AS1.24
Orals: Tue, 25 Apr | Room 1.85/86
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, 24–28 Apr 2023, EGU23-6671, https://doi.org/10.5194/egusphere-egu23-6671, 2023.
More than half of the rainfall brought to the Indian subcontinent by the summer monsoon is associated with low-pressure systems (LPSs). Yet their relationship with the Boreal Summer Intraseasonal Oscillation (BSISO) – the dominant intraseasonal forcing on the monsoon – is only superficially understood. Using reanalysis data, we explore the relationship between the BSISO and LPS intensity, propagation, and precipitation, and associated underlying mechanisms.
The BSISO has a large impact on mean monsoon vorticity and rainfall as it moves northward – maximising both in phases 2-3 over southern India and phases 5-6 over northern India – but a much weaker relationship with total column water vapour.
We present evidence that LPS genesis also preferentially follows these phases of the BSISO.
We identify significant relationships between BSISO phase and LPS precipitation and propagation: for example, during BSISO phase 5, LPSs over north India produce 51% heavier rainfall and propagate northwestward 20% more quickly.
Using a combination of moisture flux linearisation and quasigeostrophic theory, we show that these relationships are driven by changes to the underlying dynamics, rather than the moisture content or thermodynamic structure, of the monsoon.
Using the example of LPSs over northern India during BSISO phase 5, we show that the vertical structure of anomalous vorticity can be split into contributions from the BSISO background circulation and the nonlinear response of the LPS to anomalous BSISO circulation. Complementary hypotheses emerge about the source of this nonlinear vorticity response: nonlinear frictional convergence and secondary barotropic growth. We show that both are important. The BSISO imparts greater meridional shear on the background state, supporting LPS intensification. The BSISO background and nonlinear LPS response both contribute significantly to anomalous boundary layer convergence, and we show through vortex budget arguments that the former supports additional LPS intensification in boundary layer while the latter supports faster westward propagation.
This work therefore yields important insights into the scale interactions controlling one of the dominant synoptic systems contributing to rainfall during the monsoon.
How to cite: Hunt, K. and Turner, A.: Nonlinear intensification of monsoon low pressure systems by the BSISO, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-37, https://doi.org/10.5194/egusphere-egu23-37, 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 Apr 2023, EGU23-1014, https://doi.org/10.5194/egusphere-egu23-1014, 2023.
Orals: Wed, 26 Apr | Room 1.85/86
The Himalayas are an essential driver of the monsoon and climate system. However, river flooding during the monsoon impacts the most densely populated region of Himalayan downstream regions annually. Previous studies also reported elevation-dependent warming, rainfall changes, ice-sheet melting, and extremes in the Himalayas. Nevertheless, due to complicated orography, Himalayan precipitation dynamics remain quantitatively limited on a spatial scale compared to other monsoon regions. In the context of climate change, recent studies show how melting glaciers and snow, along with monsoonal rains causing recurrent floods, play a role. This study examined the last 43 years (1979-2021) to emphasize the interannual variability. We found a robust signal over in the Eastern Himalayas, where the orographic features and process plays a dominant role. Further analysis indicates Monsoonal rainfall is the main factor, rather than melting snow for these unusually extreme years. Regional monsoonal circulation connected to Walker circulation controls the variability of Himalayan monsoonal rainfall via circulation linkages. Our findings illustrate the wet and dry response mechanisms in the eastern Himalayas. The conclusions are drawn from this work highlight the role of natural variability, which might help understand Himalayan floods and their predictability.
Keywords – Himalayas, Interannual variability, Monsoon dynamics, Orographic features, River floods
How to cite: Kad, P. and Ha, K.-J.: Dynamics and characteristics of monsoonal orographic rainfall variability over Eastern Himalaya, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4950, https://doi.org/10.5194/egusphere-egu23-4950, 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 Apr 2023, EGU23-191, https://doi.org/10.5194/egusphere-egu23-191, 2023.
Monsoon systems are transporting water vapour and energy across the globe, making them a central component of the global circulation system. Changes in different forcing parameters have the potential to fundamentally change the monsoon characteristics as indicated in various paleoclimatic records. Here, we use the Atmosphere Model version 2 developed at the Geophysical Fluid Dynamics Laboratory (GFDL-AM2) and couple it with a slab ocean to analyse the monsoon's sensitivity to changes in different forcing parameters on a planet with idealized topography. This Monsoon Planet concept of an Aquaplanet with a broad zonal land stripe allows to reduce the influence of topography and to access the relevant meridional monsoon dynamics. In the simulations that enable monsoon dynamics, a bimodal rainfall distribution develops during the monsoon months with one maximum over the tropical ocean and the other one over land. The intensity and expansion of the land monsoon depends on the relative height of a local maximum in the surface pressure field that is acting as a barrier and determines the landward moisture transport. This dynamic is emerging during the course of one year, but also occurs when varying different parameters in a sensitivity analysis (slab ocean depth, sulfate aerosols, carbon dioxide, solar constant, land albedo). This structure of a bimodal rainfall distribution and a pressure-barrier located between the two maxima is also present in the Westafrican monsoon.
How to cite: Katzenberger, A., Levermann, A., Feulner, G., and Petri, S.: Monsoon Planet: Bimodal rainfall distribution due to barrier-structure in pressure field, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9398, https://doi.org/10.5194/egusphere-egu23-9398, 2023.
Solar activity affects Asian summer monsoon (ASM) at various time scales. However, it remains unknown if and how solar activity can influence ASM on the centennial time scale. Using the Community Earth System Model, we conduct a solar activity forced Holocene transient simulation with an acceleration factor of 10 and show that during the middle‒late Holocene ASM precipitation exhibits a significant 300‒600-year periodicity under solar forcing. This model-produced multi-centennial variation is also suggested by proxy data. The leading mode of the multi-centennial ASM variation shows a “wet tropics–dry subtropics” pattern, which lags the corresponding solar activity by about a quarter cycle. The western North Pacific (WNP) circulation system is responsible for the multi-centennial ASM variation, through enhancing the climatological south Asia‒WNP monsoon trough. We suggest that solar activity modulates the zonal SST gradients of the tropical Pacific, inducing the anomalous WNP cyclone and enhancing ASM precipitation in a delayed mode.
How to cite: Sun, W., Liu, J., Wang, B., Chen, D., Ning, L., and Yan, M.: Holocene multi-centennial variations of the Asian summer monsoon triggered by solar activity, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10346, https://doi.org/10.5194/egusphere-egu23-10346, 2023.
The South American monsoon (SAM) is the main source of precipitation over most of the continent, including subtropical regions, affecting the most populated areas and those with largest contribution to agricultural production and hydroelectric power generation. Precipitation extremes with strongest social and economic impacts occur during the monsoon season. Therefore, there is great interest for reliable projections of its future behavior and for a theoretical framework able to explain it. Over the tropical region, observed trends in precipitation are not consistent and vary according to the period of analysis, while in the subtropical southeast South America (SESA) a positive trend has been observed. Most recent projections from CMIP6 multimodel ensembles show very weak signal of SAM precipitation change. The IPCC AR6 findings indicate that there are no projected precipitation changes, but that the onset of monsoon will be delayed into the 21st century and extreme events will be more frequent.
The fact that the models do not show a significant future change in the total precipitation accumulated in the entire monsoon season does not mean that there are no changes of great interest in different phases of this season. Detecting them requires analysis of greater temporal resolution and an additional focus: El Niño (EN). There are two aspects that prompted the approach used in the present study: i) model projections of future SST indicate a warming pattern in the central-east equatorial Pacific leading to a more El Niño-like future climate; ii) the El Niño effects over 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.
A set of twelve CMIP6 selected models was 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 a satisfactory ENSO. The changes projected by the ensemble of seven models that best reproduced ENSO and also the climatology of SA indicate a more EN-like future climate. Consistently, the main 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 and thermodynamical processes trigger the changes from spring to summer. In central-northern-eastern Amazon dry conditions prevail throughout the season, while in SESA, precipitation increases.
The results of the all-model ensemble, although consistent with the seven-model ensemble, are weaker and less satisfactory. The most intense changes are projected by the model that best simulates ENSO and its impacts on AS.
How to cite: Grimm, A. M. and Padoan, D.: The projected future of the South American monsoon, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-810, https://doi.org/10.5194/egusphere-egu23-810, 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 Apr 2023, EGU23-696, https://doi.org/10.5194/egusphere-egu23-696, 2023.
Cyclonic low‐pressure systems (LPS) are the dominant synoptic‐scale rain-bearing system of the South Asian summer monsoon. Traditionally categorized by intensity as monsoon lows, monsoon depressions, and more intense cyclonic storms, LPS produce intense rainfall and floods in some of the world’s most densely populated regions. Yet the contribution of the relatively weak lows vs. the stronger depressions to extreme rainfall and its trends remains unknown; this knowledge gap is particularly troubling because historical trends in LPS have been difficult to assess due to changes in the observing network. Future projections have also remained highly uncertain due to the inability of many coarse-resolution climate models to accurately simulate LPS.
Here we use satellite and gauge-based precipitation estimates with atmospheric reanalyses to show that precipitation in monsoon depressions has become more intense in recent decades. This intensification has occurred as humidity over parts of India increased more rapidly than nearly anywhere else on Earth. Precipitation in depressions has risen at a relative rate larger than that of specific humidity, suggesting that upward motion in depressions has become more intense; vertical motion trends in a state-of-the-art reanalysis, which incorporates nearly all long-term climate forcings, are consistent with this hypothesis. We also examine changes in South Asian LPS precipitation simulated by an ensemble of high-resolution global models, which we find skillfully represent these storms. Future trends in total LPS precipitation, including in monsoon depressions, lie near an approximate Clausius–Clapeyron rate (7%/K) in the multi-model mean. This change in LPS rain rates contributes to a projected future increase in seasonal mean and extreme precipitation over South Asian land. Adaptation to future changes in human exposure to hydrological extremes thus requires careful monitoring, accurate multi-decadal projections, and skilful short-term forecasts of the interaction of the humidity field with the dynamics of monsoon LPS.
How to cite: Sasidharan Nair, V., R. Boos, W., D. Risser, M., A. O’Brien, T., A. Ullrich, P., and D. Collins, W.: Rise in Rainfall of South Asian Monsoon Low-Pressure Systems, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-16193, https://doi.org/10.5194/egusphere-egu23-16193, 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 21st 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, 24–28 Apr 2023, EGU23-7258, https://doi.org/10.5194/egusphere-egu23-7258, 2023.
Economically based on rain-fed agriculture and grazing, the Sahel region is extremely vulnerable to possible shifts in its rainfall pattern associated with the West African monsoon. Short-term climate prediction, i.e. in the horizon of 5 to 30 years, the so-called "decadal" time scale, is highly relevant for economic and structural planning.
In this study we evaluate the impact of model initialization in the skill and reliability with which Sahel rainfall amounts are predicted by different models at decadal timescales. Using initialized and uninitialized (historical simulations) hindcasts from height (8) CMIP6 models and their ensemble mean, our results show that the impact of initialisation depends on the model. Half of the models (MPI-ESM1-2-LR, MRI-ESM2-0, IPSL-CM6A-LR and EC-Earth3) show skillful MSSS scores in their uninitialized experiments and the initialization does not provide further improvements. This situation is very different from the previous coordinated exercise performed within CMIP5 suggesting an enhanced role of external forcing on the interannual to decadal modulation of the Sahel rainfall in CMIP6. However, for NorCMP1 while initialized predictions show skillful MSSS scores, uninitialized ones show very weak scores, suggesting the initialization improves the forced response and the internal variability in this model. Our results also show that the MRI-ESM2-0 model is more skillful than the multi-model ensemble.
In addition to MSSS scores, the reliability of these prediction systems is also tested. Preliminary results show positive impacts of the initialization in five out of height models. In general, our results suggest that the initialization does not greatly improve MSSS scores but it does enhance reliability.
How to cite: Ndiaye, C., Mignot, J., and Mohino, E.: How robust are CMIP6 models in predicting Sahel rainfall at decadal time-scales ?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13834, https://doi.org/10.5194/egusphere-egu23-13834, 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, 24–28 Apr 2023, EGU23-9482, https://doi.org/10.5194/egusphere-egu23-9482, 2023.
Posters on site: Wed, 26 Apr, 14:00–15:45 | Hall X5
Southeast (SE) Asia is located in a transitional zone where hydroclimate is controlled by both the Indian and East Asian summer monsoon branches. Recent proxy-based studies and climate models suggest that the hydroclimate of SE Asia may be out of phase with neighboring regions, such as India and China. However, we lack sufficient proxy records to verify this postulation or to identify spatial and temporal variations. This study reconstructs both past temperatures and effective moisture in Central Vietnam during Marine Isotope Stage 3 (approximately 50,00 to 30,000 years BP) to determine how these two climate variables relate in the past. Terrestrial temperatures and precipitation are reconstructed using biomarkers (branched glycerol dialkyl glycerol tetraethers, brGDGT) and compound-specific isotope analyses (carbon and hydrogen-isotopic values of leaf wax n-acids, δ13Cwax, and δDwax) from a buried peat deposit in the Central Highlands of Vietnam. The brGDGTs-derived annual temperatures range from 22.9 to 26.2°C and show a warming trend coincident with a weakening of summer insolation. A coincident and gradual enrichment of δ13Cwax from 47 to 33 kyr BP suggests a transition from C3 to C4 vegetation dominance. Such a response could signal an overall decrease in precipitation or a shift in the seasonality of precipitation. The δDwax data, however, do not indicate an overall drying trend, which supports the idea that a shift in the seasonality of rainfall, along with higher annual temperatures, is driving the vegetation change. In addition, the δDwax records may exhibit a trend opposite to a site in Thailand. We argue that the isotopic variability in the precipitation of Central Vietnam reflects the shift in moisture sources along with the shift in seasonality. In this case, an increase in amount of precipitation derived from the South China Sea in winter months is marked by rain enriched in δ2H-which could also be interpreted as a decrease in precipitation. The increase in rainfall during winter monsoon months (e.g. winter) in the Central Highlands of Vietnam does not appear to reach Thailand. We recommend that the precipitation proxies should be applied with knowledge of regional climate context and argue that better geographic representation of monsoonal climates is necessary to fully understand and model this critical climate system.
How to cite: Tran, T., Stevens-Landon, L., Tierney, J., Murphy, P., and Vu-Van, T.: Precipitation and temperature variability in Vietnam during Marine Isotope Stage 3 from terrestrial biomarkers, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-546, https://doi.org/10.5194/egusphere-egu23-546, 2023.
The Indo-Pacific warm pool (IPWP) is enclosed by a 28 ◦C isotherm and plays a vital role in controlling tropical circulations. However, the effects of changes in regional warm pool sea surface temperatures (SSTs) on the circulations remain unexplored. To do this, we divided the IPWP into the Indian and Pacific sectors and distinguished their responses to natural variability and global warming. And then, we examined the impacts of the interannual variability (IAV) in warm pool SST on the tropical Hadley, Walker, and monsoon circulations. The Hadley circulation was affected by warm pool SST warming, i.e., warmer SSTs over the warm pool strengthened the upward branch of Hadley circulation, whereas the downward branch was weakened and strengthened in the Northern and Southern Hemispheres. Walker circulation was strengthened (weakened) in the warming (natural) mode. Consequently, the Walker circulation is weakened since the natural variability of warm pool SST plays a more dominant role than the warming trend of SSTs over the warm pool. It is notable that warm pool warming has little impact on monsoon circulation. Our findings highlight the different roles of the IAV of warm pool regions in each tropical circulation as part of the warming trend and natural variability. Furthermore, an increase in precipitation is limited up to a specific SST, although SST becomes warmer. We defined this specific SST as Saturation Threshold SST (STT). Under a warming climate, future changes in STT over the IPWP and its mechanism will be shortly shown in this presentation.
How to cite: Kim, H.-R. and Ha, K.-J.: Impact of the Indo-Pacific Warm Pool on the Tropical Circulations and Changes in Saturation Threshold SST, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4743, https://doi.org/10.5194/egusphere-egu23-4743, 2023.
Owing to the complicated spatial-temporal characteristics of East Asian precipitation (EAP), climate models have limited skills in simulating the modern Asian climate. This consequently leads to large uncertainties in simulations of the past EAP variation and future projections. Here, we explore the performance of the newly developed Alfred Wegener Institute Climate Model, version 3 (AWI-CM3) in simulating the climatological summer EAP. To test whether the model’s skill depends on its atmosphere resolution, we design two AWI-CM3 simulations with different horizontal resolutions. The result shows that both simulations have acceptable performance in simulating the summer mean EAP, generally better than the majority of individual models participating in the Climate Modelling Intercomparison Project (CMIP6). However, for the monthly EAP from June to August, AWI-CM3 exhibits a decayed skill, which is due to the sub-seasonal movement of the western Pacific subtropical high bias. The higher resolution AWI-CM3 simulation shows an overall improvement relative to the one performed at a relatively lower resolution in all aspects taken into account regarding the EAP. We conclude that AWI-CM3 is a suitable tool for exploring the EAP for the observational period. Having verified the model’s skill for modern climate, we suggest employing the AWI-CM3, especially with high atmosphere resolution, also for applications in paleoclimate studies and future projections.
How to cite: Shi, J., Stepanek, C., Sein, D., Streffing, J., and Lohamnn, G.: East Asian summer precipitation in AWI-CM3: Comparison with observations and CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9428, https://doi.org/10.5194/egusphere-egu23-9428, 2023.
The phase shift of climatic systems in decadal or interdecadal scale, also called as regime shift has occurred in East Asian Summer Monsoon (EASM) in the past. For example, the shifts of the late 1970s, mid 1990s, and early 2000s are the typical examples. Before and after these shifts, dominant teleconnection mode affecting the EASM had changed. On the other hand, the shift of early 2000s has not extensively investigated. Here, it is examined the characteristics of this particular shift in relation to variability of East Asian jet during summer. First, regime shifts earlier and in the early 2000s are detected based on the variance of summer East Asian jet. Second, the teleconnection pattern that influence summer East Asian jet was changed from the Atlantic-Eurasian (AEA) pattern to distinctly different zonal pattern around extratropical region of Eurasian. Finally, it was found that after this regime shift the land-atmosphere coupling induced by variability of soil moisture also strengthened. It is hypothesized that enhanced linkage between jet in the upper atmosphere and surface heat flux over Inner East Asia is a key mechanism of enhancing variability of the East Asian summer jet, i.e., the regime shift in 2000s. These results imply that over drier region, the regional climatic system might response more sensitively to regional-scale change on surface level than large-scale influence, such as wave-train.
How to cite: Nam, J. and Yoon, J.-H.: Regime shift of Jet over East Asian summer Monsoon in the early 2000s: its detection and dynamical driver, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11107, https://doi.org/10.5194/egusphere-egu23-11107, 2023.
Projections of future West African monsoon (WAM) precipitation change in response to increased greenhouse gases are uncertain, and an improved understanding of the drivers of WAM precipitation change is needed to help aid model development and better inform adaptation policies in the region. Here, we address two of these drivers: the direct radiative effect of increased CO2 (referring to the impact of increased CO2 in the absence of SST changes), and the impact of a uniform SST warming. Atmosphere only models are used to investigate the response, finding that these two drivers have opposing impacts on WAM precipitation. In response to the direct radiative effect, an increase in precipitation is caused by a northward shift and a weakening of the shallow meridional circulation over West Africa, advecting less dry air into the monsoon rainband. In contrast, the uniform SST warming causes a decrease in precipitation due to a strengthening of the shallow meridional circulation and enhanced moisture gradients between the moist monsoon airmass and the dry desert airmass. These changes in the shallow meridional circulation are shown to be caused by large scale temperature changes as well as the more localised impact of a soil moisture feedback mechanism over the Sahel. It is then shown that the processes discussed are relevant to the intermodel uncertainty in WAM projections across a range of CMIP6 models.
How to cite: Mutton, H., Chadwick, R., Collins, M., and Lambert, H.: Understanding changes in West African monsoon precipitation in response to increased CO2, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3466, https://doi.org/10.5194/egusphere-egu23-3466, 2023.
The Indian summer monsoon rainfall (ISMR) has been declining since the middle of the last century. However, recently (since about 2002) it is reported to have revived. For these observed changes in the ISMR, several explanations have been reported. Among these explanations, the warming of the Indian Ocean is considered a major one. However, we still do not fully understand the response of the atmosphere to this warming. Here we report that warming in the Indian Ocean (focusing on the eastern side of it where the sea surface temperatures are climatologically very warm) drives atmospheric responses that oppose Indian summer monsoon circulation and reduces ISMR.
How to cite: Goswami, B. B.: Response of the Indian monsoon to a warming Indian ocean, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6151, https://doi.org/10.5194/egusphere-egu23-6151, 2023.
The characteristics of Indian summer monsoon (ISM) precipitation have been changing in a warming climate. We examined the intra-seasonal variability of daily mean ISM rainfall over central India in 20 CMIP6 models. The daily precipitation variance is found to have increased in the last 20 years of SSP585 runs compared to the 1981 – 2000 period of historical all forcing simulations. The mean ISM precipitation also shows an increase in the same period. The future changes in seasonal mean precipitation and the intra-seasonal variance in daily precipitation with respect to the historical period are scaled with the corresponding change in the surface temperature over the ISM domain. The changes in the seasonal mean precipitation do not show any significant relationship with the surface temperature change. However, the changes in the daily variance of ISM precipitation scale linearly with the changes in temperature. These results suggest that the ISM precipitation will become more erratic in a warming environment.
How to cite: Sandeep, S. and Kumari, N.: Increased intra-seasonal variability in Indian summer monsoon precipitation in a warming climate, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2011, https://doi.org/10.5194/egusphere-egu23-2011, 2023.
A better representation of the poorly understood sub-grid scale processes in the Global Circulation Model is imperative for the skilful simulation of the Indian Summer Monsoon (ISM). We customized the parametrizations for deep convection, gravity wave, and surface layer; incorporated them into the NCAR Community Atmosphere Model 5.0 (NCAR CAM5, base model). The modified deep convective parametrization includes dynamic tau (dynamic convective adjustment timescale), which allows a spatiotemporally varying tau instead of constant tau and the stochastic entrainment rate in place of a fixed entrainment rate. Similarly, the modified gravity wave parametrization facilitates estimating the response of upper-level gravity wave drag induced from secondary sources. Likewise, the modified surface layer parametrization enables a better representation of near-surface variables as well as surface fluxes. The simulations of default and customized NCAR CAM5 have been carried out for eleven years (one year for spin-up and the rest ten years considered for analysis). The analysis has been performed for two major climate change indicators, i.e., temperature and precipitation for the ISM season (June to September). The model simulated near-surface temperature and precipitation during ISM were evaluated against observation (Indian Meteorological Department). A significant improvement has been noted in simulating the total precipitation pattern and magnitude over India, as well as for surface air temperature, particularly over northern India. In addition, based on performance, the customized model alleviates some of the long-standing biases evident in the default NCAR CAM5 simulation over India. Furthermore, compared to the base model, the customized model realistically simulates the annual cycle of precipitation, medium and extreme precipitation rates, meridional tropospheric temperature gradient, upper (200 hPa) and lower (850 hPa) tropospheric winds, Madden Julian Oscillation, and equatorial waves. The study’s findings illustrate the significance of model parametrizations towards improving the ISM simulation. Meanwhile, the modeling framework would be essential for credible future climate projections of India and would become a vital tool for policymakers and diverse stakeholders.
Keywords: Indian Summer Monsoon, NCAR CAM5, Deep convection, Gravity wave, Surface layer
How to cite: Bhuyan, D. P. and Mishra, S. K.: Developed a climate modeling framework for India , EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12760, https://doi.org/10.5194/egusphere-egu23-12760, 2023.
The cross-equatorial oceanic heat transport (OHT) in the Indian Ocean during boreal summer is an integral component of the Indian summer monsoon (ISM). This OHT is believed to be a crucial factor in the interannual variability of ISM. Thus, a deeper understanding of OHT in climate model simulations is needed for the understanding of the simulated interannual variability of monsoon. Here we examine the Indian Ocean meridional OHT and how the OHT in the summer monsoon season impacts the development of SST patterns in the post-monsoon season. Our results show that the post-monsoon SST positively correlates with OHT during the summer monsoon in western IO and negatively correlates in the southeastern Indian Ocean. Further, it reveals that the OHT during summer monsoon explains the dipole pattern of SST in the post-monsoon over the equatorial Indian Ocean. This study also investigates how anomalous OHT during monsoon months contributes to the persistence of marine heat waves (MHW) in the post-monsoon season. Both CMIP6 models and observations suggest enhanced and persistent MHWs in the post-monsoon season are linked to stronger OHT during the summer season.
How to cite: Saran, R. and Sandeep, S.: Indian Ocean heat transport and its role in developing SST pattern in the post-monsoon season in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8550, https://doi.org/10.5194/egusphere-egu23-8550, 2023.
Recent studies using coupled model simulations and observation datasets suggest a poleward shift and overall weakening of Indian Summer Monsoon (ISM) circulation (Sandeep and Ajayamohan, 2015). Their investigation using experiments from the fifth phase of coupled model Inter-comparison project (CMIP5) indicate a poleward migration of the monsoon low-level jet (LLJ), with the magnitude of shift linked to the degree of warming.
Here we investigate the changes in monsoon LLJ in multiple reanalysis datasets as well as historical and future scenario simulations of the sixth phase of coupled model Inter-comparison project (CMIP6). The latitudinal location of LLJ is defined as the latitude of zero absolute vorticity over the Arabian Sea, following Tomas and Webster (1997). Although all reanalysis datasets show a poleward shift in LLJ since late 1970s, the magnitude of shift varies among them. The multi model ensemble of CMIP6 historical simulations show a northward shift of 0.4 degrees in LLJ. The ensemble mean of SSP585 simulations show a northward shift in LLJ by 0.8 degrees in the 2081 - 2100 period. The changes in the latitudinal position of LLJ and the land-sea temperature difference are significantly correlated, with a Pearson correlation coefficient of 0.81 and 0.67 for the ensemble means of historical and SSP585 runs, respectively. This suggests that the underlying dynamics of the monsoon circulation is changing in a warming climate.
References
Sandeep, S., & Ajayamohan, R. S. (2015). Poleward shift in Indian summer monsoon low level jetstream under global warming. Climate Dynamics, 45(1), 337-351.
Tomas, R. A., & Webster, P. J. (1997). The role of inertial instability in determining the location and strength of near‐equatorial convection. Quarterly Journal of the Royal Meteorological Society, 123(542), 1445-1482.
How to cite: Sreepriya, S., Mirle AchutaRao, K., and Sandeep, S.: Investigating the Causes of Poleward Shift in Monsoon Low-level Jet, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14372, https://doi.org/10.5194/egusphere-egu23-14372, 2023.
Tibet plateau plays very crucial roles in globle climate system, and the precipitation is one of the main factors in Tibet plateau climate system, Based on GPCC monthly precipitation data and ERA5 monthly precipitation reanalysis data from 1961 to 2016, this study analyzes the spatio-temporal distribution and evolution of plateau precipitation during the May-September monsoon period under the background of global warming. Try to analyze the mechanism reason affecting precipitation variation in different regions. The precipitation in the South part of Tibet plateau began to increase in May, and advance to the northwest part of Tibet plateau during the July and August, and began to back to the south in September. According to The decomposition of the empirical orthogonal function (EOF), we divided the Tibet plateau into north and south two parts by the mount tunggula, on the interannual variability, percentage of precipitation in the monsoon plateau exists reverse change relation, precipitation showed a trend of slight decrease in the south part of plateau, plateau in northern precipitation shows ascendant trend on decadal scale, rate of precipitation in the plateau there are shocks between 3 to 5 years in 7-9 or 11 years. There is a north-south inverse change rate in the precipitation in the plateau during the monsoon period. The analysis of the relationship between the monthly precipitation data and the atmospheric circulation in the south and north of the Plateau shows that the precipitation in the south of the Plateau is affected by the South Asian monsoon, while the precipitation in the north of the plateau is related to the Rossby wave of the subtropical westerly jet. In other words, the precipitation in the southern part of the plateau is mainly controlled by summer risk, while the precipitation in the northern part of the plateau is affected by subtropical westerly winds.
Keyword: Tibetan Plateau，Precipitation，Circulation，South asia monsoon，Jet stream
How to cite: Wang, H. and Hu, Z.: The North-South variation and mechanism of the precipitation over the Tibetan Plateau during the monsoon in the past 60 years, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1193, https://doi.org/10.5194/egusphere-egu23-1193, 2023.
Significant spatial and temporal deviations from the seasonal mean precipitation, such as severe droughts (deficient rainfall) and floods (excess rainfall), have a major influence on India. The non-linear energy interactions between the various scale atmospheric systems are important as they play a crucial role in the Indian summer monsoon variability. Since studies have yet to look at the whole energy budget of the southwest monsoon, it is necessary to accurately capture these energy exchanges to represent the monsoon circulation better. In this study, we found the exact nature of complex non-linear energy interactions of synoptic-scale mainly low-pressure systems (LPSs) and Intraseasonal Oscillation (lSO) 30-60 day scale with other scales, including the seasonal mean, Indian Ocean Dipole (IOD), and El Niño–Southern Oscillation (ENSO) using the in-scale and out of scale energetics. These energy interactions are calculated in the frequency domain for the Indian monsoon region using the ECMWF ERA-5 data for 72 years (1950-2021) during the monsoon season (JJAS). Since the seasonal mean kinetic energy is highly correlated with the seasonal mean rainfall, we explored how these energy exchanges vary during excess and deficient rainfall years. We also found that the ISO and synoptic scale systems influence the interannual variability of the Indian Summer Monsoon mainly through the interactions with the mean flow. In addition, the monsoon mean flow and most energy exchanges show a significant relationship at the upper level (200 hPa) and lower level (850 hPa) atmosphere.
How to cite: Dhishana, R. and Dubey, S. K.: Characteristics of spectral energetics during excess and deficient rainfall years in India, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13539, https://doi.org/10.5194/egusphere-egu23-13539, 2023.
The Indian monsoon is critical since it supplies most of the water for drinking, sanitation, industry and agriculture for a billion people. The onset of monsoon typically starts in southern India by 1 June, taking up to 6 weeks to cover the country. Meanwhile, during the monsoon, variations on time scales of a week or more give rise to periods of excess and reduced rainfall, known as active and break events.
Being able to better predict the onset of the rains, their progression, and of active and break events in the monsoon would be of great. The timing of monsoon onset is already known to be influenced by tropical variability such as the Madden-Julian Oscillation. New research has shown that the mid-latitudes also exert a powerful control, but the full extent of this extratropical role in monsoon onset progression and in the timing of active and break periods is poorly quantified and understood.
The team behind the new MiLCMOP project earlier led the INCOMPASS field campaign to India, taking new measurements and generating new hypotheses on how the monsoon is controlled, including the concept that monsoon progression can be described as a “tug-of-war” between tropical and extratropical airmasses. This "tug-of-war" is an unsteady process, with a back and forth of the two airmasses before the moist tropical flow takes over for the rest of the season.
This poster describes some of the preliminary results on which the project is designed and explains the approach that MiLCMOP will use, including established techniques and development of new metrics to quantify the interactions between monsoon progression and extratropical forcing. These methods will include use of vorticity budgets and Lagrangian feature tracking, applied to reanalysis and model data in case study years of fast and slow onset behaviour, to determine the dominant mechanisms controlling monsoon progression. New model experiments will be designed and performed to isolate the mechanisms by which extratropical drivers affect monsoon onset and its progression. Finally, novel causal inference techniques will be used to disentangle the effects of extratropical drivers from those in the tropics.
The MiLCMOP project will eventually answer the following key questions: (1) How are the pace and steadiness of Indian monsoon progression affected by interactions with the extratropics? (2) What are the mechanisms of extratropical control on monsoon progression and variability? (3) In what way do the causal extratropical and tropical drivers of ISM progression offset or reinforce each other and can the competing roles of tropical and extratropical processes be generalised to other monsoons?
How to cite: Turner, A., Volonte, A., and Kretschmer, M.: Mid-Latitude Controls on Monsoon Onset and Progression (the MiLCMOP project), EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9295, https://doi.org/10.5194/egusphere-egu23-9295, 2023.
As one of the most significant circulation systems over the Northern Hemisphere in the cold season, the East Asian winter monsoon (EAWM) has been broadly investigated from the seasonal-mean perspective, while subseasonal variations in the EAWM still remain ambiguous. Based on Season-reliant Empirical Orthogonal Function (S-EOF) analysis, this study shows that the subseasonal strength reversal of the EAWM (SR-EAWM), featuring a weaker (or stronger) EAWM in early winter (December) and a stronger (or weaker) EAWM in late winter (January-February), is a distinct leading mode of the month-to-month variation of the EAWM. The weak-to-strong SR-EAWM is characterized by an anomalous low over Eurasia and a weakened East Asian major trough (EAT) in early winter, with an intensified Siberian High and a deepened EAT in late winter. The SR-EAWM is preceded by surface air temperature anomalies over Davis Strait (DST) and those over central-eastern North America (CENAT) in September-October. The DST mainly influences the SR-EAWM in early winter through a “sea ice bridge” of the November Baffin Bay sea ice concentration anomaly (BBSIC). The BBSIC could intensify the DST in December by altering surface heat flux, thus exciting a downstream atmospheric response and modulating the strength of the EAT in early winter. The preceding CENAT affects the SR-EAWM in late winter by inducing an “ocean bridge” of the western North Atlantic sea surface temperature anomaly (WNASST). The WNASST can persist into late winter and then significantly affects the SR-EAWM by regulating Eurasian circulation anomalies and the downstream EAT. The bridge roles of the BBSIC and WNASST can be further verified by a linear baroclinic model. Finally, two physical-empirical models are established using the DST/BBSIC and the CENAT indices, respectively. Both exhibit promising prediction skills. The results highlight that the DST, BBSIC, and CENAT are crucial predictability sources for the SR-EAWM.
How to cite: Zhong, W. and Wu, Z.: Subseasonal strength reversal of the East Asian winter monsoon, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-49, https://doi.org/10.5194/egusphere-egu23-49, 2023.
The complexity of the South China Sea (SCS) summer monsoon (SCSSM) onset is mainly reflected in the interaction of multiscale processes that include the seasonal cycle, 10-25-day ISO (HISO), 30-60-day ISO (LISO). In this study, the characteristics and mechanism of the HISO and LISO and their interaction with the background field are investigated when they trigger the SCSSM onset base on newly released reanalysis and remote sensing data for the period of 1979–2020.
The SCSSM onsets always are triggered by the second westward HISO or first northward LISO when the control of subtropical high pressure weakens on the SCS. The first HISO can be seen as a signal that the control is weak enough, and the SCSSM is about to onset. The SCSSM can also be established without the effects of the HISO or LISO, but the date would be put off. Based on the budget analysis of column-integrated moist static energy (MSE), the interaction between the easterly trade winds and the zonal gradient of MSE anomalies is considered the dominant reason for the HISO that can successively propagate westward from the western North Pacific. The SSTa-induced turbulent heat flux and the interaction between the mean southerly and the meridional gradient of the MSE anomaly are both important for the northward LISO from the equatorial Indian Ocean when it triggers the SCSSM onset. For the simulation and forecasting of the SCSSM onset, we put more emphasis on the role of the HISO because it is a more active process.
How to cite: Ma, T. and Yu, W.: The complexity of South China Sea summer monsoon onset, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-658, https://doi.org/10.5194/egusphere-egu23-658, 2023.
It has been argued in recent studies that the source of dry air originating over the desert regions of the West Asia i.e. Middle East intrudes over the continental India during the boreal summer resulting in more prolonged dry spells over India. Singh and Sandeep (2021, Clim Dyn) showed the existence of a huge reservoir of moist deficit air over the northern Arabian Sea at 850-hPa. In addition to this, it has been argued that low level jet undergo weakening and broadening prior to monsoon break phase in feedback to an increased barotropic instability. Furthermore, the monsoon low-level jet which transports the moisture to the continental landmass in the active phase acts as a main carrier in transporting this dry air towards the continental India during the break phase of the summer monsoon. In order to investigate the thermodynamic effects of dry air intrusion activity during dry phases of the Indian Summer Monsoon (ISM), isentropic analysis is performed on climate models simulations of Coupled Model Intercomparison Project Phase 6 (CMIP6). Here, we analyze the specific humidity and wind fields at 316 K isentropic level. The negative specific humidity anomalies of multi models average (MMA) signifies the pattern of dry air advection which shows that a large fraction of the moisture deficit is being transported to the continental India from the northern Arabian Sea, and only a small contribution comes from West Asia. The lead-lag composites of anomalous wind vectors and relative vorticity of MMA at 316 K isentrope clearly show a weakening of the monsoon circulation associated with the break conditions. The anomalous anti-cyclonic circulation pattern propagates westwards from the Bay of Bengal which is a well known feature of the monsoon break spells.
KEYWORDS: Low level jet; Dry air intrusion; Indian Summer Monsoon
How to cite: Singh, R. and Sandeep, S.: A thermodynamical study of dry air intrusion activity over India during dry phases of summer monsoon in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7948, https://doi.org/10.5194/egusphere-egu23-7948, 2023.
The balance of forces for the South Asian Summer Monsoon (SASM) gives valuable insights for the understanding of the mean circulation and the changes it has undergone in the past. In this study, we have analyzed the zonal momentum balance for SASM for the last few decades (1950-2010) using reanalysis data to understand the changes in different forces and relate them with the changes in the associated circulation. In the lower level (925 hPa), the Pressure Gradient Force (PGF), Coriolis Force (CF), and Residual Force (RF, which includes the unresolved sub-grid scale process and frictional terms) are found to be the dominant terms of the zonal momentum balance for SASM with a magnitude of order 10-4m sec-2 whereas, horizontal advection and eddy force terms are negligible with one or more order lesser in magnitude. The residual force can be estimated by Rayleigh friction induced by turbulence, particularly over ocean points, which, however, is not a good measure of the same over the land points because of high irregularity. The momentum balance at the upper level (200 hPa) is between the PGF, CF, and the advection term, unlike the lower level, where the residual force does not seem to be dominant. In the free troposphere, the Convective Momentum Transfer or in other words convective friction is a good estimator of the RF, which represents the vertical transport of momentum. The changes in SASM circulation in the past can be apprehended by looking into the changes in these vital forces that drive the motion.
How to cite: Upadhyaya, P. and Mishra, S. K.: Zonal Momentum Balance in South Asian Summer Monsoon: Forces and Changing Winds, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12313, https://doi.org/10.5194/egusphere-egu23-12313, 2023.
Posters virtual: Wed, 26 Apr, 14:00–15:45 | vHall AS
The sea surface temperature inter-hemispheric dipole (SSTID) is an important variability mode of global SST anomalies, characterized by an anti-phase variation of SST between the two hemispheres. In this study, the decadal variation of the Northern Hemisphere summer monsoon (NHSM) is found to be strongly regulated by the SSTID, with positive (negative) phases of the SSTID corresponding to the strengthening (weakening) of NHSM. Both observation and SST-forced atmospheric model simulations suggest that the SSTID related thermal forcing modulates the NHSM by causing planetary-scale atmospheric circulation adjustments. Positive SSTID events lead to coherent increase (decrease) of surface air temperature over the entire Northern (Southern) Hemisphere, increasing the inter-hemispheric thermal contrast (ITC). As sea level pressure changes are just opposite to air temperature, the increase of ITC enhances the inter-hemispheric pressure gradient (Southern Hemisphere minus Northern Hemisphere), leading to the strengthening of summer monsoonal circulation and the increase of monsoon rainfall in the Northern Hemisphere.
How to cite: Li, J., Xue, J., Wang, B., Yu, Y., Sun, C., and Mao, J.: Multidecadal variation of Northern Hemisphere summer monsoonforced by the SST inter-hemispheric dipole, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-3429, https://doi.org/10.5194/egusphere-egu23-3429, 2023.
The Indian Ocean tripole (IOT) is an independent mode of ocean–atmosphere circulation centered on the tropical Indian Ocean. This study explores the physical mechanisms of the IOT affecting the western United States climate variation during the boreal summer. We find that the IOT is significantly correlated with both western United States summer surface temperature and precipitation anomalies. During positive IOT events, the westerly wind anomalies over the northern Indian Ocean are intensified by two cross-equator airflows over the tropical eastern Indian Ocean and the east coast of Africa. The resulting convergence of air over the northern Bay of Bengal–Indochina Peninsula–northern South China Sea (NBB–IP–NSCS) region (80°–125°E, 15°–25°N) exacerbates the surplus precipitation there. Serving as a heat source, these NBB–IP–NSCS precipitation anomalies can excite a circum-global teleconnection-like (CGT–like) pattern that propagates eastward from west-central Asia towards North America along the Asia subtropical westerly jet, further influencing local circulation anomalies. Development of strong anticyclonic circulation over the western United States enhances descending motion and divergence there, resulting in negative precipitation anomalies. This circulation anomaly also induces the diabatic heating anomalies through allowing more solar radiation to reach the ground surface, further increasing the surface temperature anomalies. Meanwhile, the increased tropospheric temperature also raises local surface temperatures by modulating the adiabatic air expansion and compression. Ultimately, the CGT-like pattern associated with NBB–IP–NSCS precipitation anomalies sets up an atmospheric bridge by which the IOT can impact summer climate in the western United States.
How to cite: Zhang, Y. and Li, J.: Climatic effects of the Indian Ocean tripole on the western United States in boreal summer, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2159, https://doi.org/10.5194/egusphere-egu23-2159, 2023.
