Asian Monsoon Research Articles

Late Miocene drainage reorganization on the NE Tibetan Plateau linked to growth of the Qilian Shan revealed by coupled carbonate Sr-silicate Nd isotopic tracers

The tectonically active NE Tibetan Plateau, which is located at the confluent area of the humid Asian monsoon region and dry Asian inlands, provides excellent archives of drainage pattern evolution in response to regional changes in topography and climate. However, the uplift of the NE Tibetan Plateau and formation of modern drainage patterns are still disputed. In this study, based on the SrNd isotopic distribution in modern surface water and sediments, we present integrated records of 87Sr/86Sr ratios in carbonate and 87Sr/86Sr-εNd(0) values in silicate of the Huaitoutala section in the northern Qaidam Basin to reveal the source-to-sink history of particulate and dissolved loads since ∼15 Ma. Our results show that changes in SrNd isotopes at ca. 10 Ma and ca. 5 Ma are indicative of two stages of source alteration and reorganization of the drainage system. Specifically, a remarkable increase in 87Sr/86Sr ratios in carbonate and silicate together with a decrease in εNd(0) values since ca. 10 Ma suggests a tectonic denudation event of Qilian Shan at ∼10 Ma. The subsequent declines in 87Sr/86Sr ratios in carbonate and silicate together with an increase in εNd(0) values since ca. 5 Ma, which approach the modern 87Sr/86Sr ratios near the study section site, suggest that a modern-like drainage pattern in the northern Qaidam Basin might have formed in response to tectonic uplift of Qilian Shan at ca. 5 Ma. Using coupled carbonate Sr-silicate SrNd isotopes, we provide reliable provenance tracers of drainage reorganization and the evolution of source-to-sink systems caused by tectonic deformation.

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.

Far-Reaching Effects of Okhotsk Sea Ice Area on Sea Surface Heat Flux, Lower Atmosphere, and Ocean Mixed Layer

Abstract The impact of interannual variations in sea ice area in the Okhotsk Sea was investigated through a composite analysis of years with extensive and limited sea ice areas (referred to as heavy and light ice years, respectively), using atmospheric and oceanic reanalysis data. The comparison of heavy and light ice-year composites in February revealed a substantial decrease in upward surface turbulent heat flux in the Okhotsk Sea (∼−250 W m−2) and a notable increase in a surprisingly extensive region in the western North Pacific (30–120 W m−2), spanning 2300 km from the ice edge. These differences were consistent with the decrease in surface air temperature and specific humidity, suggesting that during heavy ice years, cold and dry air blowing from Siberia to the North Pacific via the Okhotsk Sea undergoes less modification over larger sea ice areas, remaining colder and drier in the North Pacific and thereby enhancing the heat flux. Such advection can be associated with the Asian winter monsoon and migratory cyclones. Cloud cover and surface radiation flux altered consistently with these differences, although longwave and shortwave radiation largely counterbalanced each other. Additionally, the Pacific storm track exhibited variation. In accordance with the heat flux difference, sea surface temperature decreased, and the ocean mixed layer deepened around the subarctic during heavy ice years. These findings suggest that sea ice area in the Okhotsk Sea influences the lower atmosphere and surface ocean in the North Pacific. Such impacts could further affect ocean nutrient circulation, ecosystems, and atmospheric teleconnections. Significance Statement Sea ice cover influences heat exchange between the atmosphere and ocean. Advection of less heated air over larger sea ice areas can enhance heat exchange downwind, and vice versa. We found that this was effective in the Okhotsk Sea, situated along the path of the Asian winter monsoon. Upward surface heat flux increased significantly across a vast region of the North Pacific associated with colder and drier air during years with larger sea ice areas. Additionally, cloud cover, Pacific storm track, sea surface temperature, and ocean mixed layer depth exhibited significant differences associated with sea ice area. These variations could impact ocean heat content, nutrient circulation, and primary production in the North Pacific, and atmospheric circulation in North America and Europe.

A multiproxy reconstruction of Asian winter monsoon variability since the last glacial from southeast offshore Hainan Island, South China Sea

The evolution of East Asian winter monsoon (EAWM) in tropical oceans has been a key issue in the climatic coupling between high- and low-latitudes. This study aims to investigate the EAWM variability using multiproxy of sediment core XB19S40 in the southeast offshore of Hainan Island, South China Sea. Alkenone derived sea surface temperature (SST), sediment grain size, and elemental ratios show consistent changes over the last 20.7 ka. There were synchronous abrupt changes in the SST, Zr/Rb, K/Ti, and End-members (EMs) during the Bølling-Allerød warming period, Younger Dryas and 8.2 ka cooling events. The Zr/Rb and EM2 (18.7–84.6 μm, 40.1 μm) changes reflect the EAWM and are highly coincident with records in high latitudes. These changes suggest that the EAWM gradually weakened from the last glacial maximum to early Holocene, experienced minor strengthening in the late Holocene but has diminished since 1 ka. In contrast, the indicators K/Ti and EM1 (0.2–18.7 μm, 9.9 μm) suggest that the East Asian summer monsoon is in the opposite phase. Spectral analysis demonstrates that the intensity of East Asian monsoons in the SCS were mainly affected by solar activity on a centennial-scale. The results of this study provide new insight into the complexity of the EAWM system and emphasize the importance of understanding multiple components of the climate system.

Evaluation of vertical transport in ERA5 and ERA-Interim reanalysis using high-altitude aircraft measurements in the Asian summer monsoon 2017

Abstract. During the Asian monsoon season, greenhouse gases and pollution emitted near the ground are rapidly uplifted by convection up to an altitude of ∼ 13 km, with slower ascent and mixing with the stratospheric background above. Here, we address the robustness of the representation of these transport processes in different reanalysis data sets using ERA5, ERA-Interim and ERA5 1∘×1∘. This transport assessment includes the mean age of air from global three-dimensional simulations by the Lagrangian transport model CLaMS (Chemical Lagrangian Model of the Stratosphere), as well as different trajectory-based transport times and associated ascent rates compared with observation-based age of air and ascent rates of long-lived trace gases from airborne measurements during the Asian summer monsoon 2017 in Nepal. Our findings confirm that the ERA5 reanalysis yields a better representation of convection than ERA-Interim, resulting in different transport times and air mass origins at the Earth's surface. In the Asian monsoon region above 430 K, the mean age of air driven by ERA-Interim is too young, whereas the mean age of air from ERA5 1∘×1∘ is too old but somewhat closer to the observations. The mean effective ascent rates derived from ERA5 and ERA5 1∘×1∘ back trajectories are in good agreement with the observation-based mean ascent rates, unlike ERA-Interim, which is much faster above 430 K. Although a reliable CO2 reconstruction is a challenge for model simulations, we show that, up to 410 K, the CO2 reconstruction using ERA5 agrees best with high-resolution in situ aircraft CO2 measurements, indicating a better representation of Asian monsoon transport in the newest ECMWF reanalysis product, ERA5.

Evolution of the Asian monsoon recorded by Holocene aeolian sediments in the northeastern Tibetan Plateau

Aeolian sediments in the northeastern Tibetan Plateau may contain information about changes of the atmospheric circulation changes in the region, and can be used to investigate the varying characteristics of the East Asian monsoons and the evolution of the regional environment. In this study, parameterized grain size end-member analysis is used to analyze the grain size of climate change-sensitive aeolian profiles in Zeku. Then, the environmental significance indicated by each end-members and the environmental evolution in the northeastern Tibetan Plateau since the Holocene were explored by combining magnetic susceptibility, elements and other indexes. On this basis, the factors affecting aeolian sediments under the Asian monsoon are analyzed. The following conclusions were drawn: (1) The grain size components of the sediments in the ZK profile were divided into three end-members: EM1 is related to weathering and pedogenesis, and can indirectly indicate the dry and humid changes of the regional environment; Both EM2 and EM3 are near-source dust accumulations that relatively indicate the strength of regional aeolian activities. (2) The environmental evolution can be divided into the following three phases: 9.4–4.3 ka BP was a geological period with strong summer winds, a warm and humid climate, and weaker aeolian activities. 4.3–1.5 ka BP entered the westerlies control period, the climate developed to cool and dry, aeolian activities were more frequent, and there were secondary climate fluctuations in 4–2.0 ka BP and 1.7–1.5 ka BP. From 1.5 ka BP to present, the climate has become further cold and dry, and aeolian activities have intensified. (3) At the suborbital scale, the succession of sedimentary phase in the Northeastern Tibetan Plateau is controlled by a combination of monsoon strength and sources changes.

Meta-analysis provides insights into the origin and evolution of East Asian evergreen broad-leaved forests.

Evergreen broad-leaved forests (EBLFs) are dominated by a monsoon climate and form a distinct biome in East Asia with notably high biodiversity. However, the origin and evolution of East Asian EBLFs (EAEBLFs) remain elusive despite the estimation of divergence times for various representative lineages. Using 72 selected generic-level characteristic lineages, we constructed an integrated lineage accumulation rate (LAR) curve based on their crown ages. According to the crown-based LAR, the EAEBLF origin was identified at least as the early Oligocene (c. 31.8 million years ago (Ma)). The accumulation rate of the characteristic genera peaked at 25.2 and 6.4 Ma, coinciding with the two intensification periods of the Asian monsoon at the Oligocene - Miocene and the Miocene - Pliocene boundaries, respectively. Moreover, the LAR was highly correlated with precipitation in the EAEBLF region and negatively to global temperature, as revealed through time-lag cross-correlation analyses. An early Oligocene origin is suggested for EAEBLFs, bridging the gap between paleobotanical and molecular dating studies and solving conflicts among previous estimates based on individual representative lineages. The strong correlation between the crown-based LAR and the precipitation brought about by the Asian monsoon emphasizes its irreplaceable role in the origin and development of EAEBLFs.

Concurrent Asian monsoon strengthening and early modern human dispersal to East Asia during the last interglacial.

The relationship between initial Homo sapiens dispersal from Africa to East Asia and the orbitally paced evolution of the Asian summer monsoon (ASM)-currently the largest monsoon system-remains underexplored due to lack of coordinated synthesis of both Asian paleoanthropological and paleoclimatic data. Here, we investigate orbital-scale ASM dynamics during the last 280 thousand years (kyr) and their likely influences on early H. sapiens dispersal to East Asia, through a unique integration of i) new centennial-resolution ASM records from the Chinese Loess Plateau, ii) model-based East Asian hydroclimatic reconstructions, iii) paleoanthropological data compilations, and iv) global H. sapiens habitat suitability simulations. Our combined proxy- and model-based reconstructions suggest that ASM precipitation responded to a combination of Northern Hemisphere ice volume, greenhouse gas, and regional summer insolation forcing, with cooccurring primary orbital cycles of ~100-kyr, 41-kyr, and ~20-kyr. Between ~125 and 70 kyr ago, summer monsoon rains and temperatures increased in vast areas across Asia. This episode coincides with the earliest H. sapiens fossil occurrence at multiple localities in East Asia. Following the transcontinental increase in simulated habitat suitability, we suggest that ASM strengthening together with Southeast African climate deterioration may have promoted the initial H. sapiens dispersal from their African homeland to remote East Asia during the last interglacial.

Separation of the Asian summer monsoon and local precipitation from stalagmite oxygen isotope records in Eastern China during the last millennium

The Asian summer monsoon (ASM) regulates water vapor and heat transport across the East Asian continent and has a significant impact on patterns of regional precipitation. However, assessment of the long-term association between the monsoon and precipitation in different regions remains difficult due to spatial chronological differences and uncertainties in interpretation of climate proxies. This study provides a novel insight into the interpretation of stalagmite oxygen isotopes (δ18O) records and obtained synchronous ASM index series and precipitation variations in Northern China (NC), Central China (CC), and Southwestern China (SWC) over the last 1134 years (850–1983CE). The results show that the ASM index sequence is consistent with the trend evident in the monsoon intensity records from the region upstream of the water vapor source, which represents the response to the inter-hemispheric temperature dominated by effective solar radiation (ESR). Reconstruction precipitation data were in general agreement with the existing paleo-hydrological records of the regions, confirming the reliability and chronological accuracy of the reconstruction. Correlation analysis highlighted the significant regional heterogeneity in the destabilizing relationship between precipitation and the monsoon over the past millennium in Eastern China at sub-centennial scales. Specifically, the precipitation-monsoon relationships in three regions, NC, CC, and SWC, might be related to the North Atlantic Oscillation (NAO), the position of the intertropical convergence zone (ITCZ), and solar activity, respectively. Moreover, at the millennial scale, a certain positive correlation between precipitation and ASM in NC and SWC was identified, whereas there was a weaker correlation between precipitation and ASM in CC. This study represents an attempt to further interpret the stalagmite δ18O record, which can be generalized to larger and longer time scales.

A strange negative Indian Ocean Dipole (IOD) in 2022

The Indian Ocean Dipole (IOD) is a prominent pattern of climatic mode in the Indian Ocean that contributes to the changes in Rainfall pattern, Sea Surface Temperature (SST), and Surface Salinity (SSS) in the Indian Ocean. Recently in the scenario of Climate Change, this IOD may experience shifting in the phases development that link to the Indo-Pacific teleconnection. Here, we analyse the changes of negative IOD patterns in 2022 through the SST, SSS, rainfall pattern, and surface current system. This study showed the negative IOD was initiated earlier and coincided with the Asian monsoon during May 2022. The IOD is faster ended in October 2022 when the Australian Monsoon was generated. The Rainfall pattern at the Eastern Indian Ocean (EIO) and the Indonesian Archipelago experienced an intensive pattern from May up to December 2022. During the negative IOD phases, the precipitation at the EIO is more concentrated in Java and less precipitation in Sumatra. The warmer SST and fresh surface salinity in the Indonesian Archipelago are wider following the Pacific SST. This reveals the Indo-Pacific teleconnection is stronger during 2022 via the surface ocean patterns and the coincidence of stronger westerly wind and easterly wind in the maritime continent.

From Quaternary Science to Earth System Science

This article details how AN Zhisheng and his colleagues, through decades of dedicated research, began with the study of loess in China and integrated it with climatic and environmental researches. AN and his team developed a novel research approach to identifying the dynamics of changes of natural environments, including loess environment in East Asia. Their work has not only highlighted the global significance of Chinese loess as a record for global climate change, but also led to the formulation of the monsoon control theory regarding environmental changes in East Asia, as well as insights into Asian and global monsoon dynamics. The paradigm shift from a singular discipline of Quaternary science into a comprehensive Earth system science has happened. Their contributions have been instrumental in promoting sustainable development of the society in the Chinese Loess Plateau and western China.

Non-ENSO Precursors for Northwestern Pacific Summer Monsoon Variability with Implications for Predictability

Abstract The influence of El Niño–Southern Oscillation (ENSO) in the Asian monsoon region can persist through the post-ENSO summer, after the sea surface temperature (SST) anomalies in the tropical Pacific have dissipated. The long persistence of coherent post-ENSO anomalies is caused by a positive feedback due to interbasin ocean–atmospheric coupling, known as the Indo-western Pacific Ocean capacitor (IPOC) effect, although the feedback mechanism itself does not necessarily rely on the antecedence of ENSO events, suggesting the potential for substantial internal variability independent of ENSO. To investigate the respective role of ENSO forcing and non-ENSO internal variability, we conduct ensemble “forecast” experiments with a full-physics, globally coupled atmosphere–ocean model initialized from a multidecadal tropical Pacific pacemaker simulation. The leading mode of internal variability as represented by the forecast-ensemble spread resembles the post-ENSO IPOC, despite the absence of antecedent ENSO forcing by design. The persistent atmospheric and oceanic anomalies in the leading mode highlight the positive feedback mechanism in the internal variability. The large sample size afforded by the ensemble spread allows us to identify robust non-ENSO precursors of summer IPOC variability, including a cool SST patch over the tropical northwestern Pacific, a warming patch in the tropical North Atlantic, and downwelling oceanic Rossby waves in the tropical Indian Ocean south of the equator. The pathways by which the precursors develop into the summer IPOC mode and the implications for improved predictability are discussed.

An application of cluster analysis in investigating characteristics of the South and Southeast Asian monsoon onset in ENSO years

An application of cluster analysis in investigating characteristics of the South and Southeast Asian monsoon onset in ENSO years

Last glacial to Holocene sedimentation patterns and coccolithophore dynamics in the northwestern Bay of Bengal in response to South Asian monsoon strengthening.

Last glacial to Holocene sedimentation patterns and coccolithophore dynamics in the northwestern Bay of Bengal in response to South Asian monsoon strengthening.

Phylogeography of Pterocarya hupehensis reveals the evolutionary patterns of a Cenozoic relict tree around the Sichuan Basin.

Environmental factors such as mountain tectonic movements and monsoons can enhance genetic differentiation by hindering inter- and intra-specific gene flow. However, the phylogeographic breaks detected within species may differ depending on the different molecular markers used, and biological traits may be a major confounding factor. Pterocarya hupehensis is a vulnerable relict species distributed throughout the Sichuan Basin. Here, we investigated the phylogeographic patterns and evolutionary history of P. hupehensis using chloroplast DNA and restriction site-associated DNA sequencing data from 18 populations around the Sichuan Basin. The 24 chloroplast haplotypes separated into western and eastern lineages at approximately 16.7 Mya, largely coincident with a strengthening of the East Asian monsoon system during the early to middle Miocene. Both cpDNA and nuclear DNA datasets consistently identified distinct western and eastern lineages whose phylogeographic break conformed to the boundary of the Sino-Himalayan and Sino-Japanese forest sub-kingdoms. However, in contrast to the nuclear gene data, the cpDNA data revealed further divergence of the eastern lineage into northern and southern groups along the Yangtze River, a result that likely reflects differences in the extent of pollen vs seed dispersal. During the temperature decline in the penultimate (Riss) glacial period of the Pleistocene epoch, P. hupehensis experienced a genetic bottleneck event, and ecological niche modeling suggests that a subsequent population expansion occurred during the last interglacial period. Our findings not only establish a basis for conservation of this species, but also serve as a case study for the effects of geography and climate change on the evolutionary history of wind-pollinated relict plants.

Response of mangrove development to paleoclimate variation over the past 3,550 years in Phang Nga Province, Thailand

High-resolution reconstruction of the development of mangrove forests in the late Holocene is the basis for scientifically predicting the fate of mangroves. In this paper, we analyze the contribution of mangrove-derived organic matter (CMOM) from sediment core G in Phang Nga Province, Thailand by using a ternary end-member mixing model based on stable organic carbon isotope (δ13C) and the molar ratio of total organic carbon to total nitrogen (C/N (molar)). The variations of the CMOM in Phang Nga Province over the past ∼3,550 cal yr BP indicate that mangrove forests underwent three periods of flourishing: ∼3,550–2,720 cal yr BP, ∼1,960–1,240 cal yr BP and ∼500–0 cal yr BP, and two periods of degradation: ∼2,720–1,960 cal yr BP and ∼1,240–500 cal yr BP. Of the potential factors that affect mangrove development, relative sea level (RSL) changes and regional hydrological conditions did not have notable effects on mangrove flourishing/degradation. However, paleoclimate variations, especially the air temperature and rainfall controlled by the Indian Summer Monsoon (ISM) were the primary factors. The mangrove development reconstructed by the CMOM in Phang Nga Province, Thailand was extremely synchronized with that in the Yingluo Bay and Qinzhou Bay of Southwest China and on the west coast of India Peninsula. This study provides evidence that mangrove development controlled by the Asian monsoon in tropical Asia has regional synchronism.

Identification of cross-equatorial northerly surge (CENS) relationship to very heavy rain events in Jakarta

CENS is a meteorological phenomenon in the form of surface wind flow originating from the South China Sea (SCS), very strongly across the equator. CENS is closely related to the increase in average rainfall in parts of Indonesia. There has been very heavy rainfall on January 18, 2022 in the Jakarta area which resulted in flooding. Therefore, this study focuses on seeing if there is a CENS relationship that is factor in increasing rainfall in the Jakarta area on January 18, 2022. The results showed that the NCS began to form on January 16-17, 2022 with a Siberian High of 1040.6 hPa and the propagation of cold air masses from Asia to the south which was an indicator of CENS. The next day, the CENS phenomenon began to occur, characterized by meridional wind speeds in the 105˚- 115˚BT region of less than -5 m/s. This phenomenon triggers the strengthening of the northeast Asian monsoon flow so that the area south of the equator, including Jakarta, gets a large moisture transport from the SCS. This causes the potential for the formation of high convective clouds to trigger very heavy rain and cause flooding in the Jakarta area on January 18, 2022.

Exploring Recent Research on Definitions of the Myanmar Southwest Monsoon: A Comprehensive Bibliographic Overview

Accurately pinpointing the onset and withdrawal dates of the monsoon bears significant implications for diverse sectors. However, this task is fraught with challenges due to the intricate interannual and spatial variabilities. The Myanmar Southwest Monsoon (MSwM) index emerges as a widely adopted tool for delineating the initiation and cessation of the southwest monsoon across its respective domains. The confluence of advancements in satellite remote sensing, climate modeling, and data assimilation techniques presents opportunities for refining monsoon intensity indices. Additionally, a nuanced comprehension of the intricate interactions between the monsoon and climate teleconnections stands to enhance the assessment of monsoon intensity. Distinguished as a distinct entity from the major South Asian monsoon system, the MSwM follows a seasonal cycle intricately linked to mainland Indochina. The thermal influence of the Tibetan Plateau amplifies both meridional and zonal land-sea thermal contrasts, underscoring the importance of monitoring and anticipating monsoons. The MSwM, a product of complex interactions involving the earth, ocean, atmosphere, hydrosphere, biosphere, and cryosphere, exerts a profound impact on regional facets such as agriculture, ecology, chemistry, economics, and society. As technological strides continue, especially in satellite observations and climate models, avenues open up for refining our understanding of monsoon dynamics. The limitations inherent in the current coupled model of the land-sea-atmosphere system underscore the anticipation for advancements in modeling studies. Research in this domain not only contributes to the regulation of human activities but also fosters a harmonious coexistence between humanity and nature, safeguarding the human habitat.

PDO and AMO Modulation of the ENSO–Asian Summer Monsoon Teleconnection During the Last Millennium

AbstractObservations show that the teleconnection between the El Niño‐Southern Oscillation (ENSO) and the Asian summer monsoon (ASM) is non‐stationary. However, the underlying mechanisms are poorly understood due to inadequate availability of reliable, long‐term observations. This study uses two state‐of‐the‐art data assimilation‐based reconstructions of last millennium climate to examine changes in the ENSO–ASM teleconnection; we investigate how modes of (multi‐)decadal climate variability (namely, the Pacific Decadal Oscillation, PDO, and the Atlantic Multidecadal Oscillation, AMO) modulate the ENSO–ASM relationship. Our analyses reveal that the PDO exerts a more pronounced impact on ASM variability than the AMO. By comparing different linear regression models, we find that including the PDO in addition to ENSO cycles can improve prediction of the ASM, especially for the Indian summer monsoon. In particular, dry (wet) anomalies caused by El Niño (La Niña) over India become enhanced during the positive (negative) PDO phases due to a compounding effect. However, composite differences in the ENSO–ASM relationship between positive and negative phases of the PDO and AMO are not statistically significant. A significant influence of the PDO/AMO on the ENSO–ASM relationship occurred only over a limited period within the last millennium. By leveraging the long‐term paleoclimate reconstructions, we document and interrogate the non‐stationary nature of the PDO and AMO in modulating the ENSO–ASM relationship.

Northward expansion of Cenozoic Asian humid climate recorded by sporopollen

The distribution pattern of arid–humid climate in the modern northwest–southeast confrontation in Asia evolved from the zonal distribution pattern of the Paleogene. However, this evolutionary process remains poorly constrained by geological evidence. In this paper, we analyze the characteristics of sporopollen assemblages recorded in Late Eocene strata of the Jianchuan Basin in southwest China. We show that vegetation patterns exhibited a remarkable shift from tropical–subtropical sparse forest to subtropical–temperate mixed broad-leaved–coniferous forest at ∼40.6 Ma; this shift is further indicated by the rapid decrease in drought-tolerant plants, mostly Ephedra and Chenopodiaceae, evidence that the previous hot–dry climate was replaced by a warm–humid climate at this time. This evidence suggests that the Asian monsoon began to affect the study area at ∼40.6 Ma. We then comprehensively integrated sporopollen results from several basins across East Asia, revealing that the frontal edge of the humid monsoon climate moved intermittently to the northwest after the Late Eocene, and may have advanced to the Lunpola–Lanzhou line by the early Late Oligocene, this being similar to the position of the leading edge of the modern monsoon. Moreover, the frontal edge of the humid monsoon climate may have extended as far as the Qaidam Basin during the Middle Miocene, but then gradually retreated to the position of the modern monsoon front. The uplift of the Tibetan Plateau and global climate change may have played dominant roles in this evolution.