Near-inertial waves, mesoscale eddies, and fronts are ubiquitous in the ocean. Classical theory predicts that the interaction between the fast, unbalanced waves and the slow, balanced eddies is usually weak. A new theory demonstrates, however, that this interaction can be strong in regions of frontogenesis, where mesoscale strain drives a cross-front ageostrophic circulation and rapidly intensifies thermal wind shear. This change in geostrophic flow modifies the polarization relation of near-inertial waves that are present, making their horizontal velocity rectilinear, and resulting in a Reynolds stress that draws kinetic energy from the eddies. The kinetic energy transferred from eddies is ultimately lost to the ageostrophic circulation, hence the near-inertial waves play a catalytic role in loss-of-balance. In the process the waves lose all of their energy. Scaling arguments based on a simple theoretical model for the interaction suggest that it could play a significant role in closing the global kinetic energy budgets for both near-inertial waves and eddies. To correctly assess the impact of this process on global energy balances, however, the physics of the mechanism must be understood without the simplifying assumption used in the model of a spatially homogeneous front and wave field. This project aims to do this using a hierarchy of hydrostatic and non-hydrostatic numerical simulations of spatially localized fronts and wave fields. Two dimensional simulations will be performed that are designed to study the modifications of the waves and isolate the wave-induced changes to the mean flow. These will not, however, allow the changes in mean flow to feedback on the wave dynamics. Three-dimensional hydrostatic simulations without this constraint will be used to investigate these feedbacks and quantify the wave-induced adjustments to the eddy kinetic energy. The ultimate fate of the kinetic energy lost from the wave and eddy fields to presumably small-scale turbulence will be investigated with high-resolution non-hydrostatic simulations.
Intellectual Merit: The theory that forms the basis of the proposed research merges studies of frontal dynamics, internal wave physics, and wave mean flow interactions, yielding rich new phenomena that may shed light on one of the fundamental problems in geophysical fluid dynamics of how kinetic energy is transferred from balanced to unbalanced motions and dissipated. At the same time, the work provides a mechanism for the removal of the kinetic energy in the near-inertial wave field, a problem that is not fully understood.
Broader Impacts: The proposed research tackles one of the outstanding questions in physical oceanography ? how the kinetic energy in the mesoscale is dissipated. This is important because how eddies lose their kinetic energy affects their properties, with consequences for the large-scale circulation and hence climate. The research points to a pathway where kinetic energy from eddies and internal waves drives mixing at fronts, with implications for nutrient fluxes, primary productivity, and water mass transformation. Insights from this study will guide the development of parameterizations for the dissipation of eddies and near-inertial waves for use in global circulation models. The project will be used to train a graduate student and includes mentoring of a postdoctoral researcher. The research results will be incorporated into lectures and outreach activities that provoke interest and fascination in the ocean circulation.