NSFGEO-NERC: Energy transfer between submesoscale vortices and resonantly-forced inertial motions in the northern Gulf of Mexico

Submesoscale flows and near-inertial motions are ubiquitous features of the upper ocean. Recently developed theories have posited that the interaction of submesoscale flows and near-inertial motions could play an important role in closing the energy budgets of both the balanced circulation and the unbalanced waves. These theories have yet to be fully tested in the field due to the challenges of observing both types of motions in a relatively controlled setting. The northern Gulf of Mexico is a natural laboratory that is ideal for doing this. Here, the Mississippi-Atchafalaya river plume forms a rich field of submesoscale eddies and fronts, which in the summer are driven by a land-sea breeze that forces inertial motions at near resonance.

The proposed research will involve intensive field campaigns utilizing novel observational techniques that will be closely integrated with idealized and realistic numerical simulations to study the interaction of submesoscale eddies and near-inertial motions in the Mississippi-Atchafalaya river plume. The objective will be to characterize, quantify, and understand the energy exchanges between balanced and unbalanced motions and the turbulent cascade that can result from the interaction. More specifically, the numerical and observational experiments will be designed to test the hypotheses that periodic straining of isopycnals by inertial motions in fronts, subduction of low potential vorticity water, and the propagation, trapping, and reflection of near-inertial waves interacting with submesoscale eddies facilitate the energy exchange and result in enhanced mixing throughout the water column.

The proposed research tackles one of the outstanding questions in physical oceanography of how the kinetic energy in the balanced circulation is dissipated, in particular through wave-mean flow interactions. This question has been studied mostly theoretically for idealized flow configurations, but it has not been explored observationally in a controlled setting, as proposed here. The research will integrate observations with submesoscale-resolving simulations and large-eddy simulations, a combination that has proven to be effective in understanding the fundamental physics of complex, multi- scale observed flows.

Our results have both global importance in understanding fundamental physics important in the upper ocean worldwide, and concrete coastal implications for improved predictability of nearshore mixing and its biological implications. Submesoscale processes can significantly affect the stratification and heat content of the upper ocean, and thus play an important role in air-sea fluxes. As a result, they could influence the strength of monsoons and hurricane formation and intensification - the latter being of particular concern in this region in light of the unprecedented devastation from Hurricane Harvey last year. In terms of biogeochemistry, there is recent, compelling evidence that submesoscale features on the Texas-Louisiana shelf break up summertime bottom hypoxia, making the field patchy and transient (DiMarco et al., 2010; Zhang and Hetland, 2018). Altering the submesoscale through interactions with near-inertial motions field will also change dispersion relevant to oil spills (e.g., Marta-Almeida et al., 2013), and cross-shore transport and transformation of river-borne materials (e.g., McKee et al., 2004).

This project will train and support two graduate and two postdocs and involves an international collaboration with a PI at Cambridge University in the UK, funded by NERC. The breadth of this project (spanning from high-tech autonomy through physics and the environment) is naturally appealing to many undergraduate and high-school students. We plan to provide research experiences to 2-3 undergraduates through the TAMU Summer Research Program for Undergraduates, for which applications from underrepresented groups in the Earth Sciences and from local community colleges will be especially encouraged. We will engage regional schools that serve underrepresented groups through visits and seminars. Locally, we will use UT Rio Grand Valley - a Hispanic Serving Institution - as a model for this, with the assistance of Chip Brier in the School of Earth, Environmental, and Marine Sciences at UTRGV (see attached letter). We plan to expand this to engage students from Texas A&M at Prairie View, a Historically Black University. An additional six undergraduate students and two high-school teachers will be recruited through OSU to participate in field work and this project will be made available for senior-year engineering students' theses.