Circulation and water masses of the southwest Pacific: WOCE Section P11, Papua New Guinea to Tasmania

Abstract

The circulation near the western boundary of the South Pacie c is described on the basis of water properties and geostrophic velocities measured on a meridional section along 155E through the East Australian and Coral Basins. The section was occupied in winter 1993 as part of the World Ocean Circulation Experiment (WOCE, Section P11S). The primary objective of P11 was to quantify the zonal e ows entering and leaving the western boundary of the basin. The primary ine ow to the Tasman and Coral seas is supplied by the South Equatorial Current (SEC), which crosses the P11 section as a wide band of westward e ow between 14 and 18S with a total geostrophic transport of 55 Sv (1 Sv 5 10 6 m 3 /s) relative to the bottom. The SEC bifurcates at theAustralian coast near 18S: 29 Sv turns south to feed the EastAustralian Current (EAC), and 26 Sv recirculates in the Gulf of Papua New Guinea as a low-latitude western boundary current (the Great Barrier Reef and New Guinea Coastal Undercurrents, GBRUC/NGCUC). The NGCUC closes the tropical gyre, and carries South Pacie c water around the Louisiade Archipelago and through the Solomon Sea to the equator. The core of the EAC lies over the continental slope between 18S and 30S. A system of deep-reaching, recirculating eddies or gyres is located offshore of the EAC. At 30S the EAC separates from the coast, and the steeply sloping isopycnals associated with the current persist from the surface to the bottom.The total geostrophic transport of the EAC after separation is 57 Sv relative to the bottom. After separation from the coast, more than half of the EAC (33 Sv) recirculates north and then west, crossing P11 again at 28S. The remainder (24 Sv) continues east as a meandering jet across the Tasman Sea (the Tasman Front). The circulation in the southern part of the Tasman Sea is dominated by transient eddies and standing gyres.An anticyclonic circulation facilitates the exchange of water between the Tasman Sea and the Southern Ocean. About 10 Sv of subantarctic water spreads north to 36‐ 38S, then recirculates back to the west to merge with a weak southward e ow of modie ed subtopical water near the Tasmanian coast. The circulation in the deeper layers consists of a weak northward deep western boundary current, a cyclonic recirculation e lling the Tasman Basin, and a net export of about 3 Sv of deep water to the Coral Sea. The transport of mode and intermediate water in the low-latitude western boundary current crossing P11 is similar to the transport in these density classes further ‘ ‘ upstream’ ’ in the subtropical gyre at 32S. This suggests that the mode and intermediate waters entering the Pacie c from the south to compensate the export through the Indonesian passages are carried north to the tropical western Pacie c primarily along isopycnals.