Intraseasonal variability of extreme rainfall events (EREs) during the South Asian Summer Monsoon season is dominated by the Boreal Summer Intraseasonal Oscillation (BSISO). However, deviations from its canonical north-eastward propagation are poorly understood, posing challenges to the prediction of EREs and climate modeling. Here, we combine a climate network-based approach determining regions of synchronously occurring EREs with
a clustering analysis of zonal and meridional BSISO propagation patterns which reveals three distinct modes: canonical north-eastward, eastward-blocked, and stationary propagation. We show that Pacific sea surface temperature background states determine the propagation mode. In particular, El Niño (La Niña)-like conditions favor the stationary (eastward-blocked) mode by modifying the zonal and meridional overturning circulation structures and the strength of the BSISO Kelvin wave component. The uncovered mechanism for BSISO diversity has implications for the predictability of large, spatially extensive EREs in South Asia and the development of early warning signals on a time horizon of 3-5 weeks.

How to cite: Strnad, F., Schlör, J., Geen, R., Boers, N., and Goswami, B.: Boreal Summer Intraseasonal Oscillation extreme rainfall propagation modulated by Pacific sea surface temperatures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6671, https://doi.org/10.5194/egusphere-egu23-6671, 2023.

The northward propagating intraseasonal Oscillation (ISO) is one of the dominant modes of tropical variability during Boreal summers. Several mechanisms have been proposed to explain northward propagation. Yet the factors that decide the ISO rainfall over a particular region remains elusive. in this  study we show that the ISO rainfall anomalies weaken across the south Bay of Bengal (SBoB) before they re-strengthen over the north Bay of Bengal (NBoB). We use the moisture budget to understand the reason for the same. We find that the horizontal moisture flux convergence predominantly controls the ISO rainfall anomalies over the two regions. Further analyses reveal that the convergence of background moisture by the ISO wind perturbations decides the ISO rainfall structure. We hypothesize that the weaker rainfall anomalies in the SBoB result from the weaker background column relative humidity and moisture, which do not allow the initial dynamic perturbations to grow as fast as they do in an environment with stronger background relative humidity and moisture (NBoB). 

How to cite: Kottapalli, A. and Pn, V.: Role of background moisture in dictating the Intraseasonal Rainfall over Bay of Bengal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-426, https://doi.org/10.5194/egusphere-egu23-426, 2023.

Monsoon intraseasonal oscillation (MISO) is an important aspect of the monsoon variability on various timescales, accounting for short-term variability as well as about 40% of total seasonal rainfall variance. MISO plays an important role in modulating the active (wet) and break (dry) spells of monsoon, and its low-frequency component has a time period of 30-60 days and exhibits northward propagation from the equatorial Indian Ocean to the Himalayan foothills. This northward propagation is generally attributed to generation of positive barotropic vorticity to the north of the previous convection centre. However, using ERA5 reanalysis composites we show that the relation between convection centre and positive barotropic vorticity undergoes significant change as MISO propagates away from the equator. Close to the equator (0-15°N), barotropic voriticty is either in-phase or leads rainfall, whereas further poleward (15°N-25°N), this relationship reverses and rainfall leads vorticity by 1-2 days. This contrast is closely tied to changes in the vertical structure of vorticity: near the equator, the vorticity maximum lies in the middle troposphere, while poleward of 15°N it is in the lower troposphere. The vorticity budget at each pressure level reveals the importance of vertical advection of vorticity for its near-barotropic structure, together with the importance of thermodynamic influences on vorticity, especially poleward where the vortex stretching term grows. Such findings point to the central role of feedback on the dynamics from the thermodynamic processes away from the equator. Furthermore, it closely ties the ability of models to reproduce MISO to their ability to represent convective processes.

How to cite: Masiwal, R., Dixit, V., and Seshadri, A. K.: Role of thermodynamic processes in driving Monsoon Intraseasonal Oscillations (MISO) away from the Equator, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-436, https://doi.org/10.5194/egusphere-egu23-436, 2023.

The present study is aimed to investigate the rainfall characteristics of Monsoon Deep Depressions (MDD) originating over the Bay of Bengal (BoB) basin using a coupled ocean-atmospheric model (COAWST) and a stand-alone atmospheric (WRF) model with a lead time of up to 72h. It is found that though the tracks of the four MDDs considered in the study have been reasonably simulated, the intensity was overestimated in both sets of simulations compared to India Meteorological Department (IMD) best estimates. Upon decomposition of the contributors to the rainrate for the composite of the storms in the deep depression (DD) phase, it was found that the moisture sources/sinks play a more important role than the cloud sources/sinks in modulating the rainfall processes. Further analysis of the moisture sources/sinks showed that the horizontal and vertical advection are the major drivers in modulating the contribution of the moisture sources/sinks. The validation of rainfall using CMORPH datasets suggested that the coupled simulations had a higher skill in rainfall prediction. Furthermore, the composite of different components of moisture sources/sinks (especially vertical advection) was found to be more realistically simulated in COAWST compared to WRF upon validation with MERRA datasets. Analysis of the composite energetics showed that scarcity of bulk kinetic energy in the later hours of the DD phase in COAWST led to the dissipation of the storm core, which led to better prediction of rainfall. On the other hand, a re-intensification of the storm core by means of condensational heating led to an overestimation of rainfall in WRF, which finally resulted in lower skill in rainfall prediction. In spite of the stand-alone atmospheric model capturing the horizontal moisture incursion in the lower levels significantly, the better representation of the vertical structure enabled the coupled model to capture the precipitation features more realistically, increasing skill in rainfall prediction.

How to cite: Chakraborty, T., Pattnaik, S., and Baisya, H.: A Numerical Study to Investigate Precipitation Features of Monsoon Deep Depressions over Bay of Bengal: Comparison of Coupled and Control Simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11257, https://doi.org/10.5194/egusphere-egu23-11257, 2023.

The Australian monsoon's wet season is associated with sequences of wet and dry conditions known as bursts and breaks, which usually have timescales of a week or two. There are several hypotheses for the physical processes involved in monsoon bursts, ranging from the effects of the Madden-Julian Oscillation to extratropical influences.

We analyse rainfall bursts in Northern Australia using a moist static energy (MSE) budget framework. First, we separate the bursts into pre-monsoon, monsoon, and post-monsoon based on simple monsoon onset and retreat criteria. We then apply ERA5 data to calculate the MSE budget for each burst and construct composite bursts for each of the three types.

We find that the horizontal advection of MSE over the tropical northern Australian convergence zone is the most critical term in the budget for the day-to-day precipitation variation. An analysis of the MSE-related gross moist stability (GMS) reveals that the GMS framework is able to predict periods of convective growth and decay before and after monsoon bursts, with the exception of the pre-monsoon bursts which do not follow the characteristic evolution of tropical convective systems. We hypothesise that this is because pre-monsoon bursts have a stronger extratropical influence. We find that the growth phase of convection in monsoon and post-monsoon bursts is associated with a notable reduction of the advection of dry air into the monsoon region. We show that this is likely the result of a rearrangement of the circulation ahead of the burst.  

How to cite: Mohanty, S., Jakob, C., and Singh, M.: Australian Summer Monsoon Bursts: A Moist Static Energy Budget Perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2469, https://doi.org/10.5194/egusphere-egu23-2469, 2023.

Many global-climate models have substantial biases in their predictions of the Asian monsoon. For example, the Met Office Unified Model predicts a  monsoon trough that is too zonal and therefore underestimates summer rainfall over south and east Asia. These errors have persisted over many cycles of research-to-operations, and appear robust to significant developments in all major parametrizations in the model. Here, we address a simple question: why are these biases systematic? That is, why have they not been removed by optimization of parameters in the model's physics? Using a Perturbed Parameter Ensemble of AMIP simulations, we show that a strong constraint exists which prevents the Unified Model from simultaneously producing an unbiased monsoon and unbiased global top-of-atmosphere radiation fluxes. We use this constraint to define a scalar parameter, the "structural bias"  of the ensemble, the magnitude of which measures the conflict between the constraints and therefore how "untunable" the model is. We identify the drivers of this parameter, show that it is related to an inability to independently affect the properties of tropical and extra-tropical clouds, and suggest ways in which it could be reduced in future model versions.

How to cite: Furtado, K., Martin, G., Sexton, D., Rostron, J., and Field, P.: Why is there a systematic bias in the Asian Monsoon in the Met Office Unified Model?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6573, https://doi.org/10.5194/egusphere-egu23-6573, 2023.

State-of-the-art models show significant climatological biases in their simulation of East Asian Summer Monsoon (EASM) rainfall, with biases even more pronounced in atmosphere-only simulations versus simulations with a coupled-ocean. It has further been noted that systematic evapotranspiration biases occur locally over East Asia, and globally over land, in simulations both with and without a coupled ocean. Here, we explore a possible role for evapotranspiration in EASM precipitation biases.

Idealized model simulations are presented in which the parameterization of land evaporation is modified. The results suggest a feedback whereby excessive evapotranspiration over East Asia can result in cooling of land, a weakened monsoon low, and a shift of rainfall from the Philippine Sea to China, moistening land and further fueling evapotranspiration. Cross-model regressions against evapotranspiration over China indicate that a similar pattern of behavior is seen in Atmosphere Model Intercomparison Project (AMIP) simulations.

In AMIP, the feedback is not explained by a too-intense global hydrological cycle or by differences in radiative processes. Analysis of land-only simulations indicates that evapotranspiration biases are present even when models are forced with prescribed meteorological conditions. These biases are strengthened when the land model is coupled to the atmosphere, suggesting a role for land-model errors in driving atmospheric biases. Coupled atmosphere-ocean models are shown to have similar evapotranspiration biases to those in AMIP over China, but different precipitation biases, including a northward shift in the Intertropical Convergence Zone over the Pacific and Atlantic oceans.

How to cite: Geen, R., Pietschnig, M., Agrawal, S., Dey, D., Lambert, F. H., and Vallis, G.: Land evaporation biases link to East Asian rainfall shifts across AMIP simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2580, https://doi.org/10.5194/egusphere-egu23-2580, 2023.

The simulation of precipitation in the African Sahel region is challenging for current global climate models. These models conventionally work with grids with horizontal resolution larger than 100 km and therefore must use parametrization schemes to simulate deep convection. The nextGEMS project, on the other hand, performs global simulations with two new climate models (adapted versions of ICON and IFS) with fine resolutions of a few kilometers. At such high resolution, deep convection is resolved, which allows for a much more realistic representation of precipitation. This is particularly promising for simulating convection in the African Sahel, where most precipitation originates from mesoscale convective systems resolved at these simulation scales. 

We present a preliminary analysis of the cycle two nextGEMS simulations focusing on Sahel precipitation and the West African monsoon. We show that some characteristics of precipitation, such as low autocorrelation with one day lag, are much closer to measurements compared to conventional climate models. We also discuss some of the problems that still persist in the simulations and compare the two models depending on different features of Sahel precipitation.

How to cite: Spät, D., Schuhbauer, D., and Voigt, A.: Characteristics of African Sahel Precipitation in global storm-resolving Climate Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13727, https://doi.org/10.5194/egusphere-egu23-13727, 2023.

In this study, we present the results of a regional model (regional spectral model-regional ocean model [(RSM-ROMS]) simulation of the South Asian Summer Monsoon (SASM). The RSM-ROMS integration is carried out at 20 km grid spacing over a period of 25 years (1986–2010). The simulation is forced by global atmospheric and oceanic reanalysis. The RSM-ROMS simulation shows a realistic alignment of the simulated rainfall along the orographic features of the domain. Furthermore, the RSM-ROMS simulates the observed feature of convection over continental SASM region being more vigorous with dominance of mixed warm and cold phase hydrometeors in contrast to the dominance of the warm rain process in the neighboring tropical oceans. Similarly, the upper ocean features of contrasting mixed layer and thermocline depths between the northern and equatorial Indian Ocean are also simulated in the RSM-ROMS. Intra-Seasonal Oscillation (ISO) of the SASM at 10–20 and 20–70 days are also simulated in the RSM-ROMS with many of its features verifying with observations. For example, the 20–70 days ISO are of higher amplitude and its meridional propagation is slower in Bay of Bengal compared to that over Arabian Sea. Additionally, RSM-ROMS shows 12.3 Monsoon Low Pressure Systems (LPSs) per season that is comparable to 14.6 per season from observations. Furthermore, the observed intraseasonal contrasts of LPS between the wet and dry spells of ISO is also reproduced in the RSM-ROMS.

How to cite: Misra, V. and Jayasankar, C. B.: Dynamic Downscaling the South Asian Summer Monsoon From a Global Reanalysis Using a Regional Coupled Ocean-Atmosphere Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1014, https://doi.org/10.5194/egusphere-egu23-1014, 2023.

The heterogeneities arising out of surface variabilities, land-sea contrasts, aerosol concentrations, and the influence of orography define the intricate characteristics of regional monsoon systems. The amount of precipitation India receives during the boreal summer monsoon season can be modulated by land surface processes due to its influence on moisture availability and atmospheric stability. This study investigates the impact of vegetation changes on the seasonal mean precipitation over Indian land using fully coupled global climate model (GCM) simulations with idealized land cover. In addition, an  energetics framework is employed to unravel the physical mechanisms/pathways connecting vegetation and rainfall. In general, evaporation enhances with an increase in forest cover. However, this does not translate to a similar increase in all-India averaged precipitation. Using the energetics approach, we find that precipitation changes primarily happens via three different thermodynamic pathways. We also find the regions where each pathway is dominant. The relative dominance of these pathways in various areas leads to spatial inhomogeneities in the precipitation response due to vegetation changes. Human intervention, including agricultural expansion, has reshaped the landscape of India in the last century, altering the nature of land-atmosphere interactions. The results from this study, that land cover plays a significant role in modulating the regional characteristics of seasonal monsoon precipitation, are particularly important in this context. The findings in this study also have broader ramifications since the dominant region-specific mechanisms identified are expected to be valid for other forcings and are not just limited to the scenarios considered here. A unified framework connecting these various forcings with monsoon variability would be of great practical importance, and the present study is an advancement in this regard.

How to cite: Samuel, J. B., Chakraborty, A., and Paleri, A.: Impact of vegetation on the boreal summer monsoon precipitation over India: an energetics viewpoint., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3747, https://doi.org/10.5194/egusphere-egu23-3747, 2023.

Low-pressure systems (LPS) are convectively coupled vortices that contribute nearly half of the summer monsoon rainfall over the Indian subcontinent. About one-third of the boreal summer monsoon LPS are caused by downstream amplification of westward propagating disturbances from the western North Pacific (WNP). Analysis of downstream LPS events from 1979 to 2017 reveals that 43% of them are caused by extratropical stratospheric air intrusions over the WNP. Stratospheric air intrusions lead to high tropospheric potential vorticity (PV), and the downstream vortex seeds are observed to initiate and intensify to the southwest of the PV anomalies. The PV anomalies can deform the temperature in its neighborhood and cause adiabatic lifting, which in turn can induce and intensify low-level cyclonic vortices. The subsequent intensification of the low-level vortex is aided by deep convection, observed to the southwest of the PV anomaly, through vortex stretching and low-level PV generation by diabatic heating.

How to cite: Selvakumar, V., Ettamal, S., and Sukumaran, S.: Extratropical Stratospheric Air Intrusions Over the Western North Pacific and the Genesis of Downstream Monsoon Low-Pressure Systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2322, https://doi.org/10.5194/egusphere-egu23-2322, 2023.

The lower tropospheric circulation over the eastern Mediterranean during boreal summer is markedly characterized by northerly winds known as the Etesians (Tyrlis et al., 2013). These winds are accompanied by large scale subsidence and clear skies, and can mitigate the emergence of heat waves by bringing colder air from the Eurasian landmass. Here, we employ Causal Effect Networks, obtained by applying the Peter and Clark Momentary Conditional Independence (PCMCI) causal discovery algorithm (Runge, 2018), to identify causal precursors of the Etesians both in mid-latitude circulation fields and tropical convective activity at two different intraseasonal time scales (3 and 7-day average). We identify wave train activity over the North Atlantic and North American region and convective activity over the Arabian Sea and western coast of the Indian peninsula to be causal precursors of Etesians winds defined as 850 hPa meridional wind variations over the eastern Mediterranean at a lag of 3-to-6 days. In general, the influence of tropical drivers, i.e. the Indian summer monsoon (ISM) system,  is found to be stronger than that of the mid-latitude wave train, thus corroborating the hypothesis that the ISM affects the circulation over the Mediterranean and Northeast Africa, as suggested by the monsoon-desert mechanisms (Rodwell and Hoskins, 1996). Moreover, at longer time scales (7 to 14-day lag), the main causal influence comes from tropical convective activity over the Indian peninsula, while the effect of the mid-latitude circulation weakens and becomes not significant. We finally employ event coincidence analysis to explore the relationship between Etesians and heat extremes in the eastern Mediterranean and assess the presence of trends in the strength of Etesians outbreaks at intraseasonal variability in the historical period.

References

Rodwell, M. J. and Hoskins, B.: Monsoons and the dynamics of deserts, Q. J. R. Meteorol. Soc., 122, 1385–1404, 1996.

Runge, J.: Causal network reconstruction from time series: From theoretical assumptions to practical estimation, 28, 075310, https://doi.org/10.1063/1.5025050, 2018.

Tyrlis, E., Lelieveld, J., and Steil, B.: The summer circulation over the eastern Mediterranean and the Middle East: Influence of the South Asian monsoon, Clim. Dyn., 40, 1103–1123, https://doi.org/10.1007/s00382-012-1528-4, 2013.

How to cite: Di Capua, G., Diedrich, D., Tyrlis, E., Matei, D., and Donner, R. V.: Indian summer monsoon versus mid-latitude drivers of boreal summer tropospheric circulation and heat extremes in the eastern Mediterranean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12805, https://doi.org/10.5194/egusphere-egu23-12805, 2023.

The Western (WHF) and Eastern Himalayan foothills (EHF) are two densely populated regions that experience extreme precipitation events during the Indian summer monsoon (ISM) season lasting typically from June to September [1, 2]. Therefore, a better understanding of the processes controlling ISM intraseasonal variability is of great relevance.

In our present work we identify and quantify causal relationships at short lead-times (three to nine days) between characteristic remote and local climate patterns and the precipitation over the WHF and EHF. More specifically we apply the so-called response-guided causal precursor detection (RGCPD) scheme that builds on the Peter and Clark momentary conditional dependence (PCMCI) algorithm [3]. The employed method is based on concepts of information theory and statistical mechanics, and allows to identify strongly interdependent climate patterns associated with the ISM and to distinguish between spurious and truly causal links. Finally, causal effect networks (CENs) visually summarise the identified causal links between different variables, indicating the directionality, time lag and magnitude of the causal effect.

Our analysis reveals that WHF rainfall variability is influenced by mid-latitude teleconnections such as the circumglobal teleconnection index and seems to be driven by similar precursors and time scales as the precipitation over central India [4]. In contrast, CENs indicate that the EHF rainfall is characterised by faster dynamics compared to the WHF and whilst it is also driven by mid-latitude teleconnections, a different set of atmospheric processes appears to play a major role in its variability. Specifically, a unique and strong causal connection to the tropical western Pacific is revealed, manifesting itself in the geopotential height at 500 hPa and the mean sea-level pressure. A thorough analysis of this signal indicates a Gill-type response to a heat sink over the equatorial Pacific, that may be associated with the Madden-Julian oscillation (MJO) and suggests a link between suppressed MJO phases and enhanced rainfall activity over the EHF region. Thus, our analysis hints to a connection between break spells of the ISM, where large parts of the Indian landmass experience reduced precipitation activity, and enhanced rainfall activity over the EHF region.

References
[1] Vellore, R., et al., On the anomalous precipitation enhancement over the Himalayan foothills during monsoon breaks, Clim. Dynam. 43, 2009-2031 (2014).
[2] Vellore, R., et al., Monsoon - extratropical circulation interactions in Himalayan extreme rainfall, Clim. Dynam. 46, 3517-3564 (2016).
[3] Runge, J., Causal network reconstruction from time series: From theoretical assumptions to practical estimation, Chaos 28, 075310 (2018).
[4] Di Capua, G., et al., Tropical and mid-latitude teleconnections interacting with the Indian summer monsoon rainfall: a theory-guided causal effect network approach, Earth Syst. Dyn., 11, 17-34 (2020).

How to cite: Aviles Podgurski, L. E., Di Capua, G., and Donner, R. V.: Causal Drivers Behind Enhanced Rainfall Activity OverNorthern Indian, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13868, https://doi.org/10.5194/egusphere-egu23-13868, 2023.

Monsoon influences the well-being of billions of people in tropical and subtropical regions. The accelerated climate change and monsoon coupling with other large-scale climatic phenomena make their prediction challenging. Therefore, improvement in understanding and prediction of monsoons has become essential. Recent studies have emphasized the role of arctic region in influencing the tropical climate and its potential to cause more persistent extreme events. Therefore, it is imperative to explore the arctic region for its strategic advantage and combat climate change. In this direction, our work aims to unravel the association between the Arctic region and the Indian summer monsoon (ISM). We quantify the influence of the Arctic region on Indian summer monsoon rainfall (ISMR) using statistical parameters. The sea ice extent and Arctic Oscillation Index were correlated with the precipitation in India at seasonal and monthly scale. The Arctic Oscillation index was able to explain around 7-10% of variability in precipitation. The increased magnitude and frequency of precipitation in India are significantly related to decreased sea ice extent indicated by negative correlation coefficient ranging from 0.3-0.6.

How to cite: kulkarni, S. and Agarwal, A.: Unraveling the association between artic region and Indian summer monsoon – an empirical study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2111, https://doi.org/10.5194/egusphere-egu23-2111, 2023.

Understanding the natural variability of Indian summer monsoon (ISM) is a crucial aspect relevant for decadal climate predictions and climate change studies. The multidecadal variability of ISM is known to have a close association with the Atlantic multidecadal oscillations (AMO). Several teleconnection pathways have been suggested to explain the co-variability of the AMO and ISM in multidecadal timescales. One hypothesis is that the AMO modulates the interannual North Atlantic Oscillation (NAO) mode and there by influences the monsoon via Eurasian temperature modulations. Direct atmospheric teleconnection, across Eurasia, through upper-level circulation anomalies has also been attributed to the observed AMO-ISM relationship. Another possibility is the AMO modulating the monsoon via the Pacific pathway through the atmospheric bridge mechanism and associated modulations of the Hadley-Walker circulations. The Last millennium (LM) (851-1848) climate simulations part of the PMIP3/CMIP5 gives an opportunity to better understand the fidelity of climate models in capturing the AMO-ISM teleconnection mechanisms. In this study we explore how well the proposed mechanisms are represented in eight global climate models (GCM) LM simulations. Such a study, assessing the validity of different AMO-monsoon teleconnection mechanisms in different model climates provides crucial information about how reliable the respective GCMs may be in making decadal climate predictions.

How to cite: Dutta, A., Sivankutty, R., and Joseph Mani, N.: Investigating the Atlantic-Indian summer monsoon multidecadal teleconnections in the PMIP3/CMIP5 Last Millennium simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-191, https://doi.org/10.5194/egusphere-egu23-191, 2023.

CMIP6 future climate projections consistently show a drying trend during the onset of the South American monsoon, which has the potential for large ecological and societal impacts in this region. This trend is also present in a high-resolution regional convection-permitting simulation over the South American domain. Here, the processes responsible for this drying trend are examined using a number of idealised experiments and analysis techniques. The main driver is shown to be remote sea surface temperature (SST) warming - rather than local radiative or plant physiological responses to increased CO₂ - with both large-scale uniform SST warming and patterned regional warming playing important roles. The role of uniform SST warming on the South American monsoon onset is examined in more detail using a moist static energy budget approach, building on hypotheses from a previous single model study. The atmospheric circulation response to patterned SST warming is examined using a local overturning circulation partioning technique, allowing a link between the South American monsoon region and specific regions of ocean warming to be identified.

How to cite: Chadwick, R., Garcia-Franco, J., and Alves, L.: Processes controlling the South American Monsoon response to Climate Change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2025, https://doi.org/10.5194/egusphere-egu23-2025, 2023.

The climate of a great part of South America presents two well-defined seasons: one dry, in general, from April to September, and another wet, from October to March, which characterizes a monsoon regime. As most of the energy generation in this monsoon region is hydroelectric, precipitation is a target of several studies. In this context, the South America Monsoon (SAM) lifecycle (onset, demise, and duration) in projections of eight global climate models (GCMs) of the Coupled Model Intercomparison Project Phase 6 (CMIP6) is analyzed in this study using two approaches: (a) the original GCM outputs downloaded from the Earth System Grid Federation (ESGF) and (b) after application of the statistical downscaling (SD) technique. Daily precipitation data from the Climate Prediction Center (CPC), with a horizontal resolution of 0.5o, are used as a reference. So, the final resolution of the GCMs after applying the Quantile Delta Mapping (QDM) is the same as CPC. SAM lifecycle is identified with a similar methodology from Liebman and Marengo published in 2001, which is based on the accumulated daily precipitation anomalies. The rainy season is considered to be the period during which precipitation exceeds its climatological annual average, then a positive slope indicates the rainy season. Note that this methodology is proper to be applied in projections because it does not assume any threshold. Initial results indicate a shorter lifetime of SAM at the end of the century.  The authors thank the Programa de P&D regulado pela ANEEL: empresa Engie Brasil Energia e Companhia Energética Estreito, MC&E, FAPEMIG, CAPES and CNPq for the financial support.

How to cite: Reboita, M., Ferreira, G., and Ribeiro, J. G.: CMIP6 projections of the South American Monsoon Lifecycle: comparison with pre and post statistical downscaling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-715, https://doi.org/10.5194/egusphere-egu23-715, 2023.

The changes in the Indian summer monsoon rainfall under anthropogenic climate change would have a large socio-economic impact. The thermodynamic effect of the climate change on future monsoon rainfall is well understood with an overall increase in precipitation as the atmosphere moistens. Understanding the dynamical effect of climate change especially from the changes in the drivers of the monsoon remains challenging. Here we show that the observed western Indian monsoon rainfall has an increasing trend over the last 120 years. We find this observed trend is connected with the trend in the tropical Pacific zonal sea surface temperature (SST) gradient, where the western tropical Pacific or the warm pool region of the Pacific Ocean is warming faster than the eastern side. Applying a storyline approach to the future evolution of the zonal tropical Pacific SST gradient in 38 global climate models from the latest Coupled Model Intercomparison Project phase 6, we find a consistent connection in the models between the western Indian monsoon rainfall change and the strength of the change in the zonal tropical Pacific SST gradient under global warming. The models which warm more in the western compared to the eastern side of the tropical Pacific have higher rainfall increases over western India during the monsoon season. This link is associated with an anomalous easterly wind coming from the western tropical Pacific and converging over western India, leading to higher rainfall in both observations and models. This result suggests that future changes in the western Indian monsoon rainfall would depend on the changes in the strength of the zonal gradient of the tropical Pacific Ocean SST.

How to cite: Ghosh, R. and Shepherd, T. G.: Strengthening tropical Pacific zonal temperature gradient linked with increasing West Indian Monsoon rainfall, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5985, https://doi.org/10.5194/egusphere-egu23-5985, 2023.

The urban areas can modify the local and regional climate through various processes. They can indeed modify the water cycle and precipitations, either through the modification of land-use, or through effects induced by the emissions of anthropogenic aerosols. The thermodynamical perturbations induced by the presence of urban land-use, including the urban heat island effect, are known to induce rainfall modification due to perturbation of the flow and enhancement of the convective activity. However, this impact has yet to be clarified in a large scale, highly energetic system like the Asian Monsoon system. Using the high resolution meso-scale atmospheric model Meso-NH, we investigated the impact of urban land-use on the precipitation during the Indian Summer Monsoon, including the influence on extreme events. The results of this study will be presented and discussed.

How to cite: Falga, R. and Wang, C.: Modeling the impact of the urban land-use on the Indian Summer Monsoon precipitation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14204, https://doi.org/10.5194/egusphere-egu23-14204, 2023.

The study addresses the role of climate change on the interaction between the Indian Summer monsoon rainfall (ISMR) and western north Pacific (WNP) convective activities. We have examined two high-resolution climate model simulations, with and without anthropogenic forcing (i.e., HIST and HISTNAT), using a variable resolution model. The study is supplemented by detailed diagnostics and innovative techniques like causal network analysis which bring out the interaction of convective activities between the two regions which is altered by the influence of anthropogenically forced climate change. Our results shows that the weakening of ISMR re-orient the cross-equatorial winds along with large-scale moisture transport towards the western tropical Pacific which significantly increase the genesis potential index (GPI) over the region by 9.6%. Further we noted the probability of the occurrence of extremely low sea-level pressure (SLP) i.e., SLP < 995.5 hPa around areas near Taiwan and part of Chinese mainland is significantly higher by 10.3 % in the HIST simulation as compared to that of HISTNAT. The use of causal effect network analysis showed a significant causative link between the Indian monsoon circulation index (IMI), WNP tropical cyclone activity (GPI) and winds over the tropical Indo-Pacific (IPWND). The results show a weakening of the IMI can lead to possible enhancement of GPI and IPWND, with a certain time-lag. It is noteworthy to mention that the time lag of interaction between the IMI, GPI and IPWND are different in the two simulations with a significantly shorter time scales in HIST (~ 5 days) compared to that of HISTNAT where it is significantly larger (>20 days).

How to cite: Sagar, A., Krishnan, R., and Sabin, T. P.: Weakening of South Asian monsoon circulation and its interaction with western north Pacific tropical convective activities in a changing climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-696, https://doi.org/10.5194/egusphere-egu23-696, 2023.

Future changes in Sahel precipitation are uncertain because of large differences between projections of various climate models. We assess the effect of climate change on Sahel precipitation in summer and for the end of the 21^st century. We show that uncertainty in Sahel precipitation is associated with uncertainty at simulating future changes in surface air temperature over the northern Hemisphere. We point out the Atlantic Ocean and Euro-Mediterranean surface air temperature as drivers of the Sahel precipitation change uncertainty. We use a storyline approach, a statistical method, to construct scenarios of changes in Sahel precipitation, whose differences only depend on future changes in Atlantic Ocean and Euro-Mediterranean surface air temperature. We show that uncertainty in changes in Atlantic Ocean and Euro-Mediterranean surface air temperature explains up to 50% of Sahel precipitation change uncertainty. The approach also allows selecting models to better understand uncertainty in Sahel precipitation change, focusing on the mechanisms at play. We suggest that reducing uncertainty in the future warming of the North Atlantic and the Euro-Mediterranean areas would then allow reducing uncertainty in future changes in Sahel precipitation.

How to cite: Monerie, P.-A., Biasutti, M., Mignot, J., Mohino, E., Pohl, B., and Zappa, G.: Uncertainty in Sahel precipitation change: a storyline approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7258, https://doi.org/10.5194/egusphere-egu23-7258, 2023.

Many factors have been suggested to explain variability and change in Sahel rainfall. Of those, sea surface temperature (SST) in the Eastern Mediterranean Sea (EMS) and zonal moisture flux south of the Sahel show strong correlations. Based on observational and reanalysis data on temperature, pressure, wind and moisture flux, this paper identifies a mechanism that explains both correlations. The mechanism hinges on the Jebel Marra massif and the Ethiopian highlands, where the mesoscale convective systems (MCSs) develop that bring most of the rain to the Sahel. We find that cold SST anomalies in the EMS between June and September cause a greater trans-Sahara temperature contrast and coincide with high pressure over Libya, resulting in stronger northerlies towards Sudan. This prevents Tropical Atlantic moisture from reaching the MCS genesis region, which reduces the seasonal northward spread of Sahel rainfall and of the Atlantic intertropical convergence zone, which in turn suppresses the development of the west-African westerly jet and the African westerly jet and inhibits Atlantic moisture from reaching the MCS genesis region, thus further reducing Sahel rainfall. Anomalous moisture transport from the Mediterranean does not play a role. Mediterranean SST variability raises questions about the future development of Sahel rainfall. If a new dry period materialises, this will have substantial implications on food production in the region. There are however opportunities for mitigating against the effects of such a dry period.

How to cite: Gaasbeek, T., van der Ent, R., Coumou, D., Haarsma, R., and Keulers, S.: Where the north wind meets the sea: rainfall variability and change and its implications for food security in the Sahel, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9482, https://doi.org/10.5194/egusphere-egu23-9482, 2023.