Pacific Polar Biome
331
the western Bering Sea ceased almost entirely, and the East Kamchatka Current was
seriously weakened. This entirely unexpected change in the oceanographic regime must
have modified significantly the ecology of the whole Bering Sea basin because subsurface
water masses must have cooled strongly.
Although the Alaska Stream loses its characteristic highly stratified structure in passing
between the Aleutian Islands, dispersion of low-salinity shelf water from the eastern
quadrant results in strong stratification of the central gyre. Thus, the surface water mass of
the Bering Sea, as in the Sea of Okhotsk, has low salinity and lies above a stable halocline
at 100–300 m (Dodimead et al., 1967; Takenouti and Ohtani, 1974). In both seas, there
is a significant input of freshwater over the inner continental shelf both from ice melt
(see later discussion) and from coastal runoff and rainfall. Over the shelf regions of the
eastern Bering Sea, during summer stratification, the subthermocline water is subject to
tidal mixing—strong or weak according to the velocity of tidal streams. The water above
the thermocline remains wind-mixed.
Water from the western boundary current of the NW Pacific enters the Okhotsk
Sea between islands in the Kuril chain and is circulated cyclonically around the basin.
Additionally, the Okhotsk Sea receives water from the Japan Sea in the extreme south
from the Tsushima Current. Over the Bering Sea shelf, flow is stronger inshore and also
toward the shelf break than in the more slowly moving water of the midshelf domain.
The general circulation of both seas is highly eddying and involves the formation of
many quasipermanent major eddies. The surface circulation of each sea is dominated by
a central cyclonic gyre situated over the deeper parts of the basin, the gyres of the two seas
being connected by southward flow of the East Kamchatka Current along the east coast
of the peninsula (Dodimead et al., 1967). This subsequently passes around the central
gyre of the Okhotsk Sea to augment the southward flow of the cold Oyashio. In both
seas, cold water leaves to the southwest of the basin along the Asiatic coastline.
The Alaskan shelf receives significant inputs of freshwater from the Yukon and other
rivers, whereas the Okhotsk Sea receives water only from the Anadyr, and that only into
the effluent current from the basin, as this passes behind the island of Sakhalin.
These seas are subject to polar atmospheric conditions, with some influence of the
Asiatic monsoon in the southwestern Okhotsk Sea in summer. Even though they lie at
about the same latitude as the seas off Western Europe, they are partially ice-covered
in winter and each, therefore, supports a seasonally migrating marginal ice zone (MIZ)
with all that that implies for their ecology. Ice formation begins off the river mouths
of the Bering Sea and, by the end of winter during cold years, ice extends over 75% of
the entire sea. Ice coverage of the Sea of Okhotsk is relatively less complete and broken
pack ice is more typical. During the formation of ice, the near-surface salinity increases,
and this induces local density-driven flows that are sufficiently strong as to modify mean
currents.
Ice cover duration is highly variable between years in the Bering Sea, varying from
2 to 28 weeks (mean 20 weeks) in the midshelf domain and the date when retreat is
initiated in the south varies from mid-March to June (Schumacher and Stabeno, 1998);
total coverage varies by as much as 40% about the mean. Ice coverage of the Okhotsk Sea
also varies strongly between years, from complete to the line of the Kuril Islands to <25%
coverage. These between-year variations appear to be forced by changes in storm tracks
across the region that themselves represent the regional atmospheric response to changes
in the Pacific Decadal Oscillation (PDO), positively correlated with the relative strength
of the atmospheric Aleutian Low. The PDO was consistently negative from the early
1940s until 1976, when a change occurred in the Bering Sea that initiated a 12-year period
of positive PDO values (e.g., Schumacher et al., 2003). During this period, ice coverage
was significantly reduced, and mixed-layer temperatures on the inner shelf were higher
than previously. In 1989, the PDO returned to negative values and regional ice cover
331
the western Bering Sea ceased almost entirely, and the East Kamchatka Current was
seriously weakened. This entirely unexpected change in the oceanographic regime must
have modified significantly the ecology of the whole Bering Sea basin because subsurface
water masses must have cooled strongly.
Although the Alaska Stream loses its characteristic highly stratified structure in passing
between the Aleutian Islands, dispersion of low-salinity shelf water from the eastern
quadrant results in strong stratification of the central gyre. Thus, the surface water mass of
the Bering Sea, as in the Sea of Okhotsk, has low salinity and lies above a stable halocline
at 100–300 m (Dodimead et al., 1967; Takenouti and Ohtani, 1974). In both seas, there
is a significant input of freshwater over the inner continental shelf both from ice melt
(see later discussion) and from coastal runoff and rainfall. Over the shelf regions of the
eastern Bering Sea, during summer stratification, the subthermocline water is subject to
tidal mixing—strong or weak according to the velocity of tidal streams. The water above
the thermocline remains wind-mixed.
Water from the western boundary current of the NW Pacific enters the Okhotsk
Sea between islands in the Kuril chain and is circulated cyclonically around the basin.
Additionally, the Okhotsk Sea receives water from the Japan Sea in the extreme south
from the Tsushima Current. Over the Bering Sea shelf, flow is stronger inshore and also
toward the shelf break than in the more slowly moving water of the midshelf domain.
The general circulation of both seas is highly eddying and involves the formation of
many quasipermanent major eddies. The surface circulation of each sea is dominated by
a central cyclonic gyre situated over the deeper parts of the basin, the gyres of the two seas
being connected by southward flow of the East Kamchatka Current along the east coast
of the peninsula (Dodimead et al., 1967). This subsequently passes around the central
gyre of the Okhotsk Sea to augment the southward flow of the cold Oyashio. In both
seas, cold water leaves to the southwest of the basin along the Asiatic coastline.
The Alaskan shelf receives significant inputs of freshwater from the Yukon and other
rivers, whereas the Okhotsk Sea receives water only from the Anadyr, and that only into
the effluent current from the basin, as this passes behind the island of Sakhalin.
These seas are subject to polar atmospheric conditions, with some influence of the
Asiatic monsoon in the southwestern Okhotsk Sea in summer. Even though they lie at
about the same latitude as the seas off Western Europe, they are partially ice-covered
in winter and each, therefore, supports a seasonally migrating marginal ice zone (MIZ)
with all that that implies for their ecology. Ice formation begins off the river mouths
of the Bering Sea and, by the end of winter during cold years, ice extends over 75% of
the entire sea. Ice coverage of the Sea of Okhotsk is relatively less complete and broken
pack ice is more typical. During the formation of ice, the near-surface salinity increases,
and this induces local density-driven flows that are sufficiently strong as to modify mean
currents.
Ice cover duration is highly variable between years in the Bering Sea, varying from
2 to 28 weeks (mean 20 weeks) in the midshelf domain and the date when retreat is
initiated in the south varies from mid-March to June (Schumacher and Stabeno, 1998);
total coverage varies by as much as 40% about the mean. Ice coverage of the Okhotsk Sea
also varies strongly between years, from complete to the line of the Kuril Islands to <25%
coverage. These between-year variations appear to be forced by changes in storm tracks
across the region that themselves represent the regional atmospheric response to changes
in the Pacific Decadal Oscillation (PDO), positively correlated with the relative strength
of the atmospheric Aleutian Low. The PDO was consistently negative from the early
1940s until 1976, when a change occurred in the Bering Sea that initiated a 12-year period
of positive PDO values (e.g., Schumacher et al., 2003). During this period, ice coverage
was significantly reduced, and mixed-layer temperatures on the inner shelf were higher
than previously. In 1989, the PDO returned to negative values and regional ice cover