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Recent Advances in Understanding Multi-scale Climate Variability of the Asian Monsoon
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.
Journal Article
Monsoon rains, great rivers and the development of farming civilisations in Asia
\"The Asian monsoon and associated river systems supply the water that sustains a large portion of humanity, and has enabled Asia to become home to some of the oldest and most productive farming systems on Earth. This book uses climate data and environmental models to provide a detailed review of variations in the Asian monsoon since the mid-Holocene, and its impacts on farming systems and human settlement. Future changes to the monsoon due to anthropogenically-driven global warming are also discussed. Faced with greater rainfall and more cyclones in South Asia, as well as drying in North China and regional rising sea levels, understanding how humans have developed resilient strategies in the past to climate variations is critical. Containing important implications for the large populations and booming economies in the Indo-Pacific region, this book is an important resource for researchers and graduate students studying the climate, environmental history, agronomy and archaeology of Asia\"-- Provided by publisher.
Book
Understanding Future Change of Global Monsoons Projected by CMIP6 Models
Projecting future change of monsoon rainfall is essential for water resource management, food security, disaster mitigation, and infrastructure planning. Here we assess the future change and explore the causes of the changes using 15 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The multimodel ensemble projects that, under the shared socioeconomic pathway (SSP) 2–4.5, the total land monsoon rainfall will likely increase in the Northern Hemisphere (NH) by about 2.8% per one degree Celsius of global warming (2.8% °C−1) in contrast to little change in the Southern Hemisphere (SH; −0.3% °C−1). In addition, in the future the Asian–northern African monsoon likely becomes wetter while the North American monsoon becomes drier. Since the humidity increase is nearly uniform in all summer monsoon regions, the dynamic processes must play a fundamental role in shaping the spatial patterns of the global monsoon changes. Greenhouse gas (GHG) radiative forcing induces a “NH-warmer-than-SH” pattern, which favors increasing the NH monsoon rainfall and prolonging the NH monsoon rainy season while reducing the SH monsoon rainfall and shortening the SH monsoon rainy season. The GHG forcing induces a “land-warmer-than-ocean” pattern, which enhances Asian monsoon low pressure and increases Asian and northern African monsoon rainfall, and an El Niño–like warming, which reduces North American monsoon rainfall. The uncertainties in the projected monsoon precipitation changes are significantly related to the models’ projected hemispheric and land–ocean thermal contrasts as well as to the eastern Pacific Ocean warming. The CMIP6 models’ common biases and the processes by which convective heating drives monsoon circulation are also discussed.
Journal Article
Impacts of dynamic and thermal forcing by the Tibetan Plateau on the precipitation distribution in the Asian arid and monsoon regions
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.
Journal Article
Future Changes and Controlling Factors of the Eight Regional Monsoons Projected by CMIP6 Models
An accurate prediction of land monsoon precipitation (LMP) is critical for the sustainable future of the planet as it provides water resources for more than two-thirds of the global population. Here, we show that the ensemble mean of 24 CMIP6 (phase 6 of the Coupled Model Intercomparison Project) models projects that, under the Shared Socioeconomic Pathway 2–4.5 (SSP2–4.5) scenario, summer LMP will very likely increase in South Asia (∼4.1% °C−1), likely increase in East Asia (∼4.6% °C−1) and northern Africa (∼2.9% °C−1), and likely decrease in North America (∼−2.3% °C−1). The annual mean LMP in three Southern Hemisphere monsoon regions will likely remain unchanged due to significantly decreased winter precipitation. Regional mean LMP changes are dominated by the change in upward moisture transport with moderate contribution from evaporation and can be approximated by the changes of the product of the midtropospheric ascent and 850-hPa specific humidity. Greenhouse gas (GHG)-induced thermodynamic effects increase moisture content and stabilize the atmosphere, tending to offset each other. The spatially uniform increase of humidity cannot explain markedly different regional LMP changes. Intermodel spread analysis demonstrates that the GHG-induced circulation changes (dynamic effects) are primarily responsible for the regional differences. The GHGs induce a warm land–cool ocean pattern that strengthens the Asian monsoon, and a warm North Atlantic and Sahara that enhances the northern African monsoon, as well as an equatorial central Pacific warming that weakens the North American monsoon. CMIP6 models generally capture realistic monsoon rainfall climatology, but commonly overproduce summer rainfall variability. The models’ biases in projected regional SST and land–sea thermal contrast likely contribute to the models’ uncertainties in the projected monsoon rainfall changes.
Journal Article
Intraseasonal, Seasonal, and Interannual Characteristics of Regional Monsoon Simulations in CESM2
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
Journal Article
Recent Progress in Studies of the Variabilities and Mechanisms of the East Asian Monsoon in a Changing Climate
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.
Journal Article
Three types of East Asian summer rainfall associated with monsoon circulation and tropical cyclone activities: unique features and major influential factors
Most previous studies have investigated East Asian summer rainfall as one element. This study investigates the East Asian summer monsoon (EASM), Western North Pacific (WNP) tropical cyclones (TCs), and the concurrent rainfall occurring between May and September from 1983 to 2021. Three distinct types of rainfall are identified: monsoon-only rainfall, TC-only rainfall, and monsoon-TC joint rainfall (MS-TC rainfall), each exhibiting its own unique characteristics. Monsoon-only rainfall is characterized by positive anomalies along the East Asian subtropical front, while a decrease in TC activity leads to negative rainfall anomalies in the tropical WNP. TC-only rainfall, on the other hand, contributes to positive rainfall anomalies in the tropical WNP, as more TCs exhibit westward movement. In the case of MS-TC rainfall, positive precipitation anomalies are observed in the Philippines, the Korean Peninsula, and southern Japan. These anomalies can be attributed to an enhanced northward movement of TCs during such rainfall events. During strong monsoon-only (TC-only) rainfall years, the tropical western Pacific experiences anticyclonic (cyclonic) anomalies, along with a westward (eastward) shift of monsoon trough and WNP subtropical high (WNPSH). In strong MS-TC years, a localized cyclonic anomaly dominates the Philippine Basin, resulting in an eastward shift of the monsoon trough and WNPSH. Additionally, a significant increase (decrease) in vertical wind shear (VWS) is observed in the tropical WNP during strong monsoon-only (TC-only) years, while a moderate decrease is observed in strong MS-TC years. Distinct influential factors are associated with each rainfall type. Preceding positive sea surface temperature (SST) anomalies in the offshore China seas and WNP, in conjunction with the El Niño-Southern Oscillation (ENSO), play a primary role in driving interannual variations of monsoon-only rainfall. ENSO serves as the principal modulator of interannual variations in TC-only rainfall. Additionally, anomalous thermal conditions in the Maritime Continent (MC) act as major drivers for MS-TC rainfall. This study enhances our understanding of the underlying mechanisms and influential factors contributing to the diverse patterns of East Asian summer rainfall.
Journal Article