The focus of this study is to present the recent changes in BB activity over PSEA by considering decadal changes, 5-year ensemble mean analysis, and long-term trends from twenty years (2001-2020) of Moderate Resolution Imaging Spectroradiometer (MODIS) active fire counts. The results revealed that overall springtime BB activity significantly decreased over the past decade (2011-2020) compared to the previous decade (2001-2010). Surprisingly, the individual monthly analysis revealed that BB activity over PSEA decreased substantially in March, whereas in April it increased significantly over the past decade (2011 to 2020). Further, 5-year ensemble means revealed, BB activity over northern PSEA in March sharply increased during 2006–2010, and moderately increased in 2011-2015 followed by a profound decrease in 2016-2020. Whereas, in April, the BB activity showed a pronounced increase in the 2011-2015 and 2016-2020 periods over northern Laos compared to the rest of PSEA. The observed changes in the BB activity are strongly reflected in the BB aerosols and gases. The MERRA-2 reanalysis total surface mass concentration-PM2.5, BB black carbon, and MOPITT satellite observed surface carbon monoxide (CO) showed a significant decrease over northern PSEA in March and a strong enhancement was evident over northern Laos in April.  Finally, the trend analysis in BB activity shows a significant increasing trend over Laos and Cambodia, and a decreasing trend was found rest of the PSEA. These findings have important implications for future BB management strategies and regional climate in the PSEA region.

Two distinct rainfall stages over the Central Highlands (CH) of Vietnam during the rainy season have been objectively defined using the high-resolution Vietnam Gridded Precipitation dataset for 1983–2010 (28 years): a second rainy season (SRS) embedded in the conventional rainy season. Surprisingly, the pronounced interannual variation in the SRS onset date has led to three apparent regimes: an early (late) SRS with a 1 month longer (shorter) rainfall period occurring in early July (until mid-August) and a normal SRS starting in late July. Almost all the early SRS years occur during El Niño developing phases, particularly during the Niño3.4 sea surface temperature (SST) increase from January through December. Water vapor budget analyses reveal that the interannual variation in the divergent water vapor flux is in response to the warmer July tropical Pacific SST anomalies, resulting in rainfall enhancement over the CH and eventually inducing early SRS onset.