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The simulation skill of the perturbed parameter ensemble (PPE) of HadGEM3‐GC3.05 on the mean climate pattern of the Asian summer monsoon (ASM) is evaluated in this study. The sensitivities of the model bias to the perturbed parameters are investigated based on metrics. The results show that the PPE mean (PPE‐20M) could effectively capture the general ASM precipitation and wind patterns, with a correlation coefficient of 0.78. PPE‐20M mainly shows positive precipitation biases over the tropical western Pacific, southern slope of the Tibetan Plateau, Indochina Peninsula, and South China and negative precipitation biases over the Indian continent and Bay of Bengal. The magnitude of the precipitation biases is more sensitive than its pattern to the variation of the perturbed parameters. Four parameters (ent_fac_dp, qlmin, ps_cloud‐ph, and psm) are found to be crucial for simulating the ASM precipitation intensity, and their combined effects are related to the simulated precipitation biases. Plain Language Summary: The simulation of the Asian summer monsoon (ASM) has attracted much attention in recent decades. However, state‐of‐the‐art coupled models still show similar systematic biases when simulating monsoon precipitation from CMIP3 to CMIP6. Recently, a perturbed parameter ensemble (PPE) method has been applied in the development of HadGEM3‐GC3.05 with the aim of improving the overall simulation skill of the coupled model. The PPE can also be used to understand the related physical processes in the model simulations. Thus, in this study, we aim to evaluate the simulation skills of PPE on the mean climate distributions of ASM. Four parameters were found to be the most critical in the precipitation simulations, which could be potential tunable parameters to reduce the biases of precipitation intensity over the monsoon regions. This study provides possible pathways to reduce model bias that could benefit the model development community. Key Points: The perturbed parameter ensemble mean captures the climate pattern of the Asian summer monsoon (ASM) but mainly overestimates precipitation over the western PacificFour parameters (ent_fac_dp, qlmin, ps_cloud‐ph, and psm) are crucial for simulating ASM precipitation in coupled runThe joint effect of these parameters, especially the deep entrainment amplitude, are dominant control for the bias in the western Pacific [ABSTRACT FROM AUTHOR]

The role of the diurnal atmospheric circulation cycle around the eastern Indian subcontinent, leeward of the monsoon westerlies, on the hydroclimate of South Asia remains unknown. Here, we reveal that low‐level moist southerlies are greatly enhanced at night parallel to the coastline over the western Bay of Bengal (BoB), and then flow onto the Gangetic Plain enhancing onshore moisture flux and nocturnal precipitation over the Himalayas and the Meghalaya Plateau. This nocturnal surge is strongly controlled by the diurnal cycle of dynamic and thermodynamic effects around the subcontinent. At night, nocturnal westerly low‐level jets appear over the subcontinent. Strong low‐level southwesterly flow with a low‐level jet structure also appears parallel to the coastline over the western BoB, extending from the west of Sri Lanka. The low‐level westerlies from the subcontinent and the low‐level southwesterlies merge into a single strong southwesterly flow, forming the low‐level moist surge. Plain Language Summary: Nocturnal precipitation is a well‐known phenomenon around the Himalayas and the Meghalaya Plateau in South Asia in summer. Such precipitation is a major supply source for glaciers in the central–eastern Himalayas and the headwaters of major rivers such as the Ganges and the Brahmaputra. In this study, we show that low‐level moist southerlies are greatly enhanced at night parallel to the coastline over the western Bay of Bengal (BoB) and then flow onto the Gangetic Plain, enhancing moisture transport toward land and nocturnal precipitation in South Asia. Here, we refer to the phenomenon as the nocturnal southerly moist surge. This nocturnal surge is strongly affected by the diurnal cycle of the thermal and topographic effects of the Indian subcontinent. At night, a strong low‐level westerly jet appears above the nocturnal stable layer over the Indian subcontinent. Strong low‐level southwesterlies with a low‐level jet structure also appear over the western BoB, extending from the strait between the southernmost tip of the Indian subcontinent and Sri Lanka. The low‐level westerlies flowing from the subcontinent and the southwesterlies merge into a single strong southwesterly flow, forming the low‐level moist surge over the western BoB. Key Points: Nocturnal low‐level southerly moist surge occurs parallel to the western Bay of Bengal coastline and flows over the Gangetic PlainThe surge consists of strong low‐level westerlies from the Indian subcontinent and southwesterlies extending from the west of Sri LankaDynamic and thermodynamic effects of the Indian subcontinent likely play an important role in forming the nocturnal surge [ABSTRACT FROM AUTHOR]

In early summer 2020, the Meiyu‐Baiu rainfall was markedly enhanced, triggering devastating floods in Japan and central China. We examined the underlying processes using a climate model and analysis. The enhanced Meiyu‐Baiu rainfall was reasonably predicted by the climate model initialized at the end of April. The sensitivity experiment indicated that Indian Ocean (IO) warming enhanced the Meiyu‐Baiu rainfall. Moreover, we found that the warm IO condition can be traced back to the super Indian Ocean Dipole (IOD) in 2019. The IO warmth was influenced by successive processes: record strong downwelling Rossby waves excited by the IOD and tripole sea surface temperature anomalies in the tropical IO‐western Pacific, their arrival to the southwestern IO in spring, and associated modulation of monsoon flow. The results suggest that the seasonal predictability of the Meiyu‐Baiu rainfall in 2020 originated from the super IOD. Plain Language Summary: In early summer 2020, Japan and central China suffered from serious floods due to torrential rainfall associated with the intensified Meiyu‐Baiu front, which extends from central China to southern Japan. The results of climate model simulations indicated that a warm condition of the Indian Ocean (IO) was an underlying condition for the enhanced rainfall. We found that the warm IO condition can be traced back to the strong Indian Ocean Dipole (IOD) episode in 2019, which featured a pair of colder‐than‐normal and warmer‐than‐normal ocean temperatures west of the Sumatra coast and in the western IO, respectively. This IOD contributed to the IO warming in the following seasons through oceanic dynamics and monsoon modulation. Key Points: The markedly enhanced rainfall in the Meiyu‐Baiu frontal zone in early summer 2020 was associated with the warm IO conditionThe warm IO condition can be traced back to the super IOD event in 2019Ocean dynamics and associated modulation of monsoon flow in the IO sector facilitated IO warming [ABSTRACT FROM AUTHOR]

Mineral dust exerts great influence on modern climate. Eolian deposits reveal that atmospheric dust concentration is larger during the Last Glacial Maximum, especially over Asia. However, the larger forcing of dust on Asian climate is still not fully explored. In this study, climate model experiments are conducted to evaluate the direct radiative effect on East Asian monsoon. The results show that the radiative forcing of dust, amplified by snow albedo feedback over high‐latitude Eurasia, produces a wave‐like pattern in westerlies and weakens the monsoon southerly winds over northern China. Dust over expanded Last Glacial Maximum sources (northern Central Asia and northeastern Russia) is responsible for this perturbation of the westerlies; the absence of these dust sources at present explains why such impacts on circulations are not simulated for modern conditions. The dust‐induced anomalies in southerly wind speed and precipitation over northern China reach 29% and 41% of the glacial‐induced changes, respectively. Key Points: Radiative effect of dust weakens the East Asian monsoon during LGM, which is not obvious at presentDust‐induced changes in monsoon winds and precipitation are comparable with glacial‐induced onesSnow‐albedo feedback initialed by high‐latitude dust exerts significant perturbations on the westerlies and monsoon [ABSTRACT FROM AUTHOR]

This study examines the linkage between the Madden‐Julian oscillation (MJO) and the South Asian summer monsoon onset through spatiotemporal wave‐filtering and composite analyses of observational data sets during 1979–2016. We identify two major factors in determining the summer monsoon onset timing: the background conditions associated with seasonal transitions and the intraseasonal variations associated with an active MJO. The background conditions undergo sharp seasonal transition over the western Indian Ocean two to three pentads before the onset dates, while a typical monsoon onset is often associated with the arrival of the wet phase of the tropical MJO over the Indian Ocean, likely due to the promotion by the MJO‐initiated eastward propagating westerly wind anomaly. Conversely, the atmospheric circulation associated with the leading dry phase of a strong MJO during the climatological mean onset dates may lead to a delayed monsoon onset. Plain Language Summary: The Madden‐Julian oscillation (MJO) is one of the dominant modes of intraseasonal variabilities in the tropical atmosphere, while the Asian summer monsoon represents one of the most significant transitions in the large‐scale atmospheric and oceanic circulations in the Indo‐Pacific regions and beyond. This study examines the linkage between MJO and the South Asian summer monsoon onset through composite analysis of decades of observational data sets along with a global space‐time wave‐filtering technique. We identify two major factors in determining the summer monsoon onset timing: the background conditions associated with seasonal transitions and the intraseasonal variations associated with an active MJO. The onset of the Indian summer monsoon is often associated with the arrival of the wet phase of the tropical MJO over the Indian Ocean, while a delayed monsoon onset is likely caused by the dry phase of a strong MJO that suppresses background changes associated with seasonal transition during the climatological mean onset dates. Key Points: Seasonal transition of background conditions and the propagating MJO circulations are key factors in the monsoon onset timing over IndiaThe onset of the Indian summer monsoon is often associated with the arrival of the wet phase of the tropical MJO over the Indian OceanThe delayed monsoon onset is likely caused by the dry phase of a strong MJO that suppresses background changes during the climatological mean onset dates [ABSTRACT FROM AUTHOR]

Abstract: Anthropogenic aerosols are a major factor contributing to human‐induced climate change, particularly over the densely populated Asian monsoon region. Understanding the physical processes controlling the aerosol‐induced changes in monsoon rainfall is essential for reducing the uncertainties in the future projections of the hydrological cycle. Here we use multiple coupled and atmospheric general circulation models to explore the physical mechanisms for the aerosol‐driven monsoon changes on different time scales. We show that anthropogenic aerosols induce an overall reduction in monsoon rainfall and circulation, which can be largely explained by the fast adjustments over land north of 20∘N. This fast response occurs before changes in sea surface temperature (SST), largely driven by aerosol‐cloud interactions. However, aerosol‐induced SST feedbacks (slow response) cause substantial changes in the monsoon meridional circulation over the oceanic regions. Both the land‐ocean asymmetry and meridional temperature gradient are key factors in determining the overall monsoon circulation response. [ABSTRACT FROM AUTHOR]