Shaken and Stirred

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December 7, 2017 by Helen Hill

Surface buoyancy field from an MITgcm simulation in which convection and baroclinic instability coexist – image courtesy: J. Callies

story by Helen Hill


Researchers at the Massachusetts Institute of Technology have been using MITgcm to model the interplay between vertical convection and lateral exchange due to baroclinic instability.

Jörn is Assistant Professor for Environmental Science and Engineering in the Division of Geological and Planetary Sciences at Caltech. He holds a M.Sc. degree from the University of Hamburg, Germany, and a Ph.D. from the MIT–WHOI Joint Program in Oceanography. He has been using MITgcm since 2014.

Jörn Callies is an Assistant Professor for Environmental Science and Engineering in the Division of Geological and Planetary Sciences at Caltech. He holds a M.Sc. degree from the University of Hamburg, Germany, and a Ph.D. from the MIT–WHOI Joint Program in Oceanography.

Baroclinic mixed-layer instabilities have recently been recognized as an important source of submesoscale energy in deep winter mixed layers. Atmospheric cooling and surface winds result in mixing in the surface layer of the ocean. This mixing acts to mediate the transfer of heat and momentum between the atmosphere and ocean, feeding back on both the atmospheric climate and the oceanic general circulation.

Traditionally, oceanographers have looked at the evolution of the ocean mixed layer in a columnwize manner; atmospheric cooling and wind forcing leading to vertical mixing which together deepen the mixed layer into the thermocline below. Increasingly, however, it is clear that lateral exchanges contribute crucially to the dynamical balances of the mixed layer.

In their new study in the Journal of Physical Oceanography, Jörn Callies (now an assistant professor at Caltech) and his former PhD advisor Raf Ferrari (MIT) present idealized numerical simulations using MITgcm that allow the simultaneous development of both baroclinic instability and convective small-scale turbulence.

In the animation below, we see the surface buoyancy field for a moderately forced experiment. The side length of the domain is ten mixed layer depths (on the order of a kilometer.) The forcing consists of a cooling at the surface and a warming at the base of the mixed layer. In addition, there is a horizontal buoyancy gradient to allow for baroclinic instability to develop. Initially convective plumes can be seen rising up and hitting the surface — these are caused by the buoyancy forcing. A while later, however, a larger-scale baroclinic mode emerges, that grows and develops into a baroclinic eddy that generates sharp fronts and organizes the convection. The end state is a coexistence of baroclinic and convective turbulence.

Baroclinic instability in the presence of convection – animation credit: J. Callies

“The simulations were intended to mimic winter conditions in the surface ocean,” says Jörn. “Observations, models, and theory suggest that submesoscale baroclinic instabilities in the mixed layer are particularly strong in winter, when mixed layers are deep. But it has so far been unclear whether and how the slow baroclinic instabilities (time scale of a few days) interact with the much faster and smaller-scale mixed-layer turbulence that is generated by atmospheric forcing. It has previously been suggested that baroclinic instabilities only occur when the atmospheric forcing has subsided and mixed layer turbulence is weak. What these simulations show is that instead slow baroclinic and fast convective turbulence can happily coexist. That means that submesoscale baroclinic instabilities should be able to grow throughout the winter, even when atmospheric forcing is very strong, and thus be more pervasive than previously thought. This might be important for the ocean’s ability to take up heat and carbon from the atmosphere because the baroclinic eddies generated by the instability can exchange water between the mixed layer and the interior — and thereby subduct heat and carbon anomalies into the interior.”

To find out more about this work contact Jörn

This Month’s Featured Publication

Other New Publications this Month

Arnesen, Eirin (2017), On the Late Permian Thermohaline Circulation – A Study of the Ocean Circulation and its Relation to the Permian-Triassic Extinction, University of Oslo Masters Thesis

Igal Berenshtein, Claire B Paris, Hezi Gildor, Erick Fredj, Yael Amitai, Omri Lapidot, MosheKiflawi (2018), Auto-correlated directional swimming can enhance settlement success and connectivity in fish larvae, Journal of Theoretical Biology, Volume 439, 14 February 2018, Pages 76-85, doi: 10.1016/j.jtbi.2017.11.009

Vamsi K Chalamalla, Edward Santilli, Alberto Scotti, Masoud Jalali, Sutanu Sarkar (2017), SOMAR-LES: A framework for multi-scale modeling of turbulent stratified oceanic flows, Ocean Modelling, Volume 120, December 2017, Pages 101-119, doi: 10.1016/j.ocemod.2017.11.003

Fenwick C.Cooper (2017), A potential method to accelerate spin up of turbulent ocean models, Ocean Modelling Volume 120, December 2017, Pages 79-82, doi: 10.1016/j.ocemod.2017.10.008

Gulliver, Larry T. (2017), Direct and remote effects of topography and orientation, and the dynamics of mesoscale eddies, Monterey, California: Naval Postgraduate School, Masters Thesis

Satoshi Kimura, Adrian Jenkins, Heather Regan, Paul R. Holland, Karen M. Assmann, Daniel B. Whitt, Melchoir Van Wessem, Willem Jan van de Berg, Carleen H. Reijmer, Pierre Dutrieux (2017), Oceanographic Controls on the Variability of Ice-Shelf Basal Melting and Circulation of Glacial Meltwater in the Amundsen Sea Embayment, Antarctica, Journal of Geophysical Research Oceans, doi: 10.1002/2017JC012926

Benjamin Rurik Syquia Ocampo (2017), Numerical Simulations of Anelastic and Boussinesq Rotating Convection with Radial Entropy Gradient Boundary Conditions, University of Alberta, Department of Physics, Masters Dissertation

Oded Padon and Yosef Ashkenazy (2017), Interactive comment on “Non-hydrostatic effects in the Dead Sea” by Oded Padon and Yosef Ashkenazy, Ocean Sci. Discuss., doi: 10.5194/os-2017-29-AC2

Yan Wang, Andrew L. Stewart (2018), Eddy dynamics over continental slopes under retrograde winds: Insights from a model inter-comparison, Ocean Modelling
Volume 121, January 2018, Pages 1-18, doi: 10.1016/j.ocemod.2017.11.006

Do you have news about research using MITgcm? We are looking for contributions to these pages. If you have an interesting MITgcm project (ocean, atmosphere, sea-ice, physics, biology or otherwise) that you want to tell people about, get in touch. To make a post, contact Helen