374
Chapter 11: The Pacific Ocean
Defining Characteristics of Regional Oceanography
Circulation in the western part of this province is cyclonic because when the NEC
encounters the eastern coast of the Philippines, it bifurcates north and south into the
Kuroshio and the Mindenau Currents. The latter branch flows toward the southeast along
the shelf edge and then back across the ocean. This cyclonic flow, together with wind
stress curl associated with the NE Monsoon, results in the generation of the Mindenau
Dome at about 7
N 130
E. Further to the southeast, of course, the flow of the SEC around
the southern subtropical gyre of the Pacific Ocean is encountered. The weakly flowing
SECC at about 7–10
S (at 165
E) is scarcely observable as a slope in the thermocline
topography (Godfrey et al., 1993; Gouriou, 1993), which is otherwise dominated by the
westward flow of the SEC along the equator in both hemispheres, requiring the existence
of a pycnocline ridge aligned along the equator.
These diverse circulation features of the western tropical Pacific are included within
a single province to respect the dominant role played there by regional meteorology in
regulating stratification in the ocean that lies below the heavy convective cloud cover of
the low-pressure cell of the Walker circulation. The zonal, near-equatorial cloud band of
the ITCZ (between the northeast and southeast trades) and the cloud band of the South
Pacific Convergence Zone (between the southwest and southeast trades) meet the west
winds coming out of the Indo-Pacific archipelago in this region, and this conjunction
forms the largest region of persistent cloudiness in the tropics. The resulting heavy rainfall
leads to an excess of precipitation over evaporation of 50–150 cm y
−1 , so that a lens of
warm, brackish surface water is formed, to be further diluted by the eastward advection
of low-salinity surface water from the Indonesian archipelago. Similar conditions occur
elsewhere only in two much smaller regions, to the west of Central America and in the
northern Caribbean. As defined by the 29
C surface isotherm, the Western Pacific Warm
Pool varies in size on the decadal scale (Yan et al., 1992).
The surface isohaline layer overlies a halocline at an average depth of 30 m within
a deeper thermostad that reaches down to about 75 m (Lukas and Lindstrom, 1991;
Sprintall and Tomzcak, 1992). The Ekman layer, therefore, corresponds to the low-salinity
surface layer while the deeper part of the thermostad forms a “barrier layer” between
halocline and thermocline. Since there is negligible vertical temperature gradient within
the halocline, vertical heat flux does not occur across it (Lindstrom et al., 1987) but,
because of the relatively calm winds over the western Pacific, the diurnal temperature
cycle in the near-surface layer may be quite large, exceeding 3
C (Soloviev and Lukas,
1997). The recent WEPOCS surveys have changed the classical view that the Pacific mixed
layer deepens progressively from east to west; in fact, the mixed layer deepens westward
only to the date line, then remains at about 100 m across the western Pacific. At 15–20%
of the WEPOCS stations, however, recent wind bursts had mixed the brackish water
down to the thermocline: under these circumstances the barrier layer is eroded, some
nutrient flux is induced, and the consequent profile resembles the classic view of western
Pacific stratification.
It is in this province that the westerly wind anomalies that mark the onset of an El Niño
event first develop and, during strong negative anomalies in the Southern Oscillation,
conditions characteristic of the WARM province may come to lie so far east that we
may find it convenient to evoke the concept of a trans-Pacific WARM-ENSO province.
During at least some ENSO events, as occurred during 1986 and 1987, the effect of the
Rossby upwelling wave in the WARM province is to shoal the thermocline until it is
coincident with the bottom of the surface mixed layer; when this occurs, the “barrier
layer” no longer exists and the nutricline comes to lie at the bottom of the wind mixed
layer so that vertical flux of nutrients into the euphotic zone must be enhanced (Radenac
and Rodier, 1996).
Chapter 11: The Pacific Ocean
Defining Characteristics of Regional Oceanography
Circulation in the western part of this province is cyclonic because when the NEC
encounters the eastern coast of the Philippines, it bifurcates north and south into the
Kuroshio and the Mindenau Currents. The latter branch flows toward the southeast along
the shelf edge and then back across the ocean. This cyclonic flow, together with wind
stress curl associated with the NE Monsoon, results in the generation of the Mindenau
Dome at about 7
N 130
E. Further to the southeast, of course, the flow of the SEC around
the southern subtropical gyre of the Pacific Ocean is encountered. The weakly flowing
SECC at about 7–10
S (at 165
E) is scarcely observable as a slope in the thermocline
topography (Godfrey et al., 1993; Gouriou, 1993), which is otherwise dominated by the
westward flow of the SEC along the equator in both hemispheres, requiring the existence
of a pycnocline ridge aligned along the equator.
These diverse circulation features of the western tropical Pacific are included within
a single province to respect the dominant role played there by regional meteorology in
regulating stratification in the ocean that lies below the heavy convective cloud cover of
the low-pressure cell of the Walker circulation. The zonal, near-equatorial cloud band of
the ITCZ (between the northeast and southeast trades) and the cloud band of the South
Pacific Convergence Zone (between the southwest and southeast trades) meet the west
winds coming out of the Indo-Pacific archipelago in this region, and this conjunction
forms the largest region of persistent cloudiness in the tropics. The resulting heavy rainfall
leads to an excess of precipitation over evaporation of 50–150 cm y
−1 , so that a lens of
warm, brackish surface water is formed, to be further diluted by the eastward advection
of low-salinity surface water from the Indonesian archipelago. Similar conditions occur
elsewhere only in two much smaller regions, to the west of Central America and in the
northern Caribbean. As defined by the 29
C surface isotherm, the Western Pacific Warm
Pool varies in size on the decadal scale (Yan et al., 1992).
The surface isohaline layer overlies a halocline at an average depth of 30 m within
a deeper thermostad that reaches down to about 75 m (Lukas and Lindstrom, 1991;
Sprintall and Tomzcak, 1992). The Ekman layer, therefore, corresponds to the low-salinity
surface layer while the deeper part of the thermostad forms a “barrier layer” between
halocline and thermocline. Since there is negligible vertical temperature gradient within
the halocline, vertical heat flux does not occur across it (Lindstrom et al., 1987) but,
because of the relatively calm winds over the western Pacific, the diurnal temperature
cycle in the near-surface layer may be quite large, exceeding 3
C (Soloviev and Lukas,
1997). The recent WEPOCS surveys have changed the classical view that the Pacific mixed
layer deepens progressively from east to west; in fact, the mixed layer deepens westward
only to the date line, then remains at about 100 m across the western Pacific. At 15–20%
of the WEPOCS stations, however, recent wind bursts had mixed the brackish water
down to the thermocline: under these circumstances the barrier layer is eroded, some
nutrient flux is induced, and the consequent profile resembles the classic view of western
Pacific stratification.
It is in this province that the westerly wind anomalies that mark the onset of an El Niño
event first develop and, during strong negative anomalies in the Southern Oscillation,
conditions characteristic of the WARM province may come to lie so far east that we
may find it convenient to evoke the concept of a trans-Pacific WARM-ENSO province.
During at least some ENSO events, as occurred during 1986 and 1987, the effect of the
Rossby upwelling wave in the WARM province is to shoal the thermocline until it is
coincident with the bottom of the surface mixed layer; when this occurs, the “barrier
layer” no longer exists and the nutricline comes to lie at the bottom of the wind mixed
layer so that vertical flux of nutrients into the euphotic zone must be enhanced (Radenac
and Rodier, 1996).