Direct Observation and Model Validation of Gravity Drainage and Flushing of First Year Sea-ice in an Arctic Coastal Polynya

Coastal Arctic Polynyas play an important role in forming dense water and contribute to 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. During the first 20h of measures, two sudden and brief salinity anomalies reaching 0.2 ppt are 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, can not be related with sea-ice formation. Therefore, the signal is related with a desalination of first year sea-ice of 1-5ppt. Atmospheric conditions change from intense heat loss from the open polynya (-400 W/m2) six days before, as we could confirm by satellite, to moderate heat gain of around 75 W/m2. This change suggests ice transitions to a permeable state. One of the events is associated with flushing by meltwater according to snow melting rate values. The other event occurs before melting and then it may be associated with gravity drainage. Using 1-D enthalpy model this hyphotesis is confirmed, showing that the core of the ice remains colder than the sea water allowing temperature instabilities and consequently brine convection. Those processes in data set observation had been hardly documentated in spite of highly relevance in improving the knowledge of first year sea-ice thermodynamics.