呂宋海峽到台灣東部海域黑潮水團演化之動力過程研究

Abstract

The variability of the water masses associated with the Kuroshio are analyzed through three separate studies: (1) The spatial variations of the Kuroshio Tropical Water (KTW) from the Luzon Strait to the east of Taiwan is studied from climatology data; (2) the temporal variability of the intermediate water (IW) east of Taiwan is elucidated through the use of two years of salinity minimum time series; (3) hydrographic and current velocity data collected from 9 surveys east of Taiwan are used to supplement the results from the previous two studies. The first study examines the evolution of KTW from the Luzon Strait to the I-Lan Ridge northeast of Taiwan. Historical conductivity temperature depth (CTD) profiles are analyzed based on root mean square (rms) difference of the salinity along isopycnals. This analysis enables water masses in the Kuroshio and the vicinity, as well as their interactions, to be tracked and distinguished. Vertical and horizontal eddy diffusivities are then calculated from hydrographic and current velocity data to elucidate the dynamics underlying the KTW interactions with the surrounding water masses. Changes in KTW properties mainly occur in the southern half of the Luzon Strait, while moderate variations are observed east of Taiwan on the eastern flank of the Kuroshio. A front separating the KTW from the South China Sea Tropical Water (SCSTW) on the Kuroshio western side, is observed east of Taiwan. The estimated horizontal eddy diffusivity is high (Kh~102 m2 s-1) all along the Kuroshio path, due to the high lateral current shear along the Kuroshio flanks. The vertical diffusivity is the dominant factor responsible for the modification of KTW in the Luzon Strait and approaches 10-3 m2 s-1, with the highest value in the southern Luzon Strait. The variability of the IW east of Luzon and Taiwan is investigated using data acquired from moored instrumented lines and shipboard hydrographic and current velocity surveys. The IW is defined as the water mass with a local salinity minimum along the Kuroshio path. An empirical formula is deduced to estimate the IW salinity minimum east of Taiwan using temperature measurements around 580 m depth. The properties of the IW east of Taiwan vary greatly as a result of variable contributions from three water masses including the high salinity South China Sea Intermediate Water (SCSIW), the low salinity minimum North Pacific Intermediate Water (NPIW), and the mid-range Kuroshio Intermediate Water (KIW). Our analysis concludes that NPIW is predominantly found east of Taiwan and the northward transport of KIW from east of Luzon to east of Taiwan is not a steady process. Concurrent mooring and Pressure-sensor equipped Inverted Echo Sounder (PIES) measurements east of Luzon and Taiwan allow us to correlate the variations of the layer thickness of the Kuroshio near its origin (KLTo) east of Luzon to the nature of the IW east of Taiwan. Variations of KLTo are linked to the mesoscale eddies impinging on the Kuroshio, from the analysis of PIES data. When the Kuroshio is deep (large KLTo) KIW is transported northward across the Luzon Strait, where the KIW salinity increases presumably due to turbulence mixing, and to the east of Taiwan. When the Kuroshio is shallow (small KLTo) the KIW transport east of Luzon is diminished or southward. East of Taiwan, NPIW feeds in below the Kuroshio and is transported northward beyond the I-Lan Ridge. To further document the variability of the tropical and intermediate waters, we use the results from 9 cruises along the KTV1 line east of Taiwan, together with satellite sea surface height data. The investigation reveals that the flow of South China Sea Tropical Water (SCSTW) to the west of Kuroshio is not permanent, and the offshore shift of the Kuroshio axis strongly impacts the flow and characteristics of the water masses found in the region.