A Study Of Fronts And Frontal Eddies In The Northern Shelf Region Of The South China Sea

Researches on fronts are one of the most important research fields in physical oceanography. Photographs from space show that submesoscale eddies, ten to several ten kilometers in diameter, are common features of the seascape. These submesoscale processes are considered to play a significant role in the near-surface dynamics and are paid close attention to by more and more researchers recently.Based on seven years' (1993-1999) monthly mean SST satellite data, the feature of monthly changes for the thermal fronts in the Northern South China Sea (NSCS) is analyzed. Results show that the intraseasonal changes of the thermal fronts are distinct. Combining the wind satellite data, we analyze the effect of the northeast monsoon to the form and development of the thermal fronts. Study shows that the increase of the wind velocity of the northeast monsoon can strengthen the intensity of the thermal fronts. Through the contrast of SST features in Feb. 1998 when Kuroshio intrusion was weak and in Feb. 1999 when Kuroshio intrusion was strong, the effect of Kuroshio intrusion to the thermal fronts is analyzed. Strong Kuroshio intrusion can increase the intensity of the thermal front and can also affect the location of the thermal front.Shelf fronts in the NSCS are analyzed in further detail using the results of the field measurements in the winter cruise of 2004. Meteorology is an important factor affecting the fronts. Changes of meteorology in one to three days can lead to the corresponding changes of the fronts, such as the change of the location of frontal interface and the intensity of the front. There exists the effect of cabbeling in the frontal region of the NSCS. The maximal vertical velocity calculated by a linear model with cabbeling dynamics is 6.48m/day. Because of cabbeling and instability, several intrusive layers were observed in the cruise. The calculated lifetime of the intrusive layers under the effect of double diffusion is about 40 to 60 hours.A linear stability analysis is conducted to study instabilities of shelf front. For anideal shelf front with ideal distributions of the density and current velocity, a stability analysis based on this front is conducted. The effects of the topography, background density and velocity field to the instability of the front are also analyzed. Using the density distribution from the field measurements, the stability of the front in shelf region of the NSCS is analyzed. Results show that the temporal scales of the dominant unstable modes are on the order of one day and the spatial scales of the dominant unstable modes are on the order of 5-10 km. Baroclinic instability is a main source comparing with the barotropic instability. In the condition where current shears don't exist, big perturbations occur in the offshore side of the front; while in the condition where current shears exist, big perturbations occur in the onshore side of the front. When the wind is very week or strong, the velocity shear may be very small and the warm filaments are easier to occur in this condition. In more frequent conditions, current shears exist, for example, the surface current velocity is relatively strong and the bottom current is relatively weak. The cold filaments are easier to occur in this condition. Cold filaments and warm filaments are not just the phenomena in the sea surface. It is more likely that cold filaments and warm filaments first occur under the sea surface and then affect the sea surface gradually.Based on the former researches on the submesoscale eddy, submesoscale eddies observed in the shelf region of the NSCS are analyzed and discussed elementarily. For the occurrence of the eddies with the 'cat's-eye' pattern in the winter of 1999, it may have a relationship with the strong Kuroshio intrusion, which can lead to a strong horizontal current shear and then make eddies come into being. For eddies in other satellite imagines, reasons for their occurrence are likely the baroclinic instabilities, under the effect of which cold filaments occur and then are wound into...