Frontal Dynamics and Water Mass Transformation

The ocean is in a perpetual motion that is enabled by Water Mass Transformation (WMT) processes, i.e., the mixing of different water masses and the exchange with the atmosphere that generates other water masses with different properties. Due to the vast size of the ocean basins, these WMT processes can hardly be observed and simulated in detail. Therefore, to address the WMT processes and their influence on the ocean dynamics, we study in this project the dynamics of a regional sea, the Skagerrak, that connects the North Sea and the Baltic Sea, using it as a natural laboratory for WMT processes. The circulation in the Skagerrak consists of three inflowing waters masses, the Jutland Current from the German Bight, the Baltic Outflow and the Atlantic Sea water. These water masses are then transformed into the outflowing waters of the Norwegian Coastal Current and the Baltic Inflow. The underlying WMT processes will be quantified by a suite of observational and numerical methods. We hypothesise that frontal dynamics in the surface waters are responsible for a large part of the WMT, but we will also aim to identify deep mixing processes. A ship campaign will be organised with two research vessels during spring / summer 2025, with synoptic observations concentrating on frontal dynamics. The ship time has been requested and is currently under review by the Review Panel German Research Vessels (GPF). During the cruise drifter experiments, microstructure observations and echo sounding surveys combined with remote sensing will be carried out. These observations will be augmented by existing long-term monitoring data from various sources. Swedish colleagues will support us by own cruises coordinated with us, deploying gliders and sail drones to provide detailed information on the vertical structure of the upper Skagerrak waters. A 15-year realistic numerical hindcast model simulation for the Skagerrak region will be set up to investigate the dynamics in the region. A nesting approach allows for resolving the eddy dynamics in the entire region, including foci on specific frontal regions that had been surveyed during the cruises. An existing Eulerian WMT framework will be further developed to quantify WMT processes in the region, including numerical truncation errors. A Lagrangian particle backtracking model will be integrated into the numerical model, to understand pathways of water masses, their residence times and mixing properties. In the end, we expect to have obtained a validated and complete assessment of the WMT processes in the Skagerrak that can be generalised and transferred to other regions. To internationally coordinate and carry out the WMT research in the Skagerrak, we have formed the SkaMix Consortium with a total of 20 members from Norway, Sweden, The Netherlands and Germany by signing a joint Memorandum of Understanding. For this project, we request one PhD student for the numerical modelling. One further PhD student for the field work will be paid by our institute.