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Autumn is the transitional season when the atmospheric circulation pattern changes from summer to winter. The temperature and precipitation in Southeastern China in autumn are significantly influenced by the change in circulation patterns, and both show significant uniqueness. The clustering method can be used to observe the changes of circulation patterns in detail and to observe and analyze the transition from warm to cold seasons from a detailed view of the daily circulation pattern perspective. This method may have important research implications on how to study the generation and dissipation of extreme weather events. The Self-Organizing Maps (SOM) method is used to a 500 hPa geopotential height and 850 hPa wind and sea level pressure for 1981–2020 to identify the characteristic weather patterns (WTs) in autumn (September–November) over Southeastern China. Characteristics of the captured WTs are also analyzed in terms of the distribution characteristics of weather patterns, occurrence frequency, typical progression, precipitation and extreme precipitation (EP), temperature and extreme high temperature (EHT), and the relationship with atmospheric teleconnection. Nine WTs were identified in autumn, which represents a series of weather situations consisting of troughs and ridges. On this basis, these WTs were used to carry out the differentiation of seasonal differences between early and late autumn. The maximum mean and extreme precipitation occur in several early season patterns (WT1, WT2, WT4, and WT7). It is highly likely that extremely high temperatures occur in the WT1 and WT2 patterns. The most common progression between WTs is WT7−WT1−WT2−WT4 in the early season. This seasonality allows us to distinguish between early and late seasons based on daily weather types. A preliminary trend analysis suggests that patterns in the early season occur more frequently and last longer in the early season, and patterns in the late season occur less frequently and later. That is, the longer cool season pattern is shifting to the shorter warm season pattern. In addition, the persistence of both cool and warm patterns increased during 2001–2020 relative to 1981–2000, and the risk of both flooding and drought occurrence is on the rise. [ABSTRACT FROM AUTHOR]

It is very important to identify the possible mechanism of precipitation generation in southeastern China. To determine the weather type (WT) that is likely to generate precipitation, the Jenkinson–Collison classification method was applied to wintertime daily mean sea level pressure data across southeastern China. We found that all WTs with easterly components are prone to generate precipitation. These WTs were merged into one type named the ensemble of easterly wind components (the EE type). The persistence and transformation rules of the EE type are studied. Anticyclone type is the easiest one to convert to EE type. The persistence and transformation rules of the EE type provide favourable conditions for the formation of precipitation. Furthermore, we examined the influence of two common teleconnections (Western Pacific [WP] and Eurasian [EU]) in East Asia in winter on the WTs. When WP is in the positive phase and EU is in the negative phase, EE type weather patterns will appear frequently in southeastern China. At this time, it is most conducive to the formation of precipitation. On the one hand, warm and cold air converge in this area. On the other hand, the prevailing east wind can provide sufficient water vapour. The study implies that whether a WT is easy to produce precipitation can be judged by wind direction. The results will help us better understand the physical mechanism of precipitation generation in this area. [ABSTRACT FROM AUTHOR]

It is very important to identify the possible mechanism of precipitation generation in southeastern China. To determine the weather type (WT) that is likely to generate precipitation, the Jenkinson–Collison classification method was applied to wintertime daily mean sea level pressure data across southeastern China. We found that all WTs with easterly components are prone to generate precipitation. These WTs were merged into one type named the ensemble of easterly wind components (the EE type). The persistence and transformation rules of the EE type are studied. Anticyclone type is the easiest one to convert to EE type. The persistence and transformation rules of the EE type provide favourable conditions for the formation of precipitation. Furthermore, we examined the influence of two common teleconnections (Western Pacific [WP] and Eurasian [EU]) in East Asia in winter on the WTs. When WP is in the positive phase and EU is in the negative phase, EE type weather patterns will appear frequently in southeastern China. At this time, it is most conducive to the formation of precipitation. On the one hand, warm and cold air converge in this area. On the other hand, the prevailing east wind can provide sufficient water vapour. The study implies that whether a WT is easy to produce precipitation can be judged by wind direction. The results will help us better understand the physical mechanism of precipitation generation in this area. [ABSTRACT FROM AUTHOR]