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.

4. Isopycnal maps

Salinity on _n = 26.5 kg m^-3, an isopycnal near the pycnostad and within the Tsuchiya jet cores, as defined by the transport-weighted _n just discussed, reveals a great deal about the SSCCs sources and structure (Fig. 3). The strong S front starting just north of the equator in the western Pacific shifts poleward and weakens to the east. This S front suggests that the SSCCs are a barrier to meridional flow (Gouriou and Toole 1993), at least in the western Pacific where the front is sharp. The meridional S minimum just north of the front is the result of advection in the north SSCC. The only strong S contrast is near the northern edge of the north SSCC, suggesting that the source waters for both SSCCs are primarily from the southwestern Pacific (Tsuchiya 1981), similar to but slightly denser than the waters feeding the EUC (Tsuchiya et al. 1989). Salinity may also be an advective tracer for the south SSCC, showing a faint maximum, but dissolved oxygen concentration (not analyzed here) easily reveals both SSCCs through advectively formed isolated maxima on isopycnals (Tsuchiya 1975; Wyrtki and Kilonsky 1984).

The isopycnal _n = 26.0 kg m^-3 (Fig. 3) is near the base of the pycnocline, above the pycnostad associated with the SSCCs (Fig. 2). The poleward deepening south of the equator, the deep values near the equator, and those from 4° to 6°N are expressions of the southern component of the SEC, the EUC, and the axis between the northern component of the SEC and the NECC, respectively. However, most germane to the dynamics of the SSCCs, the surface rises steadily to the east as does the pycnocline just above it.

In contrast, _n = 26.8 kg m^-3 (Fig. 3) is below the pycnostad associated with the Tsuchiya jets, except in the far western Pacific (Fig. 2). However, while this surface is below the pycnostad, it is not so deep that it escapes the influence of the SSCCs. In fact, it is at the zone of maximum vertical shear of the SSCCs in the central Pacific, slightly above it in the western Pacific, and slightly below it in the eastern Pacific. The strong shoaling poleward of ±2° latitude in the western Pacific, shifting to poleward of ±4° latitude in the eastern Pacific, is the expression of the eastward-flowing SSCCs in each hemisphere. The weaker gradient in the Southern Hemisphere is consistent with the lower velocity and transport estimates for the SSCC there. The slight shoaling at the equator is a shallow signature of the EIC.

The most evocative map in terms of the Tsuchiya jet dynamics is that of thickness between the surfaces just discussed, _n = 26.0 and 26.8 kg m^-3 (Fig. 3), which bound the pycnostad core values. In the model presented below, _n = 26.0 kg m^-3 can be thought of as representing the pycnocline between the surface layer and the active layer, and _n = 26.8 kg m^-3 the interface between the active layer and the quiescent abyssal layer. Hence the thickness between these surfaces is that of the active layer. This thickness map dramatically illustrates the poleward shift of the SSCCs from west to east. It also shows the pycnostad between the SSCCs as it builds and spreads poleward from west to east. Finally, the poleward thinning of this layer across the axes of the SSCCs is indicative of the strong potential vorticity gradients across them. The slight decrease in thickness on the equator is the result of a constructive combination of the deep expression of the eastward-flowing EUC at the upper isopycnal and the shallow expression of the westward-flowing EIC at the lower isopycnal.

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