The dynamics of marginal seas is complex in terms of carbon dioxide absorption and release. This thesis analyzes data collected in the southern Taiwan Strait and in the South China Sea. In order to deduct the influence of temperature on the fCO2, fCO2 is normalized to the average water temperature (fCO2 mean). In the spring of 2008, in the Taiwan Strait, when salinity was smaller than approximately 33.8, measured fCO2 mean and salinity had a negative correlation; but when the salinity was higher than approximately 33.8, the correlation was positive. When salinity was smaller than apprx. 33.8, fCO2 cal. mean correlated slightly negatively to chlorophyll. This indicates that the low fCO2 cal. was not only caused by the increase of the CO2 solubility at lower temperatures, but also by the biotic photosynthesis. On the contrary, when the salinity was higher than apprx. 33.8, fCO2 cal. mean and the chlorophyll held positive correlation. It indicates that the influence of photosynthesis was reduced. In this case, the primary factor of fCO2 cal. change was due to the mixing of the high normalized dissolved inorganic carbon (NDIC=35¡ÑDIC/S) China Coastal Current with low NDIC seawater. With a raise of seawater temperature, then a decrease of the CO2 solubility, seawater became a source of carbon dioxide. In the summer of 2008, the northern South China Sea was influenced by Pearl River plume, resulting in lower fCO2 and salinity. The fCO2 of the China coast was influenced not only by the Pearl River plume, but also by the Jiulong River plume and upwelling. The Taiwan Strait water mass mainly contains the South China Sea water, a Kuroshio branch and the China Coast Current. During an El Niño year, the monsoon weakens, so that the volume of Kuroshio entering the South China Sea increases. However, for La Niña years, the monsoon strengthens, therefore the volume of the Kuroshio entering the South China Sea decreases. As a result, the Taiwan Strait water changes interannually due to different mixture of seawater of the Kuroshio and the South China Sea. The southern Taiwan Strait could be divided into the Penghu Channel and the western strait. During an El Niño summer, the Penghu Channel is occupied by waters with high temperature, salinity and pH, but low NDIC and nutrients. This is because more Kuroshio waters enter the South China Sea, then move northward to the southern Taiwan Strait. The hydrology in the Penghu Channel in normal years shows different result from season to season. In the summer, the Penghu Channel contains low temperature, salinity and pH water. In winter, waters with high salinity and pH, but low AOU, NDIC and nutrients prevail. This indicates that less Kuroshio waters enter the South China Sea in summer than in winter. The hydrology of the Penghu Channel changes decidedly from season to season in a normal year but spring, summer and fall have no clear change in the El Niño period, because more Kuroshio waters enter the South China Sea in summer. The wind effect during the El Niño period becomes weakened, have the hydrology during summer monsoon is similar to the hydrology in spring and summer. The waters of the Penghu Channel reach the highest pH, but the lowest AOU, NDIC and nutrients in winter. Older waters from upwelling move to the north in the western Strait during spring and fall in a normal year. However, during the El Niño period, possibly due to the weaker monsoon, such upwelling signal is reduced. Waters of the western strait in winter have higher temperature, salinity and pH, but lower NDIC during the El Niño period compared to a normal year. This indicates that the El Niño influences not only the Penghu Channel but also the entire southern Taiwan Strait in winter.

Recent studies have identified the North Pacific Gyre Oscillation (NPGO) as a decadal mode of climate variability that is linked to previously unexplained fluctuations of salinity, nutrient, and chlorophyll in the Northeast Pacific. The NPGO reflects changes in strength of the central and eastern branches of the subtropical gyre and is driven by the atmosphere through the North Pacific Oscillation (NPO) -the second dominant mode of sea level pressure variability. We show that Rossby waves dynamics excited by the NPO propagate the NPGO signature from the central North Pacific into the Kuroshio-Oyashio Extension (KOE), and trigger changes in strength of the KOE with a lag of 3 years. This suggests that the NPGO index can be used to track changes in the entire northern branch of the North Pacific sub-tropical gyre. These results also provide a physical mechanism to explain coherent decadal climate variations and ecosystem changes between the North Pacific eastern and western boundaries.

In this study, vertical profiles of velocity and hydrographic properties measured by the Lowered ADCP and CTD, respectively are used to calculate the vertical eddy diffusivity K based on small-scale turbulence theory. Two methods are used to estimate K, that is, the Thorpe scale analysis method (designated as Kz) and vertical wave number shear spectral method (designated as Ksh). Four different experiments with different flow conditions and bathymetry, i.e., internal tides, deep open-ocean, nonlinear internal waves and Kuroshio, are conducted and their K values are estimated and discussed. The internal tides at the mouth of Kao-Ping Submarine Canyon (KPSC) are observed during July and December (spring tide) of 2008. In each cruise the LADCP/CTD casts are repeated every two hours and last 27 and 40 hours, respectively. The results indicate the existence of strong, semi-diurnal internal tides with vertical displacement of 50~100 m and the nature of first baroclinic mode. Turbulent mixing during flood is significantly stronger than that during ebb. Note that in the winter experiments the Kz can reach 0.01 m2 s-1, which is even larger than the reported Kz values in other submarine canyons of the world, suggesting strong mixing processes are taking place in the KPSC. From the LADCP/CTD data of the joint hydrographic survey on May 2008 at SEATS station of the South China Sea, the estimated average values of Kz and Ksh in the upper 3000 m are about 3¡Ñ10-4 m2 s-1 and 1.8¡Ñ10-4 m2 s-1, respectively. The average value of Kz near the ocean bottom increases to 2.5¡Ñ10-3 m2 s-1. These estimated Kz are somewhat larger than the reported values in the open ocean. On the other hand, Kz values between 300 and 700 m deep are almost zero, indicating that turbulent mixing is inhibited in the stratified layer. Nonlinear internal waves are tracked in the South China Sea during May 2007. Our results show that after the internal solitons passed in the deep waters, the Kz profiles change significantly, surface mixing is weak, and Kz increases gradually from 400 m deep to the ocean bottom. In the shallow water region, shoaling effect of the nonlinear internal waves lead to enhanced energy dissipation and higher values of Kz, with the maximum value reaches 1 m2 s-1 near 180m depth. The flow structure of Kuroshio current between Taiwan and Lanyu is observed in October 2007. The results show that Kz in the surface layer is high (~10-2 m2 s-1), obviously due to strong Kuroshio flows there. At the 3000 m deep submarine trench near Lanyu, the Kz in the bottom layer is also very high (~ 1 m2 s-1 ), indicating that effective turbulent mixing in the bottom layer is mainly due to topography, which has similar level as the nonlinear internal waves.

Seasonal dynamics of unicellular diazotrophs were investigated in the upstream Kuroshio and the northern South China Sea (SCS). Unicellular diazotrophs had been postulated as an important N2-fixing contributor for the phenomenon of N* in the SCS where abundances of filamentous Trichodesmium and Richelia were scarced. Samples were collected during four cruises between August 2008 and August 2009 in summer (CR1310 and CR910), winter (CR886), and late spring (CR899), respectively. Sampling stations located between 21¢XN-22¢XN and 116¢XE-122¢XE in the upstream Kuroshio off southeast Taiwan and covering the shelf and basin waters of the northern SCS. The abundance of the unicellular diazotrophs was determined using whole-cell immunocytochemical method in which antibody of nitrogenase was used as the probe. Cells containing nitrogenase can be visualized and counted after the antigen-antibody reaction under microscope. Unicellular diazotrophs were classified to four types according to their sizes and shapes. For diameters of those with 1-3 £gm and in coccoid shape are called 1-3 £gm C, diameters of 1-3 £gm and in rod shape are called 1-3 £gm R, diameters of >3-10 £gm and in coccoid shape are called >3 £gm C, and diameters of >3-10 £gm and in rod shape are called >3 £gm R. Surface abundance of the unicellular diazotrophs was highest in winter in both the Kuroshio and the SCS, followed by summer, and was least in late spring. Among four cell types, 1-3 £gm C usually was the most abundant group, followed by 1-3 £gm R and >3 £gm R, and was least for the group of >3 £gm C. The abundances between groups of 1-3 £gm C and 1-3 £gm R were positively correlated. Likewise, the abundances between >3 £gm C and >3 £gm R were positively correlated. However, the total abundance of small cells (1-3 £gm C+R) was not significantly related to the large cells (>3 £gm C+R). During summer and late spring, the abundance of unicellular diazotrophs in the SCS was 1.3-2 times of that in the Kuroshio. However, in winter the abundance in the Kuroshio was 1.2 times of that in the SCS. Surface water temperature was found negatively correlated to the abundance of 1-3 £gm C, >3 £gm C, >3 £gm R, and large cells (>3 £gm C+R), respectively. Significant correlations among surface water temperature and surface chlorophyll a, [NO2+NO3], SRP and N:P ratio implicated that the dynamics of cell abundances could be attributed to the correlated ecological variables of surface water temperature. The dynamics for the abundances of >3 £gm C, >3 £gm R, and large cells (>3 £gm C+R) were suggested to relate with the fluctuation of SRP concentration. Unicellular diazotrophs accounted for 60-90 % of total unicellular cells in terms of cell number. Vertical distributions of unicellular diazotrophs in the Kuroshio and the SCS were in similar trends, with maximum abundance in deep water during summer and late spring, and on surface water during winter.

This study utilized, for the first time, the d18Osw as a tracer to investigate the seasonal variations of circulation in the Taiwan Strait (TS), which is the predominant sea passage with an average depth of 60 m connecting the East China Sea (ECS) and the South China Sea (SCS). The result shows that the circulation system in TS is mainly influenced by the inter-mixing among the China Coastal Water (CCW), the SCS water (SCSW), and the Kuroshio Water (KW). In spring, the KW dominates in TS, whereas the CCW is still observed in northwest TS. During the summer, SCSW replaces the KW and becomes the major water type in the TS, yet the KW is found to be restricted in the southwest part and the bottom of the TS. Due to the larger discharge from rivers (mainly the Yangtz River), the CCW has a more extensive distribution in the TS in summer than other seasons. In fall and winter, the CCW occupies the northern part of TS due to the stronger northeastern monsoon which limits the intrusion of the KW through the Luzon Strait to the northern TS. The two distinct water types inevitably form a front in the central TS. The hydrographic variations at Penghu Channel (PHC) were also explored in this study. The d18Osw indicates that the perennial intrusion of the KW into the PHC is varying throughout different seasons. This intrusion is found strongest in fall and winter. In summer, the upper layer of PHC is occupied chiefly by SCSW, while the KW remains at the bottom layer in PHC. By including an additional inflow of 0.5Sv from TS to ECS, this study further reconstructed a box model of the ECS, which was previously furnished by Lin(1999). The new estimates suggest that ~0.38*104 km3/year of the Kuroshio surface water (0-50m) and ~1.54*104 km3/year of the upwelled Kuroshio subsurface water (50-150m) are transported to the ECS, while ~3.83*104 km3/year of the ECS water are exported to the western Pacific Ocean.

La variabilité du contenu de chaleur hivernal de l'océan supérieur et ses mécanismes de causalité ont été étudiés en utilisant des observations et des produits de réanalyse dans le Pacifique Nord-Ouest. La relation entre la mousson d'hiver dans l'Asie de l'Est (EAWM) et l'Oscillation du Pacifique Nord (NPO) et leurs impacts sur la température de surface de la mer (SST) sont non stationnaires, avec un changement soudain en 1987/1988. L'EAWM et la NPO, qui étaient bien corrélées en 1973-1987, ne sont pratiquement plus corrélées en 1988-2002. Cette relation non stationnaire est liée au fort affaiblissement décennal de la haute pression Sibérienne après le changement de régime en 1988, ainsi qu'au changement concomitant du dipôle de la NPO positive. L'influence de l'EAWM et de la NPO sur la SST hivernale dans la région d'étude a significativement diminué après 1990. Le bilan de chaleur dans les 400 premiers mètres a été analysé à l'aide des sorties d'un modèle de la circulation océanique générale à haute résolution. Le taux de stockage de chaleur hivernal des échelles interannuelles à décennales est principalement déterminé par l'advection océanique plutôt que par le flux net de chaleur air-mer. Le rôle de l'advection de chaleur devient particulièrement important après le changement de régime en 1990, en association avec la réduction de la variabilité du flux de chaleur en surface causée par une faible variabilité de la SST. Le flux net de chaleur air-mer freine les anomalies thermiques créées par la dynamique océanique associée avec le déplacement méridien du front de l'Extension de l'Oyashio, qui est fortement corrélé avec les modes de téléconnexion de WP et PNA. / Winter upper-ocean heat content variability and its causal mechanisms are investigated using observational and reanalysis products in the western North Pacific. The relationship between the East Asian winter monsoon (EAWM) and the North Pacific Oscillation (NPO) and their impact on the sea surface temperature (SST) are nonstationary, with a sudden change at 1987/1988. During the 1973-87, the EAWM and NPO were significantly correlated to each other, but their correlation practically vanishes during the 1988-2002. This nonstationary relationship is related to the pronounced decadal weakening of the Siberian high after the 1988 regime shift as well as the concomitant positive NPO-like dipole change. The influence of EAWM and NPO to the winter SST in the study region is significantly decreased after the sudden change near-1990. The upper 400 m heat budget in the western North Pacific is analyzed using outputs from a high resolution ocean general circulation model. Winter heat storage rate on interannual to decadal time scales is mainly determined by oceanic heat advection rather than by net air-sea heat flux. The role of heat advection becomes particularly prominent after the 1990 regime shift in association with the reduced variability of surface heat flux caused by weakened SST variability. The net heat flux acts to dampen temperature anomalies caused by the ocean dynamics principally associated with the meridional shift of the Oyashio Extension front, which is significantly correlated with the West Pacific (WP) and Pacific-North America (PNA) teleconnection patterns.