The Pacific Cold Tongue: A Pathway for Interhemispheric Exchange

Abstract

Mean meridional upper-ocean temperature, salinity, and zonal velocity sections across the Pacific Ocean between 8°S and 8°N are combined with other oceanographic and air?sea flux data in an inverse model. The tropical Pacific Ocean can be divided into three regions with distinct circulation patterns: western (143°E?170°W), central (170°?125°W), and eastern (125°W?eastern boundary). In the central and eastern Pacific the downward limbs of the shallow tropical cells are 15(±13) ? 106 m3 s?1 in the north and 20(±11) ? 106 m3 s?1 in the south. The Pacific cold tongue in the eastern region results from diapycnal upwelling through all layers of the Equatorial Undercurrent, which preferentially exhausts the lightest (warmer) layers of the Equatorial Undercurrent [10(±6) ? 106 m3 s?1] between 125° and 95°W, allowing the denser (cooler) layers to upwell [9(±4) ? 106 m3 s?1] east of 95°W and adjacent to the American coast. An interhemispheric exchange of 13(±13) ? 106 m3 s?1 between the southern and northern Pacific Ocean forms the Pacific branch of the Pacific?Indian interbasin exchange. Southern Hemisphere water enters the tropical Pacific Ocean via the direct route at the western boundary and via an interior (basin) pathway. However, this water moves irreversibly into the North Pacific by upwelling in the eastern equatorial Pacific and air?sea transformation that drives poleward interior transport across 2°N.