Ting Ding

As a major winter wheat‐producing area, the Huang‐Huai Plain in China relies critically on May–June precipitation during the harvest period for food security. This study identifies preceding March–April soil moisture in the north Black Sea and Caspian Sea (NBC) region as a key precursor for precipitation anomalies in the Huang‐Huai wheat region in May–June, and reveals a significant positive correlation between them. The mechanism responsible for this lag relation involves the persistence of soil moisture, Rossby wave propagation and water vapour transport. Results suggest that the March—April NBC soil moisture anomaly is able to persist until May—June. Through this soil memory, the March–April wetter NBC soil anomaly causes the May–June cold air anomaly locally via the evaporative cooling effect, triggering upper‐level circulation anomalies and Rossby wave train propagating southeastward. The wave train drives anomalous southerly winds, enhancing water vapour transport and leading to the water vapour convergence over the Huang‐Huai region, and finally favours the positive precipitation anomaly over the Huang‐Huai region. The drier NBC soil anomaly corresponds to the opposite circulation and precipitation anomaly. For the detailed Rossby wave dynamics, the numerical experiments confirm that feedback between atmospheric circulation and temperature over the west end of China plays a critical role in guiding the Rossby wave pathway initiated by the NBC soil moisture forcing. This study provides new insights into precipitation prediction during the Huang‐Huai wheat harvest period, emphasising the remote influence of mid‐high‐latitude soil moisture on the climate prediction in the winter wheat‐producing area in China.

The air pollution transmission channel in North China, also termed the ‘2 + 26’ cities, has the most severe pollution problem in China. Based on the meteorological and environmental observation data from 2014 to 2022, the spatial–temporal features of the air quality index (AQI) in the ‘2 + 26’ cities are first compared to reveal the different changes in winter and summer. Since the issue of the Air Pollution Prevention and Control Action Plan in China in 2013, the winter AQI averaged at the ‘2 + 26’ cities shows a significant decreasing trend of −9.725 μg m ⁻³ per year from 2014 to 2022, but it shows much slight variation in summer. In summer, the ozone (O ₃ ) pollutant is the major contributor to the AQI. Results indicate that higher T _max and lower relative humidity (RH) are conducive to higher O ₃ concentration and more severe air pollution. The averaged T _max at these cities has a significant increasing trend of 0.28°C per decade, while RH decreases also significantly at a rate of −1.08% per decade. This may be the main reason why the summer O ₃ concentration maintains stability at a high level in the recent 9 years. Results also indicate that the ‘heat dome’ effect, characterised by a persistent high pressure and warming air subsidence from the upper to lower troposphere, is reinforcing in recent years, and this is favourable for the occurrence of long‐lasting dry‐type high temperatures and the enhancement of O ₃ pollution. Projections from 19 CMIP6 models show that the high pressure system will remarkably strengthen in the near, middle, and long terms under the moderate emission scenario (SSP2‐4.5). Therefore, the ‘heat dome’ effect will exacerbate the ozone pollution in the future.

There has been a notable rise in the number of wet heatwave days across most of Asia, particularly in countries with a sizeable 65+ elderly population (e.g. China and India) and in countries with a high proportion of elderly individuals (such as Japan) during 2001–2020. The frequency of dry heatwave in Asia has been observed to increase by 0.9 d per decade, whereas the wet HTs have demonstrated a considerably more pronounced trend of 1.9 d per decade. Furthermore, the growth percentage in excess of 50% is evident in the majority of regions across Asia, including West Asia, Southeast Asia, South Asia and East Asia, which collectively account for 99% of the continent’s elderly population. Relative to the period 2001–2020, the exposure to wet heatwave is projected to increase from 1.4 times to 3.2 times in EA, from 1.6 to 13 times in WA, from 1.3 to 9 times in CA, from 1.3 to 8.4 times in SA and from 2.2 to 15.7 times in SEA from 2030 to 2100. This represents a rate of increase which is more than twice that of the dry heatwave in 2100 across most regions of Asia. The rise in temperature and the growth in the population aged 65+ are responsible for the increase in exposure to wet heatwaves, and the population effect has more contribution than the climate effect across all regions.

In September 2024, the Yangtze River basin experienced a supremely extreme heatwave that broke historical records from at least 1961 and could have a severe impact on outdoor health of school children. This paper provides a timely analysis of the characteristics of the extreme heatwave in the Yangtze River basin in September 2024, its exposure to the population aged 14 years and below, and the causes that led to its occurrence, as well as its future projections. In September 2024, the regional average heatwave days in the Yangtze River basin reached 7.57 days, and the average daily maximum temperature (T_max) reached 31.53°C, both of which are much higher than the climatology and exceed the historical records. This supremely heatwave resulted in high exposure of the population aged 14 years and under, with the provinces of Sichuan, Chongqing, Hunan, and Jiangxi exposed to more than 100 million person‐days. The extreme expansion of the South Asian High (SAH) and the Western Pacific Subtropical High (WPSH) may have directly contributed to this supremely heatwave. The CMIP6 projections show that the frequency of extreme heatwaves in September similar to that in 2024 will increase in the future.

In the summer of 2024, the North China Plain was subjected to a compound drought‐flood extreme event, characterised by a prolonged and intense drought from April to June, followed by a severe flood in July. The extreme drought days in the plain are the fourth highest in June, while the precipitation is the highest in the recent half century. Based on daily observations of precipitation and combined meteorological drought data, we initially proposed a suitable approach to define the drought‐flood abrupt alternation (DFAA) events in the plain and conducted an intra‐seasonal evolution analysis. The alternation point is reached in early July. Subsequently, the severe flood rapidly replaced the drought due to the occurrence of record‐breaking intense rainfall on six consecutive days from 4 to 9 July. The DFAA in 2024 is closely associated with the monsoon rain belt and circulations, particularly the western Pacific subtropical high (WPSH). In June, the WPSH was situated considerably further south than usual, thus maintaining the rain belt in southern China. In early July, the WPSH exhibits a sudden northward shift of approximately five latitudes, which enables the transportation of water vapour to the North China Plain from the south boundary of the plain. Concurrently, the transverse trough in the middle latitudes causes a strong low vortex and precludes the transportation of moisture to the regions north of the plain, thereby engendering persistent lower‐level moisture convergence in the plain and prompting the formation of persistent intense rainfall and severe flooding.

The effect of El Niño on summer heat extremes in China has been a topic of previous studies, but the effects of different types of El Niño in developing summer on heat extremes in China have not been conclusive. We examined the effects of different types of El Niño in developing summer on heat extremes in China and found that there are spatial differences under different El Niño types. The developing eastern Pacific (EP) El Niño favours summer heat extremes from North China to central China and decreases them in southeastern China. The developing mixed‐type Pacific (MP) El Niño decreases summer heat extremes in east‐central China between 29°N and 35°N. The developing central Pacific (CP) El Niño increases summer heat extremes in east‐central China between 29°N and 37°N and decreases them in southern China. The physical mechanisms underlying the above spatially distinct effects are clarified through circulation and localised key climatic factors. Thus, the predictability of summer heat extremes in China can be improved based on judging the different El Niños in developing conditions.

Different from the general heatwaves that mainly occur in southeastern China, the super heatwaves in China are concentrated mostly in the mid–lower reaches of the Yangtze River basin. Daily maximum temperature data over land from CPC/NOAA revealed that the basin has become the centre with the highest frequency increase of super heatwaves in the East Asian monsoon regions in the 21st century. Further analyses also indicted that the extent of super heatwaves in the basin has a much higher increasing rate than that of general heatwaves by using running thresholds. The westward extension of the western Pacific subtropical high (WPSH) plays the most dominant role. Statistical results suggested that a 10‐gpm increment of the geopotential height (GPH) over the basin leads to a 0.43°C increase in the regionally averaged maximum temperature. Additionally, spatial extent of the super heatwave may expand by approximately 4%. In contrast to the stable eastern boundary of the North Africa high, the western boundary of the WPSH has significantly expanded westward in the 21st century. This expansion has led to the enhancement of the GPH over the Yangtze River basin, resulting in the super heatwave centre due to the heat‐dome effect. Projections from 29 Coupled Model Intercomparison Project Phase 6 (CMIP6) models under moderate greenhouse gas emission scenario (SSP2‐4.5) suggest that the GPH over the Yangtze River basin will continue to strengthen throughout the 21st century. This implies that the Yangtze River basin will continue to be the centre of super heatwaves in East Asia monsoon region.

Based on the observed maximum temperature (T_max), relative humidity (RH) and NCEP/NCAR reanalysis data during 1981–2021, basic temporal features and dominant atmospheric circulation patterns of dry‐type and humid‐type heatwaves in North China are investigated and compared. Statistical results indicate the dry heatwaves occur mainly in early summer (from early June to early July), that is, before the rainy season of North China, while the humid heatwaves have a high frequency in mid‐July to mid‐August. During the research period, the increasing trend of dry heatwaves is 0.67 days·decade⁻¹, while the humid heatwaves increase at a greatly higher rate of 1.85 days·decade⁻¹. For the dry heatwave, a high ridge in the subtropical westerlies plays the main role, and the northerly wind in the east of the ridge reduces the air moisture convergence over the region. However, for the humid heatwave, the westward and northward propagations of the western Pacific subtropical high (WPSH) may make the major contribution, and the southerly wind anomalies in the west of the WPSH enlarge the water vapour to the region. The adiabatic heating in subsiding air at all levels and horizontal temperature advection at lower troposphere are stronger for dry heatwaves than for humid heatwaves, which cause a higher T_max for the former type. These results highlight the diversity of the heatwaves in North China, which suggests that multiple local and large‐scale subseasonal circulations should be considered to improve the subseasonal to seasonal forecast skills for heat extremes.

Observations have revealed that the deadly “hot eastern‐pluvial western” climate pattern in Asia in summer 2022 is by no means an exception. In recent decade this pattern is becoming much more frequent owing to the notable westward extension and enhancement of the western Pacific subtropical high (WPSH). The anomalously enhanced WPSH fills the gap in geopotential heights between the Arabian Sea‐Iranian Plateau and the western Pacific, produces a heat dome over eastern Asia and intensifies the moisture convergence over western Asia by changing the water vapor transportation path. Projections of 19 CMIP6 models show that both the temperature in eastern Asia and precipitation in western Asia will remarkably increase under Shared Socioeconomic Pathway (SSP) 2‐4.5, SSP 3‐7.0, and SSP 5‐8.5 scenarios, with temperature anomalies 0.8°C, 1.5°C, and 2.4°C higher and the percentages of precipitation anomaly 6.3%, 9.4%, and 12.5% larger than the reference period in the near, middle, and long terms under the moderate emission scenario (SSP2‐4.5). Therefore, the “hot eastern‐pluvial western” Asia is becoming a new normal and exacerbating the risk of compound disasters.