The western equatorial Pacific: A water mass crossroads


  • Rana A. Fine,

  • Roger Lukas,

  • Frederick M. Bingham,

  • Mark J. Warner,

  • Richard H. Gammon


The western equatorial Pacific is a crossroads for thermocline and intermediate waters formed at higher latitudes. The role of the equatorward flowing, low-latitude western boundary currents (LLWBCs) in advecting well-ventilated (with respect to atmospheric gases), higher-latitude waters varies with density. At densities <26.5 σθ the Mindanao Current (MC) (Wyrtki, 1961; Masuzawa, 1969) advects recently ventilated water observed as tracer maxima predominantly from the North Pacific subtropical gyre (tropical water is <3 years old and the remnant subtropical mode water is <5 years); it branches into the southern Celebes Sea feeding the Indonesian throughflow and toward the east north of the equator. Between 26.5 and 26.8 σθ the MC advects predominantly North Pacific Intermediate Water (having a component that is <20 years old) mainly into the southern Celebes Sea; there is also some indication of a tracer maximum extending eastward north of the equator. However, below 26.8 σθ, South Pacific water masses appear to be stronger, so that they are the major ventilation source for the western equatorial region, including the Celebes Sea. At 27.2 σθ the New Guinea Coastal Undercurrent advects Antarctic Intermediate Water (having a component that is <25 years) into a background of older water. The presence of subtropical mode water in the western tropical North Pacific and Celebes Sea is attributed to an equatorward LLWBC in the North Pacific (and suggests a reason for the absence of 18° water in the tropical North Atlantic). The absence of a LLWBC in the North Atlantic highlights a basic difference between the circulation of the two oceans, which may be due to the different ways they import and export water. At the western boundary in the North Atlantic, warm water is imported and cold water is exported as part of the global thermohaline circulation, whereas at the western boundary in the North Pacific, warm water (above 26.8 σθ) is mainly exported to the Indian Ocean via the Indonesian throughflow and cold water is imported.