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Study Of Spatial And Temporal Characteristics Of The South China Sea Mesoscale Eddy

Posted on:2011-10-09Degree:DoctorType:Dissertation
Country:ChinaCandidate:G X ChenFull Text:PDF
GTID:1110330332996945Subject:Physical oceanography
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Ocean experiments in the 1970s revealed that mesoscale eddies with strong currents exist almost everywhere where was once thought to be low current area. Ocean eddies play important roles in dynamics as well as the transport of heat, salt, mass, momentum and other chemical properties in the ocean, and therefore affect circulation structure, mass distribution, marine organism and so on. Thus, eddies become one of the key points in the physical oceanography and also in the inter-disciplinary studies.The dissertation focuses on the distribution, variation, dynamical mechanism and the environmental effects of mesoscale eddies in the South China Sea (SCS). The dissertation not only describes the distribution and variations of eddies in the SCS as a whole, but also investigates the characteristics of representative eddies in the SCS. Multiple data are used in this dissertation, including remote sensing data (altimetry products of sea level anomaly and geostrophic flow, QuickSCAT wind product and so on), hydrographic data (conductivity–temperature–depth data covering large areas in the Philippine Sea and northern South China Sea near the Luzon Strait in September 2008, late June 2009 and so on), Argo data and assimilation data (e.g. SODA).The dissertation firstly studies the variability characteristics and mechanisms in different time scales of eddy kinetic energy (EKE) in the SCS, and then investigates the characteristics of representative eddies in two high eddy activities regions corresponding to higher EKE centers. Finally, mean properties and the spatio-temporal variability of eddies in the SCS are examined.We find that the higher EKE centers are observed to the east of Vietnam and to the southwest of Taiwan Island. We also find that the EKE structure is the consequence of the superposition of different variability components. First, interannual variability is important in the SCS. El Ni?o affects the EKE interannual signal variability in the SCS through an atmospheric bridge—wind stress curl over the SCS. Second, the seasonal cycle is the most obvious timescale affecting EKE variability, especially to the east of Vietnam, which is attributed primarily to the annual cycle there of wind stress curl. Finally, the most obvious intra-annual and mesoscale variability, which is the result of baroclinic instability of the background flow, are observed to the southwest of Taiwan Island.To the southwest of Taiwan Island, we examined the vertical structure and evolution of the Luzon Warm Eddy (LWE). The analysis shows that the LWE extends vertically to more than 500 m water depth, with a higher temperature anomaly of 5°C and lower salinity anomaly of 0.5 psu in the vicinity of the thermocline. The current speeds of the LWE are stronger in its uppermost 200 m, with a maximum speed of 0.6 m/s. Besides, we confirmed the intrusion of the Kuroshio into the SCS in September 2008. The Kuroshio obviously intrudes on the SCS from 20°N to 21°N, and mainly exists in the upper 300 m. The westward extension of the Kuroshio reaches as far west as 117°E, loops around an anticyclonic eddy and returns to the Pacific further north. The anticyclonic eddy is generated southwest of Taiwan in early July but is not induced by the Kuroshio intrusion. Furthmore, we discussed water exchange and circulation structure near the Luzon Strait in early summer 2009 and pointed out that only one meridional section data along the Luzon Strait used by many previous studies is difficult to understand the water exchange characteristics in the Luzon Strait.To the east of Vietnam, we confirmed the eddy pair is a seasonal phenomenon usually found in the summer to fall. It can reach downward to about 400 m depth but is strongest at the upper 100 m depth. In addition ,the eddy pair displays a remarkable interannual variability, which is mainly attributed to the interannual variability of the local wind stress curl. Moreover, the eddy pair has an obvious impact on the thermohaline structure of the local upper ocean. Furthermore, numerical study reveals that the cyclonic eddy is mainly attributed to the local larger positive wind stress curl, strengthened by the cyclonic circulation during summer in the northern SCS. The anticyclonic eddy is generated by the combination of local negative wind stress curl, wind stress paralleling the coastline off southeast Vietnam and heat flux and strengthened by the anticyclonic circulation during summer in the southern SCS. Finally, we investigated mean properties and the spatio-temporal variability of eddies in the SCS by analyzing more than 7000 eddies corresponding to 827 eddy tracks. Eddies are mainly generated in a northeast–southwest direction and southwest of Luzon. There is no significant difference between the numbers of two types of eddies (anticyclonic and cyclonic) in most regions. The mean radius and lifetime of eddies is 132 km and 8.8 weeks, respectively, both depending on where the eddies formed. Anticyclonic and cyclonic eddies tend to deform during their lifetimes in different ways. Furthermore, eddy propagation and evolution characteristics are examined. In the northern SCS, eddies mainly propagate southwestward along the continental slope with velocities of 5.0–9.0 cm/s, while in the central SCS, eddies tend to move with slight divergence, but still in a quasi-westward direction, with velocities of 2.0–6.4 cm/s. Eddy propagation in the western basin to the east of Vietnam is quite random, with no uniform propagate direction. In addition, the effect of eddies on the temperature profile is analyzed using 763 Argo temperature profile data. Anticyclonic eddies lower the thermocline obviously, while cyclonic eddies raise the thermocline significantly. Moreover, eddy temporal variations are examined at seasonal and interannual scales. Analysis of the seasonal cycle of eddies shows that more cyclonic and anticyclonic eddies are generated in winter and summer, respectively. At interannual scales, eddy activity might be sensitive to El Ni?o events.
Keywords/Search Tags:South China Sea, mesoscale eddy, numerical modeling, seasonal cycle, interannual variability, vertical structure, Luzon Strait, eastern Vietnam, Spatio-temporal variability
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