An abrupt increase in rainfall during the monsoon season in Asia could have a strong impact on many activities of two thirds of the world's population, including agriculture, commerce, forestry, and hydropower. Over Southeast Asia, rapid precipitation enhancement mainly occurs during commencement of the Asian summer southwest monsoon, which signifies a transition from the dry to the rainy season (Lau & Yang, 1997;Nguyen-Le et al., 2015;Zhang et al., 2002). Therefore, research in past decades has increasingly focused on the summer monsoon onset date (SMOD) or summer rainy season onset date (RSOD). For example, Zhang et al. (2002) used the observed daily rainfall over the central Indochina Peninsula (ICP) to determine the mean SMOD as being May 9, with a standard deviation of 12 days. Applying the empirical orthogonal function analysis on daily mean precipitation, Nguyen-Le et al. (2015) demonstrated that the mean summer RSOD over the eastern ICP is May 6, with a standard deviation of 13 days. However, the immediate increase in rainfall over the eastern ICP is observed in early autumn when the summer monsoon withdraws (

Based on station monthly observational statistics, the major rainfall in the Red River Delta (RRD) occurs in late summer (July–September) with conspicuous year-to-year variation. Using the ± 0.8 of the seasonal total rainfall standard deviation as criteria, seven wet and six dry years are identified over the period 1983–2015. In addition to the mean rainfall in wet years being nearly double that in dry years, the occurrence frequency of heavy rainfall days and their corresponding rainfall accumulation in wet (dry) years are higher (lower) than climatology–in particular, the average rainfall accumulation during heavy rainfall days in wet years is more than four times as much as that in dry years. As revealed from further analyses, the large variability in rainfall is attributed to the influence of tropical cyclones (TCs) and 7–24- and 30– 60-day intraseasonal oscillations (ISOs). During the wet (dry) years, the number of TCs affecting the RRD and rainfall produced by TCs are more (less), and the intensity of ISOs also enhances (reduces). It is found from the water vapor budget analyses that an anomalous cyclone (anticyclone) dominates over the Indochina Peninsula in wet (dry) years, resulting in more (less) water vapor being transported to the RRD, whereas the anomalous convergence (divergence) of water vapor flux leads to the maintenance of excessive (insufficient) rainfall over the RRD. However, the El Niño–Southern Oscillation (ENSO) forcing shows minor effects on the interannual variation in rainfall in the RRD.

Particulate matter (PM) pollution has become a major problem worldwide, with significant adversely health impacts, but the climatology of annual variations in and the effect of climate change on PM level are still not comprehensively evaluated. Here we show that the vertical motions of the East Asian winter and summer monsoon regulate the annual variation in PM over Taiwan. The PM pollution season starts and ends earlier during El Niño than during La Niña episodes. Furthermore, the onset, active, break, revival, and retreat phases of the PM pollution lifecycle are innovatively defined. Our results demonstrate that the annual seasonal cycle dominates wintertime PM pollution climate development during the active phase with a minor modulation by El Niño and La Niña episodes, whereas October and March, the seasonal transition periods, are significantly modulated by these two episodes. The findings suggest a novel dimension of the PM pollution research—lifecycle evolution effect.

Particulate matter (PM) pollution has become a major problem worldwide, with significant adverse health impacts, but the climatology of annual variations in and the effect of climate change on PM levels are still not comprehensively evaluated. Here we show that the vertical motions of the East Asian winter and summer monsoons regulate the annual variation in PM over Taiwan. The PM pollution season starts and ends earlier during El Niño episodes than during La Niña episodes. Furthermore, the onset, active, break, revival, and retreat phases of the PM pollution lifecycle are innovatively defined. Our results demonstrate that the annual seasonal cycle dominates wintertime PM pollution climate development during the active phase with a minor modulation by El Niño and La Niña episodes, whereas October and March, the seasonal transition periods, are significantly modulated by these two episodes. The findings suggest a new dimension of PM pollution research—the lifecycle evolution effect.

Based on station monthly observational statistics, the major rainfall in the Red River Delta (RRD) of Vietnam occurs in late summer (July–September) with conspicuous year-to-year variation. Using the ± 0.8 of the seasonal total rainfall standard deviation as criteria, seven wet and six dry years are identified over the period 1983–2015. In addition to the 70.5% of the seasonal total rainfall contributed by the heavy rainfall days, the distinct heavy rainfall accumulation difference between wet and dry years seems to fundamentally establish these two separated extreme wet and dry groups. As revealed from further analyses, the large variability in rainfall is attributed to the influence of tropical cyclones (TCs) and 7–24- and 30–60-day intraseasonal oscillations (ISOs); in particular, the number of TCs affecting the RRD and rainfall produced by TCs are more (less) during the wet (dry) years, and the amplitudes of ISOs are also enhanced (reduced). In many cases, heavy rainfall days are induced by the combined effect of both ISOs and TCs, while some heavy rainfall events are mainly triggered by ISOs. It is found from the water vapor budget analyses that an anomalous cyclone (anticyclone) dominates over the Indochina Peninsula in wet (dry) years, resulting in more (less) water vapor being transported to the RRD, whereas the anomalous convergence (divergence) of water vapor flux leads to the maintenance of excessive (insufficient) rainfall over the RRD. However, the El Niño–Southern Oscillation (ENSO) forcing shows minor effects on the interannual variation in rainfall in the RRD.