Modeling and parameterizing submesoscale turbulence in dense Arctic flows

Abstract

Dense gravity currents forced by surface buoyancy loss over continental shelf regions are important contributors to subsurface and abyssal ventilation throughout the World Ocean, yet remain challenging to model accurately. In this thesis, we present idealized experiments of rotating terrain-following gravity currents employing the nonhydrostatic MITgcm in z coordinates and the hydrostatic GFDL-MOM6 in z* and isopycnal coordinates. In the highest-resolution simulations, the dense flow undergoes geostrophic adjustment and forms bottom- and surface-intensified jets. The density front along the topography combined with geostrophic shear initiates submesoscale symmetric instability (SI), which leads to onset of secondary shear instability, dissipation of geostrophic energy, and irreversible mixing. We explore the impact of vertical coordinate, resolution, and parameterization of shear-driven mixing on water mass transformation. In isopycnal coordinates, limited vertical resolution in weakly stratified abyssal regions leads to inadequate representation of mixing. We develop and implement a parameterization for SI-driven turbulence to mediate this issue. The scheme is based on identifying unstable regions through a balanced Richardson number criterion and slumping the isopycnals towards a balanced state. A fraction of the released potential energy is passed to the shear mixing parameterization, so that potential energy extracted from the geostrophic flow by SI is converted to kinetic energy and used for vertical mixing. Such a scheme becomes crucial as ocean models move towards resolving mesoscale eddies and fronts but not the submesoscale phenomena they host. In the final thesis component, we examine how state-of-the-art global ocean models currently represent water transformation processes in the Arctic. We consider a 1/4 and analogous 1/8-degree model and find that ventilation by overflows as well as transformation of the warm, salty Atlantic inflow both contribute to creating highly dense waters in the Eurasian shelves. The 1/8-degree model performs better in capturing transient dense flows emanating from polynyas around coastal islands such as Novaya Zemlya; the 1/4-degree model marginally resolves overflows but has an overly diffuse vertical structure. As a next step in bridging our idealized process studies with the global simulations, we plan to perform regional mesoscale-resolving modeling with our SI parameterization to further constrain Arctic ventilation pathways.