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2020 was exceedingly difficult for humans. As the world was experienced surge waves of COVID19, East Asia was also facing a one in a century, record-breaking flood, as the result of a super 47-day Meiyu/Baiu stage of East Asian summer monsoon. As East Asian monsoons (EAM) follow a yearly cyclical pattern, we wonder which stage(s) were collateral damages of the extended Meiyu. Was it an early termination of the anomalous dry Spring, or was it a delayed northward propagation of the rain belt, i.e. late Mid-summer? The hypothesis stems from our recent finding (Dai et al., 2020) that the duration of the Spring stage is informative for the onset of Meiyu, while the duration of Meiyu is negatively correlated with that of Mid-summer, i.e., the longer the Meiyu, the shorter the Mid-summer. To verify this, we first positioned the 2020 pre-Meiyu, Meiyu, Mid-summer stages in the 40-year climatology annual cycle (Dai et al., 2020). Although neither the onset nor the termination was beyond the 40-year variance, Meiyu indeed hastened to arrive but postponed its departure. Rain belt stalled over the Yangtze river basin and southern Japan since mid-June; until the end of July, a planetary-scale anomalous high pressure band was in place encompassing the Arabian sea and north Pacific. It hindered the South Asian monsoonal flow to the South China Sea, curbing the northward propagation of the rain belt with assistance by both southeast-ward shift of South Asian High and lower level high pressure system persistent over the northern China. With these observations, we put forward a framework of ocean-atmosphere coupled mechanisms that traces back to the summer in 2019, and reveal the climate teleconnection and circulation systems that pave the road to the 2020 super Meiyu. With this study, we address the question of whether the 2020 super Meiyu was a “black swan” or a manifestation of ongoing systematic changes of the EAM annual cycle? ReferenceDai, L., Cheng, T. F., & Lu, M. (2020). Define East Asian monsoon annual cycle via a self‐organizing map‐based approach. Geophysical Research Letters, 47. e2020GL089542. https://doi.org/10.1029/2020GL089542

The identification, climatic modulation and hydrological impact of Atmospheric Rivers (ARs) is an emergent scientific topic in recent years. ARs are important and yet understudied for East Asia (EA). We use our new AR identification algorithm (Pan & Lu, 2019), to build up a comprehensive AR catalog for this region for the first time. Interesting patterns are found: (1) there is a dominant AR route, originating from the Arabian Sea, crossing over the Bay of Bengal and Indochina, South China Sea (SCS) and Southeast China (SEC), and terminating in the western North Pacific; and (2) a nine-stage annual pattern in the climatological frequency is revealed. Stage 1: mid-Mar to mid-May, the formation of Western North Pacific Subtropical Height (WNPSH) near the SCS steers and confines AR in its northwest flank over SEC. Stages 2-5: during the monsoon season from mid-May to late-Aug, the evolution of AR follow the intra-seasonal progression of Asia-Pacific monsoon (including South Asian monsoon, East Asian monsoon and western North Pacific monsoon. Stages 6-9: late-Aug to mid-Mar, ARs leave EA and only occur over the North Pacific. Over all stages, we find the contribution of AR grows significantly with more extreme rainfall (i.e., from the annual rainfall, heavy rainfall, persistent heavy rainfall to large spatial extent persistent heavy rainfall), especially in spring and early-monsoon season. This emphasizes ARs’ significant role in extreme or catastrophic rainfall events. Intriguingly, divergence of AR trajectories (also in their characteristics) occurs along the extratropical direction, and such divergent features have spatially heterogenous dependence on the leading modes of a collection of steering atmospheric and regulating climatic signals. Large divergence indicates high sensitivity of AR to transient steering; while small divergence promises high predictability of ARs, thus their associated hydrological impacts.

Atmospheric river (AR), which is defined as long, narrow and transient corridor with enhanced moisture transport, received more and more scientific attention because of its crucial roles in the global water cycle, water resource management and hydrometeorological extremes. In recent years, dozens of AR identification algorithms are proposed to detect and quantify ARs. However, limitations still exist. In this study, a novel global AR identification algorithm is developed to address some limitations among all the state-of-the-art AR algorithms. First, in the AR pathway detection, a coupled quantile and Gaussian kernel smoothing technique is implemented to define the IVT threshold to make a balance in capturing the spatiotemporal variation of IVT climatology and avoiding largely biased estimation. Second, in spite of the variety of AR shape, orientation and curvature, more reliable AR metrics (e.g., length and width) can be determined based on the smooth AR trajectory, which is generated by modifying and integrating the concept of local regression and K-nearest-neighbors. Third, a robust and resilient criterion is developed to filter the tropical moisture swell. Four, an exquisite metric (turning angle series) is proposed, which is helpful to distinguish the tropical cyclone-like (TC-like) features and quantify the AR curvature which may bridge the ARs to the atmospheric circulation system. Last but not least, another novel metric () is developed to measure the localized IVT coherence on the AR pathway. For each grid, the is defined as the inter-decile range of the IVT direction of its neighbor grids. The IVT coherent/discordant segments on the AR pathway are extracted by an image segmentation algorithm according to their spatial pattern and values. The coherent segments are more likely to carry on long-distance moisture transport, governed by the persistent and large-scale circulation system and related to hydrometeorological extreme, while discordant segments are more likely to be corresponding to the localized turbulence, low pressure system or TC-like features. So, flagging the segments into different categories will be significant in the study of the climatic modulation of AR occurrence, intensity, spatial pattern and the associated rainfall predictability. We believe that this algorithm with various metric will facilitate further quantitative investigations by the AR research community in terms of water resource management, hydrometeorological extreme predictability and climate change projections.

The Tibetan Plateau (TP), being an average of surpassing 4000 m above sea level and around 2.5 × 106 km2, is the highest and largest plateau in the world and also called as the “Third Pole”. Due to its elevated land surface and complex terrain, the TP is subjected to combined regulations of multiple climate systems and associated large-scale atmospheric circulations. In this paper, we comprehensively review the recent studies of atmospheric moisture transport versus precipitation across the TP, with the attempt to link the two, which did not receive much attention previously. This review focuses on the atmospheric moisture transport and associated circulation patterns in this region, widely adopted approaches to identify the atmospheric moisture transport, qualitative and quantitative analyses for the role of water vapor transport on the precipitation, as well as the internal physical mechanism between atmospheric moisture transport and precipitation over the TP. Moreover, directions of future research are discussed based on the following aspects, which include 1) proposing an integrated statistical-physical framework for demonstrating the influence of atmospheric moisture transport and associated circulation patterns on the precipitation, especially the extremes, in the high-cold mountainous region; 2) quantifying the contribution of atmospheric water vapor from the surrounding sources as well as the local moisture recycling on the TP's precipitation; 3) providing higher quality data for atmospheric water vapor and precipitation; 4) emphasizing on the physical mechanism sustaining the atmospheric moisture transport as well as its potential influence on the extreme precipitation, including amount, frequency, intensity and duration. It is expected that this review will be beneficial for exploring the linkage between atmospheric moisture transport versus precipitation across the TP.