A Study On The Seasonal Transition Characteristics Of Spring And Summer In The Middle And High Latitudes Of Asia And Its Impact On The Summer Weather In My Country

The issue of the spring-to-summer seasonal transition in mid-and high latitude Asia(SSTMA)is a classic research topic,which has received widespread attention for a long time.The SSTMA is a special stage of the seasonal cycle of atmospheric circulation and can be used as a starting point for the study of atmospheric circulation.It helps to understand the evolution of East Asian atmospheric circulation from spring to summer,and provides new ideas and methods to the prediction of the early summer climate in China.Using the reanalysis data and observational data,taking the SSTMA as key research issue,in this paper we put forward a new objective method of defining the spring-to-summer seasonal transition and summarize its key features and evolution rule;on this basis,we further analyze the possible thermodynamic driving processes associated with the SSTMA and its anomalies;and finally we discuss with the influences of the SSTMA on the temperature and the rainfall pattern in China,from the perspective of its early and late timings and different summer onset circulation patterns.The primary conclusions are as follows:(1)We propose a new method of defining the date of SSTMA.The area-mean 2m air temperature(T2m)in mid-and high latitude Asia(70°-160°E,50°-75°N)shows a distinct seasonal cycle.The mid-and high latitude Asia experiences an intense surface warming from mid-May to early June,which is obviously stronger than its smoothed seasonal cycle of T2 m,and then the surface warming stabilizes in the second pentad of June(P32).Thus,we define this pentad,P32,as the climatic SSTMA time.Taking climate mean P32 T2 m of this regionas the temperature threshold(280.95 K),we define the SSTMA time of each year as the pentad when the area mean T2 m rises and is closest to the temperature threshold.Apparent interannual and interdecadal variations are observed in the timing of SSTMA.(2)The mid-high latitude circulation and the corresponding weather system during the SSTMA are summarized.The SSTMA is marked by the establishments of the Northeast Asian ridge and the northern East Asian low,and the subsequent "double-blocking" circulation pattern over northeastern Asia and the Ural Mountains.Accompanying the SSTMA,the meridional gradient of near-surface temperature in the mid-and high latitude Asia is weakened and hence the corresponding high-frequency baroclinic disturbances are also weakened.In contrast,lowfrequency weather systems(including the Asian blocking high and the northeast China cold vortex)become the dominant weather system in this region.(3)The thermodynamic and dynamic factors driving the rapid warming in the mid-and high latitude Asia in association with SSTMA are revealed.In climatology,the diabatic heating is primary contributor to the intense warming associated with the SSTMA.The positive feedback between surface warming and snowmelt facilitates an intense surface warming associated with the SSTMA,which further accelerates the strong surface warming process in the mid-and high latitude Asia.The large-scale wave-train circulation affect the timing of spring-to-summer seasonal transition by modulating the horizontal linear advection term and adiabatic term.(4)The associations of the SSTMA time with the circulation anomalies over East Asia and the rainfall pattern in eastern China are clarified.In the early and late SSTMA years,the midtropospheric circulation over the Asia/North Pacific region is characterized by the OkhotskJapan(OKJ)wave pattern before P32 and then by the Pacific-Japan-like(P-J-like pattern)after P32.The WPSH is stronger(weaker)than normal in the early(late)SSTMA years.Correspondingly,the seasonal evolutions of temperature and precipitation patterns in China also show considerable differences in the early and late SSTMA years.The early summer temperature in most parts of China is higher(lower)than normal in the early(late)SSTMA years.In the early SSTMA year,the Meiyu rainfall tends to start 1 ? 2 pentads earlier than normal.In the late SSTMA year,however,an above-normal rainfall is observed in the southern side of the Yangtze River(24°–30°N)during the Meiyu period.From mid-June to early July,the north and northeast China tend to have more(less)rainfall in the early(late)SSTMA years.(5)From the perspective of different summer onset circulation patterns,this paper further expounds the influences of the anomalous spring-to-summer seasonal transitions on the summer rainfall and temperature anomalies in China.Using the self-organizing maps method(SOM),the summer onset circulation patterns over the mid-and high latitude Asia are classified into four clusters.The first to the third clusters of summer onset circulation patterns are relatively stable,which can be maintained until the end of Meiyu period.So they have a relatively stable impact on the precipitation and temperature in early summer and Meiyu period.However,the fourth cluster of summer onset circulation pattern can not be maintained stably,so the temperature and precipitation anomalies will change considerably in early summer and Meiyu period,as compared to the summer onset time.The first cluster shows the circulation characteristics of early summer onset over the mid-and high latitude Asia.In the early summer and the Meiyu period,the rainfall in South China and the middle and lower reaches of the Yangtze River is below-normal,whereas the rainfall in Huaihe River Basin is above-normal.The rainfall in northeast China is less than normal.The second cluster is characterized by a stronger trough over Lake Balkhash and a stronger ridge over northeastern Asia.The pattern in eastern China shows an obvious feature of “more in the south and less in the north”.The third cluster features a deepened trough over Lake Baikal and a deepened northern East Asian low.After the SSTMA,the temperature is remarkably cold in Central Siberia,but relatively warmer in most parts of China.In the summer onset pentad,rainfall pattern in China has a feature of“more in the south and less in the north”.During the Meiyu period,however,the rainfall in the Yangtze River Basin and northeast China is more than normal,while the rainfall in northwest China and north China is less than normal.Notably,the fourth cluster corresponds to a delayed seasonal cycle not only in the mid-and high Asia but also in the low latitude Asia.