P. Lu et al., MERIDIONAL CIRCULATION CELLS AND THE SOURCE WATERS OF THE PACIFIC EQUATORIAL UNDERCURRENT, Journal of physical oceanography, 28(1), 1998, pp. 62-84
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.