Description: Oxygen and hydrogen isotopic ratio samples were collected from sea ice and water samples. Coring and sampling were completed at six locations on July 25 for measurement of salinity and stable oxygen isotopic composition to better understand the meltwater sources for under-ice meltwater and false bottoms. Cores were collected using a corer (9 cm inner diameter; Mark II coring system, Kovacs Enterprises, US), and ice samples were collected from the bottom 10 centimeters of the ice core in 5 cm increments (0-5 and 5-10 cm above the bottom) and from the false bottom ice. Under-ice meltwater layer samples and water from directly below the false bottom were collected using a peristaltic pump. In order to minimize contamination from coring activities, under-ice meltwater samples from the meltwater layer in the void space between ice and false bottom were collected prior to coring through the false bottom. Surface melt pond water samples were collected by dipping plastic sample cups (rinsed with milliQ water) in the melt ponds where cores were collected, prior to ice coring. In total, ice samples were collected from six cores –– five of which included false bottoms –– and water samples were collected from four surface melt ponds, three under-ice melt water layers, and seawater beneath false bottoms at four locations. Locations are shown on map (Figure 2) as pink dashes. Ice samples were melted onboard the R/V Polarstern, and salinities of all water and ice samples were measured onboard using a calibrated YSI model 30 probe (salinity is given in PSS-78 scale, unitless). Vials for oxygen isotope composition were shipped to the AlfredWegener Institute (AWI) ISOLAB Facility in Potsdam, Germany, where they were analyzed for stable water isotopes with Finnigan MAT Delta-S mass spectrometers using equilibration techniques. The oxygen isotope composition is given as per mil difference relative to VSMOW (‰, Vienna Standard Mean Ocean Water), with an internal 1 sigma error better than 0.1‰ for delta 18O (Meyer et al., 2000). See attached diagram for a visualization of where the samples were collected.

Description: Temperature and heating-induced temperature differences were measured along a chain of thermistors. SIMBA 2020T60 (a.k.a. HAVOC 0101) is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the 2nd leg of MOSAiC in February 2020. The thermistor chain was 10m long and included 250 sensors with a regular spacing of 4cm. The resulting time series describes the evolution of temperature and temperature differences after two heating cycles of 30 and 120 s as a function of place, depth and time between 08 Jan 2020 and 22 Apr 2020 in sample intervals of 6 hours for temperature and 24 hours for temperature differences. The buoy was installed on the ridge crest known as SIRO. In addition to temperature, geographic position, barometric pressure, air temperature measured 1m over the ice level, tilt and compass were measured. The data set has been processed as follows: obvious inconsistencies (missing values) have been removed and unrealistic values in position have been replaced by NaN (Not a Number). This instrument was deployed as part of the project "Ridges - Safe HAVens for ice-associated Flora and Fauna in a Seasonally ice-covered Arctic OCean" (HAVOC), funded by the Research Council of Norway, project number: 280292, "Coordinated Arctic Acoustic Thermometry Experiment" (CAATEX), funded by Norwegian Research Council, project number: 280531 and "Arctic Ocean ecosystems - Applied technology, Biological interactions and Consequences in an era of abrupt climate change" (Arctic ABC Development, Research Council of Norway, project number: 245923).

Description: Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC24 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 8 January 2020. The thermistor chain was 7.36 m long and included sensors with a regular spacing of 2 cm. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 8 January 2020 and 18 March 2020 in sample intervals of 6 hours. It also contains manually estimated position of air-snow, snow-ice, and ice-water interfaces. The DTC was installed in Fort Ridge towards the level second-year ice. Ice mass balance SIMBA 2020T60 was installed at the Fort Ridge: doi:10.1594/PANGAEA.924269.

Description: Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC24 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 8 January 2020. The thermistor chain was 7.36 m long and included sensors with a regular spacing of 2 cm. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 8 January 2020 and 18 March 2020 in sample intervals of 6 hours. It also contains manually estimated position of air-snow, snow-ice, and ice-water interfaces. The DTC was installed in Fort Ridge towards the level second-year ice. Ice mass balance SIMBA 2020T60 was installed at the Fort Ridge: doi:10.1594/PANGAEA.924269.

Description: Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC25 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 8 January 2020. The thermistor chain was 5.12 m long and included sensors with a regular spacing of 2 cm. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 8 January 2020 and 23 March 2020 in sample intervals of 6 hours. It also contains manually estimated position of air-snow, snow-ice, and ice-water interfaces. The DTC was installed in Fort Ridge towards the level first-year ice. Ice mass balance SIMBA 2020T60 was installed at the Fort Ridge: doi:10.1594/PANGAEA.924269.

Description: Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC09 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 28 January 2020. The thermistor chain was 4.16 m long and included sensors with a regular spacing of 2 cm. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 27 February 2020 and 1 July 2020 in sample intervals of 6 hours. It also contains manually estimated position of air-snow, snow-ice, and ice-water interfaces. The DTCs was installed in level first-year ice next to the Fort Ridge. Ice mass balance SIMBA 2020T60 was installed at the Fort Ridge: doi:10.1594/PANGAEA.924269. Radiation station 2020R10 was installed in level first-year ice next to the Fort Ridge: doi:10.1594/PANGAEA.949124.