Hydraulic Control and Overturn Age in an Abyssal Channel
Glenn S Carter, University of Hawaii at Manoa, Oceanography, Honolulu, HI, United States, Lawrence J Pratt, WHOI, Woods Hole, United States, Matthew H Alford, Scripps Institution of Oceanography, UC San Diego, La Jolla, United States, Gunnar Voet, Scripps Institution of Oceanography, La Jolla, United States, James B Girton, Applied Physics Laboratory, University of Washington, Seattle, WA, United States, Shuwen Tan, Institute of Oceanology, Chinese Academy of Sciences, Qungdao, China, Kelly Pearson, University of Hawaii at Manoa, Honolulu, HI, United States and Jesse Cusack, Scripps Institution of Oceanography, La Jolla, CA, United States
Abstract:
The Samoan Passage is an important component of the Pacific Meridional Overturning Circulation, strongly influencing the North Pacific abyssal water properties. The deepest water exiting the northern end of the passage is 55 millidegrees Celsius warmer than the bottom water entering the passage. Although there are mixing hotspots throughout the passage, a significant proportion of the water mass modification occurs at the northern-most sill. As part of the Samoan Passage Abyssal Mixing Experiment two CTD/Lowered-ADCP tow-yo sections were conducted approximately 17 months apart (August 2012 and January 2014) over and downstream of this sill. During the tow-yos the CTD was cycled between 3500m depth and 40m above the seafloor as the ship traveled at 0.5 to 0.7 knots, which resulted in a horizontal spacing of approximately 250m. Both sections show similar flow features.
A composite Froude Number from a 2.5-layer model shows that the sill itself was hydraulically controlled. Sixteen kilometers downstream of the sill crest was another region of hydraulic control. This second control point is not associated with a sill or obvious constriction, and maybe due to a combination of change in topographic slope and entrainment of water from the layer above.
Microstructure casts were conducted concurrently with traditional (vertical) CTD casts both within and outside of the hydraulic jumps associated with these control points. The age of a turbulent event, i.e., location in a density overturn / collapse cycle, can be calculated from Thorpe and Ozmidov scales. The Thorpe scale, the rms particle displacement required to reach static stability, provides a quantitative length scale of an overturn. The Ozmidov scale represents the largest overturn unaffected by buoyancy. Profiles within the hydraulic jumps show a predominance of young overturns whereas profiles a short distance away show more mature overturns.
