Seasonal Transition And Difference Of East Asian Monsoon And Its Related Prediction

The East Asian monsoon influences one third of the total population all over the world, thus its related research is the key issue in climate dynamics. Based on a variety of reanalysis and observation data sets, the seasonal evolution features of the East Asian monsoon circulation and its physical processes are investigated. We reinvestigated the two important seasonal transition point (the subtropical East Asian monsoon onset and the South China Sea monsoon outbreak) in East Asian monsoon seasonal evolution. Further, via diagnosis and the ECHAM numerical experiments, the interannual relationship between South China Sea monsoon onset and the subtropical East Asian monsoon rainfall is discussed. And the season-dependent of Southern China rainfall change on decadal time scale is also investigated. Finally, two statistics methods for prediction of East Asian monsoon variability are provided. The main conclusions are as follows:(1) Climatologically, the East Asian subtropical monsoon (EASM) was established in the end of March and the beginning of April (around 18th-22nd pentad), the variables in thermal-wind-rainfall relationship consistently changed their sign at the same time. The wind direction changes in anti-clockwise with height in boreal winter, showing the cold advection characteristic. To the contrast, wind direction changes clockwise with height in boreal summer, suggesting a warm advection characteristic. The East Asian continental cold anticyclone moving to the East China Sea marks the establishment of East Asian subtropical monsoon. Due to the land-ocean surface thermal contrast and the seasonal transition in solar radiation, the Asian-Pacific heating type changes from west cold-east warm pattern to west warm-east cold pattern. The latter induces the southerly wind at the lower troposphere. As a result, the wind direction rotates clockwise with height with warm advection that conveys heat from the south to the north in lower troposhpere. Meanwhile, the ’thermal adaptation’process produces the upward motion and rainfall, releasing latent heat and warming the upper troposhpere. Both of these two processes have positive feedback on the thermal distribution pattern of west warm-east cold, maintaining the subtropical East Asian summer monsoon circulation.(2) In seasonal transition period (April and May), the Bay of Bengal vortex mainly moves eastward and northward. These vortexes are identified as the monsoon onset vortexes (MOV). Before the outbreak of SCSSM, the monsoon onset vortex (MOV) can be found over the Bay of Bengal. In climatological mean, MOV occurred ten days preceding the SCSSM onset, suggesting that it is a good precursor for the SCSSM onset. Particularly, in 1980,1985,1989 and 2009, there are two separate MOVs, explaining the ambiguous and difficulties in judging the outbreak date of SCS summer monsoon. The MOV is generated via local air-sea interaction, and developed under the evolution of ENSO cycle and development of convection along the Asian-Australian atmospheric bridge, and with the equatorial westerly moving northward.(3) The SCSSM onset has a significant positive relationship with the EASM rainfall. The correlation coefficient between the SCSSM onset index and the EASM precipitation index is 0.57, exceeding 99% confidence level. The earlier onset of South China Sea monsoon, the less rainfall of subtropical East Asian monsoon tends to be, and vice versa. The ECHAM numerical experiments show that the positive relationship between the SCSSM onset and the EASM precipitation is controlled by the persistent SST anomalies in the tropical Indian Ocean via the eastward propagating Kelvin wave response. The Kelvin wave can directly affect the zonal wind over South China Sea region, determining the South China Sea monsoon onset dates, meanwhile the persistent Kelvin wave generated cyclonic/anticyclonic wind shear, maintaining the Northwest Pacific cyclonic/anticyclonic anomaly which closely linked to the subtropical monsoon rainfall. This physical process controls both the South China Sea monsoon onset and East Asian subtropical monsoon precipitation, leading a positive correlation between these two.(4) The East Asian monsoon circulation has an obvious interdecadal change in the mid-1990s both in boreal spring and summer seasons. From Pre-1994 to Post-1994 period, the precipitation over South China decreases in boreal spring but increases in boreal summer. This out of phase decadal change of precipitation over Southern China is controlled by a season-dependent scenario. The La Nina like SSTA pattern in boreal spring enhances the convection over the Maritime Continent through enhanced Walker cell, thus lead decreasing of precipitation over Southern China via strengthened local Hadley cell in spring. Whereas the Indian Ocean basin wide warming in boreal summer suppresses the convection over the Maritime Continent via weakened Walker cell, and thus results in increased rainfall over Southern China through weakened local Hadley cell.(5) 10 years of independent prediction results show that the spatial-temporal projection model (STPM), compared with other statistical models, can attain higher predictive skills for forecasting the tropical convection anomalies. While based on three predictors from global perspective, the physical-based empirical model can predict South China Sea monsoon onset with a correlation coefficient of 0.81 for 8 years independent prediction. STPM can also predict South China Sea monsoon onset with a correlation coefficient of 0.71. Although the skill of STPM is not as high as PEM, it can avoid the system bias, and give an extended-range forecast for the onset of SCS monsoon, providing more detailed information of the South China Sea monsoon onset process. STPM and PEM models can achieve better skills in extended-range forecast and seasonal prediction of East Asian monsoon rainfall and its seasonal transition time.