Methodology:

Snow on sea ice and sea ice were sampled at 0.02-0.1 m vertical resolution. Samples was melted onboard the ship to measure aqueous conductivity using conductivity meters. Sp is reported in psu (practical salinity unit), approximately equivalent to the weight of dissolved inorganic matter in grams per kilogram of seawater, and has an accuracy as stated by the manufacturer of ±0.001 psu at low salinities (<1 psu).

Data collection:

Data set 1: conductivity meter SensIon 5, Hach;

Data set 2: conductivity meter Cond 315i, WTW GmbH, Germany

Data quality:

Data set 1 (see also Frey et al., 2019): measurement range of 0-200 mS cm-1 and a maximum resolution of 0.1 μS cm-1 at low conductivities (0-199.9 μS cm-1). The conductivity meter used has an automatic non-linear temperature compensation based on a NaCl solution and reference temperature of 25 degC and was calibrated with a standard salt solution (12.880 mS cm-1 at 25.1 degC), certified and traceable to NIST (REAGECON Prod. No. CSKC12880, Lot No. CS1288012K1). Conductivity values were converted into practical salinity Sp as described in Frey et al. (2019). Accuracy as stated by the manufacturer is ±0.001 psu at low salinities (<1 psu).

Data set 2 (see also Nomura et al., 2018): For calibration of salinity measurement, a Guildline PORTASAL salinometer model 8410A, standardized by International Association for the Physical Sci- ences of the Oceans (IAPSO) standard seawater (Ocean Scientific International Ltd, UK), was used. The accuracy of this sensor was ±0.003.