Africa's climate underwent significant hydroclimate changes in the Late Cenozoic. For instance, the repeated phases of aridification across the continent played a crucial role in shaping the region’s biodiversity and hominid evolution. Consequently, understanding the historical climate variations in the region becomes essential for reconstructing its paleoenvironment and paleobiological history. Moreover, past climates can be used as analogues for potential future climates and thus help us understand the implications of future climate scenarios. The precipitation seasonality and variability in the region are predominantly driven by the African monsoons, which exhibit intricate climate dynamics controlled by both regional and large-scale atmospheric teleconnections. However, due to the complexity of these dynamics and teleconnections, even state-of-the-art General Circulation Models (GCMs) still struggle to accurately reconstruct its past climate variability and provide reliable future projections.

Here, we simulate the response of the African monsoons to different late Cenozoic paleoenvironmental changes, such as atmospheric CO₂ concentration (pCO₂), orbital forcing, palaeogeography, vegetation, and orography (including the topographic evolution of the East African Rift System (EARS)). We performed time-specific simulations with a high-resolution setup of the GCM ECHAM5-wiso and the paleoenvironmental boundary conditions for the Middle Miocene climate optimum (MMCO; 16.9-14.7 Ma), Middle Miocene climate transition (MMCT; 14.7-13.8 Ma), Mid-Pliocene (MP; ~3 Ma), the Last Glacial Maximum (LGM; ~21 ka), the Mid-Holocene (MH; ~6 ka), and the pre-industrial (PI; the reference year 1850).

Furthermore, we conducted topographic sensitivity experiments of the EARS under the MMC and MMCT conditions to understand the role of tectonics in the evolution of Africa’s climate and atmospheric dynamics. We focused our analysis on disentangling the thermodynamic effects (e.g., water vapour content changes) and dynamic effects (e.g., Hadley circulation) on the monsoon changes and associated atmospheric dynamics (e.g., African Easterly Jet, Somalia Jet, Tropical Easterly Jets, low-level westerlies). Overall, the study provides an overview of hydroclimate and climate dynamics changes over Africa for the past 20 Ma, contributing to the understanding of the feedback between changes in pCO₂, orbital forcing, and tectonic events that are relevant for improving future climate prediction.

How to cite: Boateng, D. and G. Mutz, S.: African monsoon changes in the Late Cenozoic from the climate modelling perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10696, https://doi.org/10.5194/egusphere-egu24-10696, 2024.

Influenced by the northern hemisphere high-latitudes, many of the millennial-centennial scale climate changes originating in the North Atlantic have been detected even in southern hemisphere. However, the linkage between hemispheres on orbital-suborbital time scales has not been firmly examined due to the absence of records from the Southern Hemisphere. Here we present such a record from Bromfield Swamp in tropical northeastern Australia. The Australian Summer Monsoon index (AuSMI) of the last 13.5 ka was reconstructed basd on the principal component analysis (PCA) of five proxies, the Rb/Sr, Ti/Ca, Al/Ca, mean grain size and organic content. The results reflected a weak AuSM influence during the Bolling-Allerod event and with a somewhat stronger influence during the YD event. During the Holocene, there was a decreasing AuSM before ~7.8 cal kyr BP, and then it enhanced from middle to late Holocene. The AuSM change was out of phase/ in phase with the East Asian summer monsoon/ East Asian winter monsoon during the Holocene, and all of them changed parallel with the northern-southern hemisphere temperature gradient. This implied the dominance of interhemispheric thermal contrast to the highly coupled East Asian-Australian monsoon changes, by modulating the Intertropical Convergence Zone migration, which was influenced by the retreat of northern hemisphere ice sheet from early to middle Holocene and the local summer insolation changes during the late Holocene. The study highlights the likelihood that high latitude northern hemisphere played a major role in the evolution of the northeastern Australian summer monsoon.

How to cite: Shi, G., Yan, H., Zhang, W., Dodson, J., Heijnis, H., and Burrows, M.: The impacts of northern hemisphere high-latitude climate on northeastern Australian summer monsoon evolution during the Holocene, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16112, https://doi.org/10.5194/egusphere-egu24-16112, 2024.

Strength and shifts of the Western Pacific Subtropical High (WPSH) have a strong influence on East Asian Summer Monsoon (EASM) precipitation. How the WPSH changed in the past and its forcing mechanisms on different timescales are yet to be resolved. The role of the WPSH in modulating the East Asian climate can be traced from the oxygen isotopic composition of modern and fossil precipitation (d¹⁸O_prec) if the climatic significance of d¹⁸O_prec is adequately constrained. Here, we present a Holocene hydroclimatic analysis for the Western North Pacific (WNP) proximal region by combining observational data, a speleothem oxygen isotope (d¹⁸O_speleo) record from South Korea and isotope-enabled transient climate model simulation. The composite d¹⁸O_speleo record is based on two stalagmites collected from Majiri and Baeg-neyong caves. Our results shows that the interannual (based on IsoGSM simulation, 1979-2021) to interdecadal (based on i-CESM present-day run for 100 model years) variations in EASM mean amount-weighted d¹⁸O_prec values are related to the strength and zonal migration of the WPSH. The ¹⁸O-enrichment in Korean precipitation corresponds to summertime El-Niño like conditions in the tropical Pacific Ocean which suppress the convective activity in maritime continental regions and promotes the intensification of the WPSH by poleward propagating Rossby-waves. The strength of the WPSH is positively correlated to its zonal extent, and thus intensified WPSH also implies its westward migration. In this situation, the WPSH acts as a mechanical barrier to the ¹⁸O-depleted moisture contribution from the Indian Summer Monsoon (ISM) branch, and Korean precipitation receives a higher amount of moisture from the relatively ¹⁸O-enriched WNP branch. We show that the sensitivity of the Korean d¹⁸O_prec to WPSH was consistent during the pre-industrial period as well. The increasing d¹⁸O_speleo trend from around 6 ka up to present is related to an intensified and westward migrated WPSH, as evident from the i-CESM 1.2 transient Holocene run. Mg/Ca based sea surface temperature (SST) records also confirm that a decreasing of zonal SST trend (west minus east) persisted during the Mid - Late Holocene, which established an El-Nino like condition that was favorable for the WPSH intensification.

How to cite: Basu, S., Sinha, N., Jo, K.-N., Timmermann, A., Yang, Y., Yoshimura, K., Cleary, D. M., Sharma, S., and Wassenburg, J. A.: Korean precipitation isotopes reflect changes in the Western Pacific Subtropical High on interannual to millennial timescales, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14459, https://doi.org/10.5194/egusphere-egu24-14459, 2024.

This paper studies the influence of the winter NAO on the multidecadal variability of winter East Asian surface air temperature (EASAT) and its decadal prediction. The observational analysis shows that the winter EASAT and East Asian minimum SAT (EAmSAT) display strong in-phase fluctuations and a significant 60–80-year multidecadal variability, apart from a long-term warming trend. The winter EASAT experienced a decreasing trend in the last two decades, which is conducive to the occurrence of winter extremely cold events in East Asia in recent years. The winter NAO leads the detrended winter EASAT by 12–18 years with a maximumly significant positive correlation at the leading time of 15 years. Further analysis shows that ENSO may affect winter EASAT interannual variability, but does not affect the robust leading relationship between the winter NAO and EASAT. We present the coupled oceanic-atmospheric bridge (COAB) mechanism of the NAO influences on winter EASAT multidecadal variability through its accumulated delayed effect of ~15 years on the Atlantic Multidecadal Oscillation (AMO) and Africa–Asia multidecadal teleconnection (AAMT) pattern. BaseThis paper studies the influence of the winter NAO on the multidecadal variability of winter East Asian surface air temperature (EASAT) and its decadal prediction. The observational analysis shows that the winter EASAT and East Asian minimum SAT (EAmSAT) display strong in-phase fluctuations and a significant 60–80-year multidecadal variability, apart from a long-term warming trend. The winter EASAT experienced a decreasing trend in the last two decades, which is conducive to the occurrence of winter extremely cold events in East Asia in recent years. The winter NAO leads the detrended winter EASAT by 12–18 years with a maximumly significant positive correlation at the leading time of 15 years. Further analysis shows that ENSO may affect winter EASAT interannual variability, but does not affect the robust leading relationship between the winter NAO and EASAT. We present the coupled oceanic-atmospheric bridge (COAB) mechanism of the NAO influences on winter EASAT multidecadal variability through its accumulated delayed effect of ~15 years on the Atlantic Multidecadal Oscillation (AMO) and Africa–Asia multidecadal teleconnection (AAMT) pattern. Based on the COAB mechanism an NAO-based linear model for predicting winter decadal EASAT is constructed, with good hindcast performance. The winter EASAT for 2020–2034 is predicted to keep on fluctuating downward until ~2025, implying a high probability of occurrence of extremely cold events in coming winters in East Asia, and then turn towards sharp warming. The predicted 2020/21 winter EASAT is almost the same as the 2019/20 winter.d on the COAB mechanism an NAO-based linear model for predicting winter decadal EASAT is constructed, with good hindcast performance. The winter EASAT for 2020–2034 is predicted to keep on fluctuating downward until ~2025, implying a high probability of occurrence of extremely cold events in coming winters in East Asia, and then turn towards sharp warming. The predicted 2020/21 winter EASAT is almost the same as the 2019/20 winter.

How to cite: Li, J., Xie, T., Tang, X., Wang, H., Sun, C., Feng, J., Zheng, F., and Ding, R.: Multidecadal variability and decadal prediction of wintertime surface air temperature over the East Asian winter monsoon domain, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3014, https://doi.org/10.5194/egusphere-egu24-3014, 2024.

            The mean Indian summer monsoon (ISM) rainfall as well as the duration of monsoon spell have a profound impact on the agriculture practice in the country. Due to the recent increase in surface temperature, global circulation patterns exhibit considerable changes which also affects the characteristics of ISM. The present study aims to find out any long-term changes in the monsoon onset and withdrawal dates over different parts of India and the possible mechanisms behind it. During the last four decades, the trend analysis of ISM onset dates over south India and north-west (NW) India shows an early onset in both regions. However, the trends are statistically less significant. In the case of the monsoon withdrawal dates, trends over NW India and south India show a statistically significant delay of about 6 days/decade and 3.25 days/decade, respectively. As a result, the monsoon season over NW India and south India shows a lengthening of about 7.8 days/decade and 3.5 days/decade, respectively. During the withdrawal phase of the ISM, a stronger monsoon low-level jet and an enhancement of the ISM rainfall have been observed in recent decades. The enhancement in rainfall activity and the strengthening of the low-level jet in the withdrawal phase reaffirms the delayed withdrawal of the ISM in recent decades.

            The role played by factors such as Indian Ocean warming, Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) on the ISM withdrawal is examined. The AMO has changed its phase from negative to positive in recent decades, particularly after about 1998, which might have played a key role in enhancing the meridional tropospheric temperature gradient. The stronger meridional tropospheric temperature gradient and the Eurasian surface warming observed in recent decades might played a key role in the delayed monsoon withdrawal over NW India. The CESM2 large ensemble data analysis shows that both the external forcing as well as the decadal phase shift of the AMO and PDO, favour the delayed withdrawal, while the latter plays a dominant role.

How to cite: Sundaresan, A., Bódai, T., Sijikumar, S., and Satpathy, S. S.: Role of Anthropogenic Forcing and Decadal Oscillations on the Delayed Withdrawal of Indian Summer Monsoon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9447, https://doi.org/10.5194/egusphere-egu24-9447, 2024.

The predictability of Indian Summer Monsoon Rainfall at any given time period depends on the strength of its relationship with predictable drivers like the El Nino–Southern Oscillation (ENSO) that are known to undergo significant epochal variations. While the relationship between Eurasian snow cover fraction and Indian Summer Monsoon Rainfall has also shown a similar epochal variability in recent decades, its stationarity on centennial or longer timescales remains unknown. In the present work two indices of snow cover fraction have been unraveled, on the basis of the observed relationship between the dominant modes of Indian Summer Monsoon Rainfall variability and snow cover fraction over a period of 115 years (1901–2015), that encapsulate its spatio-temporal variability. It has been observed that the relationship between the snow cover fraction indices and Indian Summer Monsoon rainfall have a statistically significant increasing trend with a weak multidecadal variability superimposed on it, making significant positive correlation between the two highly probable in the coming decades. With snow cover fraction driving the North Atlantic Oscillation (NAO) that subsequently drives the Indian Summer Monsoon Rainfall variability, it has been demonstrated that the NAO plays a pivotal role in modulating the teleconnection between the Indian Summer Monsoon Rainfall and snow cover fraction on a multidecadal time scale.

Keywords: El Nino–Southern Oscillation, North Atlantic Oscillation

How to cite: Pandey, D. P., Kunz, D. M., Dwivedi, D. S., and Goswami, D. B. N.: Trend and Variability in the Long-Term Relationship Between Eurasian Snow Cover and Indian Summer Monsoon Rainfall, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9454, https://doi.org/10.5194/egusphere-egu24-9454, 2024.

The temperature and CO₂ increase due to global warming are expected to exacerbate atmospheric water demand, worsening future drought conditions. Recent studies have revealed that evapotranspiration is regulated by stomatal response in response to CO₂ increase. However, understanding droughts defined based on evapotranspiration remains incomplete as it does not adequately integrate plant responses to anticipated drought conditions. In this study, we aimed to evaluate the frequency and extent of future drought events by comparing the Evaporative Stress Index (ESI) using two potential evapotranspiration (E_p) values capturing atmospheric evaporative demand. The first E_p utilized past data and predictions from the Coupled Model Intercomparison Project Phase 6, assuming a constant surface resistance (r_s) without considering plant responses. The second E_p accounted for the sensitivity of r_s to increased CO₂. Our findings indicate a significant increase in r_s due to elevated CO₂, leading to substantial changes in drought frequency and extent. While both non-vegetative response and plant response are expected to increase in future scenarios, an ESI that ignores plant responses tends to overestimate drought risk. Therefore, our study emphasizes the importance of integrating the sensitivity of r_s to evaporative demand and CO₂ level increases when assessing drought risk.

How to cite: Ha, K.-J., Yeo, J.-H., Kim, D., and Lee, H.: Drought risks based on changes in atmospheric evaporative demand due to plant response to CO2 levels, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7210, https://doi.org/10.5194/egusphere-egu24-7210, 2024.

Tropical regions in South America are characterized by rich biodiversity, diverse climatic zones and heterogenous weather. This heterogeneity is caused by the South American Monsoon System (SAMS) and the atmospheric low-level jets (LLJ). Both atmospheric circulation features have a critical role in the distribution of moisture and precipitation. The regions located where the rainforest meets the Andes are highly affected by these LLJs. One example is the department of Madre de Dios, located in south-eastern Peru. Its economy and the well-being of the population are highly dependent on natural resources provided by the ecosystem. Hence, understanding how the SAMS and the associated LLJs will change under global warming is important for water management in the region. To investigate the climate change signals, we employ the Weather Research and Forecasting model (WRF; version 3.8.1) at convection-permitting scales (up to 1 km). Two 30-year periods of a global climate simulation are dynamically downscaled for the present (1981–2010) and the future (2071–2100). Thereby, we consider the mitigation scenario representative concentration pathway (RCP) 2.6 and the high-emission scenario RCP8.5.

The validation of the simulation for the present period indicates that while precipitation amounts fall within the range of observational datasets such as PISCO or CHIRPS, a cold bias is found from April to July compared to ERA5 or CRU. The bias in temperature is potentially caused by biases in the driving global climate simulations and by the difference in land elevation between WRF and observational datasets.

The comparison of present and future simulations shows changes in both temperature and precipitation in Madre de Dios. The climate projections indicate an increase in temperature of 1 and 3 °C under the RCP2.6 and RCP8.5 scenarios, respectively. Precipitation is projected to overall decrease in Madre de Dios. During the rainy season from September to April, the average decrease is 5 and 12 % under the RCP2.6 and RCP8.5 scenarios, respectively. During the dry season from May to August, the rain is reduced by more than 50 % in both scenarios. The general reduction in precipitation seems to be related to the changes in the SAMS under climate change, which include a less intense Bolivian High during the peak months of December and January (particularly in RCP8.5), a less intense Chaco Low in February, and a more intense Atlantic Tropical High that extends much further into the continent in both climate scenarios from April to August. These changes reduce the occurrence of LLJ events under both climate scenarios, and consequently, affecting precipitation east of the Andes.

How to cite: González-Rojí, S. J., Messmer, M., Raible, C. C., and Stocker, T. F.: Future changes in the South American Monsoon System and its consequences over south-eastern Peru, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10904, https://doi.org/10.5194/egusphere-egu24-10904, 2024.

Precipitation has a significant degree of temporal and spatial variability over the Indian region. A small change in precipitation frequency and its distribution may affect agriculture and water resources and can lead to extreme events such as flood and drought. Number of precipitating days and their spatial distribution has significant impact on many aspects of the socio-economic environment. In present study, 91-days climatology is used to enhance robustness and to reduce uncertainty of the time series. Further, Mann Kendall trend test and Pettitt’s test for change point detection is used for analysis of the number of precipitating days and corresponding precipitation over India and its sub-regions. India Meteorological Department (IMD) gridded dataset and ERA5 reanalysis dataset having resolution 0.25° x 0.25° is used for the period 1902-2020 and 1940-2020 respectively. Our results show that there is a positive trend of number of precipitating days and precipitation over northwest and negative trend over central northeast and northeast India. Indicating a westward shift of precipitation during monsoon season. Change point analysis shows majority of these changes occur after 1970. Positive precipitation anomaly is observed in the month of September over India, with the exception of the hilly and central northeast showing extension of higher precipitation from month of July-August to July-August-September. This extension is probably due to the strengthening of wind during recent time (1971-2020) which brought more moisture to the Indian landmass. Furthermore, increased moisture transfer from the Bay of Bengal has also been seen compared to the early period (1940-1970). Overall, the results of this study will help in understanding the impact of climate change on Indian summer monsoon that will assist in policy making and adapting water management practices.

How to cite: Sharma, A. and Dimri, A. P.: Shifting precipitation pattern during Indian summer monsoon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-962, https://doi.org/10.5194/egusphere-egu24-962, 2024.

The pattern of Monsoon rainfall across the Ganga-Brahmaputra-Meghna (GBM) basin is crucial for supporting various farming and ecological systems and play a significant role in affecting the basin’s food-water security, well-being, and prosperity. However, the understanding of Monsoon activity is limited due to the poor representation of large-scale processes in the climate models and their coarser resolution. This study utilises sub-daily precipitation from finer resolution CMIP6 HighResMIP models to study the changes in properties of monsoon rainfall based on the timing (onset/offset/duration) of the Monsoon and the trend in rainfall (total and extreme rainfall). All models show a delay in the monsoon but there is disagreement in trends in retreat and duration of the monsoon. Also, CMCC models project a decline in magnitude of rainfall whereas NERC models project an increasing trend. The models output is also evaluated against the reference datasets like MSWEP and ERA5 reanalyses. Our study highlights the uncertainty in climate models to capture the monsoon rainfall and disagreements in results across different horizontal resolutions and nature of models. Importantly, the delay in future Monsoon supported by all models have a strong implication on agriculture and economy of the delta.

How to cite: Ali, H. and Fowler, H.: Understanding the future Monsoon activity across the Ganga-Brahmaputra-Meghna basin using CMIP6 HighResMIP models  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15435, https://doi.org/10.5194/egusphere-egu24-15435, 2024.

The Tibetan Plateau (TP), known as Earth's “Third Pole”, has experienced significant warming since 1980. As an important component of the summer monsoon, the TP rapid warming profoundly impacts both Asian and global climate systems. While previous studies focused on surface temperature, our research uses multiple reanalysis datasets to investigate atmospheric temperature changes over the TP. All three reanalysis datasets revealed an upper tropospheric warming above the TP centered around 250 hPa. The upper tropospheric warming rate is approximately 0.3 K/decade over the 1980-2021 period, faster than those at the same latitude. An energy budget analysis is performed to attribute this warming to different processes. The primary contribution arises from the convection process, contributing around 0.4K/decade. Cloud warms the upper troposphere by an additional 0.2K/decade. Other radiative processes and adiabatic processes play counterpart roles that weaken the upper tropospheric warming. The warming center is most significant in spring. In contrast to other seasons, warming in spring primarily results from the adiabatic process, rather than the convection process. Although different in specific values, all three reanalysis datasets show a similar contribution ratio of each physical process.

How to cite: Wei, Y. and Wang, Y.: Quantifying Contributions from Different Physical Processes to the Atmospheric Warming over the Tibetan Plateau, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3865, https://doi.org/10.5194/egusphere-egu24-3865, 2024.

The East Asian summer monsoon (EASM) is an influential climate system that contributes to approximately 70% of the annual precipitation in the Asia region. Extensive research has been conducted on monsoon changes in response to future climate. In this study, we analyzed the characteristics of the EASM considering specific global warming level (GWL) using Coupled Model Inter-comparison Project 6 (CMIP6) simulations. The 30 CMIP6 models effectively captured the migration of the monsoon in present-day (PD), showing a pattern correlation coefficient of 0.91, which represents an improvement over values reported in previous studies. Dividing the monsoon period into P1 (first primary peak; 33-41 pentad) and P2 (from P1 to the withdrawal; 42-50 pentad), the frequency and amount of weak to moderate precipitation rates are predominantly higher in P2, while the frequency and amount of moderate to extreme precipitation rates are notably higher in P1. The CMIP6 models project a significant increase in precipitation under a warming climate, accompanied by a longer duration due to earlier onset and delayed termination. Under the three GWLs, the projected precipitation frequency decreases below moderate precipitation rates, while it significantly increases above strong precipitation rates. Additionally, the precipitation tendencies in both P1 and P2 are similar to those of the total period, with significant changes evident at the 3.0 °C GWL. These precipitation changes are associated with an increase in precipitation amount above the 97th percentile and influence the future changes in the EASM under a warmer climate. 

How to cite: Sun, M., Sung, H. M., Kim, J., Lee, J.-H., Shim, S., and Byun, Y.-H.: Future projection of East Asian Summer Monsoon precipitation under 1.5°C, 2°C, and 3°C global warming levels, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7608, https://doi.org/10.5194/egusphere-egu24-7608, 2024.

The monsoon onset typically starts in southern India by 1 June, taking around 6 weeks to cover the country.  During the monsoon, intraseasonal variations give rise to active and break periods in the rains.  Being able to better predict the monsoon onset, its progression, and active and break events would be of great interest to society.  The onset timing is already known to be influenced by tropical intraseasonal variability, but new research has shown that the mid-latitudes exert a powerful control, the full extent of which is not properly quantified or understood. 

The MiLCMOP project aims to answer the following: (1) How are the pace and steadiness of monsoon progression affected by interactions with the extratropics? (2) What are the mechanisms of extratropical control on monsoon progression and variability? (3) How do the causal extratropical and tropical drivers of monsoon progression offset or reinforce each other? 

Our initial work has tested a new hypothesis that monsoon progression can be described as a “tug-of-war” between tropical and extratropical airmasses.  This “tug-of-war” is unsteady, with a back and forth of the two airmasses before the moist tropical flow takes over for the season.  We demonstrate this for a case study of the 2016 season for India, while also drawing analogies with other monsoon regions, such as for the East Asian monsoon, in which we show the competition between extratropical and tropical flows in establishing the Mei Yu front as it progresses across China.

Current activities revolve around the identification of statistical relationships between monsoon onset and progression and perturbations to the subtropical westerly jet, including blocking anticyclones, meridionally propagating troughs and cyclonic features near the Tibetan Plateau.  Additional focus is also devoted to the relationship between the monsoon advancement and the strength, extent and orientation of the intrusion of mid-tropospheric dry air flowing towards India from westerly and northwesterly quadrants.

Other methods will include use of vorticity budgets and Lagrangian feature tracking in case studies of fast and slow onsets, to suggest the dominant mechanisms by which extratropical drivers affect monsoon onset and progression.  Model experiments will help isolate these mechanisms.  Finally, novel causal inference techniques will help disentangle the effects of extratropical drivers from those in the tropics. 

How to cite: Turner, A., Volonte, A., Deoras, A., and Menon, A.: Mid-Latitude Controls on Monsoon Onset and Progression (the MiLCMOP project), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19848, https://doi.org/10.5194/egusphere-egu24-19848, 2024.

This study identifies break events of the South China Sea (SCS) summer monsoon (SCSSM) based on 42 years of data from 1979 to 2020, and investigates their statistical characteristics and associated atmospheric anomalies. A total of 214 break events are identified by examining the convection evolution during each monsoon season. It is found that most events occur between June and September and show a roughly even distribution. Short-lived events (3–7 days) are more frequent, accounting for about two thirds of total events, with the residual one third for long-lived events (8–24 days).

The SCSSM break is featured by drastic variations in various atmospheric variables. Particularly, the convection and precipitation change from anomalous enhancement in adjoining periods to a substantial suppression during the break, with the differences being more than 60 W m⁻² for outgoing longwave radiation (OLR) and 10 mm d⁻¹ for precipitation. This convection/precipitation suppression is accompanied by an anomalous anticyclone in the lower troposphere, corresponding to a remarkable westward retreat of the monsoon trough from the Philippine Sea to the Indochina Peninsula, which reduces the transportation of water vapor into the SCS. Besides, the pseudo-equivalent potential temperature (θ_se) declines sharply, mainly attributable to the local specific humidity reduction caused by downward dry advection. Furthermore, it is found that the suppressed convection and anomalous anticyclone responsible for the monsoon break form near the equatorial western Pacific and then propagate northwestward to the SCS.

How to cite: Bi, M., Xu, K., and Lu, R.: Monsoon Break over the South China Sea during Summer: Statistical Features and Associated Atmospheric Anomalies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3424, https://doi.org/10.5194/egusphere-egu24-3424, 2024.

Precipitation variability over the Tibetan Plateau (TP) depends largely on the atmospheric circulation pattern associated with remote oceanic forcing, while the contribution of sea surface temperature anomaly (SSTA) in different ocean basins, especially the tropical North Atlantic (TNA), remains unclear. By using multi-source data and atmospheric general circulation model, this study reveals the individual and combined effects of the Indian Ocean Basin mode (IOBM) and TNA SSTA on the interannual variability of TP precipitation in late summer (August). During the positive phase of IOBM, warm SSTA in the Indian Ocean induces an anomalous anticyclone over the Bay of Bengal (BOBAC) via the Kelvin wave–induced Ekman divergence and the resultant positive precipitation anomaly over the southeastern TP. Simultaneously, an eastward-extending Kelvin wave triggered by the positive TNA SSTA overlaps with that caused by IOBM, further strengthening the BOBAC. In addition, the Kelvin wave triggered by TNA SSTA induces anomalous easterlies over the tropical Indo-Pacific, which contribute to the warm Indian Ocean SSTA and thus amplify the IOBM affecting TP precipitation. Moreover, the positive TNA SSTA generates a westward-extending Walker circulation anomaly that is responsible for the suppressed convection over the central Pacific, which in turn triggers a Rossby wave response and further strengthens the BOBAC. As a result, the positive precipitation anomaly over the southeastern TP is strengthened significantly. Particularly, considering the 2–3-month lead time of the IOBM and TNA SSTA, the tropical SSTA can be used as a predictor for the TP precipitation anomaly.

How to cite: Zhang, P., Duan, A., and Hu, J.: Combined Effect of the Tropical Indian Ocean and Tropical North Atlantic Sea Surface Temperature Anomaly on the Tibetan Plateau Precipitation Anomaly in Late Summer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5068, https://doi.org/10.5194/egusphere-egu24-5068, 2024.

Seasonal prediction of East Asia summer monsoon rainfall (EASMR) is in great demand but remains challenging, because the relationships between the Asian monsoon system and precursors are nonstationary and exhibit significant decadal changes. The present study aims to 1) examine decadal variations of the relationships between the EASMR and predictors used in previous studies and 2) establish a new prediction model using a Bayesian dynamical linear model (DLM), which is capable of capturing the time-evolving relationships between the predictand and predictors whereas the conventional static linear model cannot.

Two predictors were selected previously to predict the EASMR. One is the sea level pressure tendency anomalies over the tropical eastern Pacific from late spring to early summer, which represents remote forcing related to ENSO and has a stable effect on EASMR throughout the analysis period. The other is the sea surface temperature anomaly difference between the northern Indian Ocean (IO) and the WNP during spring through early summer (called IOWPSST), which denotes local air-sea interaction that affects the WNP subtropical high. Results show that the IOWPSST has strong influence on EASMR during 1979 to 2003 (period 1), while from 2004-2017 (period 2) its connection to EASMR evidently weakens. This nonstationary relationship is due to the non-persistence of the enhanced WNP subtropical high during period 2, which is associated with the positive-to-negative phase transition of the Interdecadal Pacific Oscillation since ~2000.

A new prediction model was established using the two predictors with Bayesian DLM. The cross-validation method and a 9-yr independent forward-rolling forecast is applied to test the hindcast and actual forecast ability. Results show that the Bayesian DLM has higher hindcast/forecast skill and lower mean square error compared with static linear model, suggesting that the DLM has advantage in predicting EASMR and is a promising method for seasonal prediction.

How to cite: Xing, W., Han, W., and Zhang, L.: Improving the prediction of East Asia summer monsoon precipitation using a Bayesian dynamic linear model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8515, https://doi.org/10.5194/egusphere-egu24-8515, 2024.

Tropical and subtropical precipitation impact millions of people via agriculture and rainfall driven disasters. However, a wide spread remains in future regional projections of low-latitude precipitation, dominated by uncertain shifts in rainfall, and models continue to show a variety of biases in the location and intensity of rain.

The Energy Flux Equator framework has emerged as a powerful tool in interpreting the location of low-latitude rainfall via atmospheric heat transport (AHT), which in turn can be understood through the top of atmosphere and surface energy fluxes. Recent work using a novel decomposition of the zonal-mean AHT suggests that its spatial structure is dominated by the meridional structure of the latent heat flux. Here, we apply this decomposition to investigate intermodel differences in AHT on the seasonal timescale.

We find that throughout the year, intermodel differences in total AHT and the latitude of maximum zonal mean precipitation both correlate strongly with the heat transport contribution attributed to evaporation. Curiously, spatial regressions appear to suggest that evaporation over land provides a key contribution to this spread, despite the net surface heat flux over land being close to balanced. To interrogate the causality underlying this correlation with land evaporation, we make use of the 1pctCO2-bgc simulations, in which only the carbon cycle responds to increasing carbon dioxide, with one consequence being altered evapotranspiration.

How to cite: Geen, R., Laguë, M., and Fajber, R.: Exploring the role of evaporation in atmospheric heat transport and seasonal low-latitude precipitation biases in CMIP6, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16770, https://doi.org/10.5194/egusphere-egu24-16770, 2024.

As the variability of the intensity of the tropical easterly jet (TEJ) has been extensively studied, how the location of the TEJ’s core changes is still less understood. We found that the TEJ can be identified as three locational patterns by using the cluster analysis, which exhibit an east mode (centering over the southern tip of the Indian Peninsula), a northwest mode (centering to the southeast of Arabian Peninsula), and a southwest mode (centering over the tropical western Indian Ocean). The frequency of each locational pattern is distinctly different in each summer month and change obviously from year to year. The change in the location of the TEJ’s core have an important impact on the monsoon rainfall in South Asia, Southeast Asia, and East Asia through modulating the large-scale divergence at the upper level and the vertical-meridional circulation. The decreased frequency of the northwest mode may be responsible for the drying along the Himalayas while the more frequent southwest mode may contribute to the aridification in the northern India. The results highlight the diversity of the TEJ location and can provide a different perspective in understanding the cause of the monsoon rainfall variation.

How to cite: Huang, S. and Wen, Z.: Diversity of the tropical easterly jet’s location and its influence on the Asian summer monsoon rainfall, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7526, https://doi.org/10.5194/egusphere-egu24-7526, 2024.

It has long been recognized that complex interactions and energy conversions between the atmosphere-ocean system and the solid Earth can generate strong ambient noise field, known as microseisms which can be detected worldwide. Under the vast majority of circumstances, such seismic energy is believed to be induced by tropical cyclones. Whether unidirectional propagating winds, such as monsoons, can generate microseisms lacks solid seismic evidence. Here we utilize broadband seismic data recorded by seven ocean-bottom seismometers (OBSs) deployed in the South China Sea basin and 17 terrestrial stations to systematically investigate possible influences of the summer monsoon transition on the microseisms. Spectral analyses over time reveal significant seismic energy in the secondary microseisms frequency band (0.1−0.5 Hz) during 18th to 29th May, coinciding with the period of the summer monsoon transition occurring in the South China Sea. Polarization analyses and time-space variation of offshore surface wind field indicate that the source region of the observed secondary microseisms is located at the South China Sea. Given the absence of tropical cyclones during this time, we attribute the observed strong secondary microseisms to the summer monsoon transition. When the near-surface wind field is transformed to be southwest, ocean waves are driven to propagate northeastward and interact with an opposing wave train which represents precursor waves and is reflected by coastlines, generating the secondary microseisms. This study provides solid evidence for a causal link between the monsoon transition and microseisms, highlighting the potential of applying ocean bottom seismic observations for monitoring and characterizing monsoon transition and ocean activities.

How to cite: Zhou, Y., Kong, F., Zhang, H., Liu, Z., Ding, W., and Li, J.: Summer monsoon transition induced microseisms observed at the South China Sea seabed, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7080, https://doi.org/10.5194/egusphere-egu24-7080, 2024.