教師著作

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

Browse

Author: search.filters.author.L.-Y. OeyEnd date: 2013

Search Results

Now showing 1 - 5 of 5
  • No Thumbnail Available
    Item
    Typhoon Kai-Tak: An ocean's perfect storm
    (American Meteorological Society, 2011-01-01) Chiang, T.-L.; C.-R. Wu; L.-Y. Oey
    An unusually intense sea surface temperature drop (ΔSST) of about 10.8°C induced by the Typhoon Kai-Tak is observed in the northern South China Sea (SCS) in July 2000. Observational and high-resolution SCS model analyses were carried out to study the favorable conditions and relevant physical processes that cause the intense surface cooling by Kai-Tak. Upwelling and entrainment induced by Kai-Tak account for 62% and 31% of the ΔSST, respectively, so that upwelling dominates vertical entrainment in producing the surface cooling for a subcritical storm such as Kai-Tak. However, wind intensity and propagation speed alone cannot account for the large ΔSST. Prior to Kai-Tak, the sea surface was anomalously warm and the main thermocline was anomalously shallow. The cause was a delayed transition of winter to summer monsoon in the northern SCS in May 2000. This produced an anomalously strong wind stress curl and a cold eddy capped by a thin layer of very warm surface water west of Luzon. Kai-Tak was the ocean’s perfect storm in passing over the eddy at the “right time,” producing the record SST drop and high chlorophyll-a concentration.
  • No Thumbnail Available
    Item
    Blocking and westward passage of eddies in the Luzon Strait.
    (ELSEVIER, 2010-10-01) Sheu, W.-J.; C.-R. Wu; L.-Y. Oey
    Satellite observations have shown the abundance of generally westward-propagating eddies in the subtropical regions in the North Pacific Ocean, especially north of 10衹. Eddies transport mass, and can significantly impact the circulation as well as the heat, salt and nutrient balances of the western Pacific marginal seas. This paper uses a numerical model to examine the conditions when eddies can or cannot freely propagate westward through the Luzon Strait into the South China Sea (SCS). Composite analyses on the 10-year model data show that the fates of eddies depend on the strength and path of the Kuroshio. In one path that exists mostly during fall and winter, the Kuroshio loops westward into the SCS, the potential vorticity (PV) across the current is weak, and eddies are likely to propagate freely through the Luzon Strait. In another path, which exists mostly during spring and summer, the Kuroshio tends to leap directly northward bypassing the SCS, the PV across it strengthens, and eddies are then blocked and are constrained to also follow the northward path. Nonlinear eddy-current interaction and the existence of a cyclone north of the Luzon Island during the looping phase explain why eddies of both signs can pass through the strait. It is shown also that the upstream state of the Kuroshio in the western tropical Pacific plays an important role in dictating the different paths of the Kuroshio. The looping (leaping) path is caused by a weakened (stronger) Kuroshio transport related to the northward (southward) shift of the North Equatorial Current in wintertime (summertime).
  • No Thumbnail Available
    Item
    Why are there upwellings on the northern shelf of Taiwan under northeasterly winds?
    (American Meteorological Society, 2010-06-01) Chang, Y.-L.; L.-Y. Oey; C.-R. Wu; H.-F. Lu
    Upwellings are observed on the northern shelf of Taiwan during northeasterly winds. Analytical and realistic numerical models are used to explain how vertical motions are created by divergence and convergence produced by wind acting on the vorticity field of two strong jets: the Kuroshio and the Taiwan Warm Current. The seaward increase in cyclonic vorticity near the Kuroshio’s western edge favors a stronger Ekman transport away from the jet, producing upwelling at the shelfbreak under a northeasterly wind. A similar mechanism for generating vertical motions is found across the Taiwan Warm Current. The numerical model results indicate that the vorticity effects can account for up to 30%–50% of the total variation in the surface Ekman transport. Except during summer’s weak southwesterlies, northeasterly wind is dominant over the East China Sea, suggesting that the vorticity effects may be prominent in the observed shelfbreak upwelling in nonsummer months.
  • No Thumbnail Available
    Item
    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.
  • No Thumbnail Available
    Item
    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.