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Air pollution, particularly PM2.5, poses a significant environmental and public health concern, particularly in northern Thailand, where elevated PM2.5 levels are prevalent during the dry season (January–May). This study examines the influx and patterns of transboundary biomass burning PM2.5 (TB PM2.5) in this region during the 2019 dry season using the WRF-Chem model. The model's reliability was confirmed through substantial correlations between model outputs and observations from the Pollution Control Department (PCD) of Thailand at 10 monitoring stations. The findings indicate that TB PM2.5 significantly influences local PM2.5 levels, often surpassing contributions from local sources. The influx of TB PM2.5 began in January from southern directions, intensifying and shifting northward, peaking in March with the highest TB PM2.5 proportions. Elevated levels persisted through April and declined in May. Border provinces consistently exhibited higher TB PM2.5 concentrations, with Chiang Rai province showing the highest average proportion, reaching up to 45%. On days when PM2.5 levels were classified as 'Unhealthy for Sensitive Groups' or 'Unhealthy', TB PM2.5 contributed at least 50% to the total PM2.5 at all stations. Notably, stations in Chiang Rai and Nan showed detectable TB PM2.5 even at 'Very Unhealthy' levels, underscoring the significant impact of TB PM2.5 in the northern border areas. Effective mitigation of PM2.5-related health risks requires addressing PM2.5 sources both within and beyond Thailand's borders. [ABSTRACT FROM AUTHOR]

In the last decade, a sequence of once‐in‐50 year or even once‐in‐100 year high‐impact drought events have hit East Asia. By defining a super drought with the Standardized Precipitation Evaporation Index for the time scales of 3, 6, 12 and 24 months all below −1.5, this study aims to examine the changes in super droughts and the underlying mechanism over East Asia in the past 60 years, putting the recent high‐impact droughts in the context. Super droughts in the last 10 years over East Asia have been the most expansive, with two hotspots located in the Transitional Climate Zone (TCZ) and the Southeast Asian region (SEA) regions, together accounting for 2/3 of the total. The seasonal distribution characterizes the largest contribution from summer followed by autumn and then spring and winter. Super droughts over TCZ peak in the recent two decades, which are primarily driven by the increase in potential evaporation (PET) contributing 41 and 80% in the first and second 10 years, respectively. It turns out that global warming signal can explain more than 90% of this PET increase. Over SEA, the recent decade and the period around 1990 saw the most widespread super droughts that were not spatially uniformly distributed but clustered in three subregions. Different from TCZ, the precipitation rather than PET is the most influential in governing super droughts over SEA. Out of the total variability in precipitation, about half of the super droughts in the recent decade is caused by decadal variability, while the trend mode has negligible influence. [ABSTRACT FROM AUTHOR]

The transitional climate zone (TCZ) over East Asia is an interface area between monsoon circulation and westerly circulation, making it highly susceptible to natural disasters and climate change. This study explores the spatial–temporal variation of the summer (May–September) TCZ during 1961–2018 and its response to monsoon rainy season. The summer Aridity Index (AI) instead of annual amount is used to recognize summer TCZ, with similar domain produced. However, differing from the annual total, the summer evaporation remains stable during the whole period, despite global warming. Hence, the summer TCZ has experienced little significant long‐term trends toward wetter or drier, since it is predominantly driven by precipitation that shows no clear tendency. The EOF decomposition reveals three key patterns accounting for 58.2% of the total variance: spatially uniform, south–north dipole and meridional tripole, which shows the complexity of spatial‐temporal variation in TCZ. TCZ is characterized by interdecadal mutations and periodical oscillations of 3 or 8 years. There exist remarkable 3 years interannual periods, with the greatest contribution stemming from the south‐central part of TCZ. In comparison, the northeast part exhibits a significant interdecadal band of about 8 years, which promotes the regime shift from wet to dry in the late 1990s over TCZ. Furthermore, the onset, retreat, and duration of the rainy season of TCZ in response to summer monsoon are examined, based on EEMD filtered 4 mm/day precipitation. All three indicators are subjected to strong interannual dispersion, without any significant overall trends. Only the retreat date of the western part of TCZ (100–110°E) is significantly correlated with the onset date. Dry‐wet condition in TCZ is more closely associated with the duration length of the monsoon influence. And we also notice that the westmost part of TCZ shows a greater sensitivity to monsoon activity than the east part. [ABSTRACT FROM AUTHOR]