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Abstract This study presents the linear theory of thermohaline‐shear instability, which is realized in oceanic flows that are dynamically and diffusively stable. The framework is based on the unbounded Couette model, which makes it possible to decouple the destabilizing effects of spatially uniform shear from instabilities caused by the presence of inflection points in velocity profiles. The basic state is assumed to be time dependent, which reflects the role of internal waves in controlling fine‐scale shear. Linear stability analysis suggests that conditions for thermohaline‐shear instability are met in most ocean regions where temperature and salinity concurrently increase downward. We conclude that thermohaline‐shear instability represents a plausible mechanism for the initiation of active diffusive convection, which, in turn, is essential for the formation of thermohaline staircases and maintenance of double‐diffusive interleaving.
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Initiation of diffusive layering by time-dependent shear
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.
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- Award ID(s):
- 1756491
- PAR ID:
- 10301708
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 858
- ISSN:
- 0022-1120
- Page Range / eLocation ID:
- 588 to 608
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jfm.2018.790
