THE WESTERN EQUATORIAL PACIFIC - A WATER MASS CROSSROADS

The western equatorial Pacific is a crossroads for thermocline and int ermediate waters formed at higher latitudes. The role of the equatorwa rd flowing, low-latitude western boundary currents (LLWBCs) in advecti ng well-ventilated (with respect to atmospheric gases), higher-latitud e waters varies with density. At densities <26.5 sigma(theta) the Mind anao Current (MC) (Wyrtki, 1961; Masuzawa, 1969) advects recently vent ilated water observed as tracer maxima predominantly from the North Pa cific subtropical gyre (tropical water is <3 years old and the remnant subtropical mode water is <5 years); it branches into the southern Ce lebes Sea feeding the Indonesian throughflow and toward the east north of the equator. Between 26.5 and 26.8 sigma(theta) the MC advects pre dominantly North Pacific Intermediate Water (having a component that i s <20 years old) mainly into the southern Celebes Sea; there is also s ome indication of a tracer maximum extending eastward north of the equ ator. However, below 26.8 sigma(theta), South Pacific water masses app ear to be stronger, so that they are the major ventilation source for the western equatorial region, including the Celebes Sea. At 27.2 sigm a(theta) the New Guinea Coastal Undercurrent advects Antarctic Interme diate Water (having a component that is <25 years) into a background o f older water. The presence of subtropical mode water in the western t ropical North Pacific and Celebes Sea is attributed to an equatorward LLWBC in the North Pacific (and suggests a reason for the absence of 1 8 degrees water in the tropical North Atlantic). The absence of a LLWB C in the North Atlantic highlights a basic difference between the circ ulation of the two oceans, which may be due to the different ways they import and export water. At the western boundary in the North Atlanti c, warm water is imported and cold water is exported as part of the gl obal thermohaline circulation, whereas at the western boundary in the North Pacific, warm water (above 26.8 sigma(theta)) is mainly exported to the Indian Ocean via the Indonesian throughflow and cold water is imported.