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Initiation of diffusive layering by time-dependent shear

Published online by Cambridge University Press:  06 November 2018

Justin M. Brown Open the ORCID record for Justin M. Brown [Opens in a new window]  and
Timour Radko Open the ORCID record for Timour Radko [Opens in a new window]
Justin M. Brown*
Affiliation:
Department of Oceanography, Naval Postgraduate School, 1 University Circle, Monterey, CA 93943, USA
Timour Radko
Affiliation:
Department of Oceanography, Naval Postgraduate School, 1 University Circle, Monterey, CA 93943, USA
*
†Email address for correspondence: [email protected]
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Abstract

The Arctic halocline is generally stable to the development of double-diffusive and dynamic instabilities – the two major sources of small-scale mixing in the mid-latitude oceans. Despite this, observations show the abundance of double-diffusive staircases in the Arctic Ocean, which suggests the presence of some destabilizing process facilitating the transition from smooth-gradient to layered stratification. Recent studies have shown that an instability can develop in such circumstances if weak static shear is present even when the flow is dynamically and diffusively stable. However, the impact of oscillating shear, associated with the presence of internal gravity waves, has not yet been addressed for the diffusive case. Through two-dimensional simulations of diffusive convection, we have investigated the impact of the magnitude and frequency of externally forced oscillatory shear on the thermohaline-shear instability. Simulations with stochastic shear – characterized by a continuous spectrum of frequencies from inertial to buoyancy – indicate that thermohaline layering does occur due to the presence of destabilizing modes (oscillations of near the buoyancy frequency). These simulations show that such layers appear as well-defined steps in the temperature and salinity profiles. Thus, the thermohaline-shear instability is a plausible mechanism for staircase formation in the Arctic and merits substantial future study.

JFM classification

Convection: Double diffusive convection Geophysical and Geological Flows: Internal waves Geophysical and Geological Flows: Ocean processes
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 858 , 10 January 2019 , pp. 588 - 608
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Copyright
© 2018 Cambridge University Press 

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