Title

Zonal variability of mesoscale eddies from TOPEX/POSEIDON altimetry in the North Pacific Current

Date of Award

2000

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Otis B. Brown, Committee Chair

Second Committee Member

Hans C. Graber, Committee Member

Abstract

This dissertation is an investigation of the zonal variability of mesoscale eddies in the North Pacific Current (NPC) using the TOPEX/POSEIDON (T/P) altimeter and quasi-geostrophic theory. Observations are presented in the reference frame of crossing satellite ground tracks, which allows traditional one-dimensional wavenumber spectral analysis to be coupled to wavenumber-frequency analysis of horizontally-propagating plane waves. Individual mesoscale eddies are observed and quantified using the Time-Distance Crossover Image (TDCI), a new form of Hovmoller diagram in the satellite along-track reference frame.Mesoscale eddies exist in the along-track spectrum at wavelengths of 100--800 km. Spectral slope in the range 100--400 km is well-defined, following a power law in along-track wavenumber like k-2s in the eastern NPC, and k-4s in the western NPC. New results show that mesoscale has two subranges: an isotropic regime called the submesoscale, occupied by smaller ring-like eddies unaware of the beta-effect, and an anisotropic regime, occupied by larger westward-propagating eddies with basic plane wave structure: mesoscale Rossby waves. In the eastern NPC, the transition scale is order R 1, the radius of deformation, but is 2R 1--3R1 in the western NPC. The typical period of mesoscale Rossby waves decreases from about 220 days in the east to about 150 days in the west, while wave amplitude increases by an order of magnitude. Phase speeds increase by a factor of 3--4 from east to west, but are an order of magnitude less than particle speeds at all longitudes.Two models of eddy variability in the NPC are proposed to account for these observations: an eddy generation model and a wave propagation model. The former utilizes concepts from both classical quasi-geostrophic turbulence and coherent structure turbulence to hypothesize the source of the baroclinic eddy field observed in the quiescent eastern NPC. Stochastic wind-stress curl is suggested as the ultimate energy source. An inverse or red cascade of nonlinear transfers guides this energy from the submesoscale regime to coherent structures at the deformation scale, which possess maximum baroclinic wave frequency. The east-to-west variation of Rossby wave space-time scales in the NPC is modeled with a two-layer linear dispersion relation featuring variable meridional bottom slope, a strong eastward mean flow in the surface layer, and a weak westward mean flow in deep layer. For reasonable estimates of the mean flow, the propagation model compares favorably to observations of individual Rossby waves made with TDCI.

Keywords

Physical Oceanography

Link to Full Text

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