Thermohaline Staircase

TAIT, R. I.; Howe, M.R.

doi:10.1038/231178a0

Cited by 89 publications

(33 citation statements)

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“…They are ubiquitously associated with active fingering convection, and have been observed both in laboratory experiments (Stern & Turner 1969;Krishnamurti 2003Krishnamurti , 2009) and in oceanic field measurements (Tait & Howe 1971;Schmitt et al 2005). Oceanic staircases are longlived, and can span the entire thermocline, with individual layer heights ranging from 10m-100m and horizontal extents of km-size or more.…”

Section: Introductionmentioning

confidence: 99%

Dynamics of fingering convection. Part 2 The formation of thermohaline staircases

et al. 2011

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Regions of the ocean's thermocline unstable to salt fingering are often observed to host thermohaline staircases, stacks of deep well-mixed convective layers separated by thin stably-stratified interfaces. Decades after their discovery, however, their origin remains controversial. In this paper we use 3D direct numerical simulations to shed light on the problem. We study the evolution of an analogous double-diffusive system, starting from an initial statistically homogeneous fingering state and find that it spontaneously transforms into a layered state. By analysing our results in the light of the mean-field theory developed in Paper I, a clear picture of the sequence of events resulting in the staircase formation emerges. A collective instability of homogeneous fingering convection first excites a field of gravity waves, with a well-defined vertical wavelength. However, the waves saturate early through regular but localized breaking events, and are not directly responsible for the formation of the staircase. Meanwhile, slower-growing, horizontally invariant but vertically quasi-periodic γ-modes are also excited and grow according to the γ−instability mechanism. Our results suggest that the nonlinear interaction between these various mean-field modes of instability leads to the selection of one particular γ−mode as the staircase progenitor. Upon reaching a critical amplitude, this progenitor overturns into a fully-formed staircase. We conclude by extending the results of our simulations to real oceanic parameter values, and find that the progenitor γ−mode is expected to grow on a timescale of a few hours, and leads to the formation of a thermohaline staircase in about one day with an initial spacing of the order of one to two metres.

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Section: Introductionmentioning

confidence: 99%

Dynamics of fingering convection. Part 2 The formation of thermohaline staircases

et al. 2011

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“…(approximate values near 35 0/00, 15 0 C) (Riley and Skirrow, 1965) Unfortunately the detailed temperature and salinity structure within salt fingers is not known; but since the difference of temperature and salinity across a thermocline interface may be, for example, 0.2*C and 0.03 0/00 (Tait and Howe, 1971), it is clear that the structure of the index of refraction in a salt finger field may have more inflection points than the salinity field. At best this may lead to an incorrect estimate of the diameter of the fingers; but, since the temperature must vary vertically as well as horizontally, it may result in a very confusing three-dimensional structure that would not be interpreted as fingers at all.…”

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confidence: 99%

Oceanic microstructure observed near Bermuda using a towed sensor

Magnell¹

1973

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Many hypotheses have been advanced to explain the formation of mixed layers in the ocean; the salt finger type of double-diffusive convection, in particular, has received much attention. Because of their uniquely ordered nature, salt fingers should be readily identifiable in the deep thermocline, if in fact they exist there. A relatively limited experiment could thus produce a definitive evaluation of the importance of salt finger convection in the ocean, at least in certain places and at certain times. Such an evaluation, which would help to direct future work on oceanic microstructure, was the primary objective of this thesis.A secondary and more general objective was to measure the intermittency of mixing events in the ocean; and also, by measuring the RMS gradients of temperature and salinity in such events, to evaluate directly the intensity of vertical mixing.Since a horizontally-towed sensor is essential if salt fingers are to be observed directly, a new instrument has been designed and built which can resolve fractional-centimeter structures of electrical conductivity while being towed at speeds of several knots. The design of this instrument is described in this report.The instrument has been towed in deep water near Bermuda. It is believed that salt fingers were observed in the main thermocline on several occasions; but they were so rare as to be negligible in the total vertical mixing. An analysis of one such possible salt finger event is presented.Numerous other small-scale fluctuations of electrical conductivity were observed, which can be reconciled only with a turbulent model. Intermittency statistics for these events are presented. The eddy diffusivity has been calculated from the data to be approximately 0.075 cm 2 /sec. Thesis Supervisor: Henry M. Stommel

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“…Mixed layer steps associated with diffusive convection are typically an order of magnitude thinner than those associated with salt-fingering (Schmitt, 1994;Merryfield, 2000;Ruddick and Gargett, 2004). Double diffusion is observed throughout the world's oceans including diffusive convection in the Weddell Sea in the Southern Ocean (Foster and Carmack, 1976) and the Canada Basin of the Arctic Ocean (Timmermans et al, 2003) and saltfingering in the western tropical North Atlantic (Mazeika, 1974;Boyd and Perkins, 1987), the Mediterranean outflow region of the North Atlantic (Tait and Howe, 1971), and the western Mediterranean (Johannessen and Lee, 1974). Schmid et al (2010) also report double-diffusive staircases in Lake Kivu located on the border between Rwanda and the Democratic Republic of the Congo.…”

Section: Introductionmentioning

confidence: 99%

Thermohaline Staircases in a British Columbia Fjord

Spear

Thomson

2012

Atmosphere-Ocean

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We present the first documented evidence of thermohaline staircases generated by double-diffusion in a coastal environment. Results are based on high (centimetre scale) resolution profiles of temperature, salinity and dissolved oxygen concentration collected on 20 July 2009 at seven stations along the axis of Belize Inlet on the mainland coast of British Columbia, Canada (Fig. 1). Except for stations nearest the narrow Pacific Ocean entrance to the inlet, the observed temperature and salinity gradients between 70 and 210 m depth were of the same sign, therefore conducive to double-diffusive phenomena. Thermohaline features were especially well developed near the head of the inlet. Salt-fingering staircases of approximately 10 m thickness were observed above the temperature and salinity minima at 150 m depth and diffusive-convection staircases of approximately 1 m thickness were observed below this depth. We speculate that conditions favourable to double diffusion were initiated by a mid-depth intrusion of relatively cold, low salinity, oxygen-rich water that likely originated with intense tidal mixing in the narrow oceanic passage leading to the inlet. Findings confirm the existence of double diffusion in coastal oceanic regions and indicate that Belize Inlet may be a natural laboratory for the study of thermohaline processes in the ocean.

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Thermohaline Staircase

Cited by 89 publications

References 0 publications

Dynamics of fingering convection. Part 2 The formation of thermohaline staircases

Dynamics of fingering convection. Part 2 The formation of thermohaline staircases

Oceanic microstructure observed near Bermuda using a towed sensor

Thermohaline Staircases in a British Columbia Fjord

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