Displays

How to cite: Seth, A., Giannini, A., Rojas, M., Rauscher, S., Bordoni, S., Singh, D., and Camargo, S.: A Review of Monsoon Responses to Warm Climates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3774, https://doi.org/10.5194/egusphere-egu2020-3774, 2020.

The mean and subseasonal monsoon variability is evaluated using simulations from 26 CMIP6 models in the present and future scenarios. In particular, the simulation of the monsoon trough, low pressure systems, and its relationship with seasonal rainfall, teleconnections with Pacific and Atlantic Oceans are analyzed, and the corresponding changes in the future scenario are investigated. Based on the fidelity of the model to simulate mean monsoon features, a set of models with good skill is identified. Selected good models are then used to analyze dynamical and teleconnection features. This study highlights and contrasts the performance of CMIP6 models in simulating various monsoon characteristics with CMIP5 models and further stresses the need for better water management strategies.

How to cite: Ravindran, A., Veluthedathekuzhiyil, P., and Cherumadanakadan Thelliyil, S.: South Asian Summer Monsoon Variability and its teleconnections in CMIP6 Simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4052, https://doi.org/10.5194/egusphere-egu2020-4052, 2020.

Improving projections of future changes in the global hydrological cycle is essential in order to understand the potential impacts of climate change and develop appropriate strategies of mitigation and adaptation to their socio-economics implications. This improvement requires a rigorous global climate model (GCM) evaluation, considering that several models often misrepresent fundamental processes of the global climate system. Recently, monsoons have been seen not just as independent systems that modulate the regional hydrology and climate but as a dominant global mode referred to as the Global Monsoon (GM). The GM is tied to global atmospheric circulation processes such as seasonal precipitation variations, the migration of the Inter-Tropical Convergence Zone (ITCZ), and the variability of the Hadley and Walker cells. Additionally, it can be seen as the response of the climate system to the annual solar radiation cycle. In this context, it is essential to consider not only regions with a marked seasonal change in the direction of surface winds but also the variation of precipitation in the tropics and subtropics. Reliable representations of its main characteristics are crucial for global simulations and climate change projections. 

This work assesses the ability of 64 GCMs part of three generations of the CMIP (phases 3, 5 and 6) simulating the most relevant characteristics of the global monsoon. Emphasis was placed on the GM domain and the two main modes of annual variation of precipitation and surface winds, referred to as Solstitial and Equinoctial modes. The GM wind domain and GM precipitation domain are well captured in most of the GCMs, and CMIP6 models show a significant improvement especially over the Asian-Australian monsoon (AAM) region. In order to evaluate the main modes of variability, we used projections of the model simulations onto the first two multivariate empirical orthogonal functions (MV-EOF) from observations. As a result, we find that in general, model performance is higher simulating the Solstitial mode compared to Equinoctial mode, but it has improved for both modes across the CMIP generations in terms of spatial variability and magnitude. Despite this, a regional analysis shows that performance over some regions, such as South America, does not exhibit significant improvement neither for the monsoon domain nor the annual variation modes.

We also considered the annual and seasonal mean of precipitation and surface winds, and we observed a notable improvement across CMIP generations to reproduce their spatial patterns of variability. However, biases of magnitude remain significant, mainly for global precipitation. Finally, it is relevant to point out that dispersion among GCMs was considerably reduced within CMIP6 and that we do not find a direct relationship between model performance and horizontal resolution. 

How to cite: Gómez, L. A., Hoyos, C. D., Cruz, D. C., and Webster, P. J.: Model performance in simulating the Global Monsoon: Skill evolution across CMIP generations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-919, https://doi.org/10.5194/egusphere-egu2020-919, 2020.

The Indian monsoon is a seasonal large-scale circulation system with complex dynamical and thermodynamical interactions, the physics of which is not fully understood. In particular, the advance of the monsoon over India, propagating against the mean mid-level wind field, cannot be explained by simple moisture flux arguments. 

Here we introduce an idealised two-layer model of the moisture dynamics of monsoon onset, with simple and transparent physics, based on conservation laws applied to a vertical plane (which could represent a transect from northwest to southeast India). The model allows for moisture replenishment in the lower layer (corresponding to evaporation or a moist inflow), a flux of water vapour between the layers (corresponding to convection), and along-transect advection by prescribed upper and lower-layer flows. With idealised parameterisations of replenishment and convection, the model can be written as a pair of coupled partial differential equations, which permits both analytical and numerical solutions. When an equilibrium solution is perturbed by either a change in replenishment rate, convection strength, or winds, we observe the propagation of moisture fronts in both the upper and lower layers as the solution adjusts to a new equilibrium. When these moisture fronts propagate northwestwards against the upper-layer flow, they can be viewed as the monsoon onset. Taking advantage of the simplicity of the model, which allows a wide parameter regime to be investigated efficiently, we show how the onset speed depends on the assumed timescales of the parameterised convection and lower-layer replenishment, and that physically plausible parameterisations can lead to realistic onset speeds, even in this highly idealised model.

How to cite: Recchia, L., Griffiths, S., and Parker, D.: An idealised model of the Indian monsoon onset, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10044, https://doi.org/10.5194/egusphere-egu2020-10044, 2020.

El-Niño Southern Oscillation (ENSO) affects Indian summer monsoon. Most of the worst droughts - the most recent being in 2009 - in India have been triggered by ENSO. But given the heterogeneity in rainfall patterns over India, we revisited ENSO influence on Indian summer monsoon. Our analysis based on multiple isotopic (proxy-based) and satellite data set shows significant variation in the spatiotemporal patterns of rainfall over the Indian subcontinent and adjoining oceans. We observed a weaker summer monsoon over central India and relatively stronger summer monsoon over northeast India during strong El-Niño events. Rainfall derived from isotope-enabled general circulation models reproduces weak and strong rainfall patterns during the El-Niño events over central India and northeast India, respectively. These model derived δ¹⁸O_rain (oxygen isotopic composition of rainfall) variation over central India during ENSO events mimic the weaker rainfall conditions. However, significant changes in the model derived rainfall and associated δ¹⁸O_rain is not observed over northeast India during ENSO events. Based on multiple data analysis, we infer that the long term variations (trends) in the Indian summer monsoon strength were controlled by the long term variation in ENSO during the last 50 years (1965 – 2013).

Since these observations were unprecedented and counterintuitive, we further verified our observations from the proxy records. Two speleothems (cave deposits) records from the central India and northeast India were analyzed for the rainfall variation and ENSO influence signatures. These paleo-proxy records showed a similar inverse relation of rainfall patterns over central India and northeast India during ENSO periods, confirming observed ENSO’s role on rainfall. Also, these proxy records showed a long-term pause in ENSO events or stronger La-Niña like conditions, which were persisted during 1625 – 1715 and favored stronger (weaker) rainfall over central India (northeast India).

How to cite: Singh, A., Pullabotla, K. K., and Rengaswamy, R.: Increase in summer monsoon rains in northeast India during ENSO periods: a multiproxy analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-376, https://doi.org/10.5194/egusphere-egu2020-376, 2020.

Hydrological extremes have increased in recent decades and are expected to escalate in the future. This led to global and regional water stress and drought hazards. Further down in the chain it impacts on farming, pollution, ecosystem, and socio-economic conditions. A better understanding of both quantitative and qualitative assessment of drought under changing climate is very crucial for sustainable water security and management. In the present study, over different homogeneous regions of India, using 19 Global Climate Models (GCMs) and Regional Climate Models (RCMs) 21 simulations datasets, drought climatology is prepared. The changes in drought distribution and characteristics analyzed using density functions and its probability of occurrence associated with return period derived from Standardized Precipitation Index (SPI) and Standardized Potential Evapotranspiration Index (SPEI). Each model is evaluated for biases against Multi-Model Ensembles (MME) and observational datasets for the reference period 1976-2005. Uncertainties from various sources associated with intermodal variability, including drought type and threshold, were evaluated. Under high emission (RCP8.5) scenario, both the ensembles (GCM and RCM) are showing the consistent spatiotemporal variability of precipitation and potential evapotranspiration with noticeable differences in magnitude. Biases are reduced in RCM over GCM (ensemble) with respect to observations. Modeled SPI is showing enhanced wetness derived from increased precipitation, while SPEI is exhibiting the drying trend largely associated with enhanced potential evapotranspiration under warming climate. There is an increase in the drought severity and intensity with the same return period in the future epoch. The overall analysis suggests the water scarcity and enhanced drought risks over India under unrestricted anthropogenic warming scenario.

How to cite: Saharwardi, M. S. and Kumar, P.: Drought variability and projections over India under high emission scenario with uncertainty assessment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-52, https://doi.org/10.5194/egusphere-egu2020-52, 2020.

The northern and the southern modes are two distinct principle modes that dominate the winter mean surface air temperature (Ts) variations over East Asia (EA). The cold southern mode is represented by a significant cooling south of 45°N and is linked to La Niña events. An objective criterion, which could distinguish the spatial distributions and the maximum center of sea surface temperature anomaly (SSTA), is used to classify the La Niña events into two categories: mega-La Niña and equatorial La Niña. Their impacts are inspected onto the Ts southern mode. The mega-La Niña, featured by a significant K-shape warming in the western Pacific with the maximum SSTA cooling centered in the tropical central Pacific. As a response, an anomalous barotropic high is generated over North Pacific (NP) implying a weak zonal gradient between ocean and the EA continent, which induces a neutral Ts southern mode. The equatorial La Niña characterizes a significant cooling in the tropical eastern Pacific with convective descending motions shifting eastward to the east of the dateline. The resultant low-level circulation anomalies show an anomalous subtropical NP low and a gigantic abnormal EA continent high. The strong zonal gradient results in significant northerly anomalies over EA from 55°N to southeastern China. Over the mid-upper troposphere, the anomalous subtropical NP low extends westward to the Korean Peninsula, leading to a strengthened and southward shifted EA trough. Such abnormal circulation patterns favor the intrusion of cold air to southern EA and correspond to a strong Ts southern mode. The numerical results well validate the above processes and physical mechanisms. 

How to cite: Wu, Z.: Reexamining the Relationship of La Niña and the East Asian Winter Monsoon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-159, https://doi.org/10.5194/egusphere-egu2020-159, 2020.

The Sahel is the semi-arid, transitional region separating the Sahara Desert from humid equatorial Africa, i.e. the poleward-most region to which appreciable rains from the West African monsoon extend during northern summer.  The severe drought it experienced in the 1970s and 1980s was one of the 20th century's most striking (and devastating) hydroclimatic events worldwide.  In climate model simulations of future global warming, Sahelian rainfall does anything from intense drying to even greater wettening depending on which climate model is used.  In this talk, I present recent research on rainfall in the Sahel using the moist static energy (MSE) budget -- what are the physical factors that drive its variations, and how do we expect them to change as the planet warms --- and the extent to which inferences from the Sahel can or cannot extend to other regions and other external forcings.

Using climate model simulations both of Earth's present-day conditions and of future global warming, I show that the drying influence of the Sahara Desert is a dominant factor in present-day and that this influence is strengthened with warming due to an increasing difference in moisture between the desert and the Sahel.  This enhancement of an existing moisture (and energy) gradient is a robust response of the atmosphere to mean ocean surface warming and has a firm theoretical basis.  By comparing climate model simulations of the present-day Sahel climate to real-world observations, I argue that this Sahara-driven drying mechanism is overly strong in those models that dry the Sahel most in future simulations.  This response to mean warming of global sea surface temperatures (SSTs) is readily explained using the MSE budget, whereas the Sahel rainfall response to changes in the spatial pattern of SSTs (such as during the 1970s-80s drought) are more easily interpreted via the popular energetic framework for Intertropical Convergence Zone (ITCZ) shifts.  I discuss the interplay between these and other theoretical frameworks for forced monsoon rainfall changes in the Sahel and other monsoon regions and offer ideas for refining and extending those theories.

How to cite: Hill, S.: Forced monsoon rainfall changes and the moist static energy budget: the Sahel and elsewhere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11632, https://doi.org/10.5194/egusphere-egu2020-11632, 2020.

The South Asian monsoon (SAM) precipitation has been generally regarded to exhibit contrasting responses to greenhouse gas (GHG) and anthropogenic aerosol forcing, although it is not adequately clear as to how it might respond to the combined influence of GHG and aerosol forcing.  The present study examines the individual and combined effects of global warming and anthropogenic aerosols on the SAM based on a suite of numerical experiments conducted using the IITM Earth System Model version2 (IITM-ESMv2). Four sets of 50-year model integrations are performed using IITM-ESMv2 with different anthropogenic forcings 1) Pre-Industrial control, 2) anthropogenic aerosols of 2005 3) CO2 concentrations of 2005 4) anthropogenic aerosols and CO2 of 2005. In the experiment with the elevated CO2 level of 2005, an intensification of SAM precipitation and strengthening of large-scale monsoon cross-equatorial flow is noted relative to the PI-CTL run. In contrast, the experiment with elevated anthropogenic aerosols of 2005 shows a decrease of SAM precipitation and weakening of monsoon circulation relative to the PI-CTL run. A striking result emerging from this study is the strong suppression of SAM precipitation, pronounced weakening of the monsoon circulation and suppression of organized convection in response to the combined radiative effects of elevated CO2 and anthropogenic aerosols relative to the PI-CTL run. By diagnosing the model simulations it is noted that the radiative effects in the combined forcing experiment lead to a pronounced summer-time cooling of the NH as compared to the equatorial and southern oceans which are predominantly influenced by global warming, thereby creating a north-south differential radiative forcing over the Indian longitudes.  Additionally, the influence of absorbing aerosols over South and East Asia creates a surface radiation deficit over the region, stabilizes the lower troposphere, slows down the monsoon winds and reduces surface evaporation.  Although the anticyclones over the subtropical Indian Ocean intensify in the combined forcing experiment, the model simulation shows that much of the precipitation enhancement occurs to the south of the equator over the Indian Ocean whereas the moisture transport and convergence to the north of the equator is substantially reduced. Furthermore, the combined forcing experiment shows that anomalous large-scale descent over the subcontinent reinforces the suppression of organized convection giving rise to more intense breaks and weaker active spells in the southwest monsoon on sub-seasonal time-scales. This study hints that future decreases in NH aerosol emissions could potentially reverse the ongoing decreasing trend of the observed SAM precipitation since 1950s in a purely global warming environment.

How to cite: Dey Choudhury, A., Raghavan, K., Singh, M., Panickal, S., Narayansetti, S., A.Gopinathan, P., and Vellore, R.: Combined effects of anthropogenic aerosols and global warming on the South Asian Monsoon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-923, https://doi.org/10.5194/egusphere-egu2020-923, 2020.

Monsoons are summertime circulations shaping climates and societies across the tropics and subtropics. Here the radiative effects controlling the climatological monsoon and its response to climate change are investigated using idealized simulations. The influences of clouds, water vapor and CO₂ on the monsoon are decomposed using the radiation-locking technique. Seasonal cloud and water vapor radiative effects strongly modulate the climatological monsoon, reducing net monsoon precipitation by approximately half. Warming and moistening of the monsoon by seasonal longwave cloud and water vapor effects are counteracted by a strong shortwave cloud effect. The shortwave cloud effect expedites monsoon onset by approximately 10 days, whereas longwave cloud and water vapor effects delay onset. A simple theory for monsoon onset relates monsoon onset to the efficiency of surface cooling. In climate change simulations the water vapor feedback and CO₂ forcing have similar influences on the monsoon, warming the surface and moistening the region. In contrast, clouds have a negligible effect on surface temperature yet dominate the response of the monsoon circulation to climate change. The radiation-locking simulations and analyses advance understanding of how and why radiative processes influence the monsoon, and establish a new framework for interpreting monsoon--radiation coupling in observations, in state-of-the-art models and in different climate states. Moreover, sensitivities of the monsoon to the longwave cloud feedback are found to be similar over the seasonal cycle and under CO₂ forcing, suggesting a potential emergent constraint for monsoons in a changing climate.

How to cite: Byrne, M. and Zanna, L.: Radiative effects of clouds and water vapor on the monsoon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5419, https://doi.org/10.5194/egusphere-egu2020-5419, 2020.

Tropical cyclones (TCs) in the western North Pacific (WNP) are modulated by large-scale circulation systems such monsoon trough, intraseasonal oscillation, teleconnection pattern, El Niño and Southern Oscillation, and some interdecadal oscillations. While the low-frequency, large-scale circulation produces a clustering effect on TCs, the latter in return leave marked footprints in climate mean state and variability because of large amplitudes in circulation and strong heating. In this study, we applied PV inversion technique to remove TCs from reanalysis and evaluate their contribution to mean circulation and climate variability. It is found that the mean climatological circulation (e.g., low-level monsoon trough and upper-tropospheric anticyclone) would be much weaker with TCs removed. Intraseasonal and interannual variance of certain variables could decrease by as much as 40–50 percent. An accompanied study indicated that TCs had slowed down the sea surface warming in the WNP for the past few decades because of TC-induced cooling. Our results suggest that TC effect has to be considered to understand the climate variability in the tropical atmosphere and ocean. The ensemble effect of TCs, at least in the statistical sense, has to be resolved in climate models to better simulate climate variability and produce more reliable climate projection in the TC-prone regions.

How to cite: Hsu, H.-H.: Footprints of Tropical Cyclones in WNP Summer Monsoon Variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13325, https://doi.org/10.5194/egusphere-egu2020-13325, 2020.

The EASM largely determines variations in summer precipitation in the East Asian monsoon region where approximately one-quarter of the world’s population live. A reliable East Asian summer monsoon (EASM) index covering several centuries is important in order to understand EASM dynamics. The wind-field is frequently used to calculate the EASM index during the instrumental period. However, available climate proxy data rather respond to direct precipitation changes. A gridded extended summer (May–September, MJJAS) precipitation reconstruction for China covering AD 1470–2000 is used to indirectly reconstruct two types of EASM indices (defined by the strength of the 850hPa southwesterly winds and a north-south gradient of the zonal winds), using the negative correlation between the EASM index and summer (June–August, JJA) rainfall in the middle and lower reaches of the Yangtze River of China. The two EASM indices are validated by independent historical documentary data for eastern China. The physical processes ruling the EASM variability are explored, highlighting a baroclinic structure over the middle and lower reaches of the Yangtze River. It includes an anticyclonic circulation accompanied by high pressure anomalies in the lower troposphere and a cyclonic circulation with low pressure anomaly in the upper troposphere. This is associated with a decrease in atmospheric water vapor content (due to divergence), which will decrease summer rainfall in the region, and contribute to the strengthen of the EASM variability. The dominated and inter-annual component of the EASM variation is possibly linked to the ‘ENSO-like’ sea surface temperature according to a data assimilation experiment performed with the Community Earth System Model-Last Millennium Ensemble (CESM-LME) simulation.

How to cite: Shi, F., Goosse, H., Li, J., Ljungqvist, F. C., Zhao, S., Liu, T., Yin, Q., and Guo, Z.: A reconstruction of the East Asian summer monsoon index over the half past millennium, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6488, https://doi.org/10.5194/egusphere-egu2020-6488, 2020.

Austral wet season precipitation (October through March) in the subtropical parts of Brazil is related to the strength and position of the South American Convergence Zone (SACZ), one of the main features of the South American Monsoon System. The SACZ can be defined as the aggregation of individual tropical-extratropical (TE) cloud bands. Such TE cloud bands have deep convection and heavy rainfall linking the tropical convection over the Amazon rain forest to the mid-latitude weather systems in the Southern Ocean. Utilising a cloud band identification technique, which consists of an object-based algorithm that identifies TE interactions, we detected individual weather systems and explored their associated precipitation characteristics and changes since 1980. Each event is characterised by the total precipitation within the contour of the low-value OLR. For this, we considered three different datasets: observed precipitation from various weather stations over Brazil, gridded to a 0.25° lat/lon resolution; satellite-based rainfall from TRMM (version 3B42); and reanalysis-based precipitation from ERA5. Here we explore the spatial characteristics and associated precipitation statistics of the SACZ events identified through the proposed technique. The monthly spatial signature of the selected events is similar among the three data sources and corresponds to the SACZ location. The selected events account for 25% to 50% of the total monthly precipitation during the wet season, with the largest percentages occurring in December and January. Over South-eastern Brazil, we identified a reduction in the number of events and in total precipitation during these events, resulting in a reduction of their contribution to the total precipitation climatology during the last decade. The drying trends occur mostly in December; in January, the areas with reduced precipitation migrate northward and precipitation increases over Southern Brazil, suggesting that the poleward migration of the SACZ is more pronounced during these months. These results demonstrate the relationship between synoptic systems and the changes in the location of the SACZ described in recent studies. In the next steps, we will diagnose the reanalysed and climate-simulated circulations associated with these events, identifying possible mechanisms responsible for the poleward shift of the SACZ.

How to cite: Zilli, M. and Hart, N.: Synoptic climatology and changes in precipitation associated with the South Atlantic Convergence Zone utilising a cloud band identification technique, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11038, https://doi.org/10.5194/egusphere-egu2020-11038, 2020.

Previous studies have revealed that the Tibetan Plateau (TP) can weaken the high-frequency and low-frequency transient eddies (TE) transported along the westerly jet. Here the effects of TP on East Asian summer monsoon via weakened TE are investigated based on the simulations by the NCAR Community Earth System Model, in which a nudging method is used to amplify the TP’s inhibition of TE without changing the steady dynamic and thermodynamic effects of TP. Results reveal that the weakened TE by TP weaken the East Asian westerly jet (EAWJ) and shift the jet southward via transient vorticity flux. The southward EAWJ accompanied with reduced poleward transport of moisture by TE results in less rainfall in northern East Asia but more rainfall in southern East Asia, particularly in early summer when the EAWJ is stably located over the TP and the meridional gradient of water vapor is large. Furthermore, the anomalous precipitation can move the EAWJ further southward through the anomalous diabatic heating in early summer, forming a positive feedback. Therefore, the TP’s inhibition of TE can shift the East Asian rain belt southward, different from the TP’s steady forcing which favors a poleward shift of the rain belt. It is also demonstrated that the atmospheric internal variability can lead to the south-flood-north-drought pattern of summer rainfall change over East Asia, indicating the important role of TE in East Asian summer monsoon.

How to cite: Ren, Q., Yang, S., Jiang, X., Zhang, Y., and Li, Z.: Effects of the Tibetan Plateau on East Asian Summer Monsoon via Weakened Transient Eddies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4341, https://doi.org/10.5194/egusphere-egu2020-4341, 2020.

models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). sea surface temperature

How to cite: Gan, R.: Assessing the internal variability in multi-decadal trends of summer rainfall over East Asia-Northwest Pacific with a large ensemble of GCM simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1557, https://doi.org/10.5194/egusphere-egu2020-1557, 2020.

The influence of El Niño-Southern Oscillation (ENSO) on the Indian Ocean Dipole (IOD), a coupled ocean–atmosphere mode of interannual climate variability, has been widely investigated over recent decades. However, a latest study indicates that the South China Sea summer monsoon (SCSSM) might also be responsible for IOD formation. Furthermore, an abnormal SCSSM does not always coincide with ENSO during boreal summer (June–August, JJA); consequently, the individual and combined effects of the SCSSM and ENSO on the IOD remain elusive. This study shows that the amplitude of the IOD tends to be much stronger under the coexistence of SCSSM and ENSO than that under individual SCSSM or ENSO events during JJA and autumn. The findings also indicate that the SCSSM and ENSO play the dominant role around the eastern and western poles of the IOD, respectively. An anomalous local Hadley circulation closely related to the stronger SCSSM favors anomalous southeasterly of Sumatra and Java during JJA, which enhance oceanic upwelling and subsequently result in cooling of the sea surface temperature (SST) over this area. Similarly, it can be envisaged that the contemporaneous ENSO could influence JJA SST anomalies over the western Indian Ocean via the Walker circulation coupled with oceanic variations.

How to cite: Li, J., Zhang, Y., Xue, J., Zheng, F., Wu, R., Ha, K.-J., and Feng, J.: The relative roles of the South China Sea summer monsoon and ENSO in the Indian Ocean dipole development, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1819, https://doi.org/10.5194/egusphere-egu2020-1819, 2020.

Indian monsoon, which spans through the months of June-September, brings in copious rain for the agriculture dependent country India. Monsoon low pressure systems (LPS) are the major rain bearers during the season. Apart from being a lifeline, they are also cited as a cause of disastrous floods in the country. Various approaches have been attempted to locate and track these LPS. Inconsistency exists among  them in statistics of LPS not only for the historical period, but also in future projections of these systems. We have developed an improved tracking scheme in this study. . The new approach takes into consideration geopotential height anomaly condition and is  named Automated Tracking algorithm using geopotential criteria (ATAGC). The approach is validated by comparing characteristics of LPS identified by it with those identified in previous studies. On average, around 14 LPS  each year are identified by the new approach, which comprise 9 lows, 4 depressions and about one deep depression. Further, the annual average number for LPS days is estimated as 68. The LPS mostly form over north part of Bay of Bengal and move north-westwards. Synoptic Activity Index, which quantifies LPS risk at a location in terms of both frequency and intensity of the system, shows that locations in the coastal regions of central India are highly affected by LPS. But the effect in terms of extreme rainfall is not localized near  the coast. Even though contribution of LPS towards total monsoon rainfall and total extreme precipitation has been analyzed in previous studies, the risk in terms of extreme rainfall due to LPS has not been assessed. In this study, extreme rainfall risk map in terms of average extreme precipitation and 90 percentile precipitation observed at a location in the vicinity of an LPS is determined. An average extreme rainfall of 60-100mm/day and 90 percentile extreme rainfall of 150-250mm/day is estimated at many locations in Central Indian region due to LPS. While analyzing continuous spells of rainfall, it is found  that along with LPS, topography of a region has considerable effect on the duration of the spells.

How to cite: Thomas, T. M., Bala, G., and Venkata Vemavarapu, S.: Statistics of Monsoon Low Pressure Systems in the Indian Subcontinent and Estimation of Related Extreme Rainfall Risk, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1837, https://doi.org/10.5194/egusphere-egu2020-1837, 2020.

It is known that the existence of the Tibetan Plateau (TP) intensifies the Asian summer monsoon. However, is the current subtropical location of the TP optimal for energizing the monsoon? Would monsoon dynamics become simpler if the TP were located in the tropics? A series experiments with the NCAR CESM fully-coupled model show that a change in the current subtropical TP causes apparent responses in both divergent and rotational motions of the atmosphere in the tropics and higher latitudes, respectively. When the TP is moved southward, the atmospheric response is featured by more apparent thermally-driven and divergent part of atmospheric motion, and the tropical South Asian monsoon becomes stronger. However, the subtropical East Asian monsoon becomes weaker due to the intensify of Northwest Pacific subtropical high. In the experiments in which the TP is moved northward, the subtropical East Asian monsoon strengthens at some points but the tropical South Asian monsoon weakens. Besides, variations in the meridional position of the westerlies relative to the TP lead to anomalous distribution of precipitation in East Asia. In these latter experiments, the atmospheric response is apparently featured by rotational characteristics of the atmospheric motion.

Results also show that the meridional shift of the TP would also cause changes in the African summer monsoon, whose variability is closely linked to the variations of the Asian summer monsoon.

How to cite: Wang, J. and Yang, S.: Is the Current Subtropical Position of the Tibetan Plateau Optimal for Intensifying the Asian Monsoon?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2247, https://doi.org/10.5194/egusphere-egu2020-2247, 2020.

Future greenhouse warming is expected to influence the character of global monsoon systems. However, large regional uncertainties still remain. Here we use 16 CMIP6 models to determine how the length of the summer rainy season and precipitation extremes over the Asian summer monsoon domain will change in response to greenhouse warming. Over East Asia the models simulate on average on the earlier onset and later retreat; whereas over India, the retreat will occur later. The model simulations also show an intensification of extreme rainfall events, as well as an increase of seasonal drought conditions. These results demonstrate the high volatility of the Asian summer monsoon systems and further highlight the need for improved water management strategies in this densely-populated part of the world.

How to cite: Ha, K.-J., Moon, S., Timmermann, A., and Kim, D.: Future changes of summer monsoon characteristics and evaporative demand over Asia in CMIP6 simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3269, https://doi.org/10.5194/egusphere-egu2020-3269, 2020.

The influence of the North Pacific Victoria mode (VM) on the Madden–Julian Oscillation (MJO) are examined in this analysis. The results show that the February–April (FMA) VM had a significant influence on the development and propagation of the MJO over the equatorial central–western Pacific (ECWP) during spring (March–May) between 1979 and 2017. Specifically, MJO development was favored more by positive VM events than negative VM events. One probably description for these complicated connections is that the SST gradient anomalies associated with positive VM events enhance the convergence of low-level over the ECWP, which, combined with the warm SST anomalies (SSTAs) in the equatorial central Pacific that lead to a boost in the Kelvin wave anomalies, results in the enhanced MJO activity over the ECWP. These conclusions indicate that the VM is an important factor in MJO diversity.

How to cite: Wen, T., Chen, Q., Li, J., Ding, R., Tseng, Y., and Hou, Z.: Influence of the North Pacific Victoria mode on the Madden–Julian Oscillation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3294, https://doi.org/10.5194/egusphere-egu2020-3294, 2020.

The American Monsoon System is the main source of rainfall for the tropical and sub-tropical Americas. CMIP6 climate model simulations from the MetOffice Hadley Centre (MOHC) models: HadGEMGC3.1 and the Earth System model UKESM1 were analyzed to evaluate the representation of this monsoon. Pre-industrial and historical experiments were compared to reanalyses and observations.  Several diagnostics, such as the Inter-tropical Convergence Zone (ITCZ) location, the Walker circulation and temperature and precipitation seasonal cycles in the American Monsoon System were assessed, as well as El Niño-Southern Oscillation teleconnections to the monsoon. 

These simulations reasonably represent the observed seasonal cycle of precipitation in the American Monsoon System. However, significant biases in the spatial distribution of rainfall in South America are evident. 
These biases in the South American Monsoon System are linked to temperature biases in the Amazon and Atlantic ITCZ biases. 
The midsummer drought regime in Central America, the Caribbean and southern Mexico is reproduced by all the simulations, although with a stronger intraseasonal cycle than observed.
The North American monsoon is relatively well represented by all the simulations, which is a noticeable improvement of these models compared to CMIP5. 

The overall performance of HadGEM3 at different horizontal resolutions was compared to that of UKESM1. 
This work evaluates the role of horizontal resolution and Earth System processes for monsoon representation which will be useful for interpreting scenario experiments or using CMIP6 runs for understanding variability and
teleconnections in this monsoon system.  

How to cite: García-Franco, J. L., Gray, L., and Osprey, S.: The American Monsoon System in UKESM1 and HadGEM3, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3623, https://doi.org/10.5194/egusphere-egu2020-3623, 2020.

The Victoria mode (VM) and Pacific meridional mode (PMM) are the dominant SST modes over the North Pacific. Both are forced by a North Pacific Oscillation (NPO)-like extratropical atmospheric variability, and can act as a bridge (or conduit) through which North Pacific extratropical atmospheric variability influences ENSO. Consequently, the VM shares some resemblance with the PMM. However, the VM and PMM differ in terms of their spatial structure, temporal variations, and impacts on ENSO. In contrast to the local SST mode of the PMM in the subtropical northeast Pacific, the VM, as a basin-scale SST mode of the North Pacific basin, includes large-amplitude SSTAs over the northeast Pacific, the western North Pacific (WNP), and the high-latitude North Pacific. Results indicate that SLP anomalies associated with the VM are generally located west of those associated with the PMM. In addition, the VM has a unique temporal variability, independent of the PMM. Furthermore, the VM is more closely linked to ENSO than is the PMM, possibly because the VM combines the effects of the PMM and SSTAs in the WNP. Thus, the VM represents a more reliable precursor signal than the PMM for ENSO events and may have profound implications for ENSO prediction.

How to cite: Ji, K., Zuo, H., Li, J., and Ding, R.: Analysis of the differences between the North Pacific Victoria and meridional modes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3886, https://doi.org/10.5194/egusphere-egu2020-3886, 2020.

Monsoon low pressure systems (LPS) contributes to more than half of the Indian monsoon rainfall. However most climate models fail to capture the characteristics of low pressure systems realistically. This aspect is scrutinized in a wide range of available CMIP6 model simulations using an objective LPS tracking algorithm. Broader features such as monsoon trough over which these systems forms are also analyzed. It has been found that, majority of the models fail to realistically represent these two important features. However few models that were able to capture these events in CMIP5 are able to simulate them in CMIP6 as well. We examine the dynamical features that lead to realistic simulation of LPS in these set of models. Selected good models are then used to study the characteristics of LPS in a future warming scenario. This study will help in judging the performance of models and for any future improvements.

How to cite: Veluthedathekuzhiyil, P., Ravindran, A., and Cherumadanakadan Thelliyil, S.: Simulation of Monsoon trough and low pressure systems in CMIP6 models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4049, https://doi.org/10.5194/egusphere-egu2020-4049, 2020.

The West African monsoon (WAM) originates in the Gulf of Guinea when the intertropical convergence zone (ITCZ) makes its landfall; whilst, the south Asian monsoon (SAM) originates in the Indian ocean when the ITCZ crosses the equator. The monsoonal dynamics are here studied after landfall using Gill’s tropospheric model with an implanted Ekman frictional layer (EFL). Ekman pumping increases low level convergence, making the lower monsoonal cyclone deeper and more compact that the upper anticyclone, by transferring tropospheric vorticity into the EFL. In the upper troposphere, air particles spiral-out anticyclonically away from the monsoons, subsiding over the Tropical Atlantic, the Tropical Indian ocean, or transiting into the southern hemisphere across the equator. Whilst marine air particles spiral-in cyclonically towards the WAM or the SAM, the latter appears to be a preferred ending destination in the absence of orography. The Himalayas introduced as a barrier to the monsoonal winds, strengthen the tropospheric winds by tightening the isobars. The Somali mountains (SMs), introduced as a barrier to the Ekman winds, separates the WAM and the SAM catch basins; thus, the Atlantic air particles converge towards the WAM and the Indian ocean particles converge towards the SAM. The Indian Ghats (IGs), introduced as a semi-impermeable barrier to the Ekman winds, deflect the marine air particles originated in the western Indian ocean towards the south-eastern flank of the SAM. In short, an upper single anticyclone encircles both monsoons; the Himalayas strengthen the upper-level winds by increasing the pressure gradients; the SMs split the EFL cyclone, keeping the marine air particles to the west of SMs in the WAM basin and the particles to the east of SMs in the SAM basin; the IGs guides transmit the air particles, deflecting them towards Bangladesh.

How to cite: Dalu, G., Gaetani, M., Flamant, C., and Baldi, M.: Role of friction and orography in the Asian-African monsoonal system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7619, https://doi.org/10.5194/egusphere-egu2020-7619, 2020.

Extreme weather creating a widespread humanitarian crisis over East Africa in recent decades. The seasonal cycle of precipitation over the Horn of Africa (HOA) shows bimodality with long rain and short rain. Most of the models fail to capture biannual rainfall seasonal cycles, due to circulation response to unrealistically dominate the annual mean. The Community Earth System Model (CESM) high-resolution model simulation has been employed to study the sensitivity. Precipitation distribution over HOA shows regional variations where most of the region show the bimodal distribution and the intrinsically complex. This bimodality is nominally associated with tropical rain belt, but topography and SST-forcing also play an important role in influencing the timing and intensity of seasonal rainfall. The results show that overall rainfall seasonality is increased, with intensification over high elevation. Precise representation of rainfall seasonal cycle over HOA adds confidence for future projected changes in seasonality. An important question is whether and how the seasonal cycle over HOA responds to anthropogenic forcing. We show that the future change in precipitation seasonal cycle and accumulation over HOA can be explained by the surface ocean process which module SSTs along the coastline of Somalia. The moisture convergence over low elevation land is basically regulated through the north-south SST gradient. In conclusion, future global warming leads to the intensified seasonal cycle of precipitation with a projected increase in the short rain season over east Africa. Further analysis demonstrates how topography modulates the seasonality of HOA.

How to cite: Kad, P. and Ha, K.-J.: Future change in precipitation seasonality over the Horn of Africa in high-resolution simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12782, https://doi.org/10.5194/egusphere-egu2020-12782, 2020.

How to cite: Turner, A., Shonk, J., Wilcox, L., Dittus, A., and Hawkins, E.: Uncertainty in aerosol radiative forcing impacts the simulated global monsoon in the 20th century, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16401, https://doi.org/10.5194/egusphere-egu2020-16401, 2020.

The Indian summer monsoon (ISM) has profound impacts on the economy and society since it directly affects more than a billion people in the Indian subcontinent. The development of land-ocean thermal contrast during May and June creates meridional temperature and pressure gradients and sets up the ISM circulation. The ISM circulation comprises of two branches, the Arabian Sea (AS) branch that is associated with southwesterly winds blowing from the AS towards the Indian landmass; and the Bay of Bengal (BoB) branch characterized by cyclonic circulation extending from the BoB into central and north India. These two branches dictate the advance of the ISM.

Forecast of the ISM rainfall is challenging even though it has been a research question for many decades. The Indian Meteorological Department forecasts the onset of monsoon over the Kerala, in the south [1]. The recently developed tipping element approach [2] allows forecasting the onset and withdrawal of the monsoon over Central India. However, every state in India desperately needs both forecasts: the onset and withdrawal of the monsoon. Uncertainty and delays (eg. year 2019) in the advance and withdrawal of monsoon results in farmers losing their crop investment. Further, there is no clear consensus on the effect of global warming on the monsoon timing.

Here we explore climate change effects on the advance of the ISM onset towards central India analysing observational data of Outgoing Longwave Radiation (OLR), near-surface air temperature and wind. OLR is a proxy for organized deep tropical convection, wherein low values of OLR correspond to deep clouds with low cloud-top temperatures and high values of OLR correspond to scarcity of clouds.

We use the tipping element approach [2] to reveal tipping in spatially organized rainfall. We find two tipping elements appearing in the AS and the BoB prior to the onset of monsoon in central India (MOC). Maximum fluctuations in the OLR at the tipping elements near MOC indicate deep convection within the two branches of monsoon. The abrupt transition in the OLR at the tipping elements corresponds to the transition from pre-monsoon to monsoon in Central India. We observe an interplay between the temporal dynamics of OLR at these two regions, which indicate the MOC. In these two regions, during the pre-monsoon season the average OLR closely follow each other. Subsequently, the time series of OLR in these two regions diverge from each other, which indicates MOC.

Under climate change, the temporal dynamics of OLR at these two locations show that the transition from pre-monsoon to monsoon has changed from an abrupt transition to a gradual transition in the Bay of Bengal. Furthermore, we identify different spatial patterns of near-air surface temperature, OLR and wind for early, normal and late MOC. We use these patterns as indicators for forecasting advance of ISM.

NBG and ES acknowledge the support of the EPICC project (18_II_149_Global_A_Risikovorhersage) funded by BMU

[1] https://mausam.imd.gov.in/

[2] Stolbova, V., E. Surovyatkina, B. Bookhagen, and J. Kurths (2016). GRL 43, 1–9 [doi:10.1002/2016GL068392]

How to cite: George, N. B., Surovyatkina, E., Krishnan, R., and Kurths, J.: Abrupt transition in organized convection during the monsoon onset in central India and Climate change effect, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20285, https://doi.org/10.5194/egusphere-egu2020-20285, 2020.

In this study, we estimate the changes in extreme precipitation indices over the western North Pacific and East Asia region (WNP-EA) during the spring and Mei-yu seasons in the warmer climate. Our analyses are based on two high-resolution atmospheric general circulation model simulations. The high-resolution atmospheric Model (HiRAM) was used in a series of simulations, which were forced by 4 sets of sea surface temperature (SST) changes under Representative Concentration Pathways 8.5 (RCP8.5) scenario. The Database for Policy Decision-Making for Future Climate Change (d4PDF) consists of global warming simulation outputs from MRI-AGCM3.2 with large ensemble members and multiple SST warming scenarios.

In the spring season, the changes in the spatial pattern of SDII, RX1day, and PR99 demonstrate greater enhancement over the northern flank of the climatological rainy region in both HiRAM and d4PDF, implying a northward extension of spring rain band. Besides, the changes in probability distribution display a shifting tendency that heavier extreme events occur more frequently in the warmer climate. The above changes are larger than the internal variability and uncertainty associated with SST warming patterns, indicating the robustness of the projected enhancement in precipitation intensity in the WNP-EA region. The spatial pattern for changes in CDD and total rainfall occurrence are less consistent between two datasets. In the Mei-yu season, the tendency toward more frequent extreme events in the probability distributions are consistently found in HiRAM and d4PDF. However, the changes in the spatial pattern of all indices are less consistent between HiRAM and d4PDF, implying larger uncertainty in the projection of extreme precipitation in the Mei-yu period in the warmer climate.

How to cite: Chen, C.-A. and Hsu, H.-H.: Changes in spring and Mei-yu extreme precipitation in the Western North Pacific and East Asia in the warmer climate in two high-resolution AGCMs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21514, https://doi.org/10.5194/egusphere-egu2020-21514, 2020.

This study aims to develop a large-scale climate classification for investigating the mechanisms of global climate formation in the surface. There are three types of large-scale climates, i.e., monsoon, Mediterranean and westerlies, corresponding respectively to collocation of temperature and precipitation at in-phase, anti-phase and out of phase, during seasonal cycle. The first one is called proper collocation, and the latter two are named as improper collocation, hereafter. The collocations are coupled with different seasonal moisture transport pattern with moisture divergence. Northward/southward moisture transport accompanies a moisture convergence/divergence with more/less precipitation in the season leading to different climate type. As an example, the climate around Tibetan Plateau can be attributed to four regimes, i.e., East Asia monsoon, South Asia monsoon, Central Asia and westerlies regimes, despite of the Köppen climate classification. The Central Asia regime refers to the dry climate in middle and southern part of the area, while the dry land belt with the westerlies regime extends from northern Central Asia throughout the northwestern China. The proper collocation between temperature and precipitation leads to a warm-wet climate over monsoon zones in warm season (May-October), whereas the improper one leads a hot-dry climate in Mediterranean climate areas and the dry land with the westerlies climate regime. By contrast, a mild-wet climate is in Mediterranean or quasi-Mediterranean climate areas in comparison with cold-dry climate in Asian monsoon zone during cold season (November to April). The improper collocation results in land degradation or even desertification in Mediterranean climate areas and the dry land with the westerlies regime with insufficient precipitation and over-evenly distribution of the precipitation during seasonal cycle. The improper collocation is actually made by improper dynamical and thermal dynamical collocation in regional moisture circulation associated with seasonal change of mid-latitude stationary waves in wave number and phase, which is virtually forced by large mountains and land-sea thermal contrast in the surface. Besides, analysis manifests that there exists mutually engagement between the seasonal changes in some properties of the mean moisture flows over monsoon and non-monsoon areas across Tibetan Plateau in Eurasian continent. It implies a dynamical coupling existed in large-scale moisture patterns over the earth surface.

Keywords: Large-scale climate classification, monsoon, westerlies, Mediterranean climate, Tibetan Plateau

How to cite: Dai, X.-G. and Wang, P.: A climate classification: Mediterranean, monsoon and westerlies climates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2081, https://doi.org/10.5194/egusphere-egu2020-2081, 2020.

The East Asian Summer Monsoon (EASM) is a crucial monsoon system that profoundly influences the summer climate in Southeast China (SEC). Classification of monsoon rainfall patterns is vital to physical diagnosis, rainfall prediction and identification of sites that are prone to rainfall-triggering floods. With the great endeavors on understanding the complexity of the EASM in the past decades, the traditionally accepted rainfall patterns in SEC and the relevant analyses appear outdated or even inadequate. Having highly-improved observations at hand helps update the monsoon rainfall patterns in SEC and the potential predictability.

The present study employs a nonlinear neural network classification technique, the Self-organizing map (SOM), to identify the rainfall patterns in SEC based on gauge data. Three distinct rain belts over the Huai River basin (HRB), lower Yangtze River basin (LYRB) and South Coast region (SCR) are found. Their subseasonal variability highly agrees with the stepwise progression of the East Asian Summer Monsoon (EASM) front in space and time. Analysis reveals that precipitation in the SCR and HRB rain belts undergo a regime shift after the mid-1990s, whereas the 1990s is the most active decade for the LYRB rain belt. These systematic changes are in abreast with similar changes in EASM and other climate events documented in the literature.

Additionally, a SOM-based algorithm is developed to further divide gauge stations into three groups featuring homogeneous rain belt patterns. Promising predictability of group-averaged daily rainfall is then achieved, with about 39% to 50% of the total variance explained by circulation-informed regression models, verified by both cross-validation and blind prediction. Through further diagnosis in the useful predictors, the western North Pacific subtropical high, blocking high anomalies over northeast China and the upper-level divergence over SEC, are found to best explain the variability of the rain belts. The proposed Russia-China wave pattern (western/central Russia → north of Tibetan Plateau → SEC) and teleconnection between the El Niño-Southern Oscillation and the rain belts also offer additional predictability. This study aims to set an updated benchmark on the summer monsoon rainfall patterns in SEC, from which the promising daily predictability and the informative circulation patterns are obtained. Findings from this work may also advance the understanding of the EASM rain belts, and offer insights to the source of bias for numerical simulations of daily summer monsoon rainfall in the region.

How to cite: Dai, L., Cheng, T. F., and Lu, M.: Classification and Diagnosis of Summer Monsoon Rainfall Patterns and their Potential Predictability in Southeast China , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2133, https://doi.org/10.5194/egusphere-egu2020-2133, 2020.