The distribution of intermediate water in the western equatorial Pacific during January–February 1986

Abstract

The distribution of intermediate water in the western equatorial Pacific (WEP) for the two month period January–February, 1986, is examined with hydrographic data collected by researchers on six different ships from five different countries. The WEP contains intermediate waters derived from the poleward reaches of both the North and South Pacific, and the mixing of these waters creates a complex and interleaving pattern of fronts. Antarctic Intermediate Water (AAIW) enters the WEP via the Vitiaz Strait in the New Guinea Coastal Undercurrent with a characteristic salinity minimum of about 34.55 at a density of approximately 27.2 σ φ). The salinity minimum is seen as far north as 10°N; however, distributions of oxygen indicate that newer AAI W from the South Pacific spreads only as far as 2°S to the east of 143°E and possibly further north to the west of 143°E. North Pacific Intermediate Water (NPIW) has a salinity minimum of less than 34.5. It enters the WEP via the Mindanao Current at 26.5–26.6 σ φ and spreads eastward in the North Equatorial Countercurrent (NECC) as a thin filament at about 2–3.5°N. This filament is visible as far east as 150°E. An intermediate water mass named North Pacific Tropical Intermediate Water (NPTI W) is found between two fronts at approximately 4.5 and 10°N between 26 and 27 σ φ. NPTIW is associated with a core salinity maximum and oxygen minimum at 26.8 ay and may be part of a “shadow zone”, an area of weak flow isolated from surface forcing. A companion to NPTIW. South Pacific Tropical Intermediate Water (SPTIW), spreads into the western equatorial Pacific in the Equatorial Intermediate Current along the Equator centred at 27.1 σ φ. It is identified by a lateral oxygen minimum between 2°N and 2°S as far west as 143°E.