Water availability, as measured by precipitation minus evaporation (P-E), is projected to increase in the 21st century across global monsoon regions. However, while the impacts of increased greenhouse gas (GHG) concentrations are highlighted in existing studies, the contribution of reduced anthropogenic aerosol (AA) emissions is likely to be overlooked. Here, utilizing single-forcing projections under the SSP2-4.5 scenario, we elucidate the fingerprints of GHG and AA forcings on future P-E evolution. We reveal that the future wetting trend is primarily driven by an increase in P-E during the wet season. The escalation of GHG concentrations is projected to increase P-E over Asian-African monsoon domains while decreasing it over American monsoon domains. Conversely, aerosol reductions will drive a transition from current widespread drying to future wetting. While both the GHG increase and AA reduction can elevate atmospheric moistening through radiative warming, the disparate P-E responses come from dynamic processes that favor drying trends in American monsoon domains under GHG forcing. In contrast, strengthened monsoon circulations contribute to a wetting trend in Asian monsoon domains under AA reductions, attributable to greater interhemispheric thermal contrast. Our finding highlights the importance of considering aerosol mitigation in climate risk assessment for densely populated monsoon regions.
How to cite: Jiang, J., Zhou, T., and Zhang, W.: Aerosol mitigation matters to future water availability in the global monsoon region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5379, https://doi.org/10.5194/egusphere-egu25-5379, 2025.
The Afro-Asian summer monsoon (AfroASM) sustains billions of people living in many developing countries covering West Africa and Asia, vulnerable to climate change. Future increase in AfroASM precipitation has been projected by current state-of-the-art climate models, but large inter-model spread exists. Here we show that the projection spread is related to present-day interhemispheric thermal contrast (ITC). Based on 30 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we find models with a larger ITC trend during 1981-2014 tend to project a greater precipitation increase. Since most models overestimate present-day ITC trends, emergent constraint indicates precipitation increase in constrained projection is reduced to 70% of the raw projection, with the largest reduction in West African (49%). The land area experiencing significant increases of precipitation is 57% of the raw projection. Given that the emergent constraint improves the reliability in AfroASM precipitation projections, we further investigate the impacts of the constrained projection on the potential water availability. The fractions of land area that will experience a significant increase of potential water availability are about 66% of the raw projection. We find that global surface air temperature warming plays a dominant role in the emergent constraint on precipitation changes, while the contribution from hydrological sensitivity should not be neglected. The smaller increase of potential water availability in the constrained projection than the raw projection may pose a challenge to climate change adaptation and mitigation activities related to water management and food security, although a smaller than expected increase in rainfall will also reduce the risk of extreme precipitation and flooding.
How to cite: Zhou, T. and chen, Z.: How can we narrow down the uncertainty in Afro-Asian monsoon projection?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18490, https://doi.org/10.5194/egusphere-egu25-18490, 2025.
The Indian summer monsoon (ISM) is a complex system that plays a significant role in the climate of South Asia. We used Community Earth System Model 2-Large Ensemble (CESM2-LE) simulations to explore the forced response in the mean state and interannual variability of the ISM in future projections. The model is able to reproduce the mean state and interannual variability of the ISM during historical periods. The strengthening of monsoon circulation during excess rainfall years and weakening during deficient years are also well simulated by the model. It is also noticed that though the low-level jet stream shows a weakening during deficit monsoon years, it has more eastward extension up to the western Pacific Ocean compared to excess monsoon years. In simulations for future years, the mean structure of both the low-level jet stream and the tropical easterly jet stream becomes weaker compared to historical years. However, the precipitation pattern shows an enhancement in the future periods, and also the excess rainfall years in the future can be wetter than the historical excess years. Thus, the outcomes of CESM2-LE simulations are essential for formulating better plans for handling the effects of monsoon variability and policy-making efforts aimed at mitigating the impacts in a warming world.
How to cite: Kunnath, N., Sundaresan, A., and Sivarajan, S.: Forced Response in the Mean State and Interannual Variability of the Indian Summer Monsoon in Future Projections, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-313, https://doi.org/10.5194/egusphere-egu25-313, 2025.
This study investigates the characteristics and future trends of warm rain during the winter monsoon season (December, January, February; DJF) over Indonesia, with a focus on the Java Sea. The analysis integrates satellite observations from the Tropical Rainfall Measurement Mission (TRMM), reanalysis datasets (ERA5), and model simulations from the Atmospheric General Circulation Model (AGCM). An analysis of ERA5 data (1950–2009) reveals a pronounced upward trend in SST across the broader Indonesian region (slope 0.0070) and the Java Sea (slope 0.0094), with the most significant increases occurring during DJF. Cloud Liquid Water Content (CLWC), positively correlated with SST and rainfall, is used as a proxy for warm rain. TRMM satellite observations confirm that warm rainfall corresponds spatially with CLWC distribution. AGCM simulations effectively replicate observed CLWC patterns, showing strong alignment with TRMM data, particularly over western Indonesia, including the Java Sea. Convergence patterns derived from ERA5 and AGCM data exhibit similar trends, emphasizing the role of atmospheric convergence in CLWC formation over the Java Sea. An analysis of 95th percentile CLWC values at lower atmospheric levels (1000–700 hPa) highlights a significant increase in CLWC during DJF over the northwestern Indonesian region, including the Java Sea, across 30-year intervals spanning 150 years (1950–2099). These findings underscore the critical influence of the winter monsoon on warm rain processes in the Java Sea and its connection to extreme weather events, such as flooding in Jakarta, located on the southern coast of the Java Sea.
How to cite: Harjupa, W. and Nakakita, E.: Investigation and Future Projection of Warm Rain During Winter Monsoon in Java Sea, Indonesia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4614, https://doi.org/10.5194/egusphere-egu25-4614, 2025.
This study emphasizes the role of shallow circulation in transporting lower-level moist static energy northward, thereby intensifying the onset of summer cross-equatorial circulation in the South Asia monsoon region. Previous research has suggested that the monsoon onset can be considered as a transition between an eddy-driven regime and an angular-conserving regime, and momentum budget analyses from these studies support the theory of regime transition (Bordoni and Schneider 2008; Plumb 2005; Geen et al. 2018; Shaw 2014). Additionally, studies have highlighted the significant role of boundary layer entropy during the summer monsoon period (Emanuel 1995; Plumb 2005; Nie et al. 2014), when the circulation operates within the angular-momentum-conserving regime. Adopting this boundary-layer-entropy-centric perspective, many studies emphasizing the role of topography in blocking low entropy inflow from the north and intensifying the South Asian Monsoon (Boos and Kuang 2010; Privé and Plumb 2007; Geen et al. 2014). Meanwhile, the role of synoptic systems and the early onset in the Bay of Bengal (Parker et al. 2016), as highlighted in observational data, in establishing the strong cross-equatorial summer cell remains unclear.
To bridge the gaps between observational studies and theoretical frameworks, this study investigates the mechanisms shaping the evolution of the boundary layer entropy throughout the regime transition. With the goal of understanding the interactions between convective processes and large-scale circulation, we utilize the Superparameterized Community Atmosphere Model (SPCAM), which demonstrates higher convection variability and increased precipitation near the South Asian coastal region compared to traditional global climate models, aligning well with observational data. Compared to simulations without SPCAM coupling, the SPCAM simulations show a more abrupt monsoon onset in South Asia. The sector zonal mean analysis demonstrates that the higher convection variability in SPCAM runs results in more shallow convections before the monsoon onset. Also, the shallow circulation accompanied with these shallow convections can transport higher lower-level entropy northward, causing energy convergence near the coastal region and intensifying the abruptness of the monsoon onset. In contrast, simulations without SPCAM coupling exhibit an unrealistic jump of boundary layer entropy maximum from the equator to the mountainous terrain. Our energy budget analysis highlights that the shallow overturning cell associated with the deep and shallow convections in the coastal regions holds the key for the northward migration of boundary layer entropy maximum. Such a relaxed quasi-equilibrium perspective provides an interpretation for how convection-circulation coupling contributes to the theoretical framework of regime transition.
How to cite: Chen, Y.-J., Hwang, Y.-T., Chen, W.-T., Wu, C., and Wu, D.: The role of convection-circulation coupling in expediting South Asian monsoon onset: Insights from SP-CAM, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17038, https://doi.org/10.5194/egusphere-egu25-17038, 2025.
The timing of the Indian Summer Monsoon (ISM) onset significantly impacts agriculture, food production, the economy, and livelihoods in India. Parker et al. (2016) highlighted the role of both mid-level dry northwesterly winds and low-level moist southwesterly winds in influencing the climatological ISM onset. However, the question of what drives the delay in ISM onset remains unclear and uncertain. Is it primarily due to mid-level dry northwesterly winds, low-level moist southwesterly winds, or a combination of both? In this study, we find that the weakening of low-level moist southwesterly winds is the primary factor, while the mid-level dry northwesterly winds remain unaltered during delayed onset years. This weakening of southwesterly winds is associated with the low-level circulation anomaly caused by the anomalous high pressure over the Arabian Sea, which is further linked to the weakening of the Hadley circulation. The relatively low pressure over the Mascarene High reduces the cross-equatorial pressure gradient, weakening the Hadley circulation, which in turn weakens the southwesterly winds, thereby delaying the ISM onset. Understanding the underlying mechanisms of delayed monsoon onset provides critical insights for improving Indian monsoon modelling and prediction.
How to cite: Wadhai, V., Senapati, B., and Dash, M. K.: Indian Summer Monsoon Onset Delayed by the Weakening of Hadley Circulation , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9304, https://doi.org/10.5194/egusphere-egu25-9304, 2025.
Developing an accurate definition of the onset and progression of the Indian monsoon is an outstanding research area in climate science. Determining monsoon onset dates is critical for agricultural planning and ensuring food security for billions of people in India and the world. The onset of the Indian monsoon is associated with the northward shift of the planetary-scale intertropical convergence zone (ITCZ) from the equator. ITCZ is a zone of intense convective activity, cloudiness and high precipitation girdling the Earth. As a result, the onset and progress of the Indian monsoon are interconnected on a planetary scale.
The monsoon onset definition can be classified into local and large-scale definitions. Local-scale definitions utilize daily precipitation over a small region to determine the onset. However, they are prone to bogus onsets because of pre-monsoon rains and transient weather systems. Large-scale definitions are based on precipitation and wind/cloudiness over a bigger area. However, such averaging does not guarantee separation of the ‘large-scale’ component of ITCZ precipitation from the ‘small-scale’ local contributions and is still prone to bogus onsets. Large-scale onset definitions are largely confined to defining the monsoon onset over Kerala (MoK) while representing the progression of monsoon is based entirely on local onsets. We overcome this limitation by developing a large-scale definition from small-scale local onsets interconnected on a planetary scale.
We utilize networks and their phase transitions to develop a large-scale definition. We construct time-varying spatial proximity networks based on daily precipitation, where nodes are the geographical locations in a domain encompassing India. Links are established only between nodes that are in geographical proximity and if they have undergone local onsets. Next, we estimate connected components in the network that represent clusters of local onsets. The spatiotemporal evolution, involving the growth and coalescence of clusters disentangles the true large-scale monsoon onset and progression.
We discover two abrupt phase transitions in the size of the largest cluster of the local onsets. These phase transitions are associated with the formation of large clusters representing the local onsets interconnected at a planetary scale. Thus, we unravel the setting up of the ITCZ and other synoptic-scale convective systems that facilitate consistent monsoon activity over India. We define large-scale onsets when a location becomes part of the largest cluster following the first transition.
Using lead-lag composites of precipitation for the past 84 years centred on large-scale onsets, we find that our definition captures important characteristics of the Indian monsoon usually missed by conventional large-scale definitions. During our onsets, the rainfall is strong at a large scale along the western ghats and northeast India (NEI). Further, they are followed by a rapidly northward propagating rainfall pulse, also known as the monsoon intraseasonal oscillation. Our method captures that the onset over NEI occurs before MoK, which is consistent with several recent studies but missed by conventional definitions. These new findings necessitate a reexamination of the interannual variability in the Indian monsoon, which will be discussed in the talk.
How to cite: Chopra, G., Patil, Y., Tandon, S., Goswami, B. N., and Sujith, R. I.: Unravelling large-scale onset and progression of the Indian monsoon from the evolution of clusters of local onsets using network science, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2330, https://doi.org/10.5194/egusphere-egu25-2330, 2025.
Most studies on the formation of the modern Asian monsoon focus on mechanisms arising on the Afro-Eurasian continent. While few compare the effects of other remote continents. Using a fully coupled general circulation model, this study decomposes the relative contribution of each continent on the formation, distribution and intensity of the Asian monsoon. Here we show that the existence of the North American continent is critical for the formation and intensity of the Asian summer monsoon. The mechanism involves North America acting as an additional heating center, resulting in the strengthening and extension of oceanic advection towards the Asian monsoon region. This is achieved by the Rodwell-Hoskins mechanism that strengthens the North Pacific subtropical high and through a wide-spread Northern Hemispheric heating that shifts poleward the subsidence center of Hadley circulation. This teleconnection is not dependent on the Tibetan Plateau and its impact on East Asian summer precipitation is found to be smaller but comparable to the Tibetan Plateau. The individual role of the other non-Afro-Eurasian continents was found to be less important. Previous work has shown that the Asian monsoon has global impact, including changing the climate of North America. This study firstly shows the "reversed" teleconnection that North America can have a very significant impact on the Asian monsoon. Although these experiments are idealized and based on contemporary land-sea geometry, they also highlight the role of North America in the geologic evolution of the Asian monsoon, and imply the impacts of the anthropogenic climate change of North America to the Asian monsoon in the recent history and future.
How to cite: Chen, L., Valdes, P., and Farnsworth, A.: The role of the North American continent in strengthening the Asian monsoon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1484, https://doi.org/10.5194/egusphere-egu25-1484, 2025.
The impacts of tropical systems on polar sea ice have been relatively underestimated, which could potentially offer insights into the mechanisms driving sea ice variability and enhance predictive skills regarding sea ice extent. Our recent study delves into the influence of July-August Indian Summer Monsoon (ISM) precipitation on shaping Arctic sea ice variability from August to October, alongside exploring how the December-February Australian Summer Monsoon (AUSSM) modulates simultaneous Antarctic sea ice. Our findings suggest that ISM could explain up to 20% of Arctic sea ice concentration (SIC) variance across the marginal Arctic Ocean, while AUSSM could account for roughly 10% of SIC variance in the Pacific sector of the Southern Ocean including Amunsen and Ross Seas. Insights from both observation and model experiments demonstrate that the diabatic heating associated with ISM and AUSSM can trigger the poleward propagation of Rossby waves, culminating in barotropic anomalous circulations over the Arctic and Antarctic regions. These anomalous atmospheric patterns, characterized by highs and lows, have the potential to influence surface downwelling longwave radiation and surface winds, thereby shaping sea ice variability through a combination of thermodynamic and dynamic processes.
How to cite: Zhu, J. and Wu, Z.: Atmospheric Influence of Summer Monsoon on Sea Ice Variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2173, https://doi.org/10.5194/egusphere-egu25-2173, 2025.
We present novel explainable deep learning techniques for reconstructing South Asian palaeomonsoon rainfall over the last 500 years, leveraging long instrumental precipitation records and palaeoenvironmental datasets from South and East Asia to build two types of models: dense neural networks (“regional models”) and convolutional neural networks (CNNs). The regional models are trained individually on seven regional rainfall datasets, and while they capture decadal-scale variability and significant droughts, they underestimate inter-annual variability. The CNNs, designed to account for spatial relationships in both the predictor and target, demonstrate higher skill in reconstructing rainfall patterns and produce robust spatiotemporal reconstructions. The 19th and 20th centuries were characterised by marked inter-annual variability in the monsoon, but earlier periods were characterised by more decadal- to centennial-scale oscillations. Multidecadal droughts occurred in the mid-17th and 19th centuries, while much of the 18th century (particularly the early part of the century) was characterised by above-average monsoon precipitation. Extreme droughts tend to be concentrated in southern and western India and often coincide with recorded famines. The years following large volcanic eruptions are typically marked by significantly weaker monsoons, but the sign and strength of the relationship with the El Niño–Southern Oscillation (ENSO) vary on centennial timescales. By applying explainability techniques, we show that the models make use of both local hydroclimate and synoptic-scale dynamical relationships. Our findings offer insights into the historical variability of the Indian summer monsoon and highlight the potential of deep learning techniques in palaeoclimate reconstruction.
How to cite: Hunt, K. and Harrison, S.: A novel explainable deep learning framework for reconstructing South Asian palaeomonsoons, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2815, https://doi.org/10.5194/egusphere-egu25-2815, 2025.
The Asian Summer Monsoon is fundamental for the water supply of billions of people. It has undergone major changes over the Pleistocene in response to greenhouse gas and ice sheet forcing during glacial-interglacial transitions, orbital forcing from varying obliquity and precession, and millennial-scale shifts in the ocean circulation. Yet, the spatial fingerprint of these variations and their impact on local vegetation remain uncertain. Here, we present vegetation and climate reconstructions from a pollen record in Northeastern China covering the last 70kyr with unprecedented sub-centennial resolution. During the last Glacial, its position at the ecotone between cool mixed forest and steppe led to pronounced local vegetation changes, most likely driven by varying moisture availability. The vegetation changes occur synchronously with oxygen isotope variations in Chinese speleothems. However, the timescale-dependent contributions to the total variability differ between our precipitation reconstruction and the isotope record. A regional analysis of high-resolution proxy records covering the last Glacial supports comparatively stronger contributions from orbital-scale variability along the northern monsoon edge and from millennial-scale variability in India and Southern China. This suggests that orbital forcing and Atlantic Meridional Overturning Circulation (AMOC) variations possess distinct spatial fingerprints. Climate simulations indicate that the differences are driven by stronger North Pacific sea surface temperature changes in response to orbital forcing compared to AMOC shifts. The detected spatial heterogeneity of past monsoon variations can provide valuable insights into potential regional impacts of future monsoon changes.
How to cite: Weitzel, N., Stebich, M., Adam, M., Mingram, J., and Rehfeld, K.: Timescale-dependent fingerprint of the Asian Summer Monsoon during the last Glacial and its impact on vegetation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13961, https://doi.org/10.5194/egusphere-egu25-13961, 2025.
The Hengduan Mountains region (HM), located in the Eastern Tibetan Plateau, is renowned for its rich biodiversity. High-resolution climate data from past periods are essential for gaining deeper insights into the ecological and evolutionary processes that have shaped this unique and diverse region. In this study, we applied the non-hydrostatic limited-area model COSMO, with a resolution of 12 km over East Asia, to simulate two distinct climatic periods: the mid-Pliocene (~3 Ma), representing a warmer period, and the Last Glacial Maximum (LGM; ~21 ka), a colder period, both compared to present-day conditions.
Our results reveal that, despite contrasting changes in moisture supply, both warm and cold periods experienced a weakened Indian summer monsoon, attributed to the exposure of the Indochina continental shelf during these times—caused by sea-level drops during the LGM and dynamic topography during the mid-Pliocene. During the mid-Pliocene, an earlier northward migration of the Western Jet led to an earlier onset of the Indian summer monsoon and a wetter spring in the HM. In contrast, the HM experienced increased precipitation during the LGM in both summer and winter. Increased summer precipitation was driven by enhanced moisture supply from the south, while enhanced winter precipitation, primarily in the form of snowfall at high elevations, was associated with more unstable atmospheric stratification.
The local precipitation characteristics of the HM are thus influenced by the interplay between large-scale atmospheric dynamics and regional topographical features such that, in contrast to most mid-latitude regions, the HM did not experience drying and wetting during glacial-interglacial cycles. The stability of mean precipitation across different climatic periods likely played a pivotal role in supporting the HM's high biodiversity, providing a stable and moist environment conducive to supporting diverse ecosystems.
How to cite: Xiang, R., Steger, C. R., Willett, S. D., and Schär, C.: Influence of Asian Monsoon Dynamics on Precipitation Characteristics of the Eastern Tibetan Plateau in Cold and Warm Climates: Insights from a Regional Climate Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13143, https://doi.org/10.5194/egusphere-egu25-13143, 2025.
The Indian Ocean plays an important role in modulating the global weather and climate. However, many state-of-the-art climate models can't predict the dynamically complex mechanisms of the Indian Ocean accurately. Studies show that the biases in the earlier versions of the Met Office Climate Model developed during the initial days of model simulation and persisted up to climate time scales. To investigate biases in the revised GC5 model, we analyzed 208 monthly forecasts initialized every five days from June to November (2018–2023). The spatial evolution of the SST biases over the Indian Ocean from these forecasts showed specific regions of warm and cold biases with up to a magnitude of ~ -1°C to 1°C. This regional bias formation is examined using the mixed layer heat budget analysis during the Indian summer and winter monsoons to understand the relative contribution of the various parameters in driving this variability. We have selected three warm SST bias regions, on the east coast of Africa, near the Indian Peninsula and on the west coast of Sumatra. The cold bias regions are in the northern Arabian Sea and on the west coast of Java. The primary analysis from the mixed layer heat budget shows that the warm and cold SST biases in the model are modulated mainly by some common parameters such as the net heat flux and total advection. However, further analysis showed that the total advection is more important in the warm bias regions. The vertical mixing term is also significant in generating cold SST biases and this can be a consequence of the positive wind speed biases in the model. Our study also concludes that even though the biases have comparable spatial and temporal magnitude and evolution, the parameters which modulate the SST variability have regional variations. Additionally, anomalously positive precipitation in the equatorial Indian Ocean and the west coast of India and a negative precipitation bias along the east coast of India were also identified. Hence removing these discrepancies in the SST might be crucial for accurately simulating the Indian monsoon.
How to cite: Anitha Reghunathan, A., Webber, B., Matthews, A., Copsey, D., and Rodriguéz, J.: Monsoonal mixed layer heat budget of the Indian Ocean: Understanding the biases in coupled forecast models., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3823, https://doi.org/10.5194/egusphere-egu25-3823, 2025.
This study utilizes the monitoring of the onset dates of the rainy season across India, Southeast Asia, Central America, and West Africa to predict the upcoming season. By employing a straightforward objective method based on daily rainfall data, we pinpoint the onset date of the rainy season at each grid point of the rainfall analysis by identifying the minimum on the corresponding daily cumulative anomaly curve of rainfall. To accurately estimate the onset and retreat dates for each year, we introduce a perturbation technique that generates an ensemble of 100 time series of rainfall at every grid point.
Our research demonstrates that the onset data anomalies of the rainy season are directly linked to the length and seasonal rainfall anomaly of the season in all these regions. Specifically, seasons with an earlier onset tend to be longer and wetter, while those with a later onset are shorter and drier. Furthermore, we explore the relationship between onset, retreat, seasonal length, rainfall, and various large-scale climate drivers, revealing that although these relationships are local and relatively weaker, the intrinsic connections among the variables are robust.
In this study, we leverage the 12-hour latency product of Integrated Multi-Satellite Retrievals for the Global Precipitation Mission version 6 (IMERG) for near real-time monitoring of the season's evolution. The probabilistic skill scores, assessed using the area under the relative operating characteristic curve method, confirm the high predictive skill of anomalous onset dates.
How to cite: Misra, V.: The variations of the regional monsoons and their predictability from monitoring their evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1267, https://doi.org/10.5194/egusphere-egu25-1267, 2025.
The Indian summer monsoon (ISM) is of great importance to over a billion people since it supplies over 75% of the country’s annual precipitation. Significant intraseasonal variability in rainfall affects people, with breaks responsible for causing water shortage. It is known that dry intrusions play a role in breaks; however, it is not well understood compared to the role dry intrusions play during progressions of the onset and withdrawal of the ISM. In this study, we use observations and the ERA5 reanalysis to understand the role of dry intrusions in breaks during 1940–2023. We develop an index based on moisture deficit to identify dry intrusions, and find that most breaks are associated with dry intrusions emanating from arid regions to the west and northwest of India. These dry intrusions begin to enter India around a week prior to the middle day of breaks, reaching their peak strength over north India three days prior to the middle day of breaks. Vertical profiles reveal that these are mid-level dry intrusions, which are similar to those driving the direction of the withdrawal of the ISM. As breaks evolve, these dry intrusions deepen throughout their horizontal extent and descend into the country, stabilising the troposphere and creating an unfavourable environment for deep convection. We also find that extended breaks have stronger dry intrusions as precursors. This work provides a new perspective on the causal relationship between mid-level dry intrusions and breaks. The results could help improve forecasts of breaks, ultimately benefiting stakeholders in improving long-term planning.
How to cite: Deoras, A., Turner, A., Volonté, A., Schiemann, R., Wilcox, L., and Menon, A.: The role of dry intrusions in breaks of the Indian summer monsoon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2552, https://doi.org/10.5194/egusphere-egu25-2552, 2025.
The Madden-Julian oscillation (MJO) is the predominant ocean-atmospheric phenomenon that influences the intraseasonal variabilities in the tropical atmosphere and is associated with weather extremes across the globe. This study aims to investigate the influence of MJO on extreme precipitation during Indian summer monsoon over India using ERA5 reanalysis data from 1974 to 2022. The MJO phases are calculated following Wheeler & Hendon, (2004) methodology which utilizes variables outgoing longwave radiation (OLR), zonal wind at 200hPa and 850hPa in the near-equatorial region between 15°S and 15°N. 99th percentile is used as a threshold to identify extreme precipitation. The study employs the Theil Sen slope trend test and Pettitt test for change point detection to study extremes. Further the OLR, vertically integrated moisture divergence (VIMD), and zonal wind at 850hPa are examined to study the change in dynamics. The preliminary results suggest that active phases 3 and 4 shows positive trend of extreme precipitation over southern northwest, west central and Peninsular India while active phase 2 and inactive phases 6 and 7 shows overall positive trend except for northeast India. Apart from extreme precipitation, frequency of extremes has also increased in phases 1, 2, 4, and 5. The change point analysis indicates these changes are observed after 1997. The percentage change of VIMD after change point show increased moisture availability in inactive phases which is evidently due to enhanced convective activity in recent times as also suggested by OLR. Overall, the study contributes in understanding the pattern of extremes over Indian landmass which will helps in predicting and mitigating the effect of severe weather.
How to cite: Sharma, A., Maharana, P., and Dimri, A. P.: Precipitation extremes during Madden Julian Oscillation over India, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18478, https://doi.org/10.5194/egusphere-egu25-18478, 2025.
Understanding the dynamics of precipitation in the western Himalayas (WH) during the Indian Summer Monsoon (ISM) is vital for societal well-being and effective disaster management. The region's complex terrain, diverse meteorological conditions, and observational uncertainties pose significant challenges in comprehending precipitation disparities and predicting extreme precipitation events (EPEs) across the WH. The present study provides a comprehensive investigation into the characteristics, drivers, and variability of summer monsoon precipitation, with a focus on EPEs and their underlying mechanisms in the WH. The findings reveal that EPEs, over the WH, defined as precipitation exceeding the 99th percentile, are influenced by both large-scale (61%) and convective precipitation (39%). Monsoon depressions contribute to 25.49% of these events. Atmospheric patterns such as upper-tropospheric gyres, zonal waves, and omega-type blocking emerge as key precursors, facilitating the southward extension of moisture-laden winds and enhancing low-level moisture convergence. The tropical-extratropical interactions, including the shifting of the Intertropical Convergence Zone and baroclinic wave activity characterized by zonal wave numbers 5 and 8, play a crucial role in intensifying EPE. Furthermore, High-resolution simulation using WRF demonstrate improved representation of spatiotemporal precipitation patterns, interannual variability, and EPEs compared to observational datasets. Overall, this study provides valuable scientific insights into the complex interactions governing precipitation extremes in the Himalayas. The findings enhance the understanding of ISM precipitation variability and improve the ability to predict and mitigate the impacts of extreme events in the region.
Keywords: Western Himalayas, Indian Summer Monsoon, Extreme Precipitation Events, Physical Drivers
How to cite: Saini, R. and Attada, R.: Deciphering Characteristics, Variability, and Drivers of Summer Monsoon Precipitation and Extremes over the Western Himalayas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20634, https://doi.org/10.5194/egusphere-egu25-20634, 2025.
Over the recent decades, the South Asian monsoonal environment has evolved, leading to a rise in heavy precipitation events over the Indian subcontinent. These events have increased in frequency and intensity, particularly over Western India since the 1980s. The present study employs Self-Organizing Map (SOM) clustering to examine atmospheric patterns associated with heavy rainfall over Western India, identifying two key clusters, which have shown a significant rise in occurrence since the 1970s. The first cluster is marked by a large-scale mid-level vortex stretching from the Bay of Bengal to the Arabian Sea, driven by strong easterly anomalies and low-pressure systems (LPS) along central India. In contrast, the other cluster is manifested as a localized system centred over Western India, with low geopotential heights and LPS activity, supported by moisture from the Arabian Sea and regional land evaporation. The development of the first pattern is linked to remote influences such as Indian Ocean Dipole (IOD) events, while local soil moisture conditions influence the second pattern. This study underscores the complex interactions between large-scale dynamics, land-atmosphere coupling, and extreme weather patterns, highlighting the need for enhanced understanding of multis-scale interactions and increased observational networks to improve predictions and management of hydrological extremes in Western India.
How to cite: Deychoudhury, A., Kumar Mukherjee, S., Raghavan, K., and Dey, D.: Role of large and local scale drivers in the recent rise in heavy precipitating events over western India, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20535, https://doi.org/10.5194/egusphere-egu25-20535, 2025.
Global warming has led to the intensification and increased frequency of drought events. Determining the extent to which these events are influenced by human activities is critical for developing effective strategies to address climate change. However, detecting human impacts and providing robust attribution results remain key challenges in drought research. In the summer of 2022, the Yangtze River Valley of China experienced an unprecedented extreme drought, marked by record-high surface temperatures and record-low precipitation over the past 60 years. This event caused substantial socio-economic and ecological disruptions. To assess the role of anthropogenic climate change in the intensity and frequency of such events, this study established an attribution framework based on GAMIL3.0. This study evaluated anomalies in surface temperature, precipitation, and large-scale circulation patterns during the summer of 2022. Results indicate that human activities have intensified the Western North Pacific Subtropical High and South Asian High, increasing their strength and frequency and thereby amplifying the intensity and likelihood of extreme drought events in the Yangtze River Valley. Anthropogenic forcing contributed to an additional 0.8°C rise in surface temperature (95% confidence interval: 0.1–1.5°C) and a 7.9% reduction in precipitation (-24.1% to 7.8%) during the 2022 summer. The anthropogenic forcing increased the probability of surface temperature anomalies associated with such an extreme drought event like 2022 by 1300 times (range: 87–3,001) and precipitation anomalies by 65 times (range: 1–90). This study highlights the urgent need to strengthen adaptive capacities to mitigate the impacts of extreme drought in the Yangtze River Valley.
How to cite: Zhang, L., Zhou, T., Zhang, X., Zhang, W., Li, L., and Li, L.: Attribution of the Extreme Drought Event over the Yangtze River Valley in China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10438, https://doi.org/10.5194/egusphere-egu25-10438, 2025.
The Indian Ocean dipole (IOD) is a remarkable interannual variability in the tropical Indian Ocean. The improved prediction of IOD is of a great value because of its large socioeconomic impacts. Previous studies reported that both El Ni˜ no-Southern Oscillation (ENSO) and South China Sea summer monsoon (SM) play a dominant role in the western and eastern pole of the IOD, respectively. They can be used as predictors of the IOD at 3 month lead beyond self-persistence. Here, we develop an empirical model of multi-factors in which the western pole is predicted by ENSO and persistence and the eastern pole is predicted by SM and persistence. This new empirical model outperforms largely the average level of the dynamical models from the North American multi-model ensemble (NMME) project in predicting the peak IOD in boreal autumn, with a correlation coefficient of ∼0.86 and a root mean square error of ∼0.24°C. Furthermore, the hit rate of positive culminated IOD in this new empirical model is equivalent to that in current NMME models (above 65%), much higher than that for negative culminated IOD. This improvement of skill using the empirical model suggests a perspective for better understanding and predicting the IOD.
How to cite: Li, J., Zhang, Y., Diao, Y., Wang, Q., Wu, R., Liu, T., Jin, Y., Hou, Z., and Wang, H.: Reconciling roles of the South China Sea summer monsoon and ENSO in prediction of the Indian Ocean dipole, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2285, https://doi.org/10.5194/egusphere-egu25-2285, 2025.
This study examines how the summertime Indian Ocean (IO) SST anomalies (SSTAs) affect the Indian Summer Monsoon (ISM) and its predictability in the El Niño developing years from the perspective of seasonal predictions of 1972 and 1997 when observed drastically different ISM states. The CFSv2-COLA ensemble seasonal reforecasts produce a successful ISM prediction in 1972 but a failed one in 1997. Our sensitivity experiments, in which the ocean and atmosphere are decoupled in the tropical IO with the prescribed SST, reveal that the erroneous prediction of cold IO SSTAs in 1997 exacerbates an El Niño-induced ISM drought and “correcting” these SST errors improves the ISM prediction substantially, whereas a good prediction of the summertime IO SSTAs contributes positively to the skillful ISM reforecast in 1972. It is also demonstrated that the warm IO SSTAs centered in the Arabian Sea in 1997 reduce sea-level pressures locally and steer the low-level anomalous winds to transport water vapor into the India. This regional process counters the El Niño-induced drought tendency and results in a nearly normal ISM that defies the historical El Niño-ISM relation. However, the warm SSTAs centered at the western equatorial IO in 1972 strengthen the anomalous Walker circulations originally set up by the developing El Niño in the Indo-Pacific domain, which further enhance the El Niño evolution and its teleconnection to the ISM. This inter-basin feedback process intensifies the typical El Niño-ISM relation. The spatial structure of the summer IO SSTAs may determine whether the IO regional process or the inter-basin process prevails.
How to cite: Shin, C.-S.: Influences of the Indian Ocean SST on the Indian Summer Monsoon and its Seasonal Prediction in El Niño Developing Years , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7074, https://doi.org/10.5194/egusphere-egu25-7074, 2025.
The subtropical jet stream (STJ) and tropical easterly jet (TEJ) are critical upper-tropospheric features shaping the Indian summer monsoon (ISM). BY analysing data for the period 2000–2023, this study investigates the positional dynamics of these jets and their relationship with rainfall variability over the Indian region. Using ERA5 reanalysis data and daily rainfall records from the India Meteorological Department (IMD), we analysed the zonal and meridional wind fields at 200 hPa along with rainfall observations.
Four distinct jet stream cases were examined: both southern and northern hemispheric STJs shifting: (i) equator-ward, (ii) pole-ward, (iii) northward, and (iv) southward. Results reveal that equatorward shifts of the STJ weaken the TEJ and displace it southward, reducing rainfall over central India. Conversely, poleward migration of the STJ strengthens the TEJ, driving its northward extension and intensifying monsoonal rainfall, including extreme rainfall events. Northward shifts of both hemispheric STJs enhance TEJ strength, while southward shifts suppress it, altering the spatial distribution of rainfall. Strengthening of TEJ is expected to enhance the vertical velocity and LLJ through easterly vertical shear mechanism, and result in enhanced rainfall over the central Indian region. A TEJ Index (TEJI) was developed by area-averaging the TEJ core at 200 hPa, demonstrating strong correlations with rainfall intensity.
These findings underscore the complex interplay between STJ and TEJ and their dynamical role in modulating ISM rainfall. Understanding these mechanisms provides essential insights into atmospheric circulation patterns and their influence on monsoonal extremes, aiding improved prediction and climate resilience strategies in the region.
How to cite: Sunil, R. M. and Manguttathil Gopalakrishnan, M.: Dynamical Influence of Subtropical Jet Stream (STJ) and Tropical Easterly Jet (TEJ) on Indian Summer Monsoon Rainfall, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1017, https://doi.org/10.5194/egusphere-egu25-1017, 2025.
Using 51 models of the AMIP and historical experiments of CMIP6, we investigate the inter-model diversity of atmospheric and coupled models in the strength of the Indian Summer Monsoon Rainfall (ISMR)–El Niño-Southern Oscillation (ENSO) relationship. In atmospheric models, the Walker Circulation (WC) intensity associated with the western Pacific convective activity is most responsible for the inter-model diversity. Models with strong WC have a strong ISMR–ENSO relationship via enhancing ENSO-induced anomalies of the WC and monsoon circulation. The secondary source is the monsoon circulation differences associated with meridional rainfall contrast over the Indian monsoon region. In coupled models, the primary (secondary) source is the ENSO amplitude (WC intensity). In observation, the decadal variation of WC can also explain the changes in the ISMR–ENSO relationship. This study provides a basis for improving the model performance and advances our understanding of the observed ISMR–ENSO relationship changes.
How to cite: Yu, S.-Y.: Sources of Inter-Model Diversity in the Strength of the Relationship Between the Indian Summer Monsoon Rainfall and El Niño-Southern Oscillation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5422, https://doi.org/10.5194/egusphere-egu25-5422, 2025.
The Indian summer monsoon displays intraseasonal variability with alternating "active" (intense rainfall) and "break" (deficient rainfall) phases. Analysis of Indian Meteorological Department (IMD) daily rainfall data (1979–2020, June–September) over Central India (CI) shows that active spells are more frequent during flood years (4.6 events/year) than drought years (2.3 events/year), with similar durations (3–4 days). In contrast, break spells are more frequent and prolonged in drought years (3.9 events/year, 6–7 days, occasionally exceeding 10 days) compared to flood years (1.2 events/year, 3–4 days).
Composites of mean sea level pressure reveal distinct intraseasonal dynamics between flood and drought years. During flood years, positive pressure anomalies propagate northwestward from the Bay of Bengal, while in drought years, they propagate poleward and stagnate over Central India, reducing active spell frequency. Similarly, break spells exhibit westward-moving anomalies in flood years, whereas drought years are characterized by stationary anomalies and poleward propagation.
Intraseasonal oscillations (ISOs) derived from IMD rainfall data strongly influence active and break spells. Flood years are characterized by high-frequency ISOs (HF-ISOs) with westward propagation, enhancing active spells, while drought years are dominated by low-frequency ISOs (LF-ISOs) with poleward movement, leading to prolonged breaks. Over 90% of these spells align with HF-ISO in flood years and LF-ISO in drought years. Total column water composites reveal frequent midlatitude dry air intrusions during drought years, contributing to extended break spells. Moisture budget analysis indicates that differences in mean moisture advection by mean winds drive anomalies, with positive values during flood years and negative values during drought years.
K-means clustering reveals the relationship between ISO variability and seasonal rainfall through four clusters based on variance explained by LF-ISO and HF-ISO. The cluster with the strongest LF-ISO and weakest HF-ISO records the lowest rainfall (92% of the long-term mean), while the opposite cluster experiences the highest rainfall (112% of the long-term mean). These findings align with observed HF and LF ISO intensities during flood and drought years. Strong HF-ISO activity is associated with enhanced formation and northwestward propagation of low-pressure systems from the Bay of Bengal to Central India, contributing to above-normal rainfall. Additionally, the strong HF-ISO cluster features strong low-level westerlies supported by upper-level easterlies, alongside tropospheric conditions limiting dry air intrusion from midlatitudes. In contrast, low rainfall in the cluster with large LF-ISO variance coincides with low-level easterly anomaly, and concomitantly weaker moisture transport from the Arabian Sea (AS). Clusters with maximum LF-ISO intensity feature mid-tropospheric high pressure over CI, reflecting downdrafts of dry, cold upper-level air that suppress convection and cause seasonal rainfall deficits. Midlatitude intrusions are observed in clusters with moderately strong LF-ISO intensity, accompanied by southeasterly winds northwest of CI. These intrusions are weaker, maintaining rainfall near the long-term mean.
This study underscores the contrasting active and break spells during flood and drought years, highlighting the role of ISOs, atmospheric dynamics, and thermodynamic processes.
How to cite: Jha, R., Nanjundiah, R., and Seshadri, A.: Distinct Characteristics of Active and Break Spells in Flood and Drought Years of the Indian Summer Monsoon , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6038, https://doi.org/10.5194/egusphere-egu25-6038, 2025.
The Madden-Julian Oscillation (MJO) has been a topic of great scientific interest due to its higher predictability and significant impact on global climate and weather, including the South American monsoon system. As a pillar of subseasonal predictability, it is important to investigate the influence of major interdecadal oscillations, such as the Interdecadal Pacific Oscillation (IPO) and the Atlantic Multidecadal Oscillation (AMO), to assess the potential modulation of MJO impacts by these oscillations. How do variations induced by these slower oscillations influence MJO teleconnections to South America (SA)? What is the frequency of MJO phases during the austral summer, and how might the interaction between the MJO and these oscillations affect the monsoon in SA? The combined impact of the MJO and low-frequency oscillations was characterized by composites of daily anomalies filtered in the 20-90 days band during the austral summer (DJF, rainy season), when the MJO is strongest. Composite anomalies of convection and circulation were analysed over the entire period from 1979 to 2020, as well as during two periods characterized by distinct combinations of opposite phases of the IPO and AMO: IPO(+)/AMO(-) (1979-1999, Period 1) and IPO(-)/AMO(+) (2000-2020, Period 2). Results indicate that during DJF, convection anomalies and the frequency of extreme events over SA are more pronounced in the Period 1 compared to Period 2, particularly in the central-east SA (CESA), the core monsoon region. In this region, increased (reduced) precipitation is observed during MJO phases 8 and 1 (4 and 5). Previous findings (Grimm, 2019) using Influence Function analysis, based on an extended vorticity equation model, and simulations, indicated a link between anomalous convection over the central subtropical South Pacific (CSSP) and the SA during phase 8 of the MJO, which may be responsible for the convection pattern in the CESA in phase 1. This anomalous convection in CSSP is stronger in Period 1 than in Period 2. Furthermore, there is a reversal in the sign of convective anomalies from reduced to enhanced precipitation in phase 6 over CESA from Period 1 to Period 2 and this may be associated with the change from reduced to enhanced convection over CSSP during phase 5, through teleconnections. Therefore, convection associated with MJO events during Period 1 is stronger (weaker) than in Period 2 in the CESA region during phases 8 and 1 (4, 5, 6 and 7). In contrast, in southern Brazil, positive convection anomalies persist from phases 3 to 5 in Period 1, whereas in Period 2 these anomalies are observed only in phases 3 and 4. Additionally, during Period 2, a reversal of the anomalies occurs in phases 1 and 2 compared to Period 1.
How to cite: Scheibe, L. A. and Grimm, A. M.: The Relationship of the Madden-Julian Oscillation with Interdecadal Variability Modes During the Monsoon Season in South America, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-315, https://doi.org/10.5194/egusphere-egu25-315, 2025.
Future rainfall changes in India are of paramount importance for crop production and water management, but to date, longer-term changes beyond the year 2100 have not been evaluated. Here, we leverage a 10-member extension of the CESM2 Large Ensemble under relatively strong emissions (SSP3-7.0) to identify projected rainfall changes and the underlying physical mechanism out to 2500. Our main finding is that after 2100, substantial changes occur in large-scale atmospheric circulation patterns, which are more pronounced and distinct from the changes projected over the 21st century. We test the hypothesis that under substantial thermal perturbations to the climate system after 2100, the increased atmospheric stability caused by the enhanced differential heating in the upper troposphere relative to land weakens the large-scale monsoonal circulation, while enhanced warming over the Tibetan Plateau causes a poleward shift in low-level monsoonal circulation and the climatological pressure belt. This projected shift promotes enhanced northward moisture transport, resulting in a strengthened anomalous ascending motion over northern India, ultimately leading to increased Indian summer monsoon rainfall post-2100. These changes reflect local expression of large-scale climate dynamical perturbations and provide a broader mechanistic framework for understanding long-term future climate change over India.
How to cite: Sharma, S., Ha, K.-J., Rodgers, K., Chung, E.-S., Lee, S.-S., and Nellikkattil, A. B.: Indian Summer Monsoon rainfall changes beyond the 21st century, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2958, https://doi.org/10.5194/egusphere-egu25-2958, 2025.
Future projections of South American Monsoon (SAM) precipitation from CMIP6
(Coupled Model Intercomparison Project phase 6) show a consistent drying during the early part
of the monsoon season (September to November), which is also seen in a convection-permitting
model simulation. Using a set of idealised atmosphere-only GCM experiments, this drying signal
is shown to be mainly driven by sea surface temperature (SST) changes: uniform SST warming
and patterned SST change. Different processes appear to be more important in different months
for the ensemble mean drying signal, with this primarily driven by SST pattern change in October
and by uniform SST warming in November. There is significant inter-model uncertainty in the
SAM precipitation response to each of these drivers, particularly SST pattern change. For uniform
SST warming, an existing hypothesis which suggests that SAM drying is driven by the enhanced
land-sea temperature contrast is tested, but we find that this process is not dominant. For patterned
SST warming, moderate inter-model correlations (across the coupled CMIP6 models) are found
between SAM precipitation change and changes in meridional and zonal Atlantic SST gradients.
In November, a combined zonal and meridional Atlantic SST gradient index can explain more than
half of CMIP6 inter-model uncertainty in SAM core region precipitation change.
How to cite: Chadwick, R., Good, P., Garcia-Franco, J., Alves, L., Hart, N., Zilli, M., Douville, H., Saint-Lu, M., and Medeiros, B.: Future South American monsoon changes are sensitive to Atlantic SST pattern changes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3589, https://doi.org/10.5194/egusphere-egu25-3589, 2025.
The West African Monsoon (WAM) precipitation response to increased CO2 is uncertain, with both large increases and decreases predicted by CMIP6 models. To address this, the full impact of increased CO2 has been decomposed into several drivers, three of which are shown to contribute most to the uncertainty in WAM precipitation; the direct radiative effect of increased CO2, the impact of a uniform Sea Surface Temperature (SST) warming, and the impact of a patterned SST change. Much of the uncertainty associated with the response to the direct radiative effect and uniform SST warming is shown to be related to differing changes in 700hPa moisture flux divergence associated with the shallow meridional circulation over West Africa as well as differences in a soil moisture - surface heat flux feedback over the Sahel. For the SST pattern effect, the difference between North Atlantic SSTs as well as inter-hemispheric gradients in surface temperatures are key drivers of intermodel spread.
How to cite: Mutton, H., Chadwick, R., Collins, M., Lambert, F. H., Taylor, C., Geen, R., Douville, H., and Saint-lu, M.: Understanding the Uncertainty in the West African Monsoon Precipitation Response to Increasing CO2, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10023, https://doi.org/10.5194/egusphere-egu25-10023, 2025.
Large mountain regions influence local- and global-scale atmospheric flow through mechanical forcing and changes in temperature and pressure fields. The topography of High-Mountain Asia (HMA), for example, is critical for the development of important characteristics of the Indian Monsoon. In this study, global climate model sensitivity experiments are applied to quantify the magnitude and geographical extent of the effects of HMA topography on Northern Hemisphere atmospheric flows. A series of ECHAM5-wiso (climate model) experiments were set up, in which HMA topography is reduced incrementally by 25% of its current height. All other boundary conditions, such as greenhouse gas concentrations and ice cover, are kept constant. The impact of the simulated topographic changes on the Eurasian Wave Train (EWT), which is critical for Northern Hemisphere atmospheric transport, is evaluated by examining the loading patterns from empirical orthogonal functions analyses conducted on the simulated pressure and wind fields. The impact of HMA topography on the intensity and extent of meridional flow in South Asia is assessed by examining wind speeds at different pressure levels. Changes in the EWT are particularly prominent in (Central) Asia. These may be attributed to the significant changes in pressure fields west of the Tibetan Pleateau as the topography in the HMA region is varied. Furthermore, increasing HMA topography from 0% to 100% significantly increases 1) average meridional summer wind speeds (by ≤10 m/s) at the 200hPa and 1000hPa levels, and 2) the extent of northward, monsoonal flow over the Indian subcontinent. The simulations only predict notable northward flow over western and northern India in summer if HMA topography is set to ≥50% of its modern height. The flow’s northeastern extent is restricted to 25°N-65°E in the absence of HMA topography, but reaches 33°N-75°E when it is fully developed.
How to cite: Mutz, S. G.: The Effect of High-Mountain Asia Topography on Northern Hemisphere Atmospheric Flow, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7283, https://doi.org/10.5194/egusphere-egu25-7283, 2025.
The East Asian monsoon is one of the most important features in the global climate system. Understanding the variation and moisture sources of East Asian monsoon precipitation is crucial for improving land-atmosphere interactions, developing more accurate global climate models, and optimizing the region’s water management strategies. Monsoon precipitation is produced from both local moisture through evapotranspiration and remote moisture transported from oceans via large-scale circulation. This study investigates the relative contribution of local and remote moisture sources to total monsoon precipitation in East China using model outputs from 12 CMIP6 simulations. Simulations from the historical experiments of CMIP6 are selected for the period of 1950-2014. These 12 CMIP6 models are selected based on the availability of data related to convective precipitation and soil moisture. In this study, we focus on the percentage of convective precipitation in total monsoon precipitation and soil moisture-precipitation relationship in East China. East China is divided into five regions based on their climate conditions. Our analysis suggests a significant spatial and temporal variation in the contribution of local convective precipitation to overall monsoon precipitation among different models. On average, the Southeast region shows a higher percentage of convective precipitation and a stronger soil moisture-precipitation correlation than other regions. Additionally, the percentage of convective precipitation varies significantly between models. The findings from this analysis could offer insights for enhancing the development of future climate models.
How to cite: Meng, L.: Relative contribution of local and remote sources of moisture to East Asian monsoon precipitation in CMIP6 simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2584, https://doi.org/10.5194/egusphere-egu25-2584, 2025.
Eastern China experiences substantial precipitation variability, primarily driven by the East Asian monsoon system, which is characterized by the stepwise northward progression of rainfall belt. The movement of the rainfall belt has significant socio-economic implications, necessitating precise forecasting to mitigate the risks associated with extreme weather events. This study employs Seasonal Empirical Orthogonal Function (S-EOF) analysis to examine precipitation variations, focusing on the transition of rainfall belt from spring (April-May) to summer (June-July). The results reveal that the northward shift of rainfall belt during the spring-to-summer period is strongly linked to variations in the East Asian Summer Monsoon activity. The leading mode exhibits a center of maximum rainfall in South China (SC) during spring, shifting to the middle and lower reaches of the Yangtze River basin (MLYZB) in summer, which emphasizes the spatial progression of rainfall patterns between these regions. In positive phase years, enhanced precipitation in SC during spring is related to increased moisture transported by an anomalous anticyclonic circulation over the western North Pacific (WNP). Subsequently, during summer, the enhanced rainfall moves to MLYZB along with the northward migration of the anomalous anticyclone in WNP. During negative phase years, precipitation markedly reducing in the two regions, mainly due to an anomalous cyclonic circulation over the WNP obstructs the influx of moisture from the Pacific. In summer, a cyclonic circulation over the South China Sea redirects moisture from the Indian Ocean to SC, resulting in reduced precipitation in the MLYZB. These large-scale atmospheric circulation patterns also indicate that the dominant transition of rainfall from spring to summer in eastern China can be associated with the monsoonal dynamics in the Bay of Bengal during spring. In particular, anomalous Bay of Bengal Summer Monsoon (BOBSM) activity triggers atmospheric convective heating and amplifies soil moisture anomalies in the Indochina Peninsula, thereby influencing and modulating rainfall patterns over eastern China. To further elucidate the mechanisms underlying this influence, numerical experiments are conducted to investigate the detailed processes through which BOBSM impacts the seasonal transition of rainfall in eastern China. In conclusion, this study can offer significant theoretical insights that enhance precipitation forecasting and inform extreme weather analysis.
How to cite: Ma, R., Zhang, Y., and He, C.: Leading mode and physical dynamics of spring-to-summer rainfall evolution in eastern China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14693, https://doi.org/10.5194/egusphere-egu25-14693, 2025.
This study investigates the changes in environmental conditions related to the decadal variation of genesis frequency of tropical cyclones (TCGF) over the South China Sea (SCS) during summer. Corresponding to the decadal increase in the TCGF, the seasonal mean anomalies of the genesis potential and environmental fields over the SCS are not favorable for tropical cyclogenesis, indicating their limited role in the decadal variation of SCS TCGF. It is found instead that the decadal change in tropical cyclogenesis over the SCS is more attributed to the local environmental fields associated with atmospheric intraseasonal oscillation (ISO). The decadal change in SCS TCGF is closely linked to the northward extension of ISO-related convection from the central SCS, which is contributed by the horizontal advection by background monsoon flow and the vertical advection by ISO-related vertical motions. Further analyses indicate that the anomalous upper-level cyclonic circulation over East Asia and the lower-level anticyclonic circulation over the western Pacific produce the unfavorable seasonal mean environmental fields in the SCS, whereas the resultant strong summer mean SCS monsoon flow facilitates the northward extension of ISO-related convection. This study highlights the importance of ISO activity for projections of the long-term change in SCS TC activity.
How to cite: Cai, Y., Yang, S., Wu, H., Chen, W., and Xu, J.: Environmental features related to the decadal variation of South China Sea tropical cyclogenesis in the context of summer monsoon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6560, https://doi.org/10.5194/egusphere-egu25-6560, 2025.
Analysis of ground-based and remotely retrieved precipitation data reveals that heavy Meiyu precipitation events (HMPEs) produce a relatively independent rain-belt over eastern China. A rotating calipers algorithm is applied to quantify the spatial scales of HMPEs. We find that HMPEs have regular spatial scales with an average length, width and extent of about 1400 km, 500 km and 40.00 × 104 km2, respectively, through a comprehensive assessment of different types of HMPE, illustrating that HMPEs have a size similar to that of the sub-synoptic-scale Meiyu front (1500–2000 km). Convective activities along the Meiyu front zone and the upper westerly jet stream strongly affect the position and orientation of rain-belts of HMPEs. The Meiyu front zone, strong vertical motions and large transport of warm moisture have a comparable spatial scale to the HMPE rain-belts over eastern China.
How to cite: Du, Y., Xie, Z., and Miao, Q.: Spatial Scales of Heavy Meiyu Precipitation Events in Eastern China and Associated Atmospheric Processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13901, https://doi.org/10.5194/egusphere-egu25-13901, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 5
EGU25-3896 | Posters virtual | VPS2
Dynamics and Characteristics of Climatic Extremes over East Asia Monsoon regionTue, 29 Apr, 14:00–15:45 (CEST) | vP5.12
In this talk, I will highlight our recent advances and findings in changes in climatic extremes over east Asia monsoon region. I will focus specifically on monsoon duration, intensity, rainfall extremes changes, and mechanism, with dynamic and thermodynamic factors controlling rainfall extremes over East Asia in late summer. Moreover, I will present our latest research on climatic extremes such as heatwaves based on dry conditions and stationary waves. Despite increasing future rainfall, rainfall extremes and rainfall variability in many areas, our recent studies suggest also an increase in drought risk over eastern Asia as a result of changes in evapotranspiration. However, the underlying mechanisms of heat waves and potential atmospheric and land feedbacks are still not fully understood. Through feedback attribution analysis, we found that there are dry and hot heat waves with very different underlying physical processes and feedbacks. The increasing global warming is expected to exacerbate atmospheric water demand, worsening future conditions of extreme droughts and heatwaves. Compound drought and heatwaves (DHW) events have much attention due to their notable impacts on socio-ecological systems. However, studies on the mechanisms of DHW related to land-atmosphere interaction are not still fully understood in regional aspects. Here, we investigate drastic increases in DHW from 1980 to 2019 over northern East Asia, one of the strong land-atmosphere interaction regions. Heatwaves occurring in severely dry conditions have increased after the late 1990s, suggesting that the heatwaves in northern East Asia are highly likely to be compound heatwaves closely related to drought. Moreover, the soil moisture–temperature coupling strength increased in regions with strong increases in DHW through phase transitions of both temperature and heat anomalies that determine the coupling strength. As the soil moisture decreases, the probability density of low evapotranspiration increases through evaporative heat absorption. This leads to increase evaporative stress and eventually amplify DHW since the late 1990s. Focusing on changes in stomatal conductance due to CO2 changes, our research results reveal an increase in surface resistance with CO2 elevation. Particularly under drought conditions, potential evapotranspiration tends to overestimate drought severity in the East Asian region by approximately 17% when scenarios considering vegetation are not taken into account. Additionally, intensified land-atmosphere interactions due to soil moisture deficiency lead to more frequent and amplified occurrences of compound heatwaves and droughts over northern East Asia. Understanding the relationship between soil moisture and vegetation can contribute to comprehending future severe droughts and heatwaves under diverse surface conditions with warming and moistening.
How to cite: Ha, K.-J., Yeo, J.-H., and Seo, Y.-W.: Dynamics and Characteristics of Climatic Extremes over East Asia Monsoon region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3896, https://doi.org/10.5194/egusphere-egu25-3896, 2025.
EGU25-15784 | ECS | Posters virtual | VPS2
Changes in MJO Propagation Characteristics and Regional Variations under a Changing ClimateTue, 29 Apr, 14:00–15:45 (CEST) | vP5.29
The Madden-Julian Oscillation (MJO) is a crucial atmospheric phenomenon characterized by large-scale, eastward-propagating disturbances in the tropical atmosphere. It profoundly influences global climate and weather patterns and serves as a key source of predictability for subseasonal forecasts. In particular, the propagation characteristics of the MJO are critical parameters that impacts the timing and intensity of its effects. Variability in these characteristics can alter the MJO’s interaction with other climate components, thereby affecting weather patterns. Therefore, it is essential to investigate the variability of MJO propagation characteristics.
In this study, we aim to examine the changes in propagation characteristics of the MJO, such as propagation speed, across three primary regions: Indian Ocean, Maritime Continent, western Pacific. These changes will be compared between two distinct period (P1: 1979-1998, P2: 2003-2022). Furthermore, we will investigate the mechanisms driving variations in MJO propagation speed within each tropical region and assess potential future changes using reanalysis data and model outputs. By addressing these questions, this study can contribute to improve the predictability and accuracy of climate models in representing the MJO.
How to cite: Kim, H.-R. and Ha, K.-J.: Changes in MJO Propagation Characteristics and Regional Variations under a Changing Climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15784, https://doi.org/10.5194/egusphere-egu25-15784, 2025.
EGU25-17380 | ECS | Posters virtual | VPS2
Neogene paleoclimatic evolution in Northwestern Luzon, Philippines: Insights from Lower Miocene to Lower Pliocene sedimentary recordsTue, 29 Apr, 14:00–15:45 (CEST) | vP5.33
Geochemical analyses of Neogene clastic sediments overlying the Zambales Ophiolite Complex (ZOC) in northwestern Luzon, Philippines provide insights into paleoweathering and paleoclimatic conditions. This study examines the Early Miocene Cabaluan Formation and Late Miocene to Early Pliocene Santa Cruz Formation using weathering proxies, such as the Chemical Index of Alteration (CIA), Revised Chemical Index of Alteration (CIX), Chemical Index of Weathering (CIW), and dual-elemental ratios (e.g., Al/Ti, Sc/Ti, Na/Al). Elevated CIA, CIX, and CIW values in the Cabaluan Formation indicating intense weathering suggests warm and wet conditions during the Early Miocene. Conversely, lower values and reduced Al/Ti and Sc/Ti ratios in the Santa Cruz Formation reflect a shift to cool and dry conditions at the onset of the Late Miocene period.
These findings align with regional patterns derived from similar geochemical proxies and δ18-O values in the northern South China Sea and global climatic cooling trends during the Neogene. They also highlight the influence of the East Asian Summer Monsoon (EASM) on the prevailing local weathering regime, supported by mobility indices (αᴬˡE) showing distinct elemental depletions and enrichments linked to climatic variations.
This study contributes to the scarce but growing paleoclimate studies in the Philippines using geochemical signatures in the sedimentary record. It provides a pioneering view into the Neogene paleoclimatic shift from a warm to cool climate in northwestern Luzon, Philippines, underscoring the influence of the EASM in the local and regional climatic evolution since the Early Miocene.
How to cite: Sangalang, K. J., Novero, M. J., Gabo-Ratio, J. A., Payot, B., Dimalanta, C., Alcancia, M., Jabagat, K., and Lee, Y.-H.: Neogene paleoclimatic evolution in Northwestern Luzon, Philippines: Insights from Lower Miocene to Lower Pliocene sedimentary records, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17380, https://doi.org/10.5194/egusphere-egu25-17380, 2025.
