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  1. Abstract

    The Tibetan Plateau faces changing precipitation and environmental conditions affecting alpine ecosystems and downstream freshwater sustainability. While aerosol influence has been highlighted, how human-induced greenhouse warming impacts the plateau’s moisture recycling remains unclear. Here we show that the Tibetan Plateau’s recent precipitation changes result from enhanced precipitation recycling and moisture convergence that offset the decline in monsoon- and westerly-associated moisture transport based on 40-year Lagrangian simulations and water budget analyses. Local evapotranspiration is observed to increase faster in percentage than precipitation, a trend expected to continue in future warming scenarios according to climate projections. Greenhouse gas emission causes widespread wetting while weakening the southerly monsoons across the Himalayas, heightening the sensitivity of precipitation to evapotranspiration and thereby local land surface changes. This trend exacerbates vulnerability in the water cycle of high mountain Asia, calling for proactive management to address potential risks and ensure future water and food security in Asia.

     
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  2. Abstract

    Three consecutive precipitation extremes emerged in November 2021, including India-Sri Lanka flooding, East Asian blizzard, and Canadian floods. Why the catastrophic events occurred successively and whether they will become more frequent as global warming continues are unknown. Here we show they are organized by an intraseasonal Asian/North American (ANA) teleconnection consisting of two cross-Pacific wave trains fortified by dipolar diabatic heating anomalies (“wet India-dry Philippines”). The dipolar heating anomaly is shaped by multi-scale interaction between a quasi-stationary Madden-Julian Oscillation (MJO) episode and a rapidly developed La Niña over the tropical Asian monsoon region. Numerical experiments suggest that the off-equatorial heating dipole can generate the ANA pattern resembling observations, distinct from the equatorial MJO-induced teleconnection and the La Niña-induced Pacific/North American teleconnection. Philippine cooling stimulates the circum-Pacific wave train, while Indian heating produces the eastward-propagating subtropical wave train. These wave trains persistently steered cross-Pacific atmospheric rivers channeling warm-moisture-laden air to the extratropics. We suggest that the ANA teleconnection could be a new route by which multi-scale interaction between the La Niña and quasi-stationary MJO over the tropical Asian monsoon affects extratropical East Asia and North America. This work provides a unique perspective on understanding the origins of increasing collisions of extremes worldwide within a short time as the global climate warms.

     
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  3. Abstract

    The monsoon holds great significance in Asian‐Australian civilization. Recent studies realized the link between the monsoon onset and the seasonal migration of the Intertropical Convergence Zone (ITCZ). However, no clear ITCZ band is observed in the Asian‐Australian sector due to the strong influence of topography. Instead, there exists a large‐scale (∼1,500 km) tropical convective cell––a perennial system that we hereafter coin as the “intertropical convective cell (ITCC).” Using ERA5 reanalysis and satellite‐based outgoing longwave radiation products, here we show by objective detection and tracking that the ITCC exhibits eight phases during its seasonal migration along the Asian‐Maritime land bridge. Particularly, its sudden northward jump in boreal spring coincides well with the earliest (abrupt) onset of the Asian rainy season, while its equatorward retreat heralds the overall (gradual) monsoon withdrawal. These findings demonstrate the close link of the ITCC behavior to the spring‐fall asymmetry of the monsoon. Dynamically, the off‐equatorial ITCC features a monsoon regime with a cross‐equatorial overturning circulation, differing markedly from its equatorial regime with two weak overturning cells on each side. Further budget analyses prove our hypothesis that the north‐south charging gradient of the moist static energy determines the ITCC's spring‐fall asymmetric propagation, illuminating the physical origin of the spring‐fall asymmetry in the monsoon. Our results demonstrate the usefulness of the ITCC framework in understanding the Asian‐Australian monsoon complexity in a fresh and holistic manner. The framework will facilitate monsoon diagnosis, modeling and subseasonal‐to‐seasonal forecasting in the Asian‐Australian sector.

     
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  4. Abstract

    The Indian monsoon is of utmost concern to agriculture, the economy, and the livelihoods of billions in South Asia. However, little attention has been paid to the possibility of distinct subseasonal episodes phase-locked in the Indian monsoon annual cycle. This study addresses this gap by utilizing the self-organizing map (SOM) method to objectively classify six distinct subseasonal stages based on the 850-hPa wind fields. Each subseasonal stage ranges from 23 to 90 days. The Indian summer monsoon (ISM) consists of three substages, the ISM-onset, ISM-peak, and ISM-withdrawal, altogether contributing to 82% of the annual precipitation. The three substages signify the rapid northward advance, dominance, and gradual southward retreat of southwesterlies from mid-May to early October. The winter monsoon also comprises three substages (fall, winter, and spring), distinguishable by the latitude of the Arabian Sea high pressure ridge and hydrological conditions. This study proposes two compact indices based on zonal winds in the northern and southern Arabian Sea to measure the winter and summer monsoons, respectively. These indices capture the development and turnabouts of the six SOM-derived stages and can be used for subseasonal monsoon monitoring and forecasts. The spring and the ISM-onset episodes are highly susceptible to compound hazards of droughts and heatwaves, while the greatest flood risk occurs during the ISM-peak stage. The fall stage heralds the peak season for tropical storms over the Arabian Sea and the Bay of Bengal. The annual start and end dates of the ISM-peak are highly correlated (0.6–0.8) with the criteria-based dates proposed previously, supporting the delineation of the Indian monsoon subseasonal features.

    Significance Statement

    This research explores the existence of subseasonal features in the Indian monsoon annual cycle. Through the use of machine learning, we discover that the Indian summer monsoon and winter monsoon each consist of three substages. These substages’ evolution can be measured by two compact indices proposed herein, which can aid in subseasonal monsoon monitoring and forecasts in South Asia. Pertaining to hazard adaptations, this work pinpoints the subseasonal episodes most susceptible to droughts, heatwaves, floods, and tropical storms. High correlations are obtained when validating the substages’ yearly start and end dates against those documented in the existing literature, offering credibility to the subseasonal features of the Indian monsoon.

     
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  5. Atmospheric rivers (ARs) are long, narrow synoptic scale weather features important for Earth’s hydrological cycle typically transporting water vapor poleward, delivering precipitation important for local climates. Understanding ARs in a warming climate is problematic because the AR response to climate change is tied to how the feature is defined. The Atmospheric River Tracking Method Intercomparison Project (ARTMIP) provides insights into this problem by comparing 16 atmospheric river detection tools (ARDTs) to a common data set consisting of high resolution climate change simulations from a global atmospheric general circulation model. ARDTs mostly show increases in frequency and intensity, but the scale of the response is largely dependent on algorithmic criteria. Across ARDTs, bulk characteristics suggest intensity and spatial footprint are inversely correlated, and most focus regions experience increases in precipitation volume coming from extreme ARs. The spread of the AR precipitation response under climate change is large and dependent on ARDT selection. 
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