Deep-water circulation at low latitudes in the western North Pacific

Full-depth conductivity-temperature-depth-oxygen profiler (CTDO2) data at low latitudes in the western North Pacific in winter 1999 were analyzed with water-mass analysis and geostrophic calculations. The result shows that the deep circulation carrying the Lower Circumpolar Water (LCPW) bifurcates into eastern and western branch currents after entering the Central Pacific Basin. LCPW colder than 0.98degreesC is carried by the eastern branch current, while warmer LCPW is carried mainly by the western branch current. The eastern branch current flows northward in the Central Pacific Basin, supplying water above 0.94degreesC through narrow gaps into an isolated deep valley in the Melanesian Basin, and then passes the Mid-Pacific Scamounts between 162degrees10'E and 170degrees10'E at 18degrees20'N, not only through the Wake Island Passage but also through the western passages. Except near bottom, dissolved oxygen of LCPW decreases greatly in the northern Central Pacific Basin, probably by mixing with the North Pacific Deep Water (NPDW). The western branch current flows northwestward over the lower Solomon Rise in the Melanesian Basin and proceeds westward between 10degrees40'N and 12degrees20'N at 150degreesE in the East Mariana Basin with volume transport of 4.1 Sv (I SV = 10(6) m(3) s(-1)). The current turns north, west of 150degreesE, and bifurcates around 14degreesN, south of the Magellan Seamounts, where dissolved oxygen decreases sharply by mixing with NPDW. Half of the current turns east, crosses 150degreesE at 14-15degreesN, and proceeds northward primarily between 152degreesE and 156degreesE at 18degrees20'N toward the Northwest Pacific Basin (2.1 Sv). The other half flows northward west of 150degreesE and passes 18degrees20'N just east of the Mariana Trench (2.2 Sv). It is reversed by a block of topography, proceeds southward along the Mariana Trench, then detours around the south end of the trench, and proceeds eastward along the Caroline Seamounts to the Solomon Rise, partly flowing into the West Mariana and East Caroline Basins. A deep western boundary current at 2000-3000 m depth above LCPW (10.0 Sv) closes to the coast than the deep circulation. The major part of it (8.5 Sv) turns cyclonic around the upper Solomon Rise from the Melanesian Basin and proceeds along the southern boundary of the East Caroline Basin. Nearly half of it proceeds northward in the western East Caroline Basin, joins the current from the cast, then passes the northern channel, and mostly enters the West Caroline Basin (4.6Sv), while another half enters this basin from the southern side (> 3.8 Sv). The remaining western boundary current (1.5 Sv) flows over the middle and lower Solomon Rise, proceeds westward, then is divided by the Caroline Seamounts into southern (0.9Sv) and northern (0.5 Sv) branches. The southern branch current joins that from the south in the East Caroline Basin, as noted above. The northern branch current proceeds along the Caroline Seamounts and enters the West Mariana Basin. (C) 2003 Elsevier Science Ltd. All rights reserved.