Thermohaline Staircases in a British Columbia Fjord

Abstract

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.

Staircase solutions and stability in vertically confined salt-finger convection

“…2008; Fer et al. 2010; Spear & Thomson 2012). Such structures are typical of tropical and subtropical regions where warm salty water often overlies cold fresh water and have been ascribed to the presence of salt-finger convection.…”

“…Oceanographic measurements have widely reported staircase-like structures with regions of nearly constant density in the vertical direction separated by interfaces with a sharp density gradient (Tait & Howe 1968, 1971Schmitt et al 1987Schmitt et al , 2005Padman & Dillon 1989;Muench, Fernando & Stegen 1990;Zodiatis & Gasparini 1996;Morell, Corredor & Merryfield 2006;Timmermans et al 2008;Fer et al 2010;Spear & Thomson 2012). Such structures are typical of tropical and subtropical regions where warm salty water often overlies cold fresh water and have been ascribed to the presence of salt-finger convection.…”

Approximations and Nondimensional Numbers in Physical Oceanography

“…In regions of the ocean where the vertical gradients of temperature and salinity have the same sign, this differential diffusivity can lead to the formation of sharply defined thermohaline "staircases" through the process of double diffusion (Turner, 1973;Kelley, 1990;Ruddick and Gargett, 2004;Spear and Thomson, 2012). In regions of the ocean where the vertical gradients of temperature and salinity have the same sign, this differential diffusivity can lead to the formation of sharply defined thermohaline "staircases" through the process of double diffusion (Turner, 1973;Kelley, 1990;Ruddick and Gargett, 2004;Spear and Thomson, 2012).…”