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.C.-R. WuEnd date: 2009

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Now showing 1 - 9 of 9
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    Bimodal Behavior of the Seasonal Upwelling off the northeastern coast of Taiwan
    (American Geophysical Union (AGU), 2009-03-01) Chang Y.-L.; C.-R. Wu; L.-Y. Oey
    Observations over the outer shelf and shelf break off the northeastern coast of Taiwan indicate a curious seasonal variability of upwelling. At deeper levels 100 m below the surface, upwelling is most intense in summer but weaker in winter. Nearer the surface at approximately 30 m below the surface, the opposite is true and the upwelling is stronger in winter than in summer. Results from a high-resolution numerical model together with observations and simple Ekman models are used to explain the phenomenon. It is shown that the upwelling at deeper levels (∼100 m) is primarily induced by offshore (summer) and onshore (winter) migrations of the Kuroshio, while monsoonal change in the wind stress curl, positive in winter and negative in summer, is responsible for the reversal in the seasonal variation of the upwelling near the surface (∼30 m). This mechanism reconciles previous upwelling data.
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    Spatial and Temporal Variations of the Kuroshio East of Taiwan, 1982-2005: A numerical study
    (American Geophysical Union (AGU), 2008-04-01) Hsin, Y.-C.; C.-R. Wu; P.-T. Shaw
    A 1/8 East Asian Marginal Seas model nested to a larger-domain North Pacific Ocean model is implemented over a span of 24 years from 1982 to 2005 to investigate the spatial and temporal variations of the Kuroshio east of Taiwan. Between 22 and 25 N, the mean state and variability of the Kuroshio, such as the two paths observed in the trajectories of surface drifters southeast of Taiwan and the branching of the Kuroshio northeast of Taiwan, are well reproduced by the model. Southeast of Taiwan, the Kuroshio is mostly in the top 300 m in the inshore path but extends to 600 m in the offshore path. Northeast of Taiwan, the Kuroshio follows the shelf edge in the East China Sea but may branch along a path south of the Ryukyu Islands. The latter path often meanders southward, and a significant portion of the Kuroshio transport may be diverted to this path. The Kuroshio extends from the coast to 123–123.5 E between 22 and 25 N with currents reaching a depth of 1000 m at some latitudes. The Kuroshio transports averaged over five sections east of Taiwan are 28.4 ± 5.0 Sv and 32.7 ± 4.4 Sv with and without the contribution from the countercurrent, respectively.
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    Interannual mode of sea level in the South China Sea and the roles of El Ni隳 and El Ni隳 Modoki
    (American Geophysical Union (AGU), 2008-02-01) Chang, C.-W. J.; H.-H. Hsu; C.-R. Wu; W.-J. Sheu
    ENSO-scale variation of the summer ocean circulation in the South China Sea (SCS) is investigated. The interannual mode of SSH features a north-south dipole pattern that modulates the cold jet off Vietnam. During the summers before and after the El Nin˜o, the mode has opposite signs of extrema. Strengthened circulations couple with the cold SSTAs during the El Nin˜o developing summers; weakened circulations accompany the warm SSTAs during the decaying summers. Heat advection by the basin circulation modulates the SST variation. The impact of the 1997 El Nin˜o on the SCS circulation contrasting that of 1994 and 2002 El Nin˜o Modoki is assessed. With moderate SST warming but further westward shift of the low-level convergence of the atmosphere in the equatorial Pacific, the El Nin˜o Modoki phenomenon enhanced the western North Pacific summer monsoon inside the SCS, driving stronger circulations in both the summers of 1994 and 2002.
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    Physical and geographical origins of the South China Sea Warm Current
    (American Geophysical Union (AGU), 2008-08-01) Chiang T.-L.; C.-R. Wu; S.-Y. Chao
    We examine the formation mechanism of the South China Sea Warm Current in winter, using a high-resolution, numerical model. The current, noted for its ability to flow against the prevailing northeast monsoon in winter, has received considerable attentions in recent years. The collective wisdom from previous models points to two likely generation scenarios: occasional wind relaxation or the Kuroshio intrusion. The present model consistently points to the wind relaxation as the dominant mechanism. When comparing differences between previous models and ours, we also conclude that the Kuroshio intrusion helps, but is not chiefly responsible. Tracing the current to the source, we identify the elevated sea level in the Gulf of Tonkin, induced by the northeast monsoon, as the ultimate driving force. The presence of Hainan Island bears little importance in generating the current.
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    Validation and Application of Altimetry-Derived Upper Ocean Thermal Structure in the western north Pacific Ocean for Typhoon-Intensity Forecast
    (Institute of Electrical and Electronics Engineers (IEEE), 2007-06-01) Pun, I.-F.; I-I Lin; C.-R. Wu; D.-S. Ko; W. T. Liu
    This paper uses more than 5000 colocated and near-coincident in-situ profiles from the National Oceanic and Atmospheric Administration/Global Temperature and Salinity Profile Program database spanning over the period from 2002 to 2005 to systematically validate the satellite-altimetry-derived upper ocean thermal structure in the western North Pacific ocean as such ocean thermal structure information is critical in typhoon-intensity change. It is found that this satellite-derived information is applicable in the central and the southwestern North Pacific (covering 122-170degE, 9-25degN) but not in the northern part (130-170degE, 25-40degN). However, since > 80% of the typhoons are found to intensify in the central and southern part, this regional dependence should not pose a serious constraint in studying typhoon intensification. Further comparison with the U.S. Naval Research Laboratory's North Pacific Ocean Nowcast/Forecast System (NPACNFS) hydrodynamic ocean model shows similar regional applicability, but NPACNFS is found to have a general underestimation in the upper ocean thermal structure and causes a large underestimation of the tropical cyclone heat potential (TCHP) by up to 60 kJ/cm2. After validation, the derived upper ocean thermal profiles are used to study the intensity change of supertyphoon Dianmu (2004). It is found that two upper ocean parameters, i.e., a typhoon's self-induced cooling and the during-typhoon TCHP, are the most sensitive parameters (with R 2~0.7) to the 6-h intensity change of Dianmu during the study period covering Dianmu's rapid intensification to category 5 and its subsequent decay to category 4. This paper suggests the usefulness of satellite-based upper ocean thermal information in future research and operation that is related to typhoon-intensity change in the western North Pacific
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    The Interaction of Supertyphoon Maemi (2003) With a Warm Ocean Eddy
    (American Meteorological Society, 2005-09-01) Lin, I-I; C.-C. Wu; K. A. Emanuel; I.-H. Lee; C.-R. Wu; I.-F. Pan
    Understanding the interaction of ocean eddies with tropical cyclones is critical for improving the understanding and prediction of the tropical cyclone intensity change. Here an investigation is presented of the interaction between Supertyphoon Maemi, the most intense tropical cyclone in 2003, and a warm ocean eddy in the western North Pacific. In September 2003, Maemi passed directly over a prominent (700 km 500 km) warm ocean eddy when passing over the 22°N eddy-rich zone in the northwest Pacific Ocean. Analyses of satellite altimetry and the best-track data from the Joint Typhoon Warning Center show that during the 36 h of the Maemi–eddy encounter, Maemi’s intensity (in 1-min sustained wind) shot up from 41 m s 1 to its peak of 77 m s 1. Maemi subsequently devastated the southern Korean peninsula. Based on results from the Coupled Hurricane Intensity Prediction System and satellite microwave sea surface temperature observations, it is suggested that the warm eddies act as an effective insulator between typhoons and the deeper ocean cold water. The typhoon’s self-induced sea surface temperature cooling is suppressed owing to the presence of the thicker upper-ocean mixed layer in the warm eddy, which prevents the deeper cold water from being entrained into the upper-ocean mixed layer. As simulated using the Coupled Hurricane Intensity Prediction System, the incorporation of the eddy information yields an evident improvement on Maemi’s intensity evolution, with its peak intensity increased by one category and maintained at category-5 strength for a longer period (36 h) of time. Without the presence of the warm ocean eddy, the intensification is less rapid. This study can serve as a starting point in the largely speculative and unexplored field of typhoon–warm ocean eddy interaction in the western North Pacific. Given the abundance of ocean eddies and intense typhoons in the western North Pacific, these results highlight the importance of a systematic and in-depth investigation of the interaction between typhoons and western North Pacific eddies.
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    Effects of sea level change on the upstream Kuroshio Current through the Okinawa Trough
    (American Geophysical Union (AGU), 2006-08-01) Kao, S. J.; C.-R. Wu; Y.-C. Hsin; M. Dai
    Sea-level effects on the Kuroshio Current (KC) in the Okinawa Trough (OT) are examined using a 3-D ocean model. When the sea level is −135 m (for the Last Glacial Maximum), topographic high at the southernmost OT partially blocks the KC throughflow, resulting in a 43% reduction of KC inflow. Meanwhile, meandering is enhanced and deepwater ventilation is reduced. However, the KC does not migrate to the east off the OT as suggested previously. When sea level is −40 m (for the beginning of Holocene), the modeled flow pattern resembled present KC in terms of flow path and volume transport. Sea level fluctuation may act as a major control altering the KC course in the OT, leading to significant changes in horizontal and vertical water exchange. It may alter surface water properties, biogeochemistry in both water column and sediments below, potentially, downstream KC and climate over the northeast Asia.
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    A unique seasonal pattern in phytoplankton biomass in low-latitude waters in the South China Sea
    (2005-04-01) Tseng, C.-M.; G. T. F. Wong; I-I Lin; C.-R. Wu; K.-K. Liu
    A distinctive seasonal pattern in phytoplankton biomass was observed at the South East Asian Time-series Study (SEATS) station (18°N, 116°E) in the northern South China Sea (SCS). Surface chlorophyll-a, depth-integrated chlorophyll-a and primary production were elevated to 0.3 mg/m3, ∼35 mg/m2 and 300 mg-C/m2/d, respectively, in the winter but stayed low, at ≤0.1 mg/m3, ∼15 mg/m2 and 110 mg-C/m2/d as commonly found in other low latitude waters, in the rest of the year. Concomitantly, soluble reactive phosphate and nitrate+nitrite in the mixed layer also became readily detectable in the winter. The elevation of phytoplankton biomass coincided approximately with the lowest sea surface temperature and the highest wind speed in the year. Only the combined effect of convective overturn by surface cooling and wind-induced mixing could have enhanced vertical mixing sufficiently to make the nutrients in the upper nutricline available for photosynthetic activities and accounted for the higher biomass in the winter.