教師著作

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Air-sea interaction between Tropical Cyclone and Western Boundary Current
(2008-07-11) Wu, C.-R.; Y.-L Chang
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.
Air-Sea Interaction between Typhoon Nari and Kuroshio
(2007-05-18) Wu, Chau-Ron; Chang, Yu-Lin
The 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.
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.
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).
Contrasting the evolution between two types of El Ni隳 in a data assimilation model
(Springer-Verlag, 2013-05-01) Wu, C.-R.; L.-C. Wang
Simulation outputs were used to contrast the distinct evolution patterns between two types of El Niño. The modeled isotherm depth anomalies closely matched satellite sea surface height anomalies. Results for the El Niño Modoki (central Pacific El Niño) corresponded well with previous studies which suggested that thermocline variations in the equatorial Pacific contain an east–west oscillation. The eastern Pacific El Niño experienced an additional north–south seesaw oscillation between approximately 15° N and 15° S. The wind stress curl pattern over the west-central Pacific was responsible for the unusual manifestation of the eastern Pacific El Niño. The reason why the 1982/1983 El Niño was followed by a normal state whereas a La Niña phase developed from the 1997/1998 El Niño is also discussed. In 1997/1998, the Intertropical Convergence Zone (ITCZ) retreated faster and easterly trade winds appeared immediately after the mature El Niño, cooling the sea surface temperature in the equatorial Pacific and generating the La Niña event. The slow retreat of the ITCZ in 1982/1983 terminated the warm event at a much slower rate and ultimately resulted in a normal phase.
Contrasting the flow patterns in the equatorial Pacific between two types of El Ni隳.
(Taylor & Francis: STM, Behavioural Science and Public Health Titles, 2012-11-01) Wang, L.-C.; C.-R. Wu
Outputs based on the National Centers for Environmental Prediction (NCEP) Global Ocean Data Assimilation System (GODAS) are adopted to contrast the current variations in the equatorial Pacific between two types of El Niño. The model fully resolves the equatorial currents. We found that the central Pacific El Niño (CP-El Niño) corresponds well with previous El Niño studies in that both the eastward Equatorial Undercurrent (EUC) and westward South Equatorial Current (SEC) weaken. On the other hand, the eastern Pacific El Niño (EP-El Niño) displays a distinct circulation pattern. The North Equatorial Countercurrent (NECC) strengthens in the developing phase and persists into the peak of the warm event, whereas the northern branch of the SEC (SECn) also intensifies during the mature phase and lasts for about six months. The South Equatorial Countercurrent (SECC) strengthens during the decaying phase of the EP-El Niño. The shifting of the wind stress curl associated with the thermocline variability is chiefly responsible for the unique current performance of the EP-El Niño. It is worth noting that the air–sea interaction plays an important role in the current variability not only during a CP-El Niño but also during an EP-El Niño. RÉSUMÉ [Traduit par la rédaction] Nous adoptons les sorties basées sur le système GODAS (Global Ocean Data Assimilation System) des NCEP (National Centers for Environmental Prediction) pour mettre en évidence les variations de courant dans le Pacifique équatorial entre les deux types d'El Niño. Le modèle représente complètement les courants équatoriaux. Nous trouvons que l'El Niño du centre du Pacifique (CP-El Niño) correspond bien aux études précédentes sur l'El Niño puisque le sous-courant équatorial (EUC) vers l'est et le courant sud-équatorial (SEC) vers l'ouest faiblissent. D'autre part, l'El Niño de l'est du Pacifique (EP- El Niño) affiche une configuration de circulation distincte. Le contre-courant nord-équatorial (NECC) se renforce dans la phase de développement et persiste jusqu'au maximum du réchauffement, tandis que la branche nord du SEC (SECn) s'intensifie aussi durant la phase de maturité et persiste pendant environ six mois. Le contre-courant sud-équatorial se renforce durant la phase de dissipation de l'EP-El Niño. Le changement du rotationnel de la tension du vent lié à la variabilité thermocline est principalement responsable du comportement particulier du courant de l'EP-El Niño. Il est à remarquer que l'interaction air–mer joue un rôle important dans la variabilité du courant, non seulement durant un CP-El Niño mais aussi durant un EP-El Niño.
Editorial - International Workshop on Modeling the Ocean (IWMO) special issue in Ocean Dynamics
(Springer-Verlag, 2010-04-01) Oey, L.-Y.; T. Ezer, Y. Miyazawa; C.-R. Wu
Editorial - International Workshop on Modeling the Ocean (IWMO) special issue part 2 in Ocean Dynamics
(Springer-Verlag, 2010-10-01) Oey, L.-Y.; T. Ezer; Y. Miyazawa; C.-R. Wu
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.
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. Chiang
In 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.
Field Observations of Changes in SST, Chlorophyll and POC Flux in the Southern East China Sea Before and After the Passage of Typhoon Jangmi.
(Chinese Geoscience Union, 2013-10-01) Shih, Y.-Y.; J.-S. Hsieh; G.-C. Gong; C.-C. Hung; W.-C. Chou; M.-A. Lee; K.-S. Chen; M.-H. Chen; C.-R. Wu
Severe tropical storms play an important role in triggering phytoplankton blooms, yet direct field observation of evidence of the effects of a typhoon is very rare. Sea surface temperature (SST), nitrate concentration, chlorophyll a (chl a) concentration, and particulate organic carbon (POC) flux were measured before and shortly after Typhoon Jangmi which affected the southern East China Sea (SECS) on September 28 ~ 29, 2008. In situ SST (27.5 ~ 28.0°C) on September 19 ~ 21, decreased to ~24.0°C (October 3 ~ 6) in the SECS 4 ~ 7 days after the passage of Typhoon Jangmi. In situ nitrate and chl a concentrations 7-days (on October 6) after the passage of Jangmi were 1.9 μM and 1.61 mg m-3, respectively, much higher than those (nitrate: 0.3 μM and chl a: 0.73 mg m-3) concentrations before the typhoon (September 21). The enhanced chl a concentration is thus caused by a nutrient supply via vertical mixing or upwelling in the euphotic zone. The POC flux 7-days after Jangmi’s passage was 552 ± 28 mg-C m-2 d-1, a ~2.5-fold increases before the typhoon (224 ± 33 mg-C m-2 d-1, on September 21). Our results suggest that typhoons indeed can stimulate efficient POC export out of the euphotic zone, while it is still poorly understood with regard to the total effects of a typhoon on nutrient dynamics and detailed carbon sequestration due to sampling difficulty. Therefore, successional sea-going observations ought to be conducted in the affected area after the passage of typhoons.
Fine-Resolution Modeling Studies in the Northern South China Sea
(2004-05-20) Wu, Chau-Ron; Hsin, Yi-Chia; Jiang, Tzu-Ling
Recently, 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.
Fluctuations of the thermal fronts off northeast Taiwan.
(American Geophysical Union (AGU), 2011-10-01) Hsin, Y.-C.; T.-L. Chiang; C.-R. Wu
A high-resolution sea surface temperature (SST) data derived from several satellites is used to investigate the variability of the thermal front off northeastern Taiwan. Hidden by a dominant annual cycle, the SST data cannot reveal the thermal front fluctuation in the form of Hovm闤ler diagram. An innovative methodology has been applied to the SST satellite imagery to derive the SST Standardized Index (SSTSI), capable of revealing the frontal variability with multiple time scales. Principal component analysis shows that the SSTSI variation consists mainly of two modes. Mode 1 represents a strong annual cycle related to the seasonal reversal of the monsoonal winds. The temperature gradient is enhanced in winter and a cold dome is observed off northern Taiwan in summer. Mode 2 is highly correlated with the upstream Kuroshio variability. The shoreward (seaward) migration of the thermal front takes place when the Kuroshio transport weakens (strengthens). The results are consistent with transports estimated by tidal gauge measurements, satellite altimeter-based sea level anomaly, and surface flow patterns derived from high-frequency radars. Mode 2 is coherent with the Kuroshio transport through the East Taiwan Channel at periods of 120 and 45 d with a time lag of 40 and 11 d, respectively. This 120 d fluctuation is due to the interaction between westward-propagating eddies and the Kuroshio east of Taiwan, while the 45 d signal arises from the Kuroshio's self-instability. The interannual variations of the SST pattern in winter and summer are also discussed.
The forcing mechanism leading to the Kuroshio intrusion into the South China Sea
(American Geophysical Union (AGU), 2012-07-01) Wu, C.-R.; Y.-C. Hsin
We use a high-resolution numerical model to examine the forcing mechanism responsible for Kuroshio intrusion into the South China Sea (SCS). The collective wisdom is that variations in Kuroshio intrusion are closely related to the wind, inside or outside the SCS. A series of experiments was performed to identify the wind-related forcing regulating the intrusion. The experiments demonstrated that the importance of wind inside the SCS is greater than that outside the SCS. Furthermore, the northwestward Ekman drift due to northeasterly wind in winter intensifies the upstream Kuroshio in the Luzon Strait, enhancing the Kuroshio intrusion into the SCS. In particular, the wind stress curl (WSC) off southwest Taiwan is chiefly responsible for the Kuroshio intrusion. Both the WSC and intrusion show both seasonal and intraseasonal variation. As the negative WSC off southwest Taiwan becomes stronger, it contributes to anticyclonic circulation. The enhanced anticyclonic circulation helps the development of the Kuroshio intrusion. The consistency between WSC variability and the intrusion suggests that the WSC off southwest Taiwan is essential to the Kuroshio intrusion variability.
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.
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.
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.
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.
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.