Chapter 11
The Pacific Ocean
T
he Pacific Ocean is clearly a special case, because it has an areal extent (165 ×
10
3 km
−2 ) that is almost half the total area of all oceans combined. It has a zonal
dimension across 210
of longitude, or about 60% of the circumference of the
earth. In the not-so-long-gone days of passenger travel by sea, 20 days steaming from
Panama to New Zealand and always out of sight of land except only for Pitcairn Island,
gave me a firsthand understanding of Pacific dimensions. The immense size of the Pacific
is reflected in strong zonal differences in mixed-layer depth and other physical circulation
features that are reflected in regional pelagic ecology.
A very important direct consequence of the great dimension and largely closed boundaries of this ocean is that here the major ocean circulation patterns can approach an ideal
form in response to symmetric wind patterns in the two hemispheres and to the Coriolis
effect. Here two almost-ideal subtropical gyres and their boundary currents are developed
symmetrically on either side of a quasisymmetric development of an ideal equatorial
current system having appropriate convergences, divergences, and countercurrents. If this
ocean did not exist, our oceanographic textbooks would have been forced to imagine it.
In understanding the other oceans, it is often convenient to make a comparison against
the Pacific ideal: this is true both for physics and for biogeography, in the widest sense.
The Pacific Ocean is also rich in marginal seas, ranging from relatively small, evaporative basins (Gulf of California) to large epicontinental seas (Bering Sea). The Pacific also
accommodates the vast Indo-Pacific archipelago that stretches from northern Australia
to Luzon and Formosa; flow between two oceans (Pacific and Indian) is exchanged across
this complex region of shallow shelves and small, semienclosed deep basins.
In the subarctic zone of the North Pacific, unlike its North Atlantic analogue, precipitation exceeds evaporation so that a surface layer of low salinity lies above a permanent
halocline at 100–150 m. This feature therefore constrains the depth of winter mixing,
and thus the renewal of mixed-layer nutrients. The North Pacific halocline therefore has
unique biological consequences that distinguish the subarctic regimes of the two major
oceans.
Unlike the Atlantic, the Pacific has no connection to the arctic regions in its northeastern quadrant, so the flow of the Kuroshio—the Pacific analogue of the Gulf Stream—does
not, like the Gulf Stream, lose part of its flow into high latitudes. Instead, it flows more
directly across the ocean once it has departed from the Japanese mainland, and this
has important consequences for the comparative oceanography and ecology of the two
oceans.
As in the Atlantic, the transition between the biomes of westerly and trade-wind
zones is not sharp. Though the Subtropical Convergence zone or, in a more generalized
sense, the zone of subtropical convergence fronts between the westerlies and the easterly
trades, is useful to locate this transition, it is an imprecise and moving partition. Some
winter mixing does occur equatorward of the Subtropical Convergence of the Northern
327
The Pacific Ocean
T
he Pacific Ocean is clearly a special case, because it has an areal extent (165 ×
10
3 km
−2 ) that is almost half the total area of all oceans combined. It has a zonal
dimension across 210
of longitude, or about 60% of the circumference of the
earth. In the not-so-long-gone days of passenger travel by sea, 20 days steaming from
Panama to New Zealand and always out of sight of land except only for Pitcairn Island,
gave me a firsthand understanding of Pacific dimensions. The immense size of the Pacific
is reflected in strong zonal differences in mixed-layer depth and other physical circulation
features that are reflected in regional pelagic ecology.
A very important direct consequence of the great dimension and largely closed boundaries of this ocean is that here the major ocean circulation patterns can approach an ideal
form in response to symmetric wind patterns in the two hemispheres and to the Coriolis
effect. Here two almost-ideal subtropical gyres and their boundary currents are developed
symmetrically on either side of a quasisymmetric development of an ideal equatorial
current system having appropriate convergences, divergences, and countercurrents. If this
ocean did not exist, our oceanographic textbooks would have been forced to imagine it.
In understanding the other oceans, it is often convenient to make a comparison against
the Pacific ideal: this is true both for physics and for biogeography, in the widest sense.
The Pacific Ocean is also rich in marginal seas, ranging from relatively small, evaporative basins (Gulf of California) to large epicontinental seas (Bering Sea). The Pacific also
accommodates the vast Indo-Pacific archipelago that stretches from northern Australia
to Luzon and Formosa; flow between two oceans (Pacific and Indian) is exchanged across
this complex region of shallow shelves and small, semienclosed deep basins.
In the subarctic zone of the North Pacific, unlike its North Atlantic analogue, precipitation exceeds evaporation so that a surface layer of low salinity lies above a permanent
halocline at 100–150 m. This feature therefore constrains the depth of winter mixing,
and thus the renewal of mixed-layer nutrients. The North Pacific halocline therefore has
unique biological consequences that distinguish the subarctic regimes of the two major
oceans.
Unlike the Atlantic, the Pacific has no connection to the arctic regions in its northeastern quadrant, so the flow of the Kuroshio—the Pacific analogue of the Gulf Stream—does
not, like the Gulf Stream, lose part of its flow into high latitudes. Instead, it flows more
directly across the ocean once it has departed from the Japanese mainland, and this
has important consequences for the comparative oceanography and ecology of the two
oceans.
As in the Atlantic, the transition between the biomes of westerly and trade-wind
zones is not sharp. Though the Subtropical Convergence zone or, in a more generalized
sense, the zone of subtropical convergence fronts between the westerlies and the easterly
trades, is useful to locate this transition, it is an imprecise and moving partition. Some
winter mixing does occur equatorward of the Subtropical Convergence of the Northern
327