Dense overflow from an Arctic fjord: Mean seasonal cycle, variability and wind influence

Storfjorden, an Arctic fjord in the Svalbard archipelago, is separated by a submarine sill from the adjacent shelf areas and produces one of the densest water masses in the Barents Sea. The cold and dense brine-enriched shelf water is produced through ice formation in an annually recurrent polynya in Storfjorden and overflows across the sill. We present current profiles and bottom temperature measurements from the Storfjorden sill from 2003 to 2007, which is the longest time series collected at this site, and study the interannual variability of the overflow and the influence of atmospheric forcing. The mean structure of the overflow averaged over four seasons shows that the overflow is initially strong with high volume transport, about 50 m thick and bottom-enhanced, and then gradually diminishes, becoming increasingly intermittent during the last third of the overflow season. The annual average overflow transport is about 0.03 Sv (1Sv≡106m3s-1). Overflow is observed 55% of the total record length of 958 days and cross-sill flow averaged in the bottom 20 m is greater than 10cms-1 for 49% of the overflow duration. The overflow strength increases with decreasing near-bottom temperatures. The annual variability is within 0.01 Sv whereas the seasonal variability can be as large as 0.05 Sv. In spite of the relatively constant annual overflow flux, the onset of the overflow can vary by up to 50 days. Variability on the scale of 1-2 weeks is connected to wind forcing through surface Ekman transport with significant coherence between the current at the Storfjorden sill and wind measured on Hopen Island and Edgeøya. Surface Ekman transport and the ice conditions in the Barents Sea also influence the intra-seasonal development and interannual variability of the overflow.