| Based on the daily reanalysis data of the Japan Meteorological Agency(JRA55)from1958 to 2020,the thermodynamic characteristics of the steady warming process in northern Asia before the spring and summer season transition(the 19 th to 28 th pentad),and its climatic winter onset,thermodynamic diagnosis of winter onset(the 54 th to 70 th pentad),abnormal characteristics,and possible external forcing factors affecting the early and late winter onset,were studied.The results show that:(1)Climate temperature advection and diabatic heating have opposite change trends,which successively dominate the temperature changes before the transition from spring to summer in North Asia.Before the twenty-two pentad,diabatic temperature advection dominated the warming process in North Asia.However,with the gradual enhancement of diabatic heating in the mid and high latitudes of the Eurasian continent,the climate temperature advection gradually becomes a cold advection,making diabatic heating the dominant term.The results of the linear baroclinic model indicate that not only directly heating the local atmosphere,diabatic heating also forces out cyclonic circulation anomalies and upward motion in North Asia,leading the low-pressure system originally located near New Earth Island to move towards North Asia,thereby causing diabatic cold advection in both the horizontal and vertical directions.This cold advection partially counteracts the diabatic heating effect and ultimately forms a stable warming process in North Asia.(2)The climatological winter onset in North Asia occurs in the sixty-second pentad,before which North Asia experienced a process from severe cooling to slow cooling.In this process,the diabatic heating term plays a key cooling role,while the temperature advection term plays a major heating role.The results of the linear baroclinic model indicate that not only directly cooling the local atmosphere,the diabatic cooling in the mid and high latitudes of the Eurasian continent is also conducive to the formation of southwest winds in North Asia,resulting in strong local warm advection,which partially offsets the cooling effect of the diabatic heating itself.(3)On the interannual time scale,the sea surface temperature anomalies in the Equatorial east-central Pacific and North Atlantic both affect winter onset in North Asia through anomalous wave train on the Eurasian continent,and they show significant signals one month and five months in advance,respectively,and have certain predictive significance.In addition,in the early(late)year of winter onset in North Asia,the temperature in some parts of East Asia and southern China is higher(lower)in December.In other words,the winter onset in North Asia can be a precursor signal of temperature anomalies in the above regions. |
