Coupling Modes Of East Asian Summer Monsoon Circulation And Its Possible Impacts On China Summer Rainfall

The present study used the NCEP/NCAR atmosphere reanalyzed datasets, NOAA high-resolution OISST, as well as 722 station-observed precipitations in China, and discussed the interaction among the Mongolian cyclone(MC), the western North Pacific subtropical high(WNPSH), the South Asian high(SAH) on the intraseasonal and interannual time scales, with utilizing harmonic, multi-variant empirical orthogonal function(MV-EOF), and the Atmosphere General Circulation Model of ECHAM5.4. The main concludions are summarized as follows:(1) It is identified that the existence of climatological intraseasonal oscillation in the diabatic heating, precipitation, and circulation of the East Asian Summer Monsoon(EASM). The strongest CISO signals are found in the north of the western North Pacifc, possibly because of the horizontal gradient of the diabatic heating induced by the seasonal land-sea thermal contrast. Further, the phase relationship between the diabatic heating componets mantains the EASM CISO. The coupling circulations of EASM CISO is the coaction of the MC around Lake Baikal at 850--hPa, the WNPSH at 500--h Pa, and the SAH over the Tibetan Plateau at 200--h Pa. The first mode shows that the jointly enhanced MC, WNPSH, SAH correspond to a tripole rainfall anomaly with strong Mei-Yu fronts over East Asia. The second mode, however, indicates the eastward and northwestward propagation of MC and WNPSH, respectively, with suppressed SAH, as well as a dipole rainfall over East Asia. Both the observations and numerical simulation verify the importance of daily diabatic heating and SST in mataining the CISO modes over the western North Pacific(WNP).(2) The intraseasonal coupling modes of circulations in EASM are distinct during 1997/98 ENSO cycle, featured by the different variation and coupling pattern between the lower and upper troposphere. The air-sea interaction over tropical area is active on the developing phase, wheras on the decaying phase, the center of air-sea interaction moves to northward in the WNP. It is found that the coupled modes of EASM is relatively stable during the decaying phase, wheras it become unstable during developing stage of ENSO.(3) The surface air temperature(SAT) around Lake Baikal is significant ly increased during 1965-2014, which caused the intensity of MC noticeably weakening. The intensity of SAH is remarkably strenghten due to the vertical increased temperature gradient, which may be affected by the warming of SAT around Lake Baikal, via suppressing the convective instability. The coordinated variation of MC and SAH is the major character of coupling modes on the interannual time scale, which is modulated by the variation of WNPSH.