Underway (U)CTD data were collected during an August-September 2018 expedition to the Arctic Ocean aboard the RV Akademik Tryoshnikov, and was a joint expedition between the German-Russian project CATS (Changing Arctic Transpolar System) and the US-Russian project NABOS (Nansen and Amundsen Basin Observing System). The UCTD was operated mostly in yoyo-mode during selected transects between the shelf and the basin across the continental slope of the Eurasian Basin while the ship was transiting with 8 - 14 knots. The UCTD probe records the start time of the measurements and stores 16 samples each second internally. The exact location of each profile was subsequently found based on the time stamp from the cruise track. The unpumped conductivity sensor has a slower response time than the temperature sensor, which makes the computation of salinity from conductivity and temperature potentially spiky, especially in the pycnocline or in frontal regions. We followed the recommendation of the manufacturer to calculate salinity with Seabird processing software. In shallower waters (<200m), the water column was profiled all the way to the seafloor, while in deeper waters, only the upper 200-350m were sampled. Shipboard echo soundings were not available, actual water depths at the profile locations need to be extracted from bathymetric charts (for instance IBCAO ). The UCTD was calibrated against a Seabird 911 CTD during the cruise. Temperature measurements were comparable to the ship's CTD and remained uncorrected, the processed salinities include a salinity correction based on a deviation from the ship's CTD.

The underway CTD is a self-contained free-falling probe measuring temperature, conductivity, depth while the ship is transiting. The conductivity sensor is unpumped, and has a response time that is different from that of the temperature sensor. Therefore, converting conductivity into salinity is not straightforward. We followed the recommendation of the manufacturer and computed salinity by use of the Seabird processing software as outlined in the document “UCTD salinity”.The processed data file includes temperature, pressure, and salinity. Please note that the upper 20 m were removed because the quality of the measurements cannot be guaranteed due to the 11m draft of the vessel and the associated turbulent overturning behind the ship.Additional information about the underway CTD can be found in Rudnick, D. L., and J. Klinke, 2007.The raw data are also included and can be used to reprocess the salinity data if desired.