教師著作

Permanent URI for this collectionhttp://rportal.lib.ntnu.edu.tw/handle/20.500.12235/37078

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Author: search.filters.author.Wu, C.-R.End date: 2009

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Now showing 1 - 8 of 8
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    Air-sea interaction between Tropical Cyclone Nari and Kuroshio
    (American Geophysical Union (AGU), 2008-06-01) Wu, C.-R.; Y.-L. Chang; L.-Y. Oey; C.-W.J. Chang; Y.-C. Hsin
    The air-sea interaction between tropical cyclone Nari (Sep/6 – 16/2001) and Kuroshio is studied using satellite observations and an ocean model. Nari crossed the Kuroshio several times, which caused variations in typhoon intensity. Nari weakened when it was over the shelf north of Kuroshio where cooling took place due to mixing of the shallow thermocline. The cyclonic circulation penetrated much deeper for the slowly-moving storm, regardless of Nari’s intensity. Near-inertial oscillations are 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 increases significantly after Nari’s departure. Upwelling and mixing bring nutrient-rich subsurface water to the sea surface, causing enhancement of phytoplankton bloom.
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    Transient, seasonal and interannual variability of the Taiwan Strait Current.
    (Springer Netherlands, 2007-10-01) Wu, C.-R.; S.-Y. Chao; C. Hsu
    We have constructed a fine-resolution model with realistic bathymetry to study the spatial and temporal variations of circulation in the Taiwan Strait (TS). The TS model with a resolution of 3∼10 km derives its open boundary conditions from a larger-scale model. The QSCAT/NCEP winds and AVHRR SST provide forcing at the sea surface. Because of the high resolution in model grids and forcing, the model achieves a previously unavailable level of agreement with most observations. On biweekly time scales surface-trapped current reversals often lead to Strait transport reversals if the northeasterly wind bursts in winter are sufficiently strong. On seasonal time scales the northward current is the strongest in summer since both summer monsoon and pressure gradient force are northward. The summer northward current appears to be relatively unimpeded by the Changyun Rise (CYR) and bifurcates slightly near the surface. With the arrival of the northeast monsoon in fall, downwind movement of China Coastal Water (CCW) is blocked by the northward current near 25.5°N and 120°E. In winter, the northward current weakens even more as the northeasterly monsoon strengthens. The CCW moves downwind along the western boundary; the CYR blocks part of the CCW and forces a U-shaped flow pattern in the northern Strait. Past studies have failed to reveal an anticyclonic eddy that develops on the northern flank of CYR in winter. On interannual time scales a weakened northeast monsoon during El Niño reduces advection of the cold CCW from the north and enhances intrusion of warm water from the south, resulting in warming in the TS.
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    Mesoscale eddies in the northern South China Sea
    (ELSEVIER, 2007-07-01) Wu, C.-R.; T.-L. Chiang
    A fine-grid resolution model with realistic bathymetry and forcing has been developed to study the characteristics of the mesoscale eddies for the northern South China Sea (SCS). The SCS model derives its open-boundary conditions from a larger-scale model, which minimizes errors related to the uncertainty of the Kuroshio intrusion at the open boundaries. The model results are consistent with previous observations. Model sea-surface height anomaly demonstrates that the hydrography and circulation in the northern SCS are modulated by westward-propagating mesoscale eddies originating in the vicinity of the Luzon Strait. This explains the observed intra-seasonal fluctuations at the SouthEast Asian Time-series Study (SEATS) station. The mesoscale eddies have the same propagation speed as baroclinic Rossby waves (∼0.1 ms−1). The periods of eddy shedding estimated from Strouhal number are around 40–50 days in December and 80–120 days in August, respectively. The seasonal variability of the Kuroshio intrusion results in more eddies in winter than in summer.
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    A numerical study on the formation of upwelling off northeast Taiwan
    (American Geophysical Union (AGU), 2008-08-01) Wu, C.-R.; H.-F. Lu; S.-Y. Chao
    We examined the spatial and temporal variations of upwelling off northeast Taiwan, using a fine-resolution numerical model with realistic bathymetry. The zonally running shelf break in the area deflects the Kuroshio seaward and produces upwelling on its on-shelf edge. The upwelling, in turn, manifests a cold dome or a cyclonic eddy. In depths below 150 m or so, the upwelling and hence the cyclonic eddy exist year-round. Above this depth, the eddy waxes and wanes as the upper portion of the Kuroshio migrates seaward and shoreward, respectively. The eddy event fluctuates in a wide range of timescales. Seasonally, the occurrence heavily favors summer rather than winter, because the mean Kuroshio axis migrates seaward in summer. Intraseasonally, the fluctuation contains two dominant periods centered at 70 days and 30 days. Local wind forcing and channeling by two local canyons do not affect the eddy statistics significantly.
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    Interannual variability of the South China Sea in a data assimilation model
    (American Geophysical Union (AGU), 2005-09-01) Wu, C.-R.; C.-W. J. Chang
    Sea surface height (SSH) variations in the South China Sea are examined using results from a data assimilation model. After the SSH data have had the annual cycle removed, principal component analysis illustrates two distinct anomaly patterns. The first mode, strongest off Vietnam, would affect the basin-wide gyre intensity. The second mode consists of a meridional dipole feature with a nodal line around 12衹 and is related to the development of the eastward jet and upwelling off Vietnam. Both EOF modes have significant interannual variations and are highly correlated to the ENSO events. The leading phase of mode 2 coefficient provides preliminary evidence for the air-sea interaction and the Pacific-East Asian teleconnection. An innovative mechanism is proposed to describe a weakened upwelling off Luzon in winter 1998. An extra-strong basin-wide warming in summer 1998 persists into fall and winter, preventing the formation of winter upwelling off Luzon in that year.
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    Volume transport through the Taiwan Strait : a numerical study.
    (the Chinese Geoscience Union, 2005-06-01) Wu, C.-R.; Y.-C. Hsin
    A fine grid resolution model with realistic bathymetry was constructed to study the spatial and temporal structures of flow through the Taiwan Strait where observations are limited. The model covers an expanded domain that includes the entire East China Sea and South China Sea, as well as the region occupied by the Kuroshio. The fine-resolution model derives its open boundary conditions from a larger scale Western Pacific Ocean model. Two numerical weather products from the European Center for Medium-Range Weather Forecasts and National Centers for Environmental Prediction, and one satellite observation-based wind set (QSCAT) are used to force the ocean model. Model experiments suggested that the best simulation is achieved when the model is driven by the QSCAT wind forcing. Several important features are reproduced in the model simulation. The volume transport is northward and largest in summer while minimal volume transport is southward and occurs in fall and winter. The general trend of volume transport is related to the seasonal reversal of monsoon winds. The present model also suggests that flow in the entire Taiwan Strait is to the southwest during periods of strong northeasterly wind. The annual average transport based on the present model is 1.09 Sv (1 Sv = 10 m s 6 3 1 −), which is smaller than most published values based on shipboard Acoustic Doppler Current Profiler (sb-ADCP) observations. The result suggests that sb-ADCP observations are biased toward estimates in summer and fair weather since bad weather during the winter northeast monsoon often prevents seagoing observations.
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    Intra-seasonal variation in the velocity field of the northeastern South China Sea
    (ELSEVIER, 2005-11-01) Wu, C.-R.; T. Y. Tang; S. F. Lin
    Two subsurface Acoustic Doppler Current Profilers (ADCP) were deployed in the northeastern South China Sea to study the circulation structure in the area as well as the path and process of the Kuroshio intrusion. The 48 h low-pass filtered data, for the first time, reveal significant intra-seasonal variations in the velocity field. Flow is alternately cyclonic or anticyclonic even within a single month. Local wind forcing dominated by monsoon winds fails to address the phenomena. The present study suggests that intruding current patterns are likely triggered by the strong wind stress curls. Strong negative wind stress curls off the southern tip of Taiwan introduce negative vorticity to form anticyclonic circulation in the intruding current. With diminishing wind stress curls, the intruding current weakens, forming a cyclonic circulation pattern.