Low-frequency Variations of Currents in Spring and Autumn off Southwestern Taiwan
Date Issued
2015
Date
2015
Author(s)
Tsai, Yi-Lin
Abstract
Flow patterns of the southern opening of Taiwan Strait were investigated by incorporated moored observations, wind speed measurement (from the weather station in Taiwan Strait) and sea-level measurements (from two nearby tide station), among them the moored current data include two sets of Recording Current Meters(RCMs) observations (from 1997 to 1998) and one set of ADCP data (from 2004 to 2013). Aside from these, trajectories of surface drifters and model results from the Luzon Strait Ocean Nowcast/Forecast System (LZSNFS), a data-assimilating, dynamical ocean model routinely operated by Naval Research Laboratory, USA, were used to display the simultaneous spatial structure of the flow field, which facilitates the interpretation of the significance of the variations of currents, from moored observations, associated to the larger scale flow pattern. All the time series data were pre-processed by low-passed filtration (the cut-off period of the filter used is 48 hours ), afterwards the orientation of the low-passed current data were projected to the principal and minor axes, in which the direction of the former is roughly in parallel to local isobaths. From the current data, five events could be identified, if an event is defined by the current reversal, i.e. as the direction of the low-passed current offshore Kaohsiung reverses from northwestward to southeastward within a few days. Model results from LZSNFS suggest this phenomenon is closely related to the process of the intrusion of Kuroshio from the Luzon Strait to the northeastern South China Sea (SCS), this process may induce an anti-cyclonic (clockwise) flow pattern offshore the southwest Taiwan. Statistics from the Empirical Orthogonal Function analysis (among low-pass filrated ADCP currents, sea-level and winds) and simultaneous results from LZSNFS suggest that, once an anti-cyclonic flow pattern moved into the observation region offshore southwest Taiwan, the costal sea-level near Kaohsiung descended and the low-frequency currents offshore Kaohsiung turned toward the southeast. This kind of current reversal events occurred frequently in both spring and autumn.
Subjects
Southwestern of Taiwan
principal analysis
Empirical orthogonal function
EddyKuroshio Loop Current
Type
thesis
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