Huopo Chen

A record‐breaking humid‐heat event occurred over northern China during 24 June–13 July 2025, characterized by persistent extreme wet‐bulb temperatures. The event is associated with a strong and persistent positive geopotential height anomaly, which favors the accumulation of heat and moisture. Further analyses reveal that warm sea surface temperature (SST) anomalies over the Kuroshio–Oyashio Extension (KOE) enhance upward turbulent heat fluxes, reducing lower‐tropospheric meridional temperature gradients and baroclinicity, and suppressing transient eddy activity. The associated reduction in transient eddy vorticity forcing favors positive geopotential height anomalies over the KOE, providing a dynamically consistent pathway linking SST anomalies to the large‐scale circulation. Sensitivity experiments with the Community Atmosphere Model version 5 (CAM5) reproduce the key circulation response. These results suggest that KOE SST anomalies likely contributed to the development of this extreme humid‐heat event by shaping a favorable anomalous circulation background.

Global warming has weakened the relationship between temperature and humidity, making dry‐ and humid‐heat increasingly distinct in China. Here, based on ERA5 hourly reanalysis data, the extreme dry‐ and humid‐heat events (EDHE/EHHE) at sub‐daily scale are first identified, and their spatiotemporal patterns as well as the associated underlying mechanisms across different climate regimes in China during 1961–2024 are investigated. Results indicate that both EDHE and EHHE exhibit an increasing trend, but with a greater magnitude for EHHE. Spatially, EDHE predominantly occurs in northern China, whereas EHHE mainly dominates southern China. These regional contrasts are closely linked to distinct circulation regimes and land‐atmosphere interactions. High‐frequency years of EDHE in both northern and southern China are accompanied by pronounced subsidence, which is maintained and reinforced by local land–atmosphere coupling (LAC) in both regions but considerably more strongly in southern China. During years with frequent EHHE, contrasting large‐scale environments are observed between the two regions: large‐scale circulation favors convective activities in northern China but tends to suppress convection in southern China. LAC may also contribute to EHHE in northern China, while its role in southern China remains uncertain and warrants further investigation. Overall, this study highlights pronounced regional differences in the mechanisms governing EDHE and EHHE, providing new insights into the processes driving compound heat extremes and offering guidance for improving their prediction under a warming climate.

This study delves into the interannual variability of August precipitation in northern China, revealing that the leading mode exhibits a zonal out‐of‐phase pattern, with a dividing line around 100°E. The western region (WNC) serves as a positive precipitation anomaly center, contrasting with the eastern region (ENC) as a negative anomaly center. This precipitation disparity is largely due to contrasting dynamic and moisture conditions between these regions, influenced by a quasi‐barotropic anomalous cyclonic circulation over Central Asia and anomalous anticyclonic circulation over Mongolia. Further analysis reveals that land and oceanic processes are pivotal in modulating this precipitation pattern. Specifically, soil moisture anomalies over Eurasia and sea surface temperature (SST) anomalies in the central and eastern Pacific are critical. In August, Eurasian soil moisture anomalies influence the local thermal processes, leading to eastward‐propagating Rossby wave circulations. Simultaneously, SST anomalies in the central and eastern Pacific initiate an anomalous meridional Pacific‐Japan (PJ) wave train. These circulation anomalies induce opposing conditions that promote precipitation in WNC and suppress it in ENC, thus establishing the observed out‐of‐phase precipitation pattern. In our study, Eurasian soil moisture and Pacific SST anomalies contribute approximately 44.9% and 21.2%, respectively, to the linear variance of precipitation pattern, collectively accounting for around 54.8% of the observed variation. Further investigation is required to gain a deeper understanding of this phenomenon.

The influence of anthropogenic (ANT) activity and the other external factors on extreme temperature changes over the mid–high latitudes of Asia are analysed using the different forcing simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The optimal fingerprinting technique and the probability ratio (PR) are employed to detect and quantify the influences of the external forcings on extreme temperature changes, which include annual maximum daily maximum temperature (TXx), annual minimum daily minimum temperature (TNn). Results indicate that TXx and TNn have increased from 1979 to 2014, and the simulations from historical (anthropogenic plus natural; ALL), greenhouse gas (GHG), and anthropogenic (ANT) experiments reasonably reproduce the spatiotemporal characteristics of extreme temperatures. Based on the optimal fingerprinting method, the impact of ANT forcing, in which GHG forcing is critical, can be detected in the changes of warm extremes and cold extremes. ANT and NAT forcings are separately detectable for warm extremes. GHG forcing can be separated from other ANT forcings for cold extremes but not warm extremes. Furthermore, the analysis applying the PR method shows that the probability of observed warm extremes that occur once in 20 years over the mid–high latitudes of Asia has risen by approximately three times owing to the anthropogenic influence, whereas the cold extremes became once in 50 years. Briefly, the increased anthropogenic activity has exacerbated the warm extremes and soothed the cold extremes over the mid–high latitudes of Asia during the past decades.

The social losses associated with extreme humid‐heat events (EHHE) increased sharply in recent decades. This study has explored the changing relationship between EHHE in eastern China and tropical sea surface temperature (SST) anomalies. Results show that the monopolar mode of EHHE is significantly associated with spring SST anomalies in Northwest Pacific (NWP) before the mid‐1990s, which can impact humid‐heat conditions in eastern China through an atmospheric meridional overturning circulation extending from NWP to eastern China. Then, the associated eastward shift of convection center after the mid‐1990s weakens connection between NWP SST anomalies and the monopolar mode. Further analysis reveals that the weakened Kelvin waves, induced by persistent SST anomalies from spring in tropical Indian Ocean after the mid‐1990s, cause the westward‐shifted anomalous convection in western Pacific, and then reinforce formation of the dipolar mode via exciting zonal wave trains. Numerical simulations via linear baroclinic model further validate atmospheric response to displacement of diabatic forcings.

The spring frost over northeast China significantly affects local socioeconomic development; however, the variations and underlying mechanisms remain unclear. The present study explored the spatiotemporal attributes of spring frost days (FD) over northeast China in recent decades, and the potential mechanisms underlying their interannual variations. The results indicated an evident decline in spring FD over northeast China, accompanied by substantial interannual variation. Further analyses suggested that anomalous springtime North Atlantic tripole sea surface temperature (SST) and Eurasian snow depth are critical factors controlling the interannual variations of spring FD in northeast China. Rossby wave trains can be stimulated by the anomalous North Atlantic tripole SST (NAT), which propagates eastward through the Eurasian continent to northeast China, significantly affecting the atmospheric circulation along the pathways. This results in anomalous negative geopotential heights and cyclonic circulation over northeast China, causing low‐temperature anomalies by moderating wind‐induced alterations in surface heat fluxes and cloud‐induced modifications in surface radiation. Meanwhile, Eurasian snow depth anomalies represent a consistent effect with NAT. The anomalous Eurasian snow depth may modify the atmosphere thickness by regulating surface heat flux, promoting the propagation of wave trains as a snow‐atmosphere bridge, which enhances the impact of NAT on FD. In addition, it also triggers the Rossby wave trains to spread from high‐latitude Siberia to northeast China. Both of these processes have a considerable impact on the local atmospheric circulation. Consequently, these two factors contribute to local surface cooling, thereby increasing the spring frost occurrence over northeast China.

The summer precipitation over northern China generally presents an obvious dipole pattern, but the relationship between western northern China (WNC) and eastern northern China (ENC) has experienced a significant interdecadal change, with a significant positive correlation for the periods of 1920s–1940s (P1) and 1980s–2000s (P3), while a negative correlation for 1950s–1970s (P2). Further analyses reveal that the atmospheric circulation and water vapour transport anomalies triggered by the negative phase of Pacific Decadal Oscillation (PDO), coupling with an eastward‐propagating Silk Road Pattern‐like teleconnection stimulated by the Atlantic Multidecadal Oscillation (AMO), can jointly produce the favourable conditions for summer precipitation in ENC but unfavourable for WNC during P2, comparing with P1 and P3. This results in a dipole pattern of summer precipitation over northern China, leading to a negative correlation between precipitation in these two regions. During P1 and P3, precipitation anomalies in northern China mainly exhibit strong regional consistency, with primarily positive anomalies during P1 and negative anomalies during P3. However, the instability of the precipitation relationship during P3 is greater than those of P1 and P2, which may be attributed to the enhanced impact of the interdecadal variation of ENSO at the same period.

Based on Coupled Model Intercomparison Project phase 6 (CMIP6) simulations, we found that the frequency and intensity of daytime–nighttime compound hot extremes (HEs) in the mid‐high latitudes of Asia (MHA) are expected to increase. The most significant increase is anticipated under the shared socioeconomic pathway (SSP) 5‐8.5, while the smallest increase is expected under SSP1‐2.6. Notably, unlike the decreasing trends of independent HEs since 2050 under the high emission scenarios, the compound HEs, which comprise the largest proportion, are expected to continuously increase and intensify. To better understand the impact of these changes on human society, we also focused on changes in population exposed to HEs. The findings reveal that population exposure to compound and nighttime HEs is projected to increase most rapidly under SSP3‐7.0, with estimates indicating increases of 10.06 and 3.80 times, respectively, by the end of the century. The most significant increases are expected in the mid‐latitudes, where changes in HEs are most pronounced. Climate change is the primary driver behind the rising population exposure to compound and nighttime HEs, with its impact expected to grow over time. Conversely, exposure to daytime HEs is primarily influenced by population changes, particularly in urban areas. Therefore, effective climate change mitigation and adaptive strategies are crucial to reducing future population exposure to HEs in MHA.