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Anomalies in atmospheric moisture transport are critical for drought formation, with East Asian monsoon anomalies identified as the primary driver of droughts in East Asia. However, the drought mechanism for the border region between monsoon and nonmonsoon land areas remains unclear due to complex interactions between latitudinal circulation patterns. Using a Lagrangian framework, we quantify moisture supply (MS) during spring and summer drought events within the border region, specifically Northeast China (NEC). Our results reveal that decreased MS from monsoon land areas is more crucial than nonmonsoon land areas and local areas for NEC droughts, with the local water recycling playing a more substantial role in summer than in spring. On average, summer droughts are 4 times more intense than spring droughts. Contributions to MS deficits from monsoon land areas, nonmonsoon land areas, and local areas during spring (summer) droughts are 43.2% (43.8%), 25.1% (19.2%), and 22% (33.8%), respectively. The weakened atmospheric circulation from source regions to NEC contributes to over 80% of MS deficits during droughts. Atmospheric wave trains triggered by North Atlantic sea surface temperature anomalies (SSTA) gradients, along with the weakening of the East Asian subtropical westerly jet in spring and a positive phase of the polar–Eurasian teleconnection in summer, contribute to spring and summer droughts, respectively. A sustained, record-breaking positive SSTA along the western European coast from the spring to summer excited a wave train, and resulted in the extreme 2017 spring–summer drought. These findings provide valuable insights into the drought mechanism within the interaction zone of circulation systems from different latitudes. Significance Statement: The complex interactions between atmospheric circulation patterns at different latitudes have led to an insufficient understanding of the mechanisms causing extreme drought in the border region between the monsoon and nonmonsoon land areas. This study aims to investigate the drought mechanisms in the border region from the perspective of moisture tracking. Results indicate a moisture supply deficit from the monsoon region contributes the most (approximately half) to droughts, although the majority of air parcels reaching this region originate from the nonmonsoon region. The weakening of circulation contributes to more than 80% of the moisture supply deficit during droughts, far exceeding the impact of reduced evaporation from the moisture source regions. These findings are conducive to understanding the mechanisms of extreme drought. [ABSTRACT FROM AUTHOR]

Haze pollution frequently occurs in North China almost every winter month. However, many previous studies pointed out that the interannual–decadal variations of haze in February were inconsistent with that in early winter, which demonstrated an evident subseasonal change in haze pollution. In this study, we found a close relationship between the "warm Arctic–cold Eurasia" (WACE) pattern in January and the weakened February haze pollution in North China according to monthly composites from 1980 to 2019. Associated with the warming signal in sea ice and the cooling signal in Eurasian soil temperatures, the WACE pattern sustained from January to February. The combined signal of sea ice and soil temperature anomalies and its associated anomalous cyclonic circulations over North China provided an environment conducive to the dispersion of pollutants. The relationship between the WACE pattern and the cyclonic circulation anomaly could also be detected from the large-ensemble CMIP6 simulations. Furthermore, the relationship between the January WACE pattern and the reduction of February haze pollution has been prominently enhanced since the late 1990s, which accurately corresponds to the deep Arctic warming that has emerged since the late 1990s. Significance Statement: Haze pollution frequently occurs in North China almost every winter month, but with an apparent subseasonal change. Climate anomalies in the mid- to high latitudes could regulate the variations in haze through atmospheric teleconnection. Our study found a relationship between the "warm Arctic–cold Eurasia" (WACE) pattern in January and the weakened February haze pollution in North China. The lingering signal of the WACE was associated to the anomalous cyclonic circulations over North China in February, which was conducive to the dispersion of pollutants. Furthermore, the relationship between the WACE pattern and February haze pollution has been prominently enhancing since the late 1990s, which accurately corresponds to the emergence of the deep Arctic warming accompanied by the strengthened Eurasian cooling. This investigation is of great significance for subseasonal haze prediction. [ABSTRACT FROM AUTHOR]