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Significant increase in precipitation around the middle and lower reaches of the Yangtze River valley (MLYRV) has been observed in the four months of June/July2020 and May/August 2021. The remarkable thing is, among these four months that belong to different stages of the East Asian summer monsoon (EASM) marching, it is the narrow and elongated westward extension of the Western Pacific subtropical high (WPSH) to the South China Sea (SCS) providing the direct conditions for the sustained rainy belt near MLYRV. The source of the anomalous subsidence core in the SCS which reinforces the westward trend of the WPSH is traced by using the three-pattern decomposition of the global atmospheric circulation (3P-DGAC) method. Quantitative vertical motion decomposition results show that the meridional circulation provides all the positive contributions to the subsidence. Further analysis found that the subsidence core in SCS is part of a particular tripolar meridional circulation anomaly structure which hold one subsiding branch around SCS and two ascending branches nearby the Maritime Continent (MC) and MLYRV. Such Maritime Continent-East Asia tripolar meridional circulation (MCEA-TMC) is confirmed as a typical dominant mode of meridional circulation exist in each month from May to August in the EOF results. Diagnostic study reveals the MCEA-TMC is a specific manifestation of joint effect of the multiple atmospheric factor in different latitudes. The enhanced meridional circulation from the MC to SCS is conducive to westward expansion of the WPSH and abundant water vapor to eastern China. The southward invasion of cold air in the middle-high latitudes meets the warm moist flow and forms convective convergence, limiting the northward expansion of the WPSH and stabilizing the MLYRV rain belt. This study extracted the MCEA-TMC as the stable indicator in reflecting the large scale conditions for the zonal rainy belt in the MLYRV from May to August. [ABSTRACT FROM AUTHOR]

Heat waves, droughts, and compound drought and heat waves (CDHWs) have received extensive attention because of their disastrous impacts on agriculture, ecosystems, human health, and society. Here, we computed the heat wave magnitude index (HWMI), drought magnitude index (DMI), and compound drought and heat wave magnitude index (CDHMI) for Yangtze River Valley (YRV) from July to August during 1961–2022. We compared the large-scale atmospheric circulation characteristics of different extreme events based on these indexes. The results show that the positive center with sink motion in East Asia provides a favorable circulation background for heat wave events. Drought events are mainly affected by the zonal wave train dominated by a significant negative anomaly in Siberia and a high-pressure anomaly upstream, and a anticyclonic water vapor with strong divergence over the Yangtze River basin. During CDHW events, both anomalous systems that affect heat waves and droughts appear and strengthen simultaneously. Specifically, in the middle and upper troposphere, the positive height anomaly center in YRV expands abnormally, and the "+–+" wave train over the northern 50° N region of East Asia becomes more obvious. Therefore, the positive anomaly and water vapor anomaly brought by the two circulation patterns at different latitudes are superimposed over the YRV, leading to severe CDHWs. At the same time, the warm positive eddy center and cold negative eddy center in high latitudes exhibit more stable positive pressure features, which are conducive to the persistent development and strengthening of CDHWs. In addition, the anomalous warm sea surface temperature in western Pacific moderating the favorable circulation patterns may also promote the occurrence of CDHWs in the YRV during the same period. [ABSTRACT FROM AUTHOR]

By using the hindcast and forecast data from the National Centers for Environmental Prediction Climate Forecast System version 2 (NCEP CFSv2) for the 1982–2018 period, we investigate the forecasting skills of the mid‐summer (July and August) surface air temperature (SAT) at interannual timescales in this study. Although CFSv2 predictions show a warm bias for the climatological mean SAT over the Yangtze River valley (25°–32°N, 105°–122°E), they show a consistent and great performance in predicting the interannual variability of the mid‐summer SAT over this region until 4 months in advance, where the linear correlation coefficient between the predicted and observed time series reaches +0.65, +0.51 and + 0.68 for 4, 2 and 0 months in advance, respectively. The CFSv2 predictions well simulate the linkage between the SAT anomalies over the Yangtze River valley and the anomalous atmospheric circulation aloft, including the circumglobal teleconnection and zonal extension of the Western Pacific Subtropical High. However, CFSv2 has trouble in simulating the associated vertical velocity, cloud cover and solar radiation anomalies, except for 0 months in advance. The persistent forecasting skills result from the accurate response of the circumglobal teleconnection and Western Pacific Subtropical High to the El Niño/Southern Oscillation and sea surface temperature (SST) anomalies over the mid‐latitude North Atlantic. Correspondingly, the forecasting skill, signal and signal‐to‐noise ratio are effectively improved in the years with strong mid‐summer SST anomalies over the tropical central‐eastern Pacific or mid‐latitude North Atlantic. These results are useful for understanding the predictability of the mid‐summer SAT over the Yangtze River valley. [ABSTRACT FROM AUTHOR]

Using the monthly reanalysis data sets of National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) for 1961–2015, the inter‐decadal change in inter‐annual water vapour transport over the tropical Indian Ocean–western Pacific during summer and the related mechanisms were analysed in this article. The results show that two major modes of anomalous water vapour transport over the tropical Indian Ocean–western Pacific (60°–140°E, 10°S–30°N) experienced a significant inter‐decadal change in the mid‐1980s. The first mode (EOF1) shows that the anticyclonic water vapour transport over the northwestern Pacific moves significantly southwards after the mid‐1980s. Before the mid‐1980s, the anomalous water vapour originates from the subtropical western Pacific and moves through the South China Sea to eastern China. However, the anomalous water vapour mainly originates from the tropical western Pacific and moves through the South China Sea to eastern China after the mid‐1980s. The second mode (EOF2) reflects the enhanced effect of the anomalous water vapour transport over the tropical Indian Ocean on East Asia after the mid‐1980s. Before the mid‐1980s, the distribution of the water vapour anomalies is uneven over the tropical Indian Ocean and the magnitude of water vapour is relatively small, so it has a weak effect on rainfall over East Asia. After the mid‐1980s, anomalous anticyclonic water vapour transport originates from the tropical Indian Ocean and moves along the Arabia Sea, Indian Peninsula and Indo‐China Peninsula and then forms an anomalous cyclonic water vapour transport over south China. Furthermore, the possible causes of the inter‐decadal transition in anomalous water vapour transport are investigated for the two major modes. The significant inter‐decadal transition in anomalous water vapour transport is associated with the enhanced effect of the sea surface temperature (SST). Before the mid‐1980s, the relationships between the two major modes and the SST of the tropical oceans are relatively weak. After the mid‐1980s, the EOF1 becomes significantly regulated by the El Niño‐Southern Oscillation (ENSO) in the previous winter, and the positive (negative) phase of the principal component of the first mode (PC1) corresponds to significantly positive (negative) SST anomalies over the tropical northern Indian Ocean and Maritime Continent during the same period. For EOF2, the anomalous water vapour transport over the tropical Indian Ocean is related to the enhanced inter‐annual variability of tropical Indian Ocean dipole (TIOD) after the mid‐1980s. [ABSTRACT FROM AUTHOR]

Record-breaking extreme precipitation occurred in July and August 2022 in Pakistan, causing flooding and landslides. In this study, the characteristics and mechanisms of these extreme precipitation events are investigated. The consecutive extreme precipitation events emerged in July and mid to late August. Precipitation above 500 mm occurred in the northern and southern regions of Pakistan, and the maximum precipitation was concentrated in the same regions. The diagnosis indicated that the precipitation extremes were closely tied to anomalous late spring snow cover in the western Tibetan Plateau. In July, owing to the reduction of late spring snow cover, the strengthened diabatic heating promoted the South Asian High (SAH) and its pumping effect, which contributed to abundant water vapor and strong southeasterly jet along the southwest Tibetan Plateau. Cooperating with the cyclone over Arabian Sea, extreme precipitation with a return period of 50 years occurred in Pakistan. In August, the SAH was an anomalously strong and eastward extension at 200 hPa. In the middle troposphere, the blocking high-pressure over western Russia was extremely active, and the West Pacific Subtropical High shifted anomalously westward, extending to the Tibetan Plateau. Cyclone anomalies occurred over the Arabian Sea and Bay of Bengal. With the atmospheric circulation anomaly, the southeasterly jet shifted southward, and water vapor was transported from the Arabian Sea and Bay of Bengal to south Pakistan, causing extreme precipitation with a return period of 30 years in August. • Late spring snow anomalies decreased over the western Tibetan Plateau contributes to Pakistan extreme precipitation in 2022. • The reduced snow intensifies the South Asian High and southeasterly jet which strengthen July Pakistan extreme precipitation. • The reduced snow promotes August Pakistan extreme precipitation by western Russia blocking and West Pacific Subtropical High. [ABSTRACT FROM AUTHOR]