Coastal Arctic polynyas play an important role in forming dense water and contribute to the deep ocean ventilation. Dense water formation results from brine rejection associated with sea-ice production. In March 2007, a 60m-long ice tethered mooring, recording temperature and salinity at high frequency with a relatively fine vertical resolution and a prototype float (Ice-T) measuring ice-thickness and thermal profiles in sea ice were deployed in Storfjorden in the Svalbard archipelago. This fjord hosts a latent heat polynya which is maintained open under northeasterly wind conditions. The water masses analysis reveals that the main pool of the fjord is occupied by Brine-enriched Shelf Water (BSW) over the whole water column, while the southwest part of the fjord is occupied by Arctic Water (ArW). The BSW recorded at the bottom of the mooring has a salinity of 35.2 (density of 28.31 kg m^-3), which is among the densest values of BSW reported in this region in the literature. Measurements indicate that the water column is slightly supercooled down to 30m for the first day of the record until a warm storm breaks the ice. Maximum supercooling values are 0.007±0.002°C below the in-situ freezing point with a mean value of 0.005±0.002°C at 10m. The origin of this supercooled water is investigated by analyzing the ice cover and the ocean-atmosphere heat flux during the days that preceded these observations. Although the amount of supercooling is relatively low compared with values recently reported in Storfjorden, these observations show that supercooled water can still be present under the ice several days after the cooling and freezing period. A sudden and brief salinity anomaly reaching 0.2 ppt is observed at the base of the ice. Concomitant temperature anomalies are recorded in the sea ice. The origin of this signal, which occurs during a warm storm, is analyzed. As this salinity anomaly cannot be associated with ice formation, different sea ice desalination processes are investigated, in particular flushing by meltwater and gravity drainage.