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We used local P and S waves, and regional Lg waves to investigate the Mw 4.9 Offshore Jeju Island earthquake, whose records show evidence of a complex rupture. This earthquake provides a rare window to understand the seismogenesis of moderate‐sized earthquakes on the southern Korea–East China Sea continental shelf. We computed the relative source time functions (RSTFs) by aligning the signals on the origin time of the main and its empirical Green's function (EGF) events, allowing us to use them as differential times of the EGF pair. We determined subevent locations using direct‐wave RSTFs, and captured the rupture variability of the two large subevents using waveform inversion of stacked Lg‐wave RSTFs. The first subevent rupture started by two weak nucleation phases and propagated slowly and bilaterally. Then the second subevent ruptured westward. Our analysis demonstrates that the Lg‐wave train observed at regional distances is useful in investigating detailed slip history. Plain Language Summary In‐depth studies of small‐to‐moderate‐sized earthquakes in offshore regions are often limited by sparse seismographic station coverage and a lack of close observations. Here, we demonstrate the effective use of local P and S waves and regional Lg waves to investigate the detailed rupture process of the 2021 Mw 4.9 Offshore Jeju Island, Korea earthquake. Lg wave is a guided S wave composed of a superposition of post‐critical reflections in the crust. Detailed seismological analyses using Lg waves revealed that earthquake rupture processes can be spatially complex even in areas with low seismic activity. Our analysis demonstrates that the Lg wave observed at distances greater than 150 km from the epicenter alleviates the limited local station coverage and can be a very useful signal to image the detailed slip history of the earthquake. Our analysis revealed the rupture complexity of the earthquake, expressed as a cascade of four sequential ruptures consisting of two small nucleation phases and two large subevents. Such detailed knowledge of earthquake rupture evolution is critical for understanding seismogenesis in this stable continental region setting. Key Points We demonstrate an approach to capture the complex rupture process of an offshore moderate‐sized earthquake using limited observations Direct P and S waves and Lg waves are used to image a spatiotemporal slip history of the Mw 4.9 Offshore Jeju Island, Korea earthquake Results show a cascading rupture of four subevents on a fault, offering a deeper understanding of event mechanism in low‐seismicity regions

Owing to the East Asian summer monsoon, extreme precipitation occurred frequently over the Yangtze River Basin (YRB), leading to flooding and secondary disasters. Therefore, understanding the physical mechanism and seeking predictability sources of extreme precipitation in the YRB are of scientific and practical importance. The present study examines the independent and compound impacts of leading sea surface temperature (SST) modes in the Indian Ocean and the North Atlantic Ocean on summer extreme precipitation days (EPDs) over the YRB. The Indian Ocean basin-wide uniform SST mode (the Indian Ocean Basin Mode) influences the EPDs over the YRB by inducing a Kelvin wave response and the Pacific–Japan pattern, whereas the two leading SST modes of the North Atlantic Ocean show different meridional tripole patterns with different climate impacts on East Asia. The North Atlantic SST tripole southern mode (NATS) induces quasi-stationary Rossby wave trains over mid-latitude Eurasia and the tropical waves that influence the EPDs in the YRB. The North Atlantic SST tripole northern mode (NATN) impacts the circulation anomaly over Northeast Asia through inducing different Eurasian quasi-stationary Rossby wave trains. When the IOBM and the NATS are both in the positive phase, enhanced EPDs occur over the YRB. On the one hand, the IOBM induces a Kelvin wave response, which strengthens the western North Pacific anomalous anticyclone (WNPAC). On the other hand, the NATS stimulates the mid-latitude quasi-stationary Rossby waves and results in the Northeast Asia anomalous cyclone (NEAC). The warm, moist air over the northwestern flank of the WNPAC and the cold, dry air over the southern flank of the NEAC converge in the YRB, leading to more EPDs in the region. When the IOBM and the NATN are out of phase, the Kelvin wave response in terms of the WNPAC induced by the IOBM warming is modulated by the negative phase of the NATN via quasi-stationary Rossby wave trains over mid-latitude Eurasia, resulting in more EPDs in the YRB. Based on the compound effect of different SST modes in the two ocean basins, the year-to-year EPDs over the YRB can be reconstructed reasonably well, which provides useful predictability sources for the seasonal prediction.

Compound heatwave and drought events (CHDEs) in South China (SC) have intensified in early autumn, yet their driving factor remains unclear. Based on reanalysis data and numerical experiments, this study investigates the potential influence of the summer northeastern Arctic Sea ice concentration (NEASIC) on the interannual variation of September CHDEs in the SC. Results demonstrate that positive NEASIC anomalies during summer trigger a quasi‐barotropic Rossby wave train, originating over the Greenland Sea, arching across the North Atlantic and the Mediterranean–Caspian region, and extending into East Asia. This wave dynamically drives a northward‐shifted and intensified East Asian subtropical jet and anomalous anticyclonic circulation over SC. The resulting subsidence induces moisture flux divergence, suppresses cloud cover, and enhances surface radiative forcing, explaining about 28.4% of the CHDEs variability per interquartile NEASIC increase. This mechanism enhances predictive frameworks for subtropical compound extremes, emphasizing the role of NEASIC in regional climate resilience strategies.

Understanding the variability and mechanisms of monsoon onset is extremely prominent for water management and rain-fed agriculture. Previous studies have shown a significant interdecadal advance in Asian summer monsoon (ASM) onset after the late-1990s and attributed it to the sea surface temperature anomalies (SSTA) in the tropical Pacific. However, the westward-propagating mechanisms revealed by previous studies (Walker circulation, equatorial Rossby wave response) are gradually decaying westward, which cannot explain the observational facts of stronger low-level winds over the Arabian Sea than the South China Sea. Based on longer datasets and multiple methods, this study reveals the influences of Pacific SST on the interdecadal changes of ASM onset through two eastward-propagating mechanisms: the equatorial Kelvin wave response to the SSTA in the equatorial central Pacific, and the extratropical Rossby wave train associated with SSTA in the subtropical North Pacific. These two eastward-propagating mechanisms mainly modulate the ASM onset via altering the meridional temperature gradient, which is more evident over the Arabian Sea and is more consistent with the observations. Special attention has been paid to the generation and maintenance of the extratropical Rossby wave train, which is less understood compared to the other mechanisms. This Rossby wave train can be excited by the upper-level divergence associated with the warm SSTA in the subtropical North Pacific. In addition, it can effectively gain available potential energy and kinetic energy from the basic flow, and exhibits strong positive interactions with the synoptic-scale eddies. This Rossby wave train is a newly recognized mechanism by which the extratropical Pacific SSTA influences the tropical ASM.

The leading pattern of boreal spring 250-hPa meridional wind anomalies over the North Atlantic and mid-high latitude Eurasia displays an obvious wave train. The present study documents the structure, energy source, relation to the North Atlantic sea surface temperature (SST), and impacts on Eurasian climate of this wave train during 1948–2018. This atmospheric wave train has a barotropic vertical structure with five major centers of action lying over subtropics and mid-latitudes of the North Atlantic, northern Europe, central Eurasia, and East Asia, respectively. This spring wave train can efficiently extract available potential energy from the basic mean flow. The baroclinic energy conversion process and positive interaction between synoptic-scale eddies and the mean flow both play important roles in generating and maintaining this wave train. The North Atlantic horseshoe-like (NAH) SST anomaly contributes to the persistence of the wave train via a positive air–sea interaction. Specifically, the NAH SST anomaly induces a Rossby wave-type atmospheric response, which in turn maintains the NAH SST anomaly pattern via modulating surface heat fluxes. This spring atmospheric wave train has significant impacts on Eurasian surface air temperature (SAT) and rainfall. During the positive phase of the wave train, pronounced SAT warming appears over central Eurasia and cooling occurs over west Europe and eastern Eurasia. In addition, above-normal rainfall appears over most parts of Europe and around the Lake Baikal, accompanied by below-normal rainfall to east of the Caspian Sea and over central Asia.

Based on daily precipitation data from the Chinese Meteorological Administration and reanalysis data from the National Centers for Environmental Prediction-Department of Energy, the character of low-frequency precipitation variability during the first rainy season (April–June) over South China and its corresponding atmospheric circulations in the mid-high latitudes are investigated. The results show that the precipitation anomalies during this period exhibit obvious quasi-biweekly oscillation (QBWO) features, with a period of 8–24 days. The influence of wave trains in the mid-high latitudes to low-frequency persistent heavy rain event (PHR-LF event, the 8–24-day filtered precipitation larger than one standard deviation of filtered time series and persisting at least three days over South China) is further discussed. During the first rainy season over South China, there are two low-frequency wave trains in the mid-high latitudes associated with the PHR-LF event—the wave train crossing the Eurasian continent and the wave train along the subtropical westerly jet. Analysis of wave activity flux indicates that the wave energy disperses toward eastern China along these two low-frequency wave trains from north to south and from west to east, and then propagates downward over South China. Accordingly, the disturbance of the relative vorticity of the cyclonic anomalies over eastern China is strengthened, which enhances the meridional gradient of relative vorticity. Owing to the transport of low-frequency relative vorticity and geostrophic vorticity by meridional wind, the ascending motion over South China intensifies and lasts for a long time, triggering a PHR-LF event. In addition, the tropical system is also a key factor to PHR-LF event. The QBWO of the convection over the South China Sea provide moisture for PHR-LF events, maintaining persistent rainfall and vertical ascending motion over South China.

South Asian summer monsoon (SASM) and westerlies are essential to the precipitation over Tibetan Plateau (TP) by transporting water vapor. Based on the latest reanalysis datasets, we investigate the compound mechanisms of SAMS and westerlies influencing summer precipitation over the TP. The results show that the summer precipitation over the TP is mainly modulated by two meridional Rossby wave trains which are linked to the compound impacts of SASM and westerlies over the TP (TPW). When both the SASM and the TPW are stronger, a cyclone over India and an anticyclone over Southeast Asia could jointly enhance the transport of warm and moist airflow to the TP along its southern boundary. Meanwhile, a cyclonic anomaly over the northwestern TP and an anticyclonic anomaly over the northeastern TP could induce the southward invasion of cold airflow. Consequently, the convergence of warm and cold airflows can result in more summer precipitation over the southern TP. On the contrary, when both the SASM and the TPW are weaker, a reversed pattern of above wave trains is stimulated, leading to more precipitation over the northeastern TP. Besides, with the stronger SASM and the weaker TPW configurated, anomalous moisture convergence (divergence) and ascending (descending) motion are found over the eastern (western) TP, producing more (less) precipitation over the eastern (western). Contrarily, with the weaker SASM and the stronger TPW configurated, the summer precipitation will be increased over the western TP. Overall, the compound impacts of SASM and TPW on the TP summer precipitation are significantly related to the dynamical processes instead of thermodynamic processes. Our results reveal the necessary of considering compound impacts of the SASM and TPW to explain the TP summer precipitation variation.

This study investigates the large-scale circulation anomalies induced by straight-moving tropical cyclones (TCs) over the western North Pacific (WNP) during winter. Corresponding to the straight-moving TCs, quasi-stationary wave trains appear as alternative geopotential height anomalies in the upper troposphere stretching from East Asia to the North Pacific. Specifically, the anomalous anticyclones are initially formed to the south of Japan and then lead to the subsequent anomalies over the Sea of Okhotsk and the Gulf of Alaska, respectively. Further analysis reveals that the upper-level anticyclonic anomalies are excited by negative Rossby wave sources, which are mainly attributed to the poleward vorticity advection by anomalous divergence relevant to TCs. In addition, the diagnosis indicates that the generation of wave source is caused by the product of the TC-induced divergent flows and the prominent meridional vorticity gradient in association with East Asian upper-tropospheric westerly jet. These findings imply that the tropical disturbances over the WNP, such as straight-moving TCs, can remotely affect weather over the extratropics, and thus have implications for improving the weather forecast over the extratropics through improving tropical disturbance forecast.

During the boreal spring, extratropical cyclones in East Asia exhibit the highest frequency of occurrence in northeastern China. These cyclones are characterized by a distinct cold core, which is commonly referred to as the northeastern China cold vortex (NCCV), and most of NCCV events are usually accompanied by complex and hazardous weather, significantly impacting the regional climate in northeastern China. In this study, an automatic algorithm was employed to identify spring NCCVs from 1979 to 2022 using ERA5 reanalysis dataset, and a notable annual periodicity of 3–4 years was observed based on the defined spring NCCV intensity index (NCCVI). Through the analysis of anomalous strong/weak NCCVI years, it was found that the consistent heating (cooling) in the middle and lower troposphere of central Asia (northeastern China), along with a corresponding negative anomaly of geopotential height, as well as the enhanced cyclonic circulation in northeastern China, all contribute to the intensification and maintenance of NCCV activities. Furthermore, the winter snow cover in central Asia plays a vital role in local atmospheric heating and is associated with anomalous quasi-stationary Rossby waves that propagate eastward into northeastern China. The thermal forcing caused by preceding (DJF) snow cover anomalies in central Asia was somewhat replicated in ECHAM 4.6 model simulations. However, the simulations underestimated the contribution of snow cover anomalies to the eastward propagating Rossby wave trains.

Prolonged low temperature significantly threatens transportation and planting in populous areas and is closely linked to slowly varying forcing such as sea surface temperature (SST). In this study, we reveal the impact of the north–south dipole pattern in extra-tropical North Atlantic SST (DNA) anomaly on surface air temperature (SAT) over southern China in February. The positive phase of the DNA is favorable for a positive North Atlantic Oscillation-like pattern at the troposphere via synoptic eddy vorticity forcing and triggers a southeastward-propagating Rossby wave train associated with the deepened East Asian trough and the strengthened Siberian high, which are conducive to enhancement of the East Asian winter monsoon. A stronger monsoon leads to lower-than-normal SAT over southern China by intensifying southward cold air outbreaks. Considerably, the extremely low temperature over southern China in February 2022 is explainable by this physical mechanism. Furthermore, experiments with a linear baroclinic model verify that the proposed downstream propagating Rossby wave train is mainly associated with the midlatitude North Atlantic SST anomaly. These results emphasize that the extra-tropical North Atlantic SST anomaly should be focused when interpreting and predicting the late winter SAT over southern China.