Anthropogenic climate change has led to rapid and widespread changes in the atmosphere, land, ocean, cryosphere, and biosphere, leading to more pronounced weather and climate extremes globally. Recent IPCC reports have highlighted that the probability of compound extreme events, which can amplify risk, has risen in multiple regions. However, significant gaps remain in our understanding of the drivers and mechanisms behind these events. This concept paper discusses compound events in the Asian region in the context of its unique and diverse geographical settings, and regional climatic features including the seasonal monsoons. Notably, Asia is the world’s most disaster-affected region due to weather, climate, and water-related hazards. Therefore, an integrated understanding of how climate change will impact compound events in this region is essential for effective forewarning and risk mitigation. This paper analyzes three typologies of compound events in the Asian region, illustrating their regional complexity and potential linkages to climate change. The first typology pertains to compound floods, for example, the devastating floods in the Indus River Basin and adjoining Western Himalayas during 2022 caused by the combined effects of heavy monsoon rainfall, intense pre-monsoon heatwaves, glacier melt, and modes of climate variability. The second typology relates to compound heatwave-drought events that have prominently manifested in East and South Asia, and are linked to large-scale drivers of the land-atmosphere–ocean coupled system and local feedbacks. The third typology relates to marine extremes involving the compounding effects of ocean warming, sea-level rise, marine heatwaves, and intensifying tropical cyclones. We identify key knowledge gaps in understanding and predicting compound events over the Asian region and discuss advances required in science and technology to address these gaps. We also provide recommendations for the effective utilization of climate information towards improving early warning systems and disaster risk reduction.

The present study investigates the response of west Pacific subtropical high (WPSH) to northern hemispheric warming due to natural and anthropogenic forcings. We carried out the analysis using a suite of climate model simulations for the mid-Holocene conditions based on the Paleoclimate Modeling Inter-comparison Project-3 (PMIP3) and quadrupled CO2 simulations based on the Climate Model Intercomparison Project-5 (CMIP5), representatives of natural and anthropogenic forcings. Both sets of simulations depict northern hemispheric warming and westward shift of WPSH during boreal summer. We found that the northern hemispheric warming increases the seasonal precipitation over Africa, central India and central-eastern Himalayas in the mid-Holocene and quadrupled CO2 climate, by causing northward shift of the ITCZ and enhanced moisture flux convergence. Enhanced condensational heating associated with precipitation increase over central India and eastern Himalaya, in both the climates, not only reinforces the South Asian High (SAH) in the upper-troposphere but also strengthens the WPSH and promotes its westward extension. Additionally, mid-tropospheric condensational heating over parts of northern Africa modulates the near-equatorial east–west circulation, induces anti-cyclonic circulation over equatorial Indian Ocean and Maritime continent and fosters westward extension of WPSH during boreal summer in both the climate. In short, precipitation changes induced by northern hemispheric warming modulates equatorial and subtropical east–west circulations and play a vital role in the longitudinal variations of WPSH under northern hemispheric warming climate. In a climate with strong northern hemispheric surface warming, we also noted a minimal role of warm pool SST in the westward extension of WPSH.

This study presents a detailed analysis of the 2019 record-breaking seasonal (June–September) monsoon rainfall, since 1901, over large areas of Western India, which evolved in the backdrop of an intense positive Indian Ocean Dipole (pIOD) event. Analysis of ground-station-observed daily rainfall data revealed that the unusually excessive and flood-producing monsoonal rains of 2019 over Western India were driven by three intense rain episodes occurring between late June and September viz., 1) 28th June – 12th July 2) 24th July – 11th August 3) 2nd September – 13th September, characterized by large-scale bands of organized convection. We observed an abundance of stratiform precipitation from GPM Precipitation Radar swaths during the heavy-to-extreme rain events, indicating the presence of organized mesoscale convective systems with elevated heating over the region. Our findings suggest that the continual top-heavy stratiform heating forced a Rossby-wave pattern with regional stretching of mid-tropospheric potential vorticity across South and Southeast Asia, creating a favorable environment for sustaining widespread monsoon heavy-to-extreme rain events. Results reveal that the pIOD conditions that prevailed over tropical Indian Ocean during 2019 led to enhanced cross-equatorial moisture transport and were vital in fostering deep convection and heavy precipitation in this region. Interestingly, upon analyzing long-term data, we observed a 25% increase in the spatiotemporally aggregated rainfall over Western India contributed by seasonally abundant heavy rainfall events during pIOD years, as compared to non-pIOD years. Overall, this study underscores the potential hydrological consequences of intense pIOD manifestations on intense rain episodes and their impact on the monsoon-centric regions of South and Southeast Asia, particularly in a warming world.

While observational records provide evidence for strengthening of the Pacific Walker circulation and the boreal winter Hadley cell (HC) over the last few decades, the underlying causes are not well understood. This study investigates the radiative effects of CO2 and anthropogenic aerosols on regional variations in the intensity of tropical meridional (Hadley-like) and zonal (Walker-like) overturning circulations, based on a suite of experiments using the IITM Earth System Model (IITM-ESM) along with supplementary analysis of observed and reanalysis datasets. Two key findings emerge from our study (i) strengthening of the Pacific Walker circulation with enhanced zonal sea surface temperature (SST) gradients and precipitation increase over the tropical Indo-Pacific, in response to global warming via ocean-atmosphere coupled feedbacks (ii) aerosol-induced intensification of regional meridional overturning circulation over the (0 − 120◦E) longitudinal domain resulting from inter hemispheric energy imbalance and southward shift of the southern hemispheric (SH) rainfall belt extending eastward from Africa across the Indian Ocean. Our results suggest that the combined radiative influence of increased CO2 and northern hemispheric (NH) anthropogenic aerosols reinforces the regional meridional overturning circulation, by enhancing convection over the SH and promoting widespread descent over the NH subtropics and mid-latitudes covering northern Africa, Mediterranean, parts of middle-East, West and South Asia. The present findings have important implications for the regional water resources, agriculture and environment.

The effective radius of cloud droplets ( r_eff ), representing the ratio of the third to the second moment of the droplet size distribution, is a pivotal factor in inferring the cloud optical properties, such as cloud optical thickness, and single scattering albedo, and consequently plays a vital role in the calculation of radiative properties. Effective radius holds significance not only in assessing the global radiation budget but also in incorporating the aerosol indirect effect within climate models. Hence, refining the parameterization of cloud droplet’s effective radius in earth system models (ESMs) has emerged as a critical endeavor for assessing climate change. Understanding the relationship between the gradient (k) or effective radius ratio ( β ), a dimensionless parameter, and mean diameter and relative dispersion ( ϵ ) is essential for the parameterization of effective radius. Fundamental theoretical studies using the Eulerian–Lagrangian particle-based model, guided by airborne observations, yield a spectrum of effective radius ratios ( β ) aimed at improving the parameterization of r_eff . Notably, β or the gradient (k) exhibit significant variations across shallow and convective clouds, with k ranging from 0.61 to 0.84 (mean of 0.72 at a relative dispersion of 0.35). The analysis of CMIP6 models indicate that adopting appropriate β value in the effective radius parameterization is crucial to mitigate the bias in r_eff . Thus, better parameterization of the cloud effective radius remains essential for elucidating the radiative impacts in ESMs.

This study investigates the projected changes in the upper troposphere and lower stratosphere (UTLS) water vapor over the Asian summer monsoon (ASM) region based on satellite records, numerical simulations using variable-resolution global climate model focused over south Asia (HIST-natural and anthropogenic forcing in the historical period, and FUT-following RCP4.5 in future), and Coupled Model Intercomparison Project Phase 5 (CMIP5) datasets. The simulations generally reproduced the seasonal cycle in the UTLS water vapor and regional water vapor maximum. With progressive warming in future, excessive upper tropospheric moistening is noted over the ASM region in far-future (2070–2095) climate against the HIST climate (1980–2005) with water vapor mixing ratio increasing to 7.5 ppmv relative to 5 ppmv noted in the HIST. It is further noted that projected changes in water vapor are linked to anomalous warming ( 1–4 K) in the upper tropospheric layers juxtaposed with zonally elongated ASM anticyclone and enhanced water vapor flux divergence by amplifications in rotational winds. Further, the simulations indicate robust increase in ASM upper tropospheric water vapor as compared to those at mid- and lower- troposphere in accordance with the Clausius–Clapeyron temperature dependence of moisture response to warming and amplified troposphere warming with altitude. A simple comparison between the ASM and the entire globe indicates that upper tropospheric water vapor-temperature relationship has a similar response, however, the projected variability in temperature and moisture is significantly larger (about twice) over the ASM region highlighting strong regional influence. Nonetheless, the projections indicate that ASM is a potential regional source in modulating UTLS water vapor budget in a warming climate.

Sea-level rise is an inevitable consequence of climate change due to increased external forcings and potentially devastating effects on the coastal population, low-lying islands, and marine ecosystem. To better understand and quantify the impact of climate change and variability on the global and Indian Ocean sea level, we analyze observed, reanalysis, and climate model experiments from phase five and recent phase six of the Coupled Model Intercomparison Project (CMIP). Our analysis based on CMIP5 and CMIP6 models reveals that the global and North Indian Ocean (NIO) sea-level rise is primarily driven by anthropogenic forcing and internal variability likely to have a secondary contribution. Contrary to the global sea-level rise, mainly driven by mass contribution, the past NIO sea-level rise is dominated by the thermosteric contribution caused by the collaborative contribution of anthropogenic forcing and internal variability. The observed long-term trend in thermosteric sea level during 1955-2005 in the NIO is comparable to the global mean, while the projected sea-level rise is higher than the global mean. Since the thermosteric sea level dominates the past Indian Ocean sea level, it is crucial to investigate the future sea-level projection in the Indian Ocean under global warming. Notably, by the end of the 21st century, the most significant change in total sea level in the Indian Ocean is primarily in the Arabian Sea, with a rise of about 0.76 m in the NIO and 0.75 m in the global ocean based on high emission scenario (RCP 8.5) from CMIP5 simulation. Our study highlights the dominant contributors to the total sea-level rise and its projected rise over the 21st century in the densely populated Indian Ocean region, demanding better adaptation strategies and policymaking.

The demand for effective methods to augment precipitation over arid regions of India has been increasing over the past several decades as the changing climate brings warmer average temperatures. In the fourth phase of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX IV), a scientific investigation was conducted over a rain-shadow region of the Western Ghats mountains in India. The primary objective was to investigate the efficacy of hygroscopic seeding in convective clouds and to develop a cloud seeding protocol. CAIPEEX IV followed the World Meteorological Organization (WMO) recommendations in a peer-reviewed report with physical, statistical, and numerical investigations. The initial results of the campaign in the monsoon period of 2018 and 2019 with two instrumented aircraft, a ground-based dual-polarization C-band radar, a network of rain gauges, radiosondes, and surface aerosol measurements are reported here. The hygroscopic seeding material was detected in cloud droplets and key cloud microphysical processes in the seeding hypothesis were tracked. The formidable challenges of assessing seeding impacts in convective clouds and the results from 150 seed and 122 no-seed samples of randomized experiments are illustrated. Over 5,000 cloud passes from the airborne campaign provided details about the convective cloud properties as the key indicators for a seeding strategy and the evaluation protocol. The experimental results suggest that cloud seeding can be approached scientifically to reduce uncertainty. The results from this study should interest the scientific community and policymakers concerned with climate change’s impact on precipitation and how to mitigate rainfall deficiencies.

Evapotranspiration (ET) is the primary process of water transfer in the hydrological cycle over land and is linked to water, energy and carbon cycles. While the global hydrological cycle is expected to intensify in a warming climate with enhanced ET and precipitation, the magnitude and spatial distribution of regional scale response of ET to climate change remains uncertain. Here we present an analysis of in-situ observations of ET from 23 stations in India during 1979-2008, which shows that the annual ET has declined by about 9% over the humid sub-regions of the Indo-Gangetic Plain (IGP). Additional analysis from high-resolution climate model simulations and observed climate datasets lend support to the role of aerosol-induced solar-dimming in intensifying ET reductions, in a background of decreasing monsoon precipitation and soil-moisture levels, over the IGP

While previous model sensitivity studies have mainly focused on discerning the soil moisture-precipitation feedback processes over the Indian region, the present study investigates the impact of soil moisture-temperature (SM-T) coupling on the temperature extremes (ExT) using the high-resolution (~60 km) model simulations. These simulations include the control and soil moisture (SM) sensitivity experiments (DRY-SM and WET-SM) initialized by perturbing (decreasing/increasing) SM from the historical (HIST: 1951-2010) and future 4K warming (FUT: 2051-2100) control runs. The analysis identifies the transitional regions of north-central India (NCI) as the hotspot of strong SM-T coupling. Over NCI, the HIST experiment shows an occurrence of 4-5 extreme events per year, with an average duration of 5-6 days per event and intensity exceeding 46oC. Whereas, FUT estimates indicate relatively severe, long-lasting, and more frequent extreme events. The SM sensitivity experiments reveal the significant influence of SM-T coupling on the ExT over NCI in both historical and future climates. We find that the DRY-SM results in significant enhancement of frequency, duration and intensity of ExT, in contrast to WET-SM. We note that the difference between DRY-SM and WET-SM 50-year return value of the block maxima GEV fit can reach upto 1.25oC and 3oC for historical and future climate, respectively. The enhanced (reduced) extreme temperature conditions in DRY-SM (WET-SM) simulation are caused by the intensification (abridgement) of sensible heat flux by limiting (intensifying) available total energy for evaporative cooling due to faster (slower) dissipation of positive soil moisture anomalies (also called as soil moisture memory). In addition, the influence of SM on ExT over NCI is found to be larger during the post-monsoon season as compared to the pre-monsoon and monsoon seasons.

This study investigates anomalous low cloud fractions (LCFs) in the Mascarene High(MH) environment of subtropical Indian Ocean (SIO) during June-September, and their sub-seasonal (10-90 day) circulation changes in the SIO and associated variations of the Indian summer monsoon (ISM) using observations and ERA5 circulation products based on 1999-2014 period. Periods of anomalous excess and deficits in LCFs in the SIO clearly reveal different sub-seasonal circulation attributes across the equator with precursor signals to the strength of ISM. Anomalous circulation composites from the excess LCF periods shows mean sea level pressure (MSLP) enhancements of about 2 hPa in the MH region in correspondence with increasing areal extent and intensifications in LCFs, and a net increase in low-level southerly momentum between MH and monsoon trough (MT) environments. The MSLP reinforcements in the MH are clearly demonstrated to emerge from the strength of cloud-top radiative cooling and associated winds and mass adjustments. The 10-20 [30 -50] day modes of the circulation in the SIO further elucidates zonally propagating [quasi-stationary] manifestations on MH reinforcements. There is an increase in meridional transport of moisture fluxes, by about 7 times relative to deficit LCF periods, channelled through aconduit region (15-30°S, 60-90°E) juxtaposing the cross-equatorial circulation (CEC) from both western and eastern sides of the Indian Ocean. This occurs in tandem with a zone of moisture flux convergence in the ISM region advancing poleward towards the climatological MT region - implying that excess LCF periods portend the likelihood of stronger ISM. Deficit LCF periods, on the contrary, show a mirrored scenario of the above with a net northerly low-level wind anomalies between MH and MT, pressure deficits in the MH region, and also portend the likelihood of weaker ISM. Low cloudsin the SIO are not only instrumental for MH stability, but also essential for circulation and moisture support across the equator and the signals for the strength of ISM on sub-seasonal scale.

Observations of soil moisture (SM) during excess and deficit monsoon seasons between 2000 to 2021 present a unique opportunity to understand the soil water dynamics (SWD) over core monsoon zone (CMZ) of India. This study aims to analyse SWD by investigating the SM variability, SM memory (SMM), and the coupling between surface and subsurface SM levels. Particularly intriguing are instances of concurrent monsoonal extremes, which give rise to complex SWD patterns. Usually, it is noted that a depleted convective activity and persistence of higher temperatures during the pre-monsoon season leads to lower SM, while monsoon rains and post-monsoon showers support the prevalence of higher SM conditions. The long persistent dry spells during deficit monsoon years enhances the Bowen ratio (BR) due to the high sensible heat fluxes. On the other hand, the availability of large latent heat flux during excess monsoon and post-monsoon seasons tend to decrease the BR. This enhancement or reduction in BR is due to evapotranspiration (ET), which influences the SWD by modulating the surface—subsurface SM coupling. The surface and subsurface SM coupling analysis for CMZ exhibits significant distinction in the evolution of wet and dry extremes. SM variations and persistence time scale is used as an indicator of SMM, and analysed for both surface and subsurface SM observation levels. Evidently, subsurface SM exhibits remarkably prolonged memory timescales, approximately twice that of surface SM. Furthermore, we dissect SWD linked to wet and dry extremes by analysing annual soil water balance at a local site in Pune, India. Our findings reveal that ET and deep drainage on annual scale are modulated largely by number of break events during the monsoon season. In essence, our study underscores the significance of surface–subsurface SM observations in unravelling the intricate tapestry of SWD.

This study addresses the role of human-induced climate change on the interactions of convective activities between the Indian summer monsoon (ISM) and western North Pacific (WNP) regions - an important scientific issue which has been hitherto overlooked. We have examined this problem using two numerical experiments of a high-resolution climate model, with and without anthropogenic forcing (i.e., HIST and HISTNAT) for the historical period 1951–2005, supplemented by innovative diagnostics like causal network analysis. Our findings suggest that an anthropogenically-forced weakening of the ISM circulation tends to significantly enhance the genesis potential index (GPI) of the WNP tropical cyclones by 13.5% and associated convective activities, by reorienting the large-scale flow over the tropical Indo-Pacific in a manner that is conducive for enhancement of the WNP tropical cyclogenesis. Additionally, it is found that the probability of extremely low sea-level pressure (SLP) (<995.5 hPa) around Taiwan and Chinese mainland is significantly higher by 10.3% in the anthropogenically-forced simulation as compared to the natural run. Using the model outputs from HIST and HISTNAT, we also performed a causal effect network (CEN) analysis to understand the causal connections among the three indices involved in the ISM-WNP interactions (a) Indian monsoon circulation index (IMI) which is the difference in the area-averaged 850 hPa zonal winds between the boxes (40oE-80oE, 5oN-15oN) and (70oE-90oE, 20oN-30oN) (b) WNP tropical cyclone activity expressed as the genesis potential index (GPI) averaged over the region (120oE-180oE, 5oN-30oN) and (c) Tropical Indo-Pacific wind index (IPWND) which is based on the 850 hPa zonal winds averaged over the domain (100oE-130oE, 5oN-20oN). Results from the CEN analysis indicate that an anthropogenically-forced weakening of the IMI can lead to possible strengthening of GPI and IPWND with time-lags of 5 and 7 days, respectively. In general, it is noted that the causal relationships among IMI, GPI and IPWND are associated with shorter time-lags (∼4–9 days) in HIST and longer time-lags (∼19–28 days) in HISTNAT.

In this study, a suite of ten Weather Research and Forecasting (WRF) model simulations is conducted using various choices of local and non-local representations of vertical mixing adopted in the planetary boundary layer (PBL) parameterizations to assess the boreal summertime marine layer cloud environment in the southern Indian Ocean (SIO). The local, non-local, and hybrid type PBL schemes used in the simulations are generally able to produce the pertinent features of the marine layer embedded with stratocumulus clouds in the SIO region, viz., nocturnal coverage of low clouds with warm cloud tops, well-mixed marine layer, inversion capping, strong temperature and moisture jumps across the inversion layer, cloud water generation and its coupling with cloud-top radiative cooling, as conceptually envisaged in earlier investigations. However, there is a large spatial variability noticed in the simulated low clouds over the SIO region. While the satellite diagnosed cloud liquid water paths (LWPs) are noted as high as 0.4 mm in the region 60° E–90° E, 15° S–25° S, the simulated cloud LWPs show a large mesoscale variability exhibiting a range of values between 0.04 and 2.8 mm, corresponding to cloud depths as small [high] as 200 [1700] m. It is further noted that LWPs reproduced from hybrid PBL schemes employed with eddy diffusivity mass flux (EDMF) formulation are closer to the observed estimates in the SIO region. While the non-local PBL mixing schemes tend to produce more organized sheeted low-cloud layers, the local closure schemes tend to produce more discreteness in the low-cloud decks and also exhibit substantial cloud depth variability. In addition, the coupling of in-cloud turbulence in association with cloud-top radiative cooling and sub-cloud layer turbulence are not uniquely reproduced by these schemes. The inferences from this study suggest that non-local/hybrid type of PBL parameterizations accounting for stratocumulus cloud-top driven mixing processes shows greater promise in the reproduction of low-clouds in the SIO region.

The significance of regional monsoon systems for water resources, agriculture and the economy cannot be overstated, which points to the fact that developing reliable projections of monsoon precipitation in a changing climate is central to formulating policy. However, regional monsoons are characterized by strong internal variability and their dynamics include contributions both from global climate drivers and complex local processes, which are not fully understood. Therefore, robust attributions of human influence on changes in regional monsoon precipitation are inherently challenging. In this article, we discuss the observed and projected changes in the South Asian monsoon, and the sources of uncertainty in these projections. We also discuss avenues for improving monsoon projections by a balanced approach involving basic research, building next-generation climate models and exploiting emerging techniques such as machine learning. We also suggest that the strategies for advances in this area must include specialised educational programmes in meteorology and climate science at the undergraduate level and beyond.

AbstractThe West North Pacific (WNP) is a breeding ground for tropical cyclones (TC) all through the year with peak activity during August-September, in contrast to the North Indian Ocean where tropical cyclones (TC) are observed mainly during April-June and October-December but suppressed in July-August due to the strong vertical wind shear of the Indian summer monsoon (ISM). Interannual variations in the WNP tropical cyclones (typhoons) are known to be closely linked to El Nino - Southern Oscillation (ENSO) with higher number of typhoons forming during El Nino phases as compared to La Nina; however the response of WNP typhoon to human-induced climate change is not well understood. Here, we compare two sets of high-resolution climate model simulations for the historical period 1951-2005 viz., HIST [includes both natural (solar irradiance, volcanic aerosols) and anthropogenic forcing (greenhouse gasses (GHG), aerosols and land use land cover changes)] and HISTNAT (natural-only forcing) to understand the influence of climate change on the ISM and WNP typhoon activity. It is found that an anthropogenically-forced weakening of the ISM circulation, largely due to the influence of Northern Hemispheric aerosols which oppose the GHG forcing, tends to significantly enhance the genesis potential of WNP typhoons. Our results suggest that a forced weakening of the ISM circulation reorients the large-scale flow over the tropical Indo-Pacific in a manner that is conducive for enhanced WNP typhoon genesis through enrichment of low-level relative vorticity, while the intensified mid-latitude westerlies over the Far East and North Pacific region steer the WNP typhoons to move northward with recurving tracks. Additionally the area of extremely low sea-level pressure (SLP) around Taiwan and mainland China is substantially larger in the anthropogenically- forced simulation as compared to the natural run.