Weakened Asian Monsoon Research Articles

The Interdecadal Pacific Oscillation is responsible for the linkage of decadal changes in precipitation and moisture in arid central Asia and the humid Asian monsoon region during the last millennium

Abstract. Reconstruction and observational studies imply a potential linkage of moisture and precipitation change in arid central Asia and monsoonal East Asia, in which the evolution of moisture and precipitation in central Asia is out of phase with that in northern China but in phase with that in southern China. In order to ascertain whether there is a robust linkage between the changes in climate in Asian arid regions and monsoon regions and to elucidate the underlying dynamic mechanisms, we analyzed the Last Millennium Reanalysis dataset and outputs from the Community Earth System Model Last Millennium Ensemble (CESM-LME). The results indicate a significant decadal linkage between precipitation changes in central Asia's arid region and the Asian monsoon region during the last millennium, which is primarily driven by the Interdecadal Pacific Oscillation (IPO). In spring, the positive IPO could enhance westerlies over the Mediterranean Sea and to its east, which could transport more water vapor and cause increased precipitation over central Asia. In summer, the positive IPO is accompanied by a weakened Asian monsoon and southward Asian subtropical westerly jet, which can lead to increased (decreased) summer precipitation over southern China (over northern China and South Asia). The IPO plays a dominant role in connecting the decadal variations in precipitation between arid central Asia and monsoonal Asia by modulating the precipitation of their respective major rainy seasons. Model results suggest that this decadal linkage stems entirely from the internal variability present in the CESM-LME control and all single-forcing simulations. Changes in external forcing factors do not alter this inherent linkage caused by the IPO. Moreover, based on analyses of the aridity index and soil moisture content, this relationship of precipitation variation also causes a similar decadal linkage of moisture changes in central Asia and monsoonal Asia. The differences in the multi-centennial-scale moisture and precipitation variations in the Asian arid region and the monsoon region between the Medieval Climate Anomaly and Little Ice Age are also likely caused by IPO-like sea surface temperature anomalies.

Impact of centennial-scale solar activity reduction on the weakened Asian monsoon event at 9.2 ka BP

The weakened Asian monsoon event at 9.2 ka has been documented in many proxy-based reconstructions, but its underlying causes are uncertain. In this study, we investigated this event from a regional monsoon perspective by analysing simulations of Holocene transient solar activity forcing using the Community Earth System Model and high-resolution proxies. Our results revealed two separate periods of decreasing precipitation in the western North Pacific (WNP) and South Asian (SA) monsoons from 9.5 to 9.2 ka, each lagging behind two remarkable weakening events (0.38 and 0.21 W m−2) in solar forcing. However, there was no significant change in East Asian summer monsoon precipitation at 9.2 ka based on our experiment. Moisture budget analysis indicated that the dynamic effects term (i.e., anomalous descending motion) figured prominently in decreasing WNP and SA monsoon precipitation. Such a change was affected by an anomalous WNP anticyclone, which was maintained by the ‘wind-evaporation-sea surface temperature’ feedback during the decay phase of the centennial-scale El Niño-like event. The development of centennial El Niño-like conditions was caused by a reduction in solar irradiance. Reduced solar irradiance induced cooling over northern Australia, generating anomalous zonal sea level pressure gradients and westerlies over the tropical Pacific, ultimately resulting in the development of an El Niño-like pattern.

Investigating and predicting spatiotemporal variations in vegetation cover in transitional climate zone: a case study of Gansu (China)

Vegetation ecosystems are sensitive to large-scale climate variability in climate transition zones. As a representative transitional climate zone in Northwest China, Gansu is characterized by a sharp climate and vegetation gradient. In this study, the spatiotemporal variations of vegetation over Gansu are characterized using the satellite-based normalized difference vegetation index (NDVI) observations during 2000–2020. Results demonstrate that a significant greening trend in vegetation over Gansu is positively linked with large-scale climate factors through modulating the water and energy dynamics. As a climate transition zone, the northern water-limited and southern energy-limited regions of Gansu are affected by water and energy dynamics, differently. In the water-limited region, a weakening Asian monsoon along with colder Central Pacific (CP) and warmer North Pacific (NP) Oceans enhances prevailing westerlies which bring more atmospheric moisture. The enhanced atmospheric moisture and rising temperature promote the local vegetation growth. In contrast, large-scale climate variations suppress the southwest monsoon moisture fluxes and reduce precipitation in southern energy-limited regions. In these energy-limited regions, temperature has more effects on vegetation growth than precipitation. Therefore, the greenness of vegetation is because of more available energy from higher temperatures despite overall drying conditions in the region. Based on the above mechanism, future scenarios for climate impacts on vegetation cover over Gansu region are developed based on the two latest generation from coupled climate models (Coupled Model Intercomparison Project Phase 5 and Phase 6; CMIP5 and CMIP6). In the near-term future (2021–2039), the vegetation is likely to increase due to rising temperature. However, the vegetation is expected to decrease in a long-term future (2080–2099) when the energy-limited regions become water-limited due to increasing regional temperatures and lowering atmospheric moisture flux. This study reveals an increasing desertification risk over Gansu. Similar investigations will be valuable in climate transition regions worldwide to explore how large-scale climate variability affects local ecological services under different future climate scenarios.

Tree‐ring oxygen isotope recorded precipitation variations over the past two centuries in the northeast Chinese Loess Plateau

AbstractIt was shown in previous studies that tree‐ring oxygen isotope (δ18OTR) was a proxy of regional precipitation. Using two δ18OTR chronologies of Pinus tabulaeformis Carr. in northeast Chinese Loess Plateau (CLP), we synthesized a master series which had a high correlation with regional precipitation from previous‐year September to current‐year August (PSA) (r = −0.650, N = 52, p < .001). Thus we reconstructed the PSA over the past two centuries (AD 1808–2009) for the northeast CLP. The results of calibration–verification tests indicated that the precipitation reconstruction was stable and reliable. Compared with reconstructions based on the tree‐ring width and historical documents, the PSA reconstruction reserved more decadal low‐frequency climate signals such as the drying trend since the 1930s. Superposed epoch analysis (SEA) showed that the large tropical volcanic eruptions significantly reduced PSA in the first year after the eruptions. The contrast of the regional precipitation reconstructions in the west, northeast, and southeast CLP demonstrated that spatial and temporal differences of precipitation variation. In terms of extreme climate events, major droughts in the mid‐to‐late 1920s were recorded in all three regional precipitation records, while the Ding‐Wu Disaster (AD 1876–1878) was only found in the southeast CLP. Under the background of the weakening Asian monsoon and global warming, CLP was more prone to droughts in the last five decades.

Early and late Holocene paleoenvironmental reconstruction of the Pearl River estuary, South China Sea using foraminiferal assemblages and stable carbon isotopes

Proxy reconstructions of estuarine evolution provide perspectives on regional to global environmental changes, including relative sea-level changes, climatic changes, and agricultural developments. Here, we present a new benthic foraminiferal record along with δ13C and C/N, and lithological data from a sediment core in the Pearl River estuary (Lingding Bay) adjacent to the South China Sea. The core has relatively thick Holocene sediments (>40 m) due to its location in the paleo-valley of the Pearl River. The lithologic and foraminiferal record reveal an evolution in paleoenvironment from fluvial, inner estuary to middle estuary between 11300 and 8100 cal a BP in response to rapid sea-level rise. δ13C and C/N data indicate high freshwater discharge from 10500 to 8100 cal a BP driven by a strong Asian monsoon. The middle Holocene (8100 - 3300 cal a BP) sediment is absent in the core due to subaqueous erosion resulting from the unique geomorphology of the Pearl River Delta. In the late Holocene from 3300 cal a BP to the present, the lithology and foraminiferal assemblages suggest a further evolution from outer estuary, middle estuary channel, to middle estuary shoal, resulting from deltaic progradation under stable relative sea levels. In the last 2000 years, δ13C and C/N values reveal the intensive development of agriculture coupled with the reduction of freshwater input derived from a weakening Asian monsoon.

Inconsistent sea surface temperature and salinity changing trend in the northern South China Sea since 7.0 ka BP

The South China Sea (SCS) is an important moisture and heat source in the Asian monsoon system, and its environmental changes influence economic and social stability in Southeast Asia and East Asian. Previous records on sea surface temperature (SST) and salinity (SSS) were mostly from one sediment core, and/or only showed fragmentary information. It is hard to provide insights into the mechanisms of the overall paleoceanographical environmental variability in the northern SCS. In this study, paired δ18O and Mg/Ca records of planktonic foraminifer Globigerinoides ruber from Core S0204B nearby the Luzon Strait (21°13.818′N, 118°3.077′, water depth 1822 m), and Core GHE 27L (19.85°N, 115.34°E, water depth of 1533 m) in the central northern SCS, were analyzed, so as to better understand the physical process of the SST and SSS changes in the northern SCS and its relationship with the tropical Pacific. The results demonstrate that the SST is relatively stable and seawater δ18O (δ18Osw) values, a salinity proxy, shows a decrease trend since 7.0 ka BP in Core S0204B. By contrast, SST and δ18Osw in Core GHE 27L exhibit an overall increasing trend at the same time, indicating apparent regional discrepancy on the changing trend of the SST and SSS in the northern SCS. We suggest that the thermal and hydrographic conditions in Core S0204B were primarily influenced by the outer western Pacific water. However, the thermal and hydrographic conditions in Core GHE 27L may be predominantly controlled by the weakened Asian monsoon during the mid-late Holocene, which was induced by the southern migration of Intertropical Convergence Zone (ITCZ) and stronger EI Niño Southern Oscillation.

Aromatic acids in a Eurasian Arctic ice core: a 2600-year proxy record of biomass burning

Abstract. Wildfires and their emissions have significant impacts on ecosystems, climate, atmospheric chemistry, and carbon cycling. Well-dated proxy records are needed to study the long-term climatic controls on biomass burning and the associated climate feedbacks. There is a particular lack of information about long-term biomass burning variations in Siberia, the largest forested area in the Northern Hemisphere. In this study we report analyses of aromatic acids (vanillic and para-hydroxybenzoic acids) over the past 2600 years in the Eurasian Arctic Akademii Nauk ice core. These compounds are aerosol-borne, semi-volatile organic compounds derived from lignin combustion. The analyses were made using ion chromatography with electrospray mass spectrometric detection. The levels of these aromatic acids ranged from below the detection limit (0.01 to 0.05 ppb; 1 ppb = 1000 ng L−1) to about 1 ppb, with roughly 30 % of the samples above the detection limit. In the preindustrial late Holocene, highly elevated aromatic acid levels are observed during three distinct periods (650–300 BCE, 340–660 CE, and 1460–1660 CE). The timing of the two most recent periods coincides with the episodic pulsing of ice-rafted debris in the North Atlantic known as Bond events and a weakened Asian monsoon, suggesting a link between fires and large-scale climate variability on millennial timescales. Aromatic acid levels also are elevated during the onset of the industrial period from 1780 to 1860 CE, but with a different ratio of vanillic and para-hydroxybenzoic acid than is observed during the preindustrial period. This study provides the first millennial-scale record of aromatic acids. This study clearly demonstrates that coherent aromatic acid signals are recorded in polar ice cores that can be used as proxies for past trends in biomass burning.

Centennial-scale monsoon climate fluctuations from a stalagmite record during the mid-Holocene Epoch in Fulu cave of Huaping, Yunnan, China

The mid-Holocene dynamics of the East Asian Monsoon (EAM) and Indian Monsoon (IM) are important for understanding the Holocene climate system. To clarify the relationship between EAM and IM subsystems during the mid-Holocene, a 2085-year (6,270–4,185 a BP) high-resolution record from stalagmite (FL4) in Yunnan, China was reconstructed using ICP-MS-230Th series dating and carbon and oxygen isotope analysis (δ13C and δ18O). In the study period, successive positive δ18O trends revealed a generally weakening Asian monsoon, with the monsoon climate tending toward gradual drying, especially during three centennial-scale drought events in 6,270–6,126 a BP, 5,347–5,140 a BP, and 4,810–4,620 a BP. On the other hand, the uninterrupted negative trend in δ13C signified natural improvements in vegetation overlying the cave, and a serious deficit (~2.5 ‰) in δ13C during 5,519–5,345 a BP implied a heavy rainfall event, in precise phase with δ18O, demonstrating an enhanced Indian Monsoon subsystem. The dense sawtooth-shaped pattern of the carbon and oxygen isotope records indicates that a series of decadal-scale abrupt climate changes were superimposed on the centennial-scale monsoon climate changes. FL4 stalagmite records in the mid-Holocene reflect a gradually weakening monsoon climate with superimposed decadal–centennial events, but natural improvements in local vegetation through self-adjustment.

Shrinking lakes in Tibet linked to the weakening Asian monsoon in the past 8.2 ka

Expansion or shrinkage of closed lakes is a natural response to fluctuations in precipitation and evaporation, linked closely to changes in strength or position of atmospheric circulation. In Tibet, there are many such lakes with paleo-shorelines that can be used for reconstructions of climate history. Despite the fact that many paleo-shorelines are well preserved in Tibet, dating them has been seriously hindered by various difficulties. Here we present the first optical dating chronology for a series of paleo-shorelines in Zhari Namco, the third-largest inland lake in central Tibet. Our results indicate that the lake level has dropped 128m over the past 8.2ka. Younger shorelines are found at lower altitudes, indicating that the shorelines follow a geomorphic-chronological order and a broadly continuous trend of stepwise shrinkage. The surface area of Zhari Namco has shrunk in size from 4605km2 at 8.2ka ago to 996km2 at present; 300km3 of water has been lost from this lake. Such a loss in water implies a significant reduction in precipitation over the past 8.2ka, a likely result of a weakening Asian monsoon. Following the decreasing precipitation since the early Holocene, this area has become increasingly arid.

Tropical East African climate change and its relation to global climate: A record from Lake Tanganyika, Tropical East Africa, over the past 90+ kyr

Forcing mechanisms of tropical climate in continental areas remain poorly understood, due in large part to a lack of continuous, long-term, high-fidelity records. Sediment core T97-52V from Lake Tanganyika provides new insight into the timing and mechanisms behind East African climate change over the past 90+ kyr. This record is particularly important, because, other than a recently recovered scientific drill core from Lake Malawi, there are no other continuous, well-dated records from East Africa prior to 60 ka. The high resolution age model presented here provides a large degree of age certainty for the past 45+ kyr, and our suite of proxies allows a thorough examination of Lake Tanganyika's dynamics. From core stratigraphy and chemical analyses, we present evidence of a lake level drop greater than 400 m sometime prior to ∼ 90 ka, much greater than that inferred for the LGM, suggesting a period of intense aridity sometime around 100 ka. Additionally, core T97-52 V preserves evidence of worm burrows detected by X-radiographic imagery as indicated by burrow-shaped deposits of iron oxide, indicating a shallow lake at the time of deposition of that material. Intermittently high lake levels between ∼ 78 ka and ∼ 72 ka developed at the same time as a weakened Asian monsoon and a pluvial phase in Northeast Brazil, suggesting a global reorganization of climate, possibly forced by a reduction in orbital eccentricity. Over the past 60 ka this core preserves the same events recorded in a core collected ∼ 100 km away in the southern basin of Lake Tanganyika, including an unexplained increase in biogenic silica at ∼ 37 ka, suggesting that this vast lake is responding coherently across both major bathymetric basins to regional and global climate forcing.

Trends and abrupt changes of precipitation maxima in the Pearl River basin, China

AbstractWe applied the Mann‐Kendall (MK) test and Bayesian model to systematically explore trends and abrupt changes of the precipitation series in the Pearl River basin. The results showed that no significant trends were detected for annual precipitation and summer or winter precipitation totals. Significant negative trends were identified for the number of rainy days across the Pearl River basin; significant positive trends were observed regarding precipitation intensity (PI). In particular, the precipitation totals and frequencies of extremely high precipitation events are subject to significant positive trends. In addition, the number of extremely low precipitation events was also increasing significantly. Factors affecting the changes in precipitation patterns are the weakening Asian monsoon and consequently increasing moisture transport to Southern China and the Pearl River basin. In summary, the main findings of this study are: (1) increased precipitation variability and high‐intensity rainfall was observed though rainy days and low‐intensity rainfall have decreased, and (2) the amount of rainfall has changed little but its variability has increased over the time interval divided by change points. These finds indicate potentially increased risk for both agriculture and in locations subject to flooding, both urban and rural, across the Pearl River basin. Copyright © 2009 Royal Meteorological Society

A Coupled Model Study of Glacial Asian Monsoon Variability and Indian Ocean Dipole

A coupled climate model is used to study the response of interannual variability of Asian summer monsoon, and its relation with tropical climate variability over the Pacific and Indian Oceans. Two simulations are performed, one for the present and the other for the Last Glacial Maximum. It is found that at glacial times, the weakened Asian monsoon climatology results in a significant shallowing of mean tropical thermocline in the central-western Indian Ocean. This shallower thermocline intensifies the positive ocean-atmosphere feedback in the western Indian Ocean, and therefore contributes to an enhanced Indian Ocean Dipole (IOP) variability. The increased IOD variability in turn exerts a stronger affect on the variability of South Asian monsoon rainfall, and therefore may have contributed to a significant reduction of correlation between the monsoon and the Pacific El Nino/Southern Oscillation (ENSO) variability.

Long-term trend and abrupt events of the Holocene Asian monsoon inferred from a stalagmite δ10O record from Shennongjia in Central China

A high-resolution oxygen-isotope record from a thorium-uranium-dated stalagmite from Shanbao Cave at Shennongjia reflects variations in the amount of monsoon precipitation for the period from 11.5 to 2.1 ka (1 ka = 1000 cal aBP). Between 11.5 and 9.3 ka, a sharp decrease in δ18O indicates a rapid increase in monsoon precipitation. An interval of generally high monsoon precipitation is observed between 9.3 and 4.4 ka. An arid period has prevailed between 4.4 and 2.1 ka. The long-term trend of Shanbao record appears to follow summer insolation at 33°N latitude. An abrupt decrease in monsoon precipitation around 4.3 ka is synchronous with the collapse of Neolithic culture in central China. This abrupt change could have resulted from the amplification of the gradually decreased summer insolation by the positive vegetation-atmosphere-aerosol feedback. The weakened Asian monsoon events were in concert with decreased Greenland temperature during the early Holocene, centered at 8.2, 8.6, 9.3, 10.2 and 11.0 ka. This correlation suggests that changes in low-latitude monsoon are connected with climate change in high-latitude polar region.

The depression of tropical snowlines at the last glacial maximum: What can we learn from climate model experiments?

Analyses of simulations of the last glacial maximum (LGM) made with 17 atmospheric general circulation models (AGCMs) participating in the Paleoclimate Modelling Intercomparison Project, and a high-resolution (T106) version of one of the models (CCSR1), show that changes in the elevation of tropical snowlines (as estimated by the depression of the maximum altitude of the 0 °C isotherm) are primarily controlled by changes in sea-surface temperatures (SSTs). The correlation between the two variables, averaged for the tropics as a whole, is 95%, and remains >80% even at a regional scale. The reduction of tropical SSTs at the LGM results in a drier atmosphere and hence steeper lapse rates. Changes in atmospheric circulation patterns, particularly the weakening of the Asian monsoon system and related atmospheric humidity changes, amplify the reduction in snowline elevation in the northern tropics. Colder conditions over the tropical oceans combined with a weakened Asian monsoon could produce snowline lowering of up to 1000 m in certain regions, comparable to the changes shown by observations. Nevertheless, such large changes are not typical of all regions of the tropics. Analysis of the higher resolution CCSR1 simulation shows that differences between the free atmospheric and along-slope lapse rate can be large, and may provide an additional factor to explain regional variations in observed snowline changes.