Climatology and Interannual Variability of Extratropical Cyclones in the Winter Half-Year in Northern China

Using the objective definition and auto-identification and tracking technique, the database of extratropical cyclones (ECs) in the winter half-year in northern China from 1951 to 2010 is built on the basis of the NCEP/NCAR reanalysis data. Furthermore, the climatology and interannual variability of ECs are studied. The results show that cyclogenesis is mainly focused in the central-west region of Mongolia and the eastern foot of Yablonoi Mountains. ECs are active mostly over southeast of Mongolia, southeast of Inner Mongolia, and the Songnen Plain; however, cyclolysis and rapid decay of ECs occur over the southeast of inner Mongolia and central-north of Northeast China. The frequency of ECs is the highest in early spring and middle autumn and the lowest in winter. With seasonal evolution, the active region of ECs moves eastward and southward. The eastward path is the most common track of ECs, after which comes the southeastward track, and then the northeastward one. Horizontal winds at 700 hPa can be considered as the steering flow of ECs. The rapid growth of ECs, whose standard deepening rate is greater than 0.5 Bergeron for 24 h, is mainly active over the east of Inner Mongolia, central-north of Northeast China, and the central Japan Sea. However, explosive ECs are spread mainly over the Japan Sea and marginally over the continent of China. The extremely strong EC events occurred mostly in the early spring and mid autumn before the 1980s. However, frequency of ECs was relatively low, the frequency of extremely strong ECs has increased since the 1980s. The decrease of ECs is related to the weakening of the baroclinicity in the lower troposphere of Northeast Asia.