Showing papers on "Monsoon published in 2020"

Differences between CMIP6 and CMIP5 Models in Simulating Climate over China and the East Asian Monsoon

Abstract: We compare the ability of coupled global climate models from the phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively) in simulating the temperature and precipitation climatology and interannual variability over China for the period 1961\2-2005 and the climatological East Asian monsoon for the period 1979–2005. All 92 models are able to simulate the geographical distribution of the above variables reasonably well. Compared with earlier CMIP5 models, current CMIP6 models have nationally weaker cold biases, a similar nationwide overestimation of precipitation and a weaker underestimation of the southeast\3-northwest precipitation gradient, a comparable overestimation of the spatial variability of the interannual variability, and a similar underestimation of the strength of winter monsoon over northern Asia. Pairwise comparison indicates that models have improved from CMIP5 to CMIP6 for climatological temperature and precipitation and winter monsoon but display little improvement for the interannual temperature and precipitation variability and summer monsoon. The ability of models relates to their horizontal resolutions in certain aspects. Both the multi-model arithmetic mean and median display similar skills and outperform most of the individual models in all considered aspects.

The 2018 Kerala floods: a climate change perspective

Abstract: In August 2018, the Indian state of Kerala received an extended period of very heavy rainfall as a result of a low-pressure system near the beginning of the month being followed several days later by a monsoon depression. The resulting floods killed over 400 people and displaced a million more. Here, a high resolution setup (4 km) of the Weather Research and Forecasting (WRF) model is used in conjunction with a hydrological model (WRF-Hydro, run at 125 m resolution) to explore the circumstances that caused the floods. In addition to a control experiment, two additional experiments are performed by perturbing the boundary conditions to simulate the event in pre-industrial and RCP8.5 background climates. Modelled rainfall closely matched observations over the study period, and it is found that this would this would have been about 18% heavier in the pre-industrial due to recent weakening of monsoon low-pressure systems, but would be 36% heavier in an RCP8.5 climate due to moistening of the tropical troposphere. Modelled river streamflow responds accordingly: it is shown the six major reservoirs that serve the state would have needed to have 34% more capacity to handle the heavy rainfall, and 43% had the deluge been amplified by an RCP8.5 climate. It is further shown that this future climate would have significantly extended the southern boundary of the flooding. Thus it is concluded that while climate change to date may well have mitigated the impacts of the flooding, future climate change would likely exacerbate them.

Timing and structure of the Younger Dryas event and its underlying climate dynamics.

Abstract: The Younger Dryas (YD), arguably the most widely studied millennial-scale extreme climate event, was characterized by diverse hydroclimate shifts globally and severe cooling at high northern latitudes that abruptly punctuated the warming trend from the last glacial to the present interglacial. To date, a precise understanding of its trigger, propagation, and termination remains elusive. Here, we present speleothem oxygen-isotope data that, in concert with other proxy records, allow us to quantify the timing of the YD onset and termination at an unprecedented subcentennial temporal precision across the North Atlantic, Asian Monsoon-Westerlies, and South American Monsoon regions. Our analysis suggests that the onsets of YD in the North Atlantic (12,870 ± 30 B.P.) and the Asian Monsoon-Westerlies region are essentially synchronous within a few decades and lead the onset in Antarctica, implying a north-to-south climate signal propagation via both atmospheric (decadal-time scale) and oceanic (centennial-time scale) processes, similar to the Dansgaard-Oeschger events during the last glacial period. In contrast, the YD termination may have started first in Antarctica at ∼11,900 B.P., or perhaps even earlier in the western tropical Pacific, followed by the North Atlantic between ∼11,700 ± 40 and 11,610 ± 40 B.P. These observations suggest that the initial YD termination might have originated in the Southern Hemisphere and/or the tropical Pacific, indicating a Southern Hemisphere/tropics to North Atlantic-Asian Monsoon-Westerlies directionality of climatic recovery.

Ozone pollution over China and India: seasonality and sources

Abstract: . A regional fully coupled meteorology–chemistry model, Weather Research and Forecasting model with Chemistry (WRF-Chem), was employed to study the seasonality of ozone ( O3 ) pollution and its sources in both China and India. Observations and model results suggest that O3 in the North China Plain (NCP), Yangtze River Delta (YRD), Pearl River Delta (PRD), and India exhibit distinctive seasonal features, which are linked to the influence of summer monsoons. Through a factor separation approach, we examined the sensitivity of O3 to individual anthropogenic, biogenic, and biomass burning emissions. We found that summer O3 formation in China is more sensitive to industrial and biogenic sources than to other source sectors, while the transportation and biogenic sources are more important in all seasons for India. Tagged simulations suggest that local sources play an important role in the formation of the summer O3 peak in the NCP, but sources from Northwest China should not be neglected to control summer O3 in the NCP. For the YRD region, prevailing winds and cleaner air from the ocean in summer lead to reduced transport from polluted regions, and the major source region in addition to local sources is Southeast China. For the PRD region, the upwind region is replaced by contributions from polluted PRD as autumn approaches, leading to an autumn peak. The major upwind regions in autumn for the PRD are YRD (11 %) and Southeast China (10 %). For India, sources in North India are more important than sources in the south. These analyses emphasize the relative importance of source sectors and regions as they change with seasons, providing important implications for O3 control strategies.

Monsoons, ITCZs, and the Concept of the Global Monsoon

Abstract: Earth's tropical and subtropical rainbands, such as Intertropical Convergence Zones (ITCZs) and monsoons, are complex systems, governed by both large‐scale constraints on the atmospheric general circulation and regional interactions with continents and orography, and coupled to the ocean. Monsoons have historically been considered as regional large‐scale sea breeze circulations, driven by land‐sea contrast. More recently, a perspective has emerged of a Global Monsoon, a global‐scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation. This results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone. Regional subsystems are embedded in this global monsoon, localized by surface boundary conditions. Parallel with this, much theoretical progress has been made on the fundamental dynamics of the seasonal Hadley cells and convergence zones via the use of hierarchical modeling approaches, including aquaplanets. Here we review the theoretical progress made, and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings. After summarizing the dynamical and energetic balances that distinguish an ITCZ from a monsoon, we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets. Limitations of current theories are discussed, including the need for a better understanding of the influence of zonal asymmetries and transients on the large‐scale tropical circulation.

The recent decline and recovery of Indian summer monsoon rainfall: relative roles of external forcing and internal variability

Abstract: The Indian summer monsoon (ISM) rainfall affects a large population in South Asia. Observations show a decline in ISM rainfall from 1950 to 1999 and a recovery from 1999 to 2013. While the ...

Long-term trend detection and spatiotemporal analysis of groundwater levels using GIS techniques in Lower Bhavani River basin, Tamil Nadu, India

Abstract: Groundwater resources are used in various parts of the world to meet out drinking water supply, irrigational practices and industrial applications. These valuable resources are naturally replenished by rainfall infiltration. Due to population growth and industrialization, groundwater resources are often overexploited in different parts of the world particularly in the hard rock areas. It leads to rapid declination in the groundwater level. Therefore, groundwater fluctuation with respect to space and time governs attention throughout the world for the purpose of sustainable management of water resources. In the present study, long-term trend detection and spatiotemporal variation of groundwater levels were analyzed using Geographical Information System (GIS) and performing statistical tests for the Lower Bhavani River basin, Tamil Nadu, India. For this purpose, 32 years long-term groundwater-level data (1984–2015) of 57 observation wells spread over the study area were collected from the government departments. Seasonal variation of groundwater levels was plotted spatially for pre-monsoon (March to May), post-monsoon (January and February), southwest (SW) monsoon (June to September) and northeast (NE) monsoon (October to December) seasons using GIS. The trend variation of groundwater levels was predicted by performing statistical tests such as Mann–Kendall test and Sen’s slope estimator. The present study indicates that the average annual groundwater level has lowered beyond 15 m (below ground level) during all the monsoon seasons in the year 2003 and 2004, which highlights less rainfall infiltration and overexploitation of groundwater. This leads the hard rock aquifer into stress. The study also shows that the groundwater fluctuation is very high in the southeastern and northeastern parts of the basin, and it is moderate in the northern and northwestern parts of the basin. However, the fluctuation is comparatively less in the central part of the basin because of replenishment of groundwater by the Bhavani River. The trend analysis highlights that declining water table is mostly found during SW monsoon season (summer season), which is observed more than 50% area of the basin. The places such as Emmampoondi, Kumbapanai, Kandisalai, Alukuli, Perikoduveri, P.Mettupalayam, Pudupalayam, Sathyamangalam, Nallagoundanpudur, Kullampalayam and Baguthampalayam are mostly affected by the declining trend in the groundwater level. Therefore, this study recommends for the implementation of large-scale rainwater harvesting system in the Lower Bhavani River basin to augment groundwater resources.

Future exacerbation of hot and dry summer monsoon extremes in India

Abstract: Summer monsoon (June-September) precipitation is crucial for agricultural activities in India. Extremes during the monsoon season can have deleterious effects on water availability and agriculture in the region. Here, we show that hot and dry extremes during the summer monsoon season significantly impact food production in India and find that they tend to occur during El Nino years during the observed record of 1951–2018. We then use an ensemble of climate simulations for the historic (1971–2000) and future (2006–2100) that capture this coupling between El Nino and the Indian monsoon to show that the frequency of concurrent hot and dry extremes increases by a factor of 1.5 under continued greenhouse warming during the 21st century. Despite projections of summer monsoon intensification on the order of ~10%, the rise in surface air temperatures as well as increase in rainfall variability contributes to more severe hot and dry monsoon extremes over India, thereby posing a substantial challenge to future food security in India.

Global River discharge and floods in the warmer climate of the last interglacial

Abstract: We investigate hydrology during a past climate slightly warmer than the present: the last interglacial (LIG). With daily output of preindustrial and LIG simulations from eight new climate models we force hydrological model PCR-GLOBWB and in turn hydrodynamic model CaMa-Flood. Compared to preindustrial, annual mean LIG runoff, discharge, and 100-yr flood volume are considerably larger in the Northern Hemisphere, by 14%, 25%, and 82%, respectively. Anomalies are negative in the Southern Hemisphere. In some boreal regions, LIG runoff and discharge are lower despite higher precipitation, due to the higher temperatures and evaporation. LIG discharge is much higher for the Niger, Congo, Nile, Ganges, Irrawaddy, and Pearl and lower for the Mississippi, Saint Lawrence, Amazon, Parana, Orange, Zambesi, Danube, and Ob. Discharge is seasonally postponed in tropical rivers affected by monsoon changes. Results agree with published proxies on the sign of discharge anomaly in 15 of 23 sites where comparison is possible.

Future changes in monsoon duration and precipitation using CMIP6

Abstract: Future change in summertime rainfall under a warmer climate will impact the lives of more than two-thirds of the world’s population. However, the future changes in the duration of the rainy season affected by regional characteristics are not yet entirely understood. We try to understand changes in the length of the rainy season as well as the amounts of the future summertime precipitation, and the related processes over regional monsoon domains using phase six of the Coupled Model Intercomparison Project archive. Projections reveal extensions of the rainy season over the most of monsoon domains, except over the American monsoon. Enhancing the precipitation in the future climate has various increasing rates depending on the subregional monsoon, and it is mainly affected by changes in thermodynamic factors. This study promotes awareness for the risk of unforeseen future situations by showing regional changes in precipitation according to future scenarios.

African climate response to orbital and glacial forcing in 140,000-y simulation with implications for early modern human environments

Abstract: A climate/vegetation model simulates episodic wetter and drier periods at the 21,000-y precession period in eastern North Africa, the Arabian Peninsula, and the Levant over the past 140,000 y. Large orbitally forced wet/dry extremes occur during interglacial time, ∼130 to 80 ka, and conditions between these two extremes prevail during glacial time, ∼70 to 15 ka. Orbital precession causes high seasonality in Northern Hemisphere (NH) insolation at ∼125, 105, and 83 ka, with stronger and northward extended summer monsoon rains in North Africa and the Arabian Peninsula and increased winter rains in the Mediterranean Basin. The combined effects of these two seasonally distinct rainfall regimes increase vegetation and narrow the width of the Saharan-Arabian desert and semidesert zones. During the opposite phase of the precession cycle (∼115, 95, and 73 ka), NH seasonality is low, and decreased summer insolation and increased winter insolation cause monsoon and storm track rains to decrease and the width of the desert zone to increase. During glacial time (∼70 to 15 ka), forcing from large ice sheets and lowered greenhouse gas concentrations combine to increase winter Mediterranean storm track precipitation; the southward retreat of the northern limit of summer monsoon rains is relatively small, thereby limiting the expansion of deserts. The lowered greenhouse gas concentrations cause the near-equatorial zone to cool and reduce convection, causing drier climate with reduced forest cover. At most locations and times, the simulations agree with environmental observations. These changing regional patterns of climate/vegetation could have influenced the dispersal of early humans through expansions and contractions of well-watered corridors.

Surface elevation and sedimentation dynamics in the Ganges-Brahmaputra tidal delta plain, Bangladesh: Evidence for mangrove adaptation to human-induced tidal amplification

Abstract: In the Ganges-Brahmaputra (G-B) delta, periodic flooding of the land surface during the tidal cycle coupled with enormous sediment delivery during the monsoon promotes sediment accretion and surface elevation gain through time. However, over the past several decades, widespread embankment (“polder”) construction in the G-B tidal delta plain has led to numerous environmental disturbances, including channel siltation and tide range amplification. While previous research indicates that rates of sediment accretion are relatively high in the G-B tidal delta plain, it remains unclear if and how surface elevation is maintaining pace with relative sea-level rise (RSLR) in this region. In this study, we utilize an array of surface elevation tables, sediment traps, and groundwater piezometers to provide longitudinal trends of sedimentation and elevation dynamics with respect to local platform elevation and associated hydroperiod. Two hydro-geomorphic settings of the Sundarbans mangrove forest are compared: higher elevation stream-bank and lower elevation interior. Seasonal measurements over a time span of 5 years reveal that elevation gain is occurring in all settings, with the highest rates observed at elevated stream-bank zones. Elevation gain occurs primarily in response to sediment accretion, with possible minor contributions from pore-water storage and swelling of clay minerals during the monsoon season (i.e., belowground biomass and organic contribution is minimal). As a result, elevation loss and shallow subsidence in the G-B delta is unlikely to be caused by compaction of organic-rich soils, but rather appears to be controlled by seasonal lowering of the groundwater table and compaction of clay minerals. Rates of surface elevation gain in the Sundarbans greatly exceed rates of RSLR and more closely follow rates of RSLR augmented from tide range amplification, indicating that this landscape is adapting to human-induced environmental change. The proceedings of this study underscore the adaptability of the natural G-B tidal delta plain to local environmental disturbances, with the caveat that these defenses may be lost to future upstream reductions in sediment supply.

Emergent constraints on future projections of the western North Pacific Subtropical High

Abstract: The western North Pacific Subtropical High (WNPSH) is a key circulation system controlling the summer monsoon and typhoon activities over the western Pacific, but future projections of its changes remain hugely uncertain. Here we find two leading modes that account for nearly 80% intermodel spread in its future projection under a high emission scenario. They are linked to a cold-tongue-like bias in the central-eastern tropical Pacific and a warm bias beneath the marine stratocumulus, respectively. Observational constraints using sea surface temperature patterns reduce the uncertainties by 45% and indicate a robust intensification of the WNPSH due to suppressed warming in the western Pacific and enhanced land-sea thermal contrast, leading to 28% more rainfall projected in East China and 36% less rainfall in Southeast Asia than suggested by the multi-model mean. The intensification of the WNPSH implies more future monsoon rainfall and heatwaves but less typhoon landfalls over East Asia.

Enriched East Asian oxygen isotope of precipitation indicates reduced summer seasonality in regional climate and westerlies

Abstract: Speleothem oxygen isotope records over East Asia reveal apparently large and rapid paleoclimate changes over the last several hundred thousand years. However, what the isotopic variation actually represent in terms of the regional climate and circulation is debated. We present an answer that emerges from an analysis of the interannual variation in amount-weighted annual δ18O of precipitation over East Asia as simulated by an isotope-enabled model constrained by large-scale atmospheric reanalysis fields. 18O-enriched years have reduced summer seasonality both in terms of precipitation isotopes and in the large-scale circulation. Changes occur between June and October, where the δ18O of precipitation (δ18Op) transitions from the isotopically heavier winter to the lighter summer regime. For 18O-enriched years, this transition is less pronounced. Variations in precipitation amount alone are insufficient to explain the amount-weighted annual δ18Op between 18O-enriched and 18O-depleted years. Reduced summer seasonality is also expressed in the low-level monsoonal southerlies and upper-level westerlies; for the latter, the northward migration across the Tibetan Plateau in the summer is less pronounced. Our result thus implicates the westerlies across the plateau as the proximate cause of East Asian paleomonsoon changes, manifested as a modulation of its summer peak.

The dynamic and thermodynamic processes dominating the reduction of global land monsoon precipitation driven by anthropogenic aerosols emission

Abstract: Changes in monsoon precipitation have profound social and economic impacts as more than two-thirds of the world’s population lives in monsoon regions. Observations show a significant reduction in global land monsoon precipitation during the second half of the 20th century. Understanding the cause of this change, especially possible anthropogenic origins, is important. Here, we compare observed changes in global land monsoon precipitation during 1948–2005 with those simulated by 5 global climate models participating in the Coupled Model Inter-comparison Project-phase 5 (CMIP5) under different external forcings. We show that the observed drying trend is consistent with the model simulated response to anthropogenic forcing and to anthropogenic aerosol forcing in particular. We apply the optimal fingerprinting method to quantify anthropogenic influences on precipitation and find that anthropogenic aerosols may have contributed to 102% (62–144% for the 5–95% confidence interval) of the observed decrease in global land monsoon precipitation. A moisture budget analysis indicates that the reduction in precipitation results from reduced vertical moisture advection in response to aerosol forcing. Since much of the monsoon regions, such as India and China, have been experiencing rapid developments with increasing aerosol emissions in the past decedes, our results imply a further reduction in monsoon precipitation in these regions in the future if effective mitigations to reduce aerosol emissions are not deployed. The observed decline of aerosol emission in China since 2006 helps to alleviate the reducing trend of monsoon precipiptaion.

Seven centuries of reconstructed Brahmaputra River discharge demonstrate underestimated high discharge and flood hazard frequency.

Abstract: The lower Brahmaputra River in Bangladesh and Northeast India often floods during the monsoon season, with catastrophic consequences for people throughout the region. While most climate models predict an intensified monsoon and increase in flood risk with warming, robust baseline estimates of natural climate variability in the basin are limited by the short observational record. Here we use a new seven-century (1309–2004 C.E) tree-ring reconstruction of monsoon season Brahmaputra discharge to demonstrate that the early instrumental period (1956–1986 C.E.) ranks amongst the driest of the past seven centuries (13th percentile). Further, flood hazard inferred from the recurrence frequency of high discharge years is severely underestimated by 24–38% in the instrumental record compared to previous centuries and climate model projections. A focus on only recent observations will therefore be insufficient to accurately characterise flood hazard risk in the region, both in the context of natural variability and climate change. This study investigates flood hazards of the Brahmaputra River, Bangladesh. Based on a tree ring reconstruction of seasonal river discharge, climate modelling, and historic documentation of flood events, the authors suggest flood hazard risk is underestimated by ~24–38% in the present day compared to the past 700 years.

On the glacial-interglacial variability of the Asian monsoon in speleothem δ18O records.

Abstract: While Asian monsoon (AM) changes have been clearly captured in Chinese speleothem oxygen isotope (δ18O) records, the lack of glacial-interglacial variability in the records remains puzzling. Here, we report speleothem δ18O records from three locations along the trajectory of the Indian summer monsoon (ISM), a major branch of the AM, and characterize AM rainfall over the past 180,000 years. We have found that the records close to the monsoon moisture source show large glacial-interglacial variability, which then decreases landward. These changes likely reflect a stronger oxygen isotope fractionation associated with progressive rainout of AM moisture during glacial periods, possibly due to a larger temperature gradient and suppressed plant transpiration. We term this effect, which counteracts the forcing of glacial boundary conditions, the moisture transport pathway effect.

Accelerated increases in global and Asian summer monsoon precipitation from future aerosol reductions

Abstract: . There is large uncertainty in future aerosol emissions scenarios explored in the Shared Socioeconomic Pathways (SSPs), with plausible pathways spanning a range of possibilities from large global reductions in emissions to 2050 to moderate global increases over the same period. Diversity in emissions across the pathways is particularly large over Asia. Rapid anthropogenic aerosol and precursor emission reductions between the present day and the 2050s lead to enhanced increases in global and Asian summer monsoon precipitation relative to scenarios with weak air quality policies. However, the effects of aerosol reductions don't persist in precipitation to the end of the 21st century, when response to greenhouse gases dominates differences across the SSPs. The relative magnitude and spatial distribution of aerosol changes is particularly important for South Asian summer monsoon precipitation changes. Precipitation increases here are initially suppressed in SSPs 2–4.5 and 5–8.5 relative to SSP 1–1.9 and 3–7.0 when the impact of East Asian emission decreases is counteracted by that due to continued increases in South Asian emissions.