Supercooling in the mixed layer beneath Arctic pack ice from modern and historical measurements

In the central Arctic, the oceanic heat flux from the seawater to the pack ice depends mostly on the seawater temperature elevation above freezing (dT). Long-term temperature and salinity observations from modern drifting platforms (primarily SALARGOS, IOEB, JCAD buoys and SHEBA) indicate that the annual cycle of dT in the mixed layer beneath the ice pack is significantly related to the solar zenith angle (with 1 month time lag). During winter when there is no solar input, the seawater temperature is expected to be very close to the freezing temperature at a given salinity, but the observations indicate that small positive biases in dT are common, especially in the Transpolar Driftstream. Meanwhile, supercooling where dT is negative by more than 0.01 \deg C was indicated in only 0.2% out of 6450 daily average values, and in only 4 out of 30 modern platforms. In order to extend the record backward in time, dT was also computed from hydrographic data obtained by Russian North Pole (NP) drifting stations and western AIDJEX and FRAM drifting stations. Surprisingly, virtually every station and over 31% of the observations from the NP stations and over 11% of the observations from AIDJEX indicate supercooling by more than 0.01 \deg C. In this study, the methods of data collection, accuracy of the measurement techniques, and considerations of natural variability are analyzed to gain a better understanding of the data quality and to merge the modern and historical records.