In the Barents Sea, the general pattern of circulation is strongly influenced by large-scale atmospheric circulation, inflow of waters from adjacent seas, bottom topography, tides, and other factors — all of which make it rather complicated and variable (Figure 2.3.1).
Currents and transport / Circulation and inflow
Circulation is characterized by inflow of relatively warm Atlantic Water, and coastal water from the west. This inflow of Atlantic Water divides into two branches: 1) a southern branch that flows parallel to the coast and eastwards towards Novaya Zemlya; and 2) a northern branch that flows into the Hopen Trench. Coastal Water has more fresh-water runoff and a lower salinity than Atlantic Water; it also has a stronger seasonal temperature signal. In the northern region of the Sea, fresh and cold Arctic waters flow from northeast to southwest. Atlantic and Arctic water masses are separated by the Polar Front that is characterized by strong gradients in both temperature and salinity. There is a large inter-annual variability in ocean climate related to variable strength of Atlantic Water inflow, and exchange of cold Arctic water. Thus, there can be a considerable seasonal variation in hydrographic conditions (Ozhigin et al., 2011).
The inflow from the Norwegian Sea takes place through the Bear Island Channel. Close to the coast the low salinity (S _ 34.4 pss) water of the Norwegian Coastal Current (NCC) carries a substantial fraction of the runoff from the Baltic and the Norwegian coasts into the southern Barents (Figure 2.3.1). It continues eastward as the Murman Coastal Current. Additional low salinity water is added as it passes the entrance to the White Sea and the mouth of the Pechora River, so the salinity remains low (S = 34.6 pss). Most of this ‘‘coastal’’ water passes into the Kara Sea through the Kara Gate. River runoff and net precipitation are small, and the NCC is the major freshwater source for the Barents (Tantsiura, 1959; Loeng, 1991, 1992; Wassmann, 2006).
Vertical stratification of the different water masses in the Barents Sea is important for primary production. Different mixing regimes in Atlantic Water, the trench/Polar Front region, and the Melt Water/Arctic Water region are structured by different stratification mechanisms; this has implication for the phytoplankton community development and new production (Reigstad et al., 2002). Stratification of water masses in these regions may occur in several different ways: 1) through fresh surface water from melting ice along the marginal ice zone; 2) through solar heating of surface layers in Atlantic Water masses; or 3) through lateral dispersion of waters in the southern coastal region (Rey, 1981). Extensive ice formation, brine rejection in winter, and the subsequent melting of the ice in summer lead to a separation of the water column into a colder and denser deep-water, and a less saline, less dense upper layer. In the north, annual production is initiated by strong stratification developing as ice melts and light becomes available (Reigstad et al., 2002). Low salinity surface water contributes, together with inflows from the Arctic Ocean and the Kara Sea, to maintain stable stratification in the northern and eastern regions (Wassmann et al., 2006). In the central Barents Sea, ice that drifts over Atlantic Water is melted rapidly by heat from below, creating a thin, low-salinity layer and strong stratification over the Atlantic Water. This occurs throughout the year, and strong stability ensures a rapid phytoplankton bloom in the upper layer once sufficient light is present (Wassmann et al., 2006). Within the Atlantic Water, stratification is close to absent in spring, but weak stratification develops slowly from solar radiation during the summer (Reigstad et al., 2002).
Ice conditions in the Barents Sea are influenced by both Atlantic and Arctic Oceans, and by atmospheric conditions. Typically, ice coverage is at a minimum in September, when an average of 5% of the Sea is ice-covered; while maximum ice cover is in April and ranges between 35% and 85%, with an average of 61%. Long term yearly mean ice coverage is close to 40%. However, high seasonal variability in the extent of the ice is characteristic. Inter-annual variability is also large, and the extent of ice varies widely depending on whether the winter is mild or severe. During winter, the ice-covered area expands from north to south and from east to west. By the end of winter the sea ice has reached its maximum thickness (130-150 cm). The ice edge retreats northward and eastward through September, most rapidly during June-July (Ozhigin et al., 2011).