Mixing and Restratifiation in the Upper Ocean: Competing Mechanisms in the Wave-Averaged Boussinesq Equations

Haney, Sean

Graduate Thesis Or Dissertation

Mixing and Restratifiation in the Upper Ocean: Competing Mechanisms in the Wave-Averaged Boussinesq Equations Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/8910jt62q

Abstract

The ocean mixed layer serves as buffer through which the deep ocean and atmosphere communicate. Fluxes of heat, momentum, fresh water, and gases must pass through the mixed layer, and phytoplankton flourish most in the mixed layer where light is abundant. The dynamics of the mixed layer influence these fluxes and productivity of phytoplankton by altering the stratification and mean flow. Restratifying hurricane wakes provide a unique setting in which a dramatically perturbed mixed layer is observable from satellite sea surface temperature. Strong horizontal temperature fronts which border these wakes suggest that two and three dimensional, adiabatic processes play a role. These observations provide the necessary parameters to estimate wake restratification timescales by surface heat fluxes (SF), Ekman buoyancy fluxes (EBF), and mixed layer eddies (MLEs). In the four wakes observed, the timescales for SF and EBF were comparable, while MLEs were much slower. The restratification time for MLEs is reduced for deeper and narrower wakes compared with other mechanisms. Therefore, stronger mixed layer fronts make MLEs competitive with surface heat and wind forcing. Fronts are influenced by winds, waves (Langmuir circulations; LC), MLEs, and symmetric instabilities (SI). The wave averaged (Stokes drift) effects on MLEs are subtle, with aligned (anti-aligned) Stokes and geostrophic flows yielding a slight increase (decrease) in wavenumber and growth rate. Frontal effects on LC are very weak, with the primary result confirming that increased vertical stratification suppresses LC. The effect of Stokes drift on SI is profound. It changes the background ow necessary for SI, and it alters the structure of the SI themselves. Analytic stability criteria show that SI exist when the Ertel potential vorticity (PV) is negative. When the Stokes drift is aligned (anti-aligned) with the geostrophic shear, the PV is increased (reduced). This PV criterion is confirmed in more realistic settings with numerical linear stability, and with nonlinear large eddy simulations (LES). Therefore, in the presence of waves, the criterion Ri < 1 is inappropriate for the onset of SI. LES show that fronts with strongly negative PV are far more energetic than fronts that exhibit only LC.

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