Regional and Latitudinal Resistance to Mixing in the Open Oceans
61
halocline does lie shallower than the thermocline, but only by about 10 m (Sprintall and
Tomczak, 1992), so that both contribute to the high stability associated with the density
gradient. I shall return to the ecological consequences of the barrier layer later.
The discontinuities discussed in this section are perhaps sufficiently solid to suggest
that the warmer parts of the ocean may not be treated as an ecological continuum, but
what of the cooler regions where wind stress in winter may deeply mix the water column?
I propose to examine just one example of those frontal zones, discussed in the previous
chapter, where sufficient discontinuity for our purpose can be demonstrated—in this case,
at the boundary of the polar regions. These, defined by the Polar Fronts, are relatively
small and comprise only about 6% of the whole ocean. Because it is the critical defining
feature of the polar seas, we should first ask if there are definable limits to the imposition
of resistance to mixing by the polar brackish surface layer. If no discontinuity can be
located, the effect may not be useful for our present purpose and it may be difficult to
define a boundary for polar seas. We will first look for evidence of this in some relevant
oceanographic sections.
A hydrographic section worked by Hudson into the Norwegian-Greenland Sea from
the regime of Atlantic water into that of Polar water serves to illustrate the winter situation
in the open waters of these polar regions. This section crossed the Iceland Gap Front and
the Oceanic Polar Front (Johannessen, 1986) encountering (i) arctic water, north of the
polar front, which is a cold (<2
C), brackish (34.6) surface water mass with a weak N max
(1 or 2 cycles hr
−1 ) at about 75 m; (ii) subarctic water, between the two fronts, which
is isothermal (∼3
C) and isohaline (∼ 35), with a weak pycnocline at 200 m with N max
of 0.5–2.0 cycles hr
−1 ; and (iii) Atlantic water south of the Iceland Front that is warmer
(7
C), saltier (35.2), and lies above a deep pycnocline having its upper inflection at about
250–275 m and an N max of <20 cycles hr
−1 . Here, in winter, the deep mixing typical of
the northern part of the North Atlantic subtropical gyre extends into the open, stormy
waters of the polar seas. The Western Basins Section (25
W) shows the transition to the
surface low-salinity layer in the Denmark Strait.
In the Pacific, the Washington (150
W) and Hakuho Maru (170
W) sections clearly
show the Polar Frontal Zone (32–45
N, depending on longitude) to be the southern
boundary of the surface low-salinity water of the subarctic gyre. In the Southern Ocean,
the near-surface salinity gradient across the South Subtropical Convergence Zone is
especially well seen in the Knorr section along the Greenwich meridian and the Oceanus–
Melville 25
W section; south of this feature, shallow low-salinity layers are frequently
encountered in the sections. A discontinuity occurs in the southern hemisphere within
the South Subtropical Convergence Zone and in the northern hemisphere at the polar
fronts of each ocean. Here, an abrupt increase in the values of N max and a shoaling of the
pycnocline occur at the boundaries of the polar biome.
However, specification of the geographical limits of the polar seas is complicated by
seasonal changes. Even beyond the polar fronts, in winters when sea ice cover is not
established, the surface mixed layer may be overturned by thermal convection and by
wind mixing. It is only in spring that a shoal pycnocline becomes reestablished below the
polar layer of low-salinity water, and it is only during that season that we may expect to
see a distinction between the strong polar and the weak gyral pycnoclines.
It might be protested that a surface brackish layer, constraining vertical mixing, is not
a unique characteristic of high-latitude seas. After all, as has been noted previously, the
western Pacific warm pool also has a near-surface halocline in a deeper isothermal mixed
layer over an area equivalent to 81% of the polar oceans as defined previously. However,
this is really a special case because the nutricline occurs as much as 100 m deeper than
the halocline. The surface brackish layer is thus ineffective in controlling the exchange of
water across the nutricline.
61
halocline does lie shallower than the thermocline, but only by about 10 m (Sprintall and
Tomczak, 1992), so that both contribute to the high stability associated with the density
gradient. I shall return to the ecological consequences of the barrier layer later.
The discontinuities discussed in this section are perhaps sufficiently solid to suggest
that the warmer parts of the ocean may not be treated as an ecological continuum, but
what of the cooler regions where wind stress in winter may deeply mix the water column?
I propose to examine just one example of those frontal zones, discussed in the previous
chapter, where sufficient discontinuity for our purpose can be demonstrated—in this case,
at the boundary of the polar regions. These, defined by the Polar Fronts, are relatively
small and comprise only about 6% of the whole ocean. Because it is the critical defining
feature of the polar seas, we should first ask if there are definable limits to the imposition
of resistance to mixing by the polar brackish surface layer. If no discontinuity can be
located, the effect may not be useful for our present purpose and it may be difficult to
define a boundary for polar seas. We will first look for evidence of this in some relevant
oceanographic sections.
A hydrographic section worked by Hudson into the Norwegian-Greenland Sea from
the regime of Atlantic water into that of Polar water serves to illustrate the winter situation
in the open waters of these polar regions. This section crossed the Iceland Gap Front and
the Oceanic Polar Front (Johannessen, 1986) encountering (i) arctic water, north of the
polar front, which is a cold (<2
C), brackish (34.6) surface water mass with a weak N max
(1 or 2 cycles hr
−1 ) at about 75 m; (ii) subarctic water, between the two fronts, which
is isothermal (∼3
C) and isohaline (∼ 35), with a weak pycnocline at 200 m with N max
of 0.5–2.0 cycles hr
−1 ; and (iii) Atlantic water south of the Iceland Front that is warmer
(7
C), saltier (35.2), and lies above a deep pycnocline having its upper inflection at about
250–275 m and an N max of <20 cycles hr
−1 . Here, in winter, the deep mixing typical of
the northern part of the North Atlantic subtropical gyre extends into the open, stormy
waters of the polar seas. The Western Basins Section (25
W) shows the transition to the
surface low-salinity layer in the Denmark Strait.
In the Pacific, the Washington (150
W) and Hakuho Maru (170
W) sections clearly
show the Polar Frontal Zone (32–45
N, depending on longitude) to be the southern
boundary of the surface low-salinity water of the subarctic gyre. In the Southern Ocean,
the near-surface salinity gradient across the South Subtropical Convergence Zone is
especially well seen in the Knorr section along the Greenwich meridian and the Oceanus–
Melville 25
W section; south of this feature, shallow low-salinity layers are frequently
encountered in the sections. A discontinuity occurs in the southern hemisphere within
the South Subtropical Convergence Zone and in the northern hemisphere at the polar
fronts of each ocean. Here, an abrupt increase in the values of N max and a shoaling of the
pycnocline occur at the boundaries of the polar biome.
However, specification of the geographical limits of the polar seas is complicated by
seasonal changes. Even beyond the polar fronts, in winters when sea ice cover is not
established, the surface mixed layer may be overturned by thermal convection and by
wind mixing. It is only in spring that a shoal pycnocline becomes reestablished below the
polar layer of low-salinity water, and it is only during that season that we may expect to
see a distinction between the strong polar and the weak gyral pycnoclines.
It might be protested that a surface brackish layer, constraining vertical mixing, is not
a unique characteristic of high-latitude seas. After all, as has been noted previously, the
western Pacific warm pool also has a near-surface halocline in a deeper isothermal mixed
layer over an area equivalent to 81% of the polar oceans as defined previously. However,
this is really a special case because the nutricline occurs as much as 100 m deeper than
the halocline. The surface brackish layer is thus ineffective in controlling the exchange of
water across the nutricline.