NSFGEO-NERC: Stimulated Loss of Balance

Recent results obtained by the PIs show that the advection and refraction of near-inertial waves (NIWs) by mesoscale flow is necessarily accompanied by a transfer of energy from the mesoscale to NIWs: the presence of externally forced NIWs stimulates a loss of energy from the geostrophically and hydrostatically balanced component of the flow. This process, stimulated loss of balance (SLOB), should be contrasted with the much more studied and weaker process known as spontaneous loss of balance. The spontaneous version occurs at order one Rossby number and without externally forced waves. But the stimulated version is active even at small Rossby number and hence, we hypothesize, throughout the ocean. The main objectives of this proposal are to assess and develop the hypothesis that SLOB plays a major role in the mesoscale energy budget, and to investigate SLOB by components of the internal wave spectrum other than NIWs, particularly the internal tide (IT). This will be achieved by the development of new, phase-averaged models coupling the dynamics of internal waves with that of the balanced mesoscale flow, and through numerical solution of both these models and of the three-dimensional Boussinesq equations. The outcome will be a quantitative understanding of the role played by SLOB in the ocean energy budget.

The ocean plays a key role in the Earth's climate through heat transport and carbon uptake. A quantitative understanding of ocean energetics is crucial to modeling its dynamics and predicting its future behaviour. In particular, the reliability of climate predictions depends on the ability of numerical models to accurately represent these pathways in current and future conditions. One outcome of the project will be a new understanding of the energy balance of the ocean and thus the proposed research will have an impact on climate models. Operational ocean models as well as climate models rely on parameterizations of the mixing associated with internal waves. Arguably, the wave feedback studied in this proposal should also be parameterized. Parameterizing either or both effects require a simplified representa- tion of the internal-wave dynamics, which is too fast to be resolved without imposing drastic timestep reduction. The phase-averaged models that we propose to derive represent a first step towards this simplified representation. They provide a rigorously derived foundation for parameterizations (which require further, heuristic simplifications for practical implementation, such as representing the entire wave spectrum by a few discrete components). Thus, by supporting the development of new, improved parameterizations, the proposed research can contribute to the improvement of ocean models and benefit their end-users including the shipping, fishing and energy industries. The project includes the modeling of the propagation of ITs in mesoscale flows. While this modeling is motivated by SLOB, it can have a strong impact on observational oceanography. Specifically, one of the obstacles to the accurate inference of sea-surface velocity from satellite altimetry is the (strongly aliased) tidal signal, especially when this signal loses spatial coher- ence as a result of interaction with and scattering by ocean macroturbulence. Overcoming this obstacle is a pressing need with the forthcoming SWOT mission.