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The quantitative relationship between Mesoscale Convective Systems (MCSs) and floods over East Asia has not been established. In this study, MCSs are clustered into four types with Self‐Organizing Map approach. Floods in June‐August of 2000–2021 are linked with different types of MCS by automated algorithms we constructed. We find that among the major floods (potential flood peak periods), 91% (87%) are related to MCS, 65% (78%) are dominated by MCS, and 38% (20%) are dominated by multi‐types of MCS. Types 1 and 2 MCS have higher flood‐inducing efficiencies than common MCS (Type‐4). Type‐1 MCS, characterized by the least number (2% of the total number), the largest precipitation volume, longest lifetime, slowest moving, strongest precipitation, can most efficiently produce floods. Type‐2 MCS, characterized by the second largest precipitation volume, more numerous than Type‐1 particularly over land, can induce floods not only relatively efficiently but also more frequently than Type‐1. Plain Language Summary: Mesoscale Convective Systems (MCS) can cause large flood events but how they work in East Asia at climatic scale remains unexplored. In this study, we tracked MCS over the time period of 2000–2021 and grouped them into four types using the Self‐Organizing Map approach. We connected summer floods with the four types of MCS using automated algorithms. We found that most large flood events and potential flood peak periods are dominated by MCS. Type‐1 MCS, which features large land‐rain area, longest lifetime, slowest movement and strongest precipitation, is most efficient in inducing floods. A Longer lifetime not only contributes to greater total precipitation volume but also increases the overlap area, thereby enhancing the precipitation volume per area. The slow movement further increases the overlap area. The high overlap area helps reduce the proportion of precipitation absorbed by the soil. Type‐2 MCS, which features with the second largest precipitation volume, occurs more frequently than Type‐1 especially over land, and can cause floods more frequently than Type‐1. These findings can enhance our understanding of flood formation over East Asia. Key Points: Among the major floods (potential flood peak periods) occurred in 2000–2021 summer, 91% (87%) are related to Mesoscale Convective Systems (MCS), and 66% (78%) are dominated by MCSType‐1 MCS is the most efficient among the four types in causing floods for its strongest precipitation rate, longest lifetime and slowest movementLong lifetime and slow movement increase the overlap area, boosting precipitation per area and reducing the proportion of rainfall absorbed by soil [ABSTRACT FROM AUTHOR]

The modern East Asian summer monsoon (EASM) features an extension from tropical to subtropical areas. However, the fundamental process that determines the northward extension of EASM in the geological history remains unclear. Here, we showed evidence from proxy data, climate modeling, and theoretical solutions that the northward extension of EASM to today's boundary emerged no later than the Miocene. The extension was driven by the monsoon seasonal march which features stepwise northward rainfall stages. The seasonal progression of monsoon was determined by Rossby wave responses from early summer to late summer and caused by the weakening of westerly jet colliding with the Tibetan Plateau (TP). The Rossby wave responses further led to a northward migration of the western Pacific subtropical high and thereby monsoon precipitation. Our findings propose a novel physical linkage between the geological evolution of EASM and the TP uplift in the context of monsoon seasonal march. Plain Language Summary: The seasonal march of the modern East Asian summer monsoon, characterized as stepwise northward rainfall stages, is one unique feature and the most fundamental process of northward extension of East Asian summer monsoon. However, its geological history and controlling processes remain unknown. Here, we found that the seasonal progression of the monsoon emerged no later than the Miocene and was driven by the uplift of the Tibetan Plateau. The emergence of the seasonality has pushed the East Asian summer monsoon northward to today's boundary. Our results provide an essential understanding of the northward extension of the East Asian monsoon climate in geological history, highlighting the role of monsoon seasonal march. Key Points: A novel physical linkage established between the East Asian monsoon climate and Tibetan Plateau uplift on the geological scaleOrographic forcing of the uplift of Tibetan Plateau determined the emergence of monsoon seasonal marchThe seasonal progression of the monsoon advanced further inland to the present boundary [ABSTRACT FROM AUTHOR]

Regional coupled modeling is one of the frontiers of regional climate modeling, but intercomparison has not been well coordinated. In this study, a community regional climate model, WRF4, with a resolution of 15 km, was coupled with a high-resolution (0.1°) North Pacific Ocean model (LICOM_np). The performance of the regional coupled model, WRF4_LICOM, was compared to that of another regional coupled model, RegCM4_LICOM, which was a coupling of version 4 of the Regional Climate Model (RegCM4) with LICOM_np. The analysis focused on the 2005 western North Pacific summer monsoon rainfall. The results showed that the regional coupled models with either RegCM4 or WRF4 as their atmospheric model component simulated the broad features over the WNP reasonably well. Quantitative intercomparison of the regional coupled simulations exhibited different biases for different climate variables. RegCM4_LICOM exhibited smaller biases in its simulation of the averaged June–July–August SST and rainfall, while WRF4_LICOM better captured the tropical cyclone (TC) intensity, the percentage contributions of rainfall induced by TCs to the total rainfall, and the diurnal cycle of rainfall and stratiform percentages, especially over land areas. The different behaviors in rainfall simulated by the two models were partly ascribed to the behaviors in the simulated western North Pacific subtropical high (WNPSH). The stronger (weaker) WNPSH in WRF4_LICOM (RegCM4_LICOM) was driven by overestimated (underestimated) diabatic heating, which peaked at approximately 450 hPa over the region around the Philippines in association with different condensation-radiation processes. Coupling of WRF4 with LIOCM is a crucial step towards the development of the next generation of regional earth system models at the Chinese Academy of Sciences. [ABSTRACT FROM AUTHOR]

The Indian summer monsoon (ISM) rainfall affects a large population in South Asia. Observations show a decline in ISM rainfall from 1950 to 1999 and a recovery from 1999 to 2013. While the decline has been attributed to global warming, aerosol effects, deforestation, and a negative-to-positive phase transition of the interdecadal Pacific oscillation (IPO), the cause for the recovery remains largely unclear. Through analyses of a 57-member perturbed-parameter ensemble of model simulations, this study shows that the externally forced rainfall trend is relatively weak and is overwhelmed by large internal variability during both 1950–99 and 1999–2013. The IPO is identified as the internal mode that helps modulate the recent decline and recovery of the ISM rainfall. The IPO induces ISM rainfall changes through moisture convergence anomalies associated with an anomalous Walker circulation and meridional tropospheric temperature gradients and the resultant anomalous convection and zonal moisture advection. The negative-to-positive IPO phase transition from 1950 to 1999 reduces what would have been an externally forced weak upward rainfall trend of 0.01 to −0.15 mm day−1 decade−1 during that period, while the rainfall trend from 1999 to 2013 increases from the forced value of 0.42 to 0.68 mm day−1 decade−1 associated with a positive-to-negative IPO phase transition. Such a significant modulation of the historical ISM rainfall trends by the IPO is confirmed by another 100-member ensemble of simulations using perturbed initial conditions. Our findings highlight that the interplay between the effects of external forcing and the IPO needs be considered for climate adaptation and mitigation strategies in South Asia. [ABSTRACT FROM AUTHOR]

Simulating the East Asian summer monsoon (EASM) rain belt has been proven challenging for climate models. In this study, the impacts of high resolution to the simulation of spatial distributions and rainfall intensity of the EASM rain belt are revealed based on Atmospheric Model Intercomparison Project (AMIP) simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. A set of sensitivity experiments is further performed to eliminate the potential influences of differences among CMIP5 models. The results show that the high-resolution models improve the intensity and the spatial pattern of the EASM rainfall compared to the low-resolution models, further valid in the sensitivity experiments. The diagnosis of moist static energy (MSE) balance and moisture budgets is further performed to understand the mechanisms underlying the enhancements. Both analyses indicate that the improved EASM rainfall benefits from the intensified meridional convergence along the EASM rain belt simulated by the high-resolution models. In addition, such convergence is mainly contributed by intensified stationary meridional eddy northerly flows over the central northern areas of China and southerly flows over the south of Japan due to increased model resolution, which is robust in the sensitivity experiments. Further analysis indicates that the stationary meridional eddy flow changes in high-resolution simulations are related to the barotropic Rossby wave downstream of the Tibetan Plateau resulting from increased resolution. [ABSTRACT FROM AUTHOR]

This study investigated variations in summertime short-duration precipitation extremes over the middle-lower reaches of the Yangtze River valley (MLYRV) and the possible impacts of tropical cyclones (TC) on these changes over the 1961–2016 period. Three short-duration precipitation types, including all events and the events in the 75th and 95th percentiles, are defined. At the 75th percentile, both the amount and number of TC-induced extreme short-duration rainfall events exhibit slightly declining trends, and the trends are not pronounced for the 95th-percentile cases. The number and amount of TC-induced 95th-percentile events are approximately one order of magnitude larger than those of the 75th-percentile cases, indicating that TCs play a more significant role on the upper tail of the rainfall distribution over the MLYRV in summer than on the rest of the precipitation distribution. A large number of the defined non-TC precipitation events are mainly distributed over the central-south MLYRV and southeastern regions. The spatial patterns of TC-induced rainfall resemble the contributions of TC-induced precipitation events to the total summer rainfall, with large values located in southeastern regions of the MLYRV. The spatial distribution of 95th-percentile events is principally attributed to the frequency of TC activities and to the TC tracks. Most of the TCs that trigger 95th-percentile events make landfall over the southeast MLYRV, and the corresponding large frequency of TC tracks is mainly situated over the ocean to the east of Taiwan. Both environmental factors and atmospheric circulations modulate the variations in extreme short-duration rainfall events. In particular, equatorward displacement of the western Pacific subtropical high impacts TC track density, resulting in more occurrences of precipitation extremes in 95th-percentile events over the MLYRV. • TCs play a more significant role on the upper tail of the rainfall distribution over the MLYRV in summer than on the rest of the rainfall distribution. • The short-duration precipitation extremes induced by TCs are mainly distributed in southeastern regions of the MLYRV. • Equatorward displacement of the WPSH results in more occurrences of short-duration rainfall extremes in 95th-percentile events over MLYRV. [ABSTRACT FROM AUTHOR]

The reproducibility and future changes of the onset of the Asian summer monsoon were analyzed based on the simulations and projections under the Representative Concentration Pathways (RCP) scenario in which anthropogenic emissions continue to rise throughout the 21st century (i.e. RCP8.5) by all realizations from four Chinese models that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5). Delayed onset of the monsoon over the Arabian Sea was evident in all simulations for present-day climate, which was associated with a too weak simulation of the low-level Somali jet in May. A consistent advanced onset of the monsoon was found only over the Arabian Sea in the projections, where the advanced onset of the monsoon was accompanied by an increase of rainfall and an anomalous anticyclone over the northern Indian Ocean. In all the models except FGOALS-g2, the enhanced low-level Somali jet transported more water vapor to the Arabian Sea, whereas in FGOALS-g2 the enhanced rainfall was determined more by the increased wind convergence. Furthermore, and again in all models except FGOALS-g2, the equatorial SST warming, with maximum increase over the eastern Pacific, enhanced convection in the central West Pacific and reduced convection over the eastern Indian Ocean and Maritime Continent region, which drove the anomalous anticyclonic circulation over the western Indian Ocean. In contrast, in FGOALS-g2, there was minimal (near-zero) warming of projected SST in the central equatorial Pacific, with decreased convection in the central West Pacific and enhanced convection over the Maritime Continent. The broader-scale differences among the models across the Pacific were related to both the differences in the projected SST pattern and in the present-day simulations. [ABSTRACT FROM AUTHOR]

In this study, the authors calibrated the performance of the Regional Climate Model, version 3 (RegCM3), with the Massachusetts Institute of Technology (MIT)-Emanuel cumulus parameterization scheme over the Coordinated Regional Climate Downscaling Experiment (CORDEX) East Asia domain by tuning seven selected parameters based on the multiple very fast simulated annealing (MVFSA) approach. The seven parameters were selected based on previous studies using RegCM3 with the MIT-Emanuel convection scheme. The results show the simulated spatial pattern of rainfall, and the probability density function distribution of daily rainfall rates is significantly improved in the optimal simulation. Sensitivity analysis suggests that the parameter relative humidity criteria (RHC) has the largest effect on the model results. Followed by an increase of RHC, an increase of total rainfall is found over the northern equatorial western Pacific, mainly contributed by the increase of explicit rainfall. The increases of the convergence of low-level water vapor transport and the associated increases in cloud water favor the increase of explicit rainfall. The identified optimal parameters constrained by total rainfall have positive effects on the low-level circulation and surface air temperature. Furthermore, the optimized parameters based on the chosen extreme case are transferable to a normal case and the model's new version with a mixed convection scheme. [ABSTRACT FROM AUTHOR]