The model does not consider the origin of the Tsuchiya jets. One possibility is that in the western Pacific the Tsuchiya jets are generated as lobes of the EUC under the diffusive dynamics discussed by McPhaden (1984). These lobes could separate from the EUC in the east due to the poleward shift forced by the shoaling thermocline, as discussed here, and propagate nearly inertially. However, vertical friction sufficiently strong to spin up the SSCCs in the western Pacific would likely damp them out in the central Pacific. Another possibility for their origin is a current at the western boundary turning east, similar to the way in which the EUC appears to be fed in observations and models (Tsuchiya et al. 1989; Pedlosky 1991). While observations suggest the south SSCC is fed by water from the south (Tsuchiya 1981), the north SSCC appears to have contributions from both hemispheres (Bingham and Lukas 1995).
In the eastern Pacific another interesting problem arises: the termination of the Tsuchiya jets. In an inertial model of the EUC, the right strength of diffusive upwelling at the equator can remove mass from the EUC as it flows eastward, terminating it at the eastern boundary (Pedlosky 1988). It seems likely that vertical diffusive processes, at or away from the equator, could be included in a more complex model of the SSCCs to model their termination. It is possible that vertical diffusive processes could take place within the SSCCs. Alternatively, the pycnostad between the SSCCs does intersect the equator, and might allow strong vertical processes there to be connected to the SSCCs. We know of no observational study describing the fate of the north SSCC. However, observations suggest that the EUC and the south SSCC turn south at the eastern boundary to feed the Peru-Chile undercurrent and countercurrent (Lukas 1986). This observational work seems to preclude the total dominance of vertical processes in determining the fate of the EUC and the SSCCs.
One interesting feature of the observations not explained by the model is the decrease of transport-weighted n in the Tsuchiya jets from west to east. This decrease, if it is significant in the face of the many uncertainties present in the transport calculations, suggests that mixing processes in the ocean may play some role in the SSCCs dynamics. These mixing processes presumably do work to diffuse the SSCCs, altering the potential vorticity distribution within them by decreasing the magnitude of the relative vorticity on their edges. The model results suggests that the tendency toward potential vorticity conservation in the ocean overcomes the effects of mixing to keep the Tsuchiya jets focused and shifting poleward by vortex stretching under the shoaling pycnocline.
Acknowledgments. This work was funded by the Climate and Global Change Program through the NOAA Office of Global Programs and by the NOAA Pacific Marine Environmental Laboratory. LuAnne Thompson provided insightful advice on model formulation and integration. Julian McCreary and Eric Firing made numerous helpful suggestions to improve the manuscript. Nancy Soreide, Marie Schall, Dai McClurg, and Stephan Zube all helped with programming and database management. Kristene McTaggart calibrated a great deal of the PMEL CTD data used in this work.
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