Why was the heat wave in the Yangtze River valley abnormally intensified in late summer 2022?

A record-breaking heat wave hit the Yangtze River valley during the boreal summer of 2022, and caused severe social and economic losses. One prominent feature of this long-lived heat event was its persistence and abnormal intensification in August. This study investigated the physical mechanisms be responsible for the intensification of this heat event in late summer under the background of a La Niña event. The prolonged heat event was directly related to the intensification and westward extension of the western North Pacific subtropical high (WNPSH), which can be attributed to the synergistic effects of an anomalous western North Pacific (WNP) anticyclone and the eastward extension of the South Asian high (SAH) in the upper troposphere. The anomalous anticyclone in the WNP, which was induced by negative sea surface temperature anomalies in the central tropical Pacific, strengthened in August. The positive sea surface temperature anomaly in the western Pacific warm pool and enhanced in-situ convection led to anomalous high pressure over the Yangtze River valley via the local meridional circulation. Atmospheric convergence and descending motion over the Yangtze River valley was amplified in August as a result of the zonal shift in the SAH from the Iranian Plateau to the Tibetan Plateau. The Silk Road pattern (SRP) index of August 2022 was the lowest since the 1990s. The abnormal negative phase of the SRP contributed to both the zonal shift in the SAH and the westward extension of the WNPSH, which led to the abnormal intensification of the heat event over the Yangtze River valley in August 2022.


Introduction
The Yangtze River valley experienced a heat wave during the boreal summer of 2022, which was unprecedented in terms of magnitude, duration and area of influence. Both the average temperature and the number of days with a maximum temperature >35 • C in July and August 2022 were the highest since 1961. The mean rainfall in the Yangtze River valley during these months was the lowest since 1961. The abnormally low rainfall induced a summer drought, which contributed to the exceptional magnitude of the heat wave (Hao et al 2018, Zscheischler et al 2018. The prolonged extreme heat events and drought stressed water and energy supplies, causing significant social and economic losses Zhai 2020, Liu et al 2022).
As one of the worst-hit areas of heat waves, substantial efforts has been dedicated to reveal the variability and underlying mechanism responsible for the heat waves in the Yangtze River valley (Hu et al 2012, You et al 2017, Gao et al 2018, Deng et al 2019, Huang et al 2021. The heat events can be classified as Yangtze-Huai and Jiangnan type, each with significantly different atmospheric circulation background and external forcing (Wang et al 2014, Yuan et al 2018). The center of high temperature anomaly for Jianghuai type locates in the middle and lower reaches of the Yangtze River. The oceanic forcing of Jianghuai type include the anomalous negative sea surface temperature (SST) in the tropical central and eastern Pacific and Indian Ocean, while the Jiangnan type is related to the decaying phase of El Niño. A La Niña event developed since autumn 2021 and persisted through the summer of 2022. Both the spatial pattern of temperature anomalies and oceanic background of heat event in 2022 are in good accordance with the Jianghuai type.
Among all the factors affect heat waves over Yangtze River valley, the western North Pacific subtropical high (WNPSH) is the most direct circulation system (Liu et al 2019). Both the descending motion around the ridgeline of WNPSH (Kosaka et al 2012) and its modulation of East Asian monsoon rain belt contribute to the heat waves (Tao and Wei 2006, Ding et al 2010. The interannual variability of WNPSH is closely related to the tropical air-sea interaction (Chen and Zhou 2018). In addition to the Indian Ocean warming during El Niño decaying phase (Xie et al 2009, Hu et al 2012, 2013, Luo and Lau 2018, the central Pacific cooling and a positive atmosphericocean feedback has also been revealed to be responsible for the intensification of WNPSH (Wang et al 2013, Xiang et al 2013. The high-temperature anomalies in summer 2022 also showed a significant sub-seasonal variability. In June, the high-temperature anomaly was mainly located in North China. However, from late June, accompanied by a northward shift in the WNPSH, the surface temperature in the Yangtze River valley rapidly increased. Climatologically, the maximum temperature and number of high-temperature days in the Yangtze River valley peak in July (Shi et al 2008). The long-lived heat wave persisted and abnormally amplified in August 2022, especially in the upper reaches of the Yangtze River valley. Therefore, this study investigates the possible mechanisms responsible for the abnormal intensification of the heat wave in August 2022.

Data and methods
We extracted the observational daily temperatures from the daily meteorological dataset of basic meteorological elements of the China National Surface Weather Station V3.0 (Ren et al 2012). This dataset has undergone strict quality control and inconsistency checks. The daily mean temperatures from 1991 to 2022 were used to obtain the monthly mean temperatures and anomalies. We used daily and monthly reanalysis data from the National Centers for Environmental Prediction-National Center for Atmospheric Research dataset at a horizontal resolution of (2.5 • × 2.5 • ) (Kalnay et al 1996), including the geopotential height, horizontal wind and vertical velocity at different levels. We obtained SST data from the National Oceanic and Atmospheric Administration Extended Reconstructed SST version 5 (ERSSTv5) dataset (Huang et al 2017).
We defined the intensity of the South Asian high (SAH) as the geopotential height of its high-pressure center at 200 hPa within the 12 500 gpm contours (Zhang et al 2000). The intensity of the WNPSH was defined as the standardized accumulated geopotential height of all grids within the 5880 gpm contours at 500 hPa subtracted by 5870 gpm. We defined the contours of the zonal wind at u = 0, with ∂u/∂y > 0, within the 5880 gpm isoline as the ridge line of the WNPSH (Liu et al 2012).
We performed empirical orthogonal function (EOF) analysis on the meridional wind at 200 hPa within the region (20 • -60 • N, 0 • -150 • E) to obtain the Silk Road pattern (SRP;Lu et al 2002, Enomoto 2004. The SRP index was defined as the standardized time series associated with the leading EOF mode. We conducted correlation and composite analysis to examine the relationship between two variables. We assessed the statistical significance of the correlation coefficient using a two-tailed Student's t-test. The anomalies were obtained based on climatological mean of the time period 1991-2020.

Comparison of spatial temporal characteristics of the heat wave in Yangtze River valley between July and August 2022
The Yangtze River valley was affected by an unprecedented heat event in July-August 2022. The regional weighted mean temperature reaching 29.4 • C, the highest since 1961 (figures 1(c) and (f)). The mean temperatures in July, August and July-August all showed an increasing trend of 0.09, 0.11 and 0.10 • C/10a, respectively (figures 1(d)-(f)). In July, the mean temperature for most of the Yangtze River valley was 1 • C higher than normal, with two centers of high-temperature anomalies >2 • C in the lower reaches of the Yangtze River and the Sichuan Basin (figure 1(a)). The regional mean temperature of the Yangtze River valley was 28.9 • C, slightly higher than that in 2013 (28.8 • C; figure 1(d)).
The heat waves in east and central China, northern southwest China and the Tibetan Plateau significantly amplified in August, with high-temperature anomalies >2 • C over most of the Yangtze River valley and the eastern Tibetan Plateau. As the geographical center of the heat event, the temperature anomaly of eastern Sichuan and Chongqing reaches a historical 4 • C ( figure 1(b)). The regional mean temperature in August 2022 (29.9 • C) was the highest since 1961 and was significantly higher (by 1.4 • C) than the second highest temperature in 2013 (figure 1(e)).

Subseasonal variation of atmospheric circulation associated with the heat event
The WNPSH is the dominant circulation system affecting heat waves over the Yangtze River valley  (c), respectively. A westward extension and intensification of the WNPSH compared with its climatology was seen in both July and August. As a result of the simultaneous eastward extension of the Iranian high and the westward extension of the WNPSH on most days in July, the cells of the subtropical high connected and formed an intense subtropical high belt over the Asian continent. In August, the WNPSH intensified further and extended further westward, with its western end over the Tibetan Plateau. The intensity of the WNPSH in August 2022 was the highest since 1961, with anomalies more than twice the standard deviation (figure 2(e)). The amplitude of the westward extension of the WNPSH in August 2022 was among the top four years since 1961 (figure 2(f)).
In the upper troposphere, the SAH also experienced significant sub-seasonal variations during summer 2022. As the most intense and persistent anticyclonic system in the upper troposphere during the boreal summer (Mason and Anderson 1963, Dao and Chu 1964), the SAH strongly modulates the weather and climate in China (Jiang et al 2011, Wei et al 2014, Yang andLi 2016). The intensity of the SAH was above normal for both July and August 2022, both ranked as the highest since 1961 (figure 2(g)). However, the center of the SAH shifted from the Iranian Plateau to the Tibetan Plateau (figures 2(b) and (d)). The westward extension of the WNPSH is closely related to the eastward extension of the SAH (Dao andChu 1964, Feng et al 2014). The SAH usually extends eastward several days in advance of the WNPSH. The eastward extension of the SAH modulates the WNPSH both dynamically and thermally. 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 WNPSH (Ren et al 2007). Moreover, the center of SAH overlaps with WNPSH between 90 • E and 120 • E, the high pressure system over upper reaches of Yangtze River valley presents equivalent barotropic structure, causing Sichuan and Chongqing became the center of heat wave.

Climate contributor to the intensification of heat wave in late summer 2022
In order to investigate the possible mechanism responsible for the intensification of heat wave and WNPSH in late summer, comparative analysis was undertaken from the angle of air-sea interaction between July and August 2022 ( figure 3). Under the background of persistent La Niña event, central pacific cooling continues with amplified positive SST anomalies in the western Pacific warm pool in August, leading to an increase in the zonal SST gradient (figures 3(e) and (f)). A significant descending limb located around 160 • E developed in August, On the other hand, a local meridional circulation can be found over WNP (figures 3(c) and (d)), which is associated with the warm SST anomalies enhanced convection . In July, there was a rising limb between 10 • S and the equator, with descending limb between 20 • N and 30 • N. In August, the equatorial rising limb is strengthened, while convections and another rising limb developed between 10 • N and 20 • N (above the South China Sea). In the upper troposphere, strong anomalous southerlies were seen north of 30 • N in July, whereas northerlies prevailed in August. The convergence of both southerlies and northerlies near 30 • N strengthened the anomalously sinking motion along Yangtze River valley (Wei et al 2019).
One natural question is the cause of the reversal of upper tropospheric meridional wind north of 30 • N in August. The intensity of the SAH was persistently above normal and its center shifted abruptly in early August from 40  found over Northwest China, accompanied by westward extension of WNPSH. Northerlies prevail on the right flank of anticyclonic high pressure center in the upper troposphere. For the Iranian mode, the high pressure center locates over Central Asia, an anomalous low can be found over northeastern Tibetan Plateau and southerlies prevail on its right flank. The zonal shift of SAH in early August leads to not only the westward extension of WNPSH but also the reversal of meridional wind north of 30 • N. Moreover, the zonal distribution of SAH center shows a systematic eastward displacement for strong years ( figure 4(b)), and the intensity of high pressure center for SAH of Tibetan mode is significantly stronger than the Iranian mode.
Note that the latitude of the center of the climatological SAH center is near 30 • N, whereas the anomalous center of the 200 hPa geopotential height for the Tibetan and Iranian mode is at about 40 • N (figures 4(e) and (f)). This indicates that the zonal oscillation of SAH might be associated with Rossby wave activity in the middle and high latitudes of Eurasia (Ren et al 2015, Cen et al 2020). Figures 5(a) and (c) shows the meridional wind anomalies at 200 hPa for July and August 2022, respectively. A significant wave-like pattern can be seen along the Asian westerly jet in August. The wave train propagated eastward from the entrance of the Asian westerly jet and intensified above the Tibetan Plateau. The interannual variability of the SRP index was calculated and illustrated in figures 5(b) and (d) (Lu et al 2002, Enomoto et al 2004). As a component of the circumglobal teleconnection (CGT, Ding and Wang 2005), the SRP has an important role in connecting the circulation and climate anomalies between the upstream regions and East Asia. The circulation of the upper troposphere at mid-latitudes in August 2022 showed a significant negative phase  The excitation of the SRP is closely related to the air-sea system of the North Atlantic and the background state (Watanabe 2004, Hong et al 2018, 2022. The intensification of positive SST anomalies in the mid-latitudes of the North Atlantic (figures 3(e) and (f)), accompanied by the positive phase of the North Atlantic oscillation in August, contributed to the propagation of the SRP. The SRP also modulates the intensity and location of both the SAH and WNPSH (Kosaka et al 2012, Ren et al 2015, Cen et al 2020, Zhang and Chen 2021. The correlation coefficient between the SRP index and the longitude of SAH center in August was −0.39, whereas the correlation coefficients between the SRP index and the intensity and longitude of the western end of the WNPSH were −0.33 and 0.45, passing the significance test at 90 and 95%, respectively. The abnormal negative phase of the SRP in August 2022 contributed to both the eastward shift in the SAH and the intensification and westward extension of the WNPSH.

Conclusion and discussions
The Yangtze River valley experienced an unprecedentedly hot summer in 2022. One prominent feature of this prolonged heat event was its abnormal intensification in August. To investigate the possible mechanisms, we carried out a comparative analysis with respect to both air-sea interactions and the synergistic effects of tropical and mid-latitude climate factors. Our results show that, as the direct circulation system affected the heat wave over China, the abnormal WNPSH intensified and extended westward, especially in August. In the upper troposphere, although the intensity of the SAH was persistently strong, its zonal center showed a significant subseasonal variation and shifted from the Iranian mode in July to the Tibetan mode in August.
In the tropical Pacific Ocean, under the background of a continuous La Niña state, the negative SST anomalies in the central Pacific were amplified in late summer. The induced anomalous anticyclonic circulation strengthened the WNPSH. The enhanced warm SST anomalies of the WNP warm pool and the associated convective activities contributed to the high pressure over the Yangtze River valley via the local meridional circulation.
In August, as a result of the eastward shift in the center of the SAH, significant northerlies prevailed in the upper troposphere north of 30 • N, which further strengthened the anomalous descending motion and heat wave along the Yangtze River valley. A significant wave train propagating along the Asian westerly jet was seen in the mid-latitudes of Eurasia in August. The abnormal negative phase of the SRP in August favored the extension of both the SAH and WNPSH in opposite directions and intensification; the synergistic effects of the SAH and WNPSH were therefore conducive to the persistent heat wave in the Yangtze River valley (Li et al 2021).
Previous studies have indicated that the summer SAH tends to shift northwestward and weaken during La Niña events (Tan et al 2005, Zhao et al 2009, Xue et al 2018. However, the relationship between the ENSO and the location of the SAH has weakened since the 1980s, which might be associated with the reduced impact of the ENSO on the Indian summer monsoon (Cen et al 2022). The zonal shift in the SAH is more closely related to the atmospheric wave train over mid-to high latitudes of Eurasia after the 1980s.
As part of the CGT, the SRP has significant correlations with the Indian summer monsoon. The abnormal Indian summer monsoon may excite an anomalous downstream Rossby wave train extending to the North Pacific and North America (Ding and Wang 2005, Dutta and Neena 2022), with upper tropospheric anticyclonic anomalies over East Asia (Beverley et al 2021). In summer 2022, a historical flood affected vast areas of Pakistan and latent heating from this event may have contributed to the intensification of the downstream wave train. The possible relationship between flooding in Pakistan and the heat wave in the Yangtze River valley, and its underlying mechanisms, require further investigation.
The data generated and/or analyzed during the current study are not publicly available for legal/ethical reasons but are available from the corresponding author on reasonable request.