Strong East Asian Summer Monsoon Research Articles

Why Is the East Asian Summer Monsoon Extremely Strong in 2018?—Collaborative Effects of SST and Snow Cover Anomalies

In 2018, summer precipitation was above normal in North and Northwest China and below normal around the Yangtze River valley, due to an extremely strong East Asian summer monsoon (EASM). The atmospheric circulation anomalies in East Asia and key external forcing factors that influence the EASM in 2018 are explored in this paper. The results show that there existed an anomalous cyclonic circulation near the Philippines, while the western Pacific subtropical high was located more northward than its normal position. In the mid–high latitudes, a negative geopotential height anomaly center was found near the Ural Mountains, suppressing the blocking activity. Under such a circulation pattern, precipitation near the Yangtze River valley decreased because local divergence and subsidence intensified, whereas precipitation in northern China increased due to large amounts of water vapor transport by anomalously strong southerly winds. Further analyses reveal that the strong EASM circulation in 2018 might result from the joint influences of several external forcing factors. The weak La Nina event that started from October 2017, the positive North Atlantic Tripolar mode (NAT) in spring 2018, and the reduced snow cover over the Tibetan Plateau in winter 2017/18 all collaboratively contributed to formation of the cyclonic circulation anomaly near the Philippines, leading to the extremely strong EASM. Especially, the positive NAT and the reduced Tibetan snow cover may have caused the negative geopotential height anomaly near the Ural Mountains, in favor of a strong EASM. The above external factors and their reinforcing impacts on the EASM are further verified by two groups of similar historical cases.

Early-Holocene monsoon instability and climatic optimum recorded by Chinese stalagmites

The timing and duration of the Holocene East Asian summer monsoon (EASM) maximum and the interpretation of Chinese stalagmite δ18O records have long been disputed. Notably, interpretations of Holocene EASM variations are frequently based on a single record or study area and are often contradictory. In this study, we conducted stable isotope analyses of four Holocene stalagmites from Chongqing, southwest China. The results reveal differences in the timing of the Holocene EASM maximum and to try to resolve the inconsistency we analyzed and statistically integrated a total of 16 Holocene stalagmite records from 14 caves in the EASM region. The resulting synthesized Holocene stalagmite δ18O (δ18Osyn) record is in agreement with other EASM records and confirms that stalagmite δ18O records are a valid indicator of EASM intensity, rather of local precipitation amount. The δ18Osyn record shows that the EASM intensified rapidly from the onset of the early Holocene; notably, however, there were distinct EASM oscillations in the early Holocene, consisting of three abrupt millennial-scale events. This indicates that, contrary to several previous interpretations, the early Holocene EASM was unstable. Subsequently, during 8–6 kyr BP, the EASM was relatively stable and strong, with the strongest monsoon occurring during 8–7 kyr BP. This evidence of a stable and strong mid-Holocene EASM in eastern China is in accord with the classical view of a mid-Holocene Optimum in China.

Characteristics of summer extreme precipitation in the Huai River basin and their relationship with East Asia summer monsoon during 1960–2014

Based on the daily precipitation data of the Huai River basin (HRB) and NCEP/NCAR reanalysis data from 1960 to 2014, the study examines the spatio‐temporal characteristics of summer extreme precipitation (SEP), explores the statistical relationship between the changes of SEP in the HRB and the East Asian summer monsoon (EASM), and further investigates the circulation characteristics which are most conducive to extreme precipitation under different EASM backgrounds. The main results are as follows: (a) summer extreme precipitation indices (SEPI) of PNL90 and PQ90 mainly increased (although not significant) in the HRB from 1960 to 2014, and the weakening of EASM may be the reason for the upwards trend. (b) The characteristics of SEP are quite different between the north and the south of the HRB, and SEPI mainly negatively correlates with East Asian summer monsoon index (EASMI) in the south but positively correlates in the north. (c) Extreme precipitation may occur under both strong and weak summer monsoon backgrounds in the HRB, but the circulation configurations are different. The differences between high and low SEP are mainly associated with the intensity of western Pacific subtropical high (WPSH) for the weak EASM years, while the differences mainly lie in the position of WPSH for the strong EASM years.

Sources and burial of particulate organic matter in the Kuroshio mainstream and its response to climate change over the past millennium

Sedimentation rate and biogenic elements of a sediment core KET-1 recovered from Kuroshio Mainstream, east of Taiwan, were determined to investigate variation of organic carbon burial and its response to climate fluctuations. The results showed that the organic carbon burial flux was mainly depended on the sedimentation rate and the input of river particles had a dilution effect on total organic carbon (TOC) content of the sediment. Based on the organic carbon burial flux, the entire depositional process could be divided into three stages, which exactly corresponded to three specific climate periods over the past millennium. During the Medieval Warm Period (MWP, ad 1150–1250), the strong East Asian summer monsoon (EASM) caused more river input to the sea, resulting in high sedimentation rate and high organic carbon burial flux, meanwhile the dilution effect of river particles as well as low marine primary productivity resulted in relatively low proportion of marine organic matter (MOM) in the sediment, which was reflected by low TOC content. During the Little Ice Age (LIA, ad 1400–1835), lower organic burial flux and higher marine primary productivity as well as higher TOC content in the sediment were accompanied by weakened EASM. In the Modern Period (MP, ad 1835–present), however, the correlation between organic carbon burial and climate fluctuations that was found to be significant during the MWP and LIA had been greatly changed, and anthropogenic forcing was thought to be the primary driving factor. Overall, the climate variability and anthropogenic activities over the past millennium dominated the organic carbon burial in the sediment along the Kuroshio mainstream.

Interannual Variation of the Spring and Summer Precipitation over the Three River Source Region in China and the Associated Regimes

AbstractThis study analyzes the interannual and interdecadal variability of spring and summer precipitation over the Three River Source (TRS) region in China using four datasets. A general consistency is revealed among the four datasets with regard to the interannual and interdecadal variability of TRS precipitation during 1979–2015, demonstrating a confidence of the four datasets in representing the precipitation variability over the TRS region. The TRS spring and summer precipitation shows distinct interannual and interdecadal variability, with an overall increasing trend in the spring precipitation and an interdecadal oscillation in the summer precipitation. The regimes associated with the interannual variability of TRS spring and summer precipitation are further investigated. The interannual variability of TRS spring precipitation is essentially modulated by an anomalous easterly water vapor transport (WVT) branch associated with the leading mode of Eurasian spring circulation. El Niño–Southern Oscillation (ENSO) may affect the interannual variability of TRS spring precipitation by causing southerly WVT anomalies toward the TRS region. The interannual variability of TRS summer precipitation is essentially modulated by an anomalous southwesterly WVT branch over the TRS region, which is mainly associated with a Eurasian wave train connected with the summer North Atlantic Oscillation. A strong East Asian summer monsoon and an El Niño–decaying summer may also contribute to the southwesterly WVT anomalies over the TRS region.

A Centennial Episode of Weak East Asian Summer Monsoon in the Midst of the Medieval Warming

AbstractRecent paleoproxy evidence has suggested that there is a centennial episode of weaker East Asian summer monsoon (EASM) within the Medieval Warm Period, in which the EASM is generally strong. This study aims to explore the causes of this phenomenon and determine what controls the centennial variability of the EASM. With the Community Earth System Model, a suite of control and forced experiments were conducted for the past 2,000 years. The model simulates a warm period over EA (East Asia) from CE 801–1250 with a generally increased summer mean precipitation over the northern EA; however, during CE 980–1100, the EASM is significantly weaker. On the multi‐decadal to centennial time scale, a strong EASM is associated with a La Niña‐like Indo‐Pacific warming, and a weak EASM with the El Niño‐like conditions. This sea surface temperature anomaly pattern represents the leading EOF (empirical orthogonal function) mode of centennial sea surface temperature variations, which are primarily forced by solar radiation and volcanic activity. In contrast, land use/land cover and greenhouse gases as well as internal dynamics play a negligible role. The weak EASM during CE 980–1100 is attributed to the relatively low solar radiation, which leads to a prevailing El Niño‐like Indo‐Pacific cooling with the strongest cooling occurring in the equatorial western Pacific. The suppressed convection over the equatorial western Pacific directly induces a Philippine Sea anticyclone anomaly, which increases the southern China precipitation, while suppressing Philippine Sea precipitation. The latter excites a meridional teleconnection that induces anomalous northerly winds and dry conditions over northern China.

An overview of the influence of atmospheric circulation on the climate in arid and semi-arid region of Central and East Asia

The arid and semi-arid (ASA) region of Asia occupies a large area in the middle latitudes of the Northern Hemisphere, of which the main body is the ASA region of Central and East Asia (CEA). In this region, the climate is fragile and the environment is sensitive. The eastern part of the ASA region of CEA is located in the marginal zone of the East Asian monsoon and is jointly influenced by westerly circulation and the monsoon system, while in the western part of the ASA of CEA, the climate is mainly controlled by westerly circulation. To understand and predict the climate over this region, it is necessary to investigate the influence of general circulation on the climate system over the ASA region of CEA. In this paper, recent progress in understanding the relationship between the general circulation and climate change over the ASA region is systematically reviewed. Previous studies have demonstrated that atmospheric circulation represents a significant factor in climate change over the ASA region of CEA. In the years with a strong East Asian summer monsoon, the water vapor flux increases and precipitation is abundant in the southeastern part of Northwest China. The opposite situation occurs in years when the East Asian summer monsoon is weak. With the weakening of the East Asian summer monsoon, the climate tends to dry over the semi-arid region located in the monsoon marginal zone. Recently, owing to the strengthening of the South Asian monsoon, more water vapor has been transported to the ASA region of Asia. The Plateau summer monsoon intensity and the precipitation in summer exhibit a significant positive correlation in Central Asia but a negative correlation in North China and Mongolia. A significant positive correlation also exists between the westerly index and the temperature over the arid region of CEA. The change in the westerly circulation may be the main factor affecting precipitation over the arid region of Central Asia.

Contrasting relationship between the Kuroshio Extension and the East Asian summer monsoon before and after the late 1980s

Based on our previous study (Yu et al., Clim Dyn 49:1139–1156, 2017), this paper further investigates the interdecadal change in the relationship between the Kuroshio Extension (KE; 27°–37°N, 140°–158°E) and the East Asian summer monsoon (EASM) in the late 1980s. The summer KE sea surface temperature anomalies (SSTAs) show a significant positive relationship with the EASM over the period 1968–1987 (P1), but a significant negative connection with the EASM between 1991 and 2010 (P2). This interdecadal change in the KE–EASM relationship can be interpreted by considering the difference in the relationships of summer KE SSTAs with the East Asian subtropical westerly jet (EASWJ) and western North Pacific subtropical high (WNPSH) during the two periods. During P1, summertime KE SST warming is significantly related to the strengthened EASWJ and WNPSH, but it has close relationships with the weakened and northward-moving EASWJ and WNPSH during P2. These anomalous EASWJ and WNPSH associated with the summertime KE SST warming in P1 (P2) then favors increased (reduced) rainfall over the Yangtze River Valley that corresponds to a strong (weak) EASM, thereby leading to the significant positive (negative) KE–EASM relationship during this period. This change in the relationships of summer KE SSTAs with the EASWJ and WNPSH may be attributed to the increased KE SST variability associated with an enhanced Pacific Decadal Oscillation (PDO) in summer during P2, which is most probably induced by the stronger North Pacific Oscillation (NPO)-like atmospheric forcing, especially its southern pole (SP), in the preceding spring during this period. The spring NPO-like SP forces the KE SSTAs and PDO more directly during the following summer and can thus have been a better precursor for the following EASM than the full NPO-like dipole after the late 1980s.

Pollen record of early- to mid-Holocene vegetation and climate dynamics on the eastern coast of the Yellow Sea, South Korea

To understand the early- to mid-Holocene vegetation and climate dynamics on the eastern coast of the Yellow Sea, we obtained a sedimentary core with high-resolution accelerator mass spectrometry (AMS) carbon 14 (14C) data from the Gunsan coast in South Korea. The palynological analysis demonstrated that the riverine wetland meadow from 12.1 to 9.8 cal. kyr BP changed to temperate deciduous broad-leaved forest in 9.8–2.8 cal. kyr BP. In addition, the cold climate from 12.1 to 9.8 cal. kyr BP became warmer from 8.5 to 7.3 cal. kyr BP. This was followed by another relatively cold period from 7.3 to 2.8 cal. kyr BP. The temperature change was mainly in response to solar factors. However, there are two relatively humid periods from 12.1 to 9.8 and 8.5 to 7.3 cal. kyr BP, which arose for different reasons. The earlier humid period resulted from strong westerlies and a rapidly rising sea level. The later humid period was produced mainly by the strong East Asian summer monsoon (EASM) and may also be linked to La Niña–like activity. The cold ‘Younger Dryas’ event from 12.0 to 11.4 cal. kyr BP recorded in this study may have been produced by a North Atlantic meltwater pulse. This would have reduced temperatures that were already low because of weak insolation, and the strong winter monsoons would have increased the precipitation.

Mid–late Holocene changes in sedimentary organic matter on the inner shelf of the East China Sea

Marginal seas are important transitional zones for the delivery of terrestrial organic matter (TOM) from land to the open sea, and they play an important role in the carbon cycle. Tracing the source of sedimentary organic matter (SOM) deposited in marginal seas is fundamental to our understanding of the dispersal, degradation, migration, and conversion of organic matter. This paper presents high-resolution records of bulk organic matter and biomarker proxies from Core T08 that was recovered from the inner shelf of the East China Sea (ECS), and aims to identify the contributions of marine and terrestrial organic matter over the past 3725 yrs. Total organic carbon (TOC) values were low (0.50%) and showed no significant change between 3725 and 1800 yr BP (Period I), and increased continuously from 0.40% to 0.86% after 1800 yr BP (Period II: 1800–750 yr BP; Period III: 750 yr BP–present). The TMBR′ (ratio of terrestrial to marine biomarkers) and δ13CTOC (δ13C of TOC) values showed steady TOM contribution during Period I and higher TOM contribution driven by the increased Changjiang River (CR)-derived TOM under strong East Asian Summer Monsoon (EASM) and El Niño during Period II. During Period III, the increase in marine organic matter (MOM) contribution was indicated by the TMBR′, and this was caused by enhanced marine productivity related to intensified vertical mixture that was driven by the strengthened East Asian Winter Monsoon (EAWM). δ13CTOC shows a contrary trend to the TMBR′ during Period III, probably influenced by variations in the C3 vegetation type during this period. Spectral analysis of the TMBR′ series for the last 1200 yrs shows cycles with periods of 119, 75–85, and 54 yrs, confirming that climate-related events influenced the variation in SOM under the modulation of solar activity and solar irradiance at the centennial scale.

Provenance, sea-level and monsoon climate controls on silicate weathering of Yellow River sediment in the northern Okinawa Trough during late last glaciation

High resolution multi-proxy records, including geochemical and grain-size data from detrital sediments of IODP Site U1429 in the northern Okinawa Trough, provide reliable evidence for constraining sediment source and transport processes. They also allow silicate weathering and erosion controlled by sea-level change and East Asian summer monsoon evolution since 34ka to be reconstructed. Provenance proxies indicated that sediments in the northern Okinawa Trough were mainly supplied by the Yellow River middle reach at ~34–8ka. The low sea level and proximity of paleo-Yellow River mouth to the northern Okinawa Trough were the causes of the dominant Yellow River input. After ~8ka, a retreated Yellow River mouth coupled with the blocking effect of the Kuroshio Current and its branches, together with strong East Asian summer monsoon precipitation resulted in relatively strong sediment input from Kyushu to the northern Okinawa Trough. Meanwhile, Yellow River upper reach supplied more sediments to the study site than that during the ~34–8ka. On glacial-interglacial scale, silicate weathering proxies indicate that the core sediment was more weathered during the last glacial and deglacial than that in the modern Yellow River. We attribute this to the increased upper reach sediment input, which supplied more weak weathered sediment to the study site during the Holocene. Besides, composite effect of weathering during glacial and deglacial sediment production and additional weathering upon subaerial exposure of shelf deposits during low sea-level stage, as well as older weathered sediments reworking during sea-level rise could also account for this weathering regime. On multi-millennial scale, from ~34 to 18.5ka, silicate weathering was mainly controlled by the East Asian summer monsoon, with a cooling and drying climate associated with weakened summer monsoon activity. This is consistent with the reduced alteration of sediments eroded from the Yellow River basin. From ~18.5 to 8ka, strong reworking of older weathered sediments overwhelmed the East Asian summer monsoon in controlling silicate weathering, which induced a continuous increase in chemical alteration of the sediments during this period.

The Impact of Layer Perturbation Potential Energy on the East Asian Summer Monsoon

This paper analyzes the relationship between the 1000–850-hPa layer perturbation potential energy (LPPE) as the difference in local potential energy between the actual state and the reference state and the East Asian summer monsoon (EASM) using reanalysis and observational datasets. The EASM is closely related to the first-order moment term of LPPE (LPPE1) from the preceding March to the boreal summer over three key regions: the eastern Indian Ocean, the subtropical central Pacific, and midlatitude East Asia. The LPPE1 pattern (−, +, +), with negative values over the eastern Indian Ocean, positive values over the subtropical central Pacific, and positive values over East Asia, corresponds to negative LPPE1 anomalies over the south of the EASM region but positive LPPE1 anomalies over the north of the EASM region, which lead to an anomalous downward branch over the southern region but an upward branch over the northern region. The anomalous vertical motion affects the local meridional circulation over East Asia that leads to a southwesterly wind anomaly over East Asia (south of 30°N) at 850 hPa and anomalous downward motion over 100°–120°E (along 25°–35°N), resulting in a stronger EASM, more kinetic energy over the EASM region, and less boreal summer rainfall in the middle and lower reaches of the Yangtze River valley (24°–36°N, 90°–125°E). These LPPE1 anomalies in the eastern Indian Ocean and subtropical central Pacific appear to be connected to changes in local sea surface temperature through the release of latent heat.

Lagged response of summer precipitation to insolation forcing on the northeastern Tibetan Plateau during the Holocene

The precipitation changes on the northeastern Tibetan Plateau during the Holocene remain unclear due to discrepancies among different proxy records. We compared proxy records with the results from a transient simulation performed using the Kiel Climate Model forced by orbital variations, to analyse summer precipitation changes in this area during the Holocene (9.5–0 ka BP). The model results suggested increasing amounts of summer precipitation from 9.5 to ~6.2 ka BP and a persistent decline thereafter, which matched well with pollen records but was inconsistent with ostracod δ18O records. The Holocene climatic optimum lagged the Northern Hemisphere summer insolation maximum by ~3.5 ka, caused by the interplay between the East Asian summer monsoon (EASM) circulation and the mid-latitude westerlies. The ostracod δ18O values reflected the strength of the EASM circulation. A strong EASM circulation increased the transport of water vapour towards the northeastern Tibetan Plateau from the northwestern Pacific. Weakened mid-latitude westerlies increased the incursion of cold air masses into the northeastern Tibetan Plateau. During the early Holocene, relatively strong mid-latitude westerlies, resulting from an enhanced Arctic Oscillation, reduced summer precipitation on the northeastern Tibetan Plateau, in spite of a strong insolation-driven EASM circulation. The weakening EASM circulation and the strengthening westerlies together induced the decreasing trend of summer precipitation from the middle to late Holocene. In addition, summer precipitation variations were further modulated by sea-surface temperatures in the northwestern Pacific, through weakening the strength of the EASM.

Assessing the utility of visible-to-shortwave infrared reflectance spectroscopy for analysis of soil weathering intensity and paleoclimate reconstruction

Visible-to-shortwave infrared reflectance (VSWIR) spectroscopy is a fast and efficient approach for estimation of soil properties. In order to test whether soil weathering intensity can be predicted using reflectance spectra in the VSWIR range (350–2500nm), and to examine the efficacy of this methodology to supplement or substitute for traditional mineralogical and geochemical techniques of paleoclimate reconstruction, we investigated a Quaternary soil sequence at Shengli in the Sichuan Basin of southwestern China. VSWIR absorption bands can be characterized by geometrical parameters including position (P), depth (D), width (W), asymmetry (AS), and full width at half maximum (F). Our results indicate that spectral features in the visible-to-near-infrared (VNIR; 350–1000nm) range can be interpreted in terms of the presence and/or concentration of individual Fe-oxide phases. Spectrally detected water is mostly bound in clay minerals, and this single dominant water source facilitates interpretation of shortwave infrared (SWIR; 1000–2500nm) parameters. Soil spectral parameters were compared with the Fe-oxide mineralogy, major element composition, and clay mineralogy of soil samples obtained from conventional laboratory analyses. Strong correlations are found between VNIR parameters and Fe-oxide mineralogy, with D900 serving as the best proxy for total ferric iron concentration. P900 and D700/D500 exhibit similar variation trends and relationships to the hematite/(hematite+goethite) [H/(H+G)] ratio of soil samples, confirming their value as monsoonal weathering intensity proxies. Lower values of P900 and D700/D500 indicate warmer and seasonally drier pedoenvironments, reflecting a stronger East Asian summer monsoon. The utility of some SWIR parameters for assessment of weathering intensity declines in inhomogeneous soils (e.g., those having vermiform structure). Thus, spectral evaluation of weathering intensity is most effective in soils that are relatively homogeneous, for which weathering degree is highly correlated to AS2200, D2200/D1900, and AS1400. The utility of these spectral proxies depends mainly on the mineral composition of soils and the application type (i.e., laboratory versus field studies). The application of VSWIR in the field (e.g., imaging spectroscopy, or IS) shows promise for rapid, in-situ mapping of weathering intensity, thus providing a convenient approach to paleoclimate reconstruction in remote and inaccessible regions.

Heinrich events recorded in a loess–paleosol sequence from Hexigten, Inner Mongolia

We describe the last glaciation climatic history Marine Isotope Stage (MIS, 2–4) from 66.7 ka to 14.5 ka in Hexigten, northeast Inner Mongolia, North China. The climate of the region experienced frequent and significant fluctuations between dry-cold and less dry-cold during the late MIS4. The climate was generally warm and humid during early MIS3 (MIS3c) and late MIS3 (MIS3a), whereas it was cold and dry in middle MIS3 (MIS3b) and during MIS2. In this study, the cold and dry conditions were correlated with a stronger East Asian winter monsoon and strong dune activity; whereas, warm and humid conditions were related to a stronger East Asian summer monsoon (EASM) and weak dune activity. This study establishes six distinct dry and cold intervals during the last glacial period (66.7–14.5 ka) based on optically stimulated luminescence data, multi-proxies record (magnetic susceptibility, grain size analysis, Rb/Sr, SiO2/TiO2) and chemical index of alteration (CIA). The last glacial period may be correlated with Heinrich events 1 to 6 which were further confirmed by comparison with the Hulu cave stalagmites and Greenland ice core records. It is concluded that the study area was substantially affected by the EASM, as compared with the loess–desert transition zone of the Chinese Loess Plateau, especially in MIS3c and suggested that the East Asian monsoon played a pivotal role in the last glacial period climate and dune activity.

The 2016 summer floods in China and associated physical mechanisms: A comparison with 1998

The characteristics of droughts and floods in China during the summers (May–August) of 2016 and 1998 were compared in great detail, together with the associated atmospheric circulations and external-forcing factors. Following results are obtained. (1) The precipitation was mostly above normal in China in summer 2016, with two main rainfall belts located in the Yangtze River valley (YRV) and North China. Compared with 1998, a similar rainfall belt was located over the YRV, with precipitation 100% and more above normal. However, the seasonal processes of Meiyu were different. A typical “Secondary Meiyu” occurred in 1998, whereas dry conditions dominated the YRV in 2016. (2) During May–July 2016, the Ural high was weaker than normal, but it was stronger than normal in 1998. This difference resulted from fairly different distributions of sea surface temperature anomalies (SSTAs) over the North Atlantic Ocean during the preceding winter and spring of the two years. (3) Nonetheless, tropical and subtropical circulation systems were much more similar in May–July of 2016 and 1998. The circulation systems in both years were characterized by a stronger than normal and more westward-extending western Pacific subtropical high (WPSH), a weaker than normal East Asian summer monsoon (EASM), and anomalous convergence of moisture flux in the mid and lower reaches of the YRV. These similar circulation anomalies were attributed to the similar tropical SSTA pattern in the preceding seasons, i.e., the super El Nino and strong warming in the tropical Indian Ocean. (4) Significant differences in the circulation pattern were observed in August between the two years. The WPSH broke up in August 2016, with its western part being combined with the continental high and persistently dominating eastern China. The EASM suddenly became stronger, and dry conditions prevailed in the YRV. On the contrary, the EASM was weaker in August 1998 and the “Secondary Meiyu” took place in the YRV. The Madden–Julian Oscillation (MJO) was extremely active in August 2016 and stayed in western Pacific for 25 days. It triggered frequent tropical cyclone activities and further influenced the significant turning of tropical and subtropical circulations in August 2016. In contrast, the MJO was active over the tropical Indian Ocean in August 1998, conducive to the maintenance of a strong WPSH. Alongside the above oceanic factors and atmospheric circulation anomalies, the thermal effect of snow cover over the Qinghai–Tibetan Plateau from the preceding winter to spring in 2016 was much weaker than that in 1998. This may explain the relatively stronger EASM and more abundant precipitation in North China in 2016 than those in 1998.

Iron oxide characteristics of mid-Miocene Red Clay deposits on the western Chinese Loess Plateau and their paleoclimatic implications

The middle Miocene climate optimum (MMCO, 14.3–16.3Ma) was a warm climatic interval that may provide an analog for future climates in the context of ongoing global warming. In continental regions, however, reconstruction of the MMCO remains controversial due to the lack of robust proxies from continuous sedimentary sequences. In this study, we investigated the composition and relative abundance of pedogenic iron oxides in the Zhuanglang Red Clay deposits from the western Chinese Loess Plateau, using rock magnetic, color reflectance, and geochemical analyses. Our aim was to assess the climatic sensitivity of iron oxides in order to produce robust proxies for reconstructing regional temperature and moisture relationships during the MMCO. Our results indicate that the MMCO in the study region was characterized by high effective precipitation, relatively high temperature and strong pedogenesis, suggesting a strong East Asian summer monsoon (EASM). The strengthening of the EASM intensity during the MMCO is consistent with records of enhanced terrigenous chemical weathering in the South China Sea, and the occurrence of a relatively warm and humid climate on a global scale, implying the strong coupling between Asian monsoon evolution and global climate change during the middle Miocene. In addition, the magnetic susceptibility record of the Zhuanglang Red Clay demonstrates that eccentricity played the dominant role in controlling the EASM during the MMCO.

An interdecadal change in the relationship between the western North Pacific Ocean and the East Asian summer monsoon

This study reveals that the relationship between the western North Pacific Ocean (WNPO; 0–55°N, 100–165°E) and the East Asian summer monsoon (EASM) experiences a well-defined interdecadal change in the late 1980s and early 1990s. The EASM-related WNPO sea surface temperature anomaly (SSTA) pattern changes from the dipole pattern [WNPO dipole (WNPOD)] that develops over the period between 1968 and 1987 (P1) to a tripole pattern [WNPO tripole (WNPOT)] between 1991 and 2010 (P2). The positive (negative) phase of the WNPOD is characterized by warm (cold) SSTAs in the Japan Sea and Kuroshio–Oyashio Extension region, and cold (warm) SSTAs in the subtropical WNPO, whereas the positive (negative) phase of the WNPOT shows warming (cooling) in the Kuroshio Extension region (KER), and cooling (warming) in the south of Kamchatka Peninsula (SKP) and Philippine Sea (PS). During P1 (P2), the WNPOD (WNPOT) can be regarded as the first (second) leading mode of summer WNPO SST variability, and its positive phase is associated with a weakened WNPO subtropical high and thereby the deficient summer rainfall in the Yangtze River valley, together with a strong EASM, and vice versa. The change in the WNPO–EASM relationship may be caused by interdecadal changes in the relationship of the equatorial central Pacific (ECP) with the WNPO and EASM, and an increase in summer KER SST variability. During P2, because the ECP warming-induced cyclonic anomalies move northwestwards and intensify, summertime ECP warming is able to generate a strong EASM and significant cooling over the two poles of the WNPOT (SKP and PS). These strengthened impacts of the ECP on the WNPOT and EASM contribute to the strengthened WNPOT–EASM relationship during P2. In addition, summer KER SST variability increases between 1991 and 2010, and this may have enhanced the impact of the KER on the EASM during P2. These two factors probably cause the EASM-related WNPO SSTA pattern to change from the WNPOD in P1 to the WNPOT in P2.

Distinct effects of anthropogenic aerosols on the East Asian summer monsoon between multidecadal strong and weak monsoon stages

AbstractBecause industrial emissions of anthropogenic aerosols over East Asia have greatly increased in recent decades, the interactions between atmospheric aerosols and the East Asian summer monsoon (EASM) have attracted enormous attention. To further understand the aerosol‐EASM interaction, we investigate the impacts of anthropogenic aerosols on the EASM during the multidecadal strong (1950–1977) and weak (1978–2000) EASM stages using the Community Atmospheric Model 5.1. Numerical experiments are conducted for the whole period, including the two different EASM stages, with present day (PD, year 2000) and preindustrial (PI, year 1850) aerosol emissions, as well as the observed time‐varying aerosol emissions. A comparison of the results from PD and PI shows that, with the increase in anthropogenic aerosols, the large‐scale EASM intensity is weakened to a greater degree (−9.8%) during the weak EASM stage compared with the strong EASM stage (−4.4%). The increased anthropogenic aerosols also result in a significant reduction in precipitation over North China during the weak EASM stage, as opposed to a statistically insignificant change during the strong EASM stage. Because of greater aerosol loading and the larger sensitivity of the climate system during weak EASM stages, the aerosol effects are more significant during these EASM stages. These results suggest that anthropogenic aerosols from the same aerosol emissions have distinct effects on the EASM and the associated precipitation between the multidecadal weak and strong EASM stages.

Interdecadal variability in the thermal difference between western and eastern China and its association with rainfall anomalies

AbstractThis study investigated the spring thermal difference between western and eastern China and its association with the rainfall anomalies using station and reanalysis data from 1960 to 2006. The spring thermal difference between western and eastern China underwent an obvious interdecadal shift around 1979. The thermal difference between western and eastern China was small during 1960–1978, which strengthened the southwesterly wind anomalies in line with the thermal wind. This enhanced the East Asian summer monsoon. In addition, the increase in rainfall over North China and decrease in rainfall over the Yangtze River were associated with the strong East Asian summer monsoon. However, during 1979–2006, the thermal difference between western and eastern China was large, which strengthened the northeasterly wind anomalies in line with the thermal wind. This supports the notion of weakened East Asian summer monsoon associated with the decrease in rainfall over North China and increase in rainfall over the Yangtze River.