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Studies of the multi-scale climate variability of the Asian monsoon are essential to an advanced understanding of the physical processes of the global climate system. In this paper, the progress achieved in this field is systematically reviewed, with a focus on the past several years. The achievements are summarized into the following topics: (1) the onset of the South China Sea summer monsoon; (2) the East Asian summer monsoon; (3) the East Asian winter monsoon; and (4) the Indian summer monsoon. Specifically, new results are highlighted, including the advanced or delayed local monsoon onset tending to be synchronized over the Arabian Sea, Bay of Bengal, Indochina Peninsula, and South China Sea; the basic features of the record-breaking mei-yu in 2020, which have been extensively investigated with an emphasis on the role of multi-scale processes; the recovery of the East Asian winter monsoon intensity after the early 2000s in the presence of continuing greenhouse gas emissions, which is believed to have been dominated by internal climate variability (mostly the Arctic Oscillation); and the accelerated warming over South Asia, which exceeded the tropical Indian Ocean warming, is considered to be the main driver of the Indian summer monsoon rainfall recovery since 1999. A brief summary is provided in the final section along with some further discussion on future research directions regarding our understanding of the Asian monsoon variability.

The dynamic and thermal effects of the Tibetan Plateau (TP) on the precipitation in the Asian arid and monsoon regions were investigated using three numerical experiments—one using real topography, one with the whole TP removed, and one with sensible heat turned off over the TP. The results show that there are strong seasonal and regional differences in the dynamic and thermal effects of the TP on the precipitation in the Asian arid regions. The dynamic effect dominated the decrease in winter precipitation by blocking the westerly, while the thermal effect dominated the decrease in summer precipitation due to the TP-induced compensation downdraft in Central Asia and arid East Asia. The thermal effect dominated and accounted for 60% of the decrease in summer precipitation in West Asia. The results also show that both the dynamic and thermal effects of TP exhibit a more salient influence on the East Asian monsoon region than the South Asian monsoon region. The thermal effect dominated and accounted for 40% of the increase in summer precipitation due to intensification of the summer monsoon, while the dynamic effect dominated and accounted for 80% of the decrease in winter precipitation due to the northeast wind anomaly in the northern East Asian monsoon region. The anomalous wind can reach to the coast of South China and form frontal precipitation in the southern East Asian monsoon region in winter. The thermal effect dominated and accounted for 80% of the increase in precipitation in the pre-monsoon period due to intensification of the Asian summer monsoon.

Located in a monsoon domain, East Asia suffers devastating natural hazards induced by anomalous monsoon behaviors. East Asian monsoon (EAM) research has traditionally been a high priority for the Chinese climate community and is particularly challenging in a changing climate where the global mean temperature has been rising. Recent advances in studies of the variabilities and mechanisms of the EAM are reviewed in this paper, focusing on the interannual to interdecadal time scales. Some new results have been achieved in understanding the behaviors of the EAM, such as the evolution of the East Asian summer monsoon (EASM), including both its onset and withdrawal over the South China Sea, the changes in the northern boundary activity of the EASM, or the transitional climate zone in East Asia, and the cycle of the EASM and the East Asian winter monsoon and their linkages. In addition, understanding of the mechanism of the EAM variability has improved in several aspects, including the impacts of different types of ENSO on the EAM, the impacts from the Indian Ocean and Atlantic Ocean, and the roles of mid- to high-latitude processes. Finally, some scientific issues regarding our understanding of the EAM are proposed for future investigation.

The Asian Monsoonal rainfall accounts for the majority of annual regional precipitation in East and South Asia and could be remotely regulated by El Niño‐Southern Oscillation (ENSO). Besides, several paleoclimate records and simulations have indicated solar signals in the Asian Monsoon, implying the impact of solar activity on the regional monsoon precipitation. By conducting multi‐linear regression analysis to the solar irradiance forced single‐forcing experiment in the last millennium, this study presents the comparison of solar and ENSO effects on monsoonal precipitation in South and East Asia during early summer (May–June). Increased total solar irradiance during high solar activity years tends to trigger a favorable environment for developing monsoon onset, leading to more precipitation against ENSO‐related patterns over Southeast and South Asia before peak‐summer (July–August). The result supports reconstructed terrestrial records and underlines considerable influences of the solar cycle on the variation of the Asian Summer Monsoon. Plain Language Summary Previous studies have shown the influence of the solar cycle on various climate systems, such as El Niño‐Southern Oscillation (ENSO), Atlantic Oscillation, Pacific Decadal Oscillation, etc., despite the relatively small amount of solar irradiance variations to the total amount of solar insolation. In addition, paleo‐records indicate the correlation between the long‐term solar cycle and the variation of the Asian Monsoon. This study attempts to clarify the role of solar activity in monsoonal precipitation over South and East Asia. We find that during early summer (May–June), high solar activity tends to enhance the meridional temperature gradient over the northern Indian Ocean. An associated meridional circulation is thereby intensified and well‐coupled with monsoonal circulations. The coupling contributes to more moisture transport and increased precipitation over the subcontinent, contrasting with the reduced rainfall and suppressed monsoonal circulation over the northern Indian Ocean during warmer ENSO events. This result indicates that the effects of changes in solar irradiation on climate systems should be considered, particularly in the Asian Monsoon region. Furthermore, a physical explanation is provided for the solar signals in the Asian Monsoon in paleo‐evidences. Key Points The Multi‐Linear Regression analysis is used to isolate solar activity and El Niño‐Southern Oscillation (ENSO) signals from the CESM‐LME project's solar‐only experiment Sub‐seasonal analyses to the Asian Summer Monsoon are highlighted by varied responses to solar activity in early and peak summer Higher solar activity increases early summer rainfall around Southeast Asia against ENSO‐related patterns by enhancing monsoonal circulation

The spatio-temporal characteristics and driving mechanisms of the Asian summer monsoon (ASM) during the early and middle Holocene remain unclear. Moreover, the timing of maximum monsoon precipitation in the Asian monsoon margin of northwest China during this period is also a subject of considerable debate. Herein, we analyzed phytolith assemblages within 55 calcareous root tube (CRT) samples from the hinterland of the Tengger Desert to quantitatively reconstruct Holocene millennial-scale precipitation changes and discuss the potential forcing mechanisms. Our results revealed that the mean annual precipitation (MAP) was 138 ± 53 to 149 ± 18 mm during 10.0–7.0 cal kyr BP, 179 ± 26 to 192 ± 26 mm during 7.0–5.0 cal kyr BP, and 129 ± 20 to 161 ± 13 mm during 5.0–1.0 cal kyr BP. The quantitative reconstruction results suggested that precipitation in this area was low in the early Holocene, then increased to a maximum in the middle Holocene (30% higher precipitation than present), and gradually decreased in the late Holocene. Maximum monsoon precipitation did not occur until the middle Holocene in this area because the northernmost margin of the ASM reached its northernmost latitude lagging behind its maximum monsoon intensity suppressed by high-northern latitude ice volume forcing during the early Holocene despite high summer insolation. In addition to quantifying the Holocene millennial-scale precipitation changes in the northwestern margin of the ASM, this study also provides new evidence for spatial and temporal variations of the ASM during the Holocene.

The secular change of the Asian monsoon (AM)-El Niño–Southern Oscillation (ENSO) relationship has been recognized as a specter for seasonal forecast. The causes of such changes have not been well understood. How the monsoon-ENSO relationship underwent secular changes beyond instrumental period has rarely been discussed. Here we explore the multidecadal to centennial changes of the AM-ENSO relationship with the recently compiled Reconstructed Asian summer Precipitation (RAP) dataset (1470–2013) and multiple ENSO proxy indices. During the past five centuries, two leading modes of interannual variability of RAP are found to be associated with the ENSO developing and decaying phases, respectively. The mechanisms behind the modern monsoon-ENSO relationship can reasonably well explain the past monsoon behavior. In response to a developing ENSO, precipitation anomalies from the Maritime Continent (MC) via India to northern China are in phase, and this “chain reaction” tends to be largely steady since around 1620 AD when the Indian summer monsoon abruptly strengthened. Further, the strengthening of the link between developing-ENSO and Indian-northern China rainfall since 1620 AD concurred with a phase reversal of the Pacific Decadal Oscillation. During the decaying phase, however, the summer rainfall-ENSO relationship over the Yangtze River Valley-southern East China (YRV-SEC), the MC and central Asia, has gone through large multidecadal to centennial changes over the past five centuries. A remarkable reversal of sign in the AM-decaying ENSO relationship occurred roughly from 1740 to 1760 over the YRV-SEC and MC, which may be associated with the long-term strengthening of ENSO intensity. Future research should continue focusing on revealing the possible causes of the low-frequency changes in the monsoon-ENSO relationship using general circulation models and paleoclimate proxy reconstructions.

Stalagmite is one kind of secondary carbonates formed in limestone caves (speleothem). After cave water droplets containing Ca 2+ and H C O 3 − drip onto floor, carbonate in the water might become supersaturated due to CO 2 degassing under certain conditions, resulting in the formation of stalagmite in a process year after year. Stalagmite is one of important geological archives for paleoclimate research. The advantages include wide spatial distribution, suitable for U-Th and U-Pb dating, enriched in climate proxies, continuity, long time span, comparability and lower sampling cost etc. These factors have propelled stalagmite paleoclimate research to the forefront of global paleoclimatology with an irreplaceable role. The stalagmite paleoclimate study started in the western countries, mainly in Europe and America in 1960s–1970s, while the relevant research in China was progressively developed in the 1980s–1990s after the Reform and Opening up. Although there was a huge gap between the overall research level in China and western countries, a solid research foundation, as well as a number of talent teams were established during the period. In the 21st century, starting from the publication of stalagmite records from Hulu Cave in Nanjing in 2001, the stalagmite paleoclimate research in China has ushered in a flourishing development and a real leap on the basis of international cooperation, resulting in significant international impacts. The landmark achievements, including establishment of the world’s longest (640000 years) East Asian monsoon stalagmite record, as well as the longest Indian monsoon (280000 years), South American monsoon (250000 years), North American westerly climate (330000 years), Central Asian westerly climate (135000 years), and northwestern China westerly climate (500000 years), have laid a milestone in the paleoclimate study in these climate domains. Importantly, these stalagmite records have revealed the relationship of Asian monsoon variations with solar insolation climate change in polar regions, and the South American monsoon changes on orbital-suborbital timescales, which have provided new geological observations for the development of orbital-suborbital climate theory; elaborated coupling and differentiation relationships between the Asian monsoon and the westerly climate; reconstructed the history of Asian monsoon changes in the Holocene in detail, and thus the hydrological and climate variances behind Chinese and Indian civilization-cultural evolutions. Furthermore, a large number of high-resolution stalagmite records over the past 2000 years have been reconstructed, which are important for understanding short-term climate variability and magnitude, events, cycles, and thus the future climate projection. The achievements have also involved the improvements of a number of important techniques, such as U-Th dating method, the establishments of various hydroclimatic proxies, as well as the contributions to the reconstruction of the atmosphere 14 C variation history over the past ∼54000 years. On the perspective of the future, the Chinese stalagmite community should continue to develop key techniques, further clarify the hydroclimatic significance of stalagmite proxies, impel the integration of related disciplines, and concentrate on key scientific issues in global climate change and major social demands.

Impact of multi-year La Niña events on South and East Asian summer monsoon rainfall are examined in the observations and Coupled Model Intercomparison Project Phase 5 (CMIP5) models. The analysis is carried out for the successive two summers, referred to as the first and second years during the period of 1948–2016. Composite analysis suggests that La Niña related sea surface temperature cooling is slightly high in the central and eastern equatorial Pacific during the first year summer. This anomalous cooling associated with La Niña is slightly shifted towards south and south-central Pacific Ocean during the second year. An Atlantic Niño like pattern is evident in the first year unlike the second year. Negative rainfall anomalies are apparent over most of the south Asian region except Bangladesh and Sundarbans, during the first year. Moisture convergence corroborated by low-level circulation to the north of Bangladesh and central India supports the positive rainfall anomalies in the first year. A weak circulation and negative vertically integrated moisture (VIM) anomalies in the rest of the subcontinent are consistent with the negative rainfall anomalies. In addition to these changes, the Atlantic Niño has also been found to be influencing the South Asian rainfall, remotely, during the first year. In the case of the second year, positive rainfall anomalies over the south Asian monsoon region is noted. An anomalous low-level cyclonic circulation over the central Bay of Bengal enhanced the moisture transport into the Indian subcontinent, causing positive rainfall anomalies. Moreover, an anomalous upper level divergence extends from the southeast Indian Ocean, towards the Indian subcontinent, due to La Niña’s response in the second year, which is found to be weak in the first year. This clearly suggests that the enhanced rainfall over the South Asian region is influenced remotely by La Niña forcing as well as local circulation changes during both the years. The East Asian monsoon region reported a tri-pole like structure in the rainfall anomalies, with positive values over southern and central China and negative over parts of Myanmar, Thailand and Cambodia regions and north-east China—North Korea during the first year and vice-versa in the second year. A positive–negative–positive structure in the VIM anomalies is seen in the East Asian region and it supports similar rainfall anomalies during the second year. We have further examined the ability of CMIP5 models in representing multiyear La Niña teleconnections to the south and East Asian summer monsoons. Some models are able to reproduce the South Asian rainfall and circulation anomalies well in the second year, but failed to do so, in the first. The factors responsible for weak teleconnections in the models are discussed in detail.

The interpretation of stalagmite δ18O in terms of reflecting Asian summer monsoon (ASM) precipitation is still elusive. Here, we present high‐resolution stalagmite trace element ratios (X/Ca, X = Mg, Sr, Ba) records from southwest China covering 116.09 to 4.07 ka BP. δ18O, δ13C, and X/Ca values exhibit clear precessional cycles, with δ18O values reflecting ASM circulation/intensity, while X/Ca ratios capture local precipitation or evapotranspiration variations. Our results show that Northern Hemisphere summer insolation (NHSI) is the main driver of ASM intensity and precipitation phase variation, but global ice volume modulates the response magnitude of summer precipitation to insolation. During the Last Glacial Maximum, high ice volumes caused significant monsoon precipitation to decrease. In contrast to modern observations of the tripolar distribution of precipitation in China, our record is consistent with paleo‐precipitation records in southern and northern China. Plain Language Summary While it is well known that global changes have led to variations in the intensity and spatial distribution of Asian monsoon precipitation, the mechanisms behind this are not well understood. Paleoclimate records are essential for revealing the drivers behind monsoon variation. However, speleothem records from the Asian monsoon region rarely provide direct information on the amount of rainfall. Here we report on multiple indicator data sets from a stalagmite in southwestern China. It could help explore the variation of monsoon precipitation over the last ∼100,000 years. We find that the increase/decrease of Northern Hemisphere summer insolation controls the increase/decrease of Asian summer monsoon rainfall. In addition, global ice volume moderates the magnitude of rainfall response to insolation, and precipitation decreases significantly during high ice volume periods. Based on the present paleo‐precipitation records evidence, the existence of the spatial pattern of increasing/decreasing rainfall in central China corresponding to decreasing/increasing rainfall in northern and southern China remains ambiguous on the orbital scales, although the feature has been captured by some of the model simulations. Key Points Stalagmite trace elements are indicators of regional hydrological environmental variations in Southwestern China Northern Hemisphere summer insolation and global ice volume modulate the phase and amplitude variations of regional hydrological environment The meridional tripolar spatial pattern of precipitation in monsoon region in China on the orbital scale remains ambiguous

A survey of intraseasonal, seasonal, and interannual precipitation and 850 hPa winds for various monsoon regimes around the world is presented for the Community Earth System Model Version 2 (CESM2) compared to observations and the previous generation CESM1. In CESM2 the south Asian monsoon has a reduction of excessive precipitation in the western Indian Ocean and an increase of precipitation in the eastern Bay of Bengal and land areas of Vietnam, Cambodia, and Laos. The seasonal timing of the south Asian monsoon, monsoon‐ENSO connections, and monsoon intraseasonal variability all are improved compared to CESM1. For the Australian monsoon, deficient precipitation over the Maritime Continent has been improved in CESM2 with increases of precipitation over the large tropical islands of Borneo, Celebes, and Papua New Guinea and decreases over southwestern Australia. In the West African monsoon, May–June seasonal rainfall occurs more preferentially over the African coast in CESM2 as in observations, and excessive rainfall over the Ethiopian region is reduced. During July–September in the West African monsoon, deficient precipitation over equatorial Africa in CESM1 has been lessened in CESM2, and there are increases in precipitation over the Guinean coast, though there is little overall improvement in the South African monsoon. In the South American monsoon, precipitation in CESM2 is improved with increased precipitation over the Amazon in central and western Brazil. CESM2 simulates a reduction of excessive precipitation seen in CESM1 over coastal Mexico extending up into the U.S. Great Plains in the North American monsoon. Plain Language Summary A survey of simulations of regional precipitation and 850 hPa winds is presented for the Community Earth System Model Version 2 (CESM2) compared to observations and the previous generation CESM1 for the south Asian and Australian monsoons, the West African and South African monsoons, the North American monsoon, and the South American monsoon. There are mostly improvements in the seasonal timing, monsoon‐ENSO connections, distribution of seasonal mean rainfall, and intraseasonal variability in CESM2 compared to CESM1 in the monsoon regimes. Key Points Community Earth System Model Version 2 (CESM2) is compared to observations and the previous generation CESM1 for intraseasonal, seasonal, and interannual simulations of precipitation and surface winds for the south Asian and Australian monsoons, the West African and South African monsoons, the North American monsoon, and the South American monsoon Notable improvements are seen in CESM2 compared to CESM1 for most monsoon regimes The seasonal timing of the south Asian monsoon, monsoon‐ENSO connections, and monsoon intraseasonal variability all are improved compared to CESM1 and are comparable mostly favorably with observations