Modification of near-inertial waves by coupled air-sea interaction at the mesoscale and submesoscale

University Of Maryland, College Park, College Park MD

Investigators

Abstract

Near-inertial waves (NIWs) contain a significant fraction of the energy in the ocean internal wave field and form a prominent signal in near-surface ocean currents. Generated at the ocean surface by wind, their high vertical shear can modify the depth of the surface mixed-layer—a key constraint on the coupled large-scale ocean-atmosphere system—and they provide a pathway for transporting kinetic energy out of the surface ocean into the interior, where they are believed to play an important role in the maintenance of the abyssal circulation through mixing. Recent work has highlighted the sensitivity of NIWs to coupled air-sea interactions at the mesoscale and submesoscale, showing for instance dramatic reductions of near-inertial kinetic energy when surface currents are included in the calculation of the air-sea flux of momentum. However, fundamental gaps in understanding and quantification of the coupled processes remain—both in terms of the relevant physical mechanisms and in terms of the impact on ocean dynamics and energetics. The objective of this project is to use high-resolution coupled ocean-atmosphere models (both realistic and idealized), along with analysis of recent observations, to determine how coupled air-sea interactions modify the lifecycle of NIWs—from generation at the ocean surface to dissipation in the interior. In addition, outreach materials will be developed to address a current gap in science communication materials on NIWs for the general public. This will include development of an outreach video made available online and via a program that provides materials to museums and education centers across the country, and the development of a rotating table demonstration for use in teaching and outreach. The majority of the project budget will be used to support early-career scientists, including a postdoctoral researcher and a PhD student, both of whom will receive training on geophysical fluid dynamics and coupled ocean-atmosphere modeling. Finally, an undergraduate student will also receive training in physical oceanography and science communication during a summer project helping to design and implement the above-mentioned outreach materials. This research aims to improve our understanding of the coupled ocean-atmosphere system, with a specific focus on the generation and lifecycle of NIWs—a topic which touches aspects of oceanography ranging from biogeochemistry in the surface ocean to the slow abyssal overturning circulation. It is particularly timely given broader community efforts towards improved observations of near-surface currents and winds, where near-inertial variability is identified as a key target. Preliminary results highlight the need for improvement in both the conceptual understanding, and quantitative estimates, of coupled air-sea interaction effects on NIWs. For example, sea-surface temperature variability modifies surface winds, which is shown to provide a significant, previously unaccounted for, source of kinetic energy to NIWs. Likewise, coupled air-sea interactions introduce mesoscale and submesoscale variability into the surface wind-stress which in turn imprints on the spatial scales of NIWs. This leads to regions of strong horizontal divergence that generates inertial pumping and enhances the flux of kinetic energy into the interior. This project will therefore develop a more complete mechanistic understanding of how coupled air-sea interaction processes modify NIWs—extending classic conceptual models of air-sea interaction developed considering only the low-frequency balanced flow—while also quantifying the impact on both mixed-layer dynamics and interior mixing. 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.