Preprints
https://doi.org/10.5194/egusphere-2023-920
https://doi.org/10.5194/egusphere-2023-920
10 May 2023
 | 10 May 2023

Observations of strong turbulence and mixing impacting water exchange between two basins in the Baltic Sea

Julia Christin Muchowski, Martin Jakobsson, Lars Umlauf, Lars Arneborg, Bo Gustafsson, Peter Holtermann, Christoph Humborg, and Christian Stranne

Abstract. Turbulent diapycnal mixing is important for the estuarine circulation between basins of the Baltic Sea as well as for its local ecosystems, in particular with regard to eutrophication and anoxic conditions. While the interior of the basins is overall relatively calm, stratified flow over steep bathymetric features is known as a source for strong turbulent mixing. Yet, current in situ observations often cannot capture dynamic and intermittent turbulent mixing related to overflow over rough bathymetry. We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the sparsely-sampled Southern Quark located in the Åland Sea, connecting the Northern Baltic Proper with the Bothnian Sea. Our data include high resolution broadband acoustic observations of turbulent mixing, in situ microstructure profiler measurements, and current velocities from Acoustic Doppler Current Profilers and were acquired during two one-week cruises in February–March 2019 and 202. A temporally reversing non-tidal stratified flow over the steep bathymetric sill created a dynamic and extremely energetic environment. Saltier, warmer, and less oxygenated deep water south of the sill was partly blocked, the reversing flow was at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill, and sub-mesoscale processes in the surface layer leading to high spatial variability at small scales. Mixing and vertical salt flux rates were increased by 3–4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow. We suggest based on acoustic observations and in situ measurements that underlying mechanisms for the highly increased mixing across the halocline are a combination of shear and topographic lee waves which are breaking at the halocline interface. We anticipate that the resulting deep- and surface-water modification in the Southern Quark directly impacts exchange processes between the Bothnian Sea and the Northern Baltic Proper and that the observed mixing is likely important for oxygen and nutrient conditions in the Bothnian Sea. Our results contribute to the knowledge on turbulent mixing processes in the Åland Sea and can help to improve mixing parametrizations in numerical models of the area.

Journal article(s) based on this preprint

20 Dec 2023
Observations of strong turbulence and mixing impacting water exchange between two basins in the Baltic Sea
Julia Muchowski, Martin Jakobsson, Lars Umlauf, Lars Arneborg, Bo Gustafsson, Peter Holtermann, Christoph Humborg, and Christian Stranne
Ocean Sci., 19, 1809–1825, https://doi.org/10.5194/os-19-1809-2023,https://doi.org/10.5194/os-19-1809-2023, 2023
Short summary

Julia Christin Muchowski et al.

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-920', Anonymous Referee #1, 03 Aug 2023
    • AC1: 'Reply on RC1', Julia Muchowski, 03 Oct 2023
  • RC2: 'Comment on egusphere-2023-920', Anonymous Referee #2, 26 Aug 2023
    • AC2: 'Reply on RC2', Julia Muchowski, 03 Oct 2023

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-920', Anonymous Referee #1, 03 Aug 2023
    • AC1: 'Reply on RC1', Julia Muchowski, 03 Oct 2023
  • RC2: 'Comment on egusphere-2023-920', Anonymous Referee #2, 26 Aug 2023
    • AC2: 'Reply on RC2', Julia Muchowski, 03 Oct 2023

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
AR by Julia Muchowski on behalf of the Authors (03 Oct 2023)  Author's response   Author's tracked changes   Manuscript 
ED: Publish as is (20 Oct 2023) by Mehmet Ilicak
AR by Julia Muchowski on behalf of the Authors (03 Nov 2023)

Journal article(s) based on this preprint

20 Dec 2023
Observations of strong turbulence and mixing impacting water exchange between two basins in the Baltic Sea
Julia Muchowski, Martin Jakobsson, Lars Umlauf, Lars Arneborg, Bo Gustafsson, Peter Holtermann, Christoph Humborg, and Christian Stranne
Ocean Sci., 19, 1809–1825, https://doi.org/10.5194/os-19-1809-2023,https://doi.org/10.5194/os-19-1809-2023, 2023
Short summary

Julia Christin Muchowski et al.

Julia Christin Muchowski et al.

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Short summary
We show observational data of highly increased mixing and vertical salt flux rates in a sparsely sampled region of the Northern Baltic Sea. Co-located acoustic observations complement our in-situ measurements and visualize turbulent mixing with high spatial resolution. The observed mixing is generally not resolved in numerical models of the area but likely impacts the exchange of water between the adjacent basins as well as nutrient and oxygen conditions in the Bothnian Sea.