CTD buoy 2019O4 was deployed in the MOSAiC Distributed Network in the Northern Laptev Sea in early October 2019 as part of a set of eight identical ice-tethered buoy systems, each consisting of 5 Seabird SBE37IMP Microcat CTDs mounted along an inductive modem tether at depths of 10, 20, 50, 75 and 100m. The CTDs were recording oceanographic data internally at 2-minute intervals. The surface unit of the buoy prompted the instruments for an additional measurement every 10 minutes, which was then transmitted to a base station via iridium along with GPS position and time, as well as surface temperature. After 311 days of drift through the Central Arctic Ocean, 2019O4 was recovered in August 2020, and the internally recorded data from the CTDs were secured. The 10-minute buoy data and 2-minute CTD data were co-processed and merged into a combined product. A buoy flag indicates whether a measurement was taken by the buoy (1) or was recorded by the CTD itself (0). The data were quality controlled by means of outlier detection using global limits, moving average filters and manual inspection. The dataset was carefully checked for inconsistencies, especially in the salinity. Where appropriate, parameters were modified to enhance the quality. A (slightly modified) quality flagging scheme was applied according to the Ocean Data Standards Volume 3 (UNESCO 2013), where 1 = Good, 2 = Good (Modified), 3 = Questionable, 4 = Bad, 9 = no data. Finally, the data were validated against independent measurements. Details are available in the data paper indicted below.

• GearID 1: SBE37IMP, SN 21093• GearID 2: SBE37IMP, SN 21094• GearID 3: SBE37IMP, SN 21095• GearID 4: SBE37IMP, SN 21099• GearID 5: SBE37IMP, SN 21083Acknowledgments:The data were produced as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020 (grant numbers AWI_PS122_00 and AFMOSAiC-1_00). The instruments were funded through the MIDO (Multidisciplinary Ice-based Drifting Observatory) infrastructure program, and built by Pacific Gyre, USA. We thank the crews of the research vessels Akademik Fedorov, Akademik Tryoshnikov and RV Polarstern, and in particular the chief scientists, Thomas Krumpen, Markus Rex and Matthew Shupe. We thank the helicopter company Naryan-Marsky for their great logistical support that made this effort possible. We express our gratitude to all participants of these expeditions, and particularly to the MOSAiC School for their field assistance during the set-up. We appreciate the efforts of Kirstin Schulz, Julia Regnery, Ingo Schuffenhauer, Morven Muilwijk, Sandra Tippenhauer and the entire Leg 4 Team for the recovery of the instruments.