A NUMERICAL-SIMULATION OF THE MEAN WATER PATHWAYS IN THE SUBTROPICAL AND TROPICAL PACIFIC-OCEAN

A reduced-gravity, primitive-equation, upper-ocean general circulation model is used to study the mean water pathways in the North Pacific s ubtropical and tropical ocean. The model features an explicit physical representation of the surface mixed layer, realistic basin geometry, observed wind and heat flux forcing, and a horizontal grid-stretching technique and a vertical sigma coordinate to obtain a realistic simula tion of the subtropical/tropical circulation. Velocity fields, and iso pycnal and trajectory analyses are used to understand the mean flow of mixed layer and thermocline waters between the subtropics and Tropics . Subtropical/tropical water pathways are not simply direct meridional routes; the existence of vigorous zonal current systems obviously com plicates the picture. In the surface mixed layer, upwelled equatorial waters flow into the subtropical gyre mainly through the midlatitude w estern boundary current (the model Kuroshio). There is additionally an interior ocean pathway, through the Subtropical Countercurrent (an ea stward flow across the middle of the subtropical gyre), that directly feeds subtropical subduction sites. Below the mixed layer, the water p athways in the subtropical thermocline essentially reflect the anticyc lonic gyre circulation where we find that the model subtropical gyre s eparates into two circulation centers. The surface circulation also fe atures a double-cell pattern, with the poleward cell centered at about 30 degrees N and the equatorward component contained between 15 degre es and 25 degrees N. In addition, thermocline waters that can be trace d to subtropical subduction sites move toward the Tropics almost zonal ly across the basin, succeeding in flowing toward the equator only alo ng relatively narrow north-south conduits. The low-latitude western bo undary currents serve as the main southward circuit for the subducted subtropical thermocline water. However, the model does find a direct f low of thermocline water into the Tropics through the ocean interior, confined to the far western Pacific (away from the low-latitude wester n boundary currents) across 10 degrees N. This interior pathway is fou nd just to the west of a recirculating gyre in and just below the mixe d layer in the northeastern Tropics. This equatorward interior flow an d a flow that can be traced directly to the western boundary are then swept eastward by the deeper branches of the North Equatorial Counterc urrent, finally penetrating to the equator in the central and eastern Pacific. Most of these results are consistent with available observati ons and recently published theoretical and idealized numerical experim ents, although the interior pathway of subtropical thermocline water i nto the Tropics found in this experiment is not apparent in other publ ished numerical simulations. Potential vorticity dynamics are useful i n explaining the pathways taken by subtropical thermocline water as it flows into the Tropics. In particular, a large-scale zonally oriented ''island'' of homogenous potential vorticity, whose signature is dete rmined by thin isopycnal layers in the central tropical Pacific along about 10 degrees N, is dynamically linked to a circulation that does n ot how directly from the subtropics to the Tropics. This large-scale p otential vorticity feature helps to explain the circuitous pathways of the subducted subtropical thermocline waters as they approach the equ ator. Consequently, waters must first flow westward to the western bou ndary north of these closed potential vorticity contours and then most ly move southward through the low-latitude western boundary currents, flow eastward with the North Equatorial Countercurrent, and finally eq uatorward to join the Equatorial Undercurrent in the thermocline.