Preprints
https://doi.org/10.5194/egusphere-2023-3018
https://doi.org/10.5194/egusphere-2023-3018
16 Jan 2024
 | 16 Jan 2024

Influence of wind strength and direction on diffusive methane fluxes and atmospheric methane concentrations above the North Sea

Ingeborg Bussmann, Eric P. Achterberg, Holger Brix, Nicolas Brüggemann, Götz Flöser, Claudia Schütze, and Philipp Fischer

Abstract. Quantification of the diffusive methane fluxes between the coastal ocean and atmosphere is important to constrain the atmospheric methane budget. The determination of the fluxes in coastal waters is characterized by a high level of uncertainty. To improve the accuracy of the estimation of coastal methane fluxes, high temporal and spatial sampling frequencies of dissolved methane in seawater are required as well as the quantification of atmospheric methane concentrations, wind speed and wind direction above the ocean. In most cases, these atmospheric data are obtained from land-based atmospheric and meteorological monitoring stations in the vicinity of the coastal ocean methane observations.

In this study, we measured wind speed and direction as well as atmospheric methane directly on board three research vessels in the southern North Sea and compared the local and remote atmospheric and meteorological measurements on the quality of the flux data. In addition, we assessed the source of the atmospheric methane measured in the study area in the German Bight using airmass back trajectory assessments.

The choice of the wind speed data source had a strong impact on the flux calculations. Fluxes based on wind data from nearby weather stations amounted to only 58 ± 34% of values based on situ data. Using in-situ data, we calculated an average diffusive methane sea-to-air flux of 221± 351 µmol m-2 d-1 (n = 941) and 159 ± 444 µmol m-2 d-1 (n = 3028) for our study area in September 2019 and 2020, respectively. The area-weighted diffusive flux for the entire area of Helgoland Bay (3.78 x 109 m2) was 836 ± 97 and 600 ± 111 kmol d-1 for September 2019 and 2020, respectively. Using the median value of the diffusive fluxes for these extrapolations resulted in much lower values, compared to area-weighted extrapolations or mean-based extrapolations.

In general, at high wind speeds, the surface water turbulence is enhanced and the diffusive flux increases. This enhanced methane input however is quickly diluted within the air mass. Hence, a significant correlation between the methane flux and the atmospheric concentration was observed only at wind speeds < 5 m s-1.

The atmospheric methane concentration was mainly influenced by the wind direction, i.e., the origin of the transported air mass. Airmasses coming from industrial regions resulted in elevated atmospheric methane concentrations, while airmasses coming from the North Sea transported reduced methane levels. With our detailed study on the spatial distribution of methane fluxes we were able to provide a detailed and more realistic estimation of coastal methane fluxes.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Ingeborg Bussmann, Eric P. Achterberg, Holger Brix, Nicolas Brüggemann, Götz Flöser, Claudia Schütze, and Philipp Fischer

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-3018', Anonymous Referee #1, 15 Feb 2024
    • AC1: 'Reply on RC1', Ingeborg Bussmann, 12 Apr 2024
  • RC2: 'Comment on egusphere-2023-3018', Anonymous Referee #2, 25 Mar 2024
    • AC2: 'Reply on RC2', Ingeborg Bussmann, 14 Apr 2024

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-3018', Anonymous Referee #1, 15 Feb 2024
    • AC1: 'Reply on RC1', Ingeborg Bussmann, 12 Apr 2024
  • RC2: 'Comment on egusphere-2023-3018', Anonymous Referee #2, 25 Mar 2024
    • AC2: 'Reply on RC2', Ingeborg Bussmann, 14 Apr 2024
Ingeborg Bussmann, Eric P. Achterberg, Holger Brix, Nicolas Brüggemann, Götz Flöser, Claudia Schütze, and Philipp Fischer
Ingeborg Bussmann, Eric P. Achterberg, Holger Brix, Nicolas Brüggemann, Götz Flöser, Claudia Schütze, and Philipp Fischer

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Short summary
Methane (CH4) is an important greenhouse gas and contributes to climate warming. The input of CH4 from coastal areas to the atmosphere however is not well defined. Dissolved and atmospheric CH4 was determined at high spatial resolution in or above the North Sea. The atmospheric CH4 concentration was mainly influenced by wind direction. With our detailed study on the spatial distribution of CH4 fluxes we were able to provide a detailed and more realistic estimation of coastal CH4 fluxes.