Tracing Southwest Pacific Bottom Water Using Potential Vorticity and Helium-3

Stephanie M. Downes Program in Atmospheric and Oceanic Sciences, Princeton University, New Jersey, and Research School of Earth Sciences, and ARC Centre of Excellence for Climate System Science, The Australian National University, Acton, Australian Capital Territory, Australia

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Robert M. Key Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey

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Alejandro H. Orsi Department of Oceanography, Texas A&M University, College Station, Texas

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Kevin G. Speer Department of Oceanography, The Florida State University, Tallahassee, Florida

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James H. Swift Scripps Institution of Oceanography, La Jolla, California

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Abstract

This study uses potential vorticity and other tracers to identify the pathways of the densest form of Circumpolar Deep Water in the South Pacific, termed “Southwest Pacific Bottom Water” (SPBW), along the 28.2 kg m−3 surface. This study focuses on the potential vorticity signals associated with three major dynamical processes occurring in the vicinity of the Pacific–Antarctic Ridge: 1) the strong flow of the Antarctic Circumpolar Current (ACC), 2) lateral eddy stirring, and 3) heat and stratification changes in bottom waters induced by hydrothermal vents. These processes result in southward and downstream advection of low potential vorticity along rising isopycnal surfaces. Using δ3He released from the hydrothermal vents, the influence of volcanic activity on the SPBW may be traced across the South Pacific along the path of the ACC to Drake Passage. SPBW also flows within the southern limb of the Ross Gyre, reaching the Antarctic Slope in places and contributes via entrainment to the formation of Antarctic Bottom Water. Finally, it is shown that the magnitude and location of the potential vorticity signals associated with SPBW have endured over at least the last two decades, and that they are unique to the South Pacific sector.

Corresponding author address: Stephanie Downes, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia.E-mail: stephanie.downes@anu.edu.au

Abstract

This study uses potential vorticity and other tracers to identify the pathways of the densest form of Circumpolar Deep Water in the South Pacific, termed “Southwest Pacific Bottom Water” (SPBW), along the 28.2 kg m−3 surface. This study focuses on the potential vorticity signals associated with three major dynamical processes occurring in the vicinity of the Pacific–Antarctic Ridge: 1) the strong flow of the Antarctic Circumpolar Current (ACC), 2) lateral eddy stirring, and 3) heat and stratification changes in bottom waters induced by hydrothermal vents. These processes result in southward and downstream advection of low potential vorticity along rising isopycnal surfaces. Using δ3He released from the hydrothermal vents, the influence of volcanic activity on the SPBW may be traced across the South Pacific along the path of the ACC to Drake Passage. SPBW also flows within the southern limb of the Ross Gyre, reaching the Antarctic Slope in places and contributes via entrainment to the formation of Antarctic Bottom Water. Finally, it is shown that the magnitude and location of the potential vorticity signals associated with SPBW have endured over at least the last two decades, and that they are unique to the South Pacific sector.

Corresponding author address: Stephanie Downes, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia.E-mail: stephanie.downes@anu.edu.au
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