Therefore, IPT (WBPT) per unit thickness should have a functional form Fn(0) = .
Since the velocity field obtained from models or inferred from observations does not satisfy Eqn. (4.452) exactly, IPT (WBPT) per unit thickness satisfies Eqn. (4.456) approximately.
It can readily be seen from Figure 4.89 that away from the equator and the western boundary and subduction area, IPT (WBPT) per unit thickness is approximately the functional relation Fn(0). Thus, in mid-ocean, IPT (WBPT) per unit thickness multiplied by sin 0 is approximately constant.
Pathways from the subtropics to the tropics can be identified from data generated from numerical models, using either the virtual streamfunction or tracing the layer-integrated streamlines. For example, the interior pathway and the western boundary pathways for four isopycnal surfaces identified from the SODA data are shown in Figure 4.90. It is clear that the time taken for signals to reach the equatorial band is much longer for deep isopycnal surfaces. The interior pathways in the South Pacific are much wider than the corresponding pathways in the North Pacific, consistent with the relation for the barotropic pathways.
a Unit thickness IPT (Sv) b Unit thickness WBPT (Sv)
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