Multi-source Datasets Assessment Of The South China Sea Summer Monsoon And Its Subseasonal Forecasting Windows Of Opportunity

The onset of the South China Sea summer monsoon(SCSSM)marks the comprehensive establishment of the East Asian summer monsoon,which is crucial for the rainy season in China.Predicting the onset time of the SCSSM is one of the core operations of short-term climate prediction in China.The present study compares the climatological features and interannual variability of the SCSSM onset between the JRA-55,ERA5,CRA-40,and NCEP1 reanalysis datasets,and diversity characteristics of interannual variability of SCSSM in three commonly used indicators prevailing zonal westerly wind at 850 h Pa(U850),the inversion of the mean meridian temperature gradient(MTG)in the middle and upper troposphere,and the establishment of convection(OLR)in the South China Sea.We started with the relationship between the intra-seasonal oscillation(ISO)intensity and the interannual variability of the SCSSM,and emphasized the synergistic role of different cycles of ISO in SCSSM.The final focus is on the sub-seasonal prediction of the South China Sea summer wind,and the ECMWF S2 S model data are used to explain the impact of MJO activity propagation on the SCSSM onset from the perspective of sub-seasonal predictability,and then to find the key factors of the sub-seasonal prediction window of opportunity for the South China Sea summer wind in a specific year.The primary conclusions are as follows.(1)The result shows that atmospheric circulation and precipitation evolution are consistent among the four reanalysis datasets.However,the striking difference is that the upper-tropospheric warm center and precipitation in CRA-40 are greater than the others.Based on different indicators,the onset date of the SCSSM with the four reanalysis datasets is consistent in most years in which the circulation is completely coupled with convection.In these years,while the upper and low-level circulation is adjusting,convection over the South China Sea(SCS)establishes.However,in individual years,the onset date of the SCSSM defined by the three indicators is very different because the coupling between the upper and low-level circulation is indefinite.(2)When the interannual variability of the onset date defined by U850,MTG,and OLR is consistent,the SST anomaly field from April to May presents a typical ENSOtype distribution,indicating that in these years,tropical sea-air interaction is the critical factor for the SCSSM onset.In contrast,in individual years which may be more modulated by the atmospheric ISO signal,specifically when the stronger the ISO strength in the South China Sea and surrounding areas,it is more likely that the establishment of convection is ahead of the upper-air monsoon circulation transition.(3)The ECMWF S2 S model has a window of opportunity for sub-seasonal forecasting of the SCSSM with the U850 indicator in 1999.The main reason is that the model can capture the production and propagation characteristics of the MJO(MaddenJulian Oscillation)four weeks in advance in which warm SSTs in the central Indian Ocean are favorable for MJO production.The MJO in the Indian Ocean and the midlatitude anticyclonic circulation together enhance the anticyclonic circulation in the western Pacific Ocean,resulting in an unusually strong 850 h Pa easterly flow over the equator,which is not conducive to the establishment of trans-equatorial flow in the South China Sea.With the response of the Arabian Sea SST to precipitation,the Indian Ocean SST is of the east-cooling and west-warming type,and the westerly flow over the Indian Ocean strengthens,promoting the eastward transmission of the MJO convection.The MJO not only suppresses the South China Sea-Northwest Pacific monsoon convection,but also enhances the tropical convection in the East Indian Ocean,and the unusual latitudinal structure of this convection accelerates the low-level easterly wind in the South China Sea region,thus weakening the South China Sea monsoon trough,and the middle and upper layers of the South China Sea troposphere appear The anomalous temperature gradient of "cold north and warm south" in the upper troposphere of the South China Sea delayed the establishment of low-level westerly winds,which eventually led to an abnormally late summer wind outbreak in the South China Sea.