Atmosphere Ocean Science Friday Seminar

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The Batchelor Ice-Ocean Boundary Layer: a Direct Numerical Simulation-motivated, observationally-supported parameterization

Speaker: Ken Zhao, Oregon State University

Location: Warren Weaver Hall 1314

Date: Friday, October 13, 2023, 4 p.m.

Synopsis:

Glacial melt rates at ice-ocean interfaces are commonly parameterized using a shear boundary layer assumption. However, this assumption has only been demonstrated to be appropriate for horizontal ice-ocean boundaries where the exchange of heat and freshwater across the mm-scale diffusive thermal and salinity boundary layers varies proportionally with the strength of external momentum, i.e. when buoyancy-driven turbulence and the suppression of turbulence by stratification is weak.Guided by Direct Numerical Simulations of the ice-ocean boundary layer for varying geometric and ocean forcing parameters, I will present an updated understanding of the basic principles of ice-ocean boundary layers (as a diffusive freshwater layer nested within a diffusive thermal layer within a viscous velocity shear layer within a turbulent momentum layer). I will present simulation results that seek to merge the following turbulent ice-ocean boundary layer regimes: (1) meltwater-driven buoyancy, (2) meltwater-driven shear, and (3) externally-driven shear (from both horizontal and vertical sources of momentum). In the absence of externally-driven flows, a dynamical transition from buoyancy-controlled to shear-controlled boundary layers is possible for the thermal layer, but not the freshwater layer. By contrast, I find that externally-driven sources of shear can constrain both the thermal and diffusive freshwater layers.This improved understanding allows us to develop accurate predictions for the turbulence-constrained momentum, thermal, and freshwater boundary layer thicknesses, which is required to predict their fluxes and thus, ocean-driven melt rate. I propose and discuss the implications of a new observationally-supported parameterization for melt rate at ice-ocean boundaries that modify commonly-used ocean heat, freshwater, and momentum budgets (plume theory and 3-equation thermodynamics) via new simulation-derived theories of near-boundary ocean-turbulence (turbulent transfer efficiency, plume entrainment, and friction velocity).