The R/V Elisabeth Mann Borgese left the port of Rostock in the morning of the 17th April heading toward the Eastern Gotland Basin. CTD casts and ScanFish operations were carried out. Together with three moorings, they provided the first data on physical and biological parameters under SWOT swath. Unusual at time of the year, an aurora borealis was visible on 23th April.
By Luciana Fenoglio and Volker Mohrholz
The R/V Elisabeth Mann Borgese of the Leibniz-Institute for Baltic Sea Research Warnemünde (IOW) left the port of Rostock (Fig. 1) in the morning of the 17th April. The target area of this SWOT dedicated cruise was the Eastern Gotland Basin (EGB) in the Central Baltic. Few days ago our collaborators, from the VOTO initiative in Sweden, have started a glider mission to support our vessel based observations in the EGB.
The goal of the 12-day long CIREG-SWOT cruise is to study the circulation and mesoscale dynamics of the Eastern Gotland Basin (EGB) in the Central Baltic, which is covered by the SWOT cal/val 1-day orbit (Fig. 2). The area includes the Exclusive Economic Zones (EEZ) and partly territorial waters of Sweden and Latvia. Three moorings were already deployed in May 2022 in the deep part of the EGB. The moorings were maintained during another IOW cruise in March 2023.
On the transit from Rostock to the Eastern Gotland Basin some key stations of the Baltic Long-term Observation Program located in the EEZ of Denmark and Poland were covered with CTD casts (Fig.3). During the first two days heavy weather hampered the observation. However, we could start with the planned ScanFish grid in the EGB on 19th April.
Figs. 4 to 7 give some impressions of the ScanFish operations. The grid of transects measured with ScanFish below the SWOT swath is shown in Fig. 2. Remote sensing data made available via the Spasso Bulletin were used to identify the location of the major dynamic features in the Basin. On the 20th April another mooring with a vertically moving CTD profiler was deployed in the center of the EGB to gather hourly CTD profiles. First preliminary results of the hydrographic data gathered with the ScanFish are shown in figures 8 and 9. The device measures temperature, salinity, oxygen concentration, turbidity and Chlorophyll-a fluorescence. The derived density anomaly (sigma0) is also shown.
Recording from three moorings are available in the EGB since many years. An example for the integration of field and remote sensing data has been investigated for year 2020. Fig. 10 gives the moorings and the Sentinel-3A and SWOT ground tracks location. Pressure from the Gotland_NE mooring and sea level from the nearby altimeter track number 711 are shown in Fig. 11. The 1Hz altimeter data of the RADS database have been post-processed at Uni-Bonn by selecting a small rectangle covering the mooring location. A correlation of 0.87 between pressure and sea level was obtained over the 10 month interval considered. Similar studies are planned as part of the SWOT cal/val activities.
On the 23th April an aurora borealis was visible to the expedition, an unusual gift at this time of the year (Fig.10).
Leibniz-Institute for Baltic Sea Research Warnemünde, Germany (5 scientists and technicians)
University of Rostock, Germany (2 students)
University of Oldenburg, Germany (1 student)
VOTO initiative, Sweden (cooperation partner, providing the glider observations)
University of Bonn, Germany (1 scientist, online support contact with AdAC team)
Details on instrumentation:
- A CTD is used to measure the electrical conductivity, temperature, and pressure of seawater. Additionally, the instrument is equipped with sensors for oxygen concentration, turbidity and Chlorophyll-a fluorescence. The salinity is derived from conductivity, temperature and pressure data. The CTD platform consists also of an array of FreeFlow sample bottles referred to as a carousel or rosette. The CTD is lowered vertically to obtain depth profiles of the mentioned parameters.
- The Scanfish is a tow body, like a section of an aircraft wing, that is undulated behind the moving ship. The CTD in the ScanFish body delivers hydrographic data along the sawtooth shaped tow path. After interpolation these data provide a 2d picture of water properties. The ScanFish is towed with a speed of 6 to 7 knots and allows fast and high-resolution hydrographic observations.
- A mooring is a collection of devices connected to a wire and anchored on the sea floor. The instrumentation at the three mooring in the EGB consists of CTDs (conductivity, temperature depth sensors), current meters (eg. Acoustic Doppler current profilers or deprecated rotor current meters), and biological sensors to measure various parameters. Long-term moorings can be deployed for durations of two years or more, powered with alkaline or lithium battery packs.
Tosca Ballerini (firstname.lastname@example.org)