Collaborative Research: Global estimates of energy pathways and stirring by internal waves and vortical mode

The ocean primarily derives its energy from large-scale wind, tidal and solar forcing, but the ultimate distribution of heat and currents depends on how this energy is transferred from large-scale motions to small scales where it is dissipated. Some of the energy forced by winds at the ocean surface escapes to the interior through density undulations, called near-inertial waves, that can span hundreds of kilometers.

Another significant energy source for waves comes from the tide when it encounters topography and produces internal waves, known as the internal tide. It is generally well understood that these two types of waves interact, catalyzed by other flow features, and transfer energy through an internal wave field into smaller scale motions and mixing. This study will parameterize the spectrum of internal waves in terms of the large-scale forcing and other catalysts, and the predicted stirring that results, producing the first global maps of these estimates.

Parameterizations of these stirring processes will benefit large-scale ocean general circulation models (OGCMs), and a refined understanding and parameterization of the internal wave energy cascade and its implications for vertical mixing and dissipation will benefit climate models. Recognizing that submesoscale, OGCM and climate modelers at the same meeting often do not attend the same scientific sessions, the investigators will bring these two communities together by organizing a joint session at the 2024 Ocean Sciences meeting, focused on internal wave and submesoscale parameterizations in global models.

One graduate student will be trained under this project (UMassD). The ongoing efforts of the team demonstrates their commitment to education, outreach, diversity and inclusion. PI Sundermeyer currently advises (among others) five women graduate students, and will seek to recruit women and/or under- represented/minority students under this project.

PI Sundermeyer has given several presentations on ocean processes to multiple classes in the Sandwich, MA public school district. Under the present project, he and the graduate student will work with middle and high school teachers in Sandwich and New Bedford public schools to develop ocean-related learning modules. PI Early helped design and mentor in an NSF-REU program and co-founded the NWRA early-scientist mentoring program; PI Wortham is a Science Communication Fellow at Seattle Pacific Science Center; PI Lelong is a mentor with MPOWIR and active in the Seattle Chapter of SWMS.

NWRA participates every summer in Discovery Corps, the Pacific Science Center’s summer research program for high school and college students from under-represented communities. This proposal involves collaborations with OGCM modeler H. Simmons and with Mexican mathematician G. Hernandez-Duenas.

The internal wave field in the stratified interior of the ocean draws its energy primarily from winds and tides, with geostrophic motions and topographic scattering acting as catalysts. This energy cascades to small scales and directly stirs the fluid, generating vortical mode along the way, which then itself contributes to energy transfers and stirring.

Although observations indicate stirring rates of O(1) m2 s−1 at scales of O(10) km, gaps remain in our ability to predict diffusivity at these scales directly from the energy sources. The first major contribution of this study will be to clarify the roles of relevant processes in setting the shape and strength of the internal wave and vortical spectra.

Second, it will extend previous theoretical and numerical estimates of stirring from internal waves and vortical mode to more realistic conditions. By considering realistic stratification and forcing, this study will close a significant gap in our understanding of how the oceanic internal wave and vortical mode fields are formed, and how these processes stir fluid at the submesoscale.

The work here will also help close the energy budget for the ocean by quantifying the rate at which energy is extracted from various large-scale forcing, and cascaded downscale through the internal wave field. Last, it will quantify the shape and magnitude of the vortical mode field that arises naturally as part of this cascade, a result that has remained largely elusive from field observations.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.