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This study addresses how moisture in continental precipitation is recycled globally from a moisture-tracking perspective. Using a state-of-the-art three-dimensional Lagrangian model and optimized water accounting diagnostics, we complete a 40-yr (1971–2010) tracking of moisture from continental precipitation in the present-day climate. Climatologically, we conclude that 62% of continental precipitation stems from evapotranspiration through Lagrangian tracking—a measure is known as the global “continental precipitation recycling (CPR)” ratio. The result bridges the long-standing gap between the explicit (i.e., moisture trajectory–based) and implicit (i.e., water budget–based) estimates of the global CPR ratio in the literature. On the 1° grid scale, nonlocal terrestrial sources dominate precipitation in almost 70% of the land areas, most prominent in the continental interior. The CPR ratio consistently exhibits a contrasting seasonality between the mid-to-high latitudes and monsoon regions worldwide, from which two kinds of moisture source–regulated hydroclimate are generalized. Given the importance of CPR, we identify terrestrial source hotspots for continental precipitation that deserve careful governance. Using the backward “WaterSip” and a newly proposed forward “WaterDrip” diagnostics, we conceptualize and quantify two important types of cascading moisture recycling (CMR), whereby the moisture from an earlier source precipitates over an intermediate source and then re-evaporates to sustain precipitation farther downwind. The watershed-scale CMR metrics uncover the interdependence among the regional water cycles. Findings here advance the scientific understanding of the atmospheric water cycle as seen through Lagrangian tracking, and offer insights into transboundary watershed management and land-use planning to improve freshwater sustainability. Significance Statement Water is crucial for human civilization. There has been a century-long discussion on the moisture sources of continental precipitation. Using Lagrangian tracking, we show that 62% of continental precipitation stems from evapotranspiration, closing the gap with the budget-based estimate. Terrestrial sources dominate precipitation in 70% of Earth’s surface, especially in the interior of South America, Africa, and Eurasia. Global monsoon regions and the mid-to-high latitudes share a completely different source-regulated hydroclimate. Terrestrial source hotspots for continental precipitation that deserve conservation are identified. Two types of processes that formulate a cascade of regional water cycles are proposed and evaluated. Findings here advance the understanding of the origin of continental precipitation, offering insights into water and land management for freshwater sustainability.

In East Asia, summer monsoon rainbands usually stretch for thousands of kilometers in the east-west direction and are capable of producing heavy rainfall intensity of 20-40 mm day-1 on average, rendering them the culprit of many devastating historical floods. Despite the previous endeavor to understand their formation dynamics and hazards, the atmospheric water cycle of these rainband systems remains surprisingly poorly understood. In this study, we leverage backward moisture tracking to demystify the dominant moisture pathways and sources that feed the East Asian rain belt events during the warm season (April to September) from 1981 to 2018. The simulations were conducted using a semi-Lagrangian dynamical recycling model (DRM) forced by hourly-0.25˚ ERA5 reanalysis. In virtue of an Expectation-Maximization (EM)-based curve clustering, we classify up to 15 moisture pathways along four main corridors reaching the Somali Jet, South Asia, the Bay of Bengal and the Pacific basin. Long-range moisture pathways turn out to dominate high-impact monsoon rainbands, coinciding well with the role of planetary-scale atmospheric rivers in triggering extreme rainfall over East Asia. The result also highlights the importance of terrestrial moisture pathways and sources in supplying rain belts. Back-tracing the moisture pathways and atmospheric rivers unravels interesting couplings of pre-existing weather systems. The terrestrial moisture pathways over South Asia turn out to link to circumglobal wave trains at the upper levels up to a two-week lead time. The findings here bridge the knowledge gap in the regional hydrological cycle of the disastrous East Asian rain belts. The pre-existing weather systems uncovered by tracking the moisture and atmospheric rivers also provide potential predictability of heavy precipitation in East Asia.

Water is crucial for human health, food and industrial production, ecosystem services, and climate and weather systems. As a major contributor of renewable freshwater over land, humans have been studying the origin of continental precipitation for nearly a century. From the moisture budget perspective, local evapotranspiration in a vast part of the Earth’s surface is effectively smaller than local precipitation. This entails the role of moisture advection in sustaining continental precipitation. However, previous trajectory-based quantification appeared to underestimate the global “continental precipitation recycling (CPR)” ratio –– that is, the fraction of continental precipitation originating from evapotranspiration. To this end, the present study completed a 40-year (1971-2010) tracking of moisture from continental precipitation using a three-dimensional Lagrangian tracking model and optimized water accounting diagnostics. Our Lagrangian tracking confirms that 62% of continental precipitation stems from evapotranspiration, aligning well with the water budget-based estimates in the literature. Across the globe, non-local terrestrial sources dominate 1˚-scale precipitation in nearly 70% of the land areas, together with the greatest continental moisture feedback in the interior of South America, Africa and Eurasia. Seasonally, the CPR ratio anomalies are markedly different between the mid-to-high latitudes and monsoon regions worldwide, from which two kinds of moisture source-regulated hydroclimate are generalized. For transboundary water governance, perennial source hotspots for continental precipitation are identified, including the biome-rich Amazon and Congo rainforests and other major watersheds within 30˚ equatorward. Leveraging the backward “WaterSip” and the forward “WaterDrip” algorithms, we propose two ubiquitous processes of cascading moisture recycling (CMR) that formulate a cascade of regional water cycles. The watershed-scale CMR metrics quantify the hidden interdependence between the regional water cycles through moisture recycling. Overall, by closing the gap in the estimate of the CPR ratio, this work updates the understanding of the moisture recycling, feedback and cascading characteristics of the continental atmospheric water cycle. The outcome sheds light on the potential vulnerability of local precipitation in response to the modification of non-local land surface fluxes by human activities.

East Asia will face a skewed monsoon cycle with soaring flood, drought, and weather whiplash risks in a warming climate. In our objective eight-intraseasonal-monsoon-stage framework, we uncover a ‘dry-get-wetter’ paradigm in East Asia, contesting the fallen ‘rich-get-richer’ common belief. On timing, the Mid-summer and Fall periods are stretching at the expense of three delayed, shortened, and weakened winter stages, especially near the end of the twenty-first century. On threats, entire East Asia will experience up to 14–20 more heavy precipitation days during the rainy Spring to Mid-summer stages. Specifically, the Yangtze basin will suffer from an earlier pluvial period with escalating flood risks. Moreover, societal security and ecosystem resilience in the Huai-Yellow basin, South Japan, and the Korean Peninsula will be challenged by more frequent weather whiplash. Under the monsoon-stage framework, a complete moisture budget decomposition sheds light on the causes of a slower precipitation scaling and the ‘dry-get-wetter’ paradigm.

This study aims to construct a novel source–receptor (SR) network to study the atmospheric water cycle associated with the East Asian summer monsoon (EASM) circulation. Using a dynamical recycling model (DRM), 68%–74% of the wet season (April–September) precipitation in six EASM land regions is attributed. The results reveal that terrestrial sources can be equally or more competitive than oceans for several sink regions downwind in East Asia. Terrestrial sources, such as the Indian subcontinent, Indochina, Southwest China, and the eastern Tibetan Plateau, are sustained by southwesterly monsoons and contribute to appreciable fractions of precipitation in the East Asian subregions downwind. Further, southwesterly and southeasterly sources for a sink region alternately dominate the moisture supply in the early and late wet season, respectively, referred to as the “SW–SE source swing.” The SR network is found to be largely governed by the zonal oscillation of the western North Pacific subtropical high and tropical cyclones. Knowledge about the coupled circulations might promise more predictability of the strength of the affected SR pairs. Notably, enhanced moisture supplies from regions such as the Indian subcontinent and Tibetan Plateau are well correlated with an upper-level wave train from western Russia. Finally, the preceding wintertime El Niño may favor (suppress) the moisture contribution of southwesterly (southeasterly) sources in the following wet season. The findings offer insights into the EASM water cycle and the governing circulations, and also accentuate the role of upwind terrestrial sources in the downwind precipitation and freshwater resources.

This paper scrutinizes the zonal oscillation of the western North Pacific subtropical high (WNPSH) via diagnosing its two extreme phases, which are defined by the top 10% strongest (positive phase) and the weakest (negative phase) WNPSH index (WNPSHI) days during summers in 1979–2016. Key findings include the following: a tripole pattern consisting of intensified (weakened) precipitation over the Maritime Continent and the East Asian summer monsoon regions, and suppressed (strengthened) precipitation over the western North Pacific summer monsoon region during positive (negative) WNPSH phases; a westward movement of WNPSH-induced precipitation anomalies that subsequently affects eastern China, Japan, and the Korean Peninsula at different time lags; an OLR–vorticity pattern explained by atmospheric responses to thermal sources is suggested to drive the oscillation; and the competitive interaction of local air–sea feedbacks, especially during the positive phase. In addition, moderate-to-strong positive correlations between the WNPSHI and the Niño-3.4 index are found on 1–2-, 2–3-, and 3–6-yr time scales; both exhibit decadal shifts to a higher-frequency mode, suggesting the intensification of both the zonal WNPSH oscillation and the ENSO under the changing climate and their close interdecadal association. A nonlinear quasi-biennial WNPSH–ENSO relationship is identified: the positive (negative) WNPSH phase sometimes occurs during 1) a decaying El Niño (La Niña) in the preceding summer/autumn, and/or 2) a developing La Niña (El Niño) in the current summer/autumn. A full ENSO transition from moderate-to-strong El Niño to La Niña is often seen during the positive phase, offering potential in predicting ENSO events and extreme WNPSH phases and thereby the summer monsoon rainfall in East Asia.

Ozonolysis and autoxidation have been realized as important pathways for the heterogeneous ozone oxidation of polyunsaturated fatty acids, but the relative significance of these two pathways under ...

As the world population continues to rise under global warming, it becomes increasingly urgent to understand the climate system and atmospheric water cycle, which could be beneficial for assessing future freshwater resources and land-use management. Recent endeavors by the regional studies were put to identify the source-receptor network and synoptic-scale moisture transport in major river basins worldwide affected by monsoons. Notably, growing studies suggest the transboundary upwind moisture sources are, in fact, very crucial to the intensity and variability of precipitation in the downwind areas, which arouses the call for international governance over land-use and water management. Recognizing the need for international governance on moisture sources from different regional studies and considering results from many moisture-sink-orientated studies, it, however, remains largely unclear where exactly are the moisture source hotspots that are shared by most countries and societies. Such information would better facilitate the international attention, effort and policymaking to safeguard those influential moisture hotspots. Further, more scientific questions need to be addressed: how these global moisture hotspots vary in time and space for the past few decades, and how would these changes be attributed to the known climate events and even human activities.To these ends, we utilize a state-of-the-art three-dimensional Lagrangian model, the FLEXible PARTicle dispersion model (FLEXPART), to homogeneously divide the atmosphere into six million parcels with roughly equal masses and simulate their movements from 1971 to 2010. Instead of focusing on a particular sink region, all the moisture released over land is backtracked to construct a map of moisture hotspots throughout seasons. As surprising as it may seem, the results suggest the majority of global moisture source hotspots for land precipitation are also terrestrial, especially those located in the Amazon rainforest, the Congo rainforest, the Ganges river basin, the Mekong river basin and the Yangtze River basin. Most of these hotspots also situate in monsoonal domains where their strengths vary significantly across seasons. Given also significant interannual variabilities and long-term trends in the strength of these globally shared moisture hotspots, we suspect that climatic events, global warming and urbanization processes could be attributable to the changes in the hotspots. Findings from this work would advance our knowledge of the location of global moisture hotspots that are key to the precipitation over land. Understanding the possible linkages between the hotspots’ changes and climatic events and human-related activities could benefit long-term planning of regional and international strategies for securing freshwater resources.

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

This study aims to advance the understanding of the pre-existing weather regimes up to two weeks ahead of rain belt events during the East Asian summer monsoon season. To this end, we backtrack the...

There has been growing interest in studying precipitation recycling and identifying relationships between moisture sources and receptors. The network built upon the relationships is crucial for the knowledge of the atmospheric water cycle, weather prediction, and adaptation to hydroclimatic disasters. This study aims to provide an interesting perspective of a Source-to-Receptor (SR) network to study the dynamics of the East Asian Summer Monsoon (EASM). By prescribing 24 sources and 6 EASM subregions, the SR network during the wet season is quantified using the two-dimensional physically-based Dynamical Recycling Model (DRM). Results reveal that in addition to oceanic sources, land sources including the often-overlooked plateau regions play an important role in supplying moisture to most EASM subregions. A seesaw relationship of the Indian Ocean/South Asia sector from April to June and the Pacific Ocean/East Asia sector from July to September is evidenced in the intraseasonal variation of the SR network for EASM subregions including South China coast and Taiwan, Yangtze River basin, South Japan and Korean Peninsula. Conversely, weaker intraseasonal variation is seen in the SR network for the Yellow River basin and North China. During heavy rainfall days, the zonal oscillation of western North Pacific Subtropical High (WNPSH) is deemed crucial to modulate the SR network through enhanced contributions from Bay of Bengal, Indochina, Indian subcontinent and Southwest China (the Philippine Sea and western North Pacific) during the positive (negative) phase. Coupled circulations such as two distinct pressure dipoles and coherent upper-level wave trains from mid-latitudes are responsible for bridging the moisture routes. Lastly, preceding winter/springtime El Niño is likely associated with the enhanced (weakened) moisture supply from the southwesterly (Pacific Ocean) sources. Longer-term variabilities such as the Pacific Decadal Oscillation is also considered influential to the SR network. We believe that the attributable atmospheric bridges and the SR network itself can offer insights to the current understanding of EASM and model simulations of the monsoon systems and the water cycles.

The East Asian Summer Monsoon (EASM) is a crucial monsoon system that profoundly influences the summer climate in Southeast China (SEC). Classification of monsoon rainfall patterns is vital to physical diagnosis, rainfall prediction and identification of sites that are prone to rainfall-triggering floods. With the great endeavors on understanding the complexity of the EASM in the past decades, the traditionally accepted rainfall patterns in SEC and the relevant analyses appear outdated or even inadequate. Having highly-improved observations at hand helps update the monsoon rainfall patterns in SEC and the potential predictability.The present study employs a nonlinear neural network classification technique, the Self-organizing map (SOM), to identify the rainfall patterns in SEC based on gauge data. Three distinct rain belts over the Huai River basin (HRB), lower Yangtze River basin (LYRB) and South Coast region (SCR) are found. Their subseasonal variability highly agrees with the stepwise progression of the East Asian Summer Monsoon (EASM) front in space and time. Analysis reveals that precipitation in the SCR and HRB rain belts undergo a regime shift after the mid-1990s, whereas the 1990s is the most active decade for the LYRB rain belt. These systematic changes are in abreast with similar changes in EASM and other climate events documented in the literature.Additionally, a SOM-based algorithm is developed to further divide gauge stations into three groups featuring homogeneous rain belt patterns. Promising predictability of group-averaged daily rainfall is then achieved, with about 39% to 50% of the total variance explained by circulation-informed regression models, verified by both cross-validation and blind prediction. Through further diagnosis in the useful predictors, the western North Pacific subtropical high, blocking high anomalies over northeast China and the upper-level divergence over SEC, are found to best explain the variability of the rain belts. The proposed Russia-China wave pattern (western/central Russia → north of Tibetan Plateau → SEC) and teleconnection between the El Niño-Southern Oscillation and the rain belts also offer additional predictability. This study aims to set an updated benchmark on the summer monsoon rainfall patterns in SEC, from which the promising daily predictability and the informative circulation patterns are obtained. Findings from this work may also advance the understanding of the EASM rain belts, and offer insights to the source of bias for numerical simulations of daily summer monsoon rainfall in the region.