Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage

Abstract

[1] We report the first direct turbulence observations in the Samoan Passage (SP), a 40 km wide notch in the South Pacific bathymetry through which flows most of the water supplying the North Pacific abyssal circulation. The observed turbulence is 1000 to 10, 000 times typical abyssal levels -strong enough to completely mix away the densest water entering the passage-confirming inferences from previous coarser temperature and salinity sections. Accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence. Specifically, the flow accelerates substantially at the primary sill within the passage, reaching speeds as great as 0.55 m s -1 . A strong hydraulic response is seen, with layers first rising to clear the sill and then plunging hundreds of meters downward. Turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill. In addition to the primary sill, other locations along the multiple interconnected channels through the Samoan Passage also have an effect on the mixing of the dense water. In fact, quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers, suggesting that abyssal mixing depends sensitively on bathymetric details on small scales. Citation: Alford, M. H., J. B. Girton, G. Voet, G. S. Carter, J. B. Mickett, and J. M. Klymak (2013), Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage, Geophys. Res. Lett., 40,[4668][4669][4670][4671][4672][4673][4674]

A new genus and species of Phtisicidae (Crustacea: Amphipoda) from abyssal depths in the Japan Trench, with special reference to similarities with Southern Ocean genera

“…This current is remarkably fast for abyssal depths, exceeding 20 cm/s at about 3500 m depth. Alford et al (2013) o C spread over the North Pacific from around the Samoan Passage to the Japan Trench at 5000 m depth, whereas the deep water of the North Atlantic at the same abyssal depth is warmer, more than 2 o C (see Yasuhara and Danovaro, 2014: fig. 1F).…”

Turbulent Mixing in a Deep Fracture Zone on the Mid-Atlantic Ridge

“…The role of such passages is particularly relevant in the Brazil Basin, which, for AABW, is connected to other ocean basins via five confined channels (Hogg et al 1996). Similarly, interbasin passages are known to affect the transformation of deep-water masses in the Pacific (Alford et al 2013;Voet et al 2015) and Indian Oceans (MacKinnon et al 2008). The BBTRE canyon is not an interbasin passage, however, as it terminates at the MAR crest (Thurnherr et al 2005).…”

Turbulence and finestructure in a deep ocean channel with sill overflow on the mid-Atlantic ridge

“…In contrast, studies in channels with sills report particularly elevated mixing rates and large horizontal density gradients associated with the flow over the sills (Polzin et al, 1996;Ferron et al, 1998;St. Laurent and Thurnherr, 2007;Alford et al, 2013). St.…”