Ice and brine production in Storfjorden from four winters of satellite and in situ observations and modeling

Abstract

[1] Manual classification of fast ice, pack ice, and polynya (open water and thin ice) in Storfjorden from ERS-2 SAR images during winters 1998 to 2001 was used to determine model parameters in a wind-driven polynya width model. Production of ice in the classified areas was then calculated from surface heat balance. The modeled open water area occupied on average 10% of the total area and produced 58% of the total ice (T ice ). The volume of brine-enriched shelf water (BSW; V bsw ) was estimated to be in the range 0.9-1.1 Â 10 12 m 3 or 0.06-0.07 Sv (freezing period average) and 0.03-0.04 Sv (annual average). The strength of the northerly wind component seemed to dominate over net heat flux as cause of variability in T ice . Salinity of BSW (S bsw ) was found to be primarily governed by frazil ice production, whereas V bsw was mostly determined from T ice and surface salinity (initial and change during winter). Correlation studies of modeled time series of winter mean polynya area and T ice in winters 1970 to 2001 showed that interannual variability could partly be explained by variability in regional ice and ocean conditions, and partly by the strength of the southwesterlies from the North Atlantic or the North Atlantic Oscillation (NAO). Strong southerly winds (high NAO) may give less Arctic ice import and ice production in the western Barents Sea, resulting in a higher surface salinity in Storfjorden in fall. The resulting weak stability in the water column may give favorable conditions for producing large V bsw the following winter. If strong northerly winds over Storfjorden are associated with low NAO, rapid transitions from high to low NAO would give high V bsw and S bsw . The link to NAO seems to vary in time. However, the transition from weak northerly winds in winter 1999 to strong northerly winds in winter 2000 was accompanied by high observed S bsw winter 2000 in agreement with expectations.

Dense water formation and circulation in the Barents Sea

“…5). These values are similar to the amount produced in the Storfjorden polynya (Skogseth et al, 2004) and in Arctic polynyas (Cavalieri and Martin, 1994;Winsor and Björk, 2000). On Central Bank and Great Bank the vol- …”

“…A recurring polynya is also known to form in Storfjorden ( Fig. 1) producing 0.06-0.07 Sv (1 Sv ≡ 10 6 m 3 s −1 ) of cold, dense water (often referred to as brine-enriched shelf water) during winter (Skogseth et al, 2004). The Storfjorden dense water formation is further described by e.g.…”

“…According to Harms (1997) the initial stratification is dependent on the Atlantic influence on ice freezing or melting, and Backhaus et al (1997) argued that import of warm Atlantic Water (AW) onto the Barents Sea shelf may cause a negative feedback on both deep reaching convection and ice growth. The latter is important for dense water formation through input of salt (Martin and Cavalieri, 1989;Winsor and Björk, 2000;Schauer, 1995;Skogseth et al, 2004). Furthermore, it has been recognized that as the climate of the region depends greatly on the Atlantic inflow, any variations in the characteristics of this inflow would inevitably have consequences for the production of dense water (Harms, 1997).…”

“…Harms (1997) argued that ice freezing/melting is more important than the freshwater runoff, while Ellingsen et al (2009) found that ice import through the northern boundaries is of major importance to dense water characteristics. Water column stability is another source of variability (Backhaus et al, 1997;Harms, 1997;Maus, 2003;Skogseth et al, 2004). According to Harms (1997) the initial stratification is dependent on the Atlantic influence on ice freezing or melting, and Backhaus et al (1997) argued that import of warm Atlantic Water (AW) onto the Barents Sea shelf may cause a negative feedback on both deep reaching convection and ice growth.…”

“…Localized dense water formation is often related to opening of polynyas due to offshore winds and the influence of tides upon the ice pack (e.g. Midttun, 1985;Martin and Cavalieri, 1989;Schauer, 1995;Skogseth et al, 2004). West of Novaya Zemlya cold easterly winds during winter frequently open a polynya (Martin and Cavalieri, 1989) and more than 10 m of ice forms in this polynya during an average winter (Winsor and Björk, 2000).…”