes. Using a coupled bio-optical-ocean model (ROMS-Bio-Optic), the inherent optical properties of different OSCs are modelled under varying environmental conditions and the underwater light field is spectrally-resolved in a dynamic ocean. We estimate the relative contribution of these OSCs to the divergence of the heat flux and heating rates and find that phytoplankton dominates the OSC contribution to heating in spring and summer, while CDOM dominates in summer and autumn. The study shows that seasonal and spatial changes in OSCs 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. In the Pomeranian Bight, where riverine influx of CDOM is strongest, water constituent-induced heating rates in surface waters in 2018 are estimated to be between 0.8 and 0.9 K m-1 d-1 in spring and summer, predominantly as a result of increased absorption by phytoplankton and CDOM. Further offshore, OSC-induced heating rates during the same periods are estimated to be between 0.4 and 0.8 K m-1 d-1. Warmer surface waters are balanced by cooler subsurface waters. Surface heat fluxes (latent, sensible and longwave) respond to warmer sea surface temperatures with a small increase in heat loss to the atmosphere of 5 Wm-2 during the period April to September. We find relatively 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 evidence of the suitability of the ROMS-Bio-Optic model for estimating heating rates.

Bronwyn E. Cahill et al.

Status: open (until 03 Jan 2023)

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Bronwyn E. Cahill et al.

Bronwyn E. Cahill et al.


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Short summary
This work quantifies the impact of optically significant water constituents on surface heating rates and thermal energy fluxes in the Western Baltic Sea. During productive months in 2018 (April to September), we found that the combined effect of CDOM and particulate absorption contributes to sea surface heating of between 0.4 and 0.9 K m-1 d-1 and a mean loss of heat (c. 5 Wm-2) from the sea to the atmosphere. This result may be important for regional heat balance budgets.