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Chapter 11: The Pacific Ocean
subtropical gyre of the Pacific are structured just as in the North Pacific gyre, and that
a South Subtropical Countercurrent traverses the center of the gyre at 20–25
S. This lies
approximately below the zone of calms between the trades and westerlies, so that we
should anticipate different seasonality to north and south of this line. If we had the same
plenitude of studies here as we have for the North Pacific, I should certainly propose that
we recognize two provinces here as there, one in the Westerlies and one in the Trades
biomes.
Be that as it may, this southern subtropical gyre is, in its entirety, the most uniform
and seasonally stable region of the open oceans and is (as noted by Tomczak and Godfrey)
the origin of the name of the Pacific Ocean. Surface winds are as stable as the permanent
atmospheric high pressure centered approximately above Easter Island (10
S 110
W),
and the trades are weak but remarkably constant year-round so that seasonal maps of
wave height are almost invariant. The dry descending air mass that comprises the eastern
termination of the Walker circulation cell (the wet, ascending air mass over the WARM
province comprises the other pole of this atmospheric cell) maintains an evaporationprecipitation index of 40–80 cm y
−1 and thus warm, salty surface water.
The atmospheric convergence (the South Pacific Convergence Zone) between the
two major wind systems lies NW-SE from New Guinea (15–20
S) to about 30
S as
it approaches the American continent (Barry and Chorley, 1982). This atmospheric
convergence is associated with an oceanwide line of relatively heavy cloud cover and is a
feature in the distribution of wind divergence over the ocean (Lagler and O’Brien, 1980).
It aligns with a thermocline ridge in winter, indicating some divergence of surface water,
and it is aligned above a linear, zonal maximum in sea surface height variability that is
associated with a South Subtropical Countercurrent (SSTC) that passes across the central
gyre here like its counterpart of the northern hemisphere (Qiu and Chen, 2004). This
flow, described almost 40 years ago as a “Tropical Countercurrent,” has been lost sight of
since, and does not appear in any map of general circulation that I have seen. The SSTC
is most unstable in austral winter when cooling at the sea surface increases the vertical
velocity shear between this and the underlying and opposing SEC flow.
The permanent pycnocline is bowl-shaped, lying at about 300 m at 124
W, at the
center of the bowl, and rising to 150 m from 150
W to the western edge of the province
near the date line; consequently, as in the South Atlantic Province, we should note the
general baroclinic upslope of nitrate isopleths toward the edges of the gyre. A thermocline
occurs at 25–40 m over the whole province during austral summer, but to the south of
25
S it deepens to 75–100 m during late austral winter. These meridional differences are
forced by the different characteristics of the two wind systems—trades and westerlies—on
either side of the atmospheric convergence zone; the former are less strong, but very
much more predictable both as to strength and orientation, whereas the latter have high
variability associated with the continual eastward passage of weather systems across the
southern part of the gyre.
Photosynthetically active radiation at the surface in the central part of the gyre ranges
seasonally from 75 W m
−2 in June to 135 W m
−2 in December–January even as SST varies
from 23 to 19
C, the minimum being in September when MLD is deepest. Nitrate-replete
surface water lies to the north (in PEQD) and to the south (in SSTC) of this province,
whereas nitrate-depleted surface water occupies the whole of the center of the gyre, having
an annual mean value of <05 M from the surface to 150 m. The nitracline follows the
pycnocline very closely so that the 03 M isopleth lies just deeper than the depth of
sharpest gradients in the pycnocline. A recent analysis of the depth of the nitrate field
shows that the isopleth for 20 M forms a trough below the intertropical convergence
zone, lying NE-SW across the ocean and deepening toward the east: maximum depth
of this isopleth is reached at about 25
S 120
W (McClain et al., 2004). This feature lies
below the ridge in TOPEX-derived dynamic height noted earlier.
Chapter 11: The Pacific Ocean
subtropical gyre of the Pacific are structured just as in the North Pacific gyre, and that
a South Subtropical Countercurrent traverses the center of the gyre at 20–25
S. This lies
approximately below the zone of calms between the trades and westerlies, so that we
should anticipate different seasonality to north and south of this line. If we had the same
plenitude of studies here as we have for the North Pacific, I should certainly propose that
we recognize two provinces here as there, one in the Westerlies and one in the Trades
biomes.
Be that as it may, this southern subtropical gyre is, in its entirety, the most uniform
and seasonally stable region of the open oceans and is (as noted by Tomczak and Godfrey)
the origin of the name of the Pacific Ocean. Surface winds are as stable as the permanent
atmospheric high pressure centered approximately above Easter Island (10
S 110
W),
and the trades are weak but remarkably constant year-round so that seasonal maps of
wave height are almost invariant. The dry descending air mass that comprises the eastern
termination of the Walker circulation cell (the wet, ascending air mass over the WARM
province comprises the other pole of this atmospheric cell) maintains an evaporationprecipitation index of 40–80 cm y
−1 and thus warm, salty surface water.
The atmospheric convergence (the South Pacific Convergence Zone) between the
two major wind systems lies NW-SE from New Guinea (15–20
S) to about 30
S as
it approaches the American continent (Barry and Chorley, 1982). This atmospheric
convergence is associated with an oceanwide line of relatively heavy cloud cover and is a
feature in the distribution of wind divergence over the ocean (Lagler and O’Brien, 1980).
It aligns with a thermocline ridge in winter, indicating some divergence of surface water,
and it is aligned above a linear, zonal maximum in sea surface height variability that is
associated with a South Subtropical Countercurrent (SSTC) that passes across the central
gyre here like its counterpart of the northern hemisphere (Qiu and Chen, 2004). This
flow, described almost 40 years ago as a “Tropical Countercurrent,” has been lost sight of
since, and does not appear in any map of general circulation that I have seen. The SSTC
is most unstable in austral winter when cooling at the sea surface increases the vertical
velocity shear between this and the underlying and opposing SEC flow.
The permanent pycnocline is bowl-shaped, lying at about 300 m at 124
W, at the
center of the bowl, and rising to 150 m from 150
W to the western edge of the province
near the date line; consequently, as in the South Atlantic Province, we should note the
general baroclinic upslope of nitrate isopleths toward the edges of the gyre. A thermocline
occurs at 25–40 m over the whole province during austral summer, but to the south of
25
S it deepens to 75–100 m during late austral winter. These meridional differences are
forced by the different characteristics of the two wind systems—trades and westerlies—on
either side of the atmospheric convergence zone; the former are less strong, but very
much more predictable both as to strength and orientation, whereas the latter have high
variability associated with the continual eastward passage of weather systems across the
southern part of the gyre.
Photosynthetically active radiation at the surface in the central part of the gyre ranges
seasonally from 75 W m
−2 in June to 135 W m
−2 in December–January even as SST varies
from 23 to 19
C, the minimum being in September when MLD is deepest. Nitrate-replete
surface water lies to the north (in PEQD) and to the south (in SSTC) of this province,
whereas nitrate-depleted surface water occupies the whole of the center of the gyre, having
an annual mean value of <05 M from the surface to 150 m. The nitracline follows the
pycnocline very closely so that the 03 M isopleth lies just deeper than the depth of
sharpest gradients in the pycnocline. A recent analysis of the depth of the nitrate field
shows that the isopleth for 20 M forms a trough below the intertropical convergence
zone, lying NE-SW across the ocean and deepening toward the east: maximum depth
of this isopleth is reached at about 25
S 120
W (McClain et al., 2004). This feature lies
below the ridge in TOPEX-derived dynamic height noted earlier.