Collaborative Research: Tracing the Physics of Submesoscale Entrainment and Subduction

Johns Hopkins University, Baltimore MD

Investigators

Abstract

This project will investigate the exchange between the well-mixed surface layer of the ocean and the stratified interior at the so-called submesoscales; horizontal length scales of hundreds to thousands of meters, where the rotation of the Earth is still felt but is not the dominant factor like it is at larger scales. This exchange is widely recognized as one of the primary mechanisms through which submesoscale processes affect upper ocean variability and biogeochemistry. However, despite extensive work demonstrating the impacts of submesoscale entrainment and subduction on diverse topics across the field of oceanography— including for example the global mixed-layer heat budget, primary productivity, and the export of carbon to the interior—the essential physical mechanisms remain incompletely described. In particular, next steps need to include the impact of the submesoscale interactions between the surface boundary layer and the interior on large-scale models, as parameterizations of submesoscale processes explicitly avoid entrainment fluxes. This avoidance severely underestimates submesoscale impacts on biogeochemistry and climate (e.g., long-term carbon and heat uptake). Recent work, some from this group, further indicates that cross-scale interactions between submesoscale flows, surface gravity waves, and turbulence may also significantly alter the exchange at the base of the surface mixed layer. However, the balance of several possible mechanisms for this exchange has not been established. This project will quantify the effect of these multiscale interactions using new theoretical and computational tools, resulting in more complete models of submesoscale exchange between the ocean interior and surface boundary layer. Advances for both climate-scale and high-resolution Large Eddy Simulation models will be shared with the science community. New techniques in tracer diagnostics and theory will be developed, and used to clarify the role of multiscale interactions in affecting exchange between the boundary layer and interior, providing guidance towards new approaches in multiscale numerical modeling, parameterization, and ocean observing. The majority of the funds for this project will go towards supporting early career scientists: a graduate student, a postdoctoral researcher, an early career research scientist, and an early career faculty member. Reynolds-averaged numerical simulations (RANS) will quantify the impact of incorporating entrainment fluxes into a widely utilized parameterization of submesoscale mixedlayer instability, including the impact of the improved parameterization on realistic simulations incorporating ocean biogeochemistry in the Southern Ocean. Large-eddy simulations (LES) will determine the processes and parameter dependence through which submesoscale instabilities, surface gravity wave-driven effects such as Langmuir turbulence, and small-scale three-dimensional turbulence interact to modify entrainment and subduction of tracers. New insights into these multiscale interactions will be enabled by recent developments in boundary layer energetics, potential vorticity dynamics, and techniques for eddy-flux diagnostics developed as part of this project. These advances will be applied to a set of state-of-the-art large-domain LES that resolve scales of order 1 m to 10km, providing new benchmarks in exascale ocean modeling. Together these activities will simulate and quantify the essential mechanisms of entrainment and subduction at the submesoscale, their cross-scale interactions, and their accurate interpretation in observations and representation in numerical models. Community ocean and climate model improvements will be furthered through inclusion of improved and vetted representation of entrainment fluxes in two parameterizations, including the most widely used parameterization of submesoscale mixed-layer instability and a newer process parameterization. The LES that will be performed will be made available for immediate community use, providing a valuable new resource supporting other investigations into submesoscale-Langmuir-turbulence, and physical-biogeochemical interactions. 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.