THESIS
2018
xviii, 164 pages : illustrations (some color), maps (some color) ; 30 cm
Abstract
The unique three-layer cyclonic-anti-cyclonic-cyclonic (CAC) circulation and long-lived anti-cyclonic
eddy-train have been observed in South China Sea (SCS), while underlying dynamics
for the formation process are not well established. This research conducts process-oriented
numerical study to investigate (1) the formation mechanisms of the layered-circulation
associated with external forcing through straits and internal dynamics of cross-isobath transport;
and (2) the formation of long-lived eddy-train associated with boundary current separation.
The CAC circulation is generated by lateral planetary vorticity flux provided by sandwich-like
inflow-outflow-inflow intrusion through Luzon Strait. The intruding planetary vorticity is
transported by the horizontal and vertical motion...[
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The unique three-layer cyclonic-anti-cyclonic-cyclonic (CAC) circulation and long-lived anti-cyclonic
eddy-train have been observed in South China Sea (SCS), while underlying dynamics
for the formation process are not well established. This research conducts process-oriented
numerical study to investigate (1) the formation mechanisms of the layered-circulation
associated with external forcing through straits and internal dynamics of cross-isobath transport;
and (2) the formation of long-lived eddy-train associated with boundary current separation.
The CAC circulation is generated by lateral planetary vorticity flux provided by sandwich-like
inflow-outflow-inflow intrusion through Luzon Strait. The intruding planetary vorticity is
transported by the horizontal and vertical motions inside basin, and balanced by the intrinsic
flow-slope topography interaction and nonlinear advection. The vertical motions induced by
cross-isobath transport over the slope act as the source/sink of water mass and vorticity, and they
link the circulations in different layers. These vertical motions can redistribute vertically the
intruding vorticity and contribute to the formation of layered-circulation, even layered-transport
in the Luzon Strait is absent. The cross-isobath transport is induced by along-isobath bottom
friction, nonlinear advection and pressure gradient force (PGF), in which the PGF is largely
linked with nonlinear vorticity advection and beta effect of background slope current.
Driven by the northeastward separated jet current during summer, the eddy-train with three anti-cyclonic
eddies is generated along jet’s path. The inherent stratification regulates the three-dimensional
scale of anti-cyclonic eddies, and constrains their intensities and vertical extensions
by weakening the geostrophic balance. The eddy-train’s negative vorticity is originated from the
beta effect of the northward-flowing western boundary current, and from the subsequent
downstream vorticity advection along the jet. The jet separation is a necessary condition for the
formation of eddy-train, and the enhanced stratification, increased wind stress, and associated
negative wind stress curl are favorable conditions for the formation of anti-cyclonic eddies in
eddy-train.
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