Master thesis defence by Lea K. D. Sørensen – Niels Bohr Institute - University of Copenhagen

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Master thesis defence by Lea K. D. Sørensen

Oceanographic variability at a boulder reef in the Kattegat  and surrounding waters

Supervisors
Peter Ditlevsen, Centre for Ice and Climate
Christian Mohn, Department of Bioscience, Århus University
Kristine Skovgaard Madsen, Danish Meteorological Institute

Abstract
In this thesis, an analysis of long-term time series of temperature and salinity in the Kattegat and Belt Sea is carried out. This analysis provides a picture of the stratification between the dense waters from the North Sea and the lighter waters from the Baltic Sea. The long-term analysis gives an overview of the region surrounding the area of interest, the boulder reef Hatter Barn. The research shows a year to year variation and a linear trend with increasing temperature in the bottom waters of 1.46^oC over the 20 year observation period.

Hatter Barn is enclosed by very busy conventional shipping routes with 26000 ships above 300 tons passing the area in 2010. The physical environment at the Hatter Barn reef is exposed to a combination of tidal flow, local weather conditions and large-scale currents regularly affecting the whole water column due to shallow water depths not exceeding 20 m. A study of three months high temporal resolution observations of salinity and temperature was carried out. The results show great mixing events both over longer and shorter periods with large temperature and salinity fluctuations. The tidal oscillation and wind impact are analyzed. A critical wind speed threshold of about 10 m/s appeared as the major forcing mechanism of strong mixing events. Model data are compared with observations to learn about the connections of the observed temperature and salinity variability with the flow pattern in the area. Underwater noise data were used to analyze potential effects from passing ships on short-term temperature and salinity variability. There is some indication of a possible correlation which may explain the large fluctuations within minutes observed in the temperature and salinity data.