Monsoon Research Articles

Tropical Cyclone Rainfall Structure and Its Long-Term Trend over the Western North Pacific

Abstract The tropical cyclone (TC) rainfall structure over the western North Pacific is categorized into four typical types. Clusters I and IV are featured by the southward-shifted and northward-shifted rainfall centers, respectively. Meanwhile, clusters II and III are characterized by the weak and strong maximum rainfall rates near the TC center, respectively. Apart from rainfall, clusters I–IV show diverse features in locations, intensities, and lifetime stages. Both clusters I and III appear over the low-latitude regions. However, TCs in cluster III have the strongest intensity and are in the mature stage of their lifetime, while TCs in cluster I are relatively weaker and in earlier stages. On the contrary, cluster IV has the highest mean latitude positions and strong intensity, mainly in their later lifetime period. Cluster II shows the weakest TC intensity and mainly locates in the subtropics. Diverse TC locations lead to different effects of large-scale circulation on TC rainfall patterns in clusters I–IV. For clusters I and III, abundant moisture provided by the southwest monsoon causes heavy rainfall. In contrast, cluster IV is mainly affected by midlatitude westerly troughs, which is responsible for the northward-shifted rainfall center. Cluster II shows the weakest TC intensity, resulting in a small rainfall rate. Further analysis shows that the changes in the relative frequency contributed 36% of the decreasing trend of basin-mean inner-core rainfall during 1998–2019, and the shift of rainfall pattern from cluster III to cluster I due to the enhanced monsoon plays a central role.

Earlier Seasonal March of the East Asian Summer Monsoon in the Mid-Pliocene

Abstract The mid-Pliocene (∼3.3–3 million years ago) has been considered as a potential analog for future climate, but the seasonal march of the East Asian summer monsoon (EASM)—a distinctive feature of the modern EASM characterized by a northward migration of the monsoon circulation and rainband from early to late summer—remains unclear in this regard. Here, based on model simulations mainly from the Pliocene Model Intercomparison Project Phase 2, we find that the seasonal march of the mid-Pliocene EASM is about two pentads earlier than today. It is accompanied by a northward shift of the monsoon circulation and rainband in July, resulting from both extratropical and tropical processes. For the extratropical path, greener vegetation in the midlatitudes enhances the surface warming, which reduces the meridional temperature gradient. The subsequent weakening and northward shift of the subtropical westerly jet lead to weaker zonal topographic Rossby waves, resulting in an anticyclone over East Asia. For the tropical path, exposure of the Sunda and Sahul shelves weakens the Walker circulation, which suppresses the Maritime Continent convection. The subsequent weakening of the local Hadley circulation increases precipitation in the tropical western North Pacific, which excites a Pacific–Japan meridional teleconnection with anticyclonic circulation over northern East Asia. These processes lead to a northward shift of the western Pacific subtropical high and hence the monsoon rainband. Our results unravel the dominant roles of vegetation and geography in the earlier seasonal march of the mid-Pliocene EASM, and its implications for potential future changes in the EASM’s seasonal march are discussed.

Future changes in rainfall variability for the mainland Indochina southwest monsoon

Researching future changes in rainfall variability is critical for mitigating the possible effects of global warming, especially in areas where vulnerability is high, such as Indochina. While changes in mean and extreme rainfall have received a great deal of attention, rainfall variability has been poorly researched, despite its importance. We endeavored to determine the anticipated changes in rainfall variability during the mainland Indochina southwest monsoon (MSWM) by utilizing data derived from 5 ensemble models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). Employing band pass filtering techniques on daily rainfall data, we discerned variability across an expansive spectrum of temporal scales. Our research indicates that, in the event of global warming, the variability in MSWM rainfall is expected to increase by approximately 10-25% throughout the whole region. Notably, this increased unpredictability appears uniformly throughout a wide range of time intervals. Changes in average rainfall significantly aid in explaining the majority of the intermodal variance in the predicted MSWM rainfall variability. To gain further insight into this phenomenon, we examined the effects of elevated atmospheric moisture content through the estimation of modifications resulting from idealized local thermodynamic enhancement. We showed that increased atmospheric moisture, as suggested by the Clausius-Clapeyron relation, accounts for most of the predicted changes in rainfall variability at all time scales.

Diagnosing the role of atmospheric variability on the extreme summer monsoon precipitation events over India

Diagnosing the role of atmospheric variability on the extreme summer monsoon precipitation events over India

Monitoring Heavy Rainfall Events in East Asia Using High‐Resolution Isotopic Observations

AbstractIt is important to understand the mechanisms of heavy rainfall events, as such information could improve forecasting of these events and help mitigate their adverse impacts on life and property. In this study, we analyzed hourly stable isotopic compositions in water vapor (δ18Ov and d‐excessv) during heavy rainfall events in the summer monsoon season (June to September) from 2013 to 2023 in Nanjing, eastern China. These data were extracted from the longest data set of high‐resolution and continuous in situ observations of water vapor isotopes globally. Based on these data, we identified four evolution patterns of δ18Ov during heavy rainfall events, corresponding to different weather systems: slow‐declining (tropical cyclone interacting with mid‐ and high‐latitude system), W‐shaped (tropical cyclone), U‐shaped (cold vortex system), and inclined L‐shaped (upper‐level trough system). The isotopic variations suggest that heavy rainfall events in eastern China were mainly sustained by moisture from adjacent oceans (including the South China Sea and the East China Sea) and terrestrial environment rather than from the distant Indian Ocean as previously suggested. In addition, for some heavy rainfall events with an intermittent period, the nearby oceanic moisture transport alters before and after the intermittent period due to an intensity change or overall transition of low‐level weather systems. This study serves as a benchmark for tracing heavy rainfall processes in East Asia using high‐resolution water vapor isotopes.

Correction to: Regional differences in cloud characteristics at different depth, intensity and horizontal scale over South Asia during Indian summer monsoon using CloudSat and reanalysis data

Correction to: Regional differences in cloud characteristics at different depth, intensity and horizontal scale over South Asia during Indian summer monsoon using CloudSat and reanalysis data

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.

The role of antecedent southwest summer monsoon rainfall on the occurrence of premonsoon heat waves over India in the present global warming era

The role of antecedent southwest summer monsoon rainfall on the occurrence of premonsoon heat waves over India in the present global warming era

Performance of Bt cotton evaluated in relation to mulching and weed control measures in northwest India

BackgroundWeed infestation in cotton has been reported to offer severe competition and cause yield reduction to a large extent. Weeding via cultural practices is time consuming, tedious, and expensive due to long duration of cotton crop and regular monsoon rains during cotton production in India. Chemical weed control has been successfully utilized in cotton in the recent past. However, continuous use of similar herbicides leads to resistance in weeds against herbicides. And when sprayed to the field, herbicides not only suppress weeds but leave undesirable residues in the soil that are hazardous to the environment. Therefore, a study was performed at cotton research area at Chaudhary Charan Singh Haryana Agricultural University, Hisar, Haryana during two consecutive kharif seasons (2020 and 2021) to determine the most suitable and sustainable weed management strategy through the integration of chemical and cultural methods.ResultsMulching with rice straw of 7.5 t ha−1 resulted in significantly higher cotton seed yield (3 189 and 3 084 kg ha−1) and better weed control in comparison to no mulch treatments (2 990 and 2904 kg ha−1) in 2020 and 2021, respectively. Among various weed management levels, the significantly lowest cotton seed yield was recorded in untreated control (1 841 and 1 757 kg·ha−1 during 2020 and 2021, respectively) in comparison to other treatments while all other treatments were statistically at par with each other during both years of crop experimentation.ConclusionMulching with rice straw of 7.5 t·ha−1 along with a pre-emergence application of pendimethalin (active ingredient) at 1.5 kg·ha−1fb (followed by) one hoeings at 45 days after sowing (DAS) and fb glyphosate 2 kg·ha−1 (Shielded spray) at 90 DAS is a viable option for effective control of grassy and broadleaved weeds in Bt cotton in north-west India.

Transport of Exogenous Anthropogenic Atmospheric Mercury to the Tibetan Plateau Identified Using Mercury Stable Isotopes

AbstractTransport of exogenous anthropogenic mercury (Hg) is an important source of Hg pollution in the Tibetan Plateau (TP) and its downstream water ecosystems, but the origins and contributions of Hg sources remain uncertain. Here, we investigate the concentrations and isotopic compositions of gaseous elemental mercury (GEM) at four rural sites in the TP and three urban sites surrounding the TP to quantify the sources of GEM in the TP. GEM concentrations in the surrounding cities (site‐specific means: 2.36–9.12 ng m−3) were highly elevated mainly due to strong local anthropogenic emissions as indicated by their negative δ202Hg and near zero Δ199Hg and Δ200Hg signatures. GEM isotopes indicate that GEM pollution in the TP, typically observed during the summer monsoon and the pre‐monsoon, were mainly caused by trans‐boundary transport of anthropogenic Hg from surroundings. Using an Hg isotope mixing model, we estimate that exogenous anthropogenic emissions on average contributed 26 ± 5% (1sd) to the GEM in the TP. Further analysis of the transport of anthropogenic Hg emissions based on the backward trajectory and gridded anthropogenic Hg emissions suggests that 16 ± 9% and 6 ± 13% of the GEM in the TP were derived from anthropogenic sources in South Asia and China, respectively. Our study suggests that anthropogenic Hg emissions in South Asia could be effectively transported to the TP across the Himalayan range. Future studies are needed to better assess the role of rapidly increasing anthropogenic Hg emissions in South Asia on the regional to global scale atmospheric Hg cycling.

Greening of India and revival of the South Asian summer monsoon in a warmer world

While it is accepted that the tropical hydrological cycle has intensified during past interglacial periods due to changes in insolation, greenhouse gases and ice volume, their respective influences are uncertain. Here we present a pollen record from Bengal Bay to reconstruct vegetation changes in India’s core monsoon zone during two warm periods, the current and last interglacial, comparing the data with numerical model simulations to assess the influence of different forcing mechanisms. Results show tropical forest expansion between 11.7-5 ka and 127-120 ka, defining two Indian humid periods, with the last interglacial showing the strongest monsoon activity, consistent with salinity reconstructions. Model-data comparison highlights boreal summer insolation as the primary driver of vegetation dynamics and monsoon intensity during interglacial periods, with CO2 and ice-sheets having a limited effect. Vegetation remains unaffected by pre-industrial CO2 variations above 250 ppmv, a threshold value that characterizes most interglacials of the last million years.

Large eddy simulation of the combined effect of heat fluxes and wave forcing of summer monsoon on a diurnal ocean mixed layer in the north Arabian Sea

Large eddy simulation of the combined effect of heat fluxes and wave forcing of summer monsoon on a diurnal ocean mixed layer in the north Arabian Sea

Impact of shifting patterns of the South Asian High on interannual variability of Indian summer monsoon rainfall in homogeneous regions

AbstractThe movement and intensity indices of the South Asian High (SAH), an upper‐level anticyclonic‐circulation over the Tibetan Plateau, play a crucial role in Indian summer monsoon rainfall (ISMR) during June–September. The present study aims to document (i) the association between SAH and ISMR in the periods of June–July–August (JJA) and July–August–September (JAS), and (ii) the ISMR changes at different spatial and temporal scales. The Bayesian change‐point detection method identifies 1981 as change point and the dataset has been split into two groups (past climate:1940–1980 and current climate:1981–2020) to analyze temporal variations in ISMR. Results indicate that the northwest–southeast index (INW–SE), north–south index (INS) and intensity index (IINT) of the SAH are strongly correlated (˜0.67, ˜0.60, and ˜0.51, respectively) with ISMR whereas the east–west index (IEW) is negatively correlated (˜−0.52) during the JAS season. This relation was stronger in the past climate than the current climate, except for the IINT index during the JAS season. The INW–SE and INS indices are closely associated with all‐India, northwest India (NWI), and central India (CI) rainfall, whereas IINT is associated with south peninsular India (SPI) during the JAS season. The increased rainfall over NWI and SPI in the current climate is strongly associated with positive INS and IINT indices, respectively, during the JAS season. The northeast India (NEI) rainfall did not show any association with SAH indices; however, the IEW is strongly associated with increased NEI rainfall during El Niño years. A significant positive (negative) relation between meridional (zonal) wind shear and SAH indices except the IEW index is observed. Further, the positive (negative) SAH indices favor more (moderate) ISMR due to strong positive (negative) moisture anomalies. The study demonstrates the movement and intensity of the SAH in the dynamical shifts of seasonal rainfall patterns across the Indian homogeneous regions.

Constrained Projections Indicate Less Delay in Onset of Summer Monsoon over the Bay of Bengal and South China Sea

AbstractThe summer monsoon onset over the Bay of Bengal and South China Sea signals the beginning of the Asian summer monsoon, critical for local fisheries, agriculture and livelihoods, so communities are concerned about its potential changes under global warming. Previous projections have suggested a delay, but the extent of this delay remains uncertain, undermining the reliability of the projections. Here, we show a significant correlation between the projected shift in Bay of Bengal/South China Sea monsoon onset and present‐day sea surface temperature (SST) simulation over the western Pacific (WP). This emergent relationship arises from the spread of the precipitation response over the western‐central Pacific to WP SST, as more precipitation induces stronger tropical upper‐tropospheric warming, increasing westerly vertical shear near South Asia, and facilitating the onset delay. The rectified projections indicate that the delayed shift is almost halved compared to raw projections, and the intermodel uncertainty is reduced by 30%.

Seasonal intensification of oxygen minimum zone: linking Godavari River discharge to fall hypoxia in the Bay of Bengal

IntroductionThis study investigates the biogeochemical impact of Godavari River discharge (GRD) on the Bay of Bengal (BoB), focusing on the formation of an intense and shallow oxygen minimum zone (OMZ) near the river mouth during the fall season. Unlike the BoB’s typical intermediate-depth OMZ, this subsurface (~40-200 m) phenomenon is attributed to the interplay of GRD-driven nutrient enrichment, coastal upwelling, enhanced productivity, and subsequent organic matter decomposition.Data and MethodsOur analysis using the Biogeochemical-Argo floats and World Ocean Atlas 2018 data reveals that a clear shoaling and intensification of the OMZ in the fall season. Further, a comparative analysis at two geographically distinct locations highlighted the pivotal role of GRD.Results, Discussion, and ImplicationsThe location directly influenced by GRD exhibited significantly higher chlorophyll-a blooms, net primary production during the southwest monsoon, and pronounced oxygen consumption during the fall compared to the other. Our analysis suggests that GRD fuels primary productivity, leading to organic matter abundance and intense oxygen depletion in the subsurface layers, driving the observed shallow OMZ. Understanding the complex interplay between GRD, stratification, upwelling, and biogeochemical processes is crucial for predicting the impact of altered riverine inputs on coastal ecosystems, greenhouse gas emissions, and the overall health of the coastal BoB.

Burn Period: A use-inspired metric to track wildfire risk across Arizona and New Mexico in the southwest U.S.

Abstract Numerous tools and indices exist for wildland fire managers to anticipate and track changes in wildfire risk driven by variability in weather and climate conditions at hourly to seasonal scales. However, in working closely with southwest U.S. managers, we learned of a simple meteorological metric being informally used, but not widely accessible in existing tools or information products, to gauge short-term changes in wildfire risk. This metric, termed ‘Burn Period’ (BP), is the local count of hours per day with relative humidity values equal to or less than 20%. Our collaboration led to the development of an experimental tool called the ‘Burn Period Tracker’ to ease access and promote use of BP values for planning and response. This study is a climatological analysis of BP values at 124 fire weather stations across Arizona and New Mexico for the period 2000-2022 to aid in interpretation and understanding of this use-inspired metric. BP values reflect the strong seasonality in temperature and moisture deficit-driven wildfire risk across the southwest U.S., with risk climbing through the arid spring season, peaking in June, and then falling rapidly with the onset of the summer monsoon in July. Regression analyses show that short-term variability in BP values are driven by variability in low level atmospheric moisture in all months with strongest relationships during the summer after the onset of the monsoon. This study highlights the utility of BP as a short-term wildfire planning tool as well as an example of collaborative weather and climate services development.

Warming Asian Drylands Inducing the Delayed Retreat of East Asian Summer Monsoon and Intensifying Autumn Precipitation in Northern China

AbstractIn the early 21st century, there was an increase in precipitation during the retreat period of the East Asian summer monsoon (EASM). This study aims to explore the precipitation changes and their possible causes in the context of climate change. The findings indicate that the increased precipitation primarily occurred in the Yellow—Huai River valley during the early autumn. This corresponds to a delayed retreat of the EASM with a northward shift of 0.9°N after 2002. Notably, this anomalous changes in the EASM are associated with the significant warming in two Asian dryland regions. The warming of the central Asian dryland strengthens the midlatitude high‐pressure belt and the anomaly anticyclone over northeast Asia, which restrains the development of wave troughs and westerly cold air activity. Similarly, the warming China‐Mongolia dryland enhances the anomaly anticyclone over northeast Asia through the dry soil moisture feedback and reduced latitudinal temperature gradient. These two Asian drylands thereby hold the northward shift of the westerly jet stream and the northwest extension of the Japan Sea high and the western Pacific subtropical high. These changes result in maintaining the northward EASM circulation and driving the northwest water vapor flux from the northwest Pacific, leading to moisture convergence in northern China. The China‐Mongolia dryland warming also increases the land‐sea thermal contrast, which induces a dipole pattern over East Asia and drives the northward water vapor flux from the South Sea. As a result, the rapidly warming drylands restrict westerly activity and EASM retreat, ultimately leading to increased precipitation.

Structure, Propagation of the 10–25-Day and 30–60-Day Intraseasonal Oscillations and Their Role in Triggering the South China Sea Summer Monsoon Onset

Abstract The onset of the South China Sea (SCS) summer monsoon (SCSSM) is a complex process that involves multiscale variabilities, including the 10–25-day high-frequency intraseasonal oscillation (HISO) and the 30–60-day low-frequency intraseasonal oscillation (LISO) along with the seasonal variation. Using the reanalysis and satellite data during the period of 1979–2020, this study comprehensively investigates how the HISO and LISO trigger the SCSSM onset, which improves the understanding of the abrupt SCSSM onset from the point of intraseasonal view. It is clearly documented that in most years, the abrupt SCSSM onsets are triggered by the first branch of HISO and/or LISO propagating into the northern SCS along their distinctive pathways. The HISOs (LISO) originating from the northwestern Pacific Ocean (tropical eastern Indian Ocean) propagates westward (northeastward) toward central-to-northern SCS. The diagnosis based on the moist static energy (MSE) budget reveals the different governing mechanisms of HISO and LISO propagations. HISO’s westward propagation is favored by the zonal advection of the anomalous MSE by the mean easterly wind. In contrast, the surface turbulent heating by sea surface temperature (SST) is the dominant term for LISO’s northward propagation, followed by the meridional advection of the anomalous MSE by the mean southerly wind. Significance Statement This study provides the comprehensive intraseasonal view of the SCSSM onset, where the respective roles of 10–20-day HISO and 30–60-day LISO in triggering the SCSSM onset are identified and their propagating mechanisms are revealed. The prevailing easterlies in the northwestern Pacific and SST-induced turbulent heat flux in SCS are confirmed as the respective dominant processes governing their distinct propagations. These results improve the understanding of the SCSSM onset and hence have significant implication to the monitoring, simulation, and prediction of SCSSM.

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.

Rain event detection and magnitude estimation during Indian summer monsoon: Comprehensive assessment of gridded precipitation datasets across hydroclimatically diverse regions

Accurate precipitation estimates are quintessential for hydrologic modeling and climate studies. Different gridded precipitation products are available in any region, and selecting the best one is essential for hydroclimatic modeling and analysis. In the current study, observation- (APHRODITE), reanalysis- (IMDAA, ERA5-Land, PGF), satellite-based (IMERG, CHIRPS, PERSIANN-CDR), and hybrid (MSWEP) gridded precipitation products with different spatial and temporal resolutions are evaluated using several continuous, categorical, graphical, and interval-based performance measures towards detecting Indian Summer Monsoon Rainfall (ISMR) events and estimating their magnitudes for the subcontinent of India, considering IMD gauge-based gridded as reference product. We confine our analysis to the monsoon season (i.e., June to September), the principal rainy season in the Indian sub-continent. The dearth of data and limited rain gauge-based observations from non-uniform sparse monitoring networks across India necessitated grid-to-grid comparative evaluations instead of point-to-grid assessments. We propose a new ranking framework to determine the suitability of precipitation datasets for twenty-seven hydroclimatic regions comprising homogeneous rainfall zones, Köppen-Geiger climate zones, and major river basins. Results from the comprehensive evaluation suggest that (1) APHRODITE, MSWEP, and ERA5-Land best approximate precipitation event occurrences across India, (2) MSWEP and ERA5-Land are most suitable (highest rank) alternatives at the regional level, while APHRODITE is found to be next suitable dataset owing to its persistent dry bias, (3) performances of CHIRPS and IMERG have reasonably lower rank score across India, (4) close agreement of examined datasets is noted over semi-arid and sub-humid regions (e.g., peninsular and central India), whereas ERA5-Land, IMDAA, and APHRODITE fail to detect and reproduce the intensity of the events along the west coast and northeastern India, (5) PGF and PERSIANN-CDR are the least situated datasets. Moreover, the present study provides a unique and innovative perspective to characterise the precipitation over a vast topographic, ecologic, and climatic gradient region like India.