Influence of the ITCZ on the Flow of Thermocline Water from the Subtropical to the Equatorial Pacific Ocean

Abstract

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).