The dynamics of mixed layer deepening during open ocean convection

Significance 

Convective processes in the open ocean involve the intermittent exchange of large volumes of water via deep-reaching convection that mainly occur in high latitude regions of the ocean. This fascinating yet intricate interplay of water is climatically vital to the Atlantic Meridional Overturning Circulation (AMOC), which is responsible for modulating global ocean heat content, carbon intake and global marine biological activity. Acknowledgment of the pivotal climatic role of convection demands an in-depth understanding of the dynamics of the open ocean convection process and a faithful representation of the same using credible scientific models. To this end, a number of observational, numerical and experimental studies have been conducted to investigate the processes at work during an open ocean convection event. As a result, three phases of open-ocean convection have been identified: preconditioning, deep convection and lateral exchange and spreading (see attached video). These phases illustrate that the process is driven by flow features that cover a large range of temporal and spatial scales. As a consequence, accurately characterizing open ocean convection and subsequent mixed layer deepening in large-scale ocean models is challenging. In particular, it is vital to identify the cause of the mixed layer depth bias during deep convection in ocean models.

Researchers tend to parameterize smaller-scale flow processes, particularly convection and baroclinic eddies, in models. In most instances, this parameterization of convection leads to the development of an anomalously deep mixed layer following deep convection in some global ocean models. In other words, many climate model simulations over-estimate mixed layer depth during open ocean convection, resulting in excessive formation of dense water in some regions. This has flow-on effects on the AMOC and global climate. To address this, scientists from the Research School of Earth Sciences and ARC Centre of Excellence for Climate Extremes at the Australian National University: Dr. Taimoor Sohail and Dr. Andrew McC. Hogg, in collaboration with Dr. Bishakhdatta Gayen at the University of Melbourne, Melbourne, Australia, proposed to compare the representation of convection in a non-hydrostatic turbulence-resolving simulation with convection in a hydrostatic, large-scale ocean model. Their work is currently published in Journal of Physical Oceanography.

In this work, the research team focused on examining the physical processes controlling transient mixed layer depth during open ocean convection using the two different numerical models. During this investigation, an isolated destabilizing buoyancy flux was imposed at the surface of both models and a quasi-equilibrium flow allowed to develop.

The authors reported that the mixed layer depth in the turbulence-resolving and large-scale models closely aligned with existing scaling theories. Moreover, they found that the large-scale model had an anomalously deep mixed layer prior to quasi-equilibrium. This transient mixed layer depth bias was seen to be a consequence of the lack of resolved turbulent convection in the model, which in turn delayed the onset of baroclinic instability.

In summary, the Australian study compared the representation of open ocean convection in a turbulence-resolving Direct Numerical Simulation (DNS) and large-scale ocean model (MOM6). It was revealed that in the DNS, the initial growth of the mixed layer was largely diffusive, and quickly gave way to vigorous, three-dimensional turbulent convective plumes which fluxed buoyancy vertically and created a deep mixed patch. Overall, the DNS results were seen to validate existing scaling theories while also underscoring the need to develop better representations of convection in large-scale ocean models. In a statement to Advances in Engineering, first author Dr. Taimoor Sohail clarified that their findings highlight the importance of turbulence in modulating key flow features during open ocean convection, and propose a way to improve the representation of turbulent convection in future modelling studies.

The three stages of open ocean convection in the ocean, as visualised by a high-resolution Direct Numerical Simulation. The 3D iso-surface corresponds to a fixed density in the model, coloured by depth. The top surface shows the horizontal velocity field, and the side plane shows the vertical velocity at the middle of the convective plume.

About the author

Taimoor Sohail is a Post-Doctoral Research Associate in the School of Mathematics and Statistics at the University of New South Wales (UNSW) in Sydney, Australia. Mr. Sohail completed his bachelor’s degree in Mechanical Engineering at Lafayette College, Easton, PA, USA, before moving on to complete a PhD in Earth Sciences at the Australian National University (ANU) in Canberra, Australia, supervised by Dr. Bishakhdatta Gayen and Dr. Andy Hogg. Mr. Sohail’s research interests lie at the intersection of fluid dynamics and ocean sciences. He is chiefly interested in quantifying the impact of small-scale turbulence and convection on large-scale flow structures in the global ocean, and the associated flow-on effects on the global climate. In pursuing this research problem, Mr. Sohail makes use of a combination of engineering computational fluid dynamical tools and large-scale oceanographic models. In particular, he has been exploring the utility of Direct Numerical Simulations (DNS) in quantifying oceanographic processes, including overturning circulation, mixing, and open ocean convection. Most recently, Mr. Sohail has been tying in ocean observations to supplement the knowledge obtained from idealised DNS and ocean modelling studies.

Reference

Sohail, T., B. Gayen, and A. McC. Hogg, 2020: The Dynamics of Mixed Layer Deepening during Open-Ocean Convection. J. Phys. Oceanogr., 50, 1625–1641, https://doi.org/10.1175/JPO-D-19-0264.1.

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