ENVIRONMENT AND ENERGY

The SMOC (SubMesoscale Ocean Modelling for Climate) project aimed to shed light on the role of submesoscale turbulent processes in the overall functioning of the ocean. Leveraging HPC power, the researchers in particular tried to get answers to: A) how deep do submesoscale fronts penetrate and can they be a significant source of dissipation for the ocean circulation away from the surface?, and B) to which extent do submesoscale fronts participate in the transfer into the deep ocean of the near-inertial energy injected by the wind at the ocean surface?

Scientific context

As of now, many aspects of the ocean energetics remain poorly understood. One enduring conundrum concerns the energetic equilibration of the general large-scale circulation, which is made of well-known currents such as the Gulf Stream or the Antarctic Circumpolar Current ... In a nutshell, large amounts of energy injected into the ocean by the atmosphere drive flows with very large horizontal scales (1000 km or more, see Figure 1). To remain near equilibrium, the large-scale circulation must dissipate energy at roughly the same rate it receives it. On the other hand, dissipation is achieved through viscous forces acting on fluid motions with centimeter scales or less. To bridge the scale gap, a cascade of processes must transfer energy from large scales down to fine scales.

Many aspects of this downscale energy transfer are not known and even the dominant processes are not clearly identified. Because dissipation of the ocean circulation is a key source of water mass mixing, this issue has vast implications for Earth climate modelling.

Project objectives

The SMOC (SubMesoscale Ocean Modelling for Climate) project aimed to shed light on the role of submesoscale turbulent processes in the overall functioning of the ocean. Near the ocean surface, most flows spontaneously produce submesoscale density fronts whose typical horizontal scales are in the range of 1 to 10 km (figure 1 and 2). These fronts induce substantial energy dissipation and mixing. They are the analogous of atmospheric fronts present in weather systems and have been a subject of great research interest for the past decade.

The main SMOC research questions were: how deep do submesoscale fronts penetrate and can they be a significant source of dissipation for the ocean circulation away from the surface (energy pathway 1, see Fig 1)? And, to which extent do submesoscale fronts participate in the transfer into the deep ocean of the near-inertial energy injected by the wind at the ocean surface (energy pathway 2)?

Computational details

Simultaneously resolving submesoscale frontal turbulence and its accompanying large-scale environment is computationally expensive. Ocean models have not been widely used in the submesoscale regime so the sensitivity of their behaviour to different choices of numerical schemes and parameters also need to be explored. This made the SMOC project challenging and computationally demanding.

Thanks to a PRACE allocation on HPC system Hermit (then Hazel Hen) of HLRS, the SMOC investigators were able to run a large number of “submesoscale rich” simulations in two different settings suited to explore and quantify pathway 1 and 2 respectively: 1) an idealized setting (figure 2) in which the ocean is forced as smoothly as possible so that the spontaneous emergence of submesoscale turbulence and its interaction with the large-scale circulation are unperturbed by other processes, 2) a more realistic setting in which the ocean circulation and its submesoscale turbulence interact with the circulation produced by atmospheric storms.

Main results

Careful analyses of over 50 long-term simulations carried out in the course of the project have revealed the smallness of the energy dissipation achieved by submesoscale turbulence into the ocean interior (pathway 1). They have, on the other hand, revealed the importance of submesoscale flows for the propagation of oscillatory motions excited near the ocean surface by atmospheric storms into the ocean interior (pathway 2).

The vertical distribution of near-inertial dissipation diagnosed in the SMOC simulations will now be used to constraint the parameterization of this process in NEMO, the ocean module of most european climate systems.