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A declining trend in Indian summer monsoon precipitation (ISMP) in the latter half of the 20th century is a scientifically challenging and societally relevant research issue. Heavy aerosol loading over India is one of the key factors in modulating the ISMP. Using the state-of-the-state-of-the-art chemistry-climate model, ECHAM6-HAMMOZ, the impacts of South Asian anthropogenic sulfate aerosols on the Indian summer monsoon precipitation were investigated against: (1) 2010 La Niña (excess monsoon), (2) 2015 El Niño (deficit monsoon) in comparison to (3) normal monsoon 2016. Sensitivity simulations were designed with 48% enhancement in South Asian SO 2 emissions based on a trend estimated from Ozone Monitoring Instrument (OMI) satellite observations during 2006–2017. The model simulations showed that sulfate aerosols reduce ISMP by 27.5%–43.3 %, while simulations without sulfate loading enhanced ISMP by 23% in 2010 La Niña and reduction by 35% in 2015 El Niño. This paper reports that sulfate aerosols loading over India reduce precipitation by aerosol-induced direct and indirect effects by inducing atmospheric cooling, weakening in the convection, and reduction in moisture transport to Indian landmass. This paper emphasizes the necessity of alternate use of energy to reduce sulfate aerosol emissions to solve water issues in South Asia. [Display omitted] • Sulfate emission reduces Indian summer monsoon precipitation through aerosol-radiation and aerosol-cloud interactions. • Over 27% of the summer monsoon rainfall deficit in a normal year are linked to Asian anthropogenic sulfate emissions. • Sulfate-induced rainfall reduction is the maximum for La Niña year. • SO 2 loading increases atmospheric stability and weakens moisture transport to India. [ABSTRACT FROM AUTHOR]

During late July 2022, the western North Pacific subtropical high (WNPSH) and the Iran high merge persistently, leading to concurrent extreme climate events over multiple regions. Such extraordinary circulation results from two extreme anticyclonic anomalies centered over the WNP–South Asia and Central Asia, respectively. The WNP–South Asia anticyclonic anomaly is related to La Niña SSTA pattern and the enhanced convection over the tropical eastern Indian Ocean (TEIO). SSTA serves as a seasonal-mean background, with abnormally cold tropical central Pacific (TCP) favoring the anticyclonic anomaly through Rossby wave response. The TEIO convection is extremely strong during late July and induces the anticyclonic anomaly via Kelvin wave response and a local Hadley circulation. The TEIO convection anomaly during late July results from different time scales, with the seasonal mean accounting for 30% and the 25–90-day intraseasonal oscillation (ISO) for 50%. The seasonal mean TEIO convection anomaly is related to La Niña, triggered by not only the local positive SSTA but also the negative TCP SSTA through the Walker circulation. The 25–90-day ISO features a northward-propagating convection dipole over the Indian Ocean. An enhanced ISO convection center develops over the TEIO during late July, which moves northward and is gradually replaced by suppressed convection. On the other hand, the Central Asia anticyclonic anomaly is associated with the Silk Road pattern propagating eastward along the westerly jet. During late July, a remarkable anticyclonic node moves across Central Asia, cooperating with the enhanced TEIO convection to facilitate the merging of the WNPSH and the Iran high. [ABSTRACT FROM AUTHOR]

Based on monthly rainfall station data from the National Climate Prediction Center of China Meteorological Administration, reanalysis monthly data from NCEP/NCAR including the monthly mean and long-term mean values for geopotential height, winds of multi-levels, NOAA(National Oceanic and Atmospheric Administration) monthly Sea Surface Temperature (ERSSTV5), the causes of anomalous excessive precipitation in 2021/2022 winter over South China (SC) are investigated by Liang-Kleeman information flow, composite and wave flux methods. The results show that the winter precipitation over SC in 2021/2022 DJF is far more than normal, and the associated circulation patterns are obviously different from those induced by traditional La Niña, especially in the Atlantic and East Asia areas south to Lake Baikal. In 2021/2022 DJF, the Tibet Plateau (TP) exists anomalous cyclone with strong ascending motion, which directly affects the circulation patterns in Eastern China and thus the precipitation in SC. It is a significant causal relationship for the earlier season velocity potential at 850 hPa (related to the anomalous SST) over the tropical Atlantic to later DJF anomalous vertical motions over TP. Especially when the anomalies in tropical Atlantic leads one month before, the relationship is closest. The anomalies of the tropical Atlantic can stimulate the Rossby wave train which propagates downstream, corresponding to the abnormal cyclonic circulation over TP. In the early autumn to winter of 2021/2022, the 850 hPa potential velocity anomalies in the tropical Atlantic were significant with positive phase. The Rossby wave train excited by the potential velocity anomalies and then propagated along the great circle path and global jet stream (GJT) over South Asia (SA), formed an anomalous cyclonic with ascending motion over TP and lead more precipitation over SC in winter of 2021/2022. Winter rainfall, usually accompanying with lower temperature, will bring severely bad influence on traffic, power supply, communication, agriculture and people's lives. The results of this paper can provide a reference for further understanding the causes and mechanism of DJF rainfall over SC. • The winter precipitation over SC in 2021/2022 DJF is extremely more than normal, and the associated circulation patterns are obviously different from those induced by La Niña, especially in the Atlantic and East Asia areas south to Lake Baikal. • Strong ascending motion over TP(Tibet Plateau) affects the circulation patterns directly in eastern China and lead to the excessive precipitation in SC during 2021/2022 DJF. • The anomalies over the tropical Atlantic can stimulate the Rossby wave train and propagate downstream, and the Rossby wave trains allow energy to propagate to TP area with wave flux convergent, contributing certainly to the anomaly cyclone circulation there. [ABSTRACT FROM AUTHOR]

The Madden–Julian oscillation (MJO) is one of the leading modes of tropical intra-seasonal variability, which exerts significant impacts on the weather and climate across the globe, particularly in the tropics. MJO affects the Asian monsoon by producing enhanced and suppressed convection during the active and break periods, respectively. In the recent decades, the heat content of Indo-western Pacific Ocean has increased significantly, which strengthened the MJO activity. Previous studies also have shown that the expansion of Indo-western Pacific warm pool led to the warping of MJO life cycle, which decreases its residence time over the Indian Ocean (IO) and increases over the Pacific Ocean. Here we show that in the boreal summer months, MJO amplitude has strengthened during the global warming hiatus or rapid IO warming period (1999–2015) compared to the previous period (1982–1998). In the later period, MJO exhibits a faster regeneration over the western IO, and its residence time has increased in the western hemisphere and western IO but decreased in the eastern IO and eastern Pacific Ocean. The strengthening of MJO and the readjustment in its residence time are due to the local MJO feedback on the IO and the La Nina like sea surface temperature pattern in the Pacific Ocean. The prolonged MJO activity leads to bursts of rainfall over the Indian subcontinent in Phase 3 and Phase 4, influencing the active spells of the Indian summer monsoon and causing heavy rainfall over central India and East Asia. [ABSTRACT FROM AUTHOR]

The fidelity of the eight Asia-Pacific Economic Cooperation (APEC) Climate Center (APCC) models in representing the inter-annual variability and decadal shift in the northeast monsoon (NEM; October–December) rainfall over Southern Peninsular India (SPI) is evaluated. The hindcast data is used for the period of 28 years from 1983 to 2010 based on September initial conditions. The observations showed a clear inter-annual and inter-decadal variability of NEM rainfall during the study period. The analysis suggests that most of the models exhibited poor skill in representing the inter-annual variability. Only APCC model rainfall is in phase with observed SPI rainfall variations on the inter-annual time scale. It is noticed from the observed NEM rainfall time series that the period 1990–1999 (first decade) displays an above-normal rainfall and the period 2000–2010 (second decade) displays a below normal rainfall over the SPI region. It is also evident from the observations that NEM rainfall for most of the years displayed negative anomalies in the second decade including El Niño and La Niña years, while in the first decade positive anomalies are noted, suggesting the presence of decadal variability in the NEM. Rainfall variations in most of the coupled models are in phase with the observations during the second decade but are out of phase during the first decade. As evidenced from the observations that the intensified deep convection over the Indo-western Pacific region results in too far southward movement of the intertropical convergence zone (ITCZ) during the second decade. The southward shift in the strong upper-level divergence associated with lower-level convergence over the south Indian Ocean caused negative rainfall anomalies over the SPI in this decade. Further, the difference between the second and first decade demonstrates that an anomalous anticyclonic circulation over the Indian subcontinent is accountable for the vigorous dry northerly flow towards the SPI region and the resultant decadal shift in the rainfall pattern. Though the southward shift in the rainfall and large-scale circulation patterns are mildly captured by some models from decade to decade, most of the models completely misrepresented it. This study suggests that the coupled models displayed a very limited skill not only in capturing the inter-annual variability but also in representing the decadal variability of NEM rainfall. [ABSTRACT FROM AUTHOR]

This study presents the interannual variation in the summer monsoon rainfall during the El Nino Southern Oscillation (ENSO) phases, that is, El Nino and La Nina throughout the Indian subcontinent and its subregions during 1986–2010. The analysis includes the performance assessments of the regional climate model (RegCM4) in simulating Indian summer monsoon rainfall through comparison with India Meteorological Department rainfall data. Observation clearly demonstrates that the El Nino/La Nina produced weaker/strong monsoon rainfall over India and it increases the interannual variability of the southwest monsoon. The capability and skill of RegCM4's Grell and Mix99 cumulus parameterization scheme in simulating Indian summer monsoon rainfall associated with extreme climate are usually higher in "dry" conditions rather below average for the accurate simulation of "wet" conditions. Also, model performance was good in defining the characteristics, spatial distribution, and trend of a dry spell during ENSO phases with observation. The model‐simulated spatial and temporal value of wet spell during ENSO produces overestimated value over all India. Plain Language Summary: This study has been performed with the motivation of understanding the southwest monsoon drought/flood characteristics associated with El Nino and La Nina events. Climate change variability studies are of utmost importance for the planning of agriculture and economic policy. With this motivation, an attempt has been made to study the distinguishing features of Indian summer monsoon rainfall during El Nino and La Nina years over the Indian subcontinents and its subregions with Regional Climate Model (RegCM4). The finding of our study highlights the capability and skill of RegCM4's Grell cumulus parameterization scheme in simulating Indian summer monsoon associated with extreme climate are usually higher in "dry" condition rather below average for the accurate simulation of "wet" condition. The index of maximum count of consecutive dry days and consecutive wet days helps in accessing the deficient/excess rainfall over a regionalized and localized area. Key Points: The modulation of spatial and temporal south west monsoon rainfall during ENSO phases are prominentEl Nino and La Nina episodes control the probability of occurrences and length of wet spell and dry spell during monsoonThe skill of RegCM4 in simulating ISMR during the extreme ENSO phases are better in "dry" conditions rather below average for the accurate simulation of "wet" conditions [ABSTRACT FROM AUTHOR]