The period surrounding the Dragon Boat Festival each year coincides with the most concentrated precipitation in the “pre-flood season in South China” and is thus termed “dragon boat water” (DBW, from 21 May to 20 June). While the effects of atmospheric circulation and sea surface temperature (SST) have been widely explored, the mechanism of land surface forcing on the DBW in South China (SC) remains poorly understood. Therefore, based on observation and reanalysis data, this paper studies the effect of spring soil moisture (SM) over the Indian Peninsula (IP) on the DBW and the associated physical mechanism during 1992–2021. The results show that the leading mode of empirical orthogonal decomposition of precipitation during the DBW period in SC exhibits spatial coherence. Spring SM over the IP exhibits a significant positive correlation with the DBW. Spring wet soil over the IP can persist into the DBW period. This increases land surface evapotranspiration and latent heat flux, enhancing local convective activity in the middle and lower troposphere and leading to increased precipitation over the IP. The precipitation in the IP intensifies the South Asian high and extends it eastward by releasing condensational latent heat to heat the atmosphere. The thus stimulated circumglobal teleconnection like wave train causes the Northern Hemisphere to evolve into a zonal five wave structure in the upper troposphere, and SC is under the influence of the atmospheric wave activity center over East Asia. Concurrently, barotropic energy conversion over East Asia intensifies significantly, favoring the development of the quasi-barotropic structure; thereby, the western Pacific subtropical high intensifies and extends westward. Under the influence of the abnormal anticyclone over the Northwest Pacific, SC is controlled by the stronger southwesterly wind facilitating the transport of warm moisture from the South China Sea and the Bay of Bengal to SC. Significant ascending motion and stronger meridional wind are observed throughout the troposphere, conducive to more rainfall over SC during the DBW period. These results advance the understanding of climatic effects of spring SM anomalies over the IP, and provide guidance for improving the predictability of the DBW.

Based on daily precipitation observations and NCEP reanalysis data from 1961 to 2020, this study examines the interannual variability of winter extreme precipitation frequency (EPF) over Southeast China (SEC) and its possible association with sea surface temperature (SST) anomalies. The results show that winter EPF over SEC is closely linked to a north–south dipole pattern in circulation anomalies over East Asia, as well as the anomalous Philippine Sea anticyclone. The dipole pattern is characterized by a cyclonic anomaly over southern East Asia and an anticyclonic anomaly over northeastern Asia. This pattern enhances upper-level divergence and lower-level convergence over SEC, which enhances ascending motion and favors moisture convergence. The anomalous Philippine Sea anticyclone which is a classic response to El Niño, facilitates the transport of warm and humid air into SEC. Further analysis indicates that the dipole pattern is not a localized phenomenon but is instead an integral component of two large-scale Rossby wave trains: the Indo–western Pacific and East Asia (IWP–EA) pattern and the circumglobal teleconnection (CGT) pattern. The IWP–EA pattern originates over the Indo–western Pacific and is related to El Niño–Southern Oscillation (ENSO) events. The CGT pattern is influenced by both ENSO and a North Atlantic east–west SST dipole pattern (NAEW). Therefore, both El Niño and the positive phase of NAEW collaboratively excite the CGT and IWP–EA wave trains, exerting a synergistic effect on the winter EPF over SEC.

China’s Greater Bay Area (GBA) is one of the fastest urbanizing regions in the world, featured by its complex land surface and unique geography. In this study, heatwaves (HWs) in the GBA during the summers (June, July, and August) of 1961–2020 are analyzed by using observational and reanalysis datasets. The results indicate that 70% of daytime HWs occur in the northern forested areas of the GBA, 65% of nighttime HWs are observed in the cropland and forest areas around the GBA, and 75% of compound HWs occur in the urban and southern coastal areas of the GBA. Daytime HWs are featured by lower near-surface specific humidity and drier soil moisture, while nighttime HWs are often accompanied by relatively wetter conditions. For compound HWs, they are jointly affected by the conditions of the above two types. During daytime and compound HWs, soil moisture dries and recovers quickly, exacerbating the high temperatures of HWs through strong soil moisture–air temperature coupling that far exceeds the climatology. Nighttime HWs lack this coupling and are primarily driven by atmospheric factors, with high temperatures maintained mainly by increased water vapor.

Southeast China, a densely populated and economically developed region, has experienced an increase in extreme precipitation in recent years. However, the current understanding of the influencing factors and related mechanisms of extreme precipitation remains incomplete. This study investigates the possible impact of spring Eurasian snowmelt on July extreme precipitation in Southeast China, using observational and reanalysis datasets. Singular Value Decomposition (SVD) analysis was used to explore the relationship between spring snowmelt and July extreme precipitation. The dominant SVD mode reveals that significantly increased snowmelt over the high latitudes of Eurasia and decreased snowmelt over the western and eastern sides of the midlatitudes of Eurasia tend to be accompanied by a meridional dipole pattern of extreme precipitation anomalies over Southeast China, with a positive center over the Yangtze River basin (YRB) and a negative center over South China (SC), and vice versa. Further analysis indicates that the soil moisture anomaly induced by the spring snowmelt anomaly can persist until July, modulating the land surface energy budget and atmospheric circulation conditions. When a snowmelt anomaly occurs, a distinct wave train type anomalous circulation develops over Eurasia, propagating southeastward from mid–high latitudes to South China, resulting in an anomalous cyclonic circulation around the Sea of Japan and North China, and an intensified western North Pacific subtropical high (WNPSH). The anomalous sinking motion related to the strengthened WNPSH inhibits water vapor convergence and results in reduced extreme precipitation over SC. In contrast, the anomalous southwesterly winds on the western flank of the WNPSH transport warm and moist air northward and converge with the anomalous northerly flow over the YRB, contributing to intense moisture convergence, which increases precipitation potential and the likelihood of extreme rainfall. Our findings provide valuable insights for improving the understanding and prediction of July extreme precipitation in Southeast China.

Previous studies have reported a close relationship between the negative Arctic Oscillation (AO) and the PM_2.5 (particulate matter with a diameter of 2.5 μm or less) diffusion in North China in winter. Using the North China regional mean meridional wind at 850 hPa derived from the ERA5 (ECMWF Reanalysis version 5) reanalysis data in 1979–2022 as a useful substitute for station observed PM_2.5 concentration (since the latter is available only since 2014), our study detected strong/weak northerly events representing the abnormal PM_2.5 diffusion/accumulation events, and revisited the AO–PM_2.5 diffusion relationship in North China during 1979–2022. The results show that only when the AO was characterized by a 2-month continuously negative/positive phases and with twin peaks respectively before and after the diffusion/accumulation events, would there be higher occurrences of the abnormal PM_2.5 diffusion/accumulation. The second peak of negative AO acted to prolong the strong northerly winds by an average of 2 days. Further analysis reveals that the AO with twin peaks always has a footprint in the stratospheric northern annular mode (NAM) during the abnormal PM_2.5 events, and the coupling between the stratosphere and troposphere plays a critical role in the second peak of AO. Vertical propagation of baroclinically amplifying waves leads to changes in isentropic meridional mass fluxes in the stratosphere following the changes in the troposphere. The stronger/weaker poleward mass fluxes increase/decrease the polar mass in the stratosphere, which dominates the total column air mass changes and leads to the second peak of AO. Considering the subseasonal predictability of the stratospheric NAM based on existing evidence, particular attention should be paid to these AO-related abnormal PM_2.5 diffusion and accumulation events in North China because they might be more predictable at a longer lead time.

Recent studies have suggested a close relationship between early summer precipitation over Northeast China and spring land surface thermal anomalies in West Asia. However, is this relationship the same over the multidecadal timescale? This study aims to identify the long-term variation in this relationship and the accompanying atmospheric circulation anomalies by using singular value decomposition, correlation analysis, and linear regression based on the ECMWF Reanalysis v5 (ERA5) atmospheric data, ERA-Land reanalysis, and CN05 gridded observations during 1961–2020 (60 yr). It is found that an interdecadal transition of the relationship between the spring surface temperature/thermal anomalies in West Asia and early summer precipitation over Northeast China occurred around 1990, and the temperature–rainfall relationship intensified after 1990. Based on the Mann–Kendall test, the study period was divided into P1 (1961–1990) and P2 (1991–2020). Further analysis indicated significant differences in the corresponding atmospheric circulation before and after the interdecadal transition. During P2, spring land surface warming in West Asia corresponded to a significantly enhanced early summer Circumglobal Teleconnection (CGT), which in turn suppressed the Northeast China cold vortex (NECV). The changes in circulation patterns further resulted in weakened moisture transport, strengthened subsidence, reduced precipitation triggering, and eventually, weakened precipitation. Additionally, the results suggest that the interdecadal transition of the relationship and the changes in the corresponding atmospheric circulation may be related to activities of the westerly jet stream. The second princi-pal component (PC2) mode of empirical orthogonal function (EOF) of zonal wind in June over Asia demonstrated a pattern similar to that of the atmospheric circulation corresponding to land surface thermal anomalies. In addition, during P2, the PC2 mode of the westerly jet stream in June showed a strong positive correlation with the NECV, thereby suppressing precipitation over Northeast China. Therefore, it is concluded that the westerly jet stream may have affected the interdecadal transition of the temperature–rainfall relationship around 1990.

Previous research has shown that land surface thermal anomalies in West Asia (WA) can impact regional and global climate, particularly affecting China through the eastward propagation of wave trains. However, the factors driving these anomalies in WA have not been extensively studied. Based on the observation data, this work focuses on examining the impacts of Eurasian winter snow cover on winter surface air temperature (SAT) variability over WA from 1978/1979 to 2017/2018 and explores the underlying physical mechanisms. The results indicate that a crucial snow anomaly area extending from the Baltic Sea to eastern Ural significantly influences the winter SAT anomaly in WA. An anomalous increase (decrease) in winter snow cover in this key area corresponds to the anomalously warmer (cooler) SAT in WA. This relationship is primarily driven by the albedo effects of snow cover, where more (less) snow cover induces cooling (warming) of the overlying air, altering upper-level geopotential height and influencing the intensity, duration, and frequency of local blocking events. Additionally, changes in the air temperature above the key area modify the meridional temperature gradient (MTG) between high and low latitudes, affecting the mean zonal flow in the midlatitude. Diagnosis of the thermodynamic energy equation for SAT reveals that the combined effects of variations in blocking events in high latitudes and mean zonal flow in midlatitudes alter the advection of climatological temperature by anomalous winds, which is caused by the anomalous increase (decrease) of snow cover in the key area. Consequently, this leads to changes in cold advection transported to WA, contributing to the occurrence of a warmer (colder) SAT over WA in winter.

Using daily maximum temperature (T_max) data from 516 observation stations in eastern China from 1981 to 2020, this study employed a relative threshold method to define short- and long-lived heat waves (HWs) by considering regional climate differences to investigate the spatial characteristics and evolution of large-scale circulation during summer HWs. The results demonstrated spatial disparities in the frequency distribution of HWs of different durations and differences in the magnitude of duration and intensity between short- and long-lived HWs. Empirical orthogonal function analysis revealed three dominant spatial modes for both short- and long-lived HWs. The first mode showed that short-lived HWs occur prominently in both northern and southern regions, whereas long-lived HWs mainly occur in the northern region. The second mode was characterized by a meridional dipole pattern in both cases. The third mode exhibited a quadrupole pattern for short-lived HWs and a tripole pattern for long-lived HWs. Differences in the center locations of anomalies in the 500-hPa geopotential height and 850-hPa wind fields significantly influenced the temperature and precipitation anomaly distribution of typical HWs by affecting the warm column in the lower troposphere, cloud distribution, and moisture transport. Moreover, the atmospheric circulation evolution processes of typical HWs associated with the different modes of long- and short-lived HWs were linked to distinct teleconnection patterns. During the three modes of long-lived (short-lived) HWs, there was stronger (weaker) wave flux activity with multiple (single) propagation paths. Stronger westward Atlantic wave train activity at 300 hPa triggered the synergistic action of meridional and zonal wave fluxes, favoring the strengthening and maintenance of positive anomalies in geopotential height of 500 hPa. This may have contributed to the formation of long-lived HWs. These findings provide valuable insights to enhance our understanding and prediction of summer HWs.

The combined effects of global warming and the urban heat islands exacerbate the risk of urban heat stress. It is crucial to implement effective cooling measures in urban areas to improve the comfort of the thermal environment. In this study, the Weather Research and Forecasting Model (WRF), coupled with a single-layer Urban Canopy Model (UCM), was used to study the impact of heat mitigation strategies. In addition, a 5-km resolution land-cover dataset for China (ChinaLC), which is based on satellite remote sensing data, was adjusted and used, and 18 groups of numerical experiments were designed, to increase the albedo and vegetation fraction of roof/ground parameters. The experiments were conducted for four heatwave events that occurred in the summer of 2013 in the Yangtze River Delta urban agglomeration of China. The simulated results demonstrated that, for the single roof/ground schemes, the mitigation effects were directly proportional to the albedo and greening. Among all the experimental schemes, the superposed schemes presented better cooling effects. For the ground greening scheme, with similar net radiation flux and latent heat flux, its storage heat was lower than that of the roof greening scheme, resulting in more energy flux into the atmosphere, and its daytime cooling effect was not as good as that of the roof greening scheme. In terms of human thermal comfort (HTC), the improvement achieved by the ground greening scheme was better than any other single roof/ground schemes, because the increase in the relative humidity was small. The comprehensive evaluation of the mitigation effects of different schemes on the thermal environment presented in this paper provides a theoretical basis for improving the urban environment through rational urban planning and construction.