Near-bottom Thermocline in the Samoan Passage, West Equatorial Pacific

Abstract

ANTARCTIC bottom water (ABW) flows into the Pacific basin from the region between Macquarie Island and Antarctica^1–3. Dynamic considerations suggest that the bottom water flows northwards as an intense current along the western boundary of the Pacific basin^4,5. Oceanographers have noted that the Pacific is divided into at least four principal basins which are separated by relatively shallow sills that constrain the flow of water below 4,000 m (refs. 3, 6, 7). The narrow Samoan Passage, first recognized by Reid⁸, is perhaps the most important channel with depth sufficient to allow a significant flow of bottom water from the southern basin into the central basin. Bottom current velocities in the passage of 5 to 15 cm s⁻¹ towards the north have been measured⁹ and Nansen casts in the passage revealed a sharp decrease in temperature of 0.27° C in 150 m at a depth of about 4,400 m¹⁰. This benthic thermocline marks the abrupt boundary between the deep water and the ABW and supports the observation of cold bottom water flow through the passage. (In a personal communication, P. Lonsdale and J. Reid suggested that the bottom water may include entrained high salinity Atlantic deep water as well as ABW.)