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Chapter 9: The Atlantic Ocean
less well defined by surface temperature, because the surface water of the West Greenland
Current has by this time been significantly modified by Atlantic water entrained across
the Polar Front. Icebergs are glacier-fed into the Greenland coastal currents and many
survive the passage around Baffin Bay to reach Newfoundland.
The permanent ice cover of the Arctic Ocean opens significantly in summer only in
the coastal seas; in the central Arctic Ocean, even in summer, ice is from 1.0 to 3.5 m
thick, covering from 80% to 100% of the sea surface, while a snow-pack above the sea ice
ranges from 5 to 30 cm thick. In some places, open-water polynyas occur during seasons
when one would otherwise expect complete ice cover. During the summer, these polynyas
become the centers of the seasonally open water regions surrounded by complete ice
cover. They occur at locations now predictable from topography and currents and are
usually accounted for by one of two models. The simplest case is that of the “latent-heat
polynyas” that exist because of the balance between rates of ice formation and loss; in
these polynyas the latent heat occasioned by ice formation is removed by wind, often
offshore in these cases. If wind flux becomes insufficient to support this process, the
polynya will freeze over. “Sensible-heat polynyas” result from heat flux caused by vertical
motion of warmer, deep water either by wind mixing or by wind-induced upwelling;
in these, there is less modification of the density regime of the water column than in
latent-heat polynyas.
The North Water, between Greenland and Ellesmere Island, is more complex than
either simple model (Ingram et al., 2002). Here, not only is there sensible heat flux up
into surface layers from the West Greenland Coastal Current, but the southerly transport
of pack ice is interrupted by an ice bridge that forms across the narrows of Nares Strait.
Ice that is wind-drifted southward from the northern part of Baffin Bay is therefore not
replaced. However, despite the interest that it has attracted in polynya studies, the North
Water conforms to the definition of a polynya for only a few months in spring.
Around the margins of the polar ocean, the summer retreat of the ice forms an
ecologically significant MIZ, associated with an ice-edge front in the subsurface density
field where the warmer, less saline open water meets colder, more saline water along
and below the ice edge (Fig. 9.1). This frontal region may either be simple, or more
Fig. 9.1 Diagrammatic representation of a simple marginal ice zone (MIZ) of polar seas, illustrating the
nature of the ice-edge front.
Source: Redrawn from Walton Smith, 1987.
Chapter 9: The Atlantic Ocean
less well defined by surface temperature, because the surface water of the West Greenland
Current has by this time been significantly modified by Atlantic water entrained across
the Polar Front. Icebergs are glacier-fed into the Greenland coastal currents and many
survive the passage around Baffin Bay to reach Newfoundland.
The permanent ice cover of the Arctic Ocean opens significantly in summer only in
the coastal seas; in the central Arctic Ocean, even in summer, ice is from 1.0 to 3.5 m
thick, covering from 80% to 100% of the sea surface, while a snow-pack above the sea ice
ranges from 5 to 30 cm thick. In some places, open-water polynyas occur during seasons
when one would otherwise expect complete ice cover. During the summer, these polynyas
become the centers of the seasonally open water regions surrounded by complete ice
cover. They occur at locations now predictable from topography and currents and are
usually accounted for by one of two models. The simplest case is that of the “latent-heat
polynyas” that exist because of the balance between rates of ice formation and loss; in
these polynyas the latent heat occasioned by ice formation is removed by wind, often
offshore in these cases. If wind flux becomes insufficient to support this process, the
polynya will freeze over. “Sensible-heat polynyas” result from heat flux caused by vertical
motion of warmer, deep water either by wind mixing or by wind-induced upwelling;
in these, there is less modification of the density regime of the water column than in
latent-heat polynyas.
The North Water, between Greenland and Ellesmere Island, is more complex than
either simple model (Ingram et al., 2002). Here, not only is there sensible heat flux up
into surface layers from the West Greenland Coastal Current, but the southerly transport
of pack ice is interrupted by an ice bridge that forms across the narrows of Nares Strait.
Ice that is wind-drifted southward from the northern part of Baffin Bay is therefore not
replaced. However, despite the interest that it has attracted in polynya studies, the North
Water conforms to the definition of a polynya for only a few months in spring.
Around the margins of the polar ocean, the summer retreat of the ice forms an
ecologically significant MIZ, associated with an ice-edge front in the subsurface density
field where the warmer, less saline open water meets colder, more saline water along
and below the ice edge (Fig. 9.1). This frontal region may either be simple, or more
Fig. 9.1 Diagrammatic representation of a simple marginal ice zone (MIZ) of polar seas, illustrating the
nature of the ice-edge front.
Source: Redrawn from Walton Smith, 1987.