Preprints
https://doi.org/10.5194/nhess-2021-392
https://doi.org/10.5194/nhess-2021-392

  04 Jan 2022

04 Jan 2022

Review status: this preprint is currently under review for the journal NHESS.

The role of heat wave events on the occurrence and persistence of thermal stratification in the southern North Sea

Wei Chen1, Joanna Staneva1, Sebastian Grayek1, Johannes Schulz-Stellenfleth1, and Jens Greinert2 Wei Chen et al.
  • 1Institute of Coastal Systems-Analysis and Modelling, Helmholtz-Zentrum Hereon, Max-Planck-Str. 1, 21502 Geesthacht, Germany
  • 2GEOMAR Helmholtz Center for Ocean Research Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany

Abstract. Extremes in temperatures not only directly affect the marine environment and ecosystems but also have indirect impacts on hydrodynamics and marine life. The role of heat wave events responsible for the occurrence and persistence of thermal stratification was analysed using a fully coupled hydrodynamic and wave model within the framework of the Geesthacht Coupled cOAstal model SysTem (GCOAST) for the North Sea. The model results were assessed against satellite reprocessed data and in situ observations from field campaigns and fixed MARNET stations. To quantify the degree of stratification, a potential energy anomaly over the water column was calculated. A linear correlation existed between the air temperatures and the potential energy anomaly in the North Sea excluding the Norwegian Trench and the area south of 54° N latitude. Contrary to the northern part of the North Sea, where the water column is stratified in the warming season each year, the southern North Sea is seasonally stratified in years when a heatwave occurs. The influences of heatwaves on the occurrence of summer stratifications in the southern North Sea are mainly in the form of two aspects, i.e., a rapid rise in sea surface temperature at the early stage of the heatwave period and a relatively higher water temperature during summer than the multiyear mean. Another factor that enhances the thermal stratification in summer is the memory of the water column to cold spells earlier in the year. Differences between the seasonally stratified northern North Sea and the heatwave-induced stratified southern North Sea were attributed to changes in water depth.

Wei Chen et al.

Status: open (until 15 Feb 2022)

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Wei Chen et al.

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Short summary
This study is the first to link the occurrence and persistence of density stratification in the southern North Sea to the increased number of extreme marine heatwaves. We further identified the role of the cold spells at the early stage of a year to the intensity of thermal stratification in summer. In a broader context, this research will have fundamental significance for further discussion of the secondary effect of heatwave events, such as in ecosystems, fisheries, and sediment dynamics.
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