Monsoon Domain Research Articles

Global Monsoon Changes under the Paris Agreement Temperature Goals in CESM1(CAM5)

Based on experiments with the Community Earth System Model, version 1 (Community Atmosphere Model, version 5) [CESM1(CAM5)], and an observational dataset, we found that CESM1-CAM5 is able to reproduce global monsoon (GM) features, including the patterns of monsoon precipitation and monsoon domains, the magnitude of GM precipitation (GMP, the local summer precipitation), GM area (GMA), and GM percentage (the ratio of the local summer precipitation to annual precipitation). Under the Paris Agreement temperature goals, the GM in CESM1-CAM5 displays the following changes: (1) The GMA is ambiguous under the 1.5°C temperature goal and increases under the 2.0°C temperature goal. The increase mainly results from a change in the monsoon percentage. (2) The GM, land monsoon and ocean monsoon precipitation all significantly increase under both the 1.5°C and 2.0°C goals. The increases are mainly due to the enhancement of humidity and evaporation. (3) The percentages of GM, land monsoon and ocean monsoon feature little change under the temperature goals. (4) The lengths of the GM, land monsoon and ocean monsoon are significantly prolonged under the temperature goals. The increase in precipitation during the monsoon withdrawal month mainly accounts for the prolonged monsoons. Regarding the differences between the 1.5°C and 2.0°C temperature goals, it is certain that the GMP displays significant discrepancies. In addition, a large-scale enhancement of ascending motion occurs over the southeastern Tibetan Plateau and South China under a warming climate, whereas other monsoon areas experience an overall decline in ascending motion. This leads to an extraordinary wetting over Asian monsoon areas.

Evaluating the timing and structure of the 4.2 ka event in the Indian summer monsoon domain from an annually resolved speleothem record from Northeast India

Abstract. A large array of proxy records suggests that the “4.2 ka event” marks an approximately 300-year long period (∼3.9 to 4.2 ka) of major climate change across the globe. However, the climatic manifestation of this event, including its onset, duration, and termination, remains less clear in the Indian summer monsoon (ISM) domain. Here, we present new oxygen isotope (δ18O) data from a pair of speleothems (ML.1 and ML.2) from Mawmluh Cave, Meghalaya, India, that provide a high-resolution record of ISM variability during a period (∼3.78 and 4.44 ka) that fully encompasses the 4.2 ka event. The sub-annually to annually resolved ML.1 δ18O record is constrained by 18 230Th dates with an average dating error of ±13 years (2σ) and a resolution of ∼40 years, which allows us to characterize the ISM variability with unprecedented detail. The inferred pattern of ISM variability during the period contemporaneous with the 4.2 ka event shares broad similarities and key differences with the previous reconstructions of ISM from the Mawmluh Cave and other proxy records from the region. Our data suggest that the ISM intensity, in the context of the length of our record, abruptly decreased at ∼4.0 ka (∼±13 years), marking the onset of a multi-centennial period of relatively reduced ISM, which was punctuated by at least two multi-decadal droughts between ∼3.9 and 4.0 ka. The latter stands out in contrast with some previous proxy reconstructions of the ISM, in which the 4.2 ka event has been depicted as a singular multi-centennial drought.

The effect of Indian Summer Monsoon rainfall on surface water δD values in the central Himalaya

AbstractStable isotope proxy records, such as speleothems, plant‐wax biomarker records, and ice cores, are suitable archives for the reconstruction of regional palaeohydrologic conditions. But the interpretation of these records in the tropics, especially in the Indian Summer Monsoon (ISM) domain, is difficult due to differing moisture and water sources: precipitation from the ISM and Winter Westerlies, as well as snow‐ and glacial meltwater. In this study, we use interannual differences in ISM strength (2011–2012) to understand the stable isotopic composition of surface water in the Arun River catchment in eastern Nepal. We sampled main stem and tributary water (n = 204) for stable hydrogen and oxygen isotope analysis in the postmonsoon phase of two subsequent years with significantly distinct ISM intensities. In addition to the 2011/2012 sampling campaigns, we collected a 12‐month time series of main stem waters (2012/2013, n = 105) in order to better quantify seasonal effects on the variability of surface water δ18O/δD. Furthermore, remotely sensed satellite data of rainfall, snow cover, glacial coverage, and evapotranspiration was evaluated. The comparison of datasets from both years revealed that surface waters of the main stem Arun and its tributaries were D‐enriched by ~15‰ when ISM rainfall decreased by 20%. This strong response emphasizes the importance of the ISM for surface water run‐off in the central Himalaya. However, further spatio‐temporal analysis of remote sensing data in combination with stream water d‐excess revealed that most high‐altitude tributaries and the Tibetan part of the Arun receive high portions of glacial melt water and likely Winter Westerly Disturbances precipitation. We make the following two implications: First, palaeohydrologic archives found in high‐altitude tributaries and on the southern Tibetan Plateau record a mixture of past precipitation δD values and variable amounts of additional water sources. Second, surface water isotope ratios of lower elevated tributaries strongly reflect the isotopic composition of ISM rainfall implying a suitable region for the analysis of potential δD value proxy records.

Sandy Loess Records of Precipitation Changes and Monsoon Migrations in the Hunshandake Sandy Land Since the Last Glacial Maximum

AbstractThe Hunshandake Sandy Land (Hunshandake) in central Inner Mongolia, China, is located within the fringe of the East Asian summer monsoon (EASM) domains. Precipitation records in Hunshandake are essential to studying the migrations of the EASM. This work reproduced the precipitation changes since the Last Glacial Maximum (LGM) by measuring the magnetic susceptibility and carbonate content through optically stimulated luminescence dating of two sandy loess profiles within the sandy land to obtain a high‐resolution sedimentary chronological sequence. In addition, eolian topsoil samples were collected along the modern precipitation gradient and their magnetic susceptibility values were compared among the monsoon region, the monsoon marginal region, and the nonmonsoon region. The results demonstrate the following: (1) from 25 to 15 ka, precipitation was low in Hunshandake and exhibited no substantial variability or trend; from 15 to 6 ka, precipitation increased gradually and reached its maximum at approximately 6 ka; and after 6 ka, precipitation decreased again; (2) precipitation changes in Hunshandake since the LGM were mainly determined by the migration of the EASM; and (3) changes in the monsoon migrations in Hunshandake since the LGM are primarily driven by the changes in global ice volume and subordinately by solar radiation.

Global Monsoon Precipitation: Trends, Leading Modes, and Associated Drought and Heat Wave in the Northern Hemisphere

Global monsoon precipitation (GMP) brings the majority of water for the local agriculture and ecosystem. The Northern Hemisphere (NH) GMP shows an upward trend over the past decades, while the trend in the Southern Hemisphere (SH) GMP is weak and insignificant. The first three singular value decomposition modes between NH GMP and global SST during boreal summer reflect, in order, the Atlantic multidecadal oscillation (AMO), eastern Pacific (EP) El Niño, and central Pacific (CP) El Niño, when the AMO dominates the NH climate and contributes to the increased trend. However, the first three modes between SH GMP and global SST during boreal winter are revealed as EP El Niño, the AMO, and CP El Niño, when the EP El Niño becomes the most significant driver of the SH GMP, and the AMO-induced rainfall anomalies may cancel out each other within the SH global monsoon domain and thus result in a weak trend. The intensification of NH GMP is proposed to favor the occurrences of droughts and heat waves (HWs) in the midlatitudes through a monsoon–desert-like mechanism. That is, the diabatic heating associated with the monsoonal rainfall may drive large-scale circulation anomalies and trigger intensified subsidence in remote regions. The anomalous descending motions over the midlatitudes are usually accompanied by clear skies, which result in less precipitation and more downward solar radiation, and thus drier and hotter soil conditions that favor the occurrences of droughts and HWs. In comparison, the SH GMP may exert much smaller impacts on the NH extremes in spring and summer, probably because the winter signals associated with SH GMP cannot sufficiently persist into the following seasons.

What drives the decadal variation of global land monsoon precipitation over the past 50 years?

Global land monsoon precipitation (GLMP) has shown quasi‐20‐year oscillations overlying a significant downwards trend over the past 50 years (1951–2004). However, what drives this decadal variation remains an open question. Based on Coupled Model Intercomparison Project Phase 5 (CMIP5) models, in this study we explored the possible drivers for the decadal variation of GLMP and quantified their contributions. A downwards trend of −0.30 mm/day (100 years) is observed in GLMP over 1951–2004 that explains 30% of total decadal variance, and it is primarily attributed to anthropogenic aerosols (AAs) external forcing. Further analyses showed that the downwards trend in GLMP mainly results from the widespread drying over Northern Hemisphere (NH) monsoon domain. Faster growth of AAs amount in NH than Southern Hemisphere (SH) over the past 50 years results in a reduced hemispheric thermal contrast (cooler NH–warmer SH) due to more AA‐forced cooling in NH. Reduced hemispheric thermal contrast facilitates a weakened NH summer monsoon circulation and finally suppresses monsoon precipitation. Besides the downwards trend, the remaining variation of GLMP exhibited quasi‐20‐year oscillations that explain 70% of total decadal variance. The decadal oscillations are primarily attributed to internal variability, in which the central tropical Pacific sea surface temperature anomaly and Arctic Oscillation together can explain 55% of its variation.

Multiscale Atmospheric Overturning of the Indian Summer Monsoon as Seen through Isentropic Analysis

Abstract This study investigates multiscale atmospheric overturning during the 2009 Indian summer monsoon (ISM) using a cloud-permitting numerical model. The isentropic analysis technique adopted here sorts vertical mass fluxes in terms of the equivalent potential temperature of air parcels, which is capable of delineating the atmospheric overturning between ascending air parcels with high entropy and subsiding air parcels with low entropy. The monsoonal overturning is further decomposed into contributions from three characteristic scales: the basinwide ascent over the Indian monsoon domain, the regional-scale overturning associated with synoptic and mesoscale systems, and the convective-scale overturning. Results show that the convective-scale component dominates the upward mass transport in the lower troposphere while the region-scale component plays an important role by deepening the monsoonal overturning. The spatial variability of the convective-scale overturning is analyzed, showing intense convection over the Western Ghats and the Bay of Bengal while the deepest overturning is localized over northern India and the Himalayan foothills. The equivalent potential temperature in convective updrafts is higher over land than over the ocean or coastal regions. There is also substantial variability in the atmospheric overturning associated with the intraseasonal variability. The upward mass and energy transport increase considerably during the active phases of the ISM. A clear northeastward propagation in the peak isentropic vertical mass and energy transport over different characteristic regions can be found during the ISM, which corresponds to the intraseasonal oscillations of the ISM. Altogether, this study further demonstrates the utility of the isentropic analysis technique to characterize the spatiotemporal variations of convective activities in complex atmospheric flows.

Quaternary paleoenvironmental evolution of the Tengger Desert and its implications for the provenance of the loess of the Chinese Loess Plateau

The dust sources of the loess deposits of the Chinese Loess Plateau (CLP) are suggested to be the Gobi-deserts in north China and the Tibetan Plateau in southwest China. The Tengger Desert (TD), a major desert situated to the northeast of the Tibetan Plateau and to the northwest of the CLP, on the western margin of the present-day Asian Summer Monsoon domain, is an ideal area for assessing which of these potential dust source areas may be dominant. A 104-m-long drill core (BJ14) was obtained from the northwestern TD. The magnetostratigraphy, constrained by electron spin resonance dating in the upper part of the core, includes the Brunhes, Matuyama and beginning of the Gauss polarity chrons, with their boundaries at the depths of 44.57 m and 94.31 m, respectively. Sedimentary facies analysis based on lithological, grain-size, magnetic susceptibility and micropaleontological evidences indicates that the core is primarily composed of lacustrine deposits. Combined with the results of previous studies in the eastern TD and the adjacent regions, the sedimentary sequence of core BJ14 indicates that lakes occupied the northern and eastern TD during most of the early Pleistocene and that the onset of a desert environment occurred at ∼0.9 Ma. Our results suggest that the TD may have contributed only a limited amount of dust to the CLP during the early Pleistocene.

High‐resolution paleoclimatic records spanning the past 30 cal ka BP inferred from Qionghai Lake sediments in south‐west China: Insights from geochemical investigations and grain‐size characteristics

There have been few high‐resolution investigations covering the last glacial and Holocene from Sichuan, Yunnan, or south Asia area in the south‐west monsoon domain region. High‐resolution analyses of lake sediments retrieved from Qionghai Lake, Sichuan Province, were conducted using a combination of variables including total organic carbon (TOC) and total nitrogen (TN) concentration, low frequency magnetic susceptibility (χLF), particle size, carbonate content, and δ13C value of organic matter (δ13Corg), to provide a new perspective on the detailed paleoclimate history of south‐west China spanning the past 30 cal ka BP. Our findings suggest that organic matter in the Qionghai Lake was mainly derived from microorganisms, aquatic macrophytes, and terrestrial C3 plants. From 29.1 to 23.2 cal ka BP, low carbonate contents, high median grain size (Md values), and χLF values indicated a warm and wet climate. During the period 23.2 to 15.4 cal ka BP, low χLF, Md values, and high carbonate contents suggested a decrease in temperature and humidity. The cold and dry Last Glacial Maximum (LGM) and Heinrich 1 event (H1) were captured between 21.6 and 19.4 cal ka BP and 17.6 and 15.8 cal ka BP, respectively. During the interval 15.4 to 9.7 cal ka BP, low level of χLF values and Md values, together with relatively high carbonates, demonstrated that a dry climate prevailed in this period. Since 9.7 cal ka BP, the climate was warm and humid in the early to mid‐Holocene and shifted towards cold and dry condition in the late Holocene. A cold and dry period at 8.5 to 8.1 cal ka BP and warm and wet interval at 6.6 to 4.6 cal ka BP were identified, corresponding to the 8.2‐ka cold event and Holocene climatic optimum (HCO), respectively. These climate changes may reflect variations of the intensity of the south‐west summer monsoon, primarily in response to the Northern Hemisphere solar insolation on a millennial scale induced by orbital forcing. The position of the Intertropical Convergence Zone (ITCZ) may add a complementary effect on the evolution of the monsoon strength in the Holocene.

Inter‐annual variability of global monsoon precipitation in present‐day and future warming scenarios based on 33 Coupled Model Intercomparison Project Phase 5 models

Simulations and future projections of year‐to‐year variation in global monsoon precipitation (GMP), defined as the summer precipitation amount per unit area within the global monsoon domain, are investigated using the 33 models that participated in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). The inter‐annual standard deviation of GMP, which presents the amplitude of year‐to‐year variability in monsoon rainfall, is simulated well by the multi‐model ensemble (MME) mean of the 33 models and of the best five (B5) models. The B5 models show superior skills in reproducing the climatological monsoon precipitation and its inter‐annual variability. The inter‐annual standard deviations of 25‐year GMP in the present day (1979–2005) derived from the MME average of the 33 models and of the B5 models are 0.17 and 0.15 mm/day per unit area, respectively, both being close to the observation (0.16 mm/day per unit area). Consistent with the observed El Niño–Southern Oscillation (ENSO)–GMP relationship, the simulated variability of GMP is negatively correlated with the sea surface temperature in the Niño areas at the inter‐annual timescale.Projections by the 33 CMIP5 models under the RCP4.5 scenario indicate that wet and dry monsoon years would occur more intensively in a warmer climate. Around 61% (20 out of the 33) of the models show enhanced inter‐annual variability of GMP by the end of the 21st century (2072–2098) compared to the period 1979–2005. The MME of the B5 models suggests that the amplitude of projected GMP variability would increase by ~20% from present day to the late 21st century. However, the increasing rate of GMP variability detected by the 33 models’ MME is relatively small (~6%), which is even smaller than the inter‐model standard deviation, suggesting the amplitude changes in GMP variability feature non‐negligible uncertainty among the 33 models. Under global warming, the inter‐annual variation of GMP would still be tightly connected with ENSO, although the projected amplitude changes in ENSO variability are noticeably diverse across the 33 CMIP5 models. The enhanced amplitude of inter‐annual GMP variability may be attributable to the increase in mean‐state moisture associated with global warming. The dynamic effect related to changes in the variability of monsoon circulation is small and less robust among the 33 CMIP5 models.

Relationship between the Hadley Circulation and Different Tropical Meridional SST Structures during Boreal Summer

AbstractThe relationship of the Hadley circulation (HC) to different tropical sea surface temperature (SST) meridional structures during boreal summer is investigated over the period of 1979–2016. After decomposing the variations of the HC into the equatorially asymmetric HC (HEA), zonal-mean equatorially asymmetric SST (SEA), equatorially symmetric HC (HES), and equatorially symmetric SST (SES) components, the ratio of the HEA associated with SEA with respect to the HES associated with SES is around 2 across multiple reanalyses, which is a smaller ratio than in the annual and seasonal cycle. The reduced ratio of the HC to SST is due to the regional SST variation in the Asian summer monsoon (ASM) domain. The first leading mode (EOF1) of the regional SST variability in the ASM domain is dominated by a homogeneous warming pattern. This pattern is associated with an equatorially asymmetric HC, but it has an opposite direction to the climatological HEA and so weakens the HEA. The second dominant mode has an El Niño–like pattern, which resembles the distribution of the principal mode of the SST in the non-ASM region. Both modes are responsible for the variation of HES. However, the SST EOF1 in the ASM domain displays a significant upward trend, favoring a suppressed HEA, and leading to the smaller ratio of the HC to SST during boreal summer. Moreover, the variation of the SST EOF1 is closely linked with the intensity of the ASM, highlighting the potential modulation by the ASM of the relation between the HC and SST during boreal summer.

On the Holocene evolution of the Ayeyawady megadelta

Abstract. The Ayeyawady delta is the last Asian megadelta whose evolution has remained essentially unexplored so far. Unlike most other deltas across the world, the Ayeyawady has not yet been affected by dam construction, providing a unique view on largely natural deltaic processes benefiting from abundant sediment loads affected by tectonics and monsoon hydroclimate. To alleviate the information gap and provide a baseline for future work, here we provide a first model for the Holocene development of this megadelta based on drill core sediments collected in 2016 and 2017, dated with radiocarbon and optically stimulated luminescence, together with a reevaluation of published maps, charts and scientific literature. Altogether, these data indicate that Ayeyawady is a mud-dominated delta with tidal and wave influences. The sediment-rich Ayeyawady River built meander belt alluvial ridges with avulsive characters. A more advanced coast in the western half of the delta (i.e., the Pathein lobe) was probably favored by the more western location of the early course of the river. Radiogenic isotopic fingerprinting of the sediment suggests that the Pathein lobe coast does not receive significant sediment from neighboring rivers. However, the eastern region of the delta (i.e., Yangon lobe) is offset inland and extends east into the mudflats of the Sittaung estuary. Wave-built beach ridge construction during the late Holocene, similar to several other deltas across the Indian monsoon domain, suggests a common climatic control on monsoonal delta morphodynamics through variability in discharge, changes in wave climate or both. Correlation of the delta morphological and stratigraphic architecture information on land with the shelf bathymetry, as well as its tectonic, sedimentary and hydrodynamic characteristics, provides insight on the peculiar growth style of the Ayeyawady delta. The offset between the western Pathein lobe and the eastern deltaic coast appears to be driven by tectonic–hydrodynamic feedbacks as the extensionally lowered shelf block of the Gulf of Mottama amplifies tidal currents relative to the western part of the shelf. This situation probably activates a perennial shear front between the two regions that acts as a leaky energy fence. Just as importantly, the strong currents in the Gulf of Mottama act as an offshore-directed tidal pump that helps build the deep mid-shelf Mottama clinoform with mixed sediments from the Ayeyawady, Sittaung and Thanlwin rivers. The highly energetic tidal, wind and wave regime of the northern Andaman Sea thus exports most sediment offshore despite the large load of the Ayeyawady River.

East Asian Winter Monsoon Variations and Their Links to Arctic Sea Ice During the Last Millennium, Inferred From Sea Surface Temperatures in the Okinawa Trough

AbstractThe East Asian winter monsoon (EAWM) significantly impacts living conditions in a large part of Asia, and therefore, it is important to understand its major driving mechanisms. Winter sea surface temperature (SSTW) and circulation in the southern Okinawa Trough are today both primarily controlled by the EAWM. Here we present a new SSTW reconstruction for the last millennium based on a diatom record from sediment core MD05‐2908, from the continental slope of the southern Okinawa Trough off northeastern Taiwan. Our reconstruction indicates that SSTW varied between 14.1 and 19.6°C over the past 1,000 years. Changes in SSTW in the southern Okinawa Trough correspond closely to the index of warm winters based on historical documents from the East Asian monsoon domain. This implies that our SSTW record can be used to reconstruct EAWM variability during the last millennium. Comparisons with the reconstructed winter Arctic Oscillation (AO, developed from historical snow anomaly events in Eastern Asia) and Arctic sea ice cover reveal a significant positive correlation between the EAWM and AO during the time interval from 1000–1400 Common Era (C.E.), coinciding with reduced sea ice cover. However, there is no significant correlation with increased sea ice cover during the interval from 1400 to 1700 C.E. This suggests that the reduction in Arctic sea ice may periodically have played a role in strengthening the relationship between the EAWM and the AO during the last millennium and that the current and future reduction in Arctic sea ice may have significant consequences for the EAWM.

Effects of the uplifts of the main and marginal Tibetan Plateau on the Asian climate under modern and ~30 Ma boundary conditions

The surface uplift of the Tibetan Plateau (TP) had important effects on the evolution of both the regional and global climate during the Cenozoic. Geological evidence indicated that the timing of the uplifts of the main and marginal TP was probably different. However, many previous modelling studies only investigated the effects of the uplift of the whole TP on climate under modern boundary conditions and it remained unclear what was the difference between the effects of the uplifts of the main and marginal TP on climate and how changed boundary conditions influenced these effects. We used numerical modelling to investigate these questions. Our modelling results indicated that the uplifts of the main and marginal TP had different effects on climate. In particular, under modern boundary conditions, the uplift of the main TP decreased annual precipitation throughout inland Asia and in South Asia, increased annual precipitation in East Asia, and markedly changed the summer and winter low-level winds in Asia. By contrast, the uplift of the marginal TP increased the annual precipitation in South Asia, strengthened the seasonality of winds and enlarged the monsoon domain in East Asia. Thus, although the main TP was the major part of the TP in spatial terms, its uplift did not have the dominant effects on climate in all aspects. Moreover, changes in the boundary conditions – for example, from modern to ~30Ma – could further influence the effects of the uplifts of the whole TP and its sub-regions on climate, particularly for the precipitation in South Asia and monsoon domain in East Asia. These results highlighted the different effects of the uplifts of the main and marginal TP on climate and emphasized the importance of accompanied boundary conditions when investigating the effects of the uplifts of the whole TP and its sub-regions on climate.

A lacustrine record of East Asian summer monsoon and atmospheric dust loading since the last interglaciation from Lake Xingkai, northeast China

AbstractA 336-cm-long sediment core spanning the last 130 ka was recovered from Lake Xingkai on the northeastern margin of the East Asian summer monsoon domain to reveal the linkage between lacustrine depositional processes and environmental changes. Bayesian end member modeling analysis was conducted to partition and interpret the grain-size distributions of Lake Xingkai sediments. Our results suggest that the sedimentary system is characterized by three end members (EMs). EM1 and EM2, with a modal value of 13 and 10 μm, respectively, indicate the variation of local hydraulic conditions. EM3, with a modal value of 5 μm, reflects the background atmospheric dust loading. High atmospheric dust concentration generally occurred during Marine Isotope Stage (MIS) 5d, MIS 4, and early MIS 3, when the climate in the Asian dust source region was cold and dry. In contrast, low dust concentration prevailed during MIS 2, likely due to the southward shift of the westerlies driven by maximum ice volume in the high latitudes.

Influence of Latent Heating over the Asian and Western Pacific Monsoon Region on Sahel Summer Rainfall

There has been an interdecadal shift towards a less humid state in Sahel summer rainfall since the 1960s. The decreased Sahel summer rainfall was associated with enhanced summer latent heating over the South Asian and western Pacific summer monsoon region and anomalous zonal-vertical cell of the Asian summer monsoon circulation, indicating that the latent heating plays a significant role in the change in Sahel rainfall. The effects of the latent heating over different monsoon domains on the Sahel rainfall are investigated through several model experiments. Results show that the remote monsoon heating mainly affects Sahel rainfall by generating changes in the zonal-vertical atmospheric circulation.

Environmental status of the Jilantai Basin, North China, on the northwestern margin of the modern Asian summer monsoon domain during Marine Isotope Stage 3

Two drill cores were obtained from the Jilantai sub-depression (JLT(d)) and the neighboring Dengkou sub-uplift (DK(u)), within a huge, former lake basin in northern China. From an analysis of the lithology and pollen assemblages, combined with radiocarbon dating of extracted pollen and OSL dating of extracted quartz, we concluded the following: JLT(d) was continuously occupied by lakes since 85ka; however, DK(u), the neighboring sub-uplift, was covered by lakes during 80–74ka, 50–44ka, 32.5–27.5ka and <13ka, and covered by sand during 44–32.5ka and 27.5–13ka. The evidence from lithology and dating results supports the occurrence of lakes on DK(u) during Marine Isotope Stage (MIS) 3. Evidence from shorelines, previously published cores, and the sedimentary and chronological evidence presented in this paper indicate the occurrence of a sub-humid environment, characterized by the occurrence of lakes separated by dunes, in the Jilantai Basin during MIS 3. However, further work is needed to understand the environmental significance of the co-existence of lakes and dunes during MIS 3, although a sub-humid climate background during MIS 3 is supported by well-dated geological archives along the western front of the present-day Asian Summer Monsoon domain and its eastern extensional area.

Non-linear intensification of Sahel rainfall as a possible dynamic response to future warming

Abstract. Projections of the response of Sahel rainfall to future global warming diverge significantly. Meanwhile, paleoclimatic records suggest that Sahel rainfall is capable of abrupt transitions in response to gradual forcing. Here we present climate modeling evidence for the possibility of an abrupt intensification of Sahel rainfall under future climate change. Analyzing 30 coupled global climate model simulations, we identify seven models where central Sahel rainfall increases by 40 to 300 % over the 21st century, owing to a northward expansion of the West African monsoon domain. Rainfall in these models is non-linearly related to sea surface temperature (SST) in the tropical Atlantic and Mediterranean moisture source regions, intensifying abruptly beyond a certain SST warming level. We argue that this behavior is consistent with a self-amplifying dynamic–thermodynamical feedback, implying that the gradual increase in oceanic moisture availability under warming could trigger a sudden intensification of monsoon rainfall far inland of today's core monsoon region.

Hydroclimate reconstruction in central Japan over the past four centuries from tree-ring cellulose δ18O

The East Asian summer monsoon is an important part of the Earth's climate system that is characterized by variations in the strength and expansion of the summer rain band. Rainfall reconstructions in China have revealed changing patterns of rainfall during the Little Ice Age (LIA), but limited hydroclimate reconstruction around Japan have hindered detailed understanding of physical processes associated with the atmospheric system in the western North Pacific. Here, we report relative humidity variations in the Meiyu/Baiu season from 1600–1959 CE by using tree-ring cellulose oxygen isotopes from central Japan; this is the first record of the LIA from the easternmost regions of the East Asian monsoon domain. Our data suggest that the wettest period occurred around 1780–1880 CE corresponding to the final stage of the LIA. This shift was concurrent with sea surface temperature negative (positive) anomalies around the Philippines (off eastern Japan). We suggest that meridional atmospheric circulation was weak during the last stage of the LIA.

Coherent tropical-subtropical Holocene see-saw moisture patterns in the Eastern Hemisphere monsoon systems

The concept of a Global Monsoon (GM) has been proposed based on modern precipitation observations, but its application over a wide range of temporal scales is still under debate. Here, we present a synthesis of 268 continental paleo-moisture records collected from monsoonal systems in the Eastern Hemisphere, including the East Asian Monsoon (EAsM), the Indian Monsoon (IM), the East African Monsoon (EAfM), and the Australian Monsoon (AuM) covering the last 18,000 years. The overall pattern of late Glacial to Holocene moisture change is consistent with those inferred from ice cores and marine records. With respect to the last 10,000 years (10 ka), i.e. a period that has high spatial coverage, a Fuzzy c-Means clustering analysis of the moisture index records together with “Xie-Beni” index reveals four clusters of our data set. The paleoclimatic meaning of each cluster is interpreted considering the temporal evolution and spatial distribution patterns. The major trend in the tropical AuM, EAfM, and IM regions is a gradual decrease in moisture conditions since the early Holocene. Moisture changes in the EAsM regions show maximum index values between 8 and 6 ka. However, records located in nearby subtropical areas, i.e. in regions not influenced by the intertropical convergence zone, show an opposite trend compared to the tropical monsoon regions (AuM, EAfM and IM), i.e. a gradual increase. Analyses of modern meteorological data reveal the same spatial patterns as in the paleoclimate records such that, in times of overall monsoon strengthening, lower precipitation rates are observed in the nearby subtropical areas. We explain this pattern as the effect of a strong monsoon circulation suppressing air uplift in nearby subtropical areas, and hence hindering precipitation. By analogy to the modern system, this would mean that during the early Holocene strong monsoon period, the intensified ascending airflows within the monsoon domains led to relatively weaker ascending or even descending airflows in the adjacent subtropical regions, resulting in a precipitation deficit compared to the late Holocene. Our conceptual model therefore integrates regionally contrasting moisture changes into the Global Monsoon hypothesis.