Date of Award

2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

D. Randolph Watts

Abstract

The Kuroshio current is the western boundary current (WBC) of the North Pacific. It flows north from the tropics carrying heat, momentum, and vorticity and leaves the coast of Japan ∼35°N and flows east as a free jet. After leaving the coast, the Kuroshio is renamed the Kuroshio Extension and is a vigorously meandering frontal boundary characterized by mesoscale variability. Until recently, the Kuroshio Extension was vastly under observed compared to its Atlantic counterpart, the Gulf Stream. The Kuroshio Extension System Study (KESS) was a two year observational field campaign from 2004–2006 with the goal of determining what drives the variability of the Kuroshio Extension and its recirculation gyres.

This dissertation explores the relationship between mesoscale eddies and time-mean flow of the Kuroshio Extension using the KESS observations. Mesoscale eddies drive cross-frontal exchanges of heat, momentum, and potential vorticity (PV) through eddy fluxes. It was found that eddy fluxes play a role in cross-frontal heat transport, energy conversion through eddy-mean flow interactions, eddy forcing of the mean flow, and modification of Subtropical Mode Water (STMW).

In Manuscripts 1 and 2, the mean spatial structure and spatio-temporal variability of the time-mean eddy heat fluxes was determined. It was found that the dynamically-important divergent eddy heat fluxes are driven by vertical coupling between nearly depth-independent geostrophic currents, measured in the deep ocean, and the upper-ocean thermal front. Eddy heat fluxes by geostrophic currents in thermal-wind balance, which have an equivalent barotropic vertical structure and advect no heat, are completely rotational (nondivergent) and play no dynamical role in energy exchanges between eddies and the mean flow.

The mean divergent eddy heat fluxes had a complex spatial structure that was not always down-gradient. Upstream of a mean diffluent trough fluxes were strong and down-gradient, whereas downstream of the trough fluxes were weaker and up-gradient. Down- and up-gradient fluxes implied an energy conversion from the mean to eddy field and vice versa that peaked near 400 m depth at magnitude 10 x 10-3 cm2 s-3, with a depth-averaged value of 3 x 10-3 cm2 s-3.

The KESS observations captured a regime shift from a stable to unstable meandering regime. In Manuscript 2, It was found that there was a distinct difference in the spatial structure between the two regimes. The stable regime had mostly down-gradient eddy heat fluxes, while the unstable regime exhibited asymmetry in down- and up-gradient fluxes along the mean path. The mean spatial structure of the eddy heat fluxes resulted from episodic and varied mesoscale processes. One cold-core ring (CCR) formation and one CCR-jet interaction were responsible for the asymmetry in down- and up-gradient fluxes.

Unlike what has been observed in the Gulf Stream, the Kuroshio Extension has a prominent population of externally-generated eddies, not formed through local processes, that interact with the jet to drive some of biggest eddy heat flux events. The external eddies play an important role in the variability of the jet through eddy-mean flow interactions. The eddy heat fluxes that result from eddy-jet vertical coupling act in accordance with baroclinic instability by releasing available potential energy of the mean jet. The interaction is not growth from an infinitesimal perturbation, but from the start is a finite-amplitude interaction.

In Manuscript 3, the role of eddy PV fluxes in the upper ocean was determined by exploring the mean spatial structure and their role in eddy forcing and modification of STMW. Eddy PV thickness fluxes were mostly down-gradient in the upper ocean, which suggests that eddies are stabilizing the mean flow and that the Kuroshio Extension between 144°-148°E is in a region of eddy growth. The eddy PV thickness fluxes act as a drag force on the mean flow along the mean path upstream in the western half of the KESS observations. Eddy relative vorticity fluxes play a lesser role in eddy forcing than eddy thickness fluxes by adding anticyclonic curvature to the mean flow. Finally, eddy thickness fluxes were observed to bring high PV waters from the north into the low PV waters of the southern recirculation gyre. This PV exchange mixes high PV waters from the north with low PV waters of the southern recirculation gyre where STMW resides. This mechanism can explain observed increases in the STMW PV in the recirculation gyre over the 16 months of KESS observations.

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