| As the conveyor of the global ocean, ocean circulation, the large scale movement of waters in the ocean basins, makes hydrological, chemical and thermohaline elements from the ocean in a relatively long term stability. The coupling effect between ocean circulation and atmosphere can affect the variability of global and regional climate directly and indirectly. In recent decades, the factor and rule of the variability of the climate and global environment, as well as their relationship with components of the earth system have attracted great attention of scientists from all countries. Therefore, it would be of great importance for meteorology, physical oceanography and other related geoscience to determine the ocean circulation and its law of motion critically. With hundreds years of progresses and contributions made by scientists using traditional physical oceanography methods to study ocean circulations, enormous outcome has been achieved. However, due to the imperfection of data source, data can be biased or even vacant for certain areas, which leads to the incapability of seamless coverage of the Earth. In the past20years, satellite altimetry technique has been greatly developed and sustained reliability for its application in deciphering the ocean. It has greatly enriched the data sources of the oceans and makes significant contributions to the research of global ocean circulation, which revolutionarily changes the way of traditional marine surveying.Based on the global mean sea surface height model and the time series of mean sea level anomaly derived from multi-satellite altimeter data, as well as the new global gravity model, this dissertation paper investigates the theory and method of establishing mean dynamic topography model with high accuracy and resolution, extracts global and regional characteristics of ocean circulation variabilities, analyzes the characteristics of variabilities of Kuroshio and its extension axis. Besides, the relationship between ocean circulation change and climate change has also been analyzed. Main contents and achievements of this paper include:?1? The basic theory of ocean current is introduced from the view of dynamics, including equations of motion, continuity equation, boundary conditions and the geostrophic equation. Two basic methods of calculating dynamic ocean topography are discussed in this paper. Some details in the calculation of dynamic ocean topography such as unification of reference, permanent tidal correction and noise filtering method are also expounded.?2? Base on the WHU2009mean sea surface height model, four different type of earth gravity field models named GOGONS2TIMR4, GOCO03S, ITG-GRACE2010S and EIGEN-CHAMP05S are used to determine different mean dynamic topography and corresponding ocean circulation models respectively. Comparison results with CLS09and DTU10models show that the GOCE gravity field model has great advantage in detecting ocean circulation not only in its higher order parts, but also presenting well in the mid-low order parts.?3? Based on the development of determining mean dynamic topography with high resolution, a new global mean dynamic topography model named WHU2014and its corresponding ocean circulation model with6×6'resolution are established from WHU2009global mean sea surface height model and EIGEN-6C3stat earth gravity field model. Through comparing with the VM08-HR and DTU13MDT models, we find that the precision of WHU2014model is higher than VM08-HR. By comparing the outcome of three models respectively with in situ measurements from DAC, the dissertation shows that WHU2014is remarkably better than both the VM08-HR and DTU13models, especially in the Brazil-Malvinas confluence region with the most complicated ocean current characteristics.?4? The mean sea level datasets derived from multi-satellite altimeter data are combined to extract the time series of ocean circulation velocity anomaly in both U and V components. Results show that the variabilities of the U and V components decrease linearly, with the annual trend as-0.045and-0.006?mm/s?/a respectively. Spectral analysis shows that the time series of ocean circulation velocity anomaly in the U and V directions have strongly seasonal signal and weakly decadal oscillation. The seasonal signal in the U direction is mainly distributed in regions where large scale strong ocean currents exist, such as low-latitude currents around the equator, the Gulf Stream, the Kuroshio Extension and the Antarctic circumpolar Current, etc. The seasonal signal in the V direction is weak in low-latitude currents around the equator, but strong in Arabian Sea and Gulf Stream, etc. Compared with the Southern Oscillation Index ?SOI? which representing E1Nino and La Nina phenomenon after noise smoothing, it can be found that time series in the U and V components show strong correlation with the SOI index, and their correlation coefficient is-0.65and-0.62respectively, indicating that the ocean circulation change could well reflect this anomalous climatic change.?5? Time temporal and spatial distribution of global Eddy Kinetic Energy ?EKE? is extracted based on the above result. The global EKE has obviously regional difference, the value is relatively high in regions where large scale strong current exists, such as Kuroshio and its extension, Gulf Stream, Antarctic Circumpolar Current, Agulhas Current and Brazil-Malvinas Confluence etc. Moreover, the high value of global EKE is also shown in the place where meso-scale eddies exist, such as Arabian Sea and South China Sea. The globe is over all in a approximate state of energy conservation during the period of1992.10.14to2013.04.17. the linear term of the average global EKE variability is-1.04cm2/s2with a annual trend of-0.05?cm2/s2?/a. Spectrum analysis of the global EKE variability time series indicates that its most obvious characteristic are seasonal signal and quasi-10a decadal oscillation. Besides, there are also relatively obvious periods of1.3a,3.3a and6.5a.?6? The temporal and spatial variability of EKE in the South China Sea ?SCS? is analyzed. An obvious decreasing trend is seen in SCS with linear trend of-12.3cm2/s2, and the annual trend is-0.6?cm2/s2?/a from1992.10.14to2013.04.17. It is found that the structure of EKE in SCS is the consequence of the superposition of multi-variability factors, and the higher EKE centers are observed in the east Vietnam and southwest Taiwan Island. The seasonal cycle is the most obvious timescale affecting EKE variability in SCS, especially for the east of Vietnam, whose seasonal variability controls the seasonal signal of EKE variability in the whole SCS and is attributed primarily to the annual cycle of wind stress curl. The seasonal signal of EKE variability is weak in the southwest Taiwan Island where obvious inter-annual and mesoscale variability exist, which is caused by Kuroshio intrusion and the interaction of cold eddies and warm eddies. Compared with the Southern Oscillation Index ?SOI? after noise smoothing, it can be found that generally the El Nino can affect the EKE variability in SCS. Their correlation coefficient is0.38.?7? The criterion of determining the axis of Kuroshio Current by multi-satellite altimetric data has been given, and the time series of variability of Kuroshio and its extension axis has been analyzed. The axis of Kuroshio is relatively stable, with a weak trend of shifting toward north, and has obvious seasonal signal. The variability of Kuroshio axis is affected by EKE change in south Japan. Compared with the Southern Oscillation Index ?SOI? after noise smoothing, it can be found that the variability of Kuroshio axis shows strong correlation with the SOI index, and the Kuroshio axis would shift toward south when El Nino happened.?8? Two different patterns of Kuroshio Extension ?KE? jet are found in this paper. We find that the period of1993-1995,2002-2005and2010-2012are stable years, while1996-2001and2006-2009show characteristics of instability. Compared with the variability in Kuroshio region, the variability of KE jet shows more intense, with the annual trend as0.03°/a in the upstream, while0.05°/a in the downstream. The most obvious cycle is decadal oscillation in Kuroshio Extension, and the seasonal signal is also strong. It is found that the low-frequency changes of the KE jet is connected to the variability of the southern recirculation gyre, the correlation coefficient between the strength of southern recirculation gyre and KE upstream is0.5after noise smoothing. The correlation coefficient is0.75and0.69between the position of KE jet in the upstream, downstream and the southern recirculation gyre respectively. Compared with the Pacific Decadal Oscillation ?PDO? after noise smoothing, it can be found that the time series of KE jet in both upstream and downstream as well as southern recirculation gyre show strong correlation with the PDO index, with their correlation coefficient as-0.48,-0.47and-0.61respectively, while the variability of KE jet lags of about4years. |
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