MERIDIONAL CIRCULATION CELLS AND THE SOURCE WATERS OF THE PACIFIC EQUATORIAL UNDERCURRENT

A 3-1/2-layer model is used to study the meridional circulation cells that provide the source waters of the Pacific Equatorial Undercurrent (EUC). Its three active layers represent tropical, thermocline, and up per-intermediate waters, respectively, and across-interface flow betwe en the layers parameterizes the processes of upwelling, subduction, an d diapycnal mixing. Solutions are driven by climatological winds in a domain resembling the Pacific basin from 35 degrees S to 55 degrees N. An additional forcing mechanism is a specified inflow into layer 3 ac ross the open southern boundary and a compensating outflow from layers 1 and 2 along the western boundary just north of the equator; the res ulting circulation simulates the Pacific interocean circulation (IOC), in which intermediate water enters the South Pacific and the same amo unt of thermocline and tropical waters exit via the Indonesian Through flow. Five meridional cells contribute to the EUC in the main-run solu tion: north and south Subtropical Cells (STCs), north and south Tropic al Cells (TCs), and an equatorial branch of the IOC. The STCs require subtropical subduction for their existence: however, their strength is determined largely by the Ekman divergence across +/- 18 degrees, whi ch are externally specified, equatorward boundaries of the subduction regions. The STCs are the primary source of water for the model EUC, s upplying 21.9 Sv (Sv = 10(6) m(3) s(-1)) in the main run. The TCs are generated by diapycnal mixing in the tropical ocean that causes layer- 1 water to detrain into layer 2, and they contribute 6.8 Sv to the EUC . The equatorial branch of the IOC provides 8.8 Sv, and this contribut ion ensures that nearly two-thirds of the extratropical water in the E UC is of Southern Hemisphere origin. Finally, equatorial recirculation s confined to layer 2 supply an additional 3.8 Sv; these gyres exist b ecause potential vorticity conservation causes water on the flanks of the EUC to diverge from the equator as it hows into a region where lay er 2 is thicker.