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https://doi.org/10.5194/os-2020-59
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2020-59
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 Jul 2020

14 Jul 2020

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This preprint is currently under review for the journal OS.

In situ observations of turbulent ship wakes and their potential implications for vertical mixing

Amanda T. Nylund1, Lars Arneborg2, Anders Tengberg1, Ulf Mallast3, and Ida-Maja Hassellöv1 Amanda T. Nylund et al.
  • 1Department of Mechanics and Maritime Sciences, Chalmers University of Technology, Gothenburg, 412 96 Gothenburg, Sweden.
  • 2Swedish Meteorological and Hydrological Institute (SMHI), Gothenburg, 426 71 Västra Frölunda, Sweden
  • 3Department Monitoring and Exploration Technologies, Helmholtz Centre for Environmental Research, Leipzig, 04318 Leipzig, Germany

Abstract. In areas of intensive ship traffic, ships pass every ten minutes. Considering the amount of ship traffic and the fact that global maritime trade is predicted to increase, there is a need to consider all effects shipping has on the marine environment; both pollution and physical disturbances. This paper studies a previously disregarded physical disturbance, namely ship-induced vertical mixing in the turbulent wake. A characterization of the temporal and spatial scales of the turbulent wake is needed to estimate its effect on gas exchange, dispersion of pollutants, and to identify in which areas ship-induced vertical mixing could have an impact on local biogeochemical cycles. There is a lack of field measurements of turbulent wakes of real-size ships, and this study addresses that gap by in situ and ex situ measurements of the depth, width, length, intensity and longevity of the turbulent wake for ~240 ship passages of differently sized ships. A bottom-mounted Acoustic Doppler Current Profiler (ADCP) was placed at 32 m depth below the ship lane outside Gothenburg harbour, and used to measure wake depth and temporal longevity. Thermal satellite images of the Thermal Infrared Sensor (TIRS) onboard Landsat 8 were used to measure thermal wake width and spatial longevity, using satellite scenes from the major ship lane North of Bornholm, Baltic Sea. Automatic Information System (AIS) records from both the investigated areas were used to identify the ships inducing the wakes. The results from the ADCP measurements show median wake depths of ~ 10 m, and several occasions of wakes reaching depths > 18 m. The temporal longevity of the wakes had a median of around 8 min and several passages of > 20 min. The satellite analysis showed a median thermal wake length of 13.7 km, and the longest wake extended over 60 km, which would correspond to a temporal longevity of 1 h 42 min (for a ship speed of 20 knots). The median thermal wake width was 157.5 m. The measurements of the spatial and temporal scales are in line with previous studies, but the deep mixing and extensive longevity presented in this study, has not previously been documented. The results from this study have shown that ship-induced vertical mixing occurs at temporal and spatial scales large enough to imply that this process should be considered when estimating environmental impact from shipping in areas with intense ship traffic. Moreover, the possibility that deep vertical mixing could occur in a highly frequent manner highlights the need of further studies to better characterize the spatial and temporal development of the turbulent wake.

Amanda T. Nylund et al.

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Amanda T. Nylund et al.

Amanda T. Nylund et al.

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Short summary
Acoustic and satellite observations of ship wakes show that ships can mix the water column down to 30 m depth, and that a temperature signature of the wake can last for tens of km. The temporal and spatial scales of the wake imply that intensive ship traffic have the potential to affect the natural exchange of nutrient and gases in the surface layer of the ocean. Thus, ship-induced mixing should be considered when estimating environmental impact from shipping in areas with intense ship traffic.
Acoustic and satellite observations of ship wakes show that ships can mix the water column down...
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