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 Antarctica1–3. Dynamic considerations suggest that the bottom water flows northwards as an intense current along the western boundary of the Pacific basin4,5. 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.)