After being hit by several devastating heat waves in recent years, Europe experienced an exceptionally hot June in 2019 again. This June is the hottest one on record over Central Europe both in a monthly‐mean sense and in terms of the number of extremely hot days. The above‐normal hot condition is caused by an anomalous long‐lasting anticyclone in the upper troposphere, which advects warm air from the Sahel and Mediterranean region and enhances incoming solar radiation and surface turbulent fluxes. The anomalous anticyclone results from an unusually‐intensified British‐Baikal Corridor (BBC) pattern and a synoptic Rossby wave breaking (RWB) event over Europe. Three sub‐monthly heat waves are observed in the month. The first two are associated with the BBC pattern, and the third is related to the combination of the previous BBC pattern activity and the RWB event.

The Pacific–Japan (PJ) pattern, also known as the East Asia–Pacific pattern, is a teleconnection that significantly influences the East Asian summer climate on various time scales. Based on several reanalysis and observational datasets, this study suggests that the PJ pattern has experienced a distinct three-dimensional structural change in the late 1990s. Compared with those during 1979–98, the PJ pattern shifts eastward by approximately 20° during 1999–2015, and the intensity of its barotropic structure in the extratropics weakens significantly. As a result, its influences on the summer rainfall along the mei-yu band are weakened after the late 1990s. These observed changes can be attributed to three reasons. First, the location where the PJ pattern is excited shifts eastward. Second, the easterly shear of the background wind is very weak around the source region of the PJ pattern after the late 1990s, which prevents the convection-induced baroclinic mode from converting into barotropic mode and thereby from propagating into the extratropics. Third, the PJ pattern–induced rainfall anomalies are weak along the mei-yu band after the late 1990s. As a result, their feedbacks to the PJ pattern become weak and play a considerably reduced role in maintaining the structure of the PJ pattern in the midlatitudes. In contrast, the eddy energy conversion from the basic flow efficiently maintains the PJ pattern before and after the late 1990s and thereby contributes little to the observed change.

The El Niño–Southern Oscillation (ENSO) is regarded as one of the most important factors for onset of the South China Sea summer monsoon (SCSSM). Previous studies generally indicated that an El Niño event tends to result in a late onset of the SCSSM monsoon. However, this relationship has not been true in recent years, particularly when an extremely early SCSSM onset (1 May 2019) occurred following the 2018/19 El Niño event in the preceding winter. The processes of the second earliest SCSSM onset in the past 41 years were investigated using NCEP–DOE reanalysis, OLR data, and ERSST. A negative sea surface temperature and associated anticyclonic anomalies were absent over the western North Pacific in the late spring of 2019 following an El Niño event in the preceding winter. Thus, the mean circulation in the late spring of 2019 does not prevent SCSSM onset, which is in sharp contrast to the composited spring of the El Niño decaying years. The convective active and westerly phases of a 30–60-day oscillation originating from the Indian Ocean provided a favorable background for the SCSSM onset in 2019. In addition, the monsoon onset vortex over the Bay of Bengal and the cold front associated with a midlatitude trough over East Asia also played important roles in triggering the early onset of the SCSSM in 2019. No tropical cyclone appeared over the western North Pacific during April and May, and the enhancement of quasi-biweekly oscillation mainly occurs after the SCSSM onset; thus, these two factors contribute little to the SCSSM onset in 2019.

Traditionally, the boreal summer monsoon season is regarded as a fixed period of June–September (JJAS). In fact, the monsoon commencement and termination are not fixed to the annual cycle, thus the monsoon duration exhibits remarkable variations. Based on the multivariable empirical orthogonal function analysis, the commencement, termination, and duration of the Asian summer monsoon (ASM) are objectively determined in this study. The ASM duration is shown to be closely linked to the sea surface temperature anomalies in the equatorial Pacific Ocean, which tends to be shorter during an El Niño and longer during a La Niña year. Through the anomalous Walker circulation and the equatorial Rossby wave response, a developing La Niña event is generally associated with advanced commencement and delayed termination, thus a longer ASM duration. The importance of monsoon duration is illustrated further by revisiting the relationship between El Niño-southern oscillation (ENSO) and the monsoonal rainfall, which is the total rainfall within monsoon duration (from commencement to termination) rather than within JJAS. The relationship between the JJAS rainfall over South Asia and ENSO has exhibited remarkable interdecadal changes, which becomes weak since the 1980s and is recovered after the early 2000s. In sharp contrast, the linkage between the monsoonal rainfall over South Asia and ENSO remains significant and robust over the past four decades. Via attaching the rainfall during the transition periods (advanced commencement and delayed termination), the longer ASM duration during La Niña shall increase the total rainfall, which can reinforce the canonical ENSO-monsoonal rainfall relationship. The above results suggest that a reasonable definition of ASM duration may help us better understand the monsoon phenomenon and teleconnections.