What is the role of finescale processes in water exchanges in the North Sea – Baltic Sea transition zone and in the Gotland gyro circulation in center Baltic?
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 Baltic Sea consists of an overall estuarine circulation that is driven by riverine fresh water runoff into the surface layer and inflow of saline water from the North Sea. This overturn circulation is characterized by east ward spreading of high saline water in the bottom layer and a westward directed outflow of brackish water in the upper layer. Upwelling, vertical turbulent diffusion and mixing at sills lead to upward directed net transport of salt and deep water, which closes the general circulation pattern between in- and outflowing water masses. 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 this transition zone the circulation is dominated by finescale processes and include upwelling, mesoscale frontal structure, submesoscale filamentation and water transport.
The key region for the connectivity between the deep-water layer and the surface is the eastern Gotland Basin in the central Baltic, where a basin scale cyclonic gyre is maintained by the atmospheric forcing and episodic inflows of bottom water. The dynamic properties of this basin scale eddy together with the amount of inflowing saline water, the stratification, and turbulent mixing control the upward transport of deep water and salt. The energy for the turbulent mixing at the rims is delivered by the basin scale gyre, meso- and sub-mesoscale variability and internal waves and oscillations.
Finescale features are evident in sea surface signatures from ocean color satellite observations and are studied in dedicated airborne remote sensing observations and 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)