Seasonal Cycle Of East Asian Monsoon And Its Interannual Variability

The East Asian monsoon(EAM)shows a robust annual cycle(AC),characterized by the alternation of dry and wet seasons with the seasonal reversal in the monsoon circulation between winter and summer.The interannual variability of AC in phase and amplitude has a great impact on the seasonal anomaly of circulation and rainfall over East Asia.Therefore,better understanding the fundamental mechanism of the AC of EAM may improve the scientific knowledge for the seasonal forecast of EAM.On the basis of solar decline,data analyses and model simulation,the current study investigates the dominant modes of AC of EAM,examines its interannual variability,and explore the possible relationship between the AC and sub-seasonal variation of EAM.The major conclusions are summarized as follows:(1)The AC of EAM can be decomposed into the equinoctial and solstitial modes in the combined sea level pressure,850-h Pa low-level wind,and rainfall fields.The solstitial mode shows a zonal pressure contrast between the continental thermal low and the western Pacific subtropical high,reaching its peak in July and dominating the East Asian summer monsoon.The equinoctial mode shows an approximate zonal contrast between the low-level cyclone over the east of the Tibetan Plateau and the western Pacific anticyclone over the east of the Philippines.It prevails during the spring rainy season in South China and reaches its peak in April.(2)The land-sea seasonal warming in subtropical East Asia-Pacific region lags the solar decline by 1-2 months.Data analyses and numerical modelling show the hysteresis effect of sea surface temperature(SST)in the Kuroshio extension area of the Northwest Pacific.It induces a negative vorticity response of low-level atmosphere and plays a "baton" role to enhance the further northward shift of western Pacific anticyclone,and delay the rainy season in North and Northeast China to August.(3)The seasonal cycle of East Asian summer monsoon can be regarded as the alteration of equinoctial and solstitial modes.The lead�lag phase anomaly of the two modes may result in the negative correlation of rainfall anomalies in North China between spring and fall and in South China between winter and summer,which provides a potential basis for the across-seasonal prediction of rainfall.The warm phase of ENSO in winter could give rise to the reverse interseasonal rainfall anomalies in South China,while the SST anomaly in the Northwest Pacific Ocean may regulate the rainfall anomaly in North China.(4)The seasonal cycle shows a strong impact on the intraseasonal oscillation(ISO)on interannual timescale.The ISO is more active during April to August,and its temporal-spatial evolution indicates the intraseasonal active and break of East Asian summer monsoon and rainfall.The lead and lag of seasonal cycle may result in an early and late South China Sea summer monsoon(SCSSM)onset,but a strong and weak SCSSM.