Date of Award

2008

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Specialization

Physical Oceanography

Department

Oceanography

First Advisor

Kathleen Donohue

Abstract

Fine horizontal-scale surveys performed during the Kuroshio Extension System Study deployment cruise in May 2004 provide near-synoptic ADCP and CTD data along cross-jet transects just up-stream of the first meander trough of the Kuroshio Extension. An array of Current and Pressure-recording Inverted Echo Sounders (CPIES) deployed during this cruise over a ∼600x600 km region centered on the first meander trough also provide time series of bottom pressure and currents as well as acoustic travel time measurements (τ ), which are converted via the Gravest Empirical Mode method to profiles of temperature, salinity, and specific volume anomaly. This combination allows calculation of absolute (barotropic plus baroclinic) geostrophic velocity profiles, and all data are mapped via optimal interpolation to a higher resolution grid covering the entire array area. The datasets from the surveys and the CPIES are used here to analyze the mean and time-varying velocity, hydrographic, and potential vorticity structure of the Kuroshio Extension in its “weakly meandering” state in a stream-coordinate system, which avoids the lateral smearing of the jet structure that would result from an Eulerian approach.

Stream-coordinate analysis reveals a canonical baroclinic jet structure, with isotachs sloping downwards from the cyclonic side of the jet across the core to a subsurface maximum on the anticyclonic side and cross-stream gradients of down-stream velocity that are stronger on the cyclonic side. Maximum surface down-stream velocities range from 1-2 m/s, averaging around 1.4 m/s. Down-stream velocities extend to the bottom just south of the core with average magnitudes of 1-5 cm/s, but vary in magnitude and direction depending upon the presence of deep barotropic eddies. Cross-stream velocities vary in the mean with respect to location along the meander pattern. In the first meander crest, the mean cross-stream flux is towards the cyclonic side, while entering the trough it is towards the anticyclonic side. However, these cross-stream flows appear to be event-driven, with fluctuations in steepness of the meander pattern due to the passage of frontal waves a probable driving mechanism. Relative vorticity (ζ) is found in the mean from the surveys to make contributions as high as 72% of f on the cyclonic side and -41% of f on the anticyclonic side, while the “twisting” term due to vertical shear and horizontal density gradients reaches a maximum in the mean of 45% of f just north of the core. The lower horizontal resolution of the CPIES dataset produces values of ζ and the twisting term that are about 50 and 75% weaker than in the surveys, respectively. However, comparison of the structure at various phases of the meander pattern reveals differences in the distribution of relative vorticity across the core. Both datasets suggest the presence of four isopycnal potential vorticity gradient layers, where strong cross-stream gradients represent a “barrier” to cross-stream flow. These layers include, in order of decreasing cross-jet gradients, the mode water, the main thermocline, the lower thermocline/North Pacific Intermediate Water core, and a relatively homogeneous deep layer. Comparison of the Kuroshio Extension to the Gulf Stream suggests that although the two possess many qualitative structural similarities, mean velocities and gradients are generally about 30% weaker in the Kuroshio Extension, and the strong jet structure penetrates to about 25% greater depths in the Gulf Stream.

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