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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. HsinThe 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.Item Variability analysis of Kuroshio intrusion through Luzon Strait using growing hierarchical self-organizing map(Springer-Verlag, 2012-08-01) Tsui, I.-F.; C.-R. WuAn advanced artificial neural network classification algorithm is applied to 18 years of gridded mean geostrophic velocity multi-satellite data to study the Kuroshio intrusion into the South China Sea through the Luzon Strait. The results suggest that the Kuroshio intrusion may occur year round. However, intrusion is not the major characteristic of the region. The intrusion mode occurs only 25.8 % of the time. Winter intrusion events are more frequent than summer events. Both stronger intrusion (which is related to wind speed) and weaker intrusion (which may be related to the upstream Kuroshio transport) may occur during winter, but stronger intrusion is dominant. In summer, the Kuroshio intrusion is almost the weaker type. The Kuroshio intrusion through the Luzon Strait usually occurs when the Pacific decadal oscillation index is positive (72.1 % of the time). This study shows that growing hierarchical self-organizing map is a useful tool for analyzing Kuroshio intrusion through the Luzon Strait.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. OeyObservations 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.Item 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. ShawA 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.Item 雷達測波資料展示系統(2007-01-01) 吳朝榮; 楊穎堅Item Transient, seasonal and interannual variability of the Taiwan Strait Current.(Springer Netherlands, 2007-10-01) Wu, C.-R.; S.-Y. Chao; C. HsuWe 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.Item Enhanced primary production in the oligotrophic South China Sea by eddy injection in spring(American Geophysical Union (AGU), 2010-08-01) Lin, I-I; C.-C. Lien; C.-R. Wu; G. T. F. Wong; C.-W. Huang; T.-L. ChiangIn May 2003, a phytoplankton bloom of chlorophyll-a (Chl-a) concentration of 0.3–0.4 mgm−3 was observed at the centre of northern South China Sea (SCS) by NASA's Sea-viewing Wide Field-of-View sensor. As this region is remote and known to be oligotrophic in spring (Chl-a concentration typically at ∼0.05–0.08 mgm−3), it is intriguing to explore this unusual happening. Based on six different remote sensing data and numerical modelling, the results suggest that the injection of an ocean eddy is the most likely cause of the bloom. Due to long-range transport of a large (700 × 500 km) anti-cyclonic ocean eddy, coastal nutrients and plankton could be brought across hundreds of kilometres to the centre of northern SCS and impact the biogeochemistry. The open ocean part of the northern SCS basin has long been considered generally free from coastal influences. This work provides new evidence that proves otherwise. Moreover, from the perspective of physical oceanography, it is interesting to observe that, outside the monsoon seasons, there can be well-defined anti-cyclonic ocean circulation existing in the SCS without the prevailing monsoonal wind.Item 西北太平洋颱風潛熱(Tropical Cyclone Heat Potential TCHP)之研究(2004-05-20) 潘任飛; 林依依; 吳朝榮在近來的研究中有相當多的證據顯示颱風強度的增加與海洋中的暖特徵現象 (warm oceanic features)有關，並且利用颱風潛熱(TCHP)來代替傳統的海水表 面溫度(SST)來研究颱風的強度(Shay 2000, Goni & Trinanes 2003)，因為 SST只能代表的海水表面溫度而無法代表上層海洋的垂直結構(Upper ocean thermal structure)，因此TCHP能夠清楚地描述海洋的暖特徵，如暖渦(warm core ring, WCR)；早在1972年Leipper就把TCHP 定義出來，但由於當時沒有衛 星資料的幫助，一直無法運用，直到近年由於衛星遙測的進步，才被開始被應用 ，TCHP的定義是從海面到26度C等溫線的垂直溫度積分，如下：Q/sub H/(x,y,t)=.rho.C/sub p/.int./sup 0//sub (Y=26)) .DELTA.T(x,y,z,t)dz每 年有不少的強烈颱風在西北太平洋(Northwest Pacific Ocean, NWPO)產生出來 ，但目前全球對颱風強度的預測並不是很理想，因此藉著研究NWPO的TCHP和 TCHP與颱風之間的關係來提昇我們對颱風強度之預測。本研究是採用美國海軍實 驗室(US Naval Research Laboratory)所發展的三維數值模式(NPACNFS, North Pacific Ocean Nowcast/Forecast System)，此模式是修改自Princeton Ocean Model (Blumberg and Mellor, 1987)，使模式能作資料同化(data assimilation)，其作同化的資料有T/P的SSHA和由衛星所得到的海表面溫度 (SST)。整個太平洋模式蓋了16度S~60度N，99度E~77度W的區域，而模式的垂直 解析度是用26層的sigma-levels組成，在上層海洋的分層數比較密，所以此模式 對上層海洋的解析度比較高，有利於對上層海洋的研究。利用此模式的結果，可 以算出北太平洋的TCHP。本研究並把由NPACNFS 和由美國大氣海洋總署(AOML)利 用來two-layer reduced gravity model 計算出來2003 年夏天的TCHP作比較。Item Mindanao Current/Undercurrent in an Eddy-Resolving GCM(American Geophysical Union (AGU), 2012-06-01) Qu T.; T.-L. Chiang; C.-R. Wu; P. Dutrieux; D. HuAnalysis of results from an eddy-resolving general circulation model showed two subsurface velocity cores in the mean within the depth range between 400 and 1000 m below the Mindanao Current (MC). One is confined to the inshore edge at about 126.8°E and connected with the Sulawesi Sea. The other takes place somewhat offshore around 127.7°E, being closely related to the intrusion of South Pacific water. Both cores are referred to as the Mindanao Undercurrent (MUC). The MC/MUC is approximately a geostrophic flow, except on the inshore edge of the MUC where up to 50% of the mean flow can be explained by ageostrophic dynamics. In contrast with the well-defined southward flowing MC, the MUC is of high velocity variance relative to the mean. Empirical orthogonal function (EOF) analysis shows that approximately 60% of the total velocity variance is associated with two meandering modes, with their major signatures in the subthermocline. The dominant time scale of variability is 50–100 days. An ensemble of these meso-scale fluctuations provides a northward freshwater flux on the offshore edge of the Philippine coast, which to a certain extent explains why water of South Pacific origin appears to extend farther northward than the mean MUC. In the offshore velocity core of the MUC, for example, eddy induced freshwater flux is equivalent to a mean flow of about 0.3 m s−1 in the density range between 26.9 and 27.3 kg m−3, which is greater than the mean current by a factor of 6.Item Interannual modulation of the Pacific Decadal Oscillation (PDO) on the low-latitude western North Pacific.(ELSEVIER, 2013-03-01) Wu, C.-R.To investigate the interannual variability in the northwestern Pacific, an empirical mode decomposition (EMD) was applied to 17-year Absolute Dynamic Topography (ADT) data west of Luzon Island, the Philippines. The mean sea surface height in this area is an appropriate index for the Kuroshio intrusion into the South China Sea (SCS). Significant interannual fluctuations were extracted by the EMD. The interannual variability was strongly correlated with the Pacific Decadal Oscillation (PDO) index, but not the El Niño–Southern Oscillation (ENSO). This indicated the potential impact of the PDO on the circulation in the area. In the warm phase of the PDO (positive index), a southerly anomalous wind off the Philippines causes a northward shift of the North Equatorial Current Bifurcation Latitude (NECBL). This leads to a weakened Kuroshio off Luzon, favoring Kuroshio intrusion into the SCS. The northward migration of the NECBL also results in a weakened Kuroshio off southeast Taiwan and a larger Kuroshio transport off northeast Taiwan. The abundant westward propagating eddies impinging on the Kuroshio in the Subtropical Countercurrent region increases this transport. Although the ENSO has little effect on monsoonal winds during the warm PDO phase, it has a strong impact on the monsoon and meridional migration of the NECBL during the cold phase of the PDO. Therefore, NECBL variations only show a close correspondence with the ENSO during the cold PDO phase. Because the influence of the ENSO is not stationary, the impact of the PDO should be taken into account when examining interannual variability in the low-latitude western North Pacific.