Increasing Risk of a “Hot Eastern-Pluvial Western” Asia

Abstract

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.

Introduction

As the most densely populated region with great economic inequality in the world, many countries in Asia suffered huge economic losses and severe human mortality and morbidity from extreme heatwaves and floods almost every year. Since the 1950s, the heatwaves in Asia have become more frequent and more intense across most land regions, and the population exposure to the prolonged periods of excessive heat extremes will continue to increase with additional warming (IPCC, 2021; Perkins & Lewis, 2020; Rode et al., 2021; Röthlisberger & Papritz, 2023; J. Wang et al., 2020). Meanwhile, the frequency and intensity of heavy precipitation and related floods have also increased in Asia (IPCC, 2021; Wasko, 2022; W. X. Zhang et al., 2018). The trends of heatwaves and floods during recent decades both display substantial spatial heterogeneity over the globe, and the vulnerability to these changes varies by geography and by the economic circumstances of the exposed population, especially in Asia (Johnson et al., 2018; Sovacool et al., 2017; Q. H. Sun et al., 2021).

Asia has a diverse climate covering all the climatic zones from tropics to polar regions. In general, the maximum summer temperatures in West Asia and the west part of South Asia are higher than those in other Asian regions, thereby causing more frequent heatwaves (figures omitted). While South Asia, Southeast Asia, and the southern part of East Asia are the wettest regions in the continent owing to the summer monsoon activity, where the floods are also more frequent and more intense. However, in the recent decade, eastern Asia suffered from persistent heatwaves much more frequently than any other decades, such as the 2013 extreme event, the 2017 record-breaking event, the 2018 hottest summer and the 2022 unprecedented event in history (Ding et al., 2019; J. L. Jiang et al., 2023; Noh et al., 2021; X. M. Sun et al., 2014; Z. Q. Wang et al., 2023; D. Q. Zhang et al., 2023; J. M. Zhang et al., 2021). More frequent and enhanced heatwaves in eastern Asia are projected by climate models under different scenarios (Li et al., 2019; Shin et al., 2018; Zhu et al., 2020). Almost in the same period, heavy precipitation extremes and related floods are frequently observed over the western part of Asia, especially in the relatively drier regions of the continent, such as West Asia (Darand & Pazhoh, 2022; Jamali et al., 2022; Rastmanesh et al., 2020; Sherpa & Shirzaei, 2022), the Arabian Peninsula (Atif et al., 2020), and northeastern and central India (Kim et al., 2019; Mishra et al., 2019).

Both extreme heatwaves and floods in Asia are directly caused by the anomalous summer monsoon circulations. For heatwaves in eastern Asia, the most dominant circulation factor is the western Pacific subtropical high (WPSH) in the mid-low troposphere (X. L. Chen et al., 2020; Liu et al., 2019; Yeo et al., 2019), or the Pacific-Japan pattern (Noh et al., 2021), and a circumglobal teleconnection pattern or a silk road pattern (Gao et al., 2018; Z. Q. Wang et al., 2023; T. T. Zhang et al., 2022). The upper-level South Asian high (SAH) can also affect the heatwaves by modulating the position of the WPSH (Wei et al., 2019). For instance, the prolonged heat event was directly related to the intensification and westward extension of the WPSH and the eastward extension of the SAH in the upper troposphere, which can be contributed to La Nina and the Silk Road pattern (J. L. Jiang et al., 2023; D. Q. Zhang et al., 2023). For extreme precipitation events in western Asia (including South Asia and West Asia in this study), the Arabian subtropical anticyclone, known as the Arabian high (AH), and the low-level jet from central Arabian Sea to northwestern Pacific are considered to be the main contributors (Fahad et al., 2022; Karimi et al., 2022; Raziei et al., 2012). The AH plays an essential role in regulating the weather and climate in western Asia, especially in South Asia, the Arabian Peninsula and adjacent countries (Alghamdi & Harrington, 2023; de Vries et al., 2013; Esfandiari & Lashkari, 2020; Karimi et al., 2022; Raziei et al., 2012).

The exceptional summer of 2022 further aggravated the risk of the compound heatwave-flood disaster in Asia (He et al., 2023; J. Zhang et al., 2024). In eastern Asia, China experienced the most extensive and long-lasting heatwave since 1951, and Japan saw its hottest heatwave in recent 150 years. The extreme heatwave impacted hundreds of millions of people in this densely populated region, caused drought and shortage of electricity, and threatened to damage the whole ecosystems. Meanwhile, exceptional floods hit the western Asia during the same summer. In Pakistan and Afghanistan, the extensive flooding caused at least 1,700 and 700 deaths, respectively (WMO, 2022). Floods in this summer also killed 91 and 28 people in Yemen and Iran, respectively. For this “hot eastern-pluvial western” pattern, is it just an exceptional case or will it be a “new normal” in Asia? It is essential to investigate the factors responsible for these two extremes and the linkages between them. Besides, how they will change in the future under a warming climate is still unclear. Based on the gridded observations of daily maximum temperatures and precipitation, we revealed the consistent increasing frequencies of heatwaves in eastern Asia and floods in western Asia. The importance of the WPSH and the modulation effects of SAH and AH were also explored from the perspective of their linkages with large-scale atmospheric circulations. The heatwaves, precipitation and associated circulation systems were projected from 19 CMIP6 model simulations under the moderate emission scenario of SSP2-4.5.

Data and Methods Observation and Reanalysis Data

The daily gridded data sets of maximum temperature and precipitation over global land areas during 1979–2022 are acquired from the Global Surface Air Temperature and Global Daily Unified Gauge-Based Analysis of Precipitation data products by the Climate Prediction Center, which is provided by the Physical Sciences Laboratory of National Oceanic and Atmospheric Administration (M. Y. Chen et al., 2008). The data has a horizontal resolution of 0.5° × 0.5° and is updated in real-time. In recent years, it has been widely used in scientific studies and climate monitoring (Alizadeh et al., 2022; Tarek et al., 2020). The monthly atmospheric circulation data for the same period are taken from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data set, including the 500 and 200-hPa geopotential heights, and the 850-hPa vertical vorticity with a horizontal resolution of 2.5° × 2.5° (Kalnay et al., 1996; Kistler et al., 2001). The European Centre's Medium-Range Weather Forecasts Reanalysis 5 data set (ERA5, Hersbach et al., 2020) has also been used here to verify the results.

Coupled Model Intercomparison Project Data

The monthly maximum temperature, precipitation and geopotential heights for the historical record (1850–2014) and future projections (2015–2100) are obtained from models of the Coupled Model Intercomparison Project Phase 6 (CMIP6; varied resolution, see Table 1 for details) (Eyring et al., 2016). Different Shared Socioeconomic Pathways (SSPs) are considered including SSP 1-1.9, SSP 2-4.5, SSP 3-7.0, and SSP 5-8.5. Among them, SSP 2-4.5, a moderate scenario based on the SSP 2 (middle of the road) and representative concentration pathway 4.5 (intermediate scenario) (Fricko et al., 2017), is primary focused in the study. The ensemble mean is the average of the output results of these 19 CMIP6 models.

Table 1 Basic Information of the 19 CMIP6 Models Used in This Study

Model	Institution/country (region)	Resolution (lat × lon)
AWI-CM-1-1-MR	AWI/Germany	192 × 384
ACCESS-CM2	CSIRO/Australia	144 × 192
ACCESS-ESM1-5	CSIRO/Australia	145 × 192
BCC-CSM2-MR	BCC/China	160 × 320
EC-Earth3	EC-Earth-Consortium/Europe	256 × 512
EC-Earth3-CC	EC-Earth-Consortium/Europe	256 × 512
EC-Earth3-Veg	EC-Earth-Consortium/Europe	256 × 512
EC-Earth3-Veg-LR	EC-Earth-Consortium/Europe	160 × 320
GFDL-CM4	NOAA-GFDL/USA	180 × 288
GFDL-ESM4	NOAA-GFDL/USA	180 × 288
FIO-ESM-2-0	FIO-QLNM/China	192 × 288
INM-CM4-8	INM/Russia	120 × 180
INM-CM5-0	INM/Russia	120 × 180
IPSL-CM6A-LR	IPSL/France	143 × 144
MIROC6	MIROC/Japan	128 × 256
MPI-ESM1-2-HR	MPI-M/Germany	192 × 384
MPI-ESM1-2-LR	MPI-M/Germany	96 × 192
MRI-ESM2-0	MRI/Japan	160 × 320
KACE-1-0-G	NIMS-KMA/Republic of Korea	144 × 192

Unless otherwise specified, the climatological period is 1991–2020.

Normalization

The normalization process in the original manuscript is the temporal normalization. For a time series X = (x₁, x₂, …, x_n), it is defined as: [Image Omitted. See PDF]

Where i = 1, 2, …, n, n is the number of samples, $${x}_{i}^{\ast }$$ is the normalization of x_i, $$\overline{x}$$ is the climatological mean of X, σ is the standard deviation (SD) of X. For this study, both the climatological mean and the SD are calculated based on a reference period of 1991–2020 according to WMO (2017).

Results Increasing Occurrences of an Extreme “Hot Eastern-Pluvial Western” Pattern in Asia: Observations

As mentioned above, in summer 2022, severe heatwaves and floods struck Asia and led to huge economic losses and severe health threat, especially to the low-income countries in this continent (WMO, 2022). From the normalized series of maximum temperature and precipitation in Asia during July–August 2022 (Figure 1a), it could be clearly seen that the extreme features of heatwaves in eastern Asia and wetness in western Asia were both significant. In most areas of eastern Asia, the normalized maximum temperature anomalies exceeded 2 SDs. In the west part of China and northeast part of India, the anomalies even exceeded 3 SDs. Due to the extreme high temperatures, severe droughts occurred with precipitation anomalies below 2 SDs. For the whole eastern Asia, the regional-averaged maximum temperature in July–August of 2022 (28.7°C) ranked the first since 1981, which was 2°C higher than the climatology (Figure 1b). Meanwhile, anomalous precipitation occurred in western Asia. In Iran, eastern Afghanistan, Pakistan, Yemen, and south of Turkmenistan, the precipitation anomalies exceeded 3 SDs. The regional-averaged precipitation over western Asia in July–August 2022 was 274.7 mm, which was also the largest since 1981 (Figure 1b).

Figure 1. (a) Distributions of the normalized maximum temperature (+) and precipitation (▪) in Asia during July–August 2022, and only the values exceeding 2 standard deviations (SDs) or less than −2 SDs are shown. The blue and red dashed rectangles represent western Asia and eastern Asia, respectively. (b) Regional-averaged maximum temperature (red curve) in eastern Asia and precipitation (blue curve) in western Asia in July–August during 1981–2022. The horizontal lines are the decadal means. The correlation coefficients between the two curves in different periods are listed in the lower right corner of panel (b). All the coefficients have passed the t-test at the 99% confidence level.

The abovementioned climate pattern of wet western Asia and hot eastern Asia in 2022 is not the only case. Similar distributions have already occurred in the recent decade. For instance, in summer of 2017, the extreme heatwave in China broke the record during 1961–2016 (J. M. Zhang et al., 2021; Zhou et al., 2019). Korea and Japan also experienced a much hotter summer (Arndt et al., 2018). Besides, western Asia was hit by floods during the same period. As reported in the WMO statement (WMO, 2018), the most serious flooding occurred in mid-August in Nepal, Bangladesh and India, causing at least 1,200 deaths. The flash floods triggered by heavy rain in northern Iran had left at least 12 people dead and two missing in that August (https://floodlist.com/asia/iran-flash-floods-august-2017). In Figure 1b, a close linkage between the heatwaves in eastern Asia and the floods in western Asia can be clearly found. The linear correlation coefficient (CC) between the maximum temperature and precipitation during July–August from 1981 to 2022 was 0.73, which was statistically significant at the 99% confidence level. After linear detrending, the CC was still significant at the same confidence level, with the value being 0.57. Considering the effect of the extreme heatwave in summer 2022 on this correlation relationship, the year 2022 was removed and the CC during 1981–2021 was 0.66 (0.48) before (after) linear detrending, which was still significant at the 99% confidence level. This indicates that the heatwave in eastern Asia and the flood in western Asia have similar inter-annual variations. Besides, both time series have been demonstrated to have synchronous increasing trends since 2000. The maximum temperature averaged in eastern Asia exhibited a significant increasing trend at 0.46°C decade⁻¹ during 2001–2022, and the decadal means during 2001–2010 and 2011–2020 were 26.9°C and 27.2°C, respectively. Meanwhile, the precipitation averaged in western Asia during 2001–2022 also increased significantly at a rate of 0.5 mm day⁻¹ per decade. The decadal means of precipitation during 2001–2010 and 2011–2020 were 2.96 and 3.32 mm day⁻¹, respectively.

Besides the average climate condition, the analysis of the extreme conditions of heatwaves in eastern Asia and floods in western Asia is also conducted. Here we use TX90p and Rx5day to analyze the extreme climate variability of the heatwaves and floods. Both of them refer to Expert Team on Climate Change Detection and Indices (Karl et al., 1999), and have been widely used in IPCC (2021) and other studies for climate extremes. TX90p is defined as the total number of days with the daily maximum temperature exceeding the 90 percentile centered on a 5-day window for the base period 1991–2020. Rx5day is calculated as the maximum consecutive 5-day precipitation in a period. Figure S1 in Supporting Information S1 shows the regional-averaged TX90p in eastern Asia and Rx5day in western Asia in July–August during 1981–2022. For the whole eastern Asia, the regional-averaged TX90 ranked the first since 1981. Meanwhile, the regional-averaged Rx5day over western Asia in July–August 2022 was also the largest since 1981. The linear CC between the TX90p and Rx5day from 1981 to 2022 was 0.66, which was statistically significant at the 99% confidence level. After linear detrending, the CC was still significant at the same confidence level, with the value being 0.51. Besides, both time series displayed the increasing trends since 2000. The TX90p increases at a rate of 1.5 days per decade during 2001–2022, and the Rx5day increased at a rate of 4.2 mm per decade in the same period. Both have exceeded the 95% significance t-test level. The results based on extreme indices are similar with the results by the maximum temperature and precipitation.

To illustrate the general circulation patterns causing the deadly heatwave in eastern Asia and extreme flood in western Asia in the summer 2022, Figure 2 schematically display the anomalies of the main circulation members based on the observed circulations from NCEP/NCAR reanalysis. Climatologically, in the lower troposphere, the Somali cross-equatorial flow turns eastward after passing through the equator, bringing water vapor from the Indian Ocean to the East Asian continent (Figure S2a in Supporting Information S1). In 2022, the wind anomalies at 850 hPa showed an anti-cyclonic pattern over the western Pacific and eastern Asian continent (Figure S2b in Supporting Information S1). The much stronger easterly anomaly at low levels stimulated a local anti-cyclonic anomaly with its center being over the Bay of Bengal, which changed the water vapor transport from eastward path (to eastern Asian continent) to westward path (to western Asia). During this period, the Somalia cross-equatorial flow was also much stronger, which enhanced the tropical westerly over the west part of tropical Indian Ocean. Owing to the anti-cyclonic anomaly center over the Bay of Bengal, the moisture transportation changed the traditional path by flowing westward and converging into western Asia instead of being transported to eastern Asian continent. In the summer 2022, the extreme flood in western Asia was caused by persistent heavy rainfall rather than an extreme synoptic process. The Somalia cross-equatorial flow moved northward to the south of western Asia and created the abundant moisture background of persistent heavy rainfall in the region.

Figure 2. The schematic diagram of the circulations at the upper, middle, and lower troposphere in July–August 2022. In the upper troposphere at 200 hPa, the blue and yellow contours show the climatological position of the South Asian high and its central position in 2022, respectively. The blue and yellow shading show the climatological position of the westerly jet (zonal wind speed exceeding 50 m s−1) and its central position in 2022, where the blue and red arrows show the corresponding positions of the westerly jet axes. In the middle troposphere at 500 hPa, blue and yellow contours show the geopotential height of the climatological mean and that in 2022, respectively. Only the contours of 5,880 gpm are given to represent the western Pacific subtropical high and Arabian high. The shading shows the geopotential height anomalies in 2022. In the lower troposphere at 850 hPa, the black arrows indicate the wind anomalies exceeding 5 m s−1 in 2022.

The above low-level pattern corresponded to a stronger and much more westward WPSH at the middle troposphere. As shown in the middle panel of Figure 2, the western boundary of the WPSH at 500 hPa reached the southwest China at a record-breaking longitude of 80°E since 1981, which was about 50 longitudes more westward than the climatology. Meanwhile, the averaged geopotential height over southern China (25°–40°N, 80°–120°E) reached the highest since 1949, with a 20-gpm positive anomaly center (figure omitted). In western Asia, the AH was also stronger than normal, but its eastern boundary was only about 5 longitudes more eastward than the climatology. Therefore, it can be inferred that in July–August 2022, the anomalous westward extension of the WPSH filled the gap between the subtropical highs over Iranian Plateau-Arabian Sea and western Pacific, thereby forming a heat dome over eastern Asia and changing the water vapor transportation path at low level.

The “opposite-direction shift” phenomenon between the WPSH and the SAH in middle-high troposphere has been revealed by many studies since 1964 (Jia & Yang, 2013; X. W. Jiang et al., 2011; Tao & Zhu, 1964; P. F. Zhang et al., 2016). Generally, the SAH extends eastward several days in advance of the WPSH. The eastward extension of the SAH modulates the WPSH by both dynamical and thermal roles. With the eastward extension of the SAH, both the descending flow forced by negative vorticity advection in the upper layer and the effect of adiabatic warming contribute to the westward extension of the WPSH (Ren et al., 2007). D. Q. Zhang et al. (2023) have also found that the eastward SAH favored and overlapped with the westward WPSH, which caused the heat wave in East Asia. This phenomenon could also be found before the occurrence of the super heatwave process in 2022 in East Asia. In that year, the super heatwave began on 13 June (D. Q. Zhang et al., 2023). Five days before the occurrence, the eastern part of SAH had a remarkable enhancement. On 11 June, the WPSH showed a slight westward expansion and on 13 June the WPSH extended westward significantly to control the middle-lower reaches of China's Yangtze River. As shown in the upper panel in Figure 2, the SAH in July–August 2022 was also much stronger and more eastward. Besides, it has been suggested that the position and intensity of the SAH are tightly related to the upper-level westerly jet since the jet is located in the northeast of the SAH (Wei et al., 2017). Therefore, the westerly jet was located more northward than normal in the summer 2022 as a result of the much stronger and more eastward SAH.

Other “hot eastern-pluvial western” years (2010, 2016, and 2019), selected by the normalized maximum temperature anomaly and precipitation anomaly exceeding 0.9 in July–August, are also analyzed in Figure S3 in Supporting Information S1. The composite anomalies of the main circulation members are similar with those in July–August 2022 at the upper, middle, and lower troposphere.

A previous study has revealed that the WPSH has a significant westward extension under the global warming, especially in the recent decade (Wu & Wang, 2015). The western boundary of WPSH is defined as the longitude degree of the most westward point grid with GPH above 5,880 gpm at 500 hPa within a region of (10°–50°N, 90°–180°E). A similar definition of the eastern boundary of AH is adopted, that is, the longitude degree of the most eastward point grid with GPH above 5,880 within (10°–50°N, 30°–90°E) at the same level. In the period of 1981–2010, the average position of western boundary was about 165°E, which moved westward to 135°E during 2011–2022 (Figure 3a). The intensities of the WPSH and the AH also exhibited enhancing trends in the same period. However, the eastern boundary of the AH was quite stable compared with the western boundary of the WPSH, which changed few between the periods of 1981–2010 and 2011–2022, with only a fluctuation of 2.5 longitudes. As a contrast, the fluctuation of the western boundary of the WPSH reached 30 longitudes. By calculating the mean square root of the 500 hPa GPH in July–August in 1981–2022, it is found that the interannual variations in the WPSH is remarkably higher than those in the AH. This difference could be attributed to the spatial air temperature difference in the lower and middle troposphere. In the eastern boundary of the AH, the GPH expansion is limited by the temperature decrease while the WPSH is prompted by increasing temperature to extend westward to cover the coastal areas of China. In the lower troposphere, the zonal wind anomalies from the east of the Arabian Sea to the western Pacific (80°–150°E) at 850 hPa were positive during most of the period of 1981–2010, and changed to be negative during 2011–2022, indicating the intensified easterly wind anomalies in the south side of the WPSH (Figure 3c). Such enhanced easterly anomalies favored the anti-cyclonic anomaly circulation on its north side, and induced the more westward and northward WPSH. Meanwhile, the meridional wind anomalies over the Arabian Sea were negative during 1981–2010, and changed to be positive during 2011–2022 (Figure 3d). This indicates that the Somalia cross-equatorial flows intensified greatly in the second period, resulting in more water vapor transporting to western Asia and causing more moisture convergence over this region.

Figure 3. Variations in July–August during 1981–2022: (a) the Arabian high (AH) and the western Pacific subtropical high (WPSH) at 500 hPa, (b) the South Asian high (SAH) at 200 hPa, (c) zonal, and (d) meridional wind anomalies at 850 hPa. In panels (a, b), the shading shows the geopotential height averaged over 20°–35°N. In panel (a), the green and blue solid (dashed) lines show the eastern boundary of the AH and western boundary of the WPSH during 2011–2022 (1981–2010), respectively. In panel (b), the blue solid (dashed) line represents the eastern boundary of the SAH during 2011–2022 (1981–2010), respectively. In panels (c, d), wind anomalies are averaged over 5°–15°N. The data are smoothed by a 3-year running average.

In short, the historical observations revealed that the anomalous westward extension of WPSH contributed the most in causing more heatwaves in eastern Asia and more floods in western Asia during 2011–2022 by filling the gap between the subtropical highs over Arabian Sea-Iranian Plateau and western Pacific in the geopotential height field. The “opposite-direction shift” phenomenon could also be found between the SAH and the WPSH. In the upper troposphere, the average position of the SAH eastern boundary was 105°E during 1981–2010, and moved eastward to 125°E during 2011–2022 (Figure 3b). The eastward shift of the SAH in the upper troposphere favored the westward extension of the WPSH in 2011–2022, causing more heat waves in eastern Asia.

More Risk of “Hot Eastern-Pluvial Western” Asia in Future: Projections of CMIP6 Model Ensembles

The increasing trends of both super heatwaves in eastern Asia and extreme floods in western Asia in the observations will be continued in the 21st century, as revealed by the CMIP6 climate projections. Figure 4a displays the time series of past and future maximum temperature during July–August in eastern Asia from the historical observations and the ensemble-mean projection of 19 CMIP6 models under the SSP2-4.5 scenario. Compared with the observations, the 19-model ensemble can reasonably reproduce the historical warming in the reference period (1995–2014). The results of 19 models all show obvious increases from 2020 to 2100 under the SSP2-4.5 scenario. The ensemble-mean maximum temperature anomaly in the near term (2021–2040) is 0.8°C higher relative to the reference period, and the increments reach 1.5°C and 2.4°C in the middle term (2041–2060) and long term (2081–2100), respectively. Therefore, more super and ultra-extreme heatwaves will occur in eastern Asia in future.

Figure 4. Projections of the maximum temperature and precipitation. (a) Time series of the maximum temperature anomalies from the historical data and the 19 CMIP6 model projections during July–August in eastern Asia under the SSP2-4.5 scenario. (b) Same as (a), but for the percentages of precipitation anomaly. Blue bars display the observation data from the Climate Prediction Center. Red line is the ensemble mean projection of the 19 models. Pink shading indicates the data range within 1 standard deviation of the 19 models. Yellow shading indicates the range between the maximum and minimum projections of all 19 models. The gray boxes indicate the near, middle, and long terms.

Similar future variation can also be found in the precipitation series in western Asia. Figure 4b shows the past and future precipitation anomalies from the historical data and the 19-model ensemble projection under the SSP2-4.5 scenario. The ensemble projection can well reproduce the increase in the precipitation in 2010s relative to that in 1981–2010. The increases in the precipitation in western Asia in the near, middle, and long terms are 0.2, 0.3, and 0.4 mm per day relative to that in 1995–2014, corresponding to the percentage increases of 6.3%, 9.4%, and 12.5%, respectively.

To comprehensively investigate the future changes for different scenarios, Figure S4a in Supporting Information S1 shows the projections of the maximum temperature in eastern Asia for SSP 1-1.9, SSP 2-4.5, SSP 3-7.0, and SSP 5-8.5, respectively. For SSP 1-1.9, the maximum temperature is basically unchanged. For SSP 2-4.5 and SSP 3-7.0, the maximum temperature gradually increase. Huge increase for the maximum temperature is found for SSP 5-8.5, which is greater than other scenarios. The future changes of the precipitation in western Asia is shown in Figure S4b in Supporting Information S1. It could be seen that the precipitation will increase remarkably in SSP 2-4.5, SSP 3-7.0, and SSP 5-8.5.

The consistent increases in heatwave and flood frequencies in eastern and western Asia in the future are associated with the changes of the key atmospheric circulation factors explored above. The performance for the related circulations by the ensemble mean of the 19 CMIP6 models is evaluated and compared with NCEP. It is found that the climatology of the position and the intensity of the WPSH and AH are quite similar in NCEP and in CMIP6, indicating the good performance of CMIP6 in the related circulations. The SAH and the 850 hPa moisture transportation are also examined at in CMIP6. All of the comparisons indicate that the CMIP6 ensemble has skills for the simulations of large-scale atmospheric circulations. From the 19-model ensemble in the middle troposphere, it can be seen that the WPSH shows an enhancing and westward extending trend from 2020 to 2100 under the SSP2-4.5 scenario, and the AH intensifies obviously in the future with a relatively stable eastern boundary (Figure 5a). In the upper level, the SAH also strengthens remarkably in the future, and the eastern boundary are projected to be more eastward (Figure 5b). Owing to the “opposite-direction shift” effect, the strengthening and eastward extending of the SAH favors a more westward extending WPSH, which will fill the gap between the subtropical highs over Arabian Sea-Iranian Plateau and western Pacific in the geopotential height field, weaken the westerly over the tropical Indian ocean and block the tropical moisture transportation to the East Asia continent while promote a westward transportation to the western Asia. Finally, the shifts of the SAH and the WPSH contribute to the increases of heatwaves in eastern Asia and floods in western Asia.

Figure 5. Projections of the Arabian high, western Pacific subtropical high, and South Asian high. The geopotential height from the historical data and the 19 CMIP6 model projections in July–August under the SSP2-4.5 scenario at (a) 500 and (b) 200 hPa.

The projections of zonal and meridional wind anomalies averaged over 5°–15°N at 850 hPa are given in Figure 6. From the projection, it could be considered that the westerly wind over the tropical Indian Ocean in the lower troposphere will be weakened from 2020 to 2100, and this will suppress the tropical moisture transportation to East Asia (Figure 6a). Meanwhile, the Somali cross-equatorial flow will be strengthened during the same period (Figure 6b), which will promote more water vapor transportation to the western Asia and result in more precipitation in the region.

Figure 6. Same to Figure 5, but for (a) zonal and (b) meridional wind anomalies at 850 hPa averaged over 5°–15°N.

The moisture transportation and regional moisture budget in the two regions are calculated. For the changed patterns by CMIP6 simulations, the water vapor budget averaged over eastern Asia and western Asia in July–August in different periods are shown in Figure S5 in Supporting Information S1. The water vapor budget is basically unchanged in eastern Asia in the reference period, near term, middle term, and long term. Even in the long term period, the increase is only about 5.6%. However, notable enhancement of the water vapor budget could be clearly seen in western Asia, with an increase of 12.1%, 17.7%, and 27.6% in the near term, middle term, and long term, respectively. Each of them has a significantly higher increasing rate than that in eastern Asia. The larger increase of water vapor budget in western Asia indicates more water vapor transportation to this region, which may result in more precipitation in the future.

Conclusions

In July–August 2022, a fatal “hot eastern-pluvial western” climate pattern occurred in Asia. Eastern Asia experienced an unprecedented heatwave event, where the maximum temperature reached a record high since 1981; while an exceptional flooding struck western Asia, and the precipitation also broke the record since 1981. Both the extremes impacted hundreds of millions of people in the densely populated region and caused thousands of deaths. By analyzing the observations and climate projections, this study pointed out that this pattern is evolving into a new normal rather than an extreme case. Historical observations both from seasonal mean of precipitation and temperatures and the extreme climate indices of TX90p and Rx5day indicate that this pattern prevails in the 21st century especially in the recent decade. The heatwave in eastern Asia and the flood in western Asia have similar inter-annual variations and increasing trends.

The above pattern is closely associated with the anomalous atmospheric circulation structure from the lower to the upper troposphere. Both the extreme case in 2022 and the circulation composition in the recent decade show an anti-cyclonic pattern of wind anomalies at 850 hPa over the western Pacific and eastern Asia continent and a more eastward SAH, which favored a much more westward WPSH to fill the gap between the subtropical highs over the Arabian Sea-Iranian Plateau and the western Pacific, thereby inducing the long-lasting heatwave by the heat-dome effect. Meanwhile, the westward extension of the WPSH stimulated a local anti-cyclonic anomaly center over the Bay of Bengal, and changed the moisture transportation path. On one hand, it weakened the moisture transportation from the Arabian Sea to the East Asia continent along the Somalia cross-equatorial flow; on the other hand, it led to the moisture convergence over the western Asia and brought more frequent and extreme floods.

Based on the 19 CMIP6 models, future changes of the “hot eastern-pluvial western” climate pattern and the associated atmospheric circulations are projected under the moderate emission scenario (SSP 2-4.5). Both the projected maximum temperatures in eastern Asia and the precipitation in western Asia increase gradually in the near, middle and long terms, with the temperature anomalies being 0.8°C, 1.5°C, and 2.4°C higher and the percentages of precipitation anomaly being 6.3%, 9.4%, and 12.5% larger relative to the reference period, respectively. This result is supported by other scenarios of SSP 3-7.0 and SSP 5-8.5. Future changes in the WPSH also show an enhancing and westward extending trend, and the SAH is projected to be stronger and more eastward. The projections indicate that more fatal heatwaves in eastern Asia and floods in western Asia will occur in the future.

Asian countries remain extremely vulnerable to heatwaves and floods owing to its large low-income population and poor financial capacity. It is undoubted that the above disasters pose challenges to the health, well-being, and livelihood of the residents in Asia. We emphasize the increasing threats from the extreme “hot eastern-pluvial western” climate pattern, which is important to improve the understandings of future climate risks and implement the adaption actions.

Acknowledgments

We would like to thank National Oceanic and Atmospheric Administration (NOAA) Physical Sciences Laboratory (PSL) for archiving and enabling public access to their data. We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which coordinated and promoted CMIP6. We thank the National Meteorological Information Centre of the China Meteorological Administration for compiling the observational climatic data. T.D. and H.G. are supported jointly by National Natural Science Foundation of China (Grant 42175078 and 42175048) and National Key R&D Program of China (2018YFC1505603). T.D. and X.L. are supported by Innovation and development project of China Meteorological Administration (Grant CXFZ2022J030).

Data Availability Statement

All the data that support the findings are publicly available. The Global Surface Air Temperature and Precipitation data are available in M. Y. Chen et al. (2008). The National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data set are available in Kalnay et al. (1996) and Kistler et al. (2001). The European Centre's Medium-Range Weather Forecasts Reanalysis 5 data set (ERA5) are available in Hersbach et al. (2020). Data from the CMIP6 models analyzed in this study are available in Eyring et al. (2016).