The present study investigates the roles of El Niño–Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) in the relationship between the East Asian winter monsoon (EAWM) and the following East Asian summer monsoon (EASM). The variability of the EAWM is divided into an ENSO-related part named EAWMEN and an ENSO-unrelated part named EAWMres. Corresponding to a weak EAWMEN, an anomalous low-level anticyclone forms over the western North Pacific (WNP) and persists from winter to the following summer. This anticyclone enhances southerlies over the coast of East Asia in summer. Hence, a weak EAWMEN tends to be followed by a strong EASM and vice versa. As such, a link is established between the EAWMEN and the EASM. The persistence of this WNP anticyclone may be mainly attributed to the sea surface temperature anomalies associated with the ENSO-related EAWM part in the tropical Indian Ocean and the extratropical North Pacific. In contrast, corresponding to a weak EAWMres, the anomalous WNP anticyclone is only seen in winter, and there is no obvious relationship between the EAWMres and the following EASM. Therefore, the observed EAWM–EASM relationship is dominated by the winter monsoon variability associated with ENSO. It is found that the EAWMEN–EASM relationship is modulated by the PDO. There tends to be a much stronger EASM after a weak EAWMEN during the positive PDO phases than during the negative PDO phases.

Compared to individual hot days/nights, compound hot extremes that combine daytime and nighttime heat are more impactful. However, past and future changes in compound hot extremes as well as their underlying drivers and societal impacts remain poorly understood. Here we show that during 1960-2012, significant increases in Northern Hemisphere average frequency (~1.03 days decade −1 ) and intensity (~0.28°C decade −1 ) of summertime compound hot extremes arise primarily from summer-mean warming. The forcing of rising greenhouse gases (GHGs) is robustly detected and largely accounts for observed trends. Observationally-constrained projections suggest an approximate eightfold increase in hemispheric-average frequency and a threefold growth in intensity of summertime compound hot extremes by 2100 (relative to 2012), given uncurbed GHG emissions. Accordingly, endof-century population exposure to compound hot extremes is projected to be four to eight times the 2010s level, dependent on demographic and climate scenarios.

Modulation of the Pacific decadal oscillation (PDO) on the behavior of the East Asian summer monsoon (EASM) in El Niño decaying years has been studied. When El Niño is in phase with the PDO (El Niño/high PDO), the low-level atmospheric anomalies are characterized by an anticyclone around the Philippines and a cyclone around Japan, inducing an anomalous tripolar rainfall pattern in China. In this case, the western Pacific subtropical high (WPSH) experiences a one-time slightly northward shift in July and then stays stationary from July to August. The corresponding anomalous tripolar rainfall pattern has weak subseasonal variations. When El Niño is out of phase with the PDO (El Niño/low PDO), however, the anomalous Philippines anticyclone has a much larger spatial domain, thereby causing an anomalous dipole rainfall pattern. Accordingly, WPSH experiences clearly two northward shifts. Therefore, the related dipole rainfall pattern has large subseasonal variations. One pronounced feature is that the positive rainfall anomalies shift northward from southern China in June to central China in July and finally to northern China in August. The different El Niño–EASM relationships are caused by the influences of PDO on the decaying speed of El Niño. During the high PDO phase, El Niño decays slowly and has a strong anchor in the north Indian Ocean warming, which is responsible for the anomalous EASM. Comparatively, during the low PDO phase, El Niño decays rapidly and La Niña develops in summer, which induces different EASM anomalies from that during the high PDO phase. Additionally, PDO changes El Niño behaviors mainly via modifying the background tropical winds.