Climate characteristics and influential systems of Meiyu to the south of the Yangtze River based on the new monitoring rules

Using the NCEP/NCAR reanalysis data, NOAA Sea Surface Temperature (SST) data, the Chinese Meiyu onset dates determined based on "Meiyu Monitoring Business Rules" released by China Meteorological Administration in 2014, and other data from 1951-2015, anomalous climate characteristics of the Meiyu onset date in three regions of South China are analyzed. The relationships between the Meiyu onset date to the south of the Yangtze River (MODSYR) with variations of atmospheric circulation in May-June and preceding SST are analyzed using the correlation and composition analysis methods. The results show that MODSYR has significant interannual variability in the recent 65 years. The mean MODSYR during 1951-2015 was on 8 June. The difference between the earliest and the latest MODSYR was 47 days. The MODSYR was usually in June (accounting for 80.0%). There exist significant differences in the large-scale circulation from the upper troposphere to the lower troposphere during the same period (May to June) between the earlier and later MODSYR years. In the early MODSYR years, the South Asia high (SAH) and East Asia subtropical westerly jet stream (SWJS) establish earlier and are stronger compared with their climatological means in the upper troposphere, the SAH moves northward to the Tibetan Plateau earlier than normal, and the SWJS also moves northward earlier. The mid-and high-latitudes is dominated by strong meridional circulation, and the first northward jump of the western Pacific subtropical high (WPSH) occurs earlier than normal. The cross-equatorial flow in the lower troposphere establishes earlier, and the onset of the South China Sea summer monsoon is earlier, too. However, the opposite is true for late MODSYR years. SST anomalies (SSTA) in the preceding winter and spring are important forcing factors that affect the MODSYR, and they are also critical early predictive signals. When the eastern Pacific SSTA is abnormally cold in the preceding winter, the SST gradient index to the East Australia (ASI) is in positive phase to the East of Australia and the spring North Atlantic Triassic (NAT) is in the positive phase, the MODSYR is earlier than normal. The relationship between the December ASI, the March NAT and MODSYR are the closest. When the December ASI is in positive phase, the first northward jump of the WPSH occurs earlier than normal. When the NAT is on positive phase in March, the SWJS will be stronger and located more northward in June, which is favorable for earlier MODSYR.