Summer Monsoon Variability Research Articles

Predictability of tropical cyclone track density in S2S reforecast

In this study, we examine the predictability of tropical cyclone (TC) track density in the subseasonal-to-seasonal (S2S) reforecast ensembles of the European Centre for Medium-range Weather Forecasts (ECMWF) using the method of average predictability time (APT). Eleven of the retrieved APT modes (APTMs) of TC track density possess an APT longer than 1 week. The most predictable of them, APTM-1, has an APT of almost three weeks and is found to be closely linked to the Boreal Summer Intraseasonal Oscillation (BSISO) and monsoon variability. Another discovery is the strong relationship between APTM-7 and the activity of mixed Rossby-gravity (MRG) waves and tropical depression (TD) type disturbances despite its short APT of ~12 days. We further carry out a simple case analysis to see how the relatively high predictability of APTM-1 manifests in the S2S model. Our work provides a new possibility for improving medium-range TC forecast skill, and has revealed how underlying tropical variability can play a role in determining TC predictability.

Rhythmic Indian Summer Monsoon variability inferred from stalagmite δ18O and δ13C record during Middle to Late Holocene transition

Rhythmic Indian Summer Monsoon variability inferred from stalagmite δ18O and δ13C record during Middle to Late Holocene transition

Indian summer monsoon variations since late MIS3: A perspective from multiproxy evidence from Lake Xingyun in southwestern China

Indian summer monsoon variations since late MIS3: A perspective from multiproxy evidence from Lake Xingyun in southwestern China

Late holocene vegetation succession and climate change in the Central Plains of China: Evidence from a high-resolution pollen record

Changes in the East Asian summer monsoon precipitation will have a significant impact on China’s politics, economy, and social development. To explore the controls and drivers of the Late Holocene (∼3350–2560 cal. yr. BP) vegetation succession and East Asian summer monsoon precipitation changes in the Central Plains of China, we conducted pollen analysis of the Pu Tian core collected from Zhengzhou, China. The results showed three periods of vegetation and East Asian summer monsoon variation: coniferous and broad-leaved mixed forest between ∼3350–3025 cal. yr. BP, increasing forest vegetation dominated by broadleaf Betula between ∼3025–2930 cal. yr. BP, decreasing forest vegetation and increasing herbaceous plants dominated by Artemisia and Poaceae between ∼2930–2560 cal. yr. BP. Weighted Averaging Partial Least Squares Regression was used to quantitatively reconstruct the average annual precipitation variation in the Central Plains of China. Our precipitation reconstruction and its comparison to other reconstructions and forcing show that it was influenced by internal (intertropical convergence zone and El Niño-Southern Oscillation) and external (Northern Hemisphere Summer Insolation) forcing. Specifically, average annual precipitation decreased significantly between ∼2980–2680 cal. yr. BP, and tree pollen (such as Betula) also decreased significantly during this period, possibly related to the “2.8 ka” event (a weak monsoon event in the Holocene). Our comparison of the published climate and environmental records of the East Asian monsoon region suggests that the 2.8 ka event exhibiting a “W” structure of “dry-wet-dry” and may have been caused by the weakening of solar activity, which affected the climate of the East Asian monsoon region through oceanic and terrestrial atmospheric circulation such as El Niño-Southern Oscillation.

Optimal Atmospheric Heat Sources for the Interannual Variability of South Asian Summer Monsoon

AbstractUsing a Green's function‐like approach, this study identifies optimal atmospheric heat sources for the two leading modes of South Asian Summer Monsoon (SASM) interannual variability. Optimal heating for the first mode, characterized by a lower‐level anomalous anticyclone over northern Bay of Bengal (BOB), is distributed over the Arabian Sea and tropical eastern Indian Ocean (EIO)‐Maritime Continent, with cooling over the BOB‐western North Pacific. In contrast, heating over the tropical southwestern Indian Ocean and equatorial Atlantic, along with cooling over the tropical EIO‐western Pacific, optimally drives the second mode, featuring an anomalous anticyclone over central‐northern India. El Niño/Southern Oscillation indirectly influences SASM by triggering heat sources resembling these optimal patterns. Other sea surface temperatures (SSTs), like those over equatorial Atlantic, can also generate similar heating structures, causing corresponding SASM anomalies. This suggests that the impact of SST modes on SASM depends on the similarity of induced heat sources to optimal patterns.

Impacts of early spring soil moisture over the Greater Mekong Subregion on the interannual variation of South China Sea summer monsoon onset

This study investigates the influence of early spring (March–April) soil moisture (SM) over the Greater Mekong Subregion (GMS) on the interannual variation of South China Sea summer monsoon (SCSSM) onset, using observational analyses and numerical experiments. It is found that when early spring SM over the GMS is wetter, westerly anomalies dominate the South China Sea, corresponding to an early onset of the SCSSM, and vice versa. The analyses of physical mechanism show that the positive anomalies of early spring SM decrease local surface air temperature by adjusting the surface latent heat flux and sensible heat flux. The persistence of anomalous ground cooling contributes to negative geopotential height anomalies in the middle troposphere, and further induces the eastward retreat of western North Pacific subtropical high in May. The anomalous cyclone over the South China Sea favors the early onset of SCSSM. The key physical processes linking the variability of early spring SM over the GMS and the following SCSSM onset are confirmed by the sensitivity experiments using a coupled atmosphere–land model (CAM6–CLM5). Specifically, the onset date of the SCSSM in the drier experiment is 11 days later than that in the wetter experiment. The present results have implications for the forecast of SCSSM onset.

Grain Size in an Alpine Lake from the Chinese Loess Plateau: Implications for Paleofloods and East Asian Summer Monsoon Variability

Reliable paleoflood proxies can help reconstruct past flood variation patterns. Here, we investigated the grain-size data of a 63 cm core retrieved from Lake Chaonaqiu, western Chinese Loess Plateau, in order to build a long time-series of flood occurrence from sedimentology that extends the period of instrumental data. Our results indicate that three parameters (mean, standard deviation and grain-size ratio of 16–63/2–16 μm) are sensitive to hydrodynamic changes in Lake Chaonaqiu, which are further linked to high-energy inflow associated with high-intensity rainfall or flood events. These three parameters’ variations were well correlated with the precipitation records reconstructed from tree-rings and historical documents in neighboring regions and overlapped with 109 historical flood events from historical documents in counties around the lake for the past 300 years. Therefore, we propose that the grain size in the sediments of Lake Chaonaqiu is a reliable paleoflood proxy. The sensitivity of flood signals to grain size may be related to the precipitation and vegetation cover in the catchment of the lake, which are further linked to the strength of the East Asian summer monsoon.

Mid to late Holocene Indian monsoon variability, aridification and civilization changes in the Deccan Plateau, India

Climate change has been a key driver throughout human history and has frequently been associated with the rise and fall of civilizations. Holocene settlement changes or population displacements were almost always preceded by changes in climate. A high-resolution sedimentary record from the western Bay of Bengal offers insights to centennial-scale mid- to late-Holocene Indian summer monsoon (ISM) variability and its role in the decline of chalcolithic human settlements on India's Deccan Plateau. Increased erosion in the Indian peninsula during the mid- to late-Holocene, due to aridification and agricultural expansion, is evidenced by higher sedimentation, more magnetic mineral content, coarser magnetic grain size, and increased sand content. The results from mineral magnetic and textural analyses reveal centennial-scale abrupt weak ISM during the Bond events in the core monsoon zone of India. The Deccan Chalcolithic civilizations flourished between ∼4.0–3.0 ka BP owing to the favorable climatic conditions, but most of the settlements were deserted after ∼3.0 ka BP. We argue that the abrupt weakening of ISM during Bond event 2 (∼3.1–2.8 ka BP) caused this collapse of Deccan Chalcolithic. The results from this study together with the published records of number of settlements and summed radiocarbon dates suggest a climate-culture link in the Deccan Plateau. The weak ISM periods in this study are coherent with the records of total solar irradiance and the percentage of hematite-stained grains from the North Atlantic and suggest solar control on these abrupt climatic events.

Thermal effect of the Southeast Asian low-latitude highlands on interannual variability in the date of the Bay of Bengal summer monsoon onset

The timing of the Bay of Bengal summer monsoon (BOBSM) onset has implications for the evolution of the Asian summer monsoon and associated precipitation. This study employs Japanese 55-year Reanalysis and NOAA's Gridded Precipitation Reconstruction over Land data to explore the spatiotemporal variation in the thermal influence of the Southeast Asian low-latitude highlands (SEALLH) and its effect on the BOBSM onset date. There is a significant correlation between pre-monsoon season (February–April) shortwave radiative heating (SWH) in the SEALLH and BOBSM onset in the interannual time scale. During the enhanced SWH, an anomalous vertical circulation, which converges and ascends in the lower troposphere over the SEALLH and converges and descends in the upper troposphere north of the Bay of Bengal (BOB), develops over the SEALLH–north of BOB during the pre-monsoon season with anticlockwise rotation. The warmer air mass resulting from the adiabatic heating is accumulated around the north of BOB by the insulation effect related to the anomalous vertical circulation. By facilitating the reversal of the land-sea thermal contrast, the warmer upper troposphere over the north of the BOB causes a 16-day earlier onset of the BOBSM than the case of weakened SWH. The findings shed light on subsequent research into the thermal effects of the SEALLH on Asian summer monsoon variability.

Middle Pleistocene weakening of the Indian summer monsoon driven by global cooling

The Indian Monsoon profoundly impacts the climate, agriculture, and economy of the Indian subcontinent and southwestern China. However, the long-term variability of the Indian summer monsoon (ISM) and its forcing mechanism in the context of the stepwise Pleistocene cooling trend are poorly understood, due to the lack of high-quality terrestrial paleoclimate records. Quaternary loess and paleosol deposits in SW China provide the opportunity to study the long-term evolution of the ISM. Based on analyses of magnetic properties and chemical weathering indices, we found that decreased chemical weathering and pedogenic intensity occurred in SW China since the Middle Pleistocene. Comparison of our results with published paleoclimate records from the Indian Ocean and SW China suggest that substantial ISM weakening occurred after ∼0.7–0.6 Ma, coincident with the Mid-Pleistocene climatic transition. We conclude that decreasing global surface temperatures associated with the major growth of Northern Hemisphere ice sheets caused the Middle Pleistocene weakening of the ISM, leading to changes in precipitation regimes in the southern Asian continent and SW China. Our results suggest that global cooling is strongly linked with the South Asian monsoon climate system, and they provide clues for predicting future climate changes in its area of influence.

Holocene dynamics of the Indian summer monsoon inferred from a high-resolution peat α-cellulose δ18O record on the Tibetan Plateau

Knowledge of the variability of the Indian summer monsoon (ISM) is crucial for deciphering hydroclimate dynamics in the Asian Monsoon region during the Holocene. In this study, we present a continuous history of the ISM over the past ∼9700 years using the high-resolution δ18O composition of α-cellulose (δ18Ocell) derived from sedges in a high-altitude peatland in the south-central part of the Tibetan Plateau (TP). The results show that the δ18Ocell values increased gradually from the early Holocene, implying a weakening trend of the ISM. However, a remarkable decrease in δ18Ocell occurred after 2.0 cal ka BP, which is consistent with the recently reported '2-kyr shift' in stalagmite δ18O. Further compilation of reconstructions across Asia demonstrates that the '2-kyr shift' suggests an anomalous intensification of the ISM during the late Holocene. In addition, we propose the concept of a '2-kyr dry event' characterized by the weakest ISM intensity, lasting for approximately a thousand years. The ISM record is influenced by boreal summer insolation and ocean-atmosphere coupling in the Pacific and Indian Oceans. The '2-kyr shift' and '2-kyr dry event' in the ISM region could be linked to the SSTs of the western Arabian Sea and the Pacific warm pool, which superimposed on the reduced solar insolation. This study highlights the δ18O in peat α-cellulose responds well to monsoon intensity at our study site and the important role that ocean-atmosphere circulation plays in regulating centennial-millennial regional hydroclimate variability.

East Asian summer monsoon evolution during Heinrich Stadial 5

The bi-millennial Heinrich Stadial 5 (HS 5) event, occurring approximately 48 thousand years ago (kyr BP, before 1950 AD), represents a notable cooling episode during the last glacial period in the Northern Hemisphere. However, our comprehension of its impact on Asian summer monsoon variability remains incomplete, largely because of the scarcity of precisely dated and high-resolution records. Here, we present stalagmite δ18O-inferred absolutely dated decadal-resolved evolution of the East Asian summer monsoon (EASM) during HS 5 from Xianyun Cave, southeastern China. Stalagmite chronology constrains the occurrence of HS 5 from 48.67 ± 0.13 to 46.42 ± 0.10 kyr BP. The Xianyun stalagmite record distinctly reveals the three-phase evolution of the EASM during HS 5. It includes a gradual onset lasting for 0.79 kyr (phase 1) from 48.67 to 47.88 kyr BP, a weak EASM oscillation phase (phase 2) with three centennial-scale intensified monsoon events during 47.88–46.94 kyr BP, followed by a gradual 520-yr recovery phase (phase 3). In phases 1 and 2, the synchronous climatic shifts and resemblance to three notable centennial-scale events observed in both the Xianyun and Greenland ice core records suggest a rapid atmospheric teleconnection between the North Atlantic and EASM regions. During phase 3, the 5-century monsoon recovery registered in the Xianyun record contrasts sharply with the rapid 60-yr rebound in the Greenland ice core record. This disagreement suggests a plausible decoupling between low- and high-latitude climates, probably related to tropical sea surface temperatures variabilities and the distinct responses of the regional Intertropical Convergence Zone to El Niño Southern Oscillation variabilities. Our study highlights the essential role of tropical ocean-atmosphere coupling systems in the termination stages of millennial-scale HS events.

Geochemical records of mudflat sediments from southern Saurashtra, Western India: Implications for Holocene climate and global teleconnection

The heat transfer from the low latitudes to high latitudes is responsible for maintaining the earth’s climate dynamics. Thus, deciphering the possible mechanism driving the variability of the Indian summer monsoon (ISM) during the Holocene Epoch has been critical to understand the hydroclimatic changes of the low latitudes. Despite several efforts, the teleconnection of ISM with the global climate dynamics remains under-represented and poorly understood. The present study aims to delineate the ISM variability and its possible forcing mechanism from western India (Gujarat). In this study, a sediment core (~65 cm long) was raised from the Jaffrabad mudflat (MIT) in western Gujarat. The sediment samples were subjected to geochemical analysis to investigate paleomonsoon, paleo-sediment source and paleoweathering changes. The results show that, with the addition of intermediate sources, the sediments were principally derived from the hinterland’s Deccan basalts. Further, the study suggested a warm and wet climate due to strong ISM during 10,650−5500 cal yr BP associated with the solar as well as orbital forcings. The weak monsoon during 5500−2700 cal yr BP has been linked with southward migration of the Intertropical Convergence Zone (ITCZ) along with the increased El Niño-like conditions. Further, the wavelet analysis revealed that a combined influence of solar, orbital and North Atlantic forcings led to monsoon variability along western India, during the Holocene Epoch. By reconciling the geochemical proxies, the present study has implications in the reconstruction of paleomonsoon and establishing the possible teleconnection with the global climate system.

Influence of Equatorial Spring Insolation on Abrupt Asian Summer Monsoon Decline at Orbital Scale

AbstractThe dominant influence of precession‐induced changes in summer insolation on orbital‐scale variability of the Asian summer monsoon (ASM) during the Holocene has been widely proposed; however, it remains unclear why the decline of the ASM started several thousand years after the peak summer insolation. Through comparisons of climate simulations and proxy records, our study reveals that the abrupt decline in the ASM coincided with an increase in spring insolation at the equator. The reduced spring insolation resulted in a cooler tropical Indian Ocean, which weakened and shifted northward the westerly jet due to decreased meridional thermal gradient. The South Asian high moved northward in conjunction with the westerly jet, causing anomalous upwards over northern South Asia, the Tibetan Plateau, as well as southwestern and northern China. The associated anomalous cyclone over the Tibetan Plateau enhanced the monsoonal moisture transport, subsequently intensifying the ASM circulation and precipitation. The ASM was enhanced by the decrease in spring insolation and was weakened by the opposite. The abrupt decline of the ASM was associated with an increase in spring insolation superimposed on a decrease in summer insolation. Consequently, orbital‐scale ASM variability is dominated by the precession not only via insolation changes in summer but also changes in spring.

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.

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.

Late Pleistocene island weathering and precipitation in the Western Pacific Warm Pool

Deciphering past climate variability in the Western Pacific Warm Pool (WPWP), the Earth’s largest heat and moisture centre, is vital for understanding the global climate system. Nevertheless, its long-term evolution remains controversial, largely due to ambiguities in existing proxy interpretations and discrepancies between records. Here, we present a weathering and erosion reconstruction from the WPWP spanning the last 140,000 years, based on the mineralogy and geochemistry of a sediment core from offshore of northern New Guinea. The paleo-weathering reconstruction is consistent with the simulated precipitation evolution on nearby islands, thereby suggesting a close coupling between climate variability and island weathering in a tropical setting. In addition, our combined data-model interpretation of WPWP climate history shows many similarities to the East Asian Summer Monsoon (EASM) variability over orbital timescales. Overall, our study highlights the critical role of precession-paced interhemispheric energy redistribution, via the West Pacific meridional sea-surface pressure gradient, in linking orbital-scale WPWP climate and EASM variability.

Obliquity-paced summer monsoon from the Shilou red clay section on the eastern Chinese Loess Plateau

Abstract The red clay of the Chinese Loess Plateau (CLP) is an important geological archive for understanding the variability in the late Neogene East Asian monsoon. The periodicity of the summer monsoon of the red clay on the eastern CLP is dominated by eccentricity cycles within the constraints of the palaeomagnetic chronological framework, whereas global climate change characteristics represented by the deep-sea oxygen isotope record at that time show a dominating obliquity cycle. Here, we analyzed the East Asian summer monsoon proxies from the Shilou red clay section with the cyclostratigraphy method. The results show that the summer monsoon variation was dominated by obliquity, the optimum deposition rate was 4.451 cm/kyr, and the floating age of the Shilou red clay section was ca. 1.7 Ma. The late Neogene East Asian summer monsoon inferred from the eastern CLP was thus paced by the obliquity cycle, which is consistent with global change.

Exploring temporal and spatial SST patterns and their impact in the Arabian Sea: Insights from the regional ocean modeling system

The Regional Ocean Modeling System (ROMS) is integrated across the Arabian Sea (AS) from 1992 to 2021 and covers 33°E to 80°E and 5°N to 32°N at high horizontal resolutions of 1/4°(∼25 km). The study demonstrates that the variability is controlled by a seasonal high-resolution setup, with superior performance for various seasons during the months of December-January-February (DJF), March-April-May (MAM), June-July-August-September (JJAS) and October–November (ON). The Sea Surface Temperature (SST) anomalies in the JJAS season werestudied. SST dominates the seasonal variability of the Indian summer monsoon (ISM). Our results show that the ROMS can simulate seasonal variability and its effects on the upper ocean properties over the AS. In addition, the impact of monthly variation is controlled. In recreating the spatio-temporal distribution of surface as well as subsurface hydrographic parameters such as surface and subsurface temperature, the simulation results are reasonable like observation and reanalysis.

Northward shift of Indian summer monsoon and intensifying winter westerlies cause stronger precipitation seasonality over Pamirs and its downstream basins in the 21st century

Hydroclimate will change over Pamirs and its downstream basins (PDB), including Indus River, Tarim River, Amu Darya and Syr Darya Basins, in response to the variation of Indian summer monsoon (ISM) and mid-latitude westerlies. However, the precipitation variation and its mechanism over PDB in the 21st century are yet not fully understood. Here, the best models ensemble selected from 25 CMIP6 models under SSP2–4.5 and SSP5–8.5 scenarios is applied to detect the precipitation variations over PDB in the 21st century. A remarkable dipolar pattern is found in both summer and winter precipitation over PDB, particularly in the central Indus River Basin and upper Amu and Syr Darya Basins. The central Indus River Basin (upper Amu and Syr Darya Basins) will experience an increasingly wet (dry) summer in response to northward ISM and a dry (wet) winter driven by mid-latitude westerlies. The amplifying dipolar pattern of seasonal precipitation thus increases the water resource vulnerability over PDB and emphasizes the role of Pamirs in modulating the water resources over surrounding basins, especially the Amu Darya and Syr Darya Basins in the future. The findings underscore the need for prioritizing policies by considering the impacts of precipitation seasonality on social planning.