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This study reveals a close connection between interannual variation of the Somalia Jet (SMJ) intensity in boreal spring and the northwest-southeast movement of the South Asian High (SAH) in the following summer based on multiple datasets and numerical experiments. It is found that the summer SAH tends to shift northwestward (southeastward) when the preceding spring SMJ is stronger (weaker). There are two ways by which the spring SMJ intensity affects the following summer SAH. One is via modulating sea surface temperature (SST) in the tropical Indian Ocean and the other is through modulating anomalous heating associated with the Indian summer monsoon rainfall. On one hand, increase in the spring SMJ intensity enhances southwesterly monsoon winds and upward latent heat flux and leads to SST decrease in the tropical western Indian Ocean. The tropical western Indian Ocean cooling further leads to a decrease (increase) in upper-level geopotential height to the southeastern (northwestern) side of the SAH, inducing a northwestward movement of the SAH. On the other hand, increase in the spring SMJ intensity strengthens the Indian summer monsoon and enhances rainfall and atmospheric heating over the Indian subcontinent. The enhanced Indian subcontinent heating induces strong upper-level positive geopotential height to the northwestern side of the SAH, contributing to a northwestward shift of the summer SAH. Results of this study indicate that variation of the spring SMJ intensity is an effective predictor in the prediction of the northwest-southeast movement of the SAH in the following summer.

Heat waves (HWs) can pose a serious threat to human health and natural ecosystems. Meanwhile, the mid-lower reaches of the Yangtze River are known as one of the most vulnerable regions suffering such disasters. At the mention of their potential drivers, the western North Pacific subtropical high (WNPSH) has been widely accepted as the key atmospheric circulation regulating these high-temperature extremes. Although considerable studies have proved the importance of the WNPSH, not enough work pays attention to the HWs which are not caused by the anomalous WNPSH. This study utilizes the instantaneous co-recurrence ratio α based on the Dynamical System (DS) method to quantify the forcing strength from the WNPSH to HWs, and classify HWs into four types according to their value of α and spatial pattern of the 500 hPa geopotential height, as high- α 1, 2 and low- α 1, 2. Results show that coverage by the WNPSH is not a necessary condition for the formation of HWs, which may also result from the anomalous South Asian high (SAH) and tropical cyclones (TCs), and the value of α can indicate whether the HWs are coupled with the WNPSH or not. Furthermore, the mechanisms behind these anomalous atmospheric systems are also different. The eastward-extended SAH in high- α HWs is associated with the latent heat of local condensation induced by the abnormal rainfall, while accompanied by an eastward-propagating wave train in low- α 1. The intensification of the WNPSH in high- α HWs are attributed to both meridional and zonal wave trains originating from the maritime Continent and Western Europe, respectively, and the anomalous TCs in low- α 2 are mainly due to the warmer sea surface temperature over the basin-wide equatorial Pacific, which may heat the bottom atmosphere and provide the convective instability.

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.

Recent observational and modeling studies have demonstrated the substantial influence of the Tibetan Plateau (TP) spring land surface temperature (LST) and subsurface temperature (SUBT) on downstream summer droughts/floods events in East Asia, highlighting the potential application of LST/SUBT on sub-seasonal to seasonal prediction (S2S). In this study, we employ the National Centers for Environment Prediction—Global Forecast System/Simplified Simple Biosphere model version 2 (GFS/SSiB2) to investigate the potential role of the late spring warm LST anomaly over the TP on the extraordinary June 1998 flood in the south of the Yangtze River region. Numerical experiments indicate that the warmer (above normal) May LST over the TP may contribute to the extreme flood of 1998 over the south of the Yangtze River region, with the LST reproducing about 57% and 64% of observed above-normal rainfall anomaly over the south of the Yangtze River region and southeastern China, respectively. Further analyses reveal a possible effect of springtime TP’s LST on summer southern and eastern Asian rainfall and identify some hot spots, suggesting that the TP’s spring LST effect is not only limited to the Yangtze River region, but to a much larger scale. The imposed warm LST/SUBT over the TP triggers a strong wave activities propagating eastward along the upper-level westerly jet, associated with an increase of the atmospheric baroclinic instability as well as a strengthening and southeastward movement of the South Asian high, leading to intensified moisture convergence and convective instability favorable to the excessive rainfall in the downstream region of East Asia. The results of the 1998 case have also been compared with the results from year of 2003, which had a very cold spring LST anomaly over the TP and a severe downstream June 2003 drought (flood) in southern (northern) of the Yangtze River Basin area. Simulation results provide further evidence of the great importance of the TP spring land surface temperature anomaly in regulating summer extreme hydroclimatic events (e.g. droughts and floods) in South and East Asia. The present study suggests that consideration of LST/SUBT anomalies has a strong potential for more skillful S2S prediction of extreme hydroclimatic events such as floods, droughts and heatwaves over both Southern and Eastern Asia.

Diagnostic analyses are performed to investigate the relationship between the upper-level Asian westerly jet stream (AWJS) and the associated rainfall pattern over the AWJS region in boreal summer on interannual time scales. Results show an out-of-phase rainfall variation over central Asia (CA) and north China (NC), which is closely related to the southeast–northwest (SE–NW) shift of the AWJS. The physical mechanisms on the relationship between the AWJS and the rainfall pattern are revealed by exploring the effects of the Indian summer monsoon (ISM) and the South Asian high (SAH). It is found that the SE–NW shifts of the AWJS and SAH associated with the ISM lead to the anomalous circulations over the midlatitudes in the AWJS region and cause rainfall anomalies over CA and NC. A weak ISM results in a southeastward shift of SAH, which is responsible for a southeastward shift of the AWJS. The anomalous atmospheric circulation associated with the southeastward located SAH produces anomalous updrafts (downdrafts) over the western (eastern) AWJS region, resulting in increased rainfall over CA and decreased rainfall over NC. It is proposed that the upper-level system SAH plays a crucial role in the teleconnection among the summer rainfall over the midlatitude AWJS region and ISM region.

A diagnostic analysis reveals that on the interannual time scale the southeast–northwest movement is a dominant feature of the South Asian high (SAH), and it is closely related to the Indian and East Asian summer monsoon rainfall. The southeastward (northwestward) shift of the SAH is closely related to less (more) Indian summer monsoon rainfall and more (less) rainfall in the Yangtze River valley (YRV) over the East Asian summer monsoon region. An anomalous AGCM is utilized to examine the effect of latent heat anomalies associated with the Asian summer monsoon rainfall on the SAH. The negative latent heat anomalies over the northern Indian Subcontinent associated with a weak Indian summer monsoon stimulates an anomalous cyclone to its northwest and an anticyclone to its northeast over the eastern Tibetan Plateau and eastern China in the upper troposphere, which is responsible for the east–west shift of the SAH and more rainfall in the YRV. The positive latent heat release associated with rainfall anomalies in the YRV excites a southward-located anticyclone over eastern China, exerting a feedback effect on the SAH and leading to a southeast–northwest shift of the SAH.

Meiyu shows substantial intraseasonal variation at periods of 10–30 days and 30–60 days, which often leads to extreme precipitation and disastrous flooding over the Yangtze River basin. Monitoring and prediction of the intraseasonal variation of Meiyu is crucial for disaster prevention and mitigation. Here, we proposed two sets of novel indices for Meiyu real-time monitoring and prediction based on the compound zonal displacements of the South Asia high (SAH) and the western Pacific subtropical high (WPH) at 10–30-day and 30–60-day period, respectively. For the 10–30-day period of Meiyu, the zonal displacement of the SAH is associated with a mid-latitude Eurasian Rossby wave train, whereas the WPH is related to the second mode of the boreal summer intraseasonal oscillation. On the 30–60-day timescale, the zonal displacement of the SAH and the WPH are both associated with the first mode of the boreal summer intraseasonal oscillation. The subtle differences in zonal displacement of the SAH and the WPH determine eight type configurations, corresponding to distinct influences on Meiyu. Meiyu intraseasonal variation can be well reconstructed by using the relationship between these two indices and rainfall anomalies pattern over China. Given that the ECMWF S2S model is more skillful in forecasting upper- and lower-level circulation than in directly forecasting precipitation, a hybrid dynamical–statistical model is conducted to subseasonal prediction of Meiyu using the ECMWF model forecast indices. The hybrid model outperforms the ECMWF model in subseasonal prediction of the Meiyu variation at 17–40-day lead times.

The year-to-year variations of Tibetan Plateau (TP) summer rainfall have tremendous climate impacts on the adjoining and even global climate, attracting extensive research attention in recent decades to understand the underlying mechanism. In this study, we investigate an open question of how El Niño–Southern Oscillation (ENSO) influences the TP precipitation. We show that the developing ENSO has significant impacts on the summer rainfall over the southwestern TP (SWTP), which is the second EOF mode of the interannual variability of summer rainfall over the TP. The moisture budget indicates that both the suppressed vertical motion and the deficit of moisture contribute to the reduction of SWTP rainfall during El Niño’s developing summer, with the former contribution 4 times larger than the latter. Moist static energy analyses indicate that the anomalous advection of climatological moist enthalpy by anomalous zonal wind is responsible for the anomalous descending motions over the SWTP. The El Niño–related southward displacements of the South Asian high and the upper-level cyclonic anomalies over the west of TP stimulated by the suppressed Indian summer monsoon precipitation are two key processes dominating the anomalous zonal moist enthalpy advection over SWTP. Mean while, the India–Burma monsoon trough is strengthened during El Niño developing summer, which prevents the advection of water vapor into the SWTP, and thus contributes to the deficit of summer SWTP rainfall. Our results help to understand the complicated ENSO-related air–sea interaction responsible for the variability of TP precipitation and have implications for seasonal prediction of the TP climate.

The decadal intensification of the South Asian high (SAH) after the late 1970s, which is determined based on the geopotential height (H), is suspicious due to the lifting effect upon H caused by global warming. The updated reanalysis datasets ERA5 and JRA-55 indicate that the anticyclone in the upper troposphere over the Tibetan Plateau is relatively weak during 1980–2018 compared to that during 1950–79. This decadal weakening of the SAH after 1979 can also be observed in the radiosonde observation data. Correspondingly, the SAH defined by eddy geopotential height (H′) reflects a consistent decadal weakening variation. The decadal weakening of SAH detected from H′ after the late 1970s matches with a decadal southward shift of the East Asian westerly jet, causing ascending motions over the Yangtze River valley and descending motions over North China. Moreover, the decadal weakening and westward shift of the SAH is accompanied with positive relative vorticity anomalies over the northwest Pacific in the upper troposphere, which implies a declining and eastward shift of the western Pacific subtropical high (WPSH) and a weakened East Asian summer monsoon (EASM). Hence, the decadal weakening of the SAH after the late 1970s may contribute to the Yangtze River flooding/North China drought pattern through its connection with other circulation systems of EASM.

Revealing the synoptic patterns related to hourly extreme precipitation (EP) is very important to deepen our recognition and understanding of EP formation. The predominant synoptic patterns associated with the summer regional hourly EP events (RHEPE) over the central‐eastern Tibetan Plateau (CETP) and Sichuan Basin (SCB) have been systematically identified. Results show the summer RHEPE over CETP and SCB are dominated by the background large‐scale circulations featured by the configuration of eastward‐extended South Asia high (SAH) and westward‐extended Western North Pacific Subtropical High (WNPSH) and their northward advance, except for that an obvious low‐level vortex imbedded in the background large‐scale circulations is mainly responsible for the summer RHEPE over SCB. The frequency and intensity of the total summer RHEPE over CETP and SCB all show an obvious increasing trend during 2000–2020, which is largely contributed by the synoptic pattern characterized by the configuration of eastward extended SAH and westward extended WNPSH. Plain Language Summary To completely indicate the synoptic patterns leading to the summer regional hourly extreme precipitation events (RHEPE) over the central‐eastern Tibetan Plateau (CETP) and Sichuan Basin (SCB), this study has identified the dominant synoptic patterns of summer RHEPE over the two regions. Results show that the configuration of eastward extended South Asia high (SAH) and westward extended Western North Pacific Subtropical High (WNPSH) and their northward advance dominates the prime circulation patterns of summer RHEPE over CETP and SCB. However, in addition to the opposing motion of the SAH and WNPSH, a low‐level vortex imbedded in the background large‐scale circulations is mainly responsible for the summer RHEPE over SCB. The findings of this study may help us to deepen our understanding of the RHEPE formation over the two regions with complex terrain and provide a base to further improve the prediction of extreme precipitation. Key Points Summer rainfall extremes over both central‐eastern Tibetan Plateau (CETP) and Sichuan Basin (SCB) are dominated by the combination of eastward extended SAH and westward extended Western North Pacific Subtropical High The summer rainfall extremes over SCB are mainly induced by a low‐level vortex imbedded in the background of large‐scale circulations The dominant synoptic patterns over CETP and SCB all contribute to the increasing trends of frequency and intensity of the summer regional hourly EP events