| The onset of South Asian summer monsoon (SASM) indicates the beginning of the rainy season in the South Asia region. It is not only critical for the local agriculture and animal husbandry, but also important for the water and life securities.Precipitation in the early rainy season (May) increases rapidly and has a large interannual variability, especially in the Tibetan Plateau (TP) region. One of the starting mechanisms of the monsoon system is the land-sea thermal contrast (LSTC)between Indian Ocean (IO) and South Asia region. Therefore, IO can be considered as a crucial factor for the intensity of the monsoon system, as well as the TP precipitation. In this study, we investigate the relationships between IO sea surface temperature (SST) and TP precipitation on the interannual timescale. Correlation maps show that IO SST variability contains a portion that is independent fro, the tropical Pacific Ocean SST and is negatively correlated with the TP precipitation.Here we define a LSTC index to determine the thermal condition over the IO and South Asia region. The SASM reveals an out of phase relationship with LSTC between land and ocean, which means it would be suppressed by the enhanced LSTC.The daily data is used to further analyze the relationship the SASM and TP precipitation in detail. Results show that the anomalous TP precipitation in May is mainly caused by the Bay of Bengal monsoon and the Indian monsoon is responsible for the TP precipitation in June. More specifically, wanner SST enlarges the LSTC between the IO and South Asia region in positive IOBM years. The SASM is weaker than the mean state, resulting in less precipitation over the TP. In negative years the opposite occurs.Precipitation in the TP reaches its peak in summer. The seasonal projection skill of a statistical downscaling model (SDM) for summer precipitation in the TP was compared with that of direct model output. The SDM, which is based on canonical correlation analysis (CCA), significantly increased the projection skill. The CCA reveals the flow patterns behind the seasonal projection skill of summer precipitation in the TP between 1961-2012 and quantifies its relative contributions. East Asia 500hPa geopotential height (ZG500), tropical Indian Ocean sea surface temperature(SST) and east Asia 850hPa meridional water vapor flux (MWVF850), obtained from the Max Planck Institute Earth System Model, low resolution (MPI-ESM-LR)simulations for phase 5 of the Coupled Model Intercomparison Project (CMIP5)under the representative concentration pathway (RCP) 2.6, RCP4.5 and RCP8.5 scenarios, are considered as potential predictors. The statistical downscaling models(SDM) are established in 1961-2005, validated in 2006-2012 and applied in 2013-2100. The ensemble canonical correlation (ECC) is also applied to improve projection skill. The following results are obtained: (1) The SDM projection skill for each predictor is higher than that of the MPI-ESM-LR climate model, and ECC performs even better. (2) Spatial correlation patterns of different predictors with influence on the TP are well recognized by CCA. The high relevance of ZG500 can be explained by the thermal adaptation theory, that of SST exhibits a canonical Indian Ocean Dipole (IOD) mode, and MWVF850 shows a simple water vapor link.(3) The amount of summer precipitation in the TP will slightly decrease under RCP2.6 by -3.4 mm/decade, whereas RCP4.5 and RCP8.5 reveal an increase by 2.4 mm/decade and 18.4 mm/decade, respectively. |
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