Source waters of the Pacific Equatorial Undercurrent

Abstract

Hydrographic and direct current measurements were made north and east Papua New Guinea in June–August 1985 and January–February 1986 as part of the Western Equatorial Pacific Ocean Circulation Study (WEPOCS). Analyses of the data indicate that the major portion of the water in the Equatorial Undercurrent at its beginning north of Papua New Guinea is supplied from the south by a narrow western boundary undercurrent (New Guinea Coastal Undercurrent) transporting high-salinity, low-tritium, high-oxygen, low-nutrient water from the Solomon Sea northwestward along the north coast of Papua New Guinea through the Vitiaz Strait. The New Guinea Coastal Undercurrent has a maximum speed of 40–70 cm s −1 at a depth of about 200 m. It is a permanent feature despite the reversals of the wind and the surface current during the period of the northwest monsoon in austral summer. Its transport through the Vitiaz Strait is as high as 8 × 10 6 m 3 s −1, which is of the same magnitude as the Equatorial Undercurrent transport at 143°E. The New Guinea Coastal Undercurrent revealed by the WEPOCS data is consistent with the low-latitude equatorward western boundary current implied in a calculation of the Sverdrup transport based on the observed wind-stress distribution for the tropical Pacific Ocean. High-salinity, low-tritium, low-oxygen, high-nutrient water which flows westward into the Bismarck Sea passing north of the Solomon Islands is entrained into the Equatorial Undercurrent north of New Ireland and returns to the east, resulting in a down-stream increae in the Undercurrent transport. Low-salinity, high-tritium, high-nutrient water of eastern North Pacific origin also contributes to the Equatorial Undercurrent in its source area west of the WEPOCS region. However, there is no evidence that northern waters are being continuously entrained into the Undercurrent in the WEPOCS region.