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Compound hazards are more destructive than the individual ones. Using observational and reanalysis datasets during 1960–2019, this study shows a remarkable concurrent relationship between extreme heatwaves (HWs) over southeastern coast of China (SECC) and tropical cyclone (TC) activities over western North Pacific (WNP). Overall, 70% of HWs co-occurred with TC activities (TC–HWs) in the past 60 years. Although the total frequency of TCs over WNP exhibited a decreasing trend, the occurrences of TC–HWs over SECC have been increasing, primarily due to the increasing HWs in the warming climate. In addition, TC–HWs are stronger and longer lasting than HWs that occur alone (AHWs). And in the long-term perspective, both AHWs and TC–HWs exhibit increasing trends, especially since the mid-1980s. The enhancement on HWs caused by TC activities is sustained until TCs make their landfalls and then collapse. Based on composite analysis, TC activities enhance HWs by modulating atmospheric circulations and triggering anomalous descending motion over southern China mainland which intensifies the western Pacific subtropical high (WPSH) and favors increased temperatures therein. Given the severe adverse impacts of TC–HWs on coastal populations, more research is needed to assess the future projections of TC–HWs, as HWs are expected to be more frequent and stronger as the climate warms, whereas TCs over WNP may occur less often.

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A large population in China has been exposed to both severe ozone (O3) pollution and extreme heat under global warming. Here, the spatiotemporal characteristics of coupled extremes in surface O3 and heat (OPCs) over China are investigated using surface observations, a process-based chemical transport model (GEOS-Chem), and multi-model simulations from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). North China Plain (NCP; 37–41∘ N; 114–120∘ E) is identified as a hot spot of OPCs, where more than half of the O3 pollution days are accompanied by high temperature extremes. OPCs over NCP exceeded 40 d during 2014–2019, exhibiting an increasing trend. Both O3 concentrations and temperatures are elevated during OPCs compared with O3 pollution days occurring individually (OPIs). Therefore, OPCs impose more severe health impacts to humans than OPIs, but the stronger health effects are mainly driven by the higher temperatures. GEOS-Chem simulations further reveal that enhanced chemical production resulting from hot and stable atmospheric conditions under anomalous weather patterns primarily contributes to the exacerbated O3 levels during OPCs. In the future, CMIP6 projections suggest increased occurrences of OPCs over NCP in the middle of this century, but by the end of this century, OPCs may decrease or increase depending on the pollutant emission scenarios. However, for all future scenarios, extreme high temperatures will play an increasingly important role in modulating O3 pollution in a warming climate.

The northeastern China cold vortex (NCCV) has an important influence on the regional rainstroms over East Asia. Using the National Centers for Environmental Prediction Final (FNL) reanalysis datase...

The station observations and reanalysis dataset are utilized to identify the cold vertex over Northeast China (NECV) in early summer (ES) and midsummer (MS) respectively. In this study, we focus on the characteristics of NECV and their distinct influences on local precipitations in ES and MS. The underpinning mechanisms are further inspected in terms of thermodynamic and dynamic processes. Results suggest that in ES (MS) the NECV is mainly located over Northern China-Southeastern Russia (Mongolia) and significantly correlated to the precipitations over the eastern (mid-western) regions of Northeast China. In the cases of extreme precipitations, NECV displays northward shift and intensification in ES and MS respectively. Meanwhile, the upper-level wind anomalies suggest a northward displacement of polar front jet and a weakened subtropical jet in ES, and an enhanced polar front jet and a southward shift of subtropical jet in MS. The wind anomalies induced by meridional temperature gradients and the Rossby wave activities transported to Northeast China encourage the development of atmospheric circulation vorticity and then promote the variations of NECV. Furthermore, the vorticity and temperature advections are favorable to the enhancement of ascending motion under quasigeostrophic approximation, which is combined with the sufficient water vapor transported from oceanic regions, triggering the regional extreme precipitations. Intriguingly, the pathways of water vapor transport and disturbance energy propagation caused by the diverse external forcings are different in ES and MS, which sheds some fresh light on the insight into the subseasonal variations of NECV and the distinctive contributions to extreme precipitations.

Driven by four global climate models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5) (i.e., CNRM-CM5, EC-EARTH, GFDL-ESM2M and MPI-ESM-LR) under the Representative Concentration Pathway 8.5 (RCP8.5) scenario, projections for future changes in heat waves over China are performed by Weather Research Forecasting (WRF) model simulations for future (FTR, 2031–2055) and present (1981–2005) periods. Six heat wave indices are applied to characterize heat waves based on their frequency, duration, magnitude, intensity, accumulated occurrence days and severity. Analyses show that notable increases in heat wave indices cover all of China. More areas will endure more frequent, longer lasting and more severe heat waves in the coming decades. The increasing tendencies of heat wave indices in the FTR period are more significant than those at present, indicating that heat waves will intensify more rapidly in the future. The impacts of climate changes on the accumulated properties of heat waves are more substantial than those on the individual aspects of heat waves. It is also projected that stronger heat waves with prolonged durations and more severe magnitudes will occur more often in the FTR period, whereas relatively weaker heat waves would occur less often. Hence, the occurrence of extreme heat waves shows a more remarkable increase than the occurrence of moderate heat waves. The changes in heat waves can be largely explained by the changes in the dominating high-pressure systems.

A novel diagnostic framework based on the wave activity of column integrated water vapor (CWV) is used to probe into the higher moments of the hydrological cycle with bearings on the extremes. Applying the CWV wave activity analysis to the historical and RCP8.5 scenario simulations by the CMIP5 models reveals a super Clausius–Clapeyron rate of increase in the wet vs. dry disparity of daily net precipitation due to the enhanced stirring length of wave activity at the poleward flank of the storm track, despite a decrease in the hydrological cycling rate (HCR) measured by the reciprocal of wave activity residence time. The local variant of CWV wave activity unravels the unique characteristics of atmospheric rivers (ARs) in terms of their transport function and locally enhanced mixing efficiency. Under RCP8.5, the local moist wave activity increases by ~40% over the northeastern Pacific and western United States by the end of the 21st century, indicating lengthening and more frequent landfalling ARs with a consequence of a ~20% increase in the related hydrological extremes $$\widetilde {(P - E)}^ +$$ ( P - E ) ̃ + in the west coast, despite a robust weakening of the local HCR. These results imply that the unusually wet winter the west coast just experienced in 2016/17 might be a harbinger of more frequent wet extremes in a warmer climate.

In an attempt to resolve the controversy as to whether Arctic sea ice loss leads to more midlatitude extremes, a metric of finite-amplitude wave activity is adopted to quantify the midlatitude wave activity and its change during the period of the drastic Arctic sea ice decline in both ERA Interim reanalysis data and a set of Atmospheric Model Intercomparison Project-type of model experiments. Neither the ensemble mean response to the trend in the SST nor that to the declining trend of Arctic sea ice can replicate the sizable midlatitude-wide increase in the total wave activity (Ae) observed in the reanalysis, leaving its explanation to the atmospheric internal variability. On the other hand, both the diagnostics of the flux of the local anticyclonic wave activity (LAWA) and atmospheric general circulation model experiments lend evidence to a possible linkage between the sea ice loss near the Barents and Kara Seas and the increasing trend of LAWA over the northern part of the central Eurasia and the associated impacts on the frequency of temperature extremes.

This study investigates the concurrent variations in the location and intensity of the East Asian subtropical jet (EASJ) and polar front jet (EAPJ) with a focus on the relationship between the variations and the atmospheric circulation in the mid-latitude region. The possible mechanisms for the concurrent variations of the EASJ and EAPJ are explored. The wintertime upper-level zonal wind variation over the landmass (50°–100°E) is characterized by two principal modes, i.e., the out-of-phase variations in the intensity and the meridional displacements of the EASJ/EAPJ. The EASJ and EAPJ are intensified (weakened) when they are located close to (far away from) each other, corresponding to a weakened (intensified) East Asian trough over the northern Pacific region and a decreased (increased) geopotential height in the mid-latitude over central Asia. The spatial pattern of 500 hPa geopotential height anomaly is similar to the Eurasian teleconnection pattern, while the pattern of sea level pressure anomaly shows a feature similar to the Arctic oscillation. Analyses of related thermal-dynamical processes indicate that the increased (decreased) diabatic heating and horizontal heat transport lead to a larger (smaller) meridional temperature gradient over the EASJ/EAPJ regions, which subsequently intensifies (weakens) the East Asian jets. Severe (weak) convective activities accompanied with warmer (colder) SST in the tropics also contribute to the intensification (weakening) of the EASJ and EAPJ. Dynamically, the strong (weak) divergence of the eddy vorticity flux along the northern Tibetan Plateau and that of the Eliassen–Palm flux of the synoptic-scale stationary waves are favorable for the intensification (weakening) of zonal winds through the wave-mean flow interactions. As a result, the EASJ and EAPJ are intensified (weakened).