Numerical Study Of Submesoscale Processes In Northern South China Sea And Luzon Strait

The dynamics of~O(1) Rossby number, and submesoscale processes at~O(1km)in the upper ocean play an important role in the vertical fluxes of mass, buoyancy, andtracers in the upper ocean. It can be instrumental in transferring energy and propertiesfrom the largely adiabatic mesoscale (~10-100km) flow field, to a scale where mixingcan occur. We applied a primitive equation ocean model to simulate submesoscaleactivities and processes over the shelf of the northern South China Sea (NSCS) with aone-way nesting technology (AGRIF) for downscaling. We diagnosed the modelresults and compared with the climatological observations which show a good matchwith each other. The temperature and density fields showed that submesoscaleactivities were ubiquitous in the NSCS shelf which corresponds to the satelliteobservations well.The vertical velocity was considerably enhanced in submesoscaleprocesses and could reach an average of58m per day in the subsurface.At this point,the mixed layer depth was deepened along the front, and the surface kinetic energyalso increased with the intense vertical movement induced by submesoscale activity.In addition, we diagnosed the effect of submesoscale activities on the lateralhorizontal diffusion in the upper mixed layer, the interation of surface kinetic energyis twice than lower mesoscale eddy permiting model results and the effectivehorizontal diffusion rate is calculated and it can reach up to~40-120m2s-1, which islarger than the general value~10m2s-1without the submesoscale activities. Twosensitive numerical experiments were conducted to check the effects of down-frontwind forcing on frontogenesis and submesoscale forming. As the obvious front showsup, we stop the down-front wind forcing, along the front jet, with the interaction between the development of mixed layer instability and the shear of frontal jet,submesoscale eddy and meaders show up and continue to develop until the lateralbuoyancy gradient becomes weak and thus the available potential energy is released.The whole process lasts nearly ten days, after that the mixed layer has a stablestratification. However, with the continuous down-front wind forcing, the secondaryeffect of high relative vorticity induces the nonlinear Ekman surface current. Thisprocess pulls the cold upwelling water into the orelative warm region and intentifiesthe submesoscale activities.Unlike the mechanism generating the submesoscale activities in Northern SouthChina Sea, the submesoscale activities generated in Luzon Strait is mainly due to theinteraction between the strong Kushio current and the existence of essential eightislands in Luzon Strait. In this dissertation, a very high resolution numerical modelwith the first application of2-WAY agrif nesting method (on a~1km grid) is used tostudy the island wakes in Luzon Strait. A comparsion between simulations with andwithout islands shows the surface enstrophy decreases substantially when the islandsin Luzon Strait in the oceanic model are removed, and the enstrophy decrease mainlytakes place in the areas aroud the islands. Sensitive numerical experiments show thatthe tidal current can affect the island wakes except the Bubuyan Island which face tothe Kuroshio incident. The mechanism of the generation of island wakes is alsodifferent among these islands, island wakes occurred around Babuyan Island andBatan island groups are mainy due to the lateral drag. While the generation ofsubmesoscale eddies and meadars around the four islands near the Luzon Islandinduced by the lateral drag and the bottom drag. Our modeled results reproduced theKarman Vortex Street (KVS) occurred behind the Babuyan Island, and we find thedirection of the incident eddies and length scale of KVS are nearly same as observedby the ASAR image. The period of eddy shedding is about0.3day and the time seriesof eddy generation, development and shedding are given. Given the submesoscaleactivities occurred around the islands can induce a high vertical velocity and thus pullthe high nutrients up to the mixed layer and feed the phytoplankton, we find that thebloom of phytoplankton in the northwest of Luzon Island is not only due to the upwelling driven by winter monsoon, but also the lateral diffusion of nutrients nearthe islands by Kuroshio intrusion. At the same time, the Lzuon cold eddy activeds inwinter season also triggers a vertical transfer of nutrients to the region where thephytoplankton blooms. In the end, we performed several numerical experiments toinvestigate the effects of island wakes on mean currents in Luzon Strait, we find thatonly if the horizontal grid finer than1/32°, and the model can simulate the currentwell and reproduce the realistic current pattern in the south of Luzon Strait. Thus,submesoscale stirring/mixing is not only important for tracers, such as temperature,salinity, nutrients, dissolved organic, and inorganic carbon, but also to the complexNorthern South China Sea circulation system and Kuroshio dynamics in Luzon Strait.