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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