教師著作
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Item South China Sea Interannual Variability(2007-05-18) Chang, Chiung-Wen June; Wu, Chau-Ron; Hsu, Huang-HsiungSituated at the pathway of East Asian monsoon system, the South China Sea (SCS) circulation is largely influenced by the seasonal reversal of the monsoonal winds, northeasterly in winter and southwesterly in summer. On a seasonal time scale, the surface oceanic circulation is cyclonic in winter and anti-cyclonic in summer. Beyond the seasonal time scale, the oceanic circulation of the SCS demonstrates an interannual variation related to El Nino/Southern Oscillation (ENSO) [e.g., Wu et al., 1998; Shaw et al., 1999]. A number of recent studies reveal that the ocean dynamics and horizontal advection in particular play a key part in the interannual variability in the SCS [Metzger and Hurlburt, 2001; Qu et al., 2004; Liu et al., 2004]. To understand the influence from the ocean dynamics on the interannual variability of sea surface temperature, sea surface height (SSH) variation, which is a proxy for the upper oceanic thermodynamics, in the South China Sea (SCS) are examined using results from an altimetric data assimilation model. After the SSH data have had the seasonal mean removed, principal component analysis illustrates two distinct anomaly patterns. The first mode shows an east-west sea-saw pattern which would affect the basin-wide gyre intensity in winter. The second mode consists of a meridional dipole feature with a nodal line around 12.degree.N and is related to the development of the eastward jet and upwelling off Vietnam in summer. Both EOF modes have significant interannual variations and are highly correlated to the ENSO events. This study demonstrates that SST variations in the SCS are largely accounted for by the upper oceanic dynamics. El Nino's influence would persist into the next summer through the existence of an anomalous atmospheric low-level anticyclone over the tropical North Pacific (Wang et al. 2000a). The persistent atmospheric anticyclone modulates wind fields in the summer SCS, thereby induces marked interannual variability in ocean circulation and SST (Xie et al., 2003). Effect combined of these two modes may explain the prolonged SCS warming from 1997 winter to 1998 summer. The second mode shows that before and after ENSO, summer SCS circulations are all enhanced but in opposite directions. Prior to the peak central-eastern Pacific warming, SCS circulation is anti-cyclonic in the south and cyclonic in the north. This structure 1) generates divergence off the Vietnam coast such increases the cold-water upwelling, 2) enhances an eastward offshore flow which carries the cold water into the central SCS. The Cold SST in SCS prior to El Nino is in agreement with findings in earlier literature (Yamagata and Masumoto, 1991). During our 10 years data analysis (1993-2002), year 1994 and 1997 are both ENSO developing years and IOD (Indian Ocean Dipole, Saji et al. 1999) event years; yet in 1994 there were colder SSTs in the SCS. Possible processes such as IOD and the internal variation of west northern Pacific Monsoon are accounted for the SCS summer variation will thereby be examined in the presentation.Item Air-Sea Interaction between Typhoon Nari and Kuroshio(2007-05-18) Wu, Chau-Ron; Chang, Yu-LinThe air-sea interaction between Tropical Cyclone Nari and Kuroshio was studied using satellite observations and a three-dimensional primitive equation ocean model. With energy and heat supplement from the Kuroshio, Nari was circling around a restricted water and sustained over an extraordinarily long period. The features that Nari strengthened as it passed over the warm Kuroshio, weakening as it met a cold dome were well revealed by TMI/SST. At certain locations along typhoon track, surface cooling of up to 5 degree C was observed after Nari's departure. Model simulation indicated significant positive vorticities and cyclonic current vectors did not occur throughout the trail of Nari. Only regions north of Kuroshio provided a better condition for developing a cyclonic eddy. As a result, the cold SST patch was only visible to the north of the Kuroshio axis. The cyclonic circulation penetrated much deeper for a slowly-moving storm, regardless of the typhoon intensity. Near-inertial frequency oscillations after typhoon departure were simulated by the model in terms of the vertical displacement of isotherms. The SST cooling caused by upwelling and vertical mixing is effective in cooling the upper ocean several days after the storm had passed. At certain locations, surface chlorophyll-a concentration increased significantly after Nari's departure. The combined action of upwelling and mixing in turn brings cold deep-layer, nutrient-rich water to the sea surface, which ignificantly increases the surface chlorophyll-a concentration.Item Surface Circulation in the East China Sea(2004-05-20) Hsin, Yi-Chia; Wu, Chau-Ron; Liang, Wen-DerItem Fine-Resolution Modeling Studies in the Northern South China Sea(2004-05-20) Wu, Chau-Ron; Hsin, Yi-Chia; Jiang, Tzu-LingRecently, there have been many new observations from expeditions and moored instruments in the northern South China Sea (NSCS), which produced several exciting new features not known before. These new findings call for a new generation of models to explore the processes responsible for these features, because most of the existing numerical models can not either resolve them or account for the physics behind them. A fine-resolution, hydrodynamic model of the NSCS area capable of resolving the temporal and spatial scales of corresponding oceanic processes and bringing together the unique oceanographic data sources available in this area is established in this study. The fine-resolution NSCS model is nesting to a North Pacific Ocean model. Description of the circulation is further improved by assimilating the TOPEX/Poseidon satellite altimeter data into the NSCS model. Data assimilation model not only reproduces the seasonal patterns in the earlier studies but also resolves mesoscale highs and lows in the region. The model results are reasonably consistent with the limited observations during previous expeditions. The spatial and temporal variations in the NSCS are discussed as well. Furthermore, satellite remote sensing data and time series data obtained from moored instruments (SEATS) will be also used for comparison with model output.Item Intra-Seasonal Variation in the Velocity Field of the Northeastern South China Sea(2004-05-20) Wu, Chau-Ron; Tang, T. Y.; Lin, S. F.; Yang, Y. J.; Liang, W. D.Two subsurface Acoustic Doppler Current Profilers (ADCP) were deployed at the northeastern South China Sea to study circulation structure in the area as well as the path and process of Kuroshio intrusion. The 48-hour low-pass filtered data reveal significant intra-seasonal variations in the velocity field. The current pattern alternates between clockwise and counterclockwise even within a single month. Local wind forcing dominated by monsoon winds fails to address the phenomena and variations. The present study suggests that wind stress curl forcing is the dominant process controlling the circulation picture in the area. While a stronger wind stress curl appeared and developed off southern tip of Taiwan, it will provide negative vorticity to the intruded current and form an anticyclonic eddy. The stronger current is always going along with the stronger wind stress curl. On the other hand, while the curl in the area looses or decays, the intruded current becomes weakened and forms a cyclonic eddy. The agreement between wind stress curl and the velocity field suggests that changes in the wind stress curl contribute to the intra-seasonal variations in the northeastern South China Sea.