144
Chapter 9: The Atlantic Ocean
Fram Strait (see BPLR) and Oceanic Polar Front (OPF), in the classical sense, for the
front that lies between Iceland and Bear Island.
Though the definition of the OPF is simple, its location at the sea surface is complex.
It is the conjunction between warm, salty, subtropical gyral water and cold, less saline
subpolar water and is thus the extension of the North Wall of the Gulf Stream. This
feature retains its coherence, meandering eastward as far as the Mid-Atlantic Ridge at
about 30
W (Dietrich, 1964). Thence, it passes north around Iceland, then meanders
strongly again as it continues northeast along the Faeroe-Iceland Ridge. It then turns
more northerly along the Jan Mayen section of the Mid-Atlantic Ridge (Johannessen,
1986; van Aken et al., 1991; Wassman et al., 1991). Along the cold side of the OPF there
is strong flow of relatively cold, low-salinity water from the west, recirculating within the
Subpolar Gyre of the Labrador-Irminger Basin (Krause, 1986).
Ice formed in the polar water of Baffin Bay and in the Fram Strait extends patchily and
irregularly into the Arctic Province during some winters, and broken pack ice is frequent
in the regions bordering on BPLR; complete ice cover occurs in the Odden region between
northeast Greenland and Iceland, and in the northern Barents Sea west of Spitzbergen.
The inner part of the Labrador Current, shoreward of the break of shelf, remains largely
ice-covered well into the summer period while the more offshore segment, comprising
(see later discussion) the western boundary current of the cyclonic gyre in the Labrador
Sea, is relatively ice-free. Some pack ice is drifted southward from the Fram Strait region
in the flow along eastern Greenland.
North of Iceland, winter mixed-layer depths are deepest in the central Norwegian Sea
(around 500 m), whereas a thermocline dome (<100 m) appears to be associated with the
northward-turning flow of Arctic water east of Iceland (Levitus, 1982). Not only is winter
mixing deepest at 50–60
N but also, because the subsurface nitrate field slopes with the
baroclinicity of the subtropical gyre, initial mixed-layer nitrate concentration in spring
are progressively higher toward the northern edge of the gyre (Glover and Brewer, 1988;
Yentsch, 1990).
The surface waters of the Greenland and Labrador Seas have a relatively lower stability
than adjacent areas so that cooling and wind stress at the surface cause unusually deep
regional winter mixing. In fact, the Subpolar Gyres are two of only three global centers of
deep convection of surface water into the global thermohaline circulation, the Weddell
Sea off Antarctica being the third. Consequently, at the end of winter a very deep trough in
the mixed-layer topography extends in an arc around the northern flanks of the subpolar
gyres, from the southern Labrador Sea to the Norwegian. It occupies a somewhat different
area according to the data and criteria used to display it (e.g., Levitus, 1982; Robinson
et al., 1979; Glover and Brewer, 1988; Woods, 1984). This process is associated with high
winter nitrate values in the mixed layer, reaching 16 M.
The actual depth of deep winter convection is very variable. Exceptionally intense
convection down to 2300 m occurred during 4 years in the early 1990s and created a
new deep pool of Labrador Sea Water but, in response to warmer winter weather in the
later years of the decade, convection reached to only half that depth: stratification was
therefore reestablished at between 150 and 1000 m (Lazier et al., 2002).
Response of the Pelagic Ecosystems
The chlorophyll images available since 1997 suffice to build a climatology of the seasonal
cycle of chlorophyll accumulation of this province, and it is now clear that our earlier
assumptions about the seasonality of the Arctic spring bloom were not satisfactory. It
has long been evident that the progression of the bloom does not represent a simple
poleward, irradiance-forced progression, although the pattern has not been easy to read.
Now, both the individual years and the climatology make it clear how blooms really
Chapter 9: The Atlantic Ocean
Fram Strait (see BPLR) and Oceanic Polar Front (OPF), in the classical sense, for the
front that lies between Iceland and Bear Island.
Though the definition of the OPF is simple, its location at the sea surface is complex.
It is the conjunction between warm, salty, subtropical gyral water and cold, less saline
subpolar water and is thus the extension of the North Wall of the Gulf Stream. This
feature retains its coherence, meandering eastward as far as the Mid-Atlantic Ridge at
about 30
W (Dietrich, 1964). Thence, it passes north around Iceland, then meanders
strongly again as it continues northeast along the Faeroe-Iceland Ridge. It then turns
more northerly along the Jan Mayen section of the Mid-Atlantic Ridge (Johannessen,
1986; van Aken et al., 1991; Wassman et al., 1991). Along the cold side of the OPF there
is strong flow of relatively cold, low-salinity water from the west, recirculating within the
Subpolar Gyre of the Labrador-Irminger Basin (Krause, 1986).
Ice formed in the polar water of Baffin Bay and in the Fram Strait extends patchily and
irregularly into the Arctic Province during some winters, and broken pack ice is frequent
in the regions bordering on BPLR; complete ice cover occurs in the Odden region between
northeast Greenland and Iceland, and in the northern Barents Sea west of Spitzbergen.
The inner part of the Labrador Current, shoreward of the break of shelf, remains largely
ice-covered well into the summer period while the more offshore segment, comprising
(see later discussion) the western boundary current of the cyclonic gyre in the Labrador
Sea, is relatively ice-free. Some pack ice is drifted southward from the Fram Strait region
in the flow along eastern Greenland.
North of Iceland, winter mixed-layer depths are deepest in the central Norwegian Sea
(around 500 m), whereas a thermocline dome (<100 m) appears to be associated with the
northward-turning flow of Arctic water east of Iceland (Levitus, 1982). Not only is winter
mixing deepest at 50–60
N but also, because the subsurface nitrate field slopes with the
baroclinicity of the subtropical gyre, initial mixed-layer nitrate concentration in spring
are progressively higher toward the northern edge of the gyre (Glover and Brewer, 1988;
Yentsch, 1990).
The surface waters of the Greenland and Labrador Seas have a relatively lower stability
than adjacent areas so that cooling and wind stress at the surface cause unusually deep
regional winter mixing. In fact, the Subpolar Gyres are two of only three global centers of
deep convection of surface water into the global thermohaline circulation, the Weddell
Sea off Antarctica being the third. Consequently, at the end of winter a very deep trough in
the mixed-layer topography extends in an arc around the northern flanks of the subpolar
gyres, from the southern Labrador Sea to the Norwegian. It occupies a somewhat different
area according to the data and criteria used to display it (e.g., Levitus, 1982; Robinson
et al., 1979; Glover and Brewer, 1988; Woods, 1984). This process is associated with high
winter nitrate values in the mixed layer, reaching 16 M.
The actual depth of deep winter convection is very variable. Exceptionally intense
convection down to 2300 m occurred during 4 years in the early 1990s and created a
new deep pool of Labrador Sea Water but, in response to warmer winter weather in the
later years of the decade, convection reached to only half that depth: stratification was
therefore reestablished at between 150 and 1000 m (Lazier et al., 2002).
Response of the Pelagic Ecosystems
The chlorophyll images available since 1997 suffice to build a climatology of the seasonal
cycle of chlorophyll accumulation of this province, and it is now clear that our earlier
assumptions about the seasonality of the Arctic spring bloom were not satisfactory. It
has long been evident that the progression of the bloom does not represent a simple
poleward, irradiance-forced progression, although the pattern has not been easy to read.
Now, both the individual years and the climatology make it clear how blooms really