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Recent years have witnessed contrasting trends in summer total rainfall (STR) over the Tibetan Plateau (TP), with an increase in the northern and a decrease in the southern TP. This study identifies four significant centers of rainfall trends: eastern TP ("region A"), Qiangtang Plateau ("B"), Qaidam Basin ("C"), and the northern foothills of the Himalayas ("D"). Heavy rainfall dominates STR trends in regions A and D, accounting for 55.6% and 52.0%, respectively. In region B, moderate and light rainfall contribute almost equally, accounting for 37.3% and 44.8% of the STR trend, respectively. Region C is primarily influenced by light rainfall, explaining 71.2% of the STR trend. Notably, the contributions of different rainfall intensities to STR in each region vary annually, with region A experiencing more heavy rainfall, region B having moderate dominance but less light rainfall, and region C and D showing reduced and increased light rainfall contributions, respectively. Mechanistically, the strengthening of the upper-level westerly jet and the South Asian High, coupled with changes in moisture transport and convective available potential energy, collectively cause variations in rainfall intensity, characterizing the spatial heterogeneity in STR in the TP. [ABSTRACT FROM AUTHOR]

In 2022, South China suffered severe flooding due to unexpected persistent heavy rainfall in the pre-flood season. The in situ cumulative precipitation amount during May–June in South China broke the record established in 1981. The present study ascribed this record-breaking event to extreme warm sea surface temperature anomalies (SSTAs) in the Northeast Pacific (NP) and the Maritime Continent (MC) region. In May, the NP warming associated with La Niña and the negative phase of the Pacific Decadal Oscillation intensified extratropical diabatic heating and induced a cyclonic anomaly over Northeast Asia, which shifted the East Asian trough (EAT) southward. Subsequently, the westerly wind accelerated to the south of the anomalous EAT and strengthened the western North Pacific subtropical high, which led to increase in rainfall over South China. In June, the rainfall over South China remained above normal owing to anomalies of the meridional monsoon circulation manifested as a response to the development of a warm SSTA in the MC region. The anomalous warming in the deep tropics suppressed the Asian summer monsoon convection through anomalous atmospheric descent over South Asia, accompanied by enhanced atmospheric ascent and increased water vapor convergence over South China. Numerical sensitivity experiments validated the above processes and indicated that historic warm SSTAs in the NP and MC regions could increase the probability of extreme persistent above-normal pre-flood rainfall over South China. The results support improved predictability of pre-flood rainfall over South China on the interannual timescale outside the equatorial central–eastern Pacific. [ABSTRACT FROM AUTHOR]

A record‐breaking flooding event occurred in Zhengzhou, Henan Province of China during 17–23 July 2021, causing hundreds of deaths and vast economic losses. Here, we evaluated the predictability of this extreme rainfall event and the impacts of tropical cyclones (TCs) using subseasonal‐to‐seasonal (S2S) operational models. On the monthly timescale, most models initialized in late June reasonably predicted the wet‐in‐north and dry‐in‐south patterns of anomalous rainfall over China in July, accompanied by the well‐predicted westward extension of the western North Pacific subtropical high (WNPSH) and eastward stretching of the South Asian High. On the weekly timescale, only four models captured the location, probability, and sudden intensification of the rainfall extremes in advance of 1 week, largely due to their reasonable prediction of WNPSH variability in mid‐latitudes. However, the S2S models still underestimated the super extremeness of this event. The prediction discrepancies came from the poor predictability of Typhoon IN‐FA and its impact on the daily evolution of the extreme rainfall event, even within a few days forecast lead. Compared with the observation, the prediction bias of tropical disturbance changed the environmental monsoon airflow to induce the earlier warning of rainfall extremes prior to the formation of IN‐FA. After the formation of IN‐FA, the prediction bias of the typhoon's moving speed distorted the typhoon location, which incorrectly predicted the moisture convergence center and underestimated their remote impacts on this heavy rainfall event. Plain Language Summary: Unprecedented heavy rainfall reaches the warming Earth more frequently, creating the need for effective risk‐warning alerts that utilize subseasonal‐to‐seasonal (S2S) forecasting to integrate information from nowcasting, weather, and seasonal predictions. On 17–23 July 2021, persistent heavy rainfall affected Henan Province of China, triggering devastating flooding and traffic chaos in Zhengzhou city. Some skillful S2S models can reasonably predict the monthly rainfall pattern in July. Although the chance of exceeding the new record daily rainfall is only approx. 0.7% in mid‐late July, they provide a high probability of this heavy weekly rainfall 1 week in advance. Nevertheless, the S2S models underestimate this super extremeness, mainly due to the poor predictability of tropical cyclones, even within a few leading days. The deficits in the tropical disturbance and moving speed of Typhoon IN‐FA in the model forecast altered the moisture transmission, underestimating their remote impacts on this extreme event. Risk management on S2S timescales is essential for many public and private sectors, as it provides pivotal lead times that save lives and property. Each extreme event exposes challenges regarding the credibility and sensitivity of the outcomes for decision‐making. Future research should improve our awareness of the challenges that remain in the S2S forecasts. Key Points: Most subseasonal‐to‐seasonal models produce the wet‐in‐north and dry‐in‐south rainfall anomalies and general circulation configuration over China in JulySudden intensification of rainfall in Henan China was predicted 1 week ahead by four models with accurate mid‐latitude adjustmentsDeficits in tropical disturbance and moving speed of Typhoon IN‐FA underestimate their remote impacts in forecasting [ABSTRACT FROM AUTHOR]

Yunnan Province in Southwest China is vulnerable to droughts due to its distinctive topography and local climate. Spring drought in Yunnan (SDY), which accounts for 70% of all drought events, causes the most severe devastation. By examining the variation characteristics of droughts in Yunnan from 1961 to 2020 in terms of the standardized precipitation evapotranspiration index (SPEI), this present study shows that droughts in Yunnan have worsened in the past 60 years on different timescales. Especially, the SDY exhibits notable interannual and interdecadal variations, with no significant long-term trend, although the spring average regional temperatures have risen at a rate of 0.33 °C/10a since 1961. Here, in order to quantify the contribution of the precipitation and temperature, the two main meteorological impact factors, to the SDY under the exacerbation of climate warming, the statistical analyses reveal that precipitation plays a more crucial role than temperature in interannual and interdecadal SDY variations. Further, a diagnostic analysis of the moisture budget equation indicates that suppressed vertical moisture advection is the most important physical process affecting the reduced rainfall amount in spring, followed by the restricted horizontal water vapor transport. Meanwhile, the weak Bay of Bengal (BOB) summer monsoon, which is likely regulated by El Niño-like sea surface temperature anomalies (SSTAs) in spring, is closely linked with the SDY. This mechanism provides the possibility of SDY predictability on a seasonal scale. [ABSTRACT FROM AUTHOR]

The rainy season typically terminates over the area of central northern China (CNC) in late August with the rapid retreat of the East Asian summer monsoon (EASM). However, the CNC suffered continuous heavy rainfall in September 2021. The monthly accumulative precipitation amount broke its historical record since 1961 and caused severe floods. The present study ascribes this extreme rainfall event to the westward inland intrusion of the western North Pacific subtropical high (WNPSH). Both case study and statistical analysis indicate that westward extension of the WNPSH is significantly positively correlated with convection over the Maritime Continent (MC) and northern Indian Peninsula (NIP) under the influence of sea surface temperature anomalies (SSTAs). Atmospheric general circulation model (AGCM) experiments validate the individual and combined effects of SSTAs in the tropical Indo-Pacific–Atlantic Oceans on WNPSH and CNC rainfall. In particular, the warm SSTAs in the Indo-Pacific warm pool increase the probability of extremely westward extension of the WNPSH and above-normal rainfall over the CNC. The cold SSTAs in the equatorial central-eastern Pacific enlarge the zonal SSTA gradient, strengthen the anomalous convection over the MC and NIP, and intensify the WNPSH and CNC rainfall anomalies. Meanwhile, the warmer tropical Atlantic anchors the anomalous WNPSH and CNC rainfall in their observed positions. A statistical model based on the tropical SSTAs in August can skillfully predict the westward extension of the WNPSH in September 2021, suggesting that the combined effects of tropical Indo-Pacific–Atlantic SSTAs in late summer possibly tend to prolong rainy season of the EASM until early autumn. [ABSTRACT FROM AUTHOR]

In 2020, the Yangtze River (YR) suffered a long‐persisting Meiyu season. The accumulated rainfall broke its record since 1961 and caused severe flooding and death in China. Our results show the sequential warm and cold Meiyu front regulated by the North Atlantic Oscillation (NAO) was responsible for this unexpected extreme Meiyu event. From 11 to 25 June with the positive NAO, the interaction between the South Asian High (SAH) and the western Pacific subtropical high maintained a warm front to strengthen the rainband north of the YR. Afterward, the coupling between SAH and midlatitude Mongolian Cyclone induced a cold front, which retreated the rainband to the south of YR from 30 June to 13 July with the negative NAO. Although the ECMWF S2S model successfully predicted the warm‐front‐related Meiyu rainband, it failed to forecast the Meiyu rainband in the cold‐front period, suggesting a great challenge of S2S forecasting on Meiyu rainfall. Key Points: Sequential warm Meiyu fronts in middle to late June and cold fronts in early to middle July directly cause record‐breaking Meiyu rainfall in 2020The NAO phase change leads to the alternation of the circulation regime of the East Asian summer monsoon from a warm‐ to cold‐front periodThe prediction skill of the ECMWF S2S model on the 2020 Meiyu rainband is higher in the warm front period but lower in the cold front period [ABSTRACT FROM AUTHOR]

Interannual variability of boreal summer monsoon (BSM) rainfall exhibits a prominent co-variation mode, which affects the regions with the highest population density in the world. The mode intensity depends on the rainfall variation over the western North Pacific (WNP) and the North America (NAM) monsoon regions. This study suggests that relative importance of the WNP and NAM rainfall anomalies is asymmetric between the strong positive (SP) and strong negative (SN) phases of this mode, which can be attributed to the distinct seasonal evolution of ENSO events. In the SP-phase, the suppressed summer rainfall over the WNP determines the mode following a decaying El Niño event. When the tropical Indian Ocean (TIO) warms and the eastern Equatorial Pacific (EEP) cools, the emanating Kelvin wave- and zonal SST difference-induced low-level WNP anticyclone thus gets enhanced to damp the local rainfall, which further affects the other members of the BSM. However, the SN-phase of this mode, showing closer association with anomalous NAM rainfall, appears with a developing El Niño event. The tropical North Atlantic (TNA) becomes colder from spring to summer, when the EEP warms due to the zonal circulation anomalies. A stronger zonal SST difference between the warm EEP and cold TNA suppresses the NAM rainfall by strengthening the low-level anticyclone in situ. Such asymmetry in different phases of the BSM co-variation mode is validated by the pre-industrial control runs in a state-of-the-art CGCM, indicating the roles of ENSO evolution in regulating the interaction between the BSM members. [ABSTRACT FROM AUTHOR]

Sequential heavy rainfall clusters and resultant severe winter flooding was observed over South China during the 2015/2016 super El Niño event. In the present, the intra‐seasonal oscillations (ISOs) related with heavy rainfall over East Asia from late autumn to early winter in 2015/2016 were investigated using the daily in situ rainfall records and the NCEP‐DOE Reanalysis data set. Results suggested that the successive heavy winter rainfall events in 2015/2016 were contributed by the ISOs with significant periods of 10–25 and 30–45 days. The wet phase of 10–25‐day ISO was jointly induced by the anomalous convergence of the low‐level water vapour associated with the tropical–subtropical interaction and the upper‐level divergence attributed to the eastwards propagating wavetrain. However in the wet phase of the 30–45‐day ISO, the anomalous upper‐level divergence was possibly caused by the weaker East Asian trough and the stronger barotropic westerly in subtropics. The 30–45‐day ISO can facilitate the eastwards propagation of the 10–25‐day ISO by providing a benefit upper‐level waveguide. Afterwards, the low‐level southwesterly surrounding the Tibetan Plateau gets enhanced to bring more moisture into South China where the winter flooding was serious in 2015/2016. In contrast, the more‐than‐normal winter rainfall was weaker in the super El Niño of 1997/1998 due to the lack of the 30–45‐day ISO. Therefore, the strongest winter rainfall anomaly over SC was ascribed to the superposition between the wet phases of 10–25 and 30–45‐day ISOs in 2015/2016. [ABSTRACT FROM AUTHOR]

The boreal summer monsoon (BSM), which includes the monsoons over India (IND), the western North Pacific (WNP), East Asia (EA), North America (NAM), and North Africa (NAF), shows prominent interannual variation (IAV) in summer precipitation and affects the areas with the largest populations in the world. In the present, the EOF analysis is used to extract the BSM dominant covariation mode during 1979-2014. This mode is featured by the out-of-phase rainfall IAV over the WNP compared with the other BSM members. The BSM covariation mode is closely associated with the upper- and lower-level coupled circulations, which are characterized by two anomalous zonal circulations over the tropical oceans coupled near the date line and an abnormal meridional cell over the WNP and EA regions, respectively. Furthermore, the strength of this mode depends on the phase relationship of rainfall IAV between the WNP and NAM monsoon regions, which is modulated by the seasonal evolution of ENSO events and the resultant SST anomalies (SSTAs) in the tropical Indian Ocean (TIO). The weaker mode is accompanied by the in-phase rainfall IAV between these two regions, along with the persisting ENSO events and stronger SSTAs in the TIO from winter to summer. In the years with fast-decaying ENSO events and the related weaker TIO SSTA, the out-of-phase rainfall IAV between the WNP and NAMregion takes place to enhance this mode.A series of AGCM sensitivity experiments could reproduce the anomalies of atmospheric circulation related to the distinct seasonal evolution of ENSO events. [ABSTRACT FROM AUTHOR]