Effects Of Tide And Wind On Changjiang River Plume Dispersal And Mixing Processes

River plumes are common features on the continental shelf around the world. They are produced by inflows from a coastal buoyancy source, such as a river or estuary. Plumes of buoyant water emptying into the sea will cause not only the horizontal dispersal of salinity and temperature, but also some vertical mixing process by the effect of the hydrological process and the external forcing. There are remarkable contrasts between plume water and ambient ocean water, such as salinity, temperature and suspending deposit. The plume water has similar property and forms front at the boundary between the plume water and the adjacent ocean water. Many dynamical processes might affect the dispersal and mixing of the river plume, and they are usually characterized by high diversity and variability. The plume region is not only affected by the variation of river runoff, such as the seasonal flood-dry variability of the runoff, but also the ocean dynamical processes, such as the flood-ebb and the spring-neap tidal cycles. Wind forcing and shelf circulation might also play important roles. The interaction of these processes causes the estuary-continental shelf processes to become more complicated. Many plume regions will never reach a steady state, since too many factors are working together on these regions. People don't know whether the observed phenomena are common features or specific cases, since there are no similar estuary-continental plume systems in the world.In order to understand the plume dynamics, we developed a ROMS model for the Changjiang estuary-East China Sea region. Based on diagnostic analysis of the historic data and the comparisons with model simulations, we have established a high resolution hydrodynamic model for the China Seas. We have conducted many numerical simulations to investigate the little known physical mechanisms of tide and wind effects on the dispersal and mixing processes of the Changjiang River plume. The physical mechanism of tidal mixing affecting the bulge region and the buoyancy coastal current is revealed. We also found that tidal mixing will substantially change the freshwater and volume transport of the buoyancy coastal current and discussed the possible mechanisms. The flood-ebb asymmetry of the interaction between wind and tide generate a semi-diurnal variability of the plume thickness, which provides some new insights to the tidal-straining theory.Tidal currents can affect plume dispersal by modulating estuarine outflow or generating turbulent mixing that reduces the density contrast between the fresh plume water and ambient shelf water. It is found that tidal induced turbulent mixing show large variability over the spring-neap tidal cycle. During the neap tides, the turbulent mixing is relatively weaker and the freshwater plume can spread horizontally and maintains its buoyancy contrast. As the tide transitions from the neap to spring tides, tidal current gets stronger and thus the turbulent mixing. The plume detachment occurs when tidally generated turbulent mixing is strong enough to break down the stratification outside the river mouth, thus causing the disintegration of the river plume into two distinct regions. This might provide another explanation of the observed freshwater patch outside the river mouth. Tide also modifies the structure and dynamics of the buoyancy-driven coastal current on the continental shelf. In the absence of tide, the buoyancy-driven coastal current downstream is a surface-trapped plume attached to the coast. In the presence of tide, however, tidal mixing generates strong vertical momentum flux and leads to the development of an offshore bottom Ekman layer that pushes the plume front away from the coast. Therefore the tide transforms the Changjiang River plume from a surface-trapped plume hugging the coast to a bottom-advected current shifting to an offshore location. More importantly, the tide changes the transport of the buoyancy-driven coastal current and the water accumulation in the bulge region substantially. In the absence of tide, the freshwater transport by the coast current accounts for about 35% of freshwater export from the Changjiang River, suggesting significant freshwater accumulation in the bulge region. In the presence of tide, however, the coastal current carries about 80% of the freshwater export from Changjiang River.In the presence of upwelling favorable wind, the freshwater spreads in the northeastward direction and the density contrast reduces with time due to the wind-induced mixing. It is found that the buoyancy water spread offshore in a quasi-steady uniform thickness in the absence of tide. In the presence of tide, the interaction of wind and tide will produce the current asymmetry over the flood-ebb tidal cycle. The offshore transport is enhanced at ebb but weakened at flood. The flood-ebb asymmetry causes water to accumulate at seaward side of the plume and deepens the front. The plume thickness varies over the flood-ebb tidal cycle, especially under strong wind forcing conditions. Downwelling favorable winds confine the plume to the coast and increase the plume thickness.The effects of Taiwan Warm Current and the summer monsoon wind on the Changjiang River plume have also been investigated. The numerical modeling shows that: in the absence of summer monsoon, the downstream buoyancy current and the upstream Taiwan Warm Current will produce shear-instability at the boundary and forms a chain of anticyclonic and clonic eddies. The anticyclonic eddies will detach from the bulge region and spread in the northeastward direction. The episodic northward winds will spread the freshwater patches further offshore. The summer monsoon will change the dispersion trajectory substantially. In the presence of summer monsoon, the baroclinic instability of the plume front is suppressed; the plume water spreads northeastward and enters into the Japan Sea.The research result outlined above mainly revealed effects of tide, wind and shelf circulation on the dispersal and mixing processes of the Chanjiang River plume through some process-oriented numerical modeling, which provide some theoretical evidence to understand the observations. Sea surface temperature (SST) in the South China Sea (SCS) is closely related to El Ni?o and Southern Oscillation (ENSO). Possible mechanisms have also been widely discussed. The influence of ENSO on the SCS SST is considered to be through the atmospheric bridge of atmospheric circulations. Compared to SST, investigations of sea level in the SCS have mainly been focused on seasonal scale. Until now, our understanding of interannual sea level variability in the SCS is still very poor. In particular, the mechanisms that are responsible for the SCS interannual sea level variations are not clear. SST anomalies are found to be increased during and after the mature phase of El Ni?o. However, since SST data reflects mainly the sea surface thermal phenomena, deep layer water temperature must be investigated in order to enhance our understanding obtained from interpretation of altimeter observations. The main purpose of this study is to examine the interannual variability of the SCS sea level and its relationship with ENSO.We have gathered the available data of the South China Sea, including Sea level observed by altimeter, seawater temperature, ocean circulation data, thermosteric sea level, and tide gauge records and established relatively long time series for each component. Based on multiple statistical methods, including EOF and wavelet coherence, we investigate the interannual variability of these environmental components in the SCS and their relationship with ENSO. We have also investigated the possible physical mechanisms from the perspective of volume transports between the SCS and adjacent oceans, air-sea interaction, and water mass cycle. The main and new results are listed as flows:(1) Both the interannual variations of the observed sea level and the thermosteric sea level are closely related to ENSO. The SCS sea level anomalies are negative during El Ni?o years and positive during La Ni?a years. Both the amplitude and the phase show difference between the observed and thermosteric sea level anomalies. The annual variation of the altimeter observed sea level corrected for thermosteric effect has amplitude of 63mm with a maximum in December. This variation should result from ocean mass exchange between the atmosphere and the continent via precipitation, evaporation and runoff. Since the SCS is not a closed basin, the water mass exchange between the SCS and the adjacent oceans may also play a role.(2) An'enigma'that the SST and sea level in the SCS have inverse response to ENSO is revealed. While SST reaches its maximum with a lag of five months behind the mature phase of El Ni?o, sea level reaches its minimum with a lag of four months. Such an'enigma'is revealed by calculating the heat expansion of seawater in the 0～700m layer. The SST is mainly determined by seawater temperature in the surface layer. The sea level is related to the heat expansion of seawater in all the layers. The thermosteric sea level anomalies are dominantly controlled by seawater temperature anomalies of the intermediate layers.(3) The volume transports between the SCS and the adjacent oceans and the anomalous Ekman pumping contribute a lot for the sea level fall in the developing stage of El Ni?o, while the mass exchange, which is dominated by precipitation, plays a more significant role in the following continuous negative sea level anomalies, which will account for about 60mm sea level fall during El Ni?o years.