Asian Summer Monsoon Research Articles

Impact of ENSO on the UTLS chemical composition in the Asian Summer Monsoon Anticyclone

The changes in the convection and background dynamics over the Asian Summer Monsoon (ASM) are mainly responsible for the changes in the concentrations of the tropospheric pollutants within the Asian Summer Monsoon Anticyclone (ASMA). In this study, we have investigated the structural and chemical composition changes of the ASMA during different El Niño–Southern Oscillation (ENSO) conditions by making use of reanalysis products and Microwave Limb Sounder (MLS) satellite observations. Our analysis mainly concentrates on the July and August months of 2015 (El Niño) and 2022 (La Niña), which are the cold (Nino Index 3.4 exceeding −0.5 in magnitude throughout the year) and warm (Nino Index exceeding +0.5 in magnitude throughout the year) phase of the ENSO. The ASMA structure during different phases of the ENSO is found to be different when compared with the long-term (2005–2023) mean. The ASMA is found to have a larger zonal extent during La Niña (2022), extending from 20° to 140° E compared to El Niño (2015), which is from 20° to 100–105°E. A significant decrease (increase) in the concentration of the tropospheric species (CO and WV) is noticed within the ASMA in El Niño (La Niña). The decrease in the tropospheric species is clearly evident throughout the upper troposphere. The northeastern edge of the ASMA at 100 hPa and 150 hPa showed quite distinct behaviour with an enhancement (reduction) in the tropospheric species during La Niña (El Niño). Interestingly, there is an increase in O3 concentrations over the northeastern edge of ASMA during 2015, which is attributed to the stratospheric intrusion. Our results also revealed the enhancement of the tropospheric species over the western Pacific region during 2015 but not in 2022. Current findings highlight dynamic shifts linked to varying ENSO phases, leading to notable alterations in both the ASMA structure and Upper Troposphere and Lower Stratosphere (UTLS) chemical composition.

Impact of Asian aerosols on the summer monsoon strongly modulated by regional precipitation biases

Abstract. Reliable attribution of Asian summer monsoon variations to aerosol forcing is critical to reducing uncertainties in future projections of regional water availability, which is of utmost importance for risk management and adaptation planning in this densely populated region. Yet, simulating the monsoon remains a challenge for climate models that suffer from long-standing biases, undermining their reliability in attributing anthropogenically forced changes. We analyze a suite of climate model experiments to identify a link between model biases and monsoon responses to Asian aerosols and associated physical mechanisms, including the role of large-scale circulation changes. The aerosol impact on monsoon precipitation and circulation is strongly influenced by a model's ability to simulate the spatio-temporal variability in the climatological monsoon winds, clouds, and precipitation across Asia, which modulates the magnitude and efficacy of aerosol–cloud–precipitation interactions, an important component of the total aerosol response. There is a strong interplay between South Asia and East Asia monsoon precipitation biases and their relative predominance in driving the overall monsoon response. We found a striking contrast between the early- and late-summer aerosol-driven changes ascribable to opposite signs and seasonal evolution of the biases in the two regions. A realistic simulation of the evolution of the large-scale atmospheric circulation is crucial to realize the full extent of the aerosol impact over Asia. These findings provide important implications for better understanding and constraining the diversity and inconsistencies of model responses to aerosol changes over Asia in historical simulations and future projections.

Influence of aerosol forcing on the seasonal march of East Asia summer monsoon

Focusing on the anthropogenic aerosols effects on the East Asian Summer Monsoon (EASM) progression, we found that aerosols delay the northward progression of EASM by one pentad in the south of the middle-lower reaches of the Yangtze River (PD1: pentads 25-35) and the North-Northeast China (PD3: pentads 38-42). However, aerosols have minimal impact on the seasonal march of EASM over the Yangtze-Huai River basin (PD2: pentads 36-37). In PD1 and PD3, aerosols reduce sea-land temperature differences. Also, a southward shift in the westerly jet causing lower-level north winds, which weakens the EASM in PD3. The delayed EASM progress shows its weakness in PD1 and PD3. While in PD2, EASM intensity weakens without affecting its northward progression. Aerosols, though affecting the position and intensity of Western Pacific Subtropical High, minimally influence its northward movement speed, hence no effect on the seasonal march of EASM in PD2.

Diversity of the tropical easterly jet’s core location

The upper-tropospheric tropical easterly jet (TEJ) is one of the most important systems in modulating the Asian summer monsoon rainfall. In addition to the intensity variability that has been extensively studied, the TEJ’s core experiences remarkable changes in the zonal and meridional directions. The TEJ can be identified as three locational patterns using the cluster analysis: the east, northwest, and southwest modes. The frequencies of the three locational modes exhibit discernable changes on the monthly and the interannual-decadal time scales. While the anomalous zonal divergent circulation with the convergent/divergent center over the tropical Indian Ocean (IO) determines the zonal location of the TEJ’s core, the meridional temperature gradient between the Eurasian continent and the tropical IO distinguishes the meridional location of the TEJ’s core. It reflects the fundamental role of the large-scale east-west and north-south thermal contrasts in the movement of the TEJ’s core location. The variability of the TEJ’s core location has distinct impacts on the summer monsoon precipitation via redistributing the upper-level divergence and modulating the monsoon meridional circulation, especially in South, Southeast, and East Asia. In conjunction with the thermal effect of the Tibetan Plateau, the meridional shift of the TEJ’s core can affect the precipitation along the south slope of the Tibetan Plateau. These findings highlight the cause of the diversified TEJ’s core location and the significant impacts on the summer monsoon rainfall.

Middle to late Holocene human societies on the eastern margin of the Eurasian Steppe, and their adaptation to environmental changes

The impact of environmental changes on the development of human societies in the Eurasian Steppe is not well understood over the Holocene timescale. In this study, we have reported the phytoliths, grain size, and weight loss on ignition for an aeolian soil profile in the Liao River Basin at the eastern margin of the Eurasian Steppe. The profiles spanned the middle and late Holocene based on AMS 14C dating. By employing the summed radiocarbon probability distribution of archaeological sites and kernel density analysis, the spatial and temporal distributions of human populations and settlements were investigated in order to analyze the rise and fall of ancient cultures. Our findings indicated that the environment in the Liao River Basin underwent three stages of change. From 6700 to 5400 cal yr BP, the climate was warm and humid. The climate suddenly became cold and dry from 5400 to 4900 cal yr BP. The climate fluctuated, with a general tendency towards cold and dry conditions, from 4900 cal yr BP to 1700 cal yr BP. In addition, human societies in the region underwent significant changes, with rapid agricultural development and population growth in the warm and humid environment of the Middle Neolithic (6700–5000 yr BP). However, during the Late Neolithic (5000–4000 yr BP), the living environment suddenly became cold and dry, which limited the development of human society and drastically reduced its population. Nevertheless, the resilience of human societies continued to increase, and in the Bronze Age (4000–2000 yr BP), even in an overall cold and dry living environment, human societies still developed and their population increased sharply. Changes in the East Asia Summer Monsoon were an important driver of cultural change, and our study emphasizes the complex human-environment interactions that shaped societal development on the Eurasian Steppe.

Decomposition of physical processes controlling EASM precipitation changes during the mid-Piacenzian: new insights into data–model integration

The mid-Piacenzian warm period (MPWP, ~3.264–3.025 Ma) has gained widespread interest due to its partial analogy with future climate. However, quantitative data–model comparison of East Asian Summer Monsoon (EASM) precipitation during the MPWP is relatively rare, especially due to problems in decoding the imprint of physical processes to climate signals in the records. In this study, pollen-based precipitation records are reconstructed and compared to the multi-model ensemble mean of the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). We find spatially consistent precipitation increase in most simulations but a spatially divergent change in MPWP records. We reconcile proxy data and simulation by decomposing physical processes that control precipitation. Our results 1) reveal thermodynamic control of an overall enhancement of EASM precipitation and 2) highlight a distinct control of thermodynamic and dynamical processes on increases of tropical and subtropical EASM precipitation, reflecting the two pathways of water vapor supply that enhance EASM precipitation, respectively.

Sedimentary evolution of the East China Sea with implications for the relationship between the East Asian winter and summer monsoon over the past 6.3 ka

Sediments in the muddy areas of marginal seas of East China can serve as natural archives of global climate and environmental changes. Studies of these sediments can offer valuable insights for paleoclimate changes. This study examined S04-2, a core located in the central of the inner-shelf muddy area of the East China Sea, to reveal the evolutionary history since 12 ka BP and investigate the relationship between the East Asian Winter Monsoon (EAWM) and East Asian Summer Monsoon (EASM). On a centennial timescale, the EASM was negatively correlated with EAWM and this relationship is mainly influenced by solar radiation, wherein increases or weakening of solar radiation led to a negative or positive correlation between the EASM and EAWM. Additionally, it was observed that the sustained negative correlation during the periods of 5.1–4.3 ka BP and 2.0–0.6 ka BP was modulated by the strong North Atlantic Oscillation (NAO), resulting in periods of heightened social sensitivity in ancient China during 2.0–0.6 ka BP.

Decoupling Warming and Monsoon Precipitation Over the Last 3110 Years in Northwestern Yunnan Province, Southwestern China

AbstractUnderstanding the variability and related dynamic mechanisms of the Indian summer monsoon evolution over the past millenniums is crucial for disaster prevention and mitigation in the context of strengthening global warming. Here, we present a high‐resolution hydroclimate reconstruction over the past 3110 years in northwestern Yunnan Province, southwestern China, based on grain size analysis using the End‐member modeling algorithm and other environmental proxies. The results indicate that the temperature in northwestern Yunnan Province generally decreased since 3110 cal. yr BP, aligning with other temperature records on regional and global scales. Precipitation in the study area remained relatively stable before 1000 cal. yr BP, but exhibited an obvious out‐phase change compared with the temperature records over multidecadal to centennial timescale. While after 1000 cal. yr BP, the precipitation steadily increased, revealing the hydroclimate pattern of notably dry Medieval Warm Period (MWP) and relatively wet Little Ice Age (LIA) in the study area. The decoupling combination of temperature and precipitation spanning the MWP and LIA in southwestern China contrasts with that in the East Asian summer monsoon area and the Indian subcontinent which is also controlled by the Indian summer monsoon, indicating the spatial heterogeneity of precipitation in the Asian summer monsoon region. The discussion about dynamic mechanisms points out that solar activity emerges as a significant factor influencing regional temperature changes, and El Niño‐Southern Oscillation and Intertropical Convergence Zone migration jointly impact the precipitation change in the study area and the hydroclimate discrepancy in the Asian summer monsoon region.

South Asian Summer Monsoon Precipitation Is Sensitive to Southern Hemisphere Subtropical Radiation Changes

AbstractWe study the sensitivity of South Asian Summer Monsoon (SASM) precipitation to Southern Hemisphere (SH) subtropical Absorbed Solar Radiation (ASR) changes using Community Earth System Model 2 simulations. Reducing positive ASR biases over the SH subtropics impacts SASM, and is sensitive to the ocean basin where changes are imposed. Radiation changes over the SH subtropical Indian Ocean (IO) shifts rainfall over the equatorial IO northward causing 1–2 mm/day drying south of equator, changes over the SH subtropical Pacific increases precipitation over northern continental regions by 1–2 mm/day, and changes over the SH subtropical Atlantic have little effect on SASM precipitation. Radiation changes over the subtropical Pacific impacts the SASM through zonal circulation changes, while changes over the IO modify meridional circulation to bring about changes in precipitation over northern IO. Our findings suggest that reducing SH subtropical radiation biases in climate models may also reduce SASM precipitation biases.

A pollen-based reconstruction of middle to late holocene precipitation dynamics in Anyang archaeological area, Central Plains, China

In the context of global warming, the dynamics of East Asian Summer Monsoon (EASM) are of great significance for predicting future climate change. The variations of humidity indicated by various climate proxies in Central China are controversial. This study relies on pollen data retrieved from lacustrine sediment at the Anshang site in Anyang archaeological area, Central Plains of China to quantitatively reconstruct the change in PANN (mean annual precipitation). The results show that the region experienced three distinct dry-wet intervals during the ∼8.54 to ∼3.26 cal ka BP: the earlier portion of the mid-Holocene (∼8.5–∼6.0 cal ka BP) witnessed the higher PANN; the latter portion of the mid-Holocene (∼6.0–∼3.6 cal ka BP) enjoyed the highest PANN of the past ∼8500 years with a noticeable cold and dry event occurred around 4.2 cal ka BP; the late Holocene (∼3.6–∼3.2 cal ka BP) experienced the lower PANN. Periodic analysis on pollen-based precipitation further reveals ∼500-year and ∼800-year cycles, matching well with regional climate records. Our regional comparison suggests that the Central Plains of China shared similar climatic controls with the southern region of China during the ∼8.54 to ∼3.26 cal ka BP. Shifts of Western Pacific Subtropical High (WPSH), driven by the ENSO (El Niño-Southern Oscillation) conditions and SST (Sea Surface Temperatures) in the western tropical Pacific, regulated the positioning of the EASM-related rain-belt during ∼8.54 to ∼3.26 cal ka BP. This study provides more reliable variations of humidity in the Central Plains under the influence of the EASM, which is helpful to predict future climate change.

Drivers of East Asian summer monsoon variability: Global oceans and the Tibetan Plateau

The East Asian summer monsoon (EASM) profoundly influences the hydroclimate, ecosystems, and water resources of a densely populated region in Asia. Anomalous EASM can lead to floods, droughts, significant economic losses, and social disruption in East Asia and Southeast Asia. Unlike the Indian and African monsoons, the EASM is shaped by both tropical and extratropical climate systems, including tropical oceanic modes, cross-equatorial airflows, the Madden-Julian Oscillation, the thermal condition of the Tibetan Plateau, the South Asia High, and the mid-latitude westerly jet. This complexity makes the EASM a challenging system to understand and predict. The variability of the EASM spans various time scales, with interannual variations contributing significantly to the total variance compared to decadal variability.

East Asian summer monsoon and early anthropogenic impacts on the lacustrine carbon cycle in the Otindag Sandy Land since the middle Holocene

Lake systems play a pivotal role in the global carbon cycle, yet the biogeochemical processes in alkaline and saline lakes have received little attention. In this study, we present a comprehensive analysis of δ13Ccarb and δ18Ocarb records from Lake Chagan in the Otindag Sandy Land, located in North China, spanning approximately 7000 years. Our aim is to explore how the lacustrine inorganic carbon cycle responded to both climate change and human activity. The findings reveal a notable highstand in Lake Chagan around 6.9–6.0 cal kyr BP, followed by a gradual regression leading to an unstable state over the past 1.6 cal kyr BP. The hydroclimate variations inferred from Lake Chagan align with other regional records, suggesting that long-term East Asian summer monsoon (EASM) precipitation trends were influenced by decreasing insolation and variations in solar irradiance on a centennial timescale. The sources of dissolved inorganic carbon (DIC) in lake water generally mirrored regional hydroclimate changes since the middle Holocene. Our evidence indicates that during periods of strong EASM activity, more allochthonous organic matter was converted to lake water DIC, coinciding with high vegetation cover and intense soil erosion. While hydroclimatic changes significantly impacted the lacustrine carbon cycle, the isotopic evidence also suggests substantial anthropogenic influences post-1.6 cal kyr BP, particularly in terms of the input of allochthonous organic matter into the lakes of the Otindag Sandy Land. These findings underscore the potential for anthropogenic-driven terrestrial carbon cycle feedback to global climate change over millennia.

Asian winter monsoon controls marine primary productivity in north Arabian Sea during the Holocene

The Arabian Sea (AS), which is strongly impacted by monsoon winds, is one of the most biologically productive regions. The forcing of Asian monsoons on Holocene productivity remains debated in north AS, an area influenced by summer and winter monsoon. A sediment core from Makran margin was utilized to analyze organic biomarkers and high-resolution bromine intensities, unveiling the forces of biological productivity through the past 8000 years. Alkenone sea surface temperature (SST) records show significant cooling in north AS and augmented SST difference between north and west AS toward the late Holocene, suggesting Asian winter monsoon (AWM) strengthening during the period. The Holocene productivity signals in north AS, indicated by the bromine to rubidium ratio and multiple biomarkers, are responsive to SST changes and thus mainly controlled by the AWM variation, not modulated by Asian summer monsoon (ASM). Millennial-to-centennial-scale variability of marine productivity during the Holocene is closely related to North Atlantic climate oscillations, mainly through atmospheric circulations in winter.

Accumulation and Quantitative Assessment of Terrigenous Organic Matter in Upper Oligocene to Lower Miocene Marine Source Rocks of the Qiongdongnan Basin.

The intense collision between marine and terrestrial agents results in the dual-source (marine and terrigenous) characteristics of marine source rocks. Our research quantitatively assessed terrestrial organic matter and revealed the crucial role of terrestrial organic materials in the organic matter enrichment of lower Miocene to upper Oligocene marine source rocks in the Qiongdognnan Basin. The quantitative assessment was achieved using partial least-squares analysis with eight biomarker parameters associated with n-alkanes, isoprenoids, bicadinanes, taraxerane, tricyclic terpanes, and gammacerane. Differential unloading of terrestrial organic materials based on sedimentary facies of the delta-marginal sea system were observed through oleanane and bicadinane contents. It should be noted that the diagnostic ratio of oleanane was excluded from the quantitative analysis due to the dual influence from differential unloading and contact with seawater of the terrestrial organic materials. Calculation results show that the terrestrial organic matter was highest in the delta front at 70%, followed by prodelta at 59% and inner shallow marine at 57%. From the late Oligocene to the early Miocene, the proportion of terrestrial organic matter in marine source rocks continuously increased, with the highest average value observed in the second member of the Sanya Formation at 69% and the lowest occurring in the third member of the Lingshui Formation at 46%. Increasing terrestrial organic material inputs and preservation driven by the East Asian summer monsoon provided first-order control of the accumulation of organic carbon in the Qiongdongnan Basin during late Oligocene to early Miocene, rather than the bioproductivity of marine algae. The redox conditions of the water column determine the enrichment extent of organic matter.

June insolation gradient and ice sheet forcing on Qaidam precipitation during the middle Piacenzian warm period

Whether the East Asian summer monsoon (EASM) migrated westward into the Asian interior during past warm periods is still unclear but is important for predicting precipitation variability in the Asian interior under future warming. Recent precipitation records from the Qaidam Basin in the Asian interior suggest that the EASM further migrated westward into the western Qaidam Basin during the middle Piacenzian warm period (mPWP: 3.264–3.025 Ma), indicating that precipitation in this ecologically fragile region was greater than it is currently. However, these precipitation records reveal weak sensitivity to insolation forcing, which is inconsistent with records from monsoon-influenced regions, hindering our understanding of orbital timescale precipitation variations in the Qaidam Basin during this Pliocene warmth. In this study, we report high-resolution (~3 kyr) magnetic parameter-based records and medium grain size records from the GS section of the western Qaidam Basin during 3.25–2.95 Ma. We interpret magnetic parameters and grain size as precipitation proxies based on their sensitivity to precipitation variations. Our precipitation records suggest that the EASM precipitation was likely the precipitation source for the Qaidam Basin during the mPWP. At the orbital timescale, precipitation records reveal dominant 20-kyr precessional cycles, consistent with records from the Chinese Loess Plateau and the South China Sea, suggesting that the June insolation gradient controlled precipitation variations in the Qaidam Basin during the mPWP. At the 40-kyr band, precipitation records show in-phase relationships with ice sheets, and we infer that Antarctic ice sheet forcing seems to also influence Qaidam precipitation via atmospheric or marine processes. These results provide the first evidence demonstrating that Qaidam precipitation was extremely sensitive to the June insolation gradient and ice sheets during the mPWP, improving our understanding of orbital timescale precipitation variations in the Asian interior during past warm periods.

Future changes in South Asian summer monsoon circulation under global warming: role of the Tibetan Plateau latent heating

The South Asian summer monsoon (SASM) is a significant monsoon system that exerts a profound impact on climate and human livelihoods. According to 38 models from the Coupled Model Intercomparison Project Phase 6, the SASM circulation is projected to weaken significantly under global warming as seen in the weakened low-level westerly wind over the northern tropical Indian Ocean; however, the associated climate dynamics is still under debate. Here, we identify that the weakened low-level westerly wind is closely related to the enhanced latent heating over the Tibetan Plateau (TP), which corresponds with increased summer precipitation in the future. The intensified TP latent heating triggers an anomalous meridional circulation with ascending motions over the plateau and descending motions to the south, leading to an anomalous low-level anticyclone over the northern tropical Indian Ocean. This anticyclone greatly weakens the prevailing low-level westerlies of the SASM through easterly anomalies at the anticyclone’s southern flank. Moisture budget analysis further reveals that increased atmospheric water vapor, rather than the vertical dynamic component, makes the largest contribution to the increased precipitation over the TP. This result confirms that the enhanced TP latent heating is a driver of atmospheric circulation change and contributes to weakening the SASM circulation.

Long range transport of South and East Asian anthropogenic aerosols counteracting Arctic warming

The large-scale convection during the Asian summer monsoon plays an important role in the rapid transport of boundary layer aerosols into the Asian summer monsoon anticyclone. Here, using the state-of-the-art ECHAM6–HAMMOZ aerosol-chemistry-climate model, we show that these aerosols are further transported to the Arctic along isentropic surfaces by the Brewer-Dobson-Circulation (BDC) during the monsoon season. Our model simulations show that East and South Asian anthropogenic emissions contribute significantly to the aerosol transported to the Arctic, which causes a higher negative net aerosol radiative forcing at the surface (dimming) of −0.09 ± 0.02 Wm−2 and −0.07 ± 0.02 Wm−2, respectively. Over the Arctic, the East Asian anthropogenic aerosols that include large amounts of sulfate cause a seasonal mean net radiative forcing at the top of the atmosphere (TOA) of −0.003 ± 0.001Wm−2 and a surface cooling of −0.56 K while the black carbon dominated aerosol from South Asia shows a positive TOA forcing of +0.004 ± 0.001Wm−2 with an only minor surface cooling of −0.043 K. Overall, the long-range transport of South Asian aerosols results in a notably warming throughout the atmospheric column but minimal temperature response at the Arctic surface. Conversely, East Asian aerosols cool the troposphere and heat the lower stratosphere in the Arctic. The Asian aerosol thus plays an ambivalent role, with the East Asian sources in particular having the potential to counteract the rapid rise in Arctic temperatures and the associated melting of snow and ice.

AirCore Observations at Northern Tibetan Plateau During the Asian Summer Monsoon

AbstractWe present data and analysis of a set of balloon‐borne sounding profiles, which includes co‐located O3, CO, CH4, and particles, over the northern Tibetan Plateau during an Asian summer monsoon (ASM) season. These novel measurements shed light on the ASM transport behavior near the northern edge of the anticyclone. Joint analyses of these species with the temperature and wind profiles and supported by back trajectory modeling identify three distinct transport processes that dominate the vertical chemical structure in the middle troposphere, upper troposphere (UT), and the tropopause region. The correlated changes in profile structures in the middle troposphere highlight the influence of the strong westerly jet. Elevated constituent concentrations in the UT identify the main level of convective transport at the upstream source regions. Observed higher altitude maxima for CH4 characterize the airmasses' continued ascent following convection. These data complement constituent observations from other parts of the ASM anticyclone.

The impact of precipitation changes on the safety of railway operations in China under the background of climate change

Global climate change has intensified the water cycle, leading to frequent extreme precipitation events, posing a significant threat to railway infrastructure and safety operations. Based on the analysis of past and future precipitation changes in China, this study investigates the impact of climate change on railway safety operations. The study reveals the following findings: (1) Under the influence of the intensified East Asian summer monsoon and the northward shift of the subtropical high during the 2017–2021 compared to the 2012–2016, precipitation has significantly decreased (120 mm) in the regions south of the Yangtze River and South China, while it has increased (60 mm) in the regions from the eastern of Northwest China to the middle and lower reaches of Yangtze River; The operational precipitation risk has decreased for Urumqi, Lanzhou, Qinghai-Tibet Group, Xi’an, and Wuhan railway bureaus (abbreviated as Bureau), while it has increased for Nanchang, Chengdu, Zhengzhou, Shanghai, and Shenyang Bureaus. Particularly noteworthy is that despite a decrease in total annual precipitation for Nanchang bureau (15.3 mm/a), the frequency of intense precipitation events has increased, leading to an increased operational precipitation risk. (2) During the 21st century, under high (SSP5-8.5), medium (SSP2-4.5), and low (SSP1-2.6) forcing scenarios, all projections indicate that most of the China will experience an increasing trend in precipitation, with significant increases in precipitation observed in the regions south of the Yangtze Rive, South and Southwest China. The higher the greenhouse gas emissions, the more pronounced the increasing trend in precipitation. (3) Compared to the 20th century, under high (SSP5-8.5), medium (SSP2-4.5), and low (SSP1-2.6) forcing scenarios, all projections indicate that the total annual precipitation hours, railway inspection, speed limit, and closure risk hours have all increased on a national scale during the 21st century. The operational precipitation risk for railways has also increased. The higher the alert level for railway precipitation (precipitation < inspection < speed limit < closure), the higher the proportion of risk hours compared to the 20th century. By the late 21st century, the railway inspection, speed limit, and closure risk hours have increased by 175%, 463%, and 647%, respectively, compared to the 20th century.

Using regional relaxation experiments to understand the development of errors in the Asian summer monsoon

Abstract. We describe the use of regional relaxation (“nudging”) experiments carried out in initialised hindcasts to shed light on the contribution from particular regions to the errors developing in the Asian summer monsoon. Results so far confirm previous hypotheses that errors in the Maritime Continent region contribute substantially to the East Asia summer monsoon (EASM) circulation errors through their effects on the western North Pacific subtropical high. Locally forced errors over the Indian region also contribute to the EASM errors. Errors arising over the Maritime Continent region also affect the circulation and sea surface temperatures in the equatorial Indian Ocean region, contributing to a persistent error pattern resembling a positive Indian Ocean dipole phase. This is associated with circulation errors over India and the strengthening and extension of the westerly jet across southeast Asia and the South China Sea into the western Pacific, thereby affecting the Asian summer monsoon (ASM) circulation and rainfall patterns as a whole. However, errors developing rapidly in the deeper equatorial Indian Ocean, apparently independently of the atmosphere errors, are also contributing to this bias pattern. Preliminary analysis of nudging increments over the Maritime Continent region suggests that these errors may at least partly be related to deficiencies in the convection and boundary layer parameterisations.