Research On The Seasonal Variation Characteristics And Mechanism Of Winter Low Temperature Events In Southern Chin

Under the background of global warming,extensive cold-precipitation weather has posed a great threat to economic development and people’s life.Therefore,analyzing the intra-seasonal variation features and mechanism of cold events in winter is of reference significance for disaster prevention and improvement of extended-range forecast.Based on the daily minimum air temperature(T_min)data and the daily precipitation data derived from the NCC/CMA combined with the NCEP/DOE reanalysis data,the main intra-seasonal oscillation（ISO）characteristics of the winter T_minin southern China are explored and 24extreme cold events with intra-seasonal features are calculated.Combined with precipitation data and RMM index,the events were further classified.The low frequency signals affecting temperature and precipitation anomalies caused by Madden-Julian Oscillation（MJO）and intra-seasonal variability（ISV）were investigated by using the phase synthesis,and the physical mechanism of the cold and precipitation weather processes was discussed.The main conclusions are as follows:（1）The winter T_minin southern China has intra-seasonal oscillation periods of 10-30 d and 30-60 d.According to whether the event was accompanied by precipitation and the intensity of MJO,it was divided into cold-precipitation events（S-CPEs,W-CPEs）and cold events（S-CEs,W-CEs）under the background of strong and weak MJO.Most of the events all occurring during the two intra-seasonal scales in cooling phases.（2）The influence mechanism of ISV on low-frequency precipitation at 10-30 d in S-CPEs and W-CPEs is the same.The abnormal southerly accompanied by low frequency cyclones in southwest China transport tropical water vapor to the north,which makes the southern China have a"pre-humidification"environment.The occurrence of S-CPEs is coupled with the eastward transmission of tropical convection,while the occurrence of W-CPEs does not have this characteristic.The suppression of convection by MJO will stimulate an anomalous anticyclone in the Western North Pacific,and the southwest wind on the north-west flank of the anticyclone is conducive to the 30-60 d low-frequency precipitation in S-CPEs.（3）Anomalous northernly caused by synergistic cooperation between low-frequency systems in middle and high latitudes are the main factors affecting temperature anomalies in the S-CPEs and W-CPEs.The MJO mainly affected the temperature anomalies at 30-60 d in S-CPEs.The convective intensity of MJO and the low-frequency signals at 200 h Pa that caused the temperature anomalies are most significant about 10 d in advance.The wave flux from low latitudes strengthens the low-frequency signals at mid-high latitudes,thus maintaining the cooling process.（4）The cold advection has a significant positive contribution to the low frequency component of S-CEs and W-CEs at 10-30 d,but the intensity of the two is significantly different.The source and path of cold air affecting the two kinds of events and the intensity of low-frequency northernly generated by the cooperation of the low frequency system associated with the ISV are different.In S-CEs,the low frequency systems have strong intensity and are located to the east.In the W-CEs,the system is weaker intensity and the location is to the west.During 30-60 d of S-CEs,the cooling process was accompanied by the eastward transmission of tropical MJO convection.MJO suppressed convection over the Maritime Continent stimulated local meridional circulation,resulting in ascending motion in southern China.Ascending adiabatic cooling played a role in the cooling process.The response to local meridional circulation in the outer tropical region has a lag of more than 10d.