The Pacific Subsurface Countercurrents and an Inertial Model

Gregory C. Johnson and Dennis W. Moore

National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory, Seattle, Washington

Journal of Physical Oceanography, 27, 2448-2459
Not subject to U.S. copyright. Published in 1997 by the American Meterological Society.

3. Mean meridional sections of properties and transports

Mean meridional potential temperature, , sections show that the thermocline (roughly from 15° to 25°C, centered near the 20°C isotherm) shoals and strengthens from west to east (Fig. 1). This feature is deep (120-200 m) and relatively weak at 165°E, intermediate in depth (80-200 m) and strength at 155°W, and shallow (50-120 m) and strong at 110°W. The poleward deepening of the thermocline south of the equator, and north of the equator to 4°-5°N, is associated with the southern and northern components of the westward-flowing South Equatorial Current (SEC), respectively. The poleward shoaling of the thermocline north of 4°-5°N is associated with the eastward-flowing NECC. The spreading of the thermocline around the equator marks the eastward flowing EUC. This spreading is discernable at 165°E and stronger at 155°W. At 110°W the EUC is shallow and south of the equator, where isotherms dip near 1°S. This southern displacement from the equator has been shown to be the result of meridional wind forcing (Philander and Delecluse 1983). Most relevant to this study is a poleward shoaling of isotherms below the thermocline centered near ±2.5° latitude at 165°E, ±3.5° at 155°W, and ±4.5°-5.5° at 110°W. This shoaling shifts poleward and upward to the east under the thermocline, marking the Tsuchiya jets. By 110°W, the thermostad near 13°C between the SSCCs is very pronounced.

Fig. 1. Mean meridional and S sections in the top 500 m of the ocean from 10°S to 10°N along 165°E, 155°W, and 110°W. Contour intervals are 1°C (thick lines 5°C) for and 0.1 PSS-78 (thick lines 0.5 PSS-78) for S. Vertical exaggeration is 4000:1.

Mean meridional S sections show a subsurface convergence of tongues toward the EUC (Fig. 1). The southern subsurface S maximum, water subducted in the southern subtropics where evaporation dominates over precipitation, is prominent in all three sections and strongest at 155°W. North of the equator, surface S decreases to the east under the intertropical convergence zone, a region of high precipitation (Ando and McPhaden 1997). Salinity distributions on isopycnal surfaces around _n = 25.5 kg m^-3, the core density of the EUC, suggests that the subsurface S minimum is almost certainly a remnant of the North Pacific Intermediate Water (Talley 1993). This fresh tongue appears to be converging along isopycnals toward the EUC, especially at 155°W, but almost certainly takes an indirect route involving the nearly zonal interior currents and nearly meridional western boundary currents. At 110°W the core of the EUC is marked by an isolated S maximum advected from the west (Hayes et al. 1983; Lukas 1986), here found at 1°S, 65 m. Most relevant to this study, isohalines are nearly vertical in the region of the north SSCC. The resulting strong deep S gradient near 2°-3°N at 165°E, 2°-4°N at 155°W, and 4°-5°N at 110°W suggests a front there and reinforces the assertion that the Tsuchiya jets are a barrier to meridional flow (Gouriou and Toole 1993). In addition, there is a well-defined S minimum just north of the front, just hinted at in the sections by a pinching of isohalines at 4°N, 5°N, and 6°N going from 165°E to 155°W to 110°W, but very apparent in an isopycnal map discussed in the next section (Fig. 3, top panel). Since S increases from west to east, this feature is probably fresher water being advected eastward on the northern edge of the north SSCC.

Mean meridional _n sections (Fig. 2) are similar in appearance to those of (Fig. 1) and are shown primarily for reference to the isopycnal maps and the model. The previous discussion of the thermocline and thermostad applies equally to the pycnocline and pycnostad, with S modifying the structure slightly. However, the mean meridional N² sections (Fig. 2) are worthy of more discussion. The surface mixed layer is characterized by very low N² and shoals from 165°E to 110°W. The pycnocline, a vertical maximum in N², is relatively thick, deep, and weak at 165°E. By 110°W it has thinned, nearly doubled in strength, and shoaled. Below the pycnocline, N² generally weakens with increasing depth, but a very distinct equatorial pycnostad associated with the SSCCs develops, here described as it strengthens from west to east. At 165°E, the 20 and 25 × 10^-6 s^-2 contours rise up at ±2° from the equator, marking a weak pycnostad centered near _n = 26.74 kg m^-3 at these latitudes. By 155°W a well-developed stability minimum is evident on both sides of the equator, with values near 12 × 10^-6 s^-2 at _n = 26.62 kg m^-3, again most extreme ±2° from the equator, but broader in the horizontal. By 110°W the minimum is less than 10 × 10^-6 s^-2 near _n = 26.47 kg m^-3. The latitudinal distribution shows a slight increase in stability on the equator, but the pycnostad extends from 5°S to 4°N. Thus, as the pycnostad builds in strength and expands in area to the east, it also shoals and its core _n values decreases by 0.27 kg m^-3.

Fig. 2. As in Fig. 1 but for _n and N². Contour intervals vary for both quantities, but thick lines are at 1.0 kg m^-3 intervals for _n and 200 × 10^-6 s^-2 intervals for N².

Geostrophic volume transport and velocity calculations are made using a reference surface of 700 dbar, near _n = 27.3 kg m^-3. This surface is deep enough to capture the Tsuchiya jets and is the mean of those used in three previous quantitative works in the region: 600 dbar at 165°E (Gouriou and Toole 1993), 1000 dbar at 155°W (Wyrtki and Kilonsky 1984), and 500 dbar at 110°W (Tsuchiya 1975). Table 1 lists the volume transport estimates of eastward flow between _n = 25.5 and 27.3 kg m^-3. The lower limit is near the reference surface and the upper limit is imposed to isolate the north SSCC from the eastward-flowing NECC. Only eastward flow between the bounding latitudes contributes to the transports in Table 1; these horizontal limits are based on a subjective examination of the mean meridional _n and N² sections so as to capture the eastward flow just poleward of the equatorial pycnostad. These horizontal limits are also well defined in that there is westward flow to either side. The SSCCs volume transports are relatively constant, except at 165°E where the north SSCC value may be high because some NECC flow is included in the estimate. The transport-weighted _n of the SSCCs (Table 1) are near the pycnostad core values discussed above, since the SSCCs reside in the pycnostad, but show only a 0.09 kg m^-3 decrease in _n to the east, a third of the decrease in those core values. The peak velocities shift poleward and shoal in both hemispheres (Table 1). Peak velocities remain strong across the Pacific except at 155°W, where the south SSCC peak velocity is weak, mostly owing to a shift in longitude of the mean station positions (Fig. 3, solid dots). At any rate, peak velocities from the mean sections are most likely lower than those in synoptic sections because of the relatively large latitudinal bin widths and temporal variability of the SSCCs core latitudes. The volume transport estimates and peak velocity statistics are in rough accord with the studies discussed in the introduction. However, the estimates presented here are improved by uniform analysis of an expanded data set.

If your browser cannot view the following table correctly, click this link for a GIF image of Table 1

Table 1. Geostrophic volume transport and velocity calculations made for the Pacific SSCCs using the mean meridional sections at each longitude, referenced to 700-dbar pressure. Only eastward flow between _n = 25.5 and 27.3 kg m^-3 within the latitude bounds given is used in the transport calculations. Volume transport and transport-weighted _n remain roughly constant from west to east. Peak velocity latitudes, depths, and magnitudes show the Tsuchiya jet cores shifting poleward, shoaling, and maintaining speed from west to east.

	Transport calculations			Peak velocities
Section (long)	Latitude bounds	Volume Transport (106 m3 s-1)	Transport weighted n (kg m-3)	Peak latitude	Peak depth (m)	Peak magnitude (m s-1)
South SSCC
165°E	5°-2°S	6.6	26.62	2.5°S	250	0.17
155°W	8°-3°S	4.1	26.67	3.5°S	250	0.07
110°W	6°-3°S	6.2	26.57	5.5°S	160	0.16
North SSCC
165°E	2°-5°N	10.3	26.58	2.5°N	240	0.26
155°W	2°-5°N	7.0	26.63	3.5°N	220	0.21
110°W	3°-6°N	7.4	26.45	4.5°N	130	0.21

Fig. 3. Salinity on _n = 26.5 kg m^-3, with contour intervals of 0.05 PSS-78 and saltier values increasingly shaded (top panel). Depth of _n = 26.0 kg m^-3, with contour intervals of 25 m and deeper values increasingly shaded (second panel from top). Depth of _n = 26.8 kg m^-3, with contour intervals of 25 m and deeper values increasingly shaded (third panel from top). Thickness between _n = 26.0 and 26.8 kg m^-3, with contour intervals of 25 m and thicker values increasingly shaded (bottom panel). All panels are objectively mapped from the values at each mean hydrographic profile location as described in the text. The mapping uses the Peters projection.

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