The variability of northern Australian rainfall is related to local processes and remote teleconnections, which operate on subseasonal to interdecadal timescales. This includes the Madden-Julian Oscillation, Indian Ocean Dipole, El Niño-Southern Oscillation (ENSO) and Interdecadal Pacific Oscillation. The influence of these climate drivers and local sea surface temperatures (SSTs) on northern Australian rainfall evolves during the wet season, from austral spring through to autumn. Our study shows that ENSO as well as SSTs in the Timor Sea, Arafura Seas and Coral Sea are the key sources of rainfall variability in the pre-monsoonal months September to November. SST indices explained up to 50% of variance in observed northern Australian rainfall in October and November between 1940 and 2023. However, the teleconnection between northern Australian rainfall and ENSO, and also the influence of local SSTs, breaks down with the onset of the Australian summer monsoon in late December. This leads to 0% explained variance in northwestern Australian rainfall and 9% explained variance in northeastern Australian rainfall in January using SST indices only. This presentation will discuss which processes and feedbacks might instead drive rainfall variability over northern Australia during the monsoon season and how they differ from pre- and post-monsoonal conditions.

How to cite: Heidemann, H., Brown, J., and Narsey, S.: What drives variability of the Australian summer monsoon?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-823, https://doi.org/10.5194/egusphere-egu24-823, 2024.

The dominant drivers of boreal summer precipitation variance in tropical and subtropical regions are the Asian and the North African Summer Monsoon. 
Despite extensive investigation into regional precipitation dynamics, the interaction between these monsoon systems remains hardly understood.
This study employs a complex climate network approach based on extreme rainfall events to uncover synchronously occurring heavy rainfall patterns. 
We identify a synchronization trend during the peak monsoon period in July, linking the rainfall in North India to that in the Sahel Zone.
Our findings indicate that La Ni\~na-like conditions in combination with the Boreal Summer Intraseasonal Oscillation (BSISO) foster the synchronization. 
The convective clouds are subsequently transported by an intensified tropical easterly jet toward North Africa, introducing unusual convection over the Sahel region.

How to cite: Strnad, F., Hunt, K., Boers, N., and Goswami, B.: Synchronization patterns of heavy rainfalls between North India and the Sahel Zone on daily timescales, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10229, https://doi.org/10.5194/egusphere-egu24-10229, 2024.

Diurnal Variability is one of the most fundamental modes of the global climate system arising from solar radiation variations. The precipitation over the Indian subcontinent region has significant diurnal variation as per the topographical settings of the landmass. Among the various regions with the maximum diurnal amplitude (MDA) of precipitation over India, MDA over Western Ghat (WG) is perfectly aligned along the coastal boundary.

The peculiarity of the WG region is the position of the coastally aligned mountain range stretching from Gujarat to Kerala with an average elevation of 1200m and during monsoon season with the presence of speedy low-level jet (LLJ), this range acts as a barrier to anchor precipitation over them. The MDA over this region tends to be positioned slightly inland, on the windward side of the hills and is closely associated with the geographic locations of mountain peaks over WG. Here we have attempted to understand the phase and amplitude of diurnal precipitation variation on two distinct physical regimes predominantly governed either by dynamics (DR) or thermodynamics (TR). Based on the speed of the (LLJ) we have identified 370 and 458 days from the monsoon season of 21 years, with dominant physical processes being dynamical and thermodynamical respectively. We found a substantial enhancement of MDA over the entire span of the WG region encompassing the region over the sea, coast, and land during TR which is in stark contrast to DR where MDA is concentrated mostly over the coastal side northern and central western Ghat region. This difference is also visible in the diurnal phase with gradual (abrupt) changes in TR (DR). During TR the weakened LLJ leads to the local thermodynamics to dominate, and very strong land and sea breezes are initiated, along with an unstable hot and humid boundary layer making favorable conditions for diurnal precipitation to take place. This is entirely different in DR wherein the stronger LLJ does not allow to establish a stronger temperature gradient between land and ocean leading to a lessening of diurnal rain. The storm-top height indicates the presence of low-level congestus (deep congestus) clouds during DR (TR). Thus, the diurnal rain, along with cloud types and involved microphysics is totally different in these two physical regimes. This study will be very useful for identifying the errors in the diurnal rain simulated by models segregated by dynamical or thermodynamical processes separately.

How to cite: Verma, U., Pokhrel, S., and Saha, S. K.: Role of Dynamical and Thermodynamical processes in shaping Diurnal Cycle of Rainfall over the Western Ghat of India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18707, https://doi.org/10.5194/egusphere-egu24-18707, 2024.

Since the last couple of decades, western India has been experiencing persistent, intense rain episodes frequently during the summer monsoon season. Most of the pluvial episodes are accompanied by diverse convective systems modulated by the background monsoon circulation. As the climate warms, the changing environmental conditions affect the nature and intensity of the weather systems. This study discusses the evolving large-scale conditions under global warming, along with the recent changes in the occurrence of a special class of heavy-precipitating synoptic systems, the mid-tropospheric cyclones (MTCs). Observed particularly over the Northeast Arabian Sea, MTCs exhibit pronounced mid-level vorticity with minimal signature at the surface. Observational results suggest significant increasing trends in deep convection and heavy precipitation over western India during the summer monsoon season. The background conditions are dominated by warming in the Arabian Sea and the Indian Ocean, accompanied by strengthening of cyclonic circulation and ascending motion at mid-level over western India. An objective vortex identification using reanalysis dataset indicates a rise in the seasonal frequency and duration of heavy precipitating mid-tropospheric cyclonic systems over western India, resulting in a significant amplification of precipitation from these systems. Furthermore, outputs from seven global climate models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are used to assess the potential changes in the large-scale patterns conducive to the development and sustenance of mid-tropospheric cyclonic systems over western India with continued global warming following the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5) scenario. The models project stronger moisture transport over western India that triggers greater moisture convergence along the Indian west coast, aided by elevated water vapor content due to local sea surface warming. We also notice an increase in seasonal mean ascent and relative vorticity, particularly, at the middle troposphere, thereby creating a favorable setting for the occurrence of MTCs and the deep convective clouds in the late 21^st century. This interplay between circulation–convection–precipitation on different spatiotemporal scales over the South Asian monsoon domain carries significant implications for assessment of regional hydrological extremes in a warming climate.

How to cite: K. Mukherjee, S., Dey Choudhury, A., and Krishnan, R.: Heavy-Precipitating Mid-Tropospheric Cyclonic Systems of the Indian Summer Monsoon in a Warming Climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-268, https://doi.org/10.5194/egusphere-egu24-268, 2024.

In the monsoon regions, atmospheric convection is typically stronger over the oceans than over land. Rainfall over land is potentially affected by the dynamic response of the atmosphere to deep convection over the adjacent oceans. Here, we show, in the case of the Indian summer monsoon, that enhanced atmospheric deep convection over the Bay of Bengal ∼2 weeks before onset, advances monsoon onset over India. Since the sea surface temperature of the Bay of Bengal is already hot during spring, warm anomalies further enhance convection that drives a convergence of low-level winds. A part of this circulation response blows from central India to the Bay of Bengal. It paves the way for monsoon circulation over India and advances the onset of monsoon. We tested this hypothesis using an atmospheric model forcing it by warm sea surface temperature anomalies over the Bay of Bengal 10-15 days before monsoon onset.

How to cite: Goswami, B. B. and Muller, C.: What advances monsoon onset over India?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7759, https://doi.org/10.5194/egusphere-egu24-7759, 2024.

In recent decades, several studies have proposed different methodologies to reliably identify the onset of the rainy season in monsoon climates, considering either a single variable, usually precipitation, or a combination of rainfall with other dynamic and thermodynamic variables (e.g. wind, specific humidity). These methodologies tend to define a single onset date, which can fail to diagnose critical characteristics of early season rainfall such as wet/dry spell phasing and intensity. Here, we propose a novel approach to identify the transition from dry to wet seasons as a period (6-8 weeks on average) with a positive gradient of the FFT-filtered precipitation, indicative of a steady increase in rainfall. This approach allows the characterisation of critical onset period rainfall characteristics, including information about wet and dry spell frequency and total precipitation, which is a valuable advance on typical single-date onset methods. Preliminary results considering observational and reanalysis datasets indicate a good agreement between the onset day identified using a traditional methodology and the onset periods over South America and Africa. A more in-depth analysis of the identified onset periods can provide further insights into the role of intraseasonal and interannual variability on the precipitation regimes. We also identified regions with distinct changes in the onset periods timing and related precipitation characteristics when considering present and future climate simulations, including simulations using convective-permitting models. For example, the method is able to distinguish that parts of eastern Brazil are projected to have a later onset period with more intense wet days, whereas in eastern Amazon the key signal is more dry days during the onset period, leading to a weaker intensity of onset. In addition to identifying the rainy season onset periods, this approach also identifies other onset periods, further classified as false onset (interval with positive filtered precipitation gradient followed by the onset period), second onset (interval with a secondary increase in the filtered precipitation gradient after the onset period), or wet spells (occurring during the dry season). These periods provide valuable information about spells of increased precipitation outside the rainy season onset, such as wet spells during the dry seasons or false onsets before the primary rainy season. When recurrent, they can indicate the influence of interannual or intraseasonal variability in off-season precipitation. As the core statistics that emerge from this approach are related to the intensity and phasing of rainfall rather than absolute amounts, future developments will focus on implementing the method in a seasonal forecast system, where only a few months of data are available, with the potential to obtain forecast skill which circumvents absolute rainfall biases.

How to cite: Zilli, M., Hart, N., and Morris, F.: Onset Periods: a novel approach to understand the onset of the monsoon season, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17834, https://doi.org/10.5194/egusphere-egu24-17834, 2024.

The Indian summer monsoon (ISM) supplies over 75% of the country’s annual precipitation, profoundly impacting lives of over a billion people. Significant variability in the timing of its onset and withdrawal has a direct impact on the agricultural sector and other users of water resources. Previous studies have shown that a wedge of mid-tropospheric dry air emanating from the midlatitudes is present over India during early summer, which is much shallower in the vertical toward the southeast of India. Following the strengthening of low-level monsoon winds during the onset, the dry air retreats from the southeast due to increased moistening by shallow cumulus congestus clouds, driving the north-westward progression of the ISM. The withdrawal of the ISM is observed to progress in a southeast direction during September–October, but there is a lack of a conceptual model. In this study, we use observations and the ERA5 reanalysis to understand the dynamics and thermodynamics of the withdrawal. We find that a mid-level dry intrusion re-appears over the northwest of India around mid-September. Vertical profiles associated with this dry air show how the most unfavourable environment for deep convection occurs in the northwest, where the withdrawal occurs first. As the withdrawal progresses, the wedge of dry air deepens throughout its horizontal extent and descends. This stabilises the troposphere, suppressing deep convection and ultimately driving the withdrawal toward the southeast. By mid-October, the dry air engulfs most of India, causing the ISM to withdraw from the entire country. Thus, the strengthening of the mid-level dry advection from the midlatitudes can explain the withdrawal of the ISM, and the mechanism driving the local withdrawal can be considered as the reverse of that at play during the progression of the onset. This work establishes a new paradigm for the withdrawal of the Indian monsoon in terms of midlatitude interactions, which could be tested for other monsoon regions.

How to cite: Deoras, A., Turner, A., Volonté, A., and Menon, A.: The role of midlatitude dry air during the withdrawal of the Indian monsoon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7033, https://doi.org/10.5194/egusphere-egu24-7033, 2024.

The interannual variability (IAV) of All India summer rainfall (AIMR) is low, with a Coefficient of variation (COV) around 9% of the long-term mean. Though regulated by global and regional sea surface temperatures, we explore the cause of low COV of AIMR due to the spatial distribution of rainfall. We find that the variability of AIMR is affected by the spatial covariance between the subregions with different rainfall characteristics, such as the arid western and wet northeast regions. By removing regions, one at a time, from the Indian region, we find that COV increases after removing the Northeast (NE) region due to negative covariance between NE and other sub-regions of India, especially Central India (CI). Further research is ongoing to explore the moisture distribution over the subregions and understand the negative covariance using a moisture tracking algorithm. We plan to investigate the contributions to rainfall distribution from oceanic and terrestrial sources. This study may reveal how the spatial distribution of rainfall influences the IAV of AIMR, emphasizing the significance of terrestrial and oceanic moisture contributions.

How to cite: Chandel, V. and Ghosh, S.: Role of spatial covariance in regulating interannual variability of Indian Summer Monsoon rainfall, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14603, https://doi.org/10.5194/egusphere-egu24-14603, 2024.

A weak Indian summer monsoon (ISM) strengthens an El Niño via generating a cyclonic circulation over the northwestern Pacific. The westerly anomaly on the southern flank of this cyclone generates eastward anomaly in the mixed layer, induces warm zonal advection, and excites oceanic downwelling Kelvin waves, deepening the thermocline in equatorial eastern Pacific and resulting in cold vertical advection. The influence of monsoon-induced Pacific wind anomaly on ENSO is mainly achieved by changing the zonal advective feedback and thermocline feedback. CMIP6 models show a large diversity for the impact of ISM on ENSO, related to the diverse amplitudes of ISM among the models. Models simulating a stronger ISM display more robust features of ISM-induced anomalous circulation over the northwestern Pacific, and the larger equatorial wind anomalies on the south flank of the anomalous circulation affect ENSO evolution more significantly by causing stronger ocean-atmosphere coupling processes.

       The future changes in the ISM’s impacts on ENSO also exhibit a large spread among the CMIP6 models. The uncertainty in the projections is linked to the diverse changes in the response of anomalous circulation over the northwestern Pacific to ISM. The models showing an increased (decreased) sensitivity of anomalous circulation over the northwestern Pacific to ISM simulate enhanced (weakened) ISM’s impacts on ENSO under global warming, even though the amplitudes of ISM remain unchanged

How to cite: Yang, S., Lin, S., and Dong, B.: Dynamical Processes of the Impact of Indian Summer Monsoon on ENSO: Observation, Model Simulation and Future Change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13998, https://doi.org/10.5194/egusphere-egu24-13998, 2024.

Previous studies agree on an impact of the Atlantic Multidecadal Variability (AMV) on total seasonal rainfall amounts over the Sahel. However, whether and how AMV affects the distribution of rainfall or the timing of the West African Monsoon is not well known. Here we seek to explore these impacts by analyzing daily rainfall outputs from climate model simulations with an idealized AMV forcing imposed in the North Atlantic, which is representative of the observed one. The setup follows a protocol largely consistent with the one proposed by the Component C of the Decadal Climate Prediction Project (DCPP-C). We start by evaluating model's performance in simulating precipitation, showing that models underestimate it over the Sahel, where the mean intensity is consistently smaller than observations. Conversely, models overestimate precipitation over the Guinea Coast, where too many rainy days are simulated. In addition, most models underestimate the average length of the rainy season over the Sahel, some due to a too late monsoon onset and others due to a too early cessation. In response to a persistent positive AMV pattern, models show an enhancement in total summer rainfall over continental West Africa, including the Sahel. Under a positive AMV phase, the number of wet days and the intensity of daily rainfall events are also enhanced over the Sahel. The former explains most of the changes in seasonal rainfall in the northern fringe, while the latter is more relevant in the southern region, where higher rainfall anomalies occur. This dominance is connected to the changes in the number of days per type of event: the frequency of both moderate and heavy events increases over the Sahel’s northern fringe. Conversely, over the southern limit, it is mostly the frequency of heavy events which is enhanced, affecting the mean rainfall intensity there. Extreme rainfall events are also enhanced over the whole Sahel in response to a positive phase of the AMV. Over the Sahel, models with stronger negative biases in rainfall amounts compared to observations show weaker changes in response to AMV, suggesting systematic biases could affect the simulated responses. The monsoon onset over the Sahel shows no clear response to AMV, while the demise tends to be delayed and the overall length of the monsoon season enhanced between 2 and 5 days with the positive AMV pattern. The effect of AMV on the seasonality of the monsoon is more consistent to the west of 10ºW, with all models showing a statistically significant earlier onset, later demise and enhanced monsoon season with the positive phase of the AMV. Our results suggest a potential for the decadal prediction of changes in the intraseasonal characteristics of rainfall over the Sahel, including the occurrence of extreme events.

How to cite: Mohino, E., Monerie, P.-A., Mignot, J., Diakhaté, M., Donat, M., Roberts, C. D., and Doblas-Reyes, F.: Impact of AMV on rainfall intensity distribution and timing of theWest African Monsoon in DCPP-C-like simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1774, https://doi.org/10.5194/egusphere-egu24-1774, 2024.

In a retrograde Earth simulation using the fully coupled MPI-ESM, we find that the climate in the Sahara goes from arid to monsoonal. By understanding this transition of the Sahara, we can gain insights into some of the key processes necessary for the existence of monsoons. We find that theories of monsoons based on land-sea thermal contrast and meridional shifts in the interhemispheric convergence zone (ITCZ) are not adequate to explain this change in the climate of the Sahara. Hence, we use the energetics of monsoons, which is based on local moist static energy and moisture budgets. In the regular forward-rotating Earth, the net energy input into the atmospheric column (NEI) is negative over the Sahara, implying a net energy import over the region. The reversed winds in the retrograde simulation advect moisture from the Arabian Sea and the equatorial Atlantic into the Sahara during the boreal summer. The greenhouse effect of water vapor instantaneously reduces the outgoing longwave radiation, thereby increasing the NEI. As NEI becomes positive, the Sahara exports energy, increasing convection (and, hence, monsoon precipitation). The increased cloud cover further enhances NEI through cloud radiative feedback, strengthening the monsoon. Therefore, we conclude that the radiative effects of water vapor and clouds are an essential ingredient for monsoons.

How to cite: Jalihal, C. and Mikolajewicz, U.: The role of water vapor and cloud radiative effects in monsoons: Perspectives from retrograde Earth simulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16987, https://doi.org/10.5194/egusphere-egu24-16987, 2024.

Climatic fingerprint of Heinrich (H) events was characterized by widespread megadroughts over the Asian monsoon region, accompanied by a systemic weakening of Asian summer monsoon. However, recent hydroclimate proxies suggest that South China experienced increased precipitation contrasting with the prevalent megadrought conditions during the Heinrich events. Our simulations performed with the HadCM3 model show that changes in insolation alone can induce spatiotemporal discrepancies in precipitation over the Asian summer monsoon region. During the H1, 3, 4, 5, 6 events, the amplification of the land-sea pressure contrast in response to a positive solar insolation gradient during boreal summer intensifies moisture transport from the ocean to the Asian monsoon region. The ensuing moisture divergence, combined with anomalous downdrafts, results in decreased precipitation in the South Asian Summer Monsoon (SASM) region, but converse situation for the East Asian Summer Monsoon (EASM) region. During the H2 event, the increased precipitation across the Yangtze River Valley sharply contrasts with the widespread drought over the ASM region. This is attributed to an enhancement of a southerly warm-moist vapor transport along the western edge of the subtropical Western North Pacific anticyclone and an enhancement of a northerly cold-dry vapor transport along the western edge of the Aleutian cyclone, which converge over the Yangtze River Valley. Our results further show an in-phase relationship between the SASM and EASM circulation strengths in response to orbital forcing. This is driven by the combined influence of the land-sea thermal contrast and the migration of the Intertropical Convergence Zone, supporting Kutzbach's hypothesis.

How to cite: Liang, M., Yin, Q., Sun, Y., Zhang, C., Wu, Z., and Liu, W.: Distinct response of Asian summer precipitation and monsoon circulation to orbital forcing during Heinrich events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11517, https://doi.org/10.5194/egusphere-egu24-11517, 2024.

Locust infestation has been a serious threat to agriculture and its occurrence of locust infestation is closely related to the climate condition, especially drought. Because agriculture was the main economic activity of China in historical time, damages on agricultural produce due to locust infestation had been recorded continuously in national chronicles for more than 2000 year. In this study, we will utilize the locust infestation records in Chinese historical documents in 1358-1911 to form temporal and spatial series, perform statistical analyses and infer possible changes in East Asian monsoon climate during this period.

We will utilize the digitized meteorological record database in China, called REACHES (Reconstructed East Asian Climate Historical Encoded Series. See Wang et a., 2018, Nature: Scientific Data, 5, 180288), to extract locust records in 1358-1911 corresponding to Ming and Qing dynasties of China to perform analysis. In a previous study (Lin et al., 2020) we had shown that the locust infestation is closely related to the general drought condition in Qing dynasty (1644-1911). In the present study we expand the total period length to include Ming dynasty. We will perform time series analysis as well as spatial analysis to understand the relation of locust infestation and other climate variables.

Previous studies of locust infestation in East Africa by United Nations show that the movement6s of locust swarms are closely related to monsoon fronts. Our preliminary analysis shows that this also appears to be the case in the movements of East Asian locusts. Thus it is possible that we can use the locust infestation series to reconstruct past changes in East Asian monsoon climate.

How to cite: Wang, P. K.: Locust infestation in China in 1358-1911 and its relation with changes in East Asian monsoon climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7856, https://doi.org/10.5194/egusphere-egu24-7856, 2024.

Understanding the impact of future land use changes on the global monsoon system is crucial for the economy, water supply and food security. Here, we use future deforestation and afforestation simulations under different SSP scenarios from 10 CMIP6 models participating in the Land Use Model Intercomparison Project (LUMIP). We apply an energy flux potential (EFP) framework to connect shifts in the Intertropical Convergence Zone and regional monsoons to changes in the atmospheric energy transport, and examine the contribution from individual flux components (latent heat flux, sensible heat flux, shortwave and longwave radiation). The linearity of this method allows us to attribute atmospheric EFP changes to different land and ocean regions without the need for additional simulations.

We find consistent zonal-mean precipitation shifts over oceanic regions across models in the deforestation and afforestation scenarios. However, changes in the global monsoon (as represented by zonal-mean precipitation changes over land) show large model dependence. The energy flux analysis reveals a consistent mechanism across models: The surface latent heat flux is the dominant driver of land use-induced changes in EFP in the tropics. In most regions and models, an increase in the latent heat flux component of EFP corresponds to tropical precipitation decrease and vice versa.

Our regional analysis reveals that remote oceanic energy-budget anomalies are the main contributor to the global EFP patterns and monsoon precipitation anomalies for all models, while land energy-budget anomalies modulate both patterns over land. Decomposing the EFP pattern into the contribution from different land regions indicates model consensus regarding the strong contribution from North and South America to the land-only anomaly, while inter-model differences primarily stem from different model responses to African land use change. These findings highlight the complexity of rainfall shifts to future land use change scenarios and also emphasize the value of the energy flux potential method to quantitatively link remote forcing to regional rainfall changes.

How to cite: Fahrenbach, N. L. S., Jnglin Wills, R., and De Hertog, S. J.: Future land use influences on the global monsoon: An energetic perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1908, https://doi.org/10.5194/egusphere-egu24-1908, 2024.

Global Earth System Models at storm-resolving resolutions (SR-ESM, with horizontal resolutions of ~4km) are being developed as part of the nextGEMS collaborative European EU’s Horizon 2020 programme.  Within the Storms & Ocean theme, we are exploring how resolving convective storms, ocean mesoscale eddies, and air-sea interaction on these scales influences tropical circulations and associated precipitation, and their variability.

In this talk, we evaluate the representation of the characteristics of the wet season over core monsoon regions in these SR-ESM, which include assessment of the seasonal cycle of precipitation, the timing of monsoon onset and retreat, and the total accumulated precipitation. These existing biases are compared to those seen in CMIP6 models and  interpreted through the lens of both local and remote moist energy diagnostics based on modern theories of monsoons. Local diagnostics include relative moist static energy (MSE) defined as the difference between local and tropical-mean near surface MSE, which has been recently introduced as a simple measure of the lower and upper-level influences on convective stability and shown to correlate well with monsoon onset dates in both CMIP6 simulations (Bombardi and Boos 2021) and idealized aquaplanet simulations we have conducted. The influence of possible remote biases, such as those of extratropical origin, are explored through analysis of the equator-to-pole MSE gradient. This contrast is central to vertically integrated energy budget frameworks that link changes in monsoonal precipitation to changes in meridional energy fluxes, which in turn scale with the meridional near-surface MSE gradients under diffusive approximations. Biases in this gradient result in smaller or greater advection of low-level MSE into monsoon regions, hence resulting in wet or dry biases, respectively, in monsoonal rainfall.

How to cite: Bordoni, S. and Tompkins, A. M.: Monsoon precipitation biases in storm-resolving NextGEMS Earth System Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10455, https://doi.org/10.5194/egusphere-egu24-10455, 2024.

The South Asian summer monsoon (SASM) is a significant monsoon system that exerts a profound impact on climate and human livelihoods. According to 38 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), the SASM circulation is projected to weaken significantly under global warming as seen in the weakened low-level westerly wind over the northern tropical Indian Ocean; however, the associated climate dynamics is still under debate. Here, we identify that the weakened low-level westerly wind is closely related to the enhanced diabatic heating over the Tibetan Plateau (TP), which corresponds with increased summer precipitation in the future. Further analyses and numerical experiments suggest that the intensified TP heating triggers an anomalous meridional circulation with ascending motions over the plateau and descending motions to the south, leading to an anomalous low-level anticyclone over the northern tropical Indian Ocean. This anticyclone greatly weakens the prevailing low-level westerly wind of the SASM through easterly anomalies at the anticyclone’s southern flank. Moisture budget analysis further reveals that increased atmospheric water vapor, rather than the vertical dynamic component, makes the largest contribution to the increased precipitation over the TP. This result confirms that the enhanced TP heating is a driver of atmospheric circulation change and contributes to weakening the SASM circulation.

How to cite: Luo, H., Wang, Z., He, C., Chen, D., and Yang, S.: Future changes in South Asian summer monsoon circulation under global warming: Role of the Tibetan Plateau heating, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5941, https://doi.org/10.5194/egusphere-egu24-5941, 2024.

Monsoons have been frequently severe in Pakistan in the last few decades, leading to extreme droughts and floods of unprecedented proportions. The wide belief is that these changing precipitation patterns are mainly due to climate change. However, considering this region's long history of floods and droughts, it is unwise to rule out the role of natural climate variability without a careful diagnosis. This study examines the contribution of oceanic and atmospheric variability to unusual precipitation distributions in Pakistan. We find that variations in sea surface temperatures in the tropical Pacific and northern Arabian Sea and internal atmospheric variability related to the circumglobal teleconnection pattern and the subtropical westerly jet stream account for 74% of monthly summer precipitation variability in the 21st century. Several of these forcings have co-occurred with record strength during episodes of extreme monsoons, compounding the overall effect. Climate change may have contributed to increased variability and the in-phase co-occurrences of the identified mechanisms, but further research is required to confirm any such connection.

How to cite: Ashfaq, M., Johnson, N., Kucharski, F., Diffenbaugh, N., Abid, A., and Evans, K.: Climate change is not the primary cause of extreme monsoons in Pakistan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4822, https://doi.org/10.5194/egusphere-egu24-4822, 2024.

Mountain snow cover is an integral part of our climate system that impacts ecosystems and the biosphere that rely on river systems. In recent years, the Hindu Kush-Himalayan regions have experienced a significant decline in snow cover, which is primarily attributed to global warming. However, understanding the nonlinear trends associated with these changes remains a challenge. Here, we explore the relationship between snow cover change and monsoon dynamics within the context of a changing climate, specifically examining the role of land-atmosphere interaction. The study's findings reveal a clear connection between the declining snow cover and monsoon circulation, which is explained through multiple models and grounded in mean state changes. This result highlights the crucial role of snow cover in the dynamics of monsoon.

How to cite: Blau, M. T., Kad, P., Turton, J. V., and Ha, K.-J.: Impact of Hindu Kush Himalayan snow cover change on Monsoon Circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11919, https://doi.org/10.5194/egusphere-egu24-11919, 2024.

Although the South American monsoon (SAM) is the main source of precipitation over most of the continent, the effects of anthropogenic climate change on it remain unclear. Most recent projections from CMIP6 multimodel ensembles show very weak signal of total SAM precipitation change, and sometimes climate change information from different sources seems confusing and contradictory for the public and decision makers. As SAM affects the most populated areas and those with largest contribution to agricultural production and hydroelectric power generation, its future behavior should be clearly detailed and supported by a dynamical framework able to explain it, so as to better serve decision-makers in planning actions to respond to climate change and adopt effective policies for climate adaptation.

The existence of a dynamic framework that explains the major climate changes projected by the best-performing models gives coherence to the different monthly changes throughout the monsoon season, which otherwise seem incomprehensible and can lead to discrepant interpretations if not understood within a correct dynamic context. The lack of significant future change in total monsoon precipitation does not mean that there are no changes of great interest in different phases of the monsoon season.

There are two aspects that prompted the approach of the present study: i) model projections of future SST indicate an El Niño-like warming pattern in the central-east equatorial Pacific; ii) the impacts of the present climate El Niño events on South America (SA) display a tendency to spring-summer reversal of precipitation anomalies in central-east SA (CESA), which results in little or no change in the total monsoon precipitation in this region.

Twelve CMIP6 selected models were evaluated not only for their simulation of South American climatology, but also for their simulation of ENSO and its impacts on SA. Several of them did not produce satisfactory ENSO. The changes projected by the ensemble of seven models that best reproduced ENSO and the climatology of SA indicate a more EN-like future climate. Consistently, the main climate changes projected for the SAM resemble the observed EN impacts, remarkably including the tendency to spring-summer reversal of precipitation anomalies in CESA, from dryer spring to wetter summer. While the total monsoon precipitation shows little or no change in this region, there is reduction of early monsoon rainfall and increase of the peak season rainfall, which results in a delay and shortening of the monsoon season. The dynamical effect of the EN-like SST changes shapes the spring response via teleconnection, and thermodynamical processes trigger the changes from spring to summer in CESA, which is part of the core monsoon region. Also coherently with EN impacts, drier conditions prevail in central-northern-eastern Amazon throughout the monsoon season thanks to changes in the Walker circulation, while in southeast SA, precipitation increases due to tropics-extratropics teleconnection.

The changes projected by the all-model ensemble are much weaker and confusing. This clear description of climate change throughout the monsoon season and its connection with intensified EN effects is easy to understand and use, as these effects are reasonably known.

How to cite: Grimm, A. M. and Padoan, D.: Towards robust and actionable information on monsoon climate change in South America, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13343, https://doi.org/10.5194/egusphere-egu24-13343, 2024.

Large-scale perturbations in land surface characteristics have been found to induce disturbances in the overlying atmosphere via land-atmosphere coupling. The perturbations can lead to changes in hydroclimatic variables, such as precipitation and air temperature, or in atmospheric circulation patterns. However, the local and remote atmospheric responses to continental-scale changes in land surface water have not been well studied in Australia. In this study, using the Community Earth System Model 2 (CESM2) of the National Center for Atmospheric Research (NCAR), we investigate the changes in Australian monsoon, which primarily impacts the northern Australian climate, in response to an extreme surface condition: the whole Australia being treated as a shallow lake in model simulations. The simulation results show that a continental-scale lake would extend the Australian monsoon season via earlier onset and later end. We find that the most significant changes in the simulated precipitation occur during the pre-monsoon period (e.g., early October to mid November). Considering that the traditional scheme used to explain monsoonal rainfall by the theory of land-sea thermal contrasts is not consistent with the simulated precipitation patterns, this study analyzes the changes in moist static energy (MSE) budget, the simulation with a hypothetical lake features an atmospheric condition that favors the formation of precipitation: increased moisture convergence and dry static energy divergence, which might be associated with the increased net energetic forcing and export of MSE. We also confirm the dominant role of atmospheric circulation in determining the variability of precipitation over northern Australia in wet season via examining the regional moisture recycling. A relative impact computation upon components in the moisture budget shows that the dynamic component of the vertical advection of moisture contributes the most to the temporal evolution of precipitation over northern Australia in wet season.

How to cite: Yang, Z., Narsey, S., Ryu, D., Peel, M., Lo, M.-H., and McColl, K.: The Impacts of Inundating Australia on Australian Monsoon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4467, https://doi.org/10.5194/egusphere-egu24-4467, 2024.