Forcing Mechanisms Of Seasonal To Decadal Variations Of Oceanic Temperature And Circulation In The Yellow Sea

The response of regional oceans to large-scale climate changes has becomea key problem in studies of global environmental and ecosystem changes.The large-scale climate change could influence the regional oceansthrough either oceanic or atmospheric pathways. As a semi-enclosedmarginal sea of the Northwestern Pacific, the Yellow Sea (YS) is locatedin the East Asian Monsoon region and is adjacent to the prominent westboundary current Krushio and its shelf circulation branches. Seawatertemperature and circulation are primary parameters representing oceandynamics. The characteristics of temperatures and circulationvariability in the YS have been extensively studied by many previousstudies based on observational, modelling and analytic approaches.Regarding the mechanisms of the variability, controversial ideas stillexist, hence further studies are required.This thesis aims to describe the characteristics and explore the forcingmechanism of the seasonal, inter-annual and decadal variations of watertemperature and circulation in the YS using a novel ocean model. The modelis based on the Nucleus of European Modelling of the Ocean (NEMO). In orderto include the influence of both large-scale and regional forcing, atwo-way nested global-regional model is used. The model has46verticallevels. The global model has a horizontal resolution of about100km. Theregional model covers the Northwest Pacific, has a horizontal resolutionof25km; The model forcing is based on an atmospheric reanalysis product.Primary analyses are focused on the simulation results during1958-2007,covering50years. The model is validated using satellite remote sensing observations of sea surface height and sea surface temperature, in situobservations of water temperature across representative sections, sealevels at tide gauges and other relevant historical data. It is shown thatthe simulation well captures the observed variability. A further modelsensitivity simulation is carried out to separate the relativecontribution of surface wind stress and heat flux in causing thevariations of the Yellow Sea Warm Current (YSWC).The simulated seasonal cycle of heat content in the YS shows that thecooling seasons covers September to February and heating season coversMarch to August. The magnitude of surface heat transport is significantlylarger than that of lateral heat transport. During both cooling andheating seasons, surface heat transport is the primary factor controllingthe variations of temperature and heat content in the YS. Variation ofthe lateral heat transport is mainly determined by changes of volumetransport instead of temperature.In terms of inter-annul and decadal changes of water temperature, changesin the regional integrated heat content are primarily caused by changesin surface heat transport. The winter temperature in the YS shifted fromcold phase to warm phase during1988/1989. Model results suggested thatthe warming is induced by decrease in latent heat flux, which is causedby the weakening of the East Asian Winter monsoon. The shift of ArcticOscillation from positive to negative phases has impacts on changes inthe location and strength of the East Asian Trough and the strength ofthe East Asian Winter Monsoon. The variation of temperature during heatingseason is influenced by shortwave radiation which is associated with thelocation of West Pacific Subtropical High affecting the cloud. The YellowSea Cold Water Mass exists under the thermocline in summer, which is mainlythe remnant of warmer winter water, but is still influenced by surfaceheating and lateral mixing. Overall, the YS is influenced by both surfaceatmospheric and lateral oceanic forcing. The atmospheric forcing playsa more significant role; hence further studies on long-term variationsof water temperature should focus on the roles played by atmospheric forcing.Results of model sensitivity study support the argument that the YellowSea Warm Current is a compensation current to the coastal currents drivenby local wind field. Variations of the YSWC and the Tsushima Warm Currentshow no direct relation with each other. Both the warm water and the YSWCin winter are forced by variations in the East Asian Winter Monsoon; butthe former is primarily forced by changes in surface heat flux, and thelater is forced mainly by changes in wind stress. Changes in watertemperature and circulation are thus de-coupled. During years when EastAsian Winter Monsoon is stronger, the YSWC is stronger and the watertemperature is lower. The axis of the YSWC shows inter-annual variabilityunder the influence of zonal wind component of the East Asia Winter Monsoon,which is related to the tilting of the East Asia Trough.