Roles of Regional Air-sea Interaction and Indo-Pacific Remote Forcing in Summer Rainfall Variability over the South China Sea

As part of the most active components of the global climate system, the South China Sea (SCS) climate has notable impacts on adjacent regions. Understanding the factors and processes of the SCS climate variability is critical for regional climate prediction and climate-related risk management. Based on both observational analysis and numerical experiments, this thesis conducts an innovative investigation of the impacts of both regional air-sea interaction and tropical Indo-Pacific remote forcing on the interannual variability of summer climate in the SCS region.;The regional atmosphere-ocean relationship is analyzed by examming correlation between precipitation and SST tendency, and precipitation and SST variations. Significant seasonality and regionality is identified in the SCS. As for the seasonal air-sea interaction, atmospheric forcing with remarkable cloud-radiation effect, wind-evaporation effect and complimentary effect of wind-driven oceanic processes is found in the transition seasons. Oceanic forcing of atmosphere occurs in early summer and in winter through modulating atmospheric stability and lower-level moisture convergence. As for the interannual atmosphere-ocean interaction, an atmospheric forcing of ocean is dominant in the northern and central SCS from April to July, which is characterized by remarkable cloud-radiation effect, wind-evaporation effect across the central basin, and wind-driven oceanic upwelling effect along the west coast. An oceanic forcing is identified in the northern SCS from November to February during which the SST feedbacks to the precipitation through modulation of the atmospheric stability and the lower-level convergence associated with the large-scale circulation. The 24 selected CMIP5 model simulations capture reasonably the climatological precipitation-SST and precipitation-SST tendency relationships in the SCS, yet their performances in simulation of the interannual air-sea relationship show a wide range of difference. Improvement should be made before the model simulations can be utilized to understand the regional atmosphere-ocean interaction in the SCS.;The interannual variability of summer rainfall over the SCS is correlated with SST anomalies in both the Indian and Pacific Ocean regions. These Indian and Pacific Ocean SST influences are distinguished by extracting three types of cases. In the equatorial central Pacific (ECP) SST forcing case, positive ECP SST anomalies induce anomalous lower-level cyclone over the western North Pacific and northern SCS through a Rossby-wave type response, leading to above-normal precipitation from the equatorial Pacific to the northern SCS. Meanwhile, negative Maritime Continent (MC) SST anomalies work together with positive ECP SST anomalies, and enhance the SCS summer rainfall variability through regional meridional and zonal vertical circulations. In the North Indian Ocean (NIO) SST forcing case, the NIO SST anomalies contribute to the SCS summer rainfall variability by modulating a regional east-west vertical circulation over the NIO through the western North Pacific. These NIO SST anomalies serve as a "medium" for an indirect impact of the preceding winter equatorial eastern Pacific (EEP) SST anomalies on the SCS summer rainfall variability. This delayed (indirect) El Nino-Southern Oscillation (ENSO) influence is distinguished from the simultaneous (direct) ECP SST influence on the SCS summer rainfall. In the co-existing ECP-NIO forcing case, the ECP and NIO SST influences concur, and the occurrence of precipitation anomalies over the SCS is due to a combined effect of both the Pacific and Indian Ocean SST anomalies. The ECP SST impact is dominant after 1990 and the NIO SST influence is relatively more important during 1980s. Moreover, the importance of the Indo-Pacific SST forcing in the SCS summer rainfall variability is supported by the numerical experiments of Community Earth System Model (CESM) with SST forcings specified in the ECP and NIO regions.