22 Nov 2022
 | 22 Nov 2022

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

Bronwyn E. Cahill, Piotr Kowalczuk, Lena Kritten, Ulf Gräwe, John Wilkin, and Jürgen Fischer

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: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1121', Anonymous Referee #1, 13 Dec 2022
    • AC1: 'Reply on RC1', Bronwyn Cahill, 05 Mar 2023
  • RC2: 'Comment on egusphere-2022-1121', Svetlana Losa, 25 Jan 2023
    • AC2: 'Reply on RC2', Bronwyn Cahill, 05 Mar 2023

Bronwyn E. Cahill et al.

Bronwyn E. Cahill et al.


Total article views: 275 (including HTML, PDF, and XML)
HTML PDF XML Total BibTeX EndNote
189 70 16 275 5 6
  • HTML: 189
  • PDF: 70
  • XML: 16
  • Total: 275
  • BibTeX: 5
  • EndNote: 6
Views and downloads (calculated since 22 Nov 2022)
Cumulative views and downloads (calculated since 22 Nov 2022)

Viewed (geographical distribution)

Total article views: 264 (including HTML, PDF, and XML) Thereof 264 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
Latest update: 26 Mar 2023
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.