A Study On The Interdecadal Mutation Mechanism Of Meiyu To Meiri In The Yangtze River Basin

Meiyu is one of the major components of East Asian summer monsoon.Changes in Meiyu activity exert profound impacts on the characteristics of rainy season in East China.Based on the diagnostic analyses of Meiyu data provided by China Meteorological Administration,ERAInterim reanalysis data,NOAA Optimum Interpolation Sea Surface Temperature(SST)and outgoing longwave radiation data,as well as the model sensitivity experients,we found that the Yangtze River Basin Meiyu onset experienced a regime shift.The critical factors and physical mechanisms responsible for the decadal changes in Meiyu onset were examined.The main conclusion are as follows:(1)The Meiyu characteristics has undergone interannual and decadal variability.Applying a sequential algorithm of change-point analysis to Meiyu characteristics(onset,intensity and retreat)over the period 1951–2016,a decadal shift of Meiyu onset timing was objectively detected around 2002.The shift was the most abrupt when the cut-off length of 13 years was used in the change-point analysis.By further analyzing the statistics of Meiyu onset,we found that Meiyu onset date averaged over the earlier epoch(E1: 1989–2001)is 6 June,8 days earlier than the climatological(1951–2016)mean state(14 June).Most(10/13)of the years in E1 with the Meiyu onset in early June(pentads 31–33).On the contrary,Meiyu onset date averaged over the later period(E2: 2002–2014)was delayed about 5 days in contrast to climatology(14June)and about a 2-week delay compared to that of E1.(2)Comparing the upper-tropospheric conditions before and after the change-point occurrence,we found that the decadal change of subtropical jet stream is one of the critical factors.The results of temperature diagnostics during onset date of E1(early June)showed that the amomalous adiabatic heating led to a warmer temperature to the south of the jet stream and a colder temperature to the north.The enhanced meridional temperature gradient led to a stronger subtropical jet stream,which was beneficial to the anticyclonic anomaly and the ascending motion over the Yangtze River area.In contrast,the adiabatic heating process during early June in E2 led to an opposite temperature distribution,which resulted in a weaker meridional temperature gradient.Thus,the circulation conditions with weak subtropical jet stream and reduced ascending motion over the Yangtze River Basin wrer disfavorable for Meiyu onset during early June in E2.(3)The critical physical processes at the lower troposphere causing the decadal change of Meiyu onset was closely linked to the subseasonal evolution anomalies of the western North Pacific(WNP)subtropical high.The changes in WNP subtropical high were attributable to the regime shift of climatological intraseasonal oscillation(CISO)intensity resulting from the decadal variations of SST.Compared to E2,the colder SST anomaly over the western tropical Pacific accompanied with reduced moisture content and weakened easterly wind shear in E1 were not conducive to the development of CISO propagating from the tropical central Pacific towards East Asia.Due to the phase-locking nature of WNP CISO activity as it intracts with annual cycle,the weakening of CISO convection(i.e.,suppressed convective anomaly/anticyclonic anomaly)appeared in early June of E1 promoted the westward enhancement of WNP subtropical high.The southwesterly anomaly related to enhanced WNP subtropical high favored the moisture transport towards the Yangtze River Basin.With this condition,Meiyu tended to initiate in early June during E1.A set of model experiments suggested that the decadal changes in SST over the western Pacificaffect significantly WNP CISO,while the SST anomalies over the Indian Ocean and North Atlantic show weak influences on CISO at the decadal timescale.