Using a Lagrangian trajectory model, contributions of moisture from the Indian Ocean (IO), the South China Sea (SCS), the adjacent land region (LD), and the Pacific Ocean (PO) to interannual summer precipitation variations in southwestern China (SWC) are investigated. Results show that, on average, the IO, SCS, LD, and PO contribute 48.8%, 21.1%, 23.6%, and 3.7% of the total moisture release in SWC, respectively. In summers with the above-normal precipitation, moisture release from the IO and SCS increases significantly by 41.4% and 15.1%, respectively. In summers with below-normal precipitation, moisture release from the IO and SCS decreases significantly by 44.2% and 24.6%, respectively. In addition, the moisture anomalies from the four source regions together explain 86.5% of the total interannual variances of SWC summer precipitation, and the IO and SCS only can explain 75.7%. Variations in moisture transport from the IO, SCS, and LD to SWC are not independent of one another and are commonly influenced by the anomalous anticyclone in the western North Pacific Ocean, which enhances the moisture transport from the IO and SCS by the anomalous southwesterlies over its northwestern quadrant but reduces that from the LD east of SWC by the anomalous westerlies along its northern edge. Anomalous warming in the tropical Atlantic Ocean can modify the Walker circulation, induce anomalous descending motion over the central tropical Pacific, and excite the anomalous anticyclone in the western North Pacific as the classic Matsuno–Gill response. The observed impacts of the tropical Atlantic warming on the anomalous anticyclone and summer precipitation in SWC can be well reproduced in an atmospheric general circulation model.

Significant anomalies in frequency of summer extreme hot day (SEHD) are broadly observed in the Asian monsoon region (AMR) in the post-ENSO summers. The delayed ENSO impacts are mainly conveyed by provoking the Indo-western Pacific Ocean capacitor (IPOC) effect that maintains the anomalous anticyclone in the western North Pacific. The related diabatic heating anomaly can trigger the westward propagating Rossby wave to the Indian subcontinent, which increases the geopotential heights, reduces the cloud cover, and thus increases the seasonal surface temperature and SEHD frequency in the southern AMR. Besides, the reduced atmospheric moisture in the western North Pacific hinders the northward propagation of intraseasonal oscillation (ISO) and modulates the occurrence frequency of individual ISO phases, contributing to the significantly increased/decreased SEHDs in eastern China/Hokkaido of Japan in the post-El Niño summers. The 25-model-ensemble mean of CMIP6 historical runs can reproduce well the observed SEHD anomalies in the southern AMR in the post-ENSO summers mainly due to the realistic simulation of ENSO impacts on the seasonal surface temperature, although a large inter-model spread exists due to different strength of IPOC effect in each model owing to model biases in the mean state of eastern tropical Pacific, the ENSO variance and teleconnection to the Indian Ocean. Furthermore, future projections under SSP5-8.5 scenario show that the delayed ENSO impacts on the southern AMR remains stable under global warming via the similar mechanism as in the observations and historical runs.

In the past decades, with the increasing frequency of extreme weather and climate events, the world has suffered huge losses. Based on NCEP/NCAR reanalysis data and China regional precipitation data provided by China Meteorological Administration, the extreme precipitation events in eastern China are defined by relative threshold method, and the temporal and spatial characteristics of summer extreme precipitation in eastern China from 1961 to 2016 are analyzed by empirical orthogonal function (EOF), and the reverse distribution of extreme precipitation in the middle and lower reaches of the Yangtze River and south China by Indian Ocean warm pool is revealed influence. The results show that the total amount and frequency of extreme precipitation in summer are concentrated in the Yangtze River Basin and south China. EOF1 decomposition of extreme precipitation reflects the interannual oscillation characteristics of reverse spatial distribution in the Yangtze River Basin and south China. The time series corresponding to EOF1 has significant interannual characteristics. The Pacific-Japan (PJ) teleconnection pattern is a circulation system that significantly affects the spatial-temporal pattern of extreme precipitation in southern China. When the PJ pattern is in the positive phase, the anticyclone controls the south China region, and restrains the convective activity, which results in the decrease of extreme precipitation. The anomalous southwest wind to the south of 30˚N and the anomalous northerly wind to the north of 30˚N converge in the middle and lower reaches of the Yangtze River. Combining with the sufficient water vapor carried by the anomalous southwest airflow at the edge of anticyclone, it is more conducive to the formation of extreme precipitation. The east propagating Kelvin wave in the warm pool of the Indian Ocean is an important reason for the formation of the PJ pattern and finally the formation of extreme precipi

The extreme precipitation (EP) in the early- and late-rainy seasons in Southern China is investigated from the perspective of moist static energy (MSE). At the synoptic timescale, the EP is accompanied by the charge-discharge paradigm of the vertically integrated MSE (<MSE>); the positive <MSE> anomaly reaches the peak one day before EP and decreases quickly during the event. The charge-discharge paradigm of <MSE> is dominated by the horizontal and vertical advection, respectively. However, synoptic systems responsible for the <MSE> charge in the early- and late-rainy seasons are different due to the different horizontal distributions of climatological MSE in the lower troposphere caused by the northward migration of solar radiation and monsoon system. At the interannual timescale, more EP in the early-/late-rainy season is associated with the higher seasonal-mean <MSE> that can be caused by the anomalous anticyclone/cyclone in the western North Pacific induced by the SST anomalies in the tropical Indian Ocean and central North Pacific/the tropical Pacific. The multi-model ensemble mean of CMIP6 models reproduces well the observed <MSE>-EP relationship in both the historical and SSP5-8.5 runs. Moreover, the mean state of <MSE> increases in the SSP5-8.5 compared to historical runs along with more frequent occurrence of EP events. Hence, <MSE> can serve as a useful metric for studying EP in Southern China at various timescales.

Accurate prediction of extreme weather and climate is crucial for their devastating impacts on our society and environment. Here, predictability of frequency of summer (June–August) extreme hot days (SEHDs) in China has been assessed in the SINTEX‐F2 seasonal forecast system during the period of 1983–2015. The hindcast had 12 ensemble members and was initialized on the first day of March, April and May, respectively. Results show that overall, the SINTEX‐F2 predicts more regions with good prediction skills at shorter lead time due to its better capture of the linear trends. Whereas, in Southwest China and eastern Tibetan Plateau, the prediction skills are consistently increased at shorter lead time even without the impacts of linear trends; the correlation coefficients between the region‐mean anomalies in the observed and predicted SEHD frequencies are 0.74, 0.68 and 0.61 at 1–3 month lead, respectively, and they remain as high as 0.56, 0.49 and 0.40 when the linear trends are removed. This is because the SINTEX‐F2 can reproduce the observed influences of Indian Ocean Basin Mode (IOBM) on the SEHD frequency. The warm IOBM can cause anomalous ascending and strong divergence in the upper troposphere over the tropical Indian Ocean and Indian subcontinent. The strong divergence causes direct convergence and descending over Southwestern China and eastern Tibetan Plateau. It also causes substantial descending over the western North Pacific that can introduce anomalous cyclonic circulation over northeastern Asia via the Pacific‐Japan or East‐Asia pattern. The latter serves as a source of wave energies, excites the Rossby waves meandering westward in the mid‐latitude, and enhances the anticyclonic circulation anomalies over Southwestern China and eastern Tibetan Plateau.