A Possible Origin Story for Layers in the Arctic Ocean

Significance 

Diffusive convection contributes substantially to heat transport in the Arctic Ocean. Specifically, a region of active diffusive layering in the Arctic halocline separates the warm North Atlantic water from the colder waters of Pacific origin in the Canadian Basin and from the polar mixed layer above them.

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. Regardless, direct observations have shown 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. In addition, recent publications 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.

Recently, Dr. Justin Brown and Professor Timour Radko from the Department of Oceanography, Naval Postgraduate School in California, presented a study where they focused on investigating the impact of the magnitude and frequency of externally forced oscillatory shear on water with Arctic stratification by using two-dimensional simulations of diffusive convection. They hoped to confirm the relevance of the thermohaline-shear mechanism to the problem of layering in the context of oscillatory shear. Their work is currently published in the Journal of Fluid Mechanics.

The authors derived the equations responsible for governing their physical system. They then described the numerical considerations, including the numerical scheme, resolution tests and typical evolution of the simulations. Lastly, they engaged in simulations where they focused on capturing the early growth, quasi-equilibrium and late-time behavior, respectively, of the subject matter.

Simulations with stochastic shear – characterized by a continuous spectrum of frequencies from inertial to buoyancy – indicated that thermohaline layering occurred due to the presence of destabilizing modes (oscillations of near the buoyancy frequency). Furthermore, the simulations showed that such layers appeared as well-defined steps in the temperature and salinity profiles. In other words, the two researchers found out that the thermohaline-shear instability could be triggered by time-dependent shear.

In summary, the relevance of the thermohaline-shear mechanism to the problem of layering, the stability analysis that has to be reproduced for oscillatory shear, has been confirmed. Simulations of time-dependent shear in a fluid stable to the formation of double-diffusive instabilities showed that three major regimes are possible for shear flows with Richardson numbers greater than unity. However, one caveat of this work is that it considered only horizontal motion for simplicity, but very rapid oscillations tend to slope in the ocean . Nevertheless, they identified a plausible explanation for how staircases can develop for representative oceanographic parameters at high latitudes. Altogether, the thermohaline-shear instability, as presented by Brown and Radko is a plausible mechanism for staircase formation in the Arctic and merits substantial future study.

About the author

Justin Brown is post-doctoral researcher in the Oceanography Department at the Naval Postgraduate School on a fellowship from the National Research Council, working under the guidance of Professor Timour Radko. He researches heat transport in the Arctic and at mid-latitudes to improve ocean modeling for climatologists. He received his Bachelor’s degree in astrophysics from Franklin and Marshall College in 2011 and his PhD in astrophysics with a focus on fluid dynamics from the University of California, Santa Cruz in 2016.

Justin has focused on the study of small-scale processes in both stars and in the ocean that strongly affect the transport of heat and chemicals with simulations of fluids run on some of the largest supercomputers in the world. Outside of work, Justin has been a frequent volunteer for scientific outreach, bringing telescopes and volunteers to elementary schools and museums and giving public talks about the evolution of stars and other topics in astronomy.

About the author

Timour Radko teaches courses in ocean dynamics, circulation analysis and wave motion at the Department of Oceanography of the Naval Postgraduate School. Previously, he worked as a research scientist at the Department of Earth, Atmospheric and Planetary Sciences (EAPS) at the Massachusetts Institute of Technology. He has been active in the area of double-diffusive convection for over twenty years and was closely involved in developing the theory surrounding this topic. Professor Radko has authored numerous papers on physical oceanography and fluid mechanics, and has received the prestigious NSF CAREER award in 2006, the NPS Merit Award for Research in 2008, and the Schieffelin (2010) and Griffin (2011) Awards for Excellence in Teaching. He received his PhD in oceanography from Florida State University in 1997.

Reference

Justin M. Brown, Timour Radko. Initiation of diffusive layering by time-dependent shear. Journal of Fluid Mechanics (2019), volume 858, page 588–608.

Go To Journal of Fluid Mechanics

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