The warm temperatures in the intermediate layer of the Arctic Ocean were first reported in 1902 by Fridtjof Nansen from observations made in 1893-1896 during his famous expedition aboard Fram. The Atlantic origin of this layer was soon recognized and reported on in 1909 by B. Helland-Hansen and Nansen. Until the 1980s, Atlantic water inflow into the Arctic Ocean was associated exclusively with the West Spitsbergen Current in the eastern Fram Strait, following the continental shelf break. Then, another route was found farther to the east, via the Barents Sea (see the figure).
Now both inflows are considered equal, transporting about 2 Sv each (1 Sverdrup = 106 m3 s-1) and merging north of the Kara Sea (Rudels et al., 1994). The West Spitsbergen Current transport is extremely variable, from 0 to 9 Sv (Woodgate et al, 2001). The Barents Sea transport varies seasonally from 1 to 3 Sv. The latest estimates of the long-term mean transport of these branches are 1-1.5 Sv for the West Spitsbergen Current and 2 Sv for the Barents Sea route (Rudels and Friedrich, 2000). Thus, the latest appreciation of the importance of the Barents Sea inflow marks a return to the views of Helland-Hansen and Nansen. Farther downstream, the propagation of Atlantic water has never been reliably measured and is rather assumed, largely from the Atlantic layer core temperature distribution. In general, the Atlantic water is believed to move cyclonically (counterclockwise) around the Arctic Ocean and exit via the western Fram Strait with the East Greenland Current. Within the Arctic Basin, the Atlantic water circulates around the Nansen, Amundsen, Makarov, and Canadian basins in boundary currents. The primary Arctic Ocean Boundary Current splits at the junction of the Lomonosov Ridge and Siberian shelf. One branch crosses the Lomonosov Ridge and flows along the East Siberian continental slope, while the other flows along the Lomonosov Ridge. From the first long-term (year-long) moorings at the junction of the Lomonosov Ridge with the Eurasian continent, the Arctic Ocean Boundary Current transport was estimated to be 5±1 Sv (Woodgate et al., 2001).
There are substantial differences between the Atlantic layer in the Eastern and Western Arctic. The Eastern Atlantic layer is much warmer, with the core temperature between 2°C and 3°C, whereas the Western Atlantic layer core temperature is generally below 0.5°C (McLaughlin et al., 1996). The boundary between these two regimes is termed the Atlantic/Pacific front. In the past, this front was located over the Lomonosov Ridge. The 1990s data revealed a large-scale shift of this front, which is presently located over the Mendeleyev and Alpha ridges (McLaughlin et al., 1996; Morison et al., 1998).
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