Turbulence and mixing in stratified tidal bottom boundary layers

Madelaine Gamble Rosevear, Gregory N. Ivey, Nicole L Jones, Bishakhdatta Gayen, Matthew David Rayson

Abstract: Ocean currents flowing over the seabed produce a turbulent bottom boundary layer (BBL) which extracts energy from the overlying flow and mediates sedimentary, chemical, and biological processes at the seafloor. The role of the BBL is especially important in relatively shallow shelf seas where stratification, mean flow, and tidally-driven oscillating currents are typically strong. Despite this importance, the dynamics and even the height of the BBL formed under stratified, tidal conditions are poorly understood. We address this knowledge gap by performing large-eddy simulations of the evolution of a tidally forced and stratified BBL, initialised from rest, over a range of geophysically relevant parameter values. The BBL has two distinct regions: an actively stirred, near-well-mixed bottom mixed layer (BML) and a weakly stirred and strongly stratified capping pycnocline. A simple steady-state empirical model, dependent on stratification, tidal period, and bottom turbulence intensity, provides a useful estimate of BML height. Motivated by appreciable growth in the BML that can occur on timescales of days to weeks, we also develop a time-dependent model for the BML height. This model accounts for the vertical structure of the BBL turbulence and reproduces the BML growth with high fidelity. We also present a simple model for the cycle-averaged eddy diffusvity, which depends on the BML height. Both the steady-state and time-dependent models of BML height—and the accompanying expression for eddy diffusivity— can be used to develop tidal BBL parameterisations and inform grid resolution requirements within ocean models, as well as guiding sampling for future observational studies.

External IDs:doi:10.22541/essoar.174888914.45007654/v1