31
Arctic Ocean Circulation and Hydrography
Since the Arctic Ocean is a largely enclosed ocean, there are
only two water masses that enter the basin from other oceans:
Pacific Water (PW) and Atlantic Water (AW). The lowsalinity PW is transported through the Bering Strait (Fig. 4)
and is mainly advected at the surface into the Amerasian Basin
and adjacent shelf regions. In addition to continental runoff
and precipitation, the low salinity PW is an important Arctic
fresh water source. Due to relatively small differences in
temperature throughout the water column of the Arctic
Ocean, the stratification is mainly determined by salinity
changes (Fig. 5). Thus, the fresh (light) waters stay in the
upper ocean and build the so called Polar Mixed Layer. Large
parts of the Arctic Ocean are covered by sea ice which is
built from these fresh surface waters at near-freezing temperatures. By sea ice formation and melt, freshwater is concentrated at the surface.
The AW is warmer but saltier than PW and Meteoric
waters comprising continental run-off and net precipitation.
Thereby, it is denser and can be found deeper in the water
column. The significant difference in salinity creates a strong
halocline between the Polar Mixed Layer and the AW layer
establishing a strong permanent stratification in the deep
basins (Fig. 5). The halocline, which is defined by high vertical salinity gradients (32.5 < S < 34.5), is thickest in the
Canada Basin (200–250 m) and thinnest in the Nansen Basin
(100–150 m). The temperatures of the halocline remain close
to the freezing point. Due to heat loss and mixing with shelf
waters there are many modifications of AW at intermediate
depths. Just below the halocline, temperatures are highest in
the Nansen Basin and decrease towards the Canada Basin
(Fig. 5). The densest waters are formed on the shelves of the
Barents Sea, where the AW subsequently releases heat to the
atmosphere and is mixed with brine rejected from newly
formed sea ice before it sinks down the shelf break into the
Nansen Basin (Fig. 4). This Arctic bottom water is the densest water of the world ocean, but can only be found in the
Arctic region (Tomczak and Godfrey 1994). Only a small
part is able to flow over the sill of the Fram Strait into the
Nordic Seas balancing the bottom/deep water formation on
the Arctic shelves (e.g., Bönisch and Schlosser 1995).
The surface circulation in the central Arctic Ocean mainly
comprises two features: the Beaufort Gyre and the Transpolar
Drift (Fig. 4). The Beaufort Gyre is an anticyclonic circulation in the Canada Basin that is forced by a high pressure
system in the lower atmosphere, the so called Beaufort High.
Fresh surface waters from the shelves and from the Pacific
accumulate in the interior of the gyre and leave the Arctic
through the Canadian Arctic Archipelago or the Fram Strait.
The Transpolar Drift is a wind-driven current that directs sea
ice and waters from the Siberian shelves and the Bering
Strait to the Fram Strait, where they exit the Arctic Ocean
into the Nordic Seas and subpolar North Atlantic.
The relatively warm and saline AW, which is the main
water source of the entire Arctic Ocean, enters the Nordic
Seas from the south. The Norwegian Atlantic Current carries
the AW through the Nordic Seas at the surface and splits into
two main branches. One branch enters the Arctic Ocean
through the Barents Sea Opening, the other, which then is
called West Spitsbergen Current, flows through the Fram
Strait. Only a part of the West Spitsbergen Current propagates further north into the Arctic Ocean, the other part recirculates close to the Fram Strait. The AW dives underneath
Fig. 4 Schematic of the
Arctic Ocean circulation
(reproduced from Carmack
et al. 2015, American
Meteorological Society, used
with permission). Blue arrows
indicate the surface
circulation, pink-blue arrows
the main pathways of the
Pacific Water at intermediate
depths and red arrows show
the Atlantic Water circulation.
GIN Seas: Greenland-IcelandScotland Seas, usually called
the “Nordic Seas”
The Physical System of the Arctic Ocean and Subarctic Seas in a Changing Climate
Arctic Ocean Circulation and Hydrography
Since the Arctic Ocean is a largely enclosed ocean, there are
only two water masses that enter the basin from other oceans:
Pacific Water (PW) and Atlantic Water (AW). The lowsalinity PW is transported through the Bering Strait (Fig. 4)
and is mainly advected at the surface into the Amerasian Basin
and adjacent shelf regions. In addition to continental runoff
and precipitation, the low salinity PW is an important Arctic
fresh water source. Due to relatively small differences in
temperature throughout the water column of the Arctic
Ocean, the stratification is mainly determined by salinity
changes (Fig. 5). Thus, the fresh (light) waters stay in the
upper ocean and build the so called Polar Mixed Layer. Large
parts of the Arctic Ocean are covered by sea ice which is
built from these fresh surface waters at near-freezing temperatures. By sea ice formation and melt, freshwater is concentrated at the surface.
The AW is warmer but saltier than PW and Meteoric
waters comprising continental run-off and net precipitation.
Thereby, it is denser and can be found deeper in the water
column. The significant difference in salinity creates a strong
halocline between the Polar Mixed Layer and the AW layer
establishing a strong permanent stratification in the deep
basins (Fig. 5). The halocline, which is defined by high vertical salinity gradients (32.5 < S < 34.5), is thickest in the
Canada Basin (200–250 m) and thinnest in the Nansen Basin
(100–150 m). The temperatures of the halocline remain close
to the freezing point. Due to heat loss and mixing with shelf
waters there are many modifications of AW at intermediate
depths. Just below the halocline, temperatures are highest in
the Nansen Basin and decrease towards the Canada Basin
(Fig. 5). The densest waters are formed on the shelves of the
Barents Sea, where the AW subsequently releases heat to the
atmosphere and is mixed with brine rejected from newly
formed sea ice before it sinks down the shelf break into the
Nansen Basin (Fig. 4). This Arctic bottom water is the densest water of the world ocean, but can only be found in the
Arctic region (Tomczak and Godfrey 1994). Only a small
part is able to flow over the sill of the Fram Strait into the
Nordic Seas balancing the bottom/deep water formation on
the Arctic shelves (e.g., Bönisch and Schlosser 1995).
The surface circulation in the central Arctic Ocean mainly
comprises two features: the Beaufort Gyre and the Transpolar
Drift (Fig. 4). The Beaufort Gyre is an anticyclonic circulation in the Canada Basin that is forced by a high pressure
system in the lower atmosphere, the so called Beaufort High.
Fresh surface waters from the shelves and from the Pacific
accumulate in the interior of the gyre and leave the Arctic
through the Canadian Arctic Archipelago or the Fram Strait.
The Transpolar Drift is a wind-driven current that directs sea
ice and waters from the Siberian shelves and the Bering
Strait to the Fram Strait, where they exit the Arctic Ocean
into the Nordic Seas and subpolar North Atlantic.
The relatively warm and saline AW, which is the main
water source of the entire Arctic Ocean, enters the Nordic
Seas from the south. The Norwegian Atlantic Current carries
the AW through the Nordic Seas at the surface and splits into
two main branches. One branch enters the Arctic Ocean
through the Barents Sea Opening, the other, which then is
called West Spitsbergen Current, flows through the Fram
Strait. Only a part of the West Spitsbergen Current propagates further north into the Arctic Ocean, the other part recirculates close to the Fram Strait. The AW dives underneath
Fig. 4 Schematic of the
Arctic Ocean circulation
(reproduced from Carmack
et al. 2015, American
Meteorological Society, used
with permission). Blue arrows
indicate the surface
circulation, pink-blue arrows
the main pathways of the
Pacific Water at intermediate
depths and red arrows show
the Atlantic Water circulation.
GIN Seas: Greenland-IcelandScotland Seas, usually called
the “Nordic Seas”
The Physical System of the Arctic Ocean and Subarctic Seas in a Changing Climate