Monsoon Evolution Research Articles

Impact of Climate Change in West Africa on Cereal Production Per Capita in 2050

Food security is a crucial issue in the Sahel and could be endangered by climate change and demographic pressure during the 21st century. Higher temperatures and changes in rainfall induced by global warming are threatening rainfed agriculture in this region while the population is expected to increase approximately three-fold until 2050. Our study quantifies the impact of climate change on food security by combining climate modelling (16 models from CMIP5), crop yield (simulated by agronomic model, SARRA-O) and demographic evolution (provided by UN projection) under two future climatic scenarios. We simulate yield for the main crops in five countries in West Africa and estimate the population pressure on crop production to assess the number of available cereal production per capita. We found that, although uncertain, the African monsoon evolution leads to an increase of rainfall in Eastern Sahel and a decrease in Western Sahel under the RCP8.5 (Representative Concentration Pathway) scenario from IPCC, leading to the higher temperature increase by the end of the 21st century. With regard to the abundance of food for the inhabitants, all the scenarios in each country show that in 2050, local agricultural production will be below 50 kg per capita. This situation can have impact on crop import and regional migration.

Characterization of two complete mitochondrial genomes of Pterocryptis anomala (Siluridae) and its phylogeny and cryptic diversity

Pterocryptis anomala (Siluridae) is an endemic fish species that is distributed in the drainage basins of Southern China. In this study, we reported the complete mitochondrial genomes (mitogenomes) of two P. anomala specimens from different rivers using next-generation sequencing. The gene composition and the structural arrangement of the mitogenomes were similar to those of most other teleosts, although we examined a longer length of control region in these mitogenomes than that in most of published Siluridae mitogenomes. Phylogenetic analyses combining our novel sequences with published mitogenomes and mitochondrial cytochrome c oxidase I (COI) sequences indicated the monophyly of Pterocryptis and a sister relationship between species P. anomala and P. cochinchinensis. However, the monophyly of Pterocryptis was not very robust as a limited number of species were included in our analyses. Additionally, we uncovered that a hidden diversity might occur in P. anomala populations. The likely causes for this cryptic diversity might be the geologic movements of intense uplift of the Qinghai–Tibetan Plateau and evolution of Asian monsoons, as well as low vagility of P. anomala. Our results yielded crucially important insights into the genetic resources, phylogeny and diversity of this endemic species.

Variability of the Indian Monsoon in the Andaman Sea Across the Miocene‐Pliocene Transition

AbstractWe reconstructed the variability of the Earth's strongest hydrological system, the Indian monsoon, over the interval 6.24 to 4.91 Ma at International Ocean Discovery Program (IODP) Expedition 353 Site U1448 in the Andaman Sea. We integrated high‐resolution benthic and planktic foraminiferal carbon and oxygen isotopes with Mg/Ca measurements of the mixed layer foraminifer Trilobatus sacculifer to reconstruct the isotopic composition of seawater (δ18Osw) and the gradient between planktic and benthic foraminiferal δ13C. A prominent increase in mixed layer temperatures of ~4°C occurred between 5.55 and 5.28 Ma, accompanied by a change from precession‐ to obliquity‐driven variability in planktic δ18O and δ18Osw. We suggest that an intensified cross‐equatorial transport of heat and moisture, paced by obliquity, led to increased summer monsoon precipitation during warm stages after 5.55 Ma. Transient cold stages were characterized by reduced mixed layer temperatures and summer monsoon failure, thus resembling late Pleistocene stadials. In contrast, an overall cooler background climate state with a strengthened biological pump prevailed prior to 5.55 Ma. These findings highlight the importance of internal feedback processes for the long‐term evolution of the Indian monsoon.

In-phase and out-of-phase behavior of the East Asian summer and winter monsoons recorded in the South Yellow Sea sediment over the past 9.5 ka

AbstractThe variability of the East Asian monsoon (EAM) during the Holocene exhibits significant regional response, and its evolution needs further discussion. A well-dated, high-resolution sea-surface temperature record based on long-chain unsaturated alkenones, grain-size data, and clay mineral assemblages from the South Yellow Sea sediment is presented to investigate the sedimentary provenance and reconstruct the EAM over the past 9.5 ka. The results show that the sediments are most likely supplied by the Huanghe. The evolution of the East Asian summer monsoon (EASM) can be divided into three periods: strong and relatively stable conditions during 9.5–7.0 ka, weakened conditions during 7.0–1.5 ka, and strengthened conditions during 1.5–0 ka. The East Asian winter monsoon (EAWM) has experienced five periods: weakened conditions during 9.5–6.7 ka, weak and relatively stable conditions during 6.7–5.6 ka, strong and relatively stable conditions during 5.6–2.6 ka, strengthened conditions during 2.6–1.5 ka, and weak and stable conditions during 1.5–0 ka. Moreover, in-phase correlation was found between the EAWM and EASM at the orbital time scale in response to orbital-driven solar insolation, but out-of-phase correlation at a centennial time scale is predominantly associated with solar activity.

A high-resolution sediment record of East Asian summer monsoon from the northern South China Sea spanning the past 7500 years

High-resolution Holocene marine sediment records are limited, which hampers our understanding of paleomonsoon evolution. Continental shelf sediment records are derived mainly from terrestrial weathering products and are thus often sensitive to paleoclimate variations. In this study, the grain size and magnetic mineral composition of a well-dated sediment core (YJ Core) from the northern inner shelf of the South China Sea (SCS) were analyzed to provide a high-resolution record of the Holocene evolution of the East Asian summer monsoon (EASM). These magnetic and grain size records indicate that EASM intensity followed a general declining trend between approximately 6800 and 2000 cal yr BP. This general pattern is synchronous with other geologic archives from monsoon regions, and can be attributed to solar radiation forcing in the Northern Hemisphere. On centennial timescales, a weak EASM closely coincides with periods of weak solar activity. In addition, spectral analysis of clays reveals five prominent cycles, with periodicities of approximately 364, 202, 158, 119, and 104 years, which correspond to solar activity cycles. The similarities between the cyclicities of the Asian monsoon signal in sedimentary records and those of solar activity demonstrate that solar forcing has a relatively large influence on the centennial-scale variability of the EASM.

Mid-Holocene to present-day evolution of the Indian monsoon in transient global simulations

The low-frequency evolution of Indian rainfall mean-state and associated interannual-to-decadal variability is discussed for the last 6000 years from a multi-configuration ensemble of fully coupled global transient simulations. This period is marked by a shift of Indian Summer Monsoon Rainfall (ISMR) distribution towards drier conditions, including extremes, and a contraction of the rainy season. The drying is larger in simulations with higher horizontal resolution of the atmosphere and revised land surface hydrology. Vegetation–climate interactions and the way runoff is routed to ocean modulate the timing of the monsoon onset but have negligible effects on the evolution of seasonal rainfall amounts in our modeling framework in which carbon cycling is always active. This drying trend is accompanied by changes in ISMR interannual-to-decadal variability decreasing over north and south India but increasing over central India (20°–25° N). The ISMR interannual-to-decadal variability is decomposed into six physically consistent regimes using a clustering technique to further characterize its changes and associated teleconnections. From 6 to 3.8 kyr BP, the century-to-century modulations in the frequency of occurrence associated to the regimes are asynchronous between the simulations. Orbitally-driven trends can only be detected for two regimes over the whole 6–0 kyr BP period. These two regimes reflect increased influence of ENSO on both ISMR and Indian Ocean Dipole as the inter-hemispheric energy gradient weakens. Severe long-term droughts are also shown to be a combination of long-term drying and internally generated low-frequency modulations of the interannual-to-decadal variability.

Temperature Control on Silicate Weathering Intensity and Evolution of the Neogene East Asian Summer Monsoon

AbstractAccurately reconstructing the evolution of the Asian monsoon is predicated on understanding the impact of temperature, precipitation, and tectonic paleogeography on silicate weathering proxies over million year timescales. We find that decreasing trends in chemical weathering proxies in both the northern and southern Tibetan Plateau match those from benthic oxygen isotope and sea surface temperature stacks since the late Miocene. In contrast, a synthesis of magnetic parameter‐based records and marine clastic flux records reveals stronger monsoon precipitation during both the warm middle Miocene and cool late Miocene, supporting model simulations showing that both paleogeography and atmospheric CO2 content are important in controlling the summer monsoon. This trend of increasing monsoon precipitation contrasts with the record of decreasing chemical weathering, suggesting that chemical weathering proxies are mainly regulated by temperature at million year timescales, rather than precipitation. These findings clarify reconstructions of the Cenozoic evolution of the East Asian summer monsoon.

Holocene tephrostratigraphic framework and monsoon evolution of East Asia: Key tephra beds for synchronising palaeoclimate records

In East Asia our understanding of Holocene climate change, forcing mechanisms and propagation, require the precise chronological control of palaeoclimate records to allow robust integration of data sets. The existing chronologies, predominantly based on 14C method, however, are not sufficient to constrain key questions about abrupt climate shifts that occur within a century in the transitions between states. Widely dispersed tephra layers allow precise dating and synchronisation of sedimentary archives, providing a chronological framework for integrating records, especially where the visible tephra record is complemented by the addition of cryptotephra (non-visible ash). Despite significant tephra studies in this region, however, a comprehensive Holocene tephra framework is not available. To address this issue, we carry out a thorough review on Holocene tephra investigations undertaken in Japan. Using widespread tephra beds we present an integrated tephra framework and suggest the way forward for establishing this as a wider approach for East Asia. The framework is based on twenty-two ash layers that are mainly from Japan, and to a lesser extent China/N Korea, S Korea and Russia. Each tephra is assessed in terms of chronology, geochemistry and distribution. The framework is compared with high resolution palaeoclimate records from East Asia. Using this we demonstrate regional variations in monsoon evolution and more importantly, the potential of tephra isochrons in constraining such variations. Given the scarce identification of tephra layers in those well-resolved palaeoclimate records, we advocate a more systematic employment of the cryptotephra method, which would potentially lead to a significant advance in East Asian tephrochronology and the correlation of palaeoclimate archives in the region.

Reconstruction of monsoon evolution in southernmost Sumatra over the past 35 kyr and its response to northern hemisphere climate changes

Reconstruction of monsoon evolution in the tropical Indian Ocean and evaluation of its influence on large-scale ocean circulation and sea-air interaction processes can help us understand climate driving mechanisms. Herein, we used a gravity core SO184-10043 to present the marine sedimentary record from the southernmost Sumatra, spanning the past 35 kyr. The age model is well constrained by 12 accelerator mass spectrometry (AMS) radiocarbon dates determined in mixed planktic foraminifera. We also measured sediment grain size and major and trace elements to reconstruct the variations in hydrodynamic conditions and chemical weathering intensity. These data were then used to reveal the evolution of the northwest (Indonesian) monsoon system over the past 35 kyr. The results showed that both monsoon current and chemical weathering coincided with summer insolation in the Northern Hemisphere and responded to glacial-interglacial climate changes in the northern Atlantic. Higher grain-size values of sensitive components, increased chemical index of alteration (CIA), and Ba/Sr ratios, together with lower CaO/MgO ratios, reflect a stronger monsoon current and chemical weathering during the late marine isotope stage 3 and Holocene. Higher intensity of chemical weathering might be attributed to a stronger northwest monsoon carrying warm and moist air from the Asian continent to Indonesian waters. Lower values of sensitive components, CIA, and Ba/Sr, along with higher CaO/MgO values, reveal a weaker monsoon current and lower intensity of chemical weathering during the last glaciation-deglaciation phase, which was controlled by a weaker monsoon system. At a millennial timescale, cold events occurred during the following intervals: 30–26 ka BP, Heinrich Stadial 1 (HS1; 18–15 ka BP), Last Glacial Maximum (LGM; 23–18 ka BP), Younger Dryas (YD; 11–10 ka BP), and 8.2 ka BP. The same cold events were identified from core SO184-10043, indicating a perfect match with climatic cooling in the Northern Hemisphere. We also observed periodicities of ~ 7.9 ka, 3.0–3.1 ka, 2.2–2.3 ka, and ~ 1.0 ka in our core records. These periodicities are coincident with solar-induced climate changes and support the hypothesis that monsoon evolution in the tropical Indian Ocean during the late Pleistocene coincided with the climate change in the Northern Hemisphere.

Seasonal variability of cyclone heat potential and cyclonic responses in the Bay of Bengal characterized using moored observatories

Cyclone Heat Potential (CHP) is an essential parameter for accurate prediction of the intensity of tropical cyclones. The variability of the heat storage in the near-surface layers and the vertical stratification near the surface due to large fresh water inputs create challenges in predicting the intraseasonal and interannual evolution of monsoons and tropical cyclones in the Bay of Bengal. This paper for the first time presents the D26- referenced cyclone heat potential observed in the Bay of Bengal during the period 2012-17 based on the in-situ data collected from 5.5 million demanding offshore instrument-hours of operation in the Ocean Moored Buoy Network for Northern Indian Ocean (OMNI) buoy network by the National Institute of Ocean Technology. It is observed that the CHP in the Bay of Bengal varied from 0-220 kJ/cm during various seasons. From the moored buoy observations, a CHP of ~ 90 kJ/cm with the D26 isotherm of minimum 100m is favorable for the intensification of the post-monsoon tropical cyclones. The responses of the D26 thermal structure during major tropical cyclone events in the Bay of Bengal are also presented.

Late Holocene Moisture Changes in the Core Area of Arid Central Asia Reflected by Rock Magnetic Records of Glacier Lake Kalakuli Sediments in the Westernmost Tibetan Plateau and their Influences on the Evolution of Ancient Silk Road

AbstractThe evolution of Ancient Silk Road (ASR) was deeply influenced by late Holocene moisture changes in Arid Central Asia (ACA). Nevertheless, controversies in Holocene moisture change pattern of ACA and poorly–constrained age models of related studies have made the discussion about late Holocene moisture changes in ACA and their influences on the evolution of ASR difficult. Recently, a high–resolution age model during the late Holocene was established for Kalakuli Lake, a small glacier lake located in the core area of ACA. A thorough rock magnetic investigation was carried out on Kalakuli Lake sediments based on this age model. The magnetic mineral assemblage of Kalakuli Lake sediments is still dominated by primary magnetite despite minor diagenetic effects. Comparisons of rock magnetic records to parameters previously used as indicator of glacier fluctuations suggest that clastic input to Kalakuli Lake was high (low) and magnetic grain size is relatively larger (smaller), when glaciers on Muztagh Ata advanced (retreated). The ARM/SIRM ratio, a magnetic grain size proxy, is directly related to lake hydrodynamics, which are ultimately controlled by glacier fluctuations on Muztagh Ata as the result of regional moisture changes. Late Holocene moisture changes indicated by the ARM/SIRM ratio are consistent with cool/wet and warm/dry oscillations indicated by the unweighted average of biomarker hydrogen isotopic data of the C26 and C28 n–alkanoic acids in a previous study about Kalakuli Lake, most moisture change records of the core area of ACA and winter insolation of the Northern Hemisphere, but opposite to Asian monsoon evolution. Given Asian monsoon and the westerlies are mutually inhibited, we propose that late Holocene moisture changes in the core area of ACA were controlled by the intensity of Asian monsoon versus the westerlies under the governance of solar insolation. Generally increased moisture since the late Holocene indicated by the ARM/SIRM ratio favored cultural exchange and integration between the western and the eastern Eurasia, which paved the way for the formation of ASR. Coincidence between significant increase in moisture at ~200 BC suggested by the ARM/SIRM ratio and the formation of ASR indicates moisture as an important factor that facilitated the formation of ASR. The onsets of three prosperity periods of ASR in the history generally correspond to periods when moisture was relatively high, nevertheless, stagnant periods of ASR do not coincide with periods when moisture was relatively low in the core area of ACA. Disorganized correlations between stagnant periods of ASR and moisture changes in the core area of ACA suggest that moisture is not the decisive factor influencing the evolution of ASR.

Sensitivity of altitudinal vegetation in southwest China to changes in the Indian summer monsoon during the past 68000 years

Reconstructing vegetation changes during a glacial-interglacial cycles in southwest (SW) China is crucial for understanding the amplitude of palaeoclimatic dynamics associated with orbital forcing and evolution of the Indian summer monsoon (ISM). However, few studies on the relationship between paleoclimate and ecosystem changes have covered all of the last glacial-interglacial cycle (or any other), which limits understanding of the ISM’s variability and its impact on mountain flora. In particular, knowledge of responses of vertical vegetation belts in margins of the high Tibetan plateau to glacial and subglacial (MIS3) climate changes is poor. To improve understanding, we have studied a continuous core extending back to MIS4 from Tengchong Qinghai volcanic lake in Yunnan Province. Identification of Quercus pollen by scanning electron microscopy showed that most of the abundant oak pollen in the core can be assigned to mountainous taxa of sclerophyllous Quercus living today on the high Tibetan plateau. Their current distribution indicates that the maximum downward shift of vertical vegetation belts during the last glacial period corresponds to a ∼5.5 °C drop in temperature, based on altitudinal lapse rates, in accordance with mean annual temperature (MAT) reconstruction from biomarkers in the same core. Our data clearly suggest that conditions were wet during MIS3, and humidity highest at 38–56 kyr. The wetting appears to have started after the Last Glacial Termination, indicating that the initial deglacial ISM intensification lagged behind rises in temperature and summer insolation. The highest charcoal concentrations and evidence of expansion of open vegetation coincide with the apparent onset of deglaciation (18-16 kyr). These observations indicate that this was the driest period, likely linked with reductions in the polar ice volume and delayed Tibetan glacier melting, which absorbed massive amounts of atmospheric heat and caused regional aridity. Our pollen data clearly correlate with rapid changes such as Younger Dryas and Heinrich Events, and marine records from the Indian Ocean, confirming climate teleconnection through ice volumes and global ocean circulation, and that the ISM is a key factor driving south-north transportation of energy and moisture. The multi-proxy data reveal with high confidence that the Holocene optimum in the region was between 9 and 4 kyr BP, and followed by weakening of the ISM accompanied by increasing anthropogenic disturbance of the vegetation.

Phased evolution and variation of the South Asian monsoon, and resulting weathering and surface erosion in the Himalaya–Karakoram Mountains, since late Pliocene time using data from Arabian Sea core

AbstractWe investigate the phased evolution and variation of the South Asian monsoon and resulting weathering intensity and physical erosion in the Himalaya–Karakoram Mountains since late Pliocene time (c.3.4 Ma) using a comprehensive approach. Neodymium and strontium isotopic compositions and single-grain zircon U–Pb age spectra reveal the sources of the deposits in the east Arabian Sea, and show a combination of sources from the Himalaya and the Karakoram–Kohistan–Ladakh Mountains, with sediments from the Indian Peninsula such as the Deccan Traps or Craton. We interpret shifts in the sediment sources to have been forced by sea-level changes that correlate with South Asian monsoon rainfall variation since late Pliocene time. We collected 908 samples from the International Ocean Discovery Program Hole U1456A, which was drilled in the east Arabian Sea. Time series of hematite content and grain size of the sediments were examined downcore. We found South Asian monsoon precipitation and weathering intensity experienced three phases from late Pliocene time. Lower monsoon precipitation, with a lower variability and strong weathering intensity, occurred during 3.4–2.4 Ma; an increased and more variable South Asian monsoon rainfall, along with strengthened but fluctuating weathering intensity, occurred at 1.8–1.1 Ma; and a reduced rainfall with lower South Asian monsoon precipitation variability and moderate weathering intensity marked the period 1.1–0.1 Ma. Maximum entropy spectral analysis and wavelet transform show that there were orbital-dominated cycles of periodsc.100 andc.41 ka in these proxy-based time series. We propose that the monsoon, sea level, global temperature and insolation together forced the weathering and erosion in SW Asia.

First documentation of seismic stratigraphy and depositional signatures of Zhongsha atoll (Macclesfield Bank), South China Sea

Carbonate platforms form informative archives for paleoclimates and their internal structures can also hold crucial information about the tectonic history and carbonate evolution of the ocean basins. The Zhongsha atoll (Macclesfield Bank) forms the largest atoll system in the South China Sea with a surface area of 23500 km 2 . However, the internal structure and evolution of this atoll system is completely unknown. 2D multichannel seismic reflection data were acquired in 2017 over the Zhongsha atoll in the South China Sea to unravel the stratigraphy, geomorphology, depositional processes, and seismic facies of one of the world's largest atoll for the first time. This Neogene carbonate platform comprises more than 1 km thick carbonate sequence and overlies a metamorphic basement. The southeastern part of the atoll comprises a fault-controlled graben system, which was formed during the Cenozoic rifting stage of the South China Sea. Most of the faults trend NE-SW and E-W and terminate at or slightly above the top of Middle Miocene strata. Atolls and abundant organic reefs initiated on the positive relief and closely mimicked the underlying topography during the Early Miocene. Shallow-water carbonates continued growing through Middle Miocene to present times. Regional uplift led to subaerial exposure, termination of platform growth and karstification during the Miocene. We also reveal a number of fluid-flow features such as vertical sub-bottom venting features (chimneys and pipes), chaotic reflection zones, which provide the first evidence of active fluid venting in the area of Zhongsha atoll. The Neogene sedimentation history of Zhongsha atoll further provide an important paleoenvironmental context for future scientific drilling to better constrain the evolution of Asia Monsoon. • First documentation of seismic stratigraphy and deposition of Zhongsha atoll (Macclesfield Bank), South China Sea. • Reefs and lagoons developed during the post-rift phase, whose onset is marked by a turning point around the time of the formation of reflector T50. • Three drowning events are identified to tentatively imply three episodes of fast sea floor spreading in the study region.

Quantitative reconstruction of the East Asian Winter Monsoon evolution over the past 100 years: Evidence from high-resolution sedimentary records of the inner continental shelf of the East China Sea

The East Asian Monsoon (EAM) is a regional factor affecting the East Asian climate and the oceanographic processes of the marginal seas along the Western Pacific. Finding proxies for the EAM intensity and reconstructing the interannual and interdecadal variations of the EAM using high-resolution records are necessary to improve our understanding of the EAM’s role in the global climate system and for predicting climate change. In this paper, high-resolution sedimentary records of sedimentary core C0702 obtained from the inner continental shelf of the East China Sea were comprehensively analyzed using a laser particle size analyzer, an ItraxTM core scanner, and a 210Pb and 137Cs radionuclide analyzer to explore potential proxies for the East Asian Winter Monsoon (EAWM). By combining the obtained results with instrumental observations of the EAM, we established a quantitative formula for the EAWM, which enables to reconstruct the evolution trend of the EAWM during the past 130 years. The sensitive grain-size component F2, with a grain-size range of 14.35–230 µm, and principal component PC1 of the sedimentary deposits of the East China Sea inner shelf can be used as EAWM proxies. The evolution of EAWM in 1880–1950 could be roughly divided into two stages: a weak EAWM period from 1882 to 1900 and a strong EAWM period from 1900 to 1945. This study improves our understanding of the variations in the EAWM on interannual and interdecadal temporal scales.

Role of the Surface Boundary Conditions in Boreal Spring on the Interannual Variability of the Multistage Evolution of the East Asian Summer Monsoon

AbstractThe seasonal movement of the upper-level jet plays a key role in the evolution of the East Asian summer monsoon (EASM). However, it remains unresolved how interannual changes in surface boundary conditions can influence the upper-level flow over East Asia, thereby modulating the onset of the EASM. Here we capture the timing of multistage evolution over East Asia using the upper-level zonal wind in a two-phase linear regression model. In addition, we show the impact of two surface boundary conditions on the timing of the EASM onset related to the strength of the upper-level zonal wind: 1) eastern Eurasian snow cover and 2) western North Pacific (WNP) sea surface temperature (SST) tendency. The eddy heat fluxes induced by the enhanced eastern Eurasian snow cover develop an anomalous anticyclonic circulation to the northwest, which causes anomalous warm southwesterly flow toward the north. These can make a reversal of the meridional temperature gradient, which results in the early monsoon onset via changes in the upper-level jet. The upper-level jet also responds to the SST tendency in April over the WNP via thermal wind balance and the resultant changes in transient eddy-induced heat transport. Our findings suggest potential sources for seasonal predictability in the interannual EASM onset dates.

Iron oxide characteristics of the Chinese loess-red clay sequences and their implications for the evolution of the East Asian summer monsoon since the Late Oligocene

The most significant climatic change in East Asia involved the formation and evolution of the coupled monsoon-arid environmental system during the Cenozoic, which was dynamically linked to global cooling and the growth of the Qinghai-Tibet Plateau. However, our understanding of monsoon evolution since the Late Oligocene remains controversial due to differences in the interpretation of various environmental proxies from loess archives. In this study, the composition and relative abundance of pedogenic iron oxides of two eolian sequences from the Lingtai section and the Zhuanglang drill core were investigated using rock magnetic, optical spectroscopy, and geochemical analyses. The results indicate that while the composition of iron oxides in the Mio-Pliocene red clay and Quaternary loess-paleosol sequences is similar, the concentrations of magnetite, maghemite, hematite and goethite are quite different between loess, paleosol and red clay. The combination of magnetic proxies (magnetic susceptibility, Fed/Fet, and color a* values) reveals that precipitation and temperature played varying roles in driving pedogenesis and chemical weathering since the Late Oligocene. During the Late Oligocene to Early Miocene, the occurrence of relatively strong pedogenesis and chemical weathering was likely the result of high temperatures rather than high precipitation. The East Asian summer monsoon intensified during the Middle Miocene Climate Optimum (16.5–14.5 Ma), in response to global climatic warming; subsequently, during the Middle to Late Miocene, the monsoon intensity gradually weakened due to global cooling. After ~8 Ma, the summer monsoon strengthened substantially and reached a maximum during 4.5–2.6 Ma, possibly as a result of the uplift of the northeastern margin of the Qinghai-Tibetan Plateau during the Late Miocene. The monsoon intensity then commenced a decreasing trend in the Early Pleistocene, but then strengthened again after ~1.0 Ma.

Sub‐seasonal prediction of rainfall over the South China Sea and its surrounding areas during spring–summer transitional season

AbstractThe sub‐seasonal characteristics and prediction of rainfall over the South China Sea and surrounding areas during spring–summer transitional season (April–May–June) are investigated using a full set of hindcasts generated by the Dynamic Extended Range Forecast operational system version 2.0 (DERF2.0) of Beijing Climate Center, China Meteorological Administration. The onset and development of Asian summer monsoon and the seasonal migration of rain belt over East Asia can be well depicted by the model hindcasts at various leads. However, there exist considerable differences between model results and observations, and model biases depend not only on the lead time, but also on the stage of monsoon evolution. In general, forecast skill drops with increasing lead time but rises again after lead time becomes longer than 30 days, possibly associated with the effect of slowly‐varying forcing or atmospheric variability. An abrupt turning point of bias development appears around mid‐May when bias growths of wind and precipitation exhibit significant changes over the northwestern Pacific and South Asia, especially over the Bay of Bengal and the South China Sea. This abrupt bias change is reasonably captured by the first two modes of multivariate empirical orthogonal function analysis, which reveals several important features associated with the bias change. This analysis may provide useful information for further improving model performance in sub‐seasonal rainfall prediction.

Characteristics of Atmospheric Heat Sources in the Tibetan Plateau-Tropical Indian Ocean Region

Investigating the temporal and spatial distributions of the atmospheric heat sources (AHS) over the Tibetan Plateau-Tropical Indian Ocean (TP-TIO) region is of great importance for the understanding of the evolution and development of the South Asian summer monsoon (SASM). This study used the Japanese 55-year Reanalysis (JRA-55) data from 1979 to 2016 and adopted statistical methods to study the characteristics of the AHS between the TP and TIO, and theirs link to the SASM on an interannual scale. The results indicated that the monthly variations of the AHS in the two regions were basically anti-phase, and that the summer AHS in the TP was obviously stronger than that in the TIO. There were strong AHS and atmospheric moisture sink (AMS) centers in both the eastern and western TP in summer. The AHS center in the east was stronger than that in the west, and the AMS centers showed the opposite pattern. In the TIO, a strong AHS center in the northwest-southeast direction was located near 10°S, 90°E. Trend analysis showed that summer AHS in the TIO was increasing significantly, especially before 1998, whereas there was a weakening trend in the TP. The difference of the summer AHS between the TP and TIO (hereafter IQ) was used to measure the thermal contrast between the TP and the TIO. The IQ showed an obvious decreasing trend. After 1998, there was a weak thermal contrast between the TP and the TIO, which mainly resulted from the enhanced AHS in the TIO. The land-sea thermal contrast, the TIO Hadley circulation in the southern hemisphere and the SASM circulation all weakened, resulting in abnormal circulation and abnormal precipitation in the Bay of Bengal (BOB).

Sixty-year quasi-period of the Asian monsoon around the Last Interglacial derived from an annually resolved stalagmite δ18O record

The 60-year quasi-periodic variability of the Asian monsoon in the mid-late Holocene has been found to be widespread in instrumental and various terrestrial records. However, whether this period exists in the older interglacial periods remains to be further studied. This paper is based on an 1840-mm-long stalagmite with clear annual laminae (with a growth rate of about 1 mm/yr) from Yongxing Cave, China, that allows for the reconstruction of the Asian monsoon evolution sequence during the period ~123.20–121.21 kyr BP. The stalagmite record closely tracks the 65°N solar insolation changes as a whole, indicating the dominant control of solar insolation on the Asian summer monsoon. After removing the solar insolation signal from the YX108 record, the δ18O residuals (Δδ18O) show 28 multidecadal monsoon cycles. The average amplitude of these fluctuations is larger than 0.8‰, with a duration typically ranging from 50 to 80 yr (about 64 yr on average), which is very similar to the mid-late Holocene Asian monsoon variability. Further, wavelet analysis displays a significant quasi-periodicity of 60 years throughout the studied period, providing further evidence for assessing whether the 60-year quasi-periodic variability of the Asian monsoon found in the Holocene exists in older periods. Our study thus suggests that this quasi-periodic variability not only occurred during the modern period but also existed in the Last Interglacial, and their influence may even be an intrinsic characteristic of the Asian monsoon system during glacial-interglacial cycles. We manifested that the interactions between solar activity and the Atlantic Multidecadal Oscillation may be the main reason for the 60-year quasi-period of the Asian monsoon.