Changes In Monsoon Precipitation Research Articles

The Early-Middle Pleistocene transition of Asian summer monsoon

The global climate cyclicity transferred from an Early Pleistocene mode dominated by the obliquity periodicity to a new Middle Pleistocene state dominated by the eccentricity periodicity in the absence of any significant changes in orbital forcing, known as the Early–Middle Pleistocene transition (EMPT), is an unresolved issue of Milankovitch orbital theory. Here we contribute new insights into the orbital characteristic and dynamics of the Asian summer monsoon precipitation changes across the EMPT. Spectral analyses of precipitation over the past 1.6 Myr reconstructed from a new loess magnetic susceptibility (χ) stack suggest that the EMPT of the summer monsoon precipitation characterized by a shift from obliquity to eccentricity rhythm occurred over a ~640 kyr long-term period from ~1.24 to 0.6 Ma rather than quasi-instantaneously on a much shorter duration. The eccentricity cyclicity initiated as early as 1.24 Ma, subsequently intensified and became the dominated pacing from 1.1 Ma, while obliquity weakened at ~1.1 Ma and significantly weakened at ~0.7 Ma. These EMPT characteristics of the summer monsoon precipitation reflect complex impacts from global climate systems and regional climate changes in the Northern and Southern Hemispheres, and in both low- and high-latitude regions.

Global Monsoon Responses to Decadal Sea Surface Temperature Variations during the Twentieth Century: Evidence from AGCM Simulations

AbstractMultidecadal variations in the global land monsoon were observed during the twentieth century, with an overall increasing trend from 1901 to 1955 that was followed by a decreasing trend up to 1990, but the mechanisms governing the above changes remain inconclusive. Based on the outputs of two atmospheric general circulation models (AGCMs) forced by historical sea surface temperature (SST) covering the twentieth century, supplemented with AGCM simulations forced by idealized SST anomalies representing different conditions of the North Atlantic and tropical Pacific, evidence shows that the observed changes can be partly reproduced, particularly over the Northern Hemisphere summer monsoon (NHSM) domain, demonstrating the modulation of decadal SST changes on the long-term variations in monsoon precipitation. Moisture budget analysis is performed to understand the interdecadal changes in monsoon precipitation, and the dynamic term associated with atmospheric circulation changes is found to be prominent, while the contribution of the thermodynamic term associated with humidity changes can lead to coincident wetting over the NHSM domain. The increase (decrease) in NHSM land precipitation during 1901–55 (1956–90) is associated with the strengthening (weakening) of NHSM circulation and Walker circulation. The multidecadal scale changes in atmospheric circulation are driven by SST anomalies over the North Atlantic and the Pacific. A warmer North Atlantic together with a colder eastern tropical Pacific and a warmer western subtropical Pacific can lead to a strengthened meridional gradient in mid-to-upper-tropospheric thickness and strengthened trade winds, which transport more water vapor into monsoon regions, leading to an increase in monsoon precipitation.

Northern Hemisphere land monsoon precipitation changes in the twentieth century revealed by multiple reanalysis datasets

Observations are required to understand monsoon changes over time. Reanalyses are useful supplements to observations; however, their ability to reveal long-term monsoon changes remains unknown. Here, we evaluate against observations the performance of two reanalysis datasets covering the whole twentieth century, the ECMWF reanalysis of the twentieth century (ERA20C) and the Twentieth Century Reanalysis Project (20CR), in terms of reproducing the climatological averages, the centennial co-variation, and long-term trends of Northern Hemisphere land monsoon rainfall (NHLMR). Their performance is compared with three other widely used reanalyses, the NCEP–NCAR reanalysis (NCEP1), the ECMWF 40-year reanalysis (ERA40), and the Japanese 55-year Reanalysis Project (JRA55), for the period after 1955. Results show that the five reanalysis datasets reasonably reproduce the climatological NHLMR and NHLM domains. The leading co-variation mode of NHLMR, with an overall increasing tendency before 1955 and a decreasing tendency afterwards, is captured by the corresponding principal components of the two long-term datasets, with some spatial biases. For the long-term precipitation trends, both the ERA20C and 20CR reproduce the increasing trend during 1901–1955, except for parts of the North African (NAF) monsoon region. However, neither dataset captures the decreasing trend of NHLMR after 1955 because of limitations in the NAF and North American (NAM) monsoon region. The NCEP1, ERA40, and JRA55 datasets also have shortcomings in the NAM region. An overall decreasing NHLMR is only shown in the NCEP1 dataset. A moisture budget analysis reveals that the long-term trends of NHLMR are dominated by the dynamic component of moisture convergence, suggesting a prominent role for atmospheric circulation changes. The influence of residual terms related to observations assimilated in the reanalyses is also discussed.

On Monsoon Precipitation Changes During the Medieval Warm Period and the Little Ice Age in Southeastern China

On Monsoon Precipitation Changes During the Medieval Warm Period and the Little Ice Age in Southeastern China

Tracing changes in monsoonal precipitation using Mg isotopes in Chinese loess deposits

The widespread aeolian deposits on the Chinese Loess Plateau (CLP) have been investigated intensively as an outstanding archive of information on past variations of the East Asian Monsoon (EAM). However, differentiating the impacts of precipitation and temperature on loess-based proxies is difficult and complex, which hampers our ability to understand the variability and dynamics of the EAM. In this study, we investigated the Mg isotope composition (δ26Mg) of the secondary carbonates in fine-grained loess samples from ten Holocene profiles and a loess-paleosol sequence, as well as in the primary carbonates of nine loess-source samples from the Asian inland deserts. Our aim was to explore the potential of the δ26Mg values of secondary carbonates in tracing precipitation changes. The results demonstrate that δ26Mg values are homogeneous (−2.07‰) in primary carbonates from different loess-sources, but display large variations in secondary carbonates of loess, increasing from −3.33‰ in the northwest CLP to −1.80‰ in the southeast CLP, and from −3.58‰ during the last glacial to −1.65‰ during the last interglacial. The variations were mainly controlled by EAM precipitation via the migration of isotopically light Mg, because Mg isotope fractionation during the formation of secondary carbonates is temperature insensitive, and the primary carbonate in different loess-sources has relatively constant δ26Mg values. This conclusion is further supported by high correlation (r2 = 0.84) between modern mean annual precipitation (MAP) and δ26Mg in the secondary carbonates of spatial loess samples. Based on this δ26Mg-MAP relationship, temporal variations in MAP were quantitatively estimated, ranging from 670 mm during the last interglacial to 270 mm during the last glacial at the southern CLP. Compared with previous estimates, our MAP exhibits a larger amplitude of glacial-interglacial fluctuations (∼400 mm) with a less uncertainty (53–76 mm), implying that δ26Mg values in secondary carbonates of loess can serve as a novel precipitation proxy to infer EAM variability and dynamics.

Response of grassland ecosystem to monsoonal precipitation variability during the Mid-Late Holocene: Inferences based on molecular isotopic records from Banni grassland, western India

Banni, located in the arid western India, is one of the largest tropical grasslands of the Asian continent. The net primary production in this grassland ecosystem is currently mediated by precipitation during the Indian summer monsoon (ISM). However, timing of the grassland expansion and its link to the intensity of monsoonal precipitation remains enigmatic due to the paucity of datasets. The major objective of this study is to understand the changes in monsoonal precipitation and vegetation for the last 4600 cal yr BP using hydrogen and carbon isotopic composition of n-alkanes (δDn-alkane and δ13Cn-alkane) measured from two core sediments (Chachi and Luna) in Banni region. The δ13CC29 and δ13CC31 values for Chachi core sediments vary from −30.9 ‰ to −27.2 ‰ and −34.4 ‰ to −25 ‰ respectively. The δ13Cn-alkane values from the core sediments are converted into %C4 plants based on a binary mixing model using the end-member δ13Cn-alkane values derived from the dominant modern vegetation in the Banni region. The prominent feature of the paleovegetation curve is the marked increase in the δ13Cn-alkane values after 2500 cal yr BP, which suggests proliferation of C4 grasses in this region. Similar changes after 2500 cal yr BP have also been observed in the δDn-alkane values. The δDC29 values are used to calculate δD value of paleoprecipitation that varied from 10 ‰ to −60.2 ‰. A significant increase in the δD values of paleoprecipitation (ca. 25 ‰) indicates a weakened ISM precipitation after ca. 2500 cal yr BP. The regional aridification and frequent fire events may have helped the expansion of C4 plant dominated grassland ecosystem in Banni region. Correlation between paleoclimatic records suggests that the southward migration of intertropical convergence zone and more frequent warm phases of El-Nino Southern Oscillation have triggered the weakening of monsoonal precipitation in the tropical region.

A 13,000-year peatland palaeohydrological response to the ENSO-related Asian monsoon precipitation changes in the middle Yangtze Valley

Quantitative reconstructions of the depth to water table (DWT) of ombrotrophic (rain-fed) peatlands are important for understanding the palaeohydrological responses of peatlands to past climate changes. This understanding can provide insights into projecting peatlands future variability and evolution. However, the postglacial DWT reconstruction of peatlands in China is challenging due to complications of the atmospheric circulation system, the scarcity of hydrological proxies, and the site-specific nature of hydrological signals. Here we present a postglacial quantitative DWT reconstruction based on the analysis of fossil phytoliths from the Dajiuhu Peatland, central China. The reconstructions were based on a phytolith-DWT calibration model using a weighted averaging partial least-squares regression analysis of peatland topsoil calibration datasets. Three shallow DWT (wet) periods at 13,000–11,500 cal yr BP, 9,600–7,500 cal yr BP, and 3,000 cal yr BP-present, and two extended deep DWT (dry) periods at 11,500–9600 cal yr BP and at 7,500–3,000 cal yr BP are found based on the cluster analysis of phytolith assemblages and reconstructed DWT changes. These five documented hydrological periods are consistent with regional precipitation reconstructions from independent records in the middle Yangtze Valley (MYV). We interpret these changes as mostly reflecting changes in ENSO at various timescales. An amplified ENSO forced a southward Western Pacific Subtropical High and caused the persistence of the Meiyu Front in the mid-lower Yangtze Valley, consistent with the intense rainfall periods in our study region. Our results indicate that phytolith records are a reliable and sensitive proxy for the calibration of water table models and the quantitatively palaeo-DWT reconstruction of peatlands and reveal a remarkable link between the local hydrological variations and the coupled atmospheric-oceanic circulation, which is significant for the prediction of future hydrological changes in the Asian monsoon region under the background of global warming.

Assessing future water–sediment interaction and critical area prioritization at sub-watershed level for sustainable management

Changes in the response of water and sediment have brought imbalances in present land resources as well as future management practices. Such critical areas need to be identified and prioritized for conservation and protection interventions. The Betwa River basin, located in the central part of India, has natural resources problems due to hydrologic changes. Thus, the present study has been focused on water–sediment interaction and prioritization of critical areas identified from a simulation using the Soil and Water Assessment Tool (SWAT). A downscaled and bias-corrected CMIP5 global climate model dataset has been used to simulate the water and sediment yields of the river basin. The analysis period was grouped into a 20-year period starting with a baseline of 1986 (from 1986 to 2005) and four future horizons, i.e. horizon 2020 (2020–2039), horizon 2040 (2040–2059), horizon 2060 (2060–2079) and horizon 2080 (2080–2099). Based on the sediment yield obtained from the SWAT simulation, identification of an empirical relationship and prioritization of critical areas were carried out. Results show that the relationship between annual water and sediment yields has a good correlation (R2 = 0.75) for horizon 2020; however, a low correlation (R2 = 0.63) was observed for horizon 2060. The analysis reveals that the water–sediment interaction will be affected due to changes of monsoon precipitation in future. Based on priority treatment, the critical sub-watersheds were identified and prioritized for the baseline as well as future climate horizons. Analysis shows that the sediment yield of sub-watersheds SW-5, SW-6, SW-11, SW-17, SW-18, SW-34 and SW-45 has increased with changing climate conditions. Further, these sub-watersheds would undergo changes in erosion class from moderate to very severe. Thus, the identified critical sub-watersheds for the baseline as well as for future climate conditions need to be considered for prioritization and management. The present approach can be used for sub-watershed-level sustainable planning and management.

Impacts of recent decadal changes in Asian aerosols on the East Asian summer monsoon: roles of aerosol\u2013radiation and aerosol\u2013cloud interactions

Anthropogenic aerosols (AA) can affect cloud and precipitation through aerosol–radiation interaction (ARI) and aerosol–cloud interaction (ACI). Over the past few decades, anthropogenic aerosol emissions have exhibited remarkable changes in the magnitude and in spatial pattern. The most significant changes are the increased emissions over both South Asia and East Asia. In this study, the atmospheric component of a state-of-the-art climate model that includes eight species of tropospheric aerosols, coupled to a multi-level mixed-layer ocean model, has been used to investigate the impacts of Asian anthropogenic aerosol precursor emission changes from 1970s to 2000s on large scale circulation and precipitation in boreal summer over East Asia. Results reveal significant changes in circulation and clouds over East Asia and over the tropical and western North Pacific (WNP). Increased Asian AA emissions lead to anomalous cyclonic circulation over the Maritime continent (MC) and anomalous anticyclonic circulation over the WNP, resulting in anomalous moisture transport convergence over the MC and therefore increased precipitation. They also lead to anomalous moisture flux divergence over both the WNP and large land areas of East Asia, especially over northern China, and therefore decreased precipitation there. These large scale circulation anomalies over the adjacent oceans are related to aerosol change induced ocean feedbacks, predominantly through ACI. It is the slow responses over the adjacent oceans (e.g., SST changes) through coupled atmosphere–ocean interaction in pre-monsoon seasons and summer that shape the changes of the East Asian summer monsoon and local precipitation. The results in this study suggest that increased Asian AA emissions from 1970s to 2000s may have played an important role for the observed southward shift of the Pacific intertropical convergence zone and precipitation belt, weakening of East Asian summer monsoon and reduced precipitation over northern China in East Asia during the latter half of the twentieth century.

If Rain Falls in India and No One Reports It, Are Historical Trends in Monsoon Extremes Biased?

AbstractWith two thirds of the total Indian population employed by the agriculture sector, changes in Indian monsoon precipitation have widespread implications for human welfare. Increased extreme precipitation since 1950 has been widely reported for central India. Major studies have relied upon the gridded daily precipitation observations provided by the India Meteorological Department (IMD), which assimilate observations from a variable network of weather stations. Replicating the IMD's interpolation method on satellite‐based precipitation observations, however, indicates that temporal changes in the observing weather station network introduce a jump in the record toward more extreme rainfall after 1975. Trends evaluated across this jump are suspect, and trends evaluated subsequent to it are insignificant (p > 0.1). This positive bias may also mask declines in average monsoon rainfall. Greater accuracy in these trends may resolve distinctions between climate model simulations of future changes. Access to the underlying data from IMD rain‐gauges would facilitate improved rainfall reconstruction.

Effect of the Atlantic Multidecadal Variability on the Global Monsoon

AbstractWe assess the effect of the Atlantic multidecadal variability (AMV) on the global monsoon using idealized simulations. Warm AMV phases are associated with a significant strengthening of monsoon precipitation over Northern Africa and India, and anomalously weak monsoon precipitation over South America. Changes in monsoon precipitation are mediated by a change in atmospheric dynamics, primarily associated with a shift in the circulation related to both an enhanced interhemispheric thermal contrast and the remote impact of AMV on the Pacific Ocean, through changes in the Walker circulation. In contrast, the thermodynamic changes are less important. Further experiments show that the impact of AMV is largely due to the tropical component of the sea surface temperature anomalies. However, the extratropical Atlantic also plays a role, especially for northern Africa. Finally, we show that the effect of AMV on monsoons is not linearly related to the magnitude of warming.

Mid-late Holocene rainfall variation in Taiwan: A high-resolution multi-proxy record unravels the dual influence of the Asian monsoon and ENSO

Taiwan is particularly sensitive to changes in monsoonal precipitation and to typhoon-induced heavy precipitation events, however, rainfall variability in Taiwan on centennial and millennial time scales during the Holocene has not been well understood. This study describes mid-Holocene rainfall features of Taiwan based on pollen, total organic carbon (TOC), total nitrogen (TN), and C/N ratio records of core MD05-2908. The step-wise increase in sedimentation rate, fern spore percentage and concentration, TOC content, and C/N ratio suggests an increasing terrestrial material supply due to the intensified rainfall in Taiwan since 6800 cal. yr BP. This rainfall pattern shows an inverse pattern to the decreasing East Asian summer monsoon (EASM) strength represented by the multi-proxy records from North China. Variation of the East Asian summer circulation and associated moisture transport may account for the long-term rainfall changes in Taiwan. Superimposed on this trend, we interpreted three prominent rainfall changes, which focus on the periods of 6800–6600, 1090–880 and 490–190 cal. yr BP. These centennial time scale rainfall variations in our records are linked to the intensity of El-Niño Southern Oscillations.

Changes in global monsoon precipitation and the related dynamic and thermodynamic mechanisms in recent decades

The aim of this study is to explore the long‐term trends in the distribution of global monsoon precipitation over recent decades using the Global Precipitation Climatology Project (GPCP) data set. In addition, the potential4 mechanisms are examined by applying the atmospheric moisture budget decomposition method to the outputs from the ERA‐Interim medium‐range weather forecasting reanalysis produced by the European Centre for Medium‐Range Weather Forecasts (ECMWF). Changes in global monsoon precipitation are primarily thermodynamically caused by changes in specific humidity and dynamically caused by changes in circulation, as well as by changes in moisture transport that are driven by transient eddies and local evaporation. The results show that the increasing trends in global summer monsoon precipitation from 1979 to 2016 were dominated by contributions from the southern Asian, South African, and North American monsoon regions and were driven by the response of atmospheric circulation to the enhanced east–west thermal contrast in the tropical Pacific. We find that changes in the sea surface temperature (SST) patterns promote increased low‐level vertical velocities in monsoon regions, leading to moisture convergence through divergent circulation anomalies when combined with the climatological humidity field. Hence, the processes result in increasing trends in monsoon precipitation. In addition, evaporation makes increasing contributions to monsoon precipitation throughout the world, except in the South American monsoon region. Moreover, subtropical regions in the East Pacific (the Atlantic) may be major sources of water vapour that have supported increases in precipitation over the southern Asian and North American monsoon regions (the South African monsoon region) in recent decades.

A study of Himalayan extreme rainfall events using WRF-Chem

The rising number of extreme rainfall events over the Himalayan foothill states of India during the recent decades has become a serious issue with the growing concern of aerosol influences. This study intends to provide some insight into aerosol and gas chemistry responses to changes in monsoon circulation and precipitation, and also assess the impact of aerosols on two recent infamous heavy rainfall events using coupled meteorology–chemistry–aerosol (WRF-Chem) model simulations. The sensitivity of aerosols and chemistry on rainfall distribution and the amount is evaluated using the simulations with and without chemistry. Results from this study show that the magnitude and spatial distribution of precipitation are significantly influenced by including aerosol and gas chemistry in the model simulations. Realistic meteorological conditions as well as rainfall amount and distribution are reproduced when aerosols and gasses are taken into account in the simulation. There is an overall enhancement of total cumulative rainfall as high as 20% due to aerosols and gas chemistry over the western Himalayan Indian states. This study shows that cloud-microphysical properties and the resulting precipitation distribution depend critically on the aerosol types and their concentrations under similar thermodynamic conditions. This study highlights the role of aerosol and gas chemistry and recognizes the importance of atmospheric chemistry in the model simulation for the analysis of Himalayan extreme precipitation events, and its further associations with the Himalayan hydrology.

High resolution monsoon precipitation changes on southeastern Tibetan Plateau over the past 2300 years

Climate changes on Southeastern Tibetan Plateau have important impacts on social and economic development, as well as the ecosystem of southwestern China and Indo-China Peninsula. Here, we collected two stalagmites from Shenqi (Miraculous) cave in southern Sichuan, China. The stalagmite δ18O record shows good coherence with local instrumental rainfall record, as well as tree ring- and pollen-based moisture reconstructions from southeastern Tibetan Plateau during overlapped time periods. As a result, we reconstructed high-resolution (∼4.9 yrs) monsoon precipitation variations on southeastern Tibetan Plateau over the past 2300 years by using the combined stalagmite δ18O record.The result reveals an overall decreasing precipitation trend, with two most notable wet periods occurred in 60–280 AD and 370–510 AD. The most remarkably dry period is the recent 200 years. Some decadal scale wet and dry intervals were also identified. The abnormal drought during 1160–1245 AD might have accelerated Dali kingdom's demise at 1253 AD. Power spectrum analysis indicated significant 373-, 187-, 22-, 12- and 11- yr cycles in our stalagmite record, suggesting the impact of solar activity. Increased monsoon precipitation on southeastern TP was observed in solar activity minima during the last millennium. We further synthesized an integrated precipitation record for southwestern China and discussed spatial patterns of precipitation over China during the last two millennia. The comparisons confirm a "dry southern and wet northern" pattern in monsoonal China during the Medieval Warm Period and a "wet southern and dry northern" pattern during the Little Ice Age and Dark Age Cold Period. Solar activity, the strength of westerly jet and summer monsoon, as well as the SST of tropical Indo-Pacific might play important roles on the rainfall spatial patterns over monsoonal China during the last 2000 years.

Origin and implications of orbital-induced sedimentary cyclicity in Pliocene well-logs of the Western Mediterranean

The climatic origin of astronomically induced sedimentary cycles in the Mediterranean and adjacent areas during the late Neogene and Quaternary remains puzzling; as cycles have been linked to concomitant but seasonally opposite changes in African summer monsoon precipitation (Eastern Mediterranean sapropels) and Atlantic regulated winter-precipitation (carbonate cycles on the Atlantic side of the Mediterranean). Particularly, little is known about the cyclic sedimentation on orbital time scales in the Western Mediterranean, with the prime exception of the Messinian sapropels from the Sorbas basin (southern Spain).Here we show that regular alternations in Pliocene downhole logs from the industrial drill-site Muchamiel-1, located along the Balearic Promontory in the Western Mediterranean, are related to eccentricity (bundles) and to obliquity and precession cycles (basic meter-scale alternations). We establish an astronomically based age model for the interval between 5.33 and 2.8 Ma, by first correlating cycle bundles to eccentricity and then the basic dominantly precession-related cycles to the 65°N summer insolation of La2004. The striking bed-to-bed similarities between the Muchamiel-1 well-logs and other records from both the Atlantic margin and the Central Mediterranean suggest that the same climatic forcing was responsible for the formation of carbonate cycles across the Western Mediterranean and adjacent Atlantic. We conclude that formation of alternating carbonate-rich/carbonate-poor beds was controlled by Western Mediterranean cyclogenetic mechanisms as well as by peri-Mediterranean precipitation associated with changes in the North Atlantic System (NAS). These findings highlight the importance of peri-Mediterranean precipitation on the sedimentary cyclicity by dictating terrigenous (clay) supply and potentially on the hydrology of the basin by providing additional freshwater required for sapropel formation. Consequently, cyclic sedimentation in the Mediterranean results from the combined effect of precipitation changes driven by (i) the North African monsoon, (ii) the Atlantic system, and (iii) intrabasinal Mediterranean atmospheric dynamics.

Detectable Impact of Local and Remote Anthropogenic Aerosols on the 20th Century Changes of West African and South Asian Monsoon Precipitation

AbstractAnthropogenic aerosols are a key driver of changes in summer monsoon precipitation in the Northern Hemisphere during the 20th century. Here we apply detection and attribution methods to investigate causes of change in the West African and South Asian monsoons separately and identify the aerosol source regions that are most important for explaining the observed changes during 1920–2005. Historical simulations with the GFDL‐CM3 model are used to derive fingerprints of aerosol forcing from different regions. For West Africa, remote aerosol emissions from North America and Europe (NAEU) are essential in order to detect the anthropogenic signal in observed monsoon precipitation changes. The changes are significantly underestimated in the model, however. While natural (volcanic) forcing seems to also play a role, the dominant contribution is found to come from aerosol‐induced changes in the interhemispheric temperature gradient and associated meridional shifts of the Intertropical Convergence Zone. For South Asia, in contrast, changes in observed monsoon precipitation cannot be explained without local emissions. Here the findings show a weakening of the monsoon circulation, driven by the increase of remote NAEU aerosol emissions until 1975, and since then by the increase in local emissions offsetting the decrease of NAEU emissions. The results show that the aerosol forcing from individual emission regions is strong enough to be detected over internal variability. They also underscore the importance of the spatial pattern of global‐aerosol emissions, which is likely to continue to change throughout the expected near‐future decline in global emissions.

Future changes in Asian summer monsoon precipitation extremes as inferred from 20-km AGCM simulations

This study examines the impacts of climate change on precipitation extremes in the Asian monsoon region during boreal summer, based on simulations from the 20-km Meteorological Research Institute atmospheric general circulation model. The model can capture the summertime monsoon rainfall, with characteristics similar to those from Tropical Rainfall Measuring Mission and Asian Precipitation-Highly-Resolved Observational Data Integration Towards Evaluation. By comparing the 2075–2099 with the present-day climate simulations, there is a robust increase of the mean rainfall in many locations due to a warmer climate. Over southeastern China, the Baiu rainband, Bay of Bengal and central India, extreme precipitation rates are also enhanced in the future, which can be inferred from increases of the 95th percentile of daily precipitation, the maximum accumulated precipitation in 5 consecutive days, the simple daily precipitation intensity index, and the scale parameter of the fitted gamma distribution. In these regions, with the exception of the Baiu rainband, most of these metrics give a fractional change of extreme rainfall per degree increase of the lower-tropospheric temperature of ~ 5 to 8.5% K−1, roughly consistent with the Clausius–Clapeyron relation. However, over the Baiu area extreme precipitation change scales as ~ 3.5% K−1 only. We have also stratified the rainfall data into those associated with tropical cyclones (TC) and those with other weather systems. The AGCM gives an increase of the accumulated TC rainfall over southeastern China, and a decrease in southern Japan in the future climate. The latter can be attributed to suppressed TC occurrence in southern Japan, whereas increased accumulated rainfall over southeastern China is due to more intense TC rain rate under global warming. Overall, non-TC weather systems are the main contributor to enhanced precipitation extremes in various locations. In the future, TC activities over southeastern China tend to further exacerbate the precipitation extremes, whereas those in the Baiu region lead to weaker changes of these extremes.

Enhanced Global Monsoon in Present Warm Period Due to Natural and Anthropogenic Forcings

In this study, we investigate global monsoon precipitation (GMP) changes between the Present Warm Period (PWP, 1900–2000) and the Little Ice Age (LIA, 1250–1850) by performing millennium sensitivity simulations using the Community Earth System Model version 1.0 (CESM1). Three millennium simulations are carried out under time-varying solar, volcanic and greenhouse gas (GHG) forcing, respectively, from 501 to 2000 AD. Compared to the global-mean surface temperature of the cold LIA, the global warming in the PWP caused by high GHG concentration is about 0.42 °C, by strong solar radiation is 0.14 °C, and by decreased volcanic activity is 0.07 °C. The GMP increases in these three types of global warming are comparable, being 0.12, 0.058, and 0.055 mm day−1, respectively. For one degree of global warming, the GMP increase induced by strong GHG forcing is 2.2% °C−1, by strong solar radiation is 2.8% °C−1, and by decreased volcanic forcing is 5.5% °C−1, which means that volcanic forcing is most effective in terms of changing the GMP among these three external forcing factors. Under volcanic inactivity-related global warming, both monsoon moisture and circulation are enhanced, and the enhanced circulation mainly occurs in the Northern Hemisphere (NH). The circulation, however, is weakened in the other two cases, and the GMP intensification is mainly caused by increased moisture. Due to large NH volcanic aerosol concentration in the LIA, the inter-hemispheric thermal contrast of PWP global warming tends to enhance NH monsoon circulation. Compared to the GHG forcing, solar radiation tends to warm low-latitude regions and cause a greater monsoon moisture increase, resulting in a stronger GMP increase. The finding in this study is important for predicting the GMP in future anthropogenic global warming when a change in natural solar or volcanic activity occurs.

South American monsoon response to iceberg discharge in the North Atlantic

Heinrich Stadials significantly affected tropical precipitation through changes in the interhemispheric temperature gradient as a result of abrupt cooling in the North Atlantic. Here, we focus on changes in South American monsoon precipitation during Heinrich Stadials using a suite of speleothem records covering the last 85 ky B.P. from eastern South America. We document the response of South American monsoon precipitation to episodes of extensive iceberg discharge, which is distinct from the response to the cooling episodes that precede the main phase of ice-rafted detritus deposition. Our results demonstrate that iceberg discharge in the western subtropical North Atlantic led to an abrupt increase in monsoon precipitation over eastern South America. Our findings of an enhanced Southern Hemisphere monsoon, coeval with the iceberg discharge into the North Atlantic, are consistent with the observed abrupt increase in atmospheric methane concentrations during Heinrich Stadials.