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ship-based
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CONWEST-DYCO

What is the role of finescale processes in water exchanges in the North Sea – Baltic Sea transition zone?

The circulation in the North Sea – Baltic Sea transition zone is dominated by finescale processes, and include upwelling events, mesoscale frontal structure, and submesoscale filamentation. Riverine freshwater input and large and regional wind forcing all contribute to the sea level variability in the Baltic Sea.

The Baltic Sea surface area is about 420,000 km2. The drainage area of the Baltic Sea is about four times larger than its surface area and is inhabited by around 85 million people. The general circulation in the Baltic Sea resamples an estuary system with freshwater surplus creating a brackish upper layer that flows out of the domain, while a bottom layer of varying thickness is established from episodic inflow of waters originating from the North Sea/Northeast Atlantic. The bottom inflow is of critical importance for many processes in the Baltic basin, such as oxygenation. The major surface and bottom water exchange gateways are located in the complex North Sea-Baltic Sea transition zone, where water exchanges occur in narrow straits and are mainly driven by sea level differences determined by barotropic pressure gradient. In the North Sea – Baltic Sea transition zone sub-mesoscale features correspond to upwelling events and water transport. In the Gotland basin, in the center of the Baltic Sea, the Gotland gyro dominates upper layer circulation and isimportant for the Baltic haline overturn circulation.

Finescale processes have an impact on water exchange and circulation in the Baltic and in its transition with the North Sea. Finescale features are evident in sea surface signatures from ocean color satellite observations and are studied in dedicated airborne remote sensing observations and also in high resolution modelling. The CONWEST-DYCO campaign will combine various in-situ observations with satellite SWOT data to understand the opportunities and limitations for the SWOT mission on monitoring the western Baltic Sea.  

Ground tracks 3 and 14 of the SWOT 1-day calval phase cover transition area Baltic-North Sea and the Gotland basin, respectively. In both regions fixed site observational platforms (buoys, subsurface moorings) are operational since many years. The vertical structure of the water column will be surveyed with CTD casts during the regular monitoring cruises of the IOW, that are performed five times a year (2 weeks cruises), and with cruises related to the mooring services. Recordings of velocity profiles, bottom pressure, and echo sounder water depth are planned during the 1-day ascending swath of SWOT. Full water depth data is available in real-time from the BSH “Fehmarn Belt” station and from the Station “Kiel Lighthouse”. Coastal tide gauges provide additional spatial context for the local observations at sea. In the eastern Gotland basin, three mooring stations are positioned under 1-day swath of SWOT and tide gauges are present in Landsort Norra in Sweden. The MARNET stations, located east of the Kadett Trench, will be used in the SWOT 21-day repeat.

Principal investigators: Luciana Fenoglio-Marc (fenoglio@geod.uni-bonn.de), Volker Mohrholz (volker.mohrholz@io-warnemuende.de), J. Staneva (joanna.staneva@hereon.de), Johannes Karstensen (jkarstensen@geomar.de)

Institutes involved in the campaigns: Institute of Geodesy and Geoinformation (University of Bonn), Leibniz Institute for Baltic Sea Research (IOW), Helmholtz Centre Geesthacht (HZG), Helmholtz Centre for Ocean Research Kiel (GEOMAR)

Contact point for the study site: Luciana Fenoglio-Marc (fenoglio@geod.uni-bonn.de)