Classification of Cold Air Outbreaks in East Asia and Potential Stratospheric Harbingers

Cold air outbreaks (CAOs) enormously influence agriculture, environment, industry, and other socioeconomic activities in East Asia. This study objectively identifies and categorizes CAOs in East Asia by employing 3-dimensional CAO detection algorithm and k-means clustering method. The analysis identifies a total of 106 CAOs from 1979 to 2017, which are classified into three distinct types based on core cold anomaly positions: South, Mixed, and North types. The South type, with the fewest occurrences and shortest lifetime, exhibits a weak warm lobe over Europe and northern Asian coast with the coldest anomalies over southern China. The Mixed type, most frequent and longest-lived, features the most extensive and coldest anomalies across Eurasia uniformly distributed over East Asia. The North type shows the coldest anomalies in northeast Asia with weaker effects in southern East Asia. The weakening trend in CAO intensity observed in East Asia is primarily due to long-term changes in the Mixed types. Anomalous temperature patterns and circulation variations are diverse with the South type associated with a pre-existing strong stratospheric polar vortex state and the Mixed and North types with a pre-existing weak vortex state. Under those stratospheric backgrounds, even similar wave activities can result in dissimilar upper-tropospheric circulation anomalies over East Asia leading to different extents and magnitudes of anomalous coldness. This study provides valuable insights into the diversity of East Asian CAOs essential for forecasting and managing associated risks in East Asia.