The Dominant Features of the Intraseasonal Oscillation Related to the Rainfall over the Yangtze River Basin in Meiyu seasons

The 10-20 and 30-60 day oscillation are the most frequent variations linked to the Yangtze River Basin rainfall during the summer season in China. The setup time of the maximum intensity of 30-60 day oscillation is often earlier in the early Meiyu season than that in the late Meiyu season. The dominant features of the 10-20 and 30-60 day oscillation related to the early and late Meiyu seasons rainfall during the summer from 1979 to 2012 are investigated accordingly, using the observed daily precipitation at 756 stations in mainland China and NCEP/NCAR reanalysis data. The results suggest that, the development of the wet phase of the Yangtze River Basin rainfall in both early and late Meiyu seasons is closely related to the opposite convective activity over the South China Sea (SCS) and south China along the East Asian (EA) coast of 30-60 and 10-20 day oscillation. The active convection is first developed in the Indian Ocean of 30-60 day oscillation, while the active convection initially appears around the Maritime Continent of 10-20 day oscillation. With respect to the different genesis of the convection anomaly, the active center moves much further northward of 30-60 day oscillation than that of 10-20 day oscillation, for the local latitude-vertical cell is associated to the Rossby wave-like coupled circulation convection system of 30-60 day oscillation and lead to the northward of the convective center along the EA coast consequently.

In the mid-high latitude, the oscillations differ as the wave trains propagation with a mid-latitude jet stream of 30-60 day oscillation and propagation along a polar jet stream of 10-20 day oscillation individually. In the early Meiyu seasons, there is a teleconnection pattern from Atlantic Ocean to the East Asian coast detected of 30-60 day oscillation, probably related to the SST anomaly in Atlantic Ocean. The high latitude oscillation of 10-20 day oscillation sources from the high latitude ocean and propagates southwestward in early Meiyu seasons, but it comes from Siberia and moves eastward to Lake Baikal in late Meiyu seasons, which can be considered as the precursors of the Meiyu setup.

The enhancement of the anticyclonic anomalous in 850 hPa over the south China is attributed to the phase locking of both tropical and mid-high latitude oscillation, leading to the anomalous southwesterly and the positive rainfall anomalies over the Yangtze River Basin during the different Meiyu seasons. Both the mid-high latitude oscillation and the tropical convective circulation of 30-60 and 10-20 day oscillation play key roles on the development of the wet phase of the Yangtze River Basin in the early and late Meiyu seasons.

The new knowledge on the structure and evolutions of the intraseasonal oscillation during the different Meiyu seasons, provides the further understanding of the Meiyu interannual variability and the empirical predictors for the extreme drough and dry events in the EA.