Title

Eddy-mean flow decomposition and eddy-diffusivity estimates in the tropical Pacific Ocean

Date of Award

2000

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Annalisa Griffa, Committee Chair

Abstract

The tropical Pacific Ocean surface current system can be characterized by a strong degree of nonstationarity due to the fast response time of equatorial and near-equatorial dynamics. The ocean-atmospheric dynamics create longitudinally coherent zonal flow with strong meridional shear in the large-scale mean and an energetic mesoscale (O(100km)) component. Parameterization of the effects of the mesoscale field depends on the separation of the large-scale mean from the observed velocity. In this dissertation, the focus is placed on the key issue: separating the flow into large-scale mean and mesoscale eddy components in order to compute meaningful eddy diffusivity estimates in flow regimes that demonstrate strong currents and strong shear.This work is divided into two parts. In Part 1, a method is developed for using Lagrangian data to estimate the diffusivity addressing the inhomogeneity of the mean flow. The spatially dependent estimate of the mean field is computed with a least squares bicubic smoothing spline interpolation scheme with an optimized roughness parameter which guarantees minimum energy in the fluctuation field at low frequencies. Numerical simulations based on a stochastic model of a turbulent shear flow are used to validate the approach in a conceptually simple but realistic scenario. The technique is validated by application to Lagrangian observations of two dynamically distinct time-space regions of the surface tropical Pacific.Part 2 is devoted to the application of the methodology described in Part 1 to spatially and temporally binned Lagrangian observations spanning surface buoy data of the near-surface tropical Pacific Ocean from 1979 through 1995. Away from the equator and strongly sheared flows, the magnitude of diffusivity is of relatively small magnitude: O(107 cm2/s). Turbulence statistics are characterized by a first order autoregressive process, (AR(1)), perturbed slightly by the existence of inertial oscillations. For these flows, the Lagrangian integral time scale is related in a simple way: T1 = kappa/sigma2. In the North Equatorial Countercurrent (NECC) and near-equatorial regions, removal of the strongly spatially-varying mean flow produces zonal diffusivity estimates also characterized by AR(1) turbulence statistics but with larger eddy diffusivities (∼20 to 76 x 107 cm2/s). Meridional turbulence statistics are influenced by tropical instability wave (TIW) activity and are more aptly modeled as a second order autoregressive (AR(2)) process. In these cases the higher meridional eddy energy, due to the coherent nature of energetic TIW's, does not increase dispersion and the simple AR(1) relationship describing the decay time scale fails.Eddy diffusivity parameterized and direct Reynolds' stress meridional heat flux divergence estimates of the shear mean flow equatorial region are computed and compared to provide a measure of confidence in the diffusivity estimates.

Keywords

Physical Oceanography

Link to Full Text

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