Modification of Antarctic Origin Bottom Water in the Samoan Passage by Turbulent Mixing
Glenn S Carter1, Matthew H Alford2, James B Girton3, Gunnar Voet4, John Mickett3 and Jody M Klymak5, (1)University of Hawaii at Manoa, Oceanography, Honolulu, HI, United States, (2)Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, United States, (3)University of Washington, Applied Physics Laboratory, Seattle, WA, United States, (4)Scripps Institution of Oceanography, La Jolla, CA, United States, (5)University of Victoria, Victoria, BC, Canada
Abstract:
The majority of the coldest bottom water entering the abyssal North Pacific passes through the Samoan Passage (169 W, 10 S), making this 100 km wide passage a key 'choke point' in the Pacific Overturning Circulation. Approximately 2 Sv of Antarctic origin bottom water (AABW) with a potential temperature (referenced to the surface) of less than 0.7 degree enters the passage, and none exits. The Samoan passage consists of two channels with most of this coldest water following the eastern channel, as the entrance to the more convoluted western channel is partially blocked by two sills. The eastern channel has a ~500-m high sill half way along and a ~300-m high sill at near the exit. In the eastern channel most of the mixing away of the <0.7 degree occurs just downstream of the second sill.
During cruises in 2012 and 2014 we took the first direct microstructure measurements in the Samoan Passage (116 in total). In the eastern channel away from the sills the highest turbulent kinetic energy dissipation rates occurred in a 400-500m thick layer of water colder than 0.7 degrees. Over the sills dissipation rates in the of waters between 0.7 and 1.0 degrees approached to 1e-7 W/kg. Dissipation rates over the western channel sills reaches 1e-8 W/kg for waters between 0.7 and 0.8 degrees (topographic blocking prevents <0.7 degree water from entering the western channel). Repeated microstructure casts and Thorpe scale estimate from shipboard CTDs and moorings indicate that the general spatial pattern and magnitude of dissipation rates profiles are fairly constant in time. Tidal modulation appear to be small compared to the northward velocity of the AABW.
