Impacts Of Advection On The Stratification In The Northern China Seas

The Bohai, Yellow and East China Seas are temperate marginal seas with broad shelves and significant seasonality at mid-latitude in the east of China. The evolution of stratification is complicated and is influenced more or less by factors like the air-sea heat flux, tides, advection, run-offs, wind fields and waves. The historical studies have demonstrated the the balance between the air-sea heat flux and tides play a dominant role to the pattern of the stratification. But it is not satisfied in some local regions like the Bohai Sea and the area adjacent to the Changjiang River Estuary where advection and run-offs are crucial, too. The seasonal stratification is also a critical factor to the ecological processes in the coastal waters.Based upon analysis of massive in situ observations in addition to the use of remote sensing data, a serial of numerical tests with primitive equation model-Regional Ocean Modeling Systems (ROMS) and several turbulence models, the thesis is dedicated to understanding of the evolution and characteristics of the stratification in the China seas north of 28°N, and clarification of the contributions from multi-factors mentioned above and their relative importance. Some typical cases are also analysized to explain the relationship between stratification processes and ecological processes. The main results of the paper are as follows:(1) The bottom cold water below the thermocline is a stable and significant water feature in summer in the Bohai Sea. The cold waters are located separately in the Liaodong Bay (LDB) and south of the Bohai Sea respectively and they are not equal in temperature, for the former is 1-3℃lower than the latter, which is named as the Asymmetric Dual-Core Cold Bottom Water (ADCCBW). ROMS is applied to investigate the mechanism of the formation and maintenance of the ADCCBW during stratification periods with a series of sensitivity tests. The paper has revealed that the ADCCBW is caused initially by the differences of the net solar radiation into the sea and the different water depth (thus heat content). The long-time endurence of the ADCCBW is mainly due to the inflow of the near bottom water from the deep channel of the northern Yellow Sea. The temperature is relatively high in the central bight of the Bohai Sea between the ADCCBW due to the relative strong tidal mixing and shallow depth. The spatial disparity of stratification yields several density circulation gyres. On the vertical average, the density circulation in the Bohai Sea in summer is much more stronger than tides and wind induced residual flows. (2)The paper has simulated the enhancement of the extension of the Changjiang Diluted Water (CDW) along the Jiangsu Shoal and in the southwestern Yellow Sea induced by the tidal diffusion during stratification periods. The pycnocline is stronger in this area influenced by the CDW extension and air-sea heat flux. The cold bottom water, referred to as the Yellow Sea Cold Water Mass (YSCWM) by historic literatures, is thus a distinct and long-term endured feature in the north and south portion of the Yellow Sea trough where tidal movements and mixing are weak. The near surface is mainly occupied by a large-scale cyclonic circulation above the YSCWM and embedded with a couple of small-scale eddies. While the central YSCWM is nearly at rest, the cold water south of 34.5°N is much easier to escape and reach the vicinity of the Cheju Island.(3)The paper has investigated the spring phytoplankton bloom occurrence in the vicinity of the central Yellow Sea when weak stratification just begins. There are two sites of blooms with different types of species of phytoplankton, which occurred in succession in the western flank and central part of the Yellow Sea trough respectively. Both of these blooms appear as subsurface Chlorophyll a maximums (SCM). The water mass and hydrodynamic environments at these two sites are different which is in favor of the growth of different phytoplankton species and in turn cause different magnitude of blooms (biomass). The bloom at the western flank the trouch is dominated by the northwestward warm and saline water at the bottom, in the meanwhile is under the prevailing cold and fresh southward coastal current at surface. It is relative weak and terminated finally by the advection. The bloom in the central trough is stronger and of long duration where is favored by the weak and periodical tidal motions and stronger stratification.(4)The paper has demonstrated a tight relationship between the stratification and the low dissolved oxygen (DO) conditions in the near bottom water off the Changjiang River Estuary. The strong pycnocline is contributed by the solar radiation, fresh water from the Changjiang River as well as the dense bottom water from the Taiwan Warm Current (TWC). The intense stratification forms a barrier and makes it difficult for the exchange of DO in the vertical direction, which finally cause a remarkable DO deficiency (hypoxia). Furthermore, forcings like wind fields, the TWC and the river runoff are believed to have significant impacts on the hypoxia distribution through the advection and diffusion processes, which influence the distribution of nutrients and dissolved organic matter in addition to the stratification. The case study with in situ observations analysis and numerical model results on the disparity of summer hypoxic zone between 1999 and 2006 presents detailed evidence to support the above hypothesis.(5)The paper has also evaluated the performance of foure different types of turbulence closure schemes in several realistic situations. The comparative studies suggest that their performance differ in different stratification conditions and should be selected carefully.