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Abrupt monsoon onsets/retreats are indispensable targets for climate prediction and future projection, but the origins of their abruptness remain elusive. This study establishes the existence of three climatological Madden-Julian Oscillation (CMJO) episodes contributing to the rapid Australian summer monsoon retreat in mid-March, the South China Sea (or East Asian) summer monsoon onset in mid-May, and the Indian summer monsoon onset in early June. The CMJO displays a dynamically coherent convection-circulation structure resembling its transitionary counterpart, demonstrating its robustness as a convectively coupled circulation system and the tendency of the transient MJOs’ phase-lock to the annual cycle. The CMJO is inactive during the boreal winter due to destructive year-to-year modulations of El Niño-Southern Oscillation. We hypothesize that the interaction between atmospheric internal variability (MJO) and the insolation-forced slow annual cycle generates the sudden monsoon withdrawal/onset during the boreal spring. Understanding the factors determining the timing and location of the MJO’s phase-locking and its variability is vital for monsoon forecasting and climate projection.

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.

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.

Recently, 2D electron gases have been observed in atomically thin semiconducting crystals, enabling the observation of rich physical phenomena at the quantum level within the ultimate thickness limit. However, the observation of 2D electron gases and subsequent quantum Hall effect require exceptionally high crystalline quality, rendering mechanical exfoliation as the only method to produce high‐quality 2D semiconductors of black phosphorus and indium selenide (InSe), which hinder large‐scale device applications. Here, the controlled one‐step synthesis of high‐quality 2D InSe thin films via chemical vapor transport method is reported. The carrier Hall mobility of hexagonal boron nitride (hBN) encapsulated InSe flakes can be up to 5000 cm²V⁻¹s⁻¹at 1.5 K, enabling to observe the quantum Hall effect in a synthesized van der Waals semiconductor. The existence of the quantum Hall effect in directly synthesized 2D semiconductors indicates a high quality of the chemically synthesized 2D semiconductors, which hold promise in quantum devices and applications with high mobility.