338
Chapter 11: The Pacific Ocean
about 2 months so that the maximum (13–14
C at OWS P) is reached in August and a
minimum (5–6
C) in March. Within the isothermal surface mixed layer, a thermocline
is established at 30–60 m in early summer, after which time the transient stratification,
which occurs in the surface layer with increasing frequency during the summer, mixes
down within a few days to join the main seasonal thermocline. The summer thermocline
remains at about the same depth until it progressively deepens and is eroded by increasing
wind stress (and decreasing heat input) during fall and early winter. Between this feature
and the permanent halocline, salinity increases progressively during summer, because of
exchange across the top of the halocline. Miller et al. (1991) suggest that Ekman suction
of 1.5–3.0 m month
−1 is also the mechanism for a continual supply of new nutrients
across the nutricline.
The western gyre is smaller, triangular in outline (look at the triangular space
between the Commander and Kuril Islands), and it, too, has a central halocline dome
although this is deeper than in the Alaskan gyre, reaching 200–300 m at 180
W. The
western, equatorward limb of the western gyre forms a coastal jet, the Kuril Current,
that heads southwest and is topographically locked to the topography of the Kuril
islands.
The southern edge of the PSAG province (along 45
N for practical purposes) is
intended to represent the division between meandering flow in the Subarctic Current
and in the warmer, saltier water of the West Wind Drift or Transition Zone (see NPTG
Province). Dodimead et al. suggest that the southern edge of subarctic conditions is
indicated by the 7
C isoline at the depth of the halocline or maximal winter mixing.
North of this line, temperature profiles in the subarctic water are consistently dicothermal,
having a thermal minimum at about 100 m depth.
Because of the early interest in this region, and the serial observations at OWS P
from the mid-1950s to 1981, we have almost unparalleled information on decadal and
longer-term trends in the physical environment. As Whitney and Freeland (1999) point
out, near-surface warming and salinity progressively declined during this period, and this
will tend to decrease the density contrast in the upper part of the water column. Hence,
we shall expect (for equivalent mixing) a reduction in annual nutrient flux, as indeed
has been observed, and also a higher mean irradiance in spring within the now-shallower
surface mixed layer.
Though nutrients will be discussed again later, it will be useful here to note that the
southern boundary of PSAG is marked by a strong surface gradient in mixed-layer nitrate
during winter. A contour of 04 M passes across the whole North Pacific basin, just
beyond the southern boundary of the PSAG province, which is almost entirely occupied
by values in the range 5–20 M. Maximum mixed-layer winter nitrates (15–20 M) occur
south of the western Aleutians, and to the south of Alaska (Anderson et al., 1969; Glover
et al., 1994).
Regional Response of the Pelagic Ecosystem
In the Pacific Subarctic, we get some confirmation of the ecological reality of the borders
of a province from biogeographic data. Between the Polar Frontal Zone and the northern
coasts, some subarctic biota have distributions that very closely match the boundaries
of PSAG (McGowan, 1971). The most important species included in this assemblage
are some copepod species that will be discussed later (N. plumchrus, N. cristatus, and
E. bungii) together with the chaetognath Sagitta elegans, the euphausiids Thysanoessa
longipes and Euphausia pacifica, and the mollusks Lima helicina, Clio polita, and Clione
limacina. The distribution envelopes of these species match the borders of PSAG very
well, with the important exception that some species also occur in the adjacent coastal
Chapter 11: The Pacific Ocean
about 2 months so that the maximum (13–14
C at OWS P) is reached in August and a
minimum (5–6
C) in March. Within the isothermal surface mixed layer, a thermocline
is established at 30–60 m in early summer, after which time the transient stratification,
which occurs in the surface layer with increasing frequency during the summer, mixes
down within a few days to join the main seasonal thermocline. The summer thermocline
remains at about the same depth until it progressively deepens and is eroded by increasing
wind stress (and decreasing heat input) during fall and early winter. Between this feature
and the permanent halocline, salinity increases progressively during summer, because of
exchange across the top of the halocline. Miller et al. (1991) suggest that Ekman suction
of 1.5–3.0 m month
−1 is also the mechanism for a continual supply of new nutrients
across the nutricline.
The western gyre is smaller, triangular in outline (look at the triangular space
between the Commander and Kuril Islands), and it, too, has a central halocline dome
although this is deeper than in the Alaskan gyre, reaching 200–300 m at 180
W. The
western, equatorward limb of the western gyre forms a coastal jet, the Kuril Current,
that heads southwest and is topographically locked to the topography of the Kuril
islands.
The southern edge of the PSAG province (along 45
N for practical purposes) is
intended to represent the division between meandering flow in the Subarctic Current
and in the warmer, saltier water of the West Wind Drift or Transition Zone (see NPTG
Province). Dodimead et al. suggest that the southern edge of subarctic conditions is
indicated by the 7
C isoline at the depth of the halocline or maximal winter mixing.
North of this line, temperature profiles in the subarctic water are consistently dicothermal,
having a thermal minimum at about 100 m depth.
Because of the early interest in this region, and the serial observations at OWS P
from the mid-1950s to 1981, we have almost unparalleled information on decadal and
longer-term trends in the physical environment. As Whitney and Freeland (1999) point
out, near-surface warming and salinity progressively declined during this period, and this
will tend to decrease the density contrast in the upper part of the water column. Hence,
we shall expect (for equivalent mixing) a reduction in annual nutrient flux, as indeed
has been observed, and also a higher mean irradiance in spring within the now-shallower
surface mixed layer.
Though nutrients will be discussed again later, it will be useful here to note that the
southern boundary of PSAG is marked by a strong surface gradient in mixed-layer nitrate
during winter. A contour of 04 M passes across the whole North Pacific basin, just
beyond the southern boundary of the PSAG province, which is almost entirely occupied
by values in the range 5–20 M. Maximum mixed-layer winter nitrates (15–20 M) occur
south of the western Aleutians, and to the south of Alaska (Anderson et al., 1969; Glover
et al., 1994).
Regional Response of the Pelagic Ecosystem
In the Pacific Subarctic, we get some confirmation of the ecological reality of the borders
of a province from biogeographic data. Between the Polar Frontal Zone and the northern
coasts, some subarctic biota have distributions that very closely match the boundaries
of PSAG (McGowan, 1971). The most important species included in this assemblage
are some copepod species that will be discussed later (N. plumchrus, N. cristatus, and
E. bungii) together with the chaetognath Sagitta elegans, the euphausiids Thysanoessa
longipes and Euphausia pacifica, and the mollusks Lima helicina, Clio polita, and Clione
limacina. The distribution envelopes of these species match the borders of PSAG very
well, with the important exception that some species also occur in the adjacent coastal