Observations of hydrography and downflow of brine‐enriched shelf water in the Storfjorden polynya, Svalbard

Abstract

Observations of hydrography and currents in the active Storfjorden polynya during fieldwork in April 2004 and 2006 are presented. The polynya adds salt from its efficient ice production, usually increasing the density ∼0.15 kg m−3. Downflow of dense water from the coastal polynya to the deeper basin enclosed by the Storfjorden sill was captured in both years. April 2006 had a period of strong heat loss and intense frazil ice growth in the polynya. This created downflow of brine‐enriched shelf water (BSW) with a maximum salinity of 35.25, i.e., an increased density ∼0.4 kg m−3 above that of the source water. In April 2004, the salinity remained lower than BSW (S > 34.8), reflecting the fresh source water in fall 2003 due to the heavy ice conditions in the western Barents Sea that year. A portion (0.05 Sv) of such source water probably enters Storfjorden through Freemansundet where the mean current transports less saline and warmer water from the northwestern Barents Sea. Freemansundet has additionally a strong tidal current peaking at 53 cm s−1, dominated by the M2 component. The tidal wave propagates through the narrow sound producing shallow water components and a locally well‐mixed water column. The estimated ice production in winter 2004, twice as large as that of winter 2006, could not overcome this less saline well‐mixed source water. BSW downflow within Storfjorden was both observed and modeled to occur in areas with steep bathymetry. Our observations show that high‐resolution hydrographical data, spacing less than 0.5 km, are necessary to resolve such downflow. Consistent with the observed polynya water salinity, current data at the sill showed weaker overflow in 2004 than in 2006. Overflow of BSW was observed on the eastern part of the Storfjorden sill in April 2006, indicating that overflow occurs across the entire sill width during the freezing period. Current, temperature, and salinity data from the overflow indicate a time lag of 12–18 days between the BSW production and the sill. This downflow–overflow link is confirmed by a numerical experiment and analytical scaling, making results applicable to other polynyas.