Southward Cold Airmass Flux Associated with the East Asian Winter Monsoon: Diversity and Impacts

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AbstractThe winter monsoon has strong impacts on East Asia via latitude-crossing southward cold airmass fluxes called cold air outbreaks (CAOs). CAOs have a high diversity in terms of meridional extent and induced weather. Using the daily cold airmass flux normalized at 50° and 30°N during 1958–2016, we categorize the CAOs into three groups: high–middle (H–M), high–low (H–L), and middle–low (M–L) latitude events. The H–L type is found to have the longest duration, and the M–L type is prone to strong CAOs regarding normalized intensity. The H–L and H–M events feature a large-scale dipole pattern of cold airmass flux over high-latitude Eurasia; the former events feature relatively strong anticyclonic circulation over Siberia, while the latter events feature cyclonic circulation over northeastern Asia. In contrast, the M–L events are characterized by a cyclonic anomaly over northeastern Asia but no obvious high-latitude precursor. The H–L events have the greatest cold anomaly in airmasses near the surface, and the M–L events mainly feature a strong northerly wind. As a result, the H–L events induce widespread long-lasting low temperatures over East Asia, while the M–L events induce a sharp temperature drop at mainly low latitudes. Both H–L and M–L events couple with the MJO to enhance rainfall over the South China Sea, while H–M events increase rainfall over southern China. Moreover, the occurrences of H–L and M–L events have experienced a long-term decrease since the 1980s that has induced a stronger warming trend in the cold extremes than in the winter mean temperature at mid–low latitudes over East Asia.