The flow of thermocline water from the subtropical to the equatorial Pacific Ocean is investigated using a 2½-layer numerical model. In this system, the lower of the two active layers represents the thermocline region of the ocean, and the upper layer simulates the near-surface region including the mixed layer. Water is allowed to move between the layers via an across-interface velocity that parameterizes the processes of upwelling and subtropical subduction. Solutions are obtained in a basin that resembles the Pacific basin, and they are forced by Hellerman and Rosenstein winds.
The primary result is that the intertropical convergence zone (ITCZ) creates a potential vorticity barrier that inhibits the direct flow of lower-layer (thermocline) water from the subtropical North Pacific to the equator. Lower-layer water must first flow to the western boundary north of the ITCZ and only then can it move equatorward in a western boundary current to join the Equatorial Undercurrent. Another result is that there is a convergence of lower-layer water onto the equator in the central ocean; however, it is associated with a pair of lower-layer recirculation cells confined within 5° of the equator and is not part of the large-scale circulations that carry subtropical water to the equator (the north and south subtropical cells).