358
Chapter 11: The Pacific Ocean
This creates regional inequalities in the scaling, or horizontal dimensions, of mesoscale
eddies and baroclinic Rossby waves.
The Subtropical Convergence along the northern edge of this province lies below the
seasonally migrating conjunction of the westerly winds and the trades. This province,
then, lies below the subtropical easterlies but nevertheless some winter mixing, forced
by the frequent extratropical cyclones passing eastward (see NPST Province), extends
as far south as the Hawaiian Islands at 20–25
N. As has been recently confirmed, an
eddy-energetic Subtropical Counter Current induces zonal flow westward in this region
at 18–25
N and is more consistent in boreal spring than at other seasons (Qiu, 1999).
At 22
N, the mixed layer deepens from 35–40 m in summer to 80–90 m in winter. But
both climatology and ALOHA data (e.g., Karl and Lukas, 1996) suggest that such winter
deepening may be a relatively brief affair, in some years involving no more than a single
month, such as January 1993, that appears as an outlier on a more moderate seasonal
cycle. Coincidentally, it is for this part of the province that we have the most abundant
information from the HOT and ALOHA time-series stations. Elsewhere, and especially
further to the southwest in the province, we should probably not encounter a significant
winter excursion of mixed-layer depth. Of course, the permanent pycnocline lies still
deeper and a permanent nitracline occurs across its density gradient. These features are
continuous and at a nearly uniform depth, close to the 1% isolume, from the eastern
edge of the province to at least the longitude of Hawaii. They are deepest in the northern
part of the province, shoalest in the south, and slope up toward the American coast.
Both circulation and some characteristics of the water column in this province are
somewhat variable, in response to the state of the ENSO index, although these effects
are more important in the equatorial and warm pool provinces to the south. SST is
significantly correlated with the SO index, and the westerly wind bursts that introduce
an ENSO event modify mixed-layer depth and strength of the westward flow of the
NEC. This lies between the Subtropical Convergence and the northern Doldrum Front of
Roden (1975) at 10
N, where salinity decreases abruptly southward into equatorial water.
Beyond 180
W, the southern part of this limb of the gyral circulation passes progressively
under the influence of the heavy precipitation which characterizes the western Pacific
“warm pool”; where these conditions are fully developed, to the south of 10
N and
west of 160
E, it is proper to recognize a unique province (see WARM), some of whose
characteristics will nevertheless be relevant to the western part of NPTG.
Flow perturbation past the Hawaiian island chain causes the formation of a von
Karman vortex street, by the generation of alternate cyclonic and anticylonic eddies
similar to those downstream of the Canaries (see NAST). However, in the case of the
Hawaiian Islands, it is likely that eddies are also formed by the effect of curl of the wind
stress, which induces upwelling and downwelling of surface water at the wind-shear lines
by strong Ekman pumping. Off Hawaii it is suggested that this process induces sufficient
vertical nutrient flux to induce features in the chlorophyll field (see the review of island
eddy wakes by Aristegui et al., 1997).
Regional Response of the Pelagic Ecosystem
We are fortunate that for this province we have two sets of excellent time-series studies,
together with some trans-Pacific meridional and zonal sections. Many expeditions, mostly
from Scripps Institute of Oceanography in California, investigated the CLIMAX area
(28–30
N) from 1968 to 1987 (Hayward, 1987). The recent HOT (Hawaii Ocean Time
Series) investigations were established at the ALOHA station, at 22
N 158
W, in 1988 with
the intention of obtaining a 20-year time series (Karl and Lukas, 1996). The following
account relies very heavily on these two projects, which are among the very best data sets
for the analysis of long-term changes in the structure of a pelagic ecosystem.
Chapter 11: The Pacific Ocean
This creates regional inequalities in the scaling, or horizontal dimensions, of mesoscale
eddies and baroclinic Rossby waves.
The Subtropical Convergence along the northern edge of this province lies below the
seasonally migrating conjunction of the westerly winds and the trades. This province,
then, lies below the subtropical easterlies but nevertheless some winter mixing, forced
by the frequent extratropical cyclones passing eastward (see NPST Province), extends
as far south as the Hawaiian Islands at 20–25
N. As has been recently confirmed, an
eddy-energetic Subtropical Counter Current induces zonal flow westward in this region
at 18–25
N and is more consistent in boreal spring than at other seasons (Qiu, 1999).
At 22
N, the mixed layer deepens from 35–40 m in summer to 80–90 m in winter. But
both climatology and ALOHA data (e.g., Karl and Lukas, 1996) suggest that such winter
deepening may be a relatively brief affair, in some years involving no more than a single
month, such as January 1993, that appears as an outlier on a more moderate seasonal
cycle. Coincidentally, it is for this part of the province that we have the most abundant
information from the HOT and ALOHA time-series stations. Elsewhere, and especially
further to the southwest in the province, we should probably not encounter a significant
winter excursion of mixed-layer depth. Of course, the permanent pycnocline lies still
deeper and a permanent nitracline occurs across its density gradient. These features are
continuous and at a nearly uniform depth, close to the 1% isolume, from the eastern
edge of the province to at least the longitude of Hawaii. They are deepest in the northern
part of the province, shoalest in the south, and slope up toward the American coast.
Both circulation and some characteristics of the water column in this province are
somewhat variable, in response to the state of the ENSO index, although these effects
are more important in the equatorial and warm pool provinces to the south. SST is
significantly correlated with the SO index, and the westerly wind bursts that introduce
an ENSO event modify mixed-layer depth and strength of the westward flow of the
NEC. This lies between the Subtropical Convergence and the northern Doldrum Front of
Roden (1975) at 10
N, where salinity decreases abruptly southward into equatorial water.
Beyond 180
W, the southern part of this limb of the gyral circulation passes progressively
under the influence of the heavy precipitation which characterizes the western Pacific
“warm pool”; where these conditions are fully developed, to the south of 10
N and
west of 160
E, it is proper to recognize a unique province (see WARM), some of whose
characteristics will nevertheless be relevant to the western part of NPTG.
Flow perturbation past the Hawaiian island chain causes the formation of a von
Karman vortex street, by the generation of alternate cyclonic and anticylonic eddies
similar to those downstream of the Canaries (see NAST). However, in the case of the
Hawaiian Islands, it is likely that eddies are also formed by the effect of curl of the wind
stress, which induces upwelling and downwelling of surface water at the wind-shear lines
by strong Ekman pumping. Off Hawaii it is suggested that this process induces sufficient
vertical nutrient flux to induce features in the chlorophyll field (see the review of island
eddy wakes by Aristegui et al., 1997).
Regional Response of the Pelagic Ecosystem
We are fortunate that for this province we have two sets of excellent time-series studies,
together with some trans-Pacific meridional and zonal sections. Many expeditions, mostly
from Scripps Institute of Oceanography in California, investigated the CLIMAX area
(28–30
N) from 1968 to 1987 (Hayward, 1987). The recent HOT (Hawaii Ocean Time
Series) investigations were established at the ALOHA station, at 22
N 158
W, in 1988 with
the intention of obtaining a 20-year time series (Karl and Lukas, 1996). The following
account relies very heavily on these two projects, which are among the very best data sets
for the analysis of long-term changes in the structure of a pelagic ecosystem.