Interdecadal Changes In The Asymmetric Impacts Of ENSO On Wintertime Rainfall Over China And Atmospheric Circulations Over Western North Pacific

Pacific decadal oscillation(PDO)and El Ni?o and South Oscillation(ENSO)are different time scale signals affecting wintertime climate in China.In this paper,based on the observed and reanalysis datasets from 1951 to 2010,the role played by Pacific decadal oscillation(PDO)in the impact of El Ni?o and South Oscillation(ENSO)on the precipitation over southern China in winter half year is investigated.And the possible physical mechanism is discussed.It is found that the PDO can significantly modulate the asymmetry in the responses of the East Asian climate to El Ni?o and La Ni?a.The response of precipitation anomaly over southern China in winter half year to ENSO is asymmetrical when the PDO is in the positive phase.Specifically,significant positive rainfall anomalies can be observed in the SC in the winter half year during El Ni?o episodes,but not reversed results in La Ni?a winter half years.However,such asymmetric responses do not appear in negative PDO phase.The asymmetric responses of ENSO to rainfall over southern China are mainly caused by the atmospheric circulation over western North Pacific(WNP)in lower troposphere.In PDO positive phase,with the influence of strong anomalous anticyclone over WNP,southern China is affected by the southwest wind and the convergence center of water vapor appears in El Ni?o winter half years.While in the La Ni?a episodes,accompanied by the weaker cyclone,southern China is dominated by cold and dry northeasterly winds.The northeasterlies hinder the water vapor from south,and the divergence of water vapor appears over southern China.However,during the negative PDO phases,the anomalous anticyclone over the WNP in El Ni?o years becomes weaker and move east,which has little influence on south China.But with stronger cyclone,south China is controlled by northeasterly wind,which causes significantly negative rainfall anomalies appeared.Moreover,sea level pressure,water vapor flux and potential height are also analysed.The results show the obvious asymmetric in PDO positive phase,and such asymmetric responses do not appear in negative PDO phase.These circulation characteristics are closely related to precipitation and circulation asymmetry in lower troposphere.Further analyses reveal that,the PDO as the interdecadal background of the SSTA fields in tropical Pacific,modulates the asymmetric impacts of ENSO on the atmospheric circulations over WNP.The warm background of the tropical Pacific in PDO positive phase causes significant difference in the amplitudes of convection anomalies in tropical western Pacific,resulting in the asymmetric responses of the WNP circulation and rainfall over southern China to El Ni?o and La Ni?a.Nevertheless,the cold background in PDO negative phase reduces the amplitude difference between El Ni?o and La Ni?a winters.A comparable convection anomalies appear in tropical western Pacific in El Ni?o and La Ni?a winters and the asymmetric responses do not occur.To figure out the possible physical mechanism of the relationship between ENSO and SC winter precipitation,the perturbation kinetic energy equation is used to investigate the difference of kinetic energy generation between El Ni?o and La Ni?a.The results show that in El Ni?o winter half years,potential energy coverts to kineic energy,causing the kinetic energy increased;While in La Ni?a winters,potential energy converts to kinetic energy and the kinetic energy decreased.It's obvious the kinetic generation shows significant asymmetric.However in the PDO negative phase,the convection between potential energy and kinetic energy is smaller.As a result,the asymmetric responses do not occur.