Oceanography The Official Magazine of
The Oceanography Society
Volume 32 Issue 04

View Issue TOC
Volume 32, No. 4
Pages 194 - 203


A Spatial Geography of Abyssal Turbulent Mixing in the Samoan Passage

Glenn S. Carter Gunnar VoetMatthew H. AlfordJames B. GirtonJohn B. MickettJody M. KlymakLarry J. PrattKelly A. Pearson-PottsJesse M. CusackShuwen Tan
Article Abstract

High levels of turbulent mixing have long been suspected in the Samoan Passage, an important topographic constriction in the deep limb of the Pacific Meridional Overturning Circulation. Along the length of the passage, observations undertaken in 2012 and 2014 showed the bottom water warmed by ~55 millidegrees Celsius and decreased in density by 0.01 kg m–3. Spatial analysis of this first-ever microstructure survey conducted in the Samoan Passage confirmed there are multiple hotspots of elevated abyssal mixing. This mixing was not just confined to the four main sills—even between sills, the nature of the mixing processes appeared to differ: for example, one sill is clearly a classical hydraulically controlled overflow, whereas another is consistent with mode-2 hydraulic control. When microstructure casts were averaged into 0.1°C conservative temperature classes, the largest dissipation rates and diapycnal diffusivity values (>10–7 W kg–1 and 10–2 m2 s–1, respectively) occurred immediately downstream of the northern sill in the eastern and deepest channel. Although topographic blocking is the primary reason that no water colder than Θ = 0.7°C is found in the western channel, intensive mixing at the entrance sills appeared to be responsible for eroding an approximately 100 m thick layer of Θ < 0.7°C water. Three examples highlighting weak temporal variability, and hence suggesting that the observed spatial patterns are robust, are presented. The spatial variability in mixing over short lateral scales suggests that any simple parameterization of mixing within the Samoan Passage may not be applicable.


Carter, G.S., G. Voet, M.H. Alford, J.B. Girton, J.B. Mickett, J.M. Klymak, L.J. Pratt, K.A. Pearson-Potts, J.M. Cusack, and S. Tan. 2019. A spatial geography of abyssal turbulent mixing in the Samoan Passage. Oceanography 32(4):194–203, https://doi.org/10.5670/oceanog.2019.425.


Alford, M.H., R. Lukas, B.M. Howe, A. Pickering, and F. Santiago-Mandujano. 2011. Moored observations of episodic abyssal flow and mixing at station ALOHA. Geophysical Research Letters 38, L15606, https://doi.org/10.1029/2011GL048075.

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. Geophysical Research Letters 40(17):4,668–4,674, https://doi.org/10.1002/grl.50684.

Becker, J.J., D.T. Sandwell, W.H.F. Smith, J. Braud, B. Binder, J. Depner, D. Fabre, J. Factor, S. Ingalls, S.-H. Kim, and others. 2009. Global bathymetry and elevation data at 30 arc seconds resolution: Srtm30_plus. Marine Geodesy 32(4):355–371, https://doi.org/10.1080/01490410903297766.

Cusack, J.M., G. Voet, M. Alford, J.B. Girton, G.S. Carter, L.J. Pratt, K.A. Pearson-Potts, and S. Tan. 2019. Persistent turbulence in the Samoan Passage. Journal of Physical Oceanography, https://doi.org/10.1175/JPO-D-19-0116.1.

Dillon, T.M. 1982. Vertical overturns: A comparison of Thorpe and Ozmidov length scales. Journal of Geophysical Research 87(C12):9,601–9,613, https://doi.org/10.1029/JC087iC12p09601.

Freeland, H. 2001. Observations of the flow of abyssal water through the Samoa Passage. Journal of Physical Oceanography 31(8):2,273–2,279, https://doi.org/10.1175/1520-0485(2001)031​<2273:OOTFOA>2.0.CO;2.

Gourestski, V., and K. Koltermann. 2004. WOCE Global Hydrographic Climatology. Berichte des BSH 35(1–52).

Gregg, M. C. 1999. Uncertainties and limitations in measuring ε and χT. Journal of Atmospheric and Oceanic Technology 16(11):1,483–1,490, https://doi.org/10.1175/1520-0426(1999)016​<1483:UALIMA>2.0.CO;2.

Gregg, M.C. 2004. Small-scale processes in straits. Deep Sea Research Part II 51:489–503, https://doi.org/​10.1016/j.dsr2.2003.08.003.

Gregg, M.C., E.A. D’Asaro, J.J. Riley, and E. Kunze. 2018. Mixing efficiency in the ocean. Annual Review of Marine Science 10:443–473, https://doi.org/​10.1146/annurev-marine-121916-063643.

Kunze, E., J.F. Dower, I. Beveridge, R. Dewey, and K.P. Barlett. 2006. Observations of biologically generated turbulence in a coastal inlet. Science 313:1,768–1,770, https://doi.org/10.1126/science.1129378.

Lavelle, J.W., I.D. Lozovatsky, and D.C. Smith IV. 2004. Tidally induced turbulent mixing at Irving Seamount: Modeling and measurements. Geophysical Research Letters 31, L10308, https://doi.org/10.1029/2004GL019706.

McDougall, T.J., and P.M. Baker. 2011. Getting Started with TEOS-10 and the Gibbs Seawater Oceanographic Toolbox. SCOR/IAPSO WG127.

Moum, J.N., M.C. Gregg, R.-C. Lien, and M.E. Carr. 1995. Comparison of turbulence kinetic energy dissipation rate estimates from two ocean microstructure profilers. Journal of Atmospheric and Oceanic Technology 12(4):346–366, https://doi.org/10.1175/​1520-0426(1995)012<0346:COTKED>2.0.CO;2.

Oakey, N.S. 1982. Determination of the rate of dissipation of turbulent energy from simultaneous temperature and velocity shear microstructure measurements. Journal of Physical Oceanography 12:56–271, https://doi.org/10.1175/1520-0485(1982)​012<0256:DOTROD>2.0.CO;2.

Osborn, T.R. 1980. Estimates of the local rate of vertical diffusion from dissipation measurements. Journal of Physical Oceanography 10:83–89, https://doi.org/10.1175/1520-0485(1980)010​<0083:EOTLRO>2.0.CO;2.

Osborn, T.R., and W.R. Crawford. 1980. An airfoil probe for measuring turbulent velocity fluctuations in water. Chapter 19 in Air-Sea Interaction: Instruments and Methods. F. Dobson, L. Hasse, and R. Davis, eds, Plenum Press.

Pawlak, G., and L. Armi. 1997. Hydraulics of two-layer arrested wedge flows. Journal of Hydraulic Research 35(5):603–618, https://doi.org/​10.1080/​00221689709498397.

Peters, H., M.C. Gregg, and J.M. Toole. 1988. On the parameterization of equatorial turbulence. Journal of Geophysical Research 93(C2):1,199–1,218, https://doi.org/10.1029/JC093iC02p01199.

Polzin, K.L., J.M. Toole, J.R. Ledwell, and R.W. Schmitt. 1997. Spatial variability of turbulent mixing in the abyssal ocean. Science 276(5309):93–96, https://doi.org/10.1126/science.276.5309.93.

Pratt, L.J. 1984. On nonlinear flow with multiple obstructions. Journal of the Atmospheric Sciences 41(7):1,214–1,225, https://doi.org/10.1175/1520-​0469​(1984)041​<1214:ONFWMO>2.0.CO;2.

Pratt, L.J., G. Voet, A. Pacini, S. Tan, M.H. Alford, G.S. Carter, J.B. Girton, and D. Menemenlis. 2019. Pacific abyssal transport and mixing: Through the Samoan Passage versus around the Manihiki Plateau. Journal of Physical Oceanography 49(6):1,577–1,592, https://doi.org/​10.1175/JPO-D-18-0124.1.

Reid, J.L., and P.F. Lonsdale. 1974. On the flow of water through the Samoan Passage. Journal of Physical Oceanography 4(1):58–73, https://doi.org/10.1175/1520-0485(1974)004​<0058:OTFOWT>2.0.CO;2.

Roemmich, D., S. Hautala, and D.L. Rudnick. 1996. Northward abyssal transport through the Samoan passage and adjacent regions. Journal of Geophysical Research 101(C6):14,039–14,055, https://doi.org/10.1029/96JC00797.

Rudnick, D.L. 1997. Direct velocity measurements in the Samoan Passage. Journal of Geophysical Research 102(C2):3,293–3,302, https://doi.org/​10.1029/96JC03286.

Sánchez-Garrido, J.C., G. Sannino, L. Liberti, J. Garcia Lafuente, and L.J. Pratt. 2011. Numerical modeling of three-dimensional stratified tidal flow over Camarinal Sill, Strait of Gibraltar. Journal of Geophysical Research 116(C12), https://doi.org/​10.1029/2011JC007093.

Shay, T.J., and M.C. Gregg. 1986. Convectively driven turbulent mixing in the upper ocean. Journal of Physical Oceanography 16(11):1,777–1,798, https://doi.org/10.1175/1520-0485(1986)016​<1777:CDTMIT>2.0.CO;2.

Thorpe, S.A. 1977. Turbulence and mixing in a Scottish Loch. Philosophical Transactions of the Royal Society of London A 286:125–181, https://doi.org/​10.1098/rsta.1977.0112.

Voet, G., J.B. Girton, M.H. Alford, G.S. Carter, J.M. Klymak, and J.B. Mickett. 2015. Pathways, volume transport and mixing of abyssal water in the Samoan Passage. Journal of Physical Oceanography 45(2):562–588, https://doi.org/​10.1175/JPO-D-14-0096.1.

Voet, G., M.H. Alford, J.B. Girton, G.S. Carter, J.B. Mickett, and J.M. Klymak. 2016. Warming and weakening of the abyssal flow through Samoan Passage. Journal of Physical Oceanography 46(8):2,389–2,401, https://doi.org/​10.1175/​JPO-D-16-0063.1.

Waterhouse, A.F., J.A. MacKinnon, J.D. Nash, M.H. Alford, E. Kunze, H.L. Simmons, K.L. Polzin, L.C. St. Laurent, O.M. Sun, R. Pinkel, and others. 2014. Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate. Journal of Physical Oceanography 44(7):1,854–1,872, https://doi.org/​10.1175/JPO-D-13-0104.1.

Wesson, J.C., and M.C. Gregg. 1994. Mixing at Camarinal Sill in the Strait of Gibraltar. Journal of Geophysical Research 99(C5):9,847–9,878, https://doi.org/10.1029/94JC00256.

Winters, K.B. 2016. The turbulent transition of a supercritical downslope flow: Sensitivity to downstream conditions. Journal of Fluid Mechanics 792:997–1,012, https://doi.org/10.1017/jfm.2016.113.

Copyright & Usage

This is an open access article made available under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution, and reproduction in any medium or format as long as users cite the materials appropriately, provide a link to the Creative Commons license, and indicate the changes that were made to the original content. Images, animations, videos, or other third-party material used in articles are included in the Creative Commons license unless indicated otherwise in a credit line to the material. If the material is not included in the article’s Creative Commons license, users will need to obtain permission directly from the license holder to reproduce the material.