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The Tibetan Plateau (TP), known as the "Third Pole," profoundly affects weather and climate at regional and global scales. In this study, we investigate the characteristics of heat waves in China and their association with the TP heat source. The results show that during the summertime from 1980 to 2020, the frequency of heat wave days that hit eastern China and northwest China increased at rates of 0.09 and 0.24 days yr−1, respectively, accompanied by an increase in the atmospheric heat source (AHS) over the TP by over 2 W m−2 yr−1 under the background of global warming. The enhanced TP heat source induces an anomalous upper-tropospheric anticyclone, which caused the western Pacific subtropical high and South Asia high to be stronger and closer to each other, causing descending motions over eastern China and consequently more heat waves. At the same time, the enhanced TP heat source weakened the westerlies, thereby favoring the occurrence and maintenance of the anticyclone centered in northwest China and creating more heat waves due to strong descending motions. Therefore, the association between the TP heat source and heat waves in China provides relevant information for studying the mechanism and future changes of heat waves. Significance Statement: The Tibetan Plateau significantly affects the surrounding weather and climate. We find that there are some linkages between the Tibetan heat source and heat waves in China. The stronger the Tibetan heat source is, the more heat waves hit China. The enhancement of the Tibetan heat source could induce an anomalous upper-tropospheric anticyclone by affecting the western Pacific subtropical high and South Asia high, consequently causing descending motions over eastern China and the resulting heat waves. The weaker westerly due to the enhanced Tibetan heat source favors the occurrence of the anticyclone centered in northwest China, leading to more heat waves by descending motions. This study reveals a potential contributor to heat waves in China, providing some clues for better prediction in the future. [ABSTRACT FROM AUTHOR]

The Indian monsoon is of utmost concern to agriculture, the economy, and the livelihoods of billions in South Asia. However, little attention has been paid to the possibility of distinct subseasonal episodes phase-locked in the Indian monsoon annual cycle. This study addresses this gap by utilizing the self-organizing map (SOM) method to objectively classify six distinct subseasonal stages based on the 850-hPa wind fields. Each subseasonal stage ranges from 23 to 90 days. The Indian summer monsoon (ISM) consists of three substages, the ISM-onset, ISM-peak, and ISM-withdrawal, altogether contributing to 82% of the annual precipitation. The three substages signify the rapid northward advance, dominance, and gradual southward retreat of southwesterlies from mid-May to early October. The winter monsoon also comprises three substages (fall, winter, and spring), distinguishable by the latitude of the Arabian Sea high pressure ridge and hydrological conditions. This study proposes two compact indices based on zonal winds in the northern and southern Arabian Sea to measure the winter and summer monsoons, respectively. These indices capture the development and turnabouts of the six SOM-derived stages and can be used for subseasonal monsoon monitoring and forecasts. The spring and the ISM-onset episodes are highly susceptible to compound hazards of droughts and heatwaves, while the greatest flood risk occurs during the ISM-peak stage. The fall stage heralds the peak season for tropical storms over the Arabian Sea and the Bay of Bengal. The annual start and end dates of the ISM-peak are highly correlated (0.6-0.8) with the criteria-based dates proposed previously, supporting the delineation of the Indian monsoon subseasonal features. [ABSTRACT FROM AUTHOR]

The relative impacts of the Tibetan Plateau–Himalaya (TPH) and the adjacent narrow mountains on the South Asian summer monsoon (SASM) are still unclear. We used a general circulation model to evaluate the relative effects of the TPH, the Hindu Kush Mountains (HKM), and the Iranian Plateau on the SASM from the perspective of the asymmetrical onset of the Bay of Bengal (BOB) summer monsoon (BOBSM) and the Indian summer monsoon (ISM). The results show that the abrupt onset of the BOBSM in May is affected by the TPH and HKM, which significantly strengthen precipitation over the BOB. By contrast, precipitation over the Indian subcontinent is suppressed by the TPH. In June, the TPH and the HKM contribute to strengthening and expanding precipitation over the Indian subcontinent and the ISM is significantly intensified. The strengthening of the SASM onset induced by the HKM is comparable with, or even larger than, that induced by the TPH. The effect of the Iranian Plateau on the SASM is much weaker and localized. The distinct role of the HKM is caused by the appearance of an anomalous low-level cyclone around the HKM and an anticyclone to the south, strengthening the India–Burma trough and the Somali jet. The topographic control of the HKM depends on the pre-existence of the TPH. We show that the sensible heat from the HKM accounts for 78.6% and 31.6% of the HKM-induced increase in precipitation during the onset of the BOBSM and ISM, respectively. Significance Statement: The South Asian summer monsoon shows a prominent asynchronous feature in that it starts earlier over the Bay of the Bengal (BOB) during May and appears over the Indian subcontinent in June. However, how the adjacent mountain ranges modulate this monsoon advance is still unclear. Our climate model experiments show that both the Hindu Kush Mountains (HKM) and the Tibet Plateau–Himalaya (TPH) contribute to the strengthening of the asymmetrical onset of the monsoon. During May, the monsoon over the BOB is significantly intensified while the precipitation over the Indian subcontinent is largely suppressed. The HKM–TPH strengthens the precipitation over the Indian subcontinent in June. The HKM-induced strengthening is comparable with, or even larger than, that induced by the TPH. [ABSTRACT FROM AUTHOR]

Monsoon low pressure systems (LPS) are synoptic-scale disturbances that form along the quasi-stationary trough of the larger-scale South Asian summer monsoon, producing a large fraction of total monsoon precipitation. Here, we use an Earth system model to investigate the influence of Tibetan and Himalayan orography (THO) on mean monsoon strength and LPS activity. The influence of THO height on LPS activity has not been investigated before. The model simulates decreased mean monsoon circulation and precipitation when THO is removed, but the number of LPS and the meridional extent of LPS activity increase; this is an unexpected, important finding given that LPS are principal rain-bearing systems of the monsoon. The decreased mean monsoon circulation and precipitation on removal of THO are attributed to enhanced mixing of dry extratropical air into the humid monsoon domain, as demonstrated by prior studies. The increase in LPS frequency and the meridional extent of LPS activity is attributed to the increase in magnitude and meridional extent of the low-level meridional shear of zonal wind, which previous work showed amplifies LPS through barotropic growth. Specifically, as monsoon precipitation decreases, the sensible heat-driven low-level trough intensifies and shifts equatorward; this strengthens the shear zone in which LPS grow. Conversely, increasing THO height decreases the magnitude and meridional extent of cyclonic shear over India, decreasing LPS frequency and the spatial extent of LPS activity while increasing total monsoon precipitation. These results demonstrate that LPS activity and total monsoon rainfall can undergo large, opposing changes in response to imposed forcings. Significance Statement: Monsoon low pressure systems (LPS) are propagating atmospheric vortices that deliver a large fraction of seasonal-mean precipitation to agricultural regions of India. The influence of Tibetan and Himalayan orography (THO) on the South Asian monsoon is an active area of research, but its effect on LPS has remained unexplored. Here, a global climate model is used to simulate LPS characteristics for different heights of THO. Flattening the THO reduces total summer monsoon precipitation but increases the number of LPS and the spatial extent of LPS activity. We attribute these LPS changes to a decrease in the magnitude and meridional extent of the hydrodynamically unstable low-level cyclonic shear zone over South Asia that occurs when orography is flattened. [ABSTRACT FROM AUTHOR]

The premonsoon circulation over South Asia in May shows remarkable interannual variations, and it can modulate the onset of the Asian summer monsoon. The present study derives two dominant stratosphere–troposphere modes over the North Atlantic in April via applying principal component (PC) analysis to regional geopotential height during 1979–2015. Both of the modes reflect the North Atlantic Oscillation (NAO)-like circulation at the midtroposphere, but the circulation patterns and planetary wave activity between the two modes are quite different. The first mode represents a stratosphere–troposphere-coupled mode. Further analysis indicates that during the years with a positive phase of the first mode, the low-level anomalous southerlies over the northern Barents Sea (BS) lead to a reduction in local sea ice, which could persist into May through the ice-albedo feedback. The BS sea ice anomalies in May could generate a southeastward propagating Rossby wave train, producing an anomalous anticyclonic circulation to the west of Qinghai–Tibet Plateau. This anomalous anticyclone induces increases in local air temperature and the meridional temperature gradient, resulting in a strengthened premonsoon circulation. Given that prediction of the premonsoon circulation over South Asia remains a challenging issue, the stratosphere–troposphere-coupling modes in April provide another potential predictability source. On the other hand, ENSO has a significant effect on the South Asian premonsoon circulations in May, and can also lead to the NAO-like circulation by influencing atmospheric Rossby waves. Such an NAO-like circulation is closely related to the second mode. [ABSTRACT FROM AUTHOR]

The southwest monsoon delivers over 70% of India's annual rainfall and is crucial to the success of agriculture across much of South Asia. Monsoon precipitation is known to be sensitive to sea surface temperature (SST) in the Bay of Bengal (BoB). Here, we use a configuration of the Unified Model of the Met Office coupled to an ocean mixed layer model to investigate the role of upper-ocean features in the BoB on southwest monsoon precipitation. We focus on the pronounced zonal and meridional SST gradients characteristic of the BoB; the zonal gradient in particular has an as-yet unknown effect on monsoon rainfall. We find that the zonal SST gradient is responsible for a 50% decrease in rainfall over the southern BoB (approximately 5 mm day−1), and a 50% increase in rainfall over Bangladesh and northern India (approximately 1 mm day−1). This increase is remotely forced by a strengthening of the monsoon Hadley circulation. The meridional SST gradient acts to decrease precipitation over the BoB itself, similarly to the zonal SST gradient, but does not have comparable effects over land. The impacts of barrier layers and high-salinity subsurface water are also investigated, but neither has significant effects on monsoon precipitation in this model; the influence of barrier layers on precipitation is felt in the months after the southwest monsoon. Models should accurately represent oceanic processes that directly influence BoB SST, such as the BoB cold pool, in order to faithfully represent monsoon rainfall. [ABSTRACT FROM AUTHOR]

Both South Asia and East Asia are the most polluted regions of the world. Unlike East Asia, the aerosol optical depth (AOD) over South Asia keeps increasing for all recent years, which calls for more attention. This study investigates the impacts of anthropogenic emissions over South Asia on the downstream regional climate during spring with the Community Earth System Model 2 (CESM2). The model results suggest that South Asian pollutants have significant impacts on East Asian spring climate, and the impacts could be even larger than locally emitted aerosols. Two possible dynamical pathways (i.e., the northern and the southern pathways) bridging South Asian aerosol forcing and East Asian climate are proposed, and both ways are associated with the black carbon (BC)-induced climate feedbacks surrounding the Tibetan Plateau (TP). The northern pathway is mainly due to the TP warming induced by the BC snow darkening effect (SDE), which significantly reduces the surface air temperature (SAT) over northern East Asia. BC-induced TP warming increases the meridional thermal gradient and accelerates the midlatitude jet stream, which favors the cold-air advection over northern East Asia. The southern pathway is associated with the BC "elevated heat pump" hypothesis, which mainly affects the precipitation in southern East Asia. BC from South Asia accumulates near the south slope of TP, inducing an abnormal ascending motion near the Bay of Bengal. A compensating anomalous sinking motion is then forced in South China, which suppresses the precipitation there. A primary observational analysis is also performed to verify both dynamical pathways. [ABSTRACT FROM AUTHOR]

Significant anomalies in frequency of summer extreme hot days (SEHDs) are broadly observed in the Asian monsoon region (AMR) in the post-ENSO summers. The delayed ENSO impacts are mainly conveyed by provoking the Indo-western Pacific Ocean capacitor (IPOC) effect that maintains the anomalous anticyclone in the western North Pacific. The related diabatic heating anomaly can trigger the westward-propagating Rossby wave to the Indian subcontinent, which increases the geopotential heights, reduces the cloud cover, and thus increases the seasonal surface temperature and SEHD frequency in the southern AMR. Besides, the reduced atmospheric moisture in the western North Pacific hinders the northward propagation of intraseasonal oscillation (ISO) and modulates the occurrence frequency of individual ISO phases, contributing to the significantly increased/decreased SEHDs in eastern China/Hokkaido, Japan, in the post-El Niño summers. The 25-model-ensemble mean of CMIP6 historical runs can reproduce well the observed SEHD anomalies in the southern AMR in the post-ENSO summers mainly due to the realistic simulation of ENSO impacts on the seasonal surface temperature, although a large intermodel spread exists due to different strengths of IPOC effect in each model owing to model biases in the mean state of the eastern tropical Pacific, the ENSO variance, and teleconnection to the Indian Ocean. Furthermore, future projections under the SSP5-8.5 scenario show that the delayed ENSO impacts on the southern AMR remain stable under global warming via a similar mechanism as in the observations and historical runs. [ABSTRACT FROM AUTHOR]

Understanding and predicting extreme weather and climate are ultimately important for adaptation and resilience. Here, we assess the prediction skills of frequency of summer extreme hot days (SEHDs) in the Asian monsoon region (AMR) by using the 1981-2014 hindcasts of the Predictive Ocean Atmosphere Model for Australia version 2 (POAMA-2) subseasonal-to-seasonal prediction system. Generally, the good prediction skills of SEHD frequency appear in the southern AMR south of 308N including the Indian subcontinent, Indo-China peninsula, and the Philippines; the anomaly correlation coefficients between the observed and predicted region-mean SEHD anomalies are 0.72, 0.72, 0.70, 0.60, 0.61, and 0.61 at 1-6 months lead, respectively, all statistically significant at the 99.9% confidence level. The high prediction skills in POAMA-2 are due to its capacity on reproducing the observed delayed ENSO impacts on the large-scale atmospheric circulation, the seasonal surface air temperature, and thus the SEHD frequency in the southern AMR via provoking the Indo--western Pacific Ocean capacitor effect. Since POAMA-2 only provides the prediction at most half a year in advance, we also conduct the hindcasts in the SINTEX-F that can reproduce well the robust ENSO--SEHD relationship and has high prediction skill of ENSO itself. Results show that skillful prediction of the region-mean SEHD frequency can be up to 14 months in advance. These results are complementary to Part I of this study that ENSO can impact the SEHDs in the AMR and provide the sources of predictability. [ABSTRACT FROM AUTHOR]

The relative contributions of cyclonic disturbances (CDs; i.e., low pressure systems, depressions, and cyclonic storms) and non-CDs to annual and seasonal rainfall are studied using 22 years of TRMM and GPM measurements during the passage of 866 CDs in the South Asia region (SAR). The changes in stratiform and convective precipitation within the cyclonic storm and in different CDs are also examined. The rainfall in the wettest regions of the SAR, the west coasts of India and Myanmar, and the slopes of the Himalayas is of non-CD origin, while CD rainfall peaks in the eastern parts of the monsoon trough and the northern Bay of Bengal (BOB). The CD rain fraction (RF) of annual and seasonal rainfall exhibits large spatial variation in the range of 4%–55%. The land–ocean dichotomy exhibited by CD RF is not uniform across India. Large CD RF is confined to the coast in some regions due to topographical barriers, but extends to 800–1000 km inland from the coast in the monsoon trough region. Low pressure systems contribute more to annual rain than depressions and cyclonic storms in the monsoon trough and the northern BOB (∼40%), particularly during the monsoon, mainly due to their frequent occurrence. The stratiform RF and occurrence are higher in CDs than in nonCDs, with the greatest contribution in central India (.80%), whereas the non-CDs are characterized by having higher convective RFs. The stratiform rain occurrence increases with intensification of CDs over both land and ocean, indicating its importance in the intensification of CDs and organizing large-scale systems. [ABSTRACT FROM AUTHOR]

Humans' essential ability to combat heat stress through sweat-based evaporative cooling is modulated by ambient air temperature and humidity, making humid heat a critical factor for human health. In this study, we relate the occurrence of extreme humid heat in two focus regions to two related modes of intraseasonal climate variability: the Madden–Julian oscillation (MJO) and the boreal summer intraseasonal oscillation (BSISO). In the Persian Gulf and South Asia during the May–June and July–August seasons, wet-bulb temperatures of 28°C are found to be almost twice as likely during certain oscillation phases than in others. Variations in moisture are found, to varying degrees, to be an important ingredient in anomalously high wet-bulb temperatures in all three areas studied, influenced by distinct local circulation anomalies. In the Persian Gulf, weakening of climatological winds associated with the intraseasonal oscillation's propagating center of convection allows for anomalous onshore advection of humid air. Anomalously high wet-bulb temperatures in the northwestern region of South Asia are closely aligned with positive specific humidity anomalies associated with the convectively active phase of the oscillation. On the southeastern coast of India, high wet-bulb temperatures are associated with convectively inactive phases of the intraseasonal oscillation, suggesting that they may be driven by increased surface insolation and reduced evaporative cooling during monsoon breaks. Our results aid in building a foundation for subseasonal predictions of extreme humid heat in regions where it is highly impactful. Significance Statement: Understanding when and why extreme humid heat occurs is essential for informing public health efforts protecting against heat stress. This analysis works to improve our understanding of humid heat variability in two at-risk regions, the Persian Gulf and South Asia. By exploring how subseasonal oscillations affect daily extreme events, this analysis helps bridge the prediction gap between weather and climate. We find that extreme humid heat is more than twice as likely during specific phases of these oscillations than in others. Extremes depend to different extents upon combinations of above-average temperature and humidity. This new knowledge of the regional drivers of humid heat variability is important to better prepare for the increasingly widespread health and socioeconomic impacts of heat stress. [ABSTRACT FROM AUTHOR]

The Indochina Peninsula (ICP) has a critical effect in shaping the Asian summer monsoon (ASM). However, the seasonal responses of the ASM to the ICP are not fully understood. This study employs a 18 atmospheric general circulation model to examine the different contributions of the ICP's orography and land-sea contrast to the ASM during the early and late summer. Results indicate that the orographic effect increases South Asian rainfall and reduces the rainfall over the South China Sea (SCS) and North China in early summer, but its influence on monsoonal circulation and rainfall is limited to East Asia in late summer. The impact of the ICP's land-sea contrast is basically opposite in the two summer stages. With the presence of the ICP, SCS rainfall is enhanced but South Asian rainfall is weakened in early summer. In late summer, however, rainfall from the ICP to the northwestern Pacific is strikingly reduced, accompanied by intensified rainfall over South Asia. Relatively, the orographic effect seems to be more important in modulating the South Asian monsoon in early summer, while the land-sea contrast is dominant in strengthening the SCS monsoon and suppressing the northwest Pacific monsoon via the interaction between the induced local circulation and multilevel ASM subsystems. In late summer, the orographic effect on the ASM is much weaker compared to the land-sea contrast, which plays a critical role by shifting the subtropical high southwestward and through the "thermal adaption" feedback mechanism. Therefore, the orographic impact of the ICP on the ASM differs from that of the land-sea contrast in the two summer stages. [ABSTRACT FROM AUTHOR]

The direct perturbation of anthropogenic aerosols on Earth's energy balance [i.e., direct aerosol radiative forcing (DARF)] remains uncertain in climate models. These uncertainties critically depend on aerosol optical properties, primarily aerosol optical depth (AOD), single scattering albedo (SSA), and the asymmetry factor g. In this study, we investigate the intermodel spread of DARF across 14 global models within phase 6 of the Coupled Model Intercomparison Project (CMIP6), using unified radiative transfer calculation and aerosol optical parameter assumptions. The global mean DARF for clear sky in 2014 with respect to 1850 is estimated as 20.77 ± 0.52 W m-2 assuming an externally mixed state and 20.68 ± 0.53 W m-2 for an internally mixed state. We further conduct a quantitative analysis and find that globally, for the external mixing assumption, AOD is the dominant factor, whose intermodel spread results in 36% of the total DARF uncertainty. For the internal mixing assumption, SSA becomes the major factor, which also leads to 36% DARF uncertainty. The g parameter and aerosol vertical distribution combined contribute to ~30% of the DARF uncertainty. Regionally, DARF uncertainty is typically more sensitive to SSA where the absorbing aerosol fraction is high, such as South Asia and central Africa. Substantial differences between model-averaged and observed aerosol optical parameters are still noticed, with external mixing in general yielding closer agreement with observations. Our results highlight the importance of aerosol scattering and absorption properties in DARF estimation. [ABSTRACT FROM AUTHOR]

Upper-tropospheric anticyclones (UTACs) emerge throughout the seasons with changing location and intensity. Here, the formation mechanisms of these UTACs, especially in the Asian–Australian–western Pacific sector, were investigated based on the diagnosis of the vorticity equation as well as the contribution of the planetary waves. During June–July–August (JJA), a vigorous UTAC corresponding to the South Asian high (SAH) forms over South Asia, to the south of the Tibetan Plateau, where intense heating associated with the Asian summer monsoon rainfall and the resultant baroclinic Rossby response are the important physical processes. Meanwhile, the produced anticyclonic vorticity is farther transported by the interhemispheric divergent wind toward the Southern Hemisphere (SH), creating the SH UTAC centered over the Maritime Continent. During December–January–February (DJF), two zonally elongated UTACs reside on each side of the equator (∼10° poleward), mainly over the Maritime Continent–western Pacific sector. Upon a closer look at the NH winter, we observed that the northern parts of UTAC cannot be explained by this vorticity balance alone. Diagnosis of the wave activity flux indicated that planetary waves emanating from the cold Eurasian continent converges around the northern parts of the UTAC with its peak in the NH winter, which weakens the subtropical jet, thus generating UTAC. Configuration of the SH summer (DJF) UTAC bears resemblance with that of SAH. These results suggest that the creation of anticyclonic vorticity and its interhemispheric transportation as well as the propagation of planetary wave are the selectively important agents for the genesis of seasonally varying UTACs. Significance Statement: Recent studies have provided evidence that the South Asian high (formerly Tibetan high) is not a purely thermally driven system only maintained over the elevated Tibetan Plateau. This study aims to understand the physical processes responsible for the genesis of the upper-tropospheric anticyclone, especially in the Asian–Australian–western Pacific sector, throughout the season. During summer in the Northern Hemisphere, deep heating caused by South Asian monsoon rainfall plays a crucial role in the genesis of the South Asian high. The wintertime anticyclone emerging over the subtropical western North Pacific is caused via remote influences anchored with tropical convection and the cold Eurasian continent in which atmospheric teleconnections are important. These findings provide new avenues for research on tropical–extratropical interactions with respect to the formation and variability of important climate phenomena. [ABSTRACT FROM AUTHOR]

The water sources and their variability responsible for the South Asian summer monsoon precipitation were analyzed using Lagrangian atmospheric water-mass trajectories. The results indicated that evaporated waters from the central and south Indian Ocean are the major contributors to the South Asian summer monsoon rainfall, followed by the contribution from the local recycling (precipitated water that evapotranspirated from the South Asian landmass), the Arabian Sea, remote sources, and the Bay of Bengal. It was also found that although the direct contribution originating from the Bay of Bengal is small, it still provides a pathway for the atmospheric water that comes from other regions. This pathway is hence only crossing over the Bay of Bengal. The outcomes further revealed that the evaporated waters originating from the central and south Indian Ocean are responsible for the net precipitation over the coastal regions of the Ganges–Brahmaputra–Meghna Delta, northeast India, Myanmar, the foothills of the Himalayas, and central-east India. Evaporated waters from the Arabian Sea are mainly contributing to the rainfall over the western coast and west-central India. Summer monsoon precipitation due to the local recycling is primarily restricted to the Indo-Gangetic plain. No recycled precipitation was observed over the mountain chain along the west coast of India (Western Ghats). The month-to-month precipitation variation over South Asia was analyzed to be linked with the Somali low-level jet variability. The interannual variability of the South Asian summer monsoon precipitation was found to be mainly controlled by the atmospheric waters that were sourced and traveled from the central and south Indian Ocean. [ABSTRACT FROM AUTHOR]

Long-lived (≥6 days) heatwaves (HWs) have strong social impacts with serious health implications. Using homogenized historical daily temperatures from China and ECMWF reanalysis data, this study investigates its frequency between 1979 and 2018 and driving mechanisms. It is found that the occurrence of HWs is strongly associated with the joint actions of the South Asian high and the western North Pacific subtropical high, which can be described by a synergy index measured by the boundary distance between the two subtropical high pressure systems. When the synergy index is positive, there are more long-lived HWs in the east of the Tibetan Plateau, the lower reaches of the Yangtze River, and the southern region in China, and vice versa. A Mann–Kendall test shows a significant interdecadal shift around 2004/05 toward increased occurrence that is consistent with enhanced subtropical high systems. This study shows the important roles of large-scale dynamic systems in regional climate extremes and their future changes. [ABSTRACT FROM AUTHOR]

The Asian monsoon has large spatial and temporal variabilities in winds and precipitation. This study reveals that the Asian monsoon also exhibits pronounced regional differences in cloud regimes and cloud–rainfall relationship at a wide range of time scales from diurnal to seasonal to interannual. Over South (East) Asia, the convectively active regime of deep convection (CD) occurs frequently in June–September (March–September) with a late-afternoon peak (morning feature). The intermediate mixture (IM) regime over South Asia mainly occurs in summer and maximizes near noon. It develops as CD at late afternoon and dissipates as convective cirrus (CC) after midnight, showing a life cycle of thermal convection in response to solar radiation. Over East Asia, IM is dominant in cold seasons with a small diurnal cycle, indicating a prevalence of midlevel stratiform clouds. Further analyses show that CD and CC contribute 80%–90% of the rainfall amount and most of the intense rainfall in the two key regions. The CD-related rainfall also accounts for the pronounced diurnal cycles of summer rainfall with a late-afternoon peak (morning feature) over northern India (Southeast China). The afternoon CD-related rainfall mainly results from thermal convection under the moderate humidity but warm conditions particularly over northern India, while the morning CD-related rainfall over Southeast China is more related to the processes with high humidity. The CD/CC-related rainfall also exhibits large interannual variations that explain ~90% of the interannual variance of summer rainfall. The interannual variations of CD/CC occurrence are positively correlated with the moist southerlies and induced convergence, especially over Southeast China, suggesting a close relationship between cloud regimes and monsoon activities. [ABSTRACT FROM AUTHOR]

Understanding the Tibetan Plateau (TP) thermal processes is of utmost significance in changing climate. This study investigates the effect of soil moisture in changing the TP thermal profile and consequently summer precipitation in South Asia (SA). Soil moisture from Special Sensor Microwave Imager (SSM/I) developed from the F-08, F-11, and F-13 fundamental climate data record and atmospheric reanalysis from ERA-Interim, MERRA-2, and NCEP/CFSR during 1988–2008 are used. A generalized linear method that assesses the reciprocal forcing between two connected fields, named the coupled manifold technique (CMT), is applied to TP soil moisture and SA summer precipitation. It is revealed that interannual variations of SA precipitation are significantly (confidence level = 99%) impacted by TP soil moisture and the explained ratio of variance in SA is 0.3–0.4. Composite analysis indicates that SA summer precipitation has positive anomalies in response to dry TP soil moisture in the previous spring and vice versa. For understanding the possible mechanism, thermal processes, relative humidity, wind components, and moisture flux anomalies were calculated for dry and wet TP soil moisture and summer precipitation. The results suggested that TP soil moisture is likely to regulate near-surface energy balance and diabatic heating profile over TP. As a result, the surrounding lower-level westerlies (easterlies) (at 850 hPa) converge (diverge), associated with divergence (convergence) at the upper troposphere (200 hPa). The westerlies (easterlies) are usually moisture-rich (moisture-deficient) and thus cause more (less) precipitation in western (eastern) SA. It is thus suggested that the spring soil moisture may affect the thermal profile of TP, affecting the summer precipitation in SA as a consequence. [ABSTRACT FROM AUTHOR]

This study aims to construct a novel source-receptor (SR) network to study the atmospheric water cycle associated with the East Asian summer monsoon (EASM) circulation. Using a dynamical recycling model (DRM), 68%- 74% of the wet season (April-September) precipitation in six EASM land regions is attributed. The results reveal that terrestrial sources can be equally or more competitive than oceans for several sink regions downwind in East Asia. Terrestrial sources, such as the Indian subcontinent, Indochina, Southwest China, and the eastern Tibetan Plateau, are sustained by southwesterly monsoons and contribute to appreciable fractions of precipitation in the East Asian subregions downwind. Further, southwesterly and southeasterly sources for a sink region alternately dominate the moisture supply in the early and late wet season, respectively, referred to as the ''SW-SE source swing.'' The SR network is found to be largely governed by the zonal oscillation of the western North Pacific subtropical high and tropical cyclones. Knowledge about the coupled circulations might promise more predictability of the strength of the affected SR pairs. Notably, enhanced moisture supplies from regions such as the Indian subcontinent and Tibetan Plateau are well correlated with an upper-level wave train from western Russia. Finally, the preceding wintertime El Niño may favor (suppress) the moisture contribution of southwesterly (southeasterly) sources in the following wet season. The findings offer insights into theEASMwater cycle and the governing circulations, and also accentuate the role of upwind terrestrial sources in the downwind precipitation and freshwater resources. [ABSTRACT FROM AUTHOR]

An accurate prediction of land monsoon precipitation (LMP) is critical for the sustainable future of the planet as it provides water resources for more than two-thirds of the global population. Here, we show that the ensemble mean of 24 CMIP6 (phase 6 of the Coupled Model Intercomparison Project) models projects that, under the Shared Socioeconomic Pathway 2-4.5 (SSP2-4.5) scenario, summer LMP will very likely increase in South Asia (~4.1% 8C21), likely increase in East Asia (~4.6% 8C21) and northern Africa (~2.9% 8C21), and likely decrease in North America (~22.3% 8C21). The annual mean LMP in three Southern Hemisphere monsoon regions will likely remain unchanged due to significantly decreased winter precipitation. Regional mean LMP changes are dominated by the change in upward moisture transport with moderate contribution from evaporation and can be approximated by the changes of the product of the midtropospheric ascent and 850-hPa specific humidity. Greenhouse gas (GHG)-induced thermodynamic effects increase moisture content and stabilize the atmosphere, tending to offset each other. The spatially uniform increase of humidity cannot explain markedly different regional LMP changes. Intermodel spread analysis demonstrates that the GHGinduced circulation changes (dynamic effects) are primarily responsible for the regional differences. The GHGs induce a warm land-cool ocean pattern that strengthens the Asian monsoon, and a warm North Atlantic and Sahara that enhances the northern African monsoon, as well as an equatorial central Pacific warming that weakens the North American monsoon. CMIP6 models generally capture realistic monsoon rainfall climatology, but commonly overproduce summer rainfall variability. The models' biases in projected regional SST and land-sea thermal contrast likely contribute to the models' uncertainties in the projected monsoon rainfall changes. [ABSTRACT FROM AUTHOR]

This study examined the responses of Asian monsoon precipitation to global warming on the regional scale, focusing on monsoon westerlies and monsoon trough. This is because the Asian monsoon precipitation is closely associated with tropical disturbances. To reproduce convective precipitation and tropical disturbances, this study used outputs of a high-resolution climate simulation. Two sets of approximately 30-yr simulations under present-day (control) and warmer climate conditions (global warming) were conducted by the 14-km Nonhydrostatic Icosahedral Atmospheric Model (NICAM) with explicitly calculated convection. For understanding the spatial pattern of future precipitation changes, a further set of a 5-yr simulation [sea surface temperature (SST) + 4 K] was also conducted. Overall, the Asian summer monsoon was well simulated by the model. Precipitation increased as a result of global warming along the monsoon trough, which was zonally elongated across northern India, the Indochina Peninsula, and the western North Pacific Ocean. This increased precipitation was likely due to an increase in precipitable water. The spatial pattern of the increased precipitation was associated with enhanced cyclonic circulations over a large area along the monsoon trough, although it was difficult to determine whether the large-scale monsoon westerly was enhanced. This enhancement can be explained by future changes in tropical disturbance activity, including weak tropical cyclones. However, over part of South Asia, circulation changes may not contribute to the increased precipitation, suggesting regional characteristics. The regional increase in precipitation along the monsoon trough was mostly explained by the uniform increase in SST, whereas SST spatial patterns are important over some regions. [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]

In simulations of the boreal summer Asian monsoon, generations of climate models show a persistent climatological wet bias over the tropical western Indian Ocean and a dry bias over South Asia. Here, focusing on the monsoon developing stages (May–June), process-based diagnostics are first applied to a suite of NCAR models and reanalysis products. Two primary factors are identified for the initiation and maintenance of the wet bias over the northwestern Indian Ocean (NWIO; 5°–15°N, 52°–67°E): (i) excessive tropospheric moisture and (ii) restrained horizontal advection of the 1000–800-hPa levels cold–dry air couplet that originates offshore of Somalia. Second, guided by the diagnostics, we hypothesized that insufficient dilution of convective updrafts is one possible candidate for model bias and performed a series of enhanced entrainment sensitivity experiments with NCAR CAM4. Over the NWIO, the results suggest that globally increasing the maximum entrainment rate εmax leads to a drier free troposphere, arrests the vertical extension of clouds, and weakens moisture–convection and cloud–radiation feedbacks; each factor contributes to a reduced wet bias. Moreover, a higher εmax leads to a reduced dry bias over South Asia through changes in the local circulation features. In CAM4, improved precipitation climatology due to increased εmax suggests that insufficient dilution is one factor, but not the only one, that contributes to systematic errors. Rather, realistic representation of boundary layer processes in climate models arising out of local ocean–atmosphere interaction processes off Somalia's coast deserves attention in reducing the NWIO wet bias. [ABSTRACT FROM AUTHOR]

The extreme concentration of population over South Asia makes it critical to accurately understand the global warming impact on the South Asian monsoon (SAM), but the complex orography of the region makes future projections of monsoon intensity technically challenging. Here we describe a series of climate projections constructed using the Weather Research and Forecasting (WRF) Model for South Asia to dynamically downscale a global warming simulation constructed using the Community Earth System Model under the representative concentration pathway 8.5 (RCP8.5) scenario. A physics-based miniensemble is employed to investigate the sensitivity of the projected change of the SAM to the implementation of different parameterization schemes in WRF. We analyze not only the changes in mean seasonal precipitation but also the impact of the warming process on precipitation extremes. All projections are characterized by a consistent increase in average monsoon precipitation and a fattening of the tail of the daily rainfall distribution (more than a 50% decrease in the return periods of 50-yr extreme rainfall events by the end of the twenty-first century). Further analysis based on one of the WRF physics ensemble members shows that both the average rainfall intensity changes and the extreme precipitation increases are projected to be slightly larger than expectations based upon the Clausius–Clapeyron thermodynamic reference of 7% °C−1 of surface warming in most parts of India. This further increase can be primarily explained by the fact that the surface warming is projected to be smaller than the warming in the midtroposphere, where a significant portion of rain originates, and dynamical effects play only a secondary role. [ABSTRACT FROM AUTHOR]

Forecasting monsoon rainfall using dynamical climate models has met with little success, partly due to models' inability to represent the monsoon climatological state accurately. In this article the nature and dynamical causes of their biases are investigated. The approach is to analyze errors in multimodel-mean climatological fields determined from CMIP5, and to carry out sensitivity experiments using a coupled model [the Coupled Model for the Earth Simulator (CFES)] that does represent the monsoon realistically. Precipitation errors in the CMIP5 models persist throughout the annual cycle, with positive (negative) errors occurring over the near-equatorial western Indian Ocean (South Asia). Model errors indicate that an easterly wind stress bias Δτ along the equator begins during April-May and peaks during November; the severity of the Δτ is that the Wyrtki jets, eastward-flowing equatorial currents during the intermonsoon seasons (April-May and October-November), are almost eliminated. An erroneous east-west SST gradient (warm west and cold east) develops in June. The structure of the model errors indicates that they arise from Bjerknes feedback in the equatorial Indian Ocean (EIO). Vertically integrated moisture and moist static energy budgets confirm that warm SST bias in the western EIO anchors moist processes that cause the positive precipitation bias there. In CFES sensitivity experiments in which Δτ or warm SST bias over the western EIO is artificially introduced, errors in the EIO are similar to those in the CMIP5 models; moreover, precipitation over South Asia is reduced. An overall implication of these results is that South Asian rainfall errors in CMIP5 models are linked to errors of coupled processes in the western EIO, and in coupled models correct representation of EIO coupled processes (Bjerknes feedback) is a necessary condition for realistic monsoon simulation. [ABSTRACT FROM AUTHOR]

The comprehensive effects of anthropogenic aerosols (sulfate, black carbon, and organic carbon) on terrestrial aridity were simulated using an aerosol-climate coupled model system. The results showed that the increase in total anthropogenic aerosols in the atmosphere from 1850 to 2010 had caused global land annual mean precipitation to decrease by about 0.19 (0.18, 0.21) mm day−1, where the uncertainty range of the change (minimum, maximum) is given in parentheses following the mean change, and reference evapotranspiration ET0 (representing evapotranspiration ability) to decrease by about 0.33 (0.31, 0.35) mm day−1. The increase in anthropogenic aerosols in the atmosphere from 1850 to 2010 had caused land annual mean terrestrial aridity to decrease by about 3.0% (2.7%, 3.6%). The areal extent of global total arid and semiarid areas had reduced due to the increase in total anthropogenic aerosols in the atmosphere from preindustrial times. However, it was found that the increase in anthropogenic aerosols in the atmosphere had enhanced the terrestrial aridity and thus resulted in an expansion of arid and semiarid areas over East and South Asia. The projected decrease in anthropogenic aerosols in the atmosphere from 2010 to 2100 will increase global land annual mean precipitation by about 0.15 (0.13, 0.16) mm day−1 and ET0 by about 0.26 (0.25, 0.28) mm day−1, thereby producing a net increase in terrestrial aridity of about 2.8% (2.1%, 3.6%) and an expansion of global total arid and semiarid areas. [ABSTRACT FROM AUTHOR]

This study identifies coherent and robust large-scale atmospheric patterns of interannual variability of the South Asian summer monsoon (SASM) in observational data. A decomposition of the water vapor budget into dynamic and thermodynamic components shows that interannual variability of SASM net precipitation ( P − E) is primarily caused by variations in winds rather than in moisture. Linear regression analyses reveal that strong monsoons are distinguished from weak monsoons by a northward expansion of the cross-equatorial monsoonal circulation, with increased precipitation in the ascending branch. Interestingly, and in disagreement with the view of monsoons as large-scale sea-breeze circulations, strong monsoons are associated with a decreased meridional gradient in the near-surface atmospheric temperature in the SASM region. Teleconnections exist from the SASM region to the Southern Hemisphere, whose midlatitude poleward eddy energy flux correlates with monsoon strength. Possible implications of these teleconnection patterns for understanding SASM interannual variability are discussed. [ABSTRACT FROM AUTHOR]

This paper analyzes a suite of global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) archives to understand the mechanisms behind a net increase in the South Asian summer monsoon precipitation in response to enhanced radiative forcing during the twenty-first century. An increase in radiative forcing fuels an increase in the atmospheric moisture content through warmer temperatures, which overwhelms the weakening of monsoon circulation and results in an increase of moisture convergence and therefore summer monsoon precipitation over South Asia. Moisture source analysis suggests that both regional (local recycling, the Arabian Sea, the Bay of Bengal) and remote (including the south Indian Ocean) sources contribute to the moisture supply for precipitation over South Asia during the summer season that is facilitated by the monsoon dynamics. For regional moisture sources, the effect of excessive atmospheric moisture is offset by weaker monsoon circulation and uncertainty in the response of the evapotranspiration over land, so anomalies in their contribution to the total moisture supply are either mixed or muted. In contrast, weakening of the monsoon dynamics has less influence on the moisture supply from remote sources that not only is a dominant moisture contributor in the historical period but is also the net driver of the positive summer monsoon precipitation response in the twenty-first century. The results also indicate that historic measures of the monsoon dynamics may not be well suited to predict the nonstationary moisture-driven South Asian summer monsoon precipitation response in the twenty-first century. [ABSTRACT FROM AUTHOR]

A robust response of South Asian summer monsoon precipitation to increasing greenhouse gas concentration during the twenty-first century is identified in 23 models from phase 5 of the Coupled Model Intercomparison Project. The pattern of this response is dominated by two dipole structures, one oriented east-west across the Maritime Continent and another oriented north-south across the equatorial Indian Ocean, and is characterized by enhanced rainfall in South Asia and diminished rainfall over the Maritime Continent. The response is robust in the sense that the same pattern has a trend that is within one standard deviation of the trend of other models. Another robust feature is that the variability of precipitation about this trend decreases in all models, and hence becomes more detectable with time. The response is negligible compared to internal variability during the twentieth century but emerges clearly by the middle of the twenty-first century. Although the pattern as a whole is robust, the response over small land areas such as India has more uncertainty, with some models disagreeing even with the sign of the response. [ABSTRACT FROM AUTHOR]

The variation of the summer monsoon onset over South Asia was investigated by using long-term data of the onset over Kerala, India, during the 64-yr period from 1948 to 2011. It was found that the onset over Kerala shows variation on a multidecadal scale. In early-onset years, the sea surface temperature (SST) anomaly over the northern Pacific Ocean was very similar to the negative Pacific decadal oscillation (PDO). The stationary wave train related to the negative PDO reaches into central Asia and generates a warm anomaly, thereby intensifying the land-sea thermal contrast, which promotes summer monsoon onset over South and Southeast Asia. The correlation between the onset date over Kerala and the PDO has strengthened since 1976. Analysis of zonal wind in the upper-level troposphere for the period 1958-2002 indicates that the change in the correlation is related to the change in the wave train path. The wave train propagating from the northern Pacific Ocean to western Russia could propagate eastward more easily in 1976-2002 than in 1958-75. [ABSTRACT FROM AUTHOR]

The direct radiative effect of absorbing aerosols consists of absorption-induced atmospheric heating together with scattering- and absorption-induced surface cooling. It is thus important to understand whether some of the reported climate impacts of anthropogenic absorbing aerosols are mainly due to the coexistence of these two opposite effects and to what extent the nonlinearity raised from such coexistence would become a critical factor. To answer these questions specifically regarding the South Asia summer monsoon with focus on aerosol-induced changes in monsoon onset, a set of century-long simulations using the Community Earth System Model, version 1.0.3 (CESM 1.0.3), of NCAR with fully coupled atmosphere and ocean components was conducted. Prescribed direct heating to the atmosphere and cooling to the surface were applied in the simulations over the Indian subcontinent, either alone or combined, during the aerosol-laden months of May and June. Over many places in the Indian subcontinent, the nonlinear effect dominates in the changes of subcloud layer moist static energy, precipitation, and monsoon onset. The surface cooling effect of aerosols appears to shift anomalous precipitative cooling away from the aerosol-forcing region and hence turn the negative feedback to aerosol-induced atmospheric heating into a positive feedback on the monsoon circulation through latent heat release over the Himalayan foothills. Moisture processes form the critical chain mediating local aerosol direct effects and onset changes in the monsoon system. [ABSTRACT FROM AUTHOR]

The tropical Indian Ocean (TIO)'s influence on the South Asia high (SAH)'s intensity experiences a decadal change in the late 1970s; after (before) the decadal shift, the influence is significant (insignificant). The present study investigates the role of tropospheric temperature in relaying the impact of sea surface temperature (SST) to the SAH and the change in the TIO's influence. During the two epochs, the local tropospheric temperature responses to the TIO warming are distinct-more significant during the second epoch. It is inferred that this change may be responsible for the strengthening of the TIO's influence on the SAH. Encouragingly, the ensemble simulations accurately capture the time of the decadal change, indicating that the enhanced influence is attributed to the SST forcing. There are two possible reasons for the change in the TIO-SAH relationship. The first reason is the change in the locations of the SST anomalies in the TIO. During the second epoch, positive SST anomalies lie in the Indian Ocean warm pool. Through the background vigorous convection and moist adjustment, the SST anomalies affect largely the tropospheric temperature and thus the SAH. The second reason is the decadal change in mean SST and the SST variability. During the recent decades, both the background SST and the variability of the TIO SST increase, which enhance the influence of the SST anomalies on the atmosphere. The influence of the remote oceanic forcing on the enhanced TIO-SAH relationship and its comparison with the contribution of the TIO SST are also discussed. [ABSTRACT FROM AUTHOR]

In the present research to identify moist processes that initiate and maintain extended monsoon breaks over South Asia moisture and moist static energy (MSE) budgets are performed on the newly available European Centre for Medium-Range Weather Forecasts Interim reanalysis (ERA-Interim) and ensemble integrations from a coupled model. The hypothesis that interaction between moist physics and regional circulation and the role of cloud-radiation feedbacks are important is tested. Budget diagnostics show that dry advection is the principal moist process to initiate extended breaks. Its sources are (i) regional anticyclonic circulation anomalies forced by equatorial Indian Ocean negative rainfall anomalies advect low MSE air from north to central India, and (ii) rainfall enhancement over tropical west Pacific forces cyclonic circulation anomalies to its northwest as a Rossby wave response, and the northerlies at the poleward flank of this circulation advect air of low MSE content from north. The dominance of anomalous wind acting on climatological moisture gradient is confirmed from an examination of the moisture advection equation. A partition of various flux terms indicates that over central India, due to an increase in upwelling shortwave and longwave fluxes, radiative cooling increases during extended breaks. Here, enhanced rainfall over the equatorial Indian Ocean promotes anomalous radiative warming due to trapping of upwelling fluxes. The differential radiative heating anchors a local Hadley circulation with descent over central India. A direct implication of this research is that observational efforts are necessary to monitor the three-dimensional moisture distribution and cloud-radiation interaction over the monsoon region that would aid in better understanding, modeling, and predicting extended monsoon breaks. [ABSTRACT FROM AUTHOR]