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The summer monsoon onset over the Bay of Bengal and South China Sea signals the beginning of the Asian summer monsoon, critical for local fisheries, agriculture and livelihoods, so communities are concerned about its potential changes under global warming. Previous projections have suggested a delay, but the extent of this delay remains uncertain, undermining the reliability of the projections. Here, we show a significant correlation between the projected shift in Bay of Bengal/South China Sea monsoon onset and present‐day sea surface temperature (SST) simulation over the western Pacific (WP). This emergent relationship arises from the spread of the precipitation response over the western‐central Pacific to WP SST, as more precipitation induces stronger tropical upper‐tropospheric warming, increasing westerly vertical shear near South Asia, and facilitating the onset delay. The rectified projections indicate that the delayed shift is almost halved compared to raw projections, and the intermodel uncertainty is reduced by 30%. Plain Language Summary: The summer monsoon onset over the Bay of Bengal and South China Sea is a crucial time for fishing, agriculture and livelihood, and is important for the further advance of the monsoon rains over tropical Asia. Previous studies have suggested that monsoon onset in these seas will be delayed in the future, but there is large uncertainty between models, undermining the reliability of the projection. This study finds that the projected shift in the onset is strongly related to a model's representation of sea surface temperatures over the western Pacific Ocean in the present day. Based on the relationship found in observations, we adjust or "constrain" the future projections of the monsoon onset. The delay to the monsoon onset using this adjustment is almost half of the original projection, and the uncertainty between models is also reduced. We suggest that a more accurate simulation of sea‐surface temperatures in the present‐day climate of the western Pacific Ocean is crucial for reliable future projections of the monsoon onset. Key Points: An observational constraint is identified for projected changes in summer monsoon onset date over the Bay of Bengal and South China SeaModels with a warmer sea surface temperature bias over the tropical western Pacific tend to project a more delayed monsoon onset in the futureThe emergent constraint indicates that the delay of monsoon onset over the Bay of Bengal and South China Sea is overestimated [ABSTRACT FROM AUTHOR]

The earliest Asian summer monsoon onset (SMO) occurs in the Bay of Bengal (BoB), heralding the coming of the rainy season. In late April or early May, the strong sea surface temperature (SST) diurnal variation accompanied by ocean surface warming triggers the SMO. However, this observed diurnal cycle intensity cannot be reasonably simulated by state‐of‐the‐art climate models, resulting in a spurious delayed SMO. To address this issue, the SST diurnal cycle parameterized by a diagnostic sublayer scheme was incorporated into a climate model named FIO‐ESM v2.0. The large diurnal amplitude of SST contributes to surface warming and changes atmospheric circulation. Consequently, the high‐pressure anomaly at high levels and an inverted trough at low levels promote more convective activity, triggering an earlier SMO. Our findings improve the ability of climate models in simulating the evolution of the Asian monsoon system. Plain Language Summary: Asian summer monsoon onset (SMO) accompanied by the coming of the rainy season has a significant impact on the lives and livelihoods of more than half of the world's population. However, state‐of‐the‐art climate models struggle to capture the major features of SMO accurately. The simulated diurnal amplitude of sea surface temperature (SST) is much smaller compared with observation, resulting in a cold bias in SST and a delayed SMO in the Bay of Bengal (BoB). Here, we incorporated a diagnostic sublayer parameterization into the climate model to account for the effect of a strong diurnal cycle. The SST in the BoB increases, and more convective activities are promoted in response to the warmer underlying surface. Changes in monsoon circulation create favorable environmental conditions for an advanced SMO. Considering the effect of strong diurnal variation provides a solution to reduce common biases of the climate model in simulating SMO. Key Points: The spurious delayed Asian summer monsoon onset (SMO) is improved by including the effect of large sea surface temperature (SST) diurnal amplitude in a climate modelStrong diurnal variation of SST contributes to upper ocean warming in the Bay of Bengal before the Asian SMOLarge‐scale circulation with favorable conditions for convection activity in response to ocean warming triggers an advanced monsoon onset [ABSTRACT FROM AUTHOR]

This study investigates the connection between significant sea surface temperature (SST) anomalies in the North Pacific during boreal spring (February–April, FMA) and the subsequent South China Sea (SCS) summer monsoon (SCSSM) onset. The SST anomalies, similar to the Pacific meridional mode (PMM), referred to as the PMM+ mode, are defined to examine the new influencing factor on the SCSSM onset. Our findings reveal that the (February–March–April, FMA) PMM+ has a noteworthy positive correlation with the subsequent May SCSSM onset date, with this correlation being minimally affected by the El Niño–Southern Oscillation (ENSO) during preceding winter. A robust positive PMM+ in boreal spring can be persist until May via atmosphere–ocean interaction. The cooling area over Western North Pacific would reduce precipitation heating, thereby generating Rossby waves that reinforce the formation of the anomalous anticyclone over the SCS. As a result, easterly winds and suppressed convection prevail over the SCS, making the SCSSM break out later than normal. Furthermore, the amplification of anticyclonic vorticity anomalies also strengthens the western North Pacific subtropical high (WNPSH) stronger and shifts its position further westward compared to normal years, thereby blocking active convection to the west of the SCS. Given the weakened relationship between El Niño–Southern Oscillation (ENSO) and the SCSSM onset in recent years, the PMM+ could be considered as a promising preceding signal for the SCSSM onset, thus holding significant implications for the SCSSM prediction efforts. [ABSTRACT FROM AUTHOR]

The South American Monsoon System, which transports moisture from Amazonia to Central-West Brazil, is an important moisture source for the summer rainy season in this region. While local evapotranspiration also contributes to the atmospheric moisture supply, the balance between local and remote sources during the onset of the rainy season remains uncertain. Our research aimed to quantify the role of local evapotranspiration in initiating the rainy season in Central-West Brazil. By utilizing data from various sources, such as remote sensing (MODIS), modern reanalysis (ECMWF's ERA5), and composite products of rainfall (CHIRPS), and analyzing them in a comparative way, we conclusively found that local evapotranspiration is the only relevant source of moisture to the atmosphere during the dry-to-wet season transition, preceding the establishment of the monsoon system. [ABSTRACT FROM AUTHOR]

Abstract The summer monsoon onset over the Bay of Bengal and South China Sea signals the beginning of the Asian summer monsoon, critical for local fisheries, agriculture and livelihoods, so communities are concerned about its potential changes under global warming. Previous projections have suggested a delay, but the extent of this delay remains uncertain, undermining the reliability of the projections. Here, we show a significant correlation between the projected shift in Bay of Bengal/South China Sea monsoon onset and present‐day sea surface temperature (SST) simulation over the western Pacific (WP). This emergent relationship arises from the spread of the precipitation response over the western‐central Pacific to WP SST, as more precipitation induces stronger tropical upper‐tropospheric warming, increasing westerly vertical shear near South Asia, and facilitating the onset delay. The rectified projections indicate that the delayed shift is almost halved compared to raw projections, and the intermodel uncertainty is reduced by 30%.

Abstract The earliest Asian summer monsoon onset (SMO) occurs in the Bay of Bengal (BoB), heralding the coming of the rainy season. In late April or early May, the strong sea surface temperature (SST) diurnal variation accompanied by ocean surface warming triggers the SMO. However, this observed diurnal cycle intensity cannot be reasonably simulated by state‐of‐the‐art climate models, resulting in a spurious delayed SMO. To address this issue, the SST diurnal cycle parameterized by a diagnostic sublayer scheme was incorporated into a climate model named FIO‐ESM v2.0. The large diurnal amplitude of SST contributes to surface warming and changes atmospheric circulation. Consequently, the high‐pressure anomaly at high levels and an inverted trough at low levels promote more convective activity, triggering an earlier SMO. Our findings improve the ability of climate models in simulating the evolution of the Asian monsoon system.

Abstract Victoria mode (VM), the second dominant mode of North Pacific sea surface temperature variability, has been identified as one of the important factors influencing the Indian summer monsoon (ISM) onset. The positive phase of the May VM delays the ISM onset by both tropical and extratropical pathways. Here, we found a significant interdecadal enhancement of their relationship since the early 1990s, which is mainly attributed to the structure changes and increased variance of the VM. In recent decades, the VM has shown more significant warm SST anomalies in the tropical central Pacific, which drive the large-scale divergent circulation more effectively. This enhanced divergent circulation leads to low-level divergence and reduced rainfall in the tropical Asian summer monsoon region. The reduced rainfall excites equatorial Rossby wave response and anomalous easterly winds in the northern Indian Ocean, delaying the ISM onset. Besides, the increased variance of the VM after 1992/1993 stimulates a stronger extratropical Rossby wave train. This stationary Rossby wave train induces a stronger cooling to the northwest of India, which weakens the land-sea thermal contrast and leads to the delayed ISM onset. This finding should be taken into account to improve short-term predictions of the monsoon onset.

The near-inertial motion in ocean surface currents directly reflects the energy transported by wind towards the surface layer, playing an important role in climate regulation and energy balance. Previous studies have mainly focused on near inertial oscillations (NIOs) induced by tropical cyclones in the Taiwan Strait, with few reports on near inertial oscillations induced by monsoon onset. Using high-frequency radar observations, we detected an amplification of NIOs induced by the winter monsoon onset. While not as strong as NIOs induced by tropical cyclones, the near-inertial current (NIC) induced by winter monsoon onset in the Taiwan Strait has peak speeds reaching up to 5.2 cm/s and explaining up to 0.7% of non-tidal variance. This study presents observational results of NIOs during three monsoon onset events, and analyzes the impact of winds and temperature changes on NIOs. Temporal and spectral analysis reveals that the monsoon onset is the primary driver behind the formation of NIOs. Results indicate that near-inertial kinetic energy is relatively lower in shallower waters, such as the Taiwan Bank, compared to deeper regions. Furthermore, by integrating the air and sea surface temperature from reanalysis products, we have examined the abrupt changes in sea surface temperature (SST) before and after monsoon onset and their correlation with NIOs. The findings suggest that temperature falling favors the intensification of NICs during monsoon onset, and a lack of significant SST changes precludes the triggering of notable NICs. These insights enhance our understanding of the mechanisms driving NIOs and their roles in seawater mixing.

Victoria mode (VM), the second dominant mode of North Pacific sea surface temperature variability, has been identified as one of the important factors influencing the Indian summer monsoon (ISM) onset. The positive phase of the May VM delays the ISM onset by both tropical and extratropical pathways. Here, we found a significant interdecadal enhancement of their relationship since the early 1990s, which is mainly attributed to the structure changes and increased variance of the VM. In recent decades, the VM has shown more significant warm SST anomalies in the tropical central Pacific, which drive the large-scale divergent circulation more effectively. This enhanced divergent circulation leads to low-level divergence and reduced rainfall in the tropical Asian summer monsoon region. The reduced rainfall excites equatorial Rossby wave response and anomalous easterly winds in the northern Indian Ocean, delaying the ISM onset. Besides, the increased variance of the VM after 1992/1993 stimulates a stronger extratropical Rossby wave train. This stationary Rossby wave train induces a stronger cooling to the northwest of India, which weakens the land-sea thermal contrast and leads to the delayed ISM onset. This finding should be taken into account to improve short-term predictions of the monsoon onset. [ABSTRACT FROM AUTHOR]

Understanding the response of the Indian summer monsoon (ISM) onset to CO2 forcing is of utmost importance for rain‐fed agriculture and water management. In this study, we utilized an idealized symmetric CO2 removal scenario from the sixth phase of the Coupled Model Intercomparison Project to analyze the reversibility of monsoon onset. The results show that ISM onset is reversible but exhibits strong asymmetry: it undergoes minimal changes during the ramp‐up phase, but experiences rapid postponement as the CO2 begins to decline; Eventually, it is delayed more than 1 week when the CO2 concentration is restored to the initial level. To investigate the possible underlying mechanisms, we decomposed the climate response to CO2 forcing into the fast and slow processes. Notably, it is the enhanced slow response, which is driven by long‐term sea surface temperature (SST) changes, that dominates the asymmetric response of ISM onset. This slow response delays the ISM onset by strengthening near‐surface poleward land‐sea moist static energy contrast, thereby weakening the lower‐tropospheric monsoonal circulation. Based on the atmospheric component model simulations, we found that both the uniform SST change and patterned SST changes in the slow response contribute to the delay of ISM onset, but the latter plays a dominant role. Our results emphasize the importance of thoroughly assessing regional hydrological cycle features when designing the CO2 removal pathways. Plain Language Summary: The 2015 Paris Agreement set a target to limit global warming to 2°C by the end of the 21st century, with a preference for achieving 1.5°C. Meeting this temperature goal requires the atmospheric CO2 concentration to peak in this century and then start declining. It is crucial to understand whether the changes in the regional hydrological cycle can be reversed when we reach the global mean temperature goal. This study focuses on examining the responses of the Indian summer monsoon (ISM) onset to idealized CO2 removal (CDR) forcing, in which the atmospheric CO2 concentration decreases symmetrically after increasing. Under the symmetric CDR pathway, the ISM onset displays reversible yet asymmetric evolution, it significantly delays when the CO2 concentration returns to the pre‐industrial level. This asymmetric response is attributed to the slow process of the climate system driven by long‐term sea surface temperature change, which leads to the delay in ISM onset by weakening the lower‐tropospheric circulation and, consequently, the precipitation. The slow response lags behind the evolution of CO2 concentration and plays a dominant role in the global mean surface temperature change during the ramp‐down phase. Understanding these mechanisms is essential for addressing climate change challenges and the impacts on regional monsoonal systems. Key Points: The idealized symmetric CO2 removal forcing leads to a reversible but asymmetric Indian summer monsoon (ISM) onsetThe delayed onset of ISM during the ramp‐down period is primarily dominated by the asymmetric and lagged slow process, which is associated with the long‐term sea surface temperature changeThe slow process delays the ISM onset by enhancing the near‐surface poleward land‐sea moist static energy contrast, which subsequently weakens the monsoonal circulation [ABSTRACT FROM AUTHOR]

The South American Monsoon System, which transports moisture from Amazonia to Central-West Brazil, is an important moisture source for the summer rainy season in this region. While local evapotranspiration also contributes to the atmospheric moisture supply, the balance between local and remote sources during the onset of the rainy season remains uncertain. Our research aimed to quantify the role of local evapotranspiration in initiating the rainy season in Central-West Brazil. By utilizing data from various sources, such as remote sensing (MODIS), modern reanalysis (ECMWF’s ERA5), and composite products of rainfall (CHIRPS), and analyzing them in a comparative way, we conclusively found that local evapotranspiration is the only relevant source of moisture to the atmosphere during the dry-to-wet season transition, preceding the establishment of the monsoon system.

Abstract Understanding the response of the Indian summer monsoon (ISM) onset to CO2 forcing is of utmost importance for rain‐fed agriculture and water management. In this study, we utilized an idealized symmetric CO2 removal scenario from the sixth phase of the Coupled Model Intercomparison Project to analyze the reversibility of monsoon onset. The results show that ISM onset is reversible but exhibits strong asymmetry: it undergoes minimal changes during the ramp‐up phase, but experiences rapid postponement as the CO2 begins to decline; Eventually, it is delayed more than 1 week when the CO2 concentration is restored to the initial level. To investigate the possible underlying mechanisms, we decomposed the climate response to CO2 forcing into the fast and slow processes. Notably, it is the enhanced slow response, which is driven by long‐term sea surface temperature (SST) changes, that dominates the asymmetric response of ISM onset. This slow response delays the ISM onset by strengthening near‐surface poleward land‐sea moist static energy contrast, thereby weakening the lower‐tropospheric monsoonal circulation. Based on the atmospheric component model simulations, we found that both the uniform SST change and patterned SST changes in the slow response contribute to the delay of ISM onset, but the latter plays a dominant role. Our results emphasize the importance of thoroughly assessing regional hydrological cycle features when designing the CO2 removal pathways.

Monsoon onset in India has always been a topic of interest for the research fraternity and various stakeholders. This study aimed to determine the monsoon onset date at the grid point scale, to obtain the trend of monsoon onset, and to unravel the spatial distribution of monsoon onset during the period 1901–2019 (especially in different climate modes). Based on observed cumulative rainfall, the piecewise linear regression model (PLRM), which employs least-squares principles, finds changepoints that signify the beginning of the monsoon season with the onset of monsoon. In this study, monsoon onset is examined with respect to several climate modes to evaluate their impact on monsoon onset. Monsoon onset is delayed in El Niño and drought years due to strong negative anomalies that are revealed by a spatial examination of monsoon onset. However, because of local atmospheric circulation impacts, there are outliers. The study also reveals areas with notable monotonic tendencies in monsoon onset, suggesting future changes in onset dates. These areas need more sophisticated frameworks for developing mitigation strategies since they should be viewed as susceptible. The comparison of the PLRM outcomes with objective methods reveals a strong correlation, confirming the accuracy of the PLRM method. Overall, the PLRM has been shown to be a useful tool for predicting the start of the monsoon on fine spatial scales and may be used in conjunction with regional climate models to anticipate the start of the monsoon in various regions of India. The results of this study could have a significant impact on regional planning and policy initiatives for sustainable development. [ABSTRACT FROM AUTHOR]

Studies of the multi-scale climate variability of the Asian monsoon are essential to an advanced understanding of the physical processes of the global climate system. In this paper, the progress achieved in this field is systematically reviewed, with a focus on the past several years. The achievements are summarized into the following topics: (1) the onset of the South China Sea summer monsoon; (2) the East Asian summer monsoon; (3) the East Asian winter monsoon; and (4) the Indian summer monsoon. Specifically, new results are highlighted, including the advanced or delayed local monsoon onset tending to be synchronized over the Arabian Sea, Bay of Bengal, Indochina Peninsula, and South China Sea; the basic features of the record-breaking mei-yu in 2020, which have been extensively investigated with an emphasis on the role of multi-scale processes; the recovery of the East Asian winter monsoon intensity after the early 2000s in the presence of continuing greenhouse gas emissions, which is believed to have been dominated by internal climate variability (mostly the Arctic Oscillation); and the accelerated warming over South Asia, which exceeded the tropical Indian Ocean warming, is considered to be the main driver of the Indian summer monsoon rainfall recovery since 1999. A brief summary is provided in the final section along with some further discussion on future research directions regarding our understanding of the Asian monsoon variability. [ABSTRACT FROM AUTHOR]

The onset date of the South China Sea (SCS) summer monsoon (SCSSM) and the generation time of the first tropical cyclone (TC) over the Bay of Bengal (BoB) during late April and early June were significantly correlated with a correlation coefficient of 0.58 during 1979–2020. The composite analysis found that under the impact of BoB TCs, an enhanced southwesterly low‐level flow transported abundant moisture from the BoB to the northern SCS. Besides, the diabatic heating related to TC convection stimulated an anticyclonic anomaly in the upper troposphere over the southern Tibetan Plateau, which was conducive to the enhancement and expansion of the South Asian high (SAH) over the Indo‐China Peninsula. The stronger easterly outflows from the eastern periphery of the SAH overlapped with low‐level water vapor convergence over the northern SCS, enhancing the development of monsoon convection. Thus, more condensation heating warmed the tropospheric atmosphere and reversed the meridional temperature gradient over the SCS, implying the SCSSM onset. [ABSTRACT FROM AUTHOR]

Abstract The onset date of the South China Sea (SCS) summer monsoon (SCSSM) and the generation time of the first tropical cyclone (TC) over the Bay of Bengal (BoB) during late April and early June were significantly correlated with a correlation coefficient of 0.58 during 1979–2020. The composite analysis found that under the impact of BoB TCs, an enhanced southwesterly low‐level flow transported abundant moisture from the BoB to the northern SCS. Besides, the diabatic heating related to TC convection stimulated an anticyclonic anomaly in the upper troposphere over the southern Tibetan Plateau, which was conducive to the enhancement and expansion of the South Asian high (SAH) over the Indo‐China Peninsula. The stronger easterly outflows from the eastern periphery of the SAH overlapped with low‐level water vapor convergence over the northern SCS, enhancing the development of monsoon convection. Thus, more condensation heating warmed the tropospheric atmosphere and reversed the meridional temperature gradient over the SCS, implying the SCSSM onset.

The South China Sea Summer Monsoon (SCSSM) onset is characterized by an apparent seasonal conversion of circulation and convection. Accordingly, various indices have been introduced to identify the SCSSM onset date. However, the onset dates as determined by various indices can be very inconsistent. It not only limits the determination of onset dates but also misleads the assessment of prediction skills. In 2021, the onset time as identified by the circulation criteria was 20 May, which is 12 days earlier than that deduced by also considering the convection criteria. The present study mainly ascribes such circulation-convection inconsistency to the activities of tropical cyclones (TCs) modulated by the Madden-Julian Oscillation (MJO). The convection of TC "Yaas" (2021) acted as an upper-level diabatic heat source to the north of the SCS, facilitating the circulation transition. Afterward, TC "Cho-wan" (2021) over the western Pacific aided the westerlies to persist at lower levels while simultaneously suppressing moist convection over the SCS. Accurate predictions using the ECMWF S2S forecast system were obtained only after the MJO formation. The skillful prediction of the MJO during late spring may provide an opportunity to accurately predict the establishment of the SCSSM several weeks in advance. [ABSTRACT FROM AUTHOR]

Most of the empirical literature assessing the impacts of climate change on agriculture has modeled crop yields as a function of the levels or deviations in the growing‐period rainfall. However, an aspect that has received little attention in the empirical literature relates to the relationship between the timing of monsoon rains and crop yields. Using a pan‐India district‐level panel dataset for 50 years, this article investigates two interrelated issues critical to understanding the impacts of weather‐induced agricultural risks and their management. It first examines the impact of the timing of monsoon onset on crop yields and then assesses the role of irrigation in mitigating its effects. The article finds that the delayed onset of monsoon is detrimental to crops, and its effects are realized beyond the rainy season. The findings also demonstrate that irrigation helps mitigate the harmful effects of delayed monsoon. Finally, to link these findings to farm‐level adjustments, the article shows that farmers explicitly adjust the timing of irrigation in response to delays in monsoon rains. [ABSTRACT FROM AUTHOR]

The northeast/winter monsoon over Malaysia often leads to extreme rainfall events and floods over the windward side of terrain due to the strong northeasterly winds. Comparatively, much less rain falls over the other are sheltered by the mountain ranges. Based on this consideration, the onset of monsoon in the region is best determined operationally using 925 hPa winds. Therefore, we propose the first, simple, single‐variable‐based method to determine the onset dates of winter monsoon is yet reliable and can be closely monitored for operational purposes. The onset date is defined in the third pentad when the average of three pentads of 925 hPa northerly wind speed is greater than 1 m·s−1, and at least one of three pentads must be greater than 2.5 m·s−1. The study also investigates the relationship between the interannual variability of onset date and El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). Late‐onset can be expected when La Niña develops in the boreal fall season. However, a similar association between monsoon onset and El Niño condition was not observed. The lead–lag correlation between the IOD and the onset dates shows that the onset is significantly negatively correlated with the IOD. During negative (positive) IOD events, a delay (early) onset is expected. Results also show that the relationship between the onset and IOD is more robust and linear than the ENSO. [ABSTRACT FROM AUTHOR]

Using the high spatiotemporal resolution (2 km-and-10 min) data from the Advanced Himawari Imager onboard the Himawari-8 satellite, this study documents the fine-scale characteristics of daytime cloud regimes (CRs) over coastal South China during the pre-summer rainy season (April–June). Six CRs (CR1–CR6) are identified based on the joint frequency distribution of cloud top brightness temperature and cloud optical thickness, namely, the optically thin-to-moderate cloud mixture, optically thin warm clouds with cirrus, optically thick warm clouds, weak convective cloud mixture, strong convective clouds, and extreme, deep convective clouds. The optically thick warm clouds are the major CR during April and May, with higher frequencies over land, especially along the urban agglomeration, rather than the offshore which may be an indicator of the higher aerosol concentrations being a contributing factor over the cities. The CRs with weak convective cloud mixtures and strong convective clouds appear more frequently over the land, while the two CRs with optically thinner clouds occur mainly offshore. Synoptic flow patterns (SPs) are objectively identified and examined focusing on those favoring the two major rain-producing CRs (CR5 and CR6) and the highly reflective CR with optically thick warm clouds (CR3). The two SPs favoring CR5 and CR6 are characterized by abundant moisture with low-level jets after monsoon onset, and a northwest high-southeast low pattern with strong dynamic convergence along the coastline, respectively. The non-convective CR3 with high reflectance is related to a SP that features the western North Pacific subtropical high extending more westward, leading to a moderate moisture supply and a wide range of convective available potential energy, but also, large convective inhibition. [ABSTRACT FROM AUTHOR]

Climatological intraseasonal oscillation (CISO), as phase-locking of transient intraseasonal oscillation (ISO) to the annual cycle, is an important seasonal evolution of the Asian summer monsoon (ASM). We identified six CISO centers related to the ASM onset: the wet CISO begins from the central-to-eastern equatorial Indian Ocean (CIO) around pentad 25, which propagates eastward to the western Pacific (WP) around pentad 26, northward to the Bay of Bengal (BoB) around pentad 28, northeastward to the South China Sea (SCS) around pentad 29, and then northward to East Asia (EA) around pentad 34, while the wet CISO over the Indian summer monsoon (ISM) region peaking around pentad 32 originates from the northwestern Indian Ocean. The CISO over the CIO at pentad 25 is related to the phase-locking of Madden-Julian Oscillation (MJO), caused by the seasonal jump of the Intertropical Convergence Zone (ITCZ) from the Southern Hemisphere to the Northern Hemisphere over the Indian Ocean, signaling the beginning of the ASM CISO. The multi-model ensemble mean (MME) of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models can well reproduce these strong CISOs over the ISM, BoB, SCS, and EA regions, although the simulated intensity is weaker and the peak wet phase is 2–3 pentads later than that in the observation. The very weak CISOs over the CIO and WP in the simulations challenge our subseasonal prediction of the ASM since its intraseasonal variability is usually forced by the tropical ISO. These CISOs would be significantly enhanced over the BoB and EA under the future global warming in a high emission scenario of the CMIP6 models, while the CISOs over the CIO and WP would be still absent. [ABSTRACT FROM AUTHOR]

Monsoon onset in India has always been a topic of interest for the research fraternity and various stakeholders. This study aimed to determine the monsoon onset date at the grid point scale, to obtain the trend of monsoon onset, and to unravel the spatial distribution of monsoon onset during the period 1901–2019 (especially in different climate modes). Based on observed cumulative rainfall, the piecewise linear regression model (PLRM), which employs least-squares principles, finds changepoints that signify the beginning of the monsoon season with the onset of monsoon. In this study, monsoon onset is examined with respect to several climate modes to evaluate their impact on monsoon onset. Monsoon onset is delayed in El Niño and drought years due to strong negative anomalies that are revealed by a spatial examination of monsoon onset. However, because of local atmospheric circulation impacts, there are outliers. The study also reveals areas with notable monotonic tendencies in monsoon onset, suggesting future changes in onset dates. These areas need more sophisticated frameworks for developing mitigation strategies since they should be viewed as susceptible. The comparison of the PLRM outcomes with objective methods reveals a strong correlation, confirming the accuracy of the PLRM method. Overall, the PLRM has been shown to be a useful tool for predicting the start of the monsoon on fine spatial scales and may be used in conjunction with regional climate models to anticipate the start of the monsoon in various regions of India. The results of this study could have a significant impact on regional planning and policy initiatives for sustainable development.

In this paper, we report on the frequency of false onsets of wet season rainfall in the case of the Northern Australian wet season and investigate the role of large‐scale tropical climate processes such as the El Niño–Southern Oscillation, Indian Ocean Dipole (IOD) and Madden–Julian Oscillation. A false onset occurs when a wet season rainfall onset criterion is met, but follow‐up rainfall is not received for weeks or months later. Our analysis of wet season rainfall data from 1950 through 2020 shows a false onset occurs, on average, between 20 and 30% of wet seasons across all of northern Australia. This increases at a regional and local level such as at Darwin, the Northern Territory (NT), and parts of Queensland's north coast to over 50%. Seasonal climate influences, such as a La Niña pattern and a negative IOD that typically expedite the wet season rainfall onset, also increase the likelihood of a false onset over northern Australia. Our analysis also finds that periods of false onsets can sometimes, but not always, coincide with periods of rapid soil moisture depletion. The false rainfall onsets that develop into flash drought can be potentially disruptive and costly and are of potential significance for agriculture and fire management in northern Australia, and in other monsoonal climates that also typically experience a slow build‐up to the seasonal monsoon. In conclusion, effective rainfall indicates that many seasons experience 'false onsets' with dry conditions after early rainfall. We propose that false onsets are a physical characteristic of the climate of northern Australia which occurs with relatively high frequency. In addition, these false onsets may sometimes co‐occur with a flash drought. [ABSTRACT FROM AUTHOR]

Regional variations of monsoon onset dates across India were analyzed for 67 years (1951–2017) under different modes of climate variations, i.e., El Niño, La Niña, and the Indian Ocean Dipole (IOD), along with flood and drought years using the objective method and statistical techniques. Monsoon onset analysis revealed that the northern, northeastern, and southern parts were highly susceptible to the early onset of La Niña, and the northern and northern northwest parts were highly susceptible to the early onset of El Niño. The onset dates were early (late) in the sub-regions of the central, southern, and northeastern (northern, northwestern, and western) parts of India during flood (drought) years. Further, onset dates in flood years occurred earlier than those in La Niña years, and onset dates in drought years were later than those in El Niño years. The onset occurrence probability and influence of the synoptic events are discussed. This research could help in understanding the onset of monsoon and its predictability for societal applications. [ABSTRACT FROM AUTHOR]

Accurately predicting the Indian monsoon is limited by inadequate understanding of the underlying processes, which feed into systematic model biases. Here we aim to understand the dynamic and thermodynamic features associated with the progression of the monsoon, using 2016 as a representative year, with the help of convection‐permitting simulations of the Met Office Unified Model. Simulations are carried out in a 4 km resolution limited‐area model, nested within a coarser global model. Two major processes thought to influence the northwestward progression of the monsoon are: (a) the interaction between the low‐level monsoon flow and a mid‐tropospheric dry‐air intrusion from the northwest, and (b) land–atmosphere interactions. We find that the 4 km limited‐area model simulates the mid‐tropospheric moistening that erodes the northwesterly dry intrusion, pushing the northern limit of moist convection northwestwards. The surface soil moisture also plays a major role at the leading edge of the monsoon progression. The heavy rains associated with the local onset wet the soil, reducing the sensitivity of surface fluxes to soil moisture and weakening the land influence on further progression of monsoon rains. The 4 km model is tested with an alternative land‐surface configuration to explore its sensitivity to land‐surface processes. We find that the choice of soil and vegetation ancillaries affects the time‐scales of soil moisture–precipitation feedback and the timing of diurnal convection, thereby affecting the local onset. We further compare these simulations with a parametrized convection run at 17 km resolution to isolate the effects of convective parametrization and resolution. The model with explicit convection better simulates the dynamic and thermodynamic features associated with the progression of the monsoon. [ABSTRACT FROM AUTHOR]

Traditionally, a delayed (early) onset of the South China Sea summer monsoon (SCSSM) has been observed to follow a warm (cold) El Niño-Southern Oscillation (ENSO) event in winter, supporting high seasonal predictability of SCSSM onset. However, the empirical seasonal forecasting skill of the SCSSM onset, solely based on ENSO, has deteriorated since 2010. Meanwhile, unexpected delayed onsets of the SCSSM have also occurred in the past decade. We attribute these changes to the Northwest Indian Ocean (NWIO) warming of the sea surface. The NWIO warming has teleconnections related to (1) suppressing the seasonal convection over the South China Sea, which weakens the impacts of ENSO on SCSSM onset and delays the start of SCSSM, and (2) favoring more high-frequency, propagating moist convective activities, which enhances the uncertainty of the seasonal prediction of SCSSM onset date. Our results yield insight into the predictability of the SCSSM onset under the context of uneven ocean warming operating within the larger-scale background state of global climate change. [ABSTRACT FROM AUTHOR]

This study investigates the influence of early spring (March–April) soil moisture (SM) over the Greater Mekong Subregion (GMS) on the interannual variation of South China Sea summer monsoon (SCSSM) onset, using observational analyses and numerical experiments. It is found that when early spring SM over the GMS is wetter, westerly anomalies dominate the South China Sea, corresponding to an early onset of the SCSSM, and vice versa. The analyses of physical mechanism show that the positive anomalies of early spring SM decrease local surface air temperature by adjusting the surface latent heat flux and sensible heat flux. The persistence of anomalous ground cooling contributes to negative geopotential height anomalies in the middle troposphere, and further induces the eastward retreat of western North Pacific subtropical high in May. The anomalous cyclone over the South China Sea favors the early onset of SCSSM. The key physical processes linking the variability of early spring SM over the GMS and the following SCSSM onset are confirmed by the sensitivity experiments using a coupled atmosphere–land model (CAM6–CLM5). Specifically, the onset date of the SCSSM in the drier experiment is 11 days later than that in the wetter experiment. The present results have implications for the forecast of SCSSM onset. • When preceding early spring soil moisture (SM) is wetter (drier) over the GMS, the SCSSM onset tends to advance (delay). • The SM changes the local thermal conditions and affects subsequent SCSSM onset by modifying the WNP subtropical high. • Simulation using a coupled atmosphere–land model (CAM6–CLM5) confirms the above observational results. [ABSTRACT FROM AUTHOR]

Although it is well established that transpiration contributes much of the water for rainfall over Amazonia, it remains unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall. Here, we use multiple independent satellite datasets to show that rainforest transpiration enables an increase of shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. This shallow convection moisture pump (SCMP) preconditions the atmosphere at the regional scale for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and wet season onset 2-3 mo before the arrival of the Intertropical Convergence Zone (ITCZ). Aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. Our results highlight the mechanisms by which interactions among land surface processes, atmospheric convection, and biomass burning may alter the timing of wet season onset and provide a mechanistic framework for understanding how deforestation extends the dry season and enhances regional vulnerability to drought.

India observes the summer monsoon in June–July–August–September (JJAS) season, and the livelihood security of a huge population depends on it. The impact of the monsoon onset timing, length of monsoon season, rainfall amount, and related extreme events is huge on the Indian economy. Therefore, understanding the inherent intricacies needed a detailed investigation. In five homogenous monsoon regions of India, the trend of monsoon onset and the length of monsoon season are examined. The association between 1) monsoon onset ∼ rainfall amount, 2) length of monsoon season ∼ rainfall amount, and 3) monsoon onset ∼ length of monsoon season is investigated. Subsequently, the behavior of rainfall and extreme excess days in the ±1 standard deviation (SD) length of monsoon season is also examined in detail. The trend for monsoon onset shows late onset in all the homogenous monsoon regions except the northeast region. The length of monsoon season is found increasing significantly with high magnitude in west central and northwest regions. A significantly strong negative correlation (∼−0.6) for monsoon onset timing ∼ length of monsoon season is observed. Therefore, the change in rainfall anomaly, extreme excess days, and rainy days is done concerning the length of the monsoon season. In the cases of the −1 SD (+1 SD) length of monsoon season, rainfall anomaly and extreme excess days are low (high) in most parts of the homogenous monsoon regions. Extreme excess days showed a significant association with rainy days, which indicates a high possibility of rainy days converting into extreme excess days. However, the increase in extreme excess days in the +1 SD length of monsoon season is limited to a great extent in JJAS and June only. Morlet wavelet power spectrum shows the delay (advance) of power in −1SD (+1 SD) length of monsoon season.

The transition phase of the Indian summer monsoon has always remained crucial for the agricultural, societal, and economic conditions of India. The present study addresses the role of South Asian High (SAH) and subtropical westerly jet (STJ) on the transition period (onset and withdrawal) of the Indian summer monsoon. For this purpose, we have utilized the outputs of Regional Climate Model (RegCM v4.6) and reanalysis ERA5 pressure level data for 24 years (1982–2005) of study. We begin our analysis with the evaluation of Tibetan Plateau (TP) heating, and its relationship with Sub-Tropical Jet (STJ). We further peruse its connection with different atmospheric factors during the seasonal transition of the monsoon. We have also tried to decipher the link between SAH and inter-annual variability of monsoon. Our analysis shows the efficiency of the model in simulating inter-annual variability of onset and withdrawal features during the monsoon period. The days of onset and withdrawal period simulated by the model have a similar mid-latitude connection when compared with the reanalysis data. The vertical structure of the Hadley cell and horizontal position of SAH have been produced realistically during the transition of the monsoon. We found that the change in meridional position of STJ has a significant impact on the arrival and departure dates of the monsoon. This repositioning of STJ in the meridional direction is strongly correlated to the upper-level high developed over the eastern periphery of the western pacific, which is an important component of the monsoon flow over Bay of Bengal. Thus, the zonal position of SAH is observed to have direct implications on the onset and withdrawal dates of India. Its consideration in future studies can potentially enhance the predictability of the monsoon. [ABSTRACT FROM AUTHOR]

The Indian monsoon is a seasonal large-scale circulation system with complex dynamical and thermodynamic interactions. The physical processes are not fully understood. In particular, the mechanisms that control the propagation of the monsoon onset across the Indian continent, against the mid-level wind field, are debated. The Indian monsoon is poorly represented in weather and climate models, with persistent systematic errorsmaking it difficult to forecast the Indian monsoon accurately on subseasonal timescales. A two-layermodel based on moisture conservation with a parameterised flux representing convection is developed and used to investigate the competition between dry advection in the upper levels, the rate of moisture replenishment at low levels, and the rate of convection fromthe lower to the upper layers. In a fixed Eulerian frame, the system is initialised at an equilibrium representing pre-onset (May) conditions. Then, changes in the rates of moist inflow and upper-level advection are introduced, triggering a transition to a new equilibrium, which reflects the full monsoon state (July-September). The two-layer model reproduces the Indian monsoon onset and its progression to the northwest, against an imposed 5m⋅s-1 wind in the upper layer. Increasing the parameter representing moist inflow induces a monsoon onset, defined as a threshold of total column moisture, with clear progression from southeast toward northwest India. A lesser wind speed in the upper layer, signifying a weakening midtropospheric dry intrusion, allows more rapid progression of themonsoon onset. A greater upper-level wind speed, associated with a strengthening dry intrusion, causes the monsoon onset to retreat. We can quantify the nature of the monsoon onset by deriving an onset speed and the time taken for the system to adjust to a new equilibrium, using analytical theory. [ABSTRACT FROM AUTHOR]

Regional variations of monsoon onset dates across India were analyzed for 67 years (1951–2017) under different modes of climate variations, i.e., El Niño, La Niña, and the Indian Ocean Dipole (IOD), along with flood and drought years using the objective method and statistical techniques. Monsoon onset analysis revealed that the northern, northeastern, and southern parts were highly susceptible to the early onset of La Niña, and the northern and northern northwest parts were highly susceptible to the early onset of El Niño. The onset dates were early (late) in the sub-regions of the central, southern, and northeastern (northern, northwestern, and western) parts of India during flood (drought) years. Further, onset dates in flood years occurred earlier than those in La Niña years, and onset dates in drought years were later than those in El Niño years. The onset occurrence probability and influence of the synoptic events are discussed. This research could help in understanding the onset of monsoon and its predictability for societal applications.

Accurate forecasting of variations in Indian monsoon precipitation and progression on seasonal time scales remains a challenge for prediction centres. We examine prediction skill for the seasonal-mean Indian summer monsoon and its onset in the European Centre for Medium-Range Weather Forecasts (ECMWF) seasonal forecasting system 5 (SEAS5). We analyse summer hindcasts initialised on 1st of May, with 51 ensemble members, for the 36-year period of 1981–2016. We evaluate the hindcasts against the Global Precipitation Climatology Project (GPCP) precipitation observations and the ECMWF reanalysis 5 (ERA5). The model has significant skill at forecasting dynamical features of the large-scale monsoon and local-scale monsoon onset tercile category one month in advance. SEAS5 shows higher skill for monsoon features calculated using large-scale indices compared to those at smaller scales. Our results also highlight possible model deficiencies in forecasting the all India monsoon rainfall. [ABSTRACT FROM AUTHOR]

Timely crop planting is a foundation for climate-resilient rice-wheat systems of the Eastern Gangetic Plains—a global food insecurity and poverty hotspot. We hypothesize that the capacity of individual farmers to plant on time varies considerably, shaped by multifaceted enabling factors and constraints that are poorly understood. To address this knowledge gap, two complementary datasets were used to characterize drivers and decision processes that govern the timing of rice planting in this region. The first dataset was a large agricultural management survey (rice-wheat: n = 15,245; of which rice: n = 7597) from a broad geographic region that was analyzed by machine learning methods. The second dataset was a discussion-based survey (n = 112) from a more limited geography that we analyzed with graph theory tools to elicit nuanced information on planting decisions. By combining insights from these methods, we show for the first time that differences in rice planting times are primarily shaped by ecosystem and climate factors while social factors play a prominent secondary role. Monsoon onset, surface and groundwater availability, and land type determine village-scale mean planting times whereas, for resource-constrained farmers who tend to plant later ceteris paribus, planting is further influenced by access to farm machinery, seed, fertilizer, and labor. Also, a critical threshold for economically efficient pumping appears at a groundwater depth of around 4.5 m; below this depth, farmers do not irrigate and delay planting. Without collective action to spread risk through synchronous timely planting, ecosystem factors such as threats posed by pests and wild animals may further deter early planting by individual farmers. Accordingly, we propose a three-pronged strategy that combines targeted strengthening of agricultural input chains, agroadvisory development, and coordinated rice planting and wildlife conservation to support climate-resilient agricultural development in the Eastern Gangetic Plains. [ABSTRACT FROM AUTHOR]

This study investigates the climatology of the monsoon break following the onset of the summer rainy season over Luzon Island (120–122.5°E, 13–22°N) in the Philippines from 1979–2017. The first post‐onset monsoon break is remarkable in stations located over the north and central Luzon Island and occurs climatologically in early June. Composite analysis of the large‐scale circulation features during the monsoon break period shows that this break is associated with the westward extension of the western North Pacific Subtropical High (WNPSH), which weakened the monsoon southwesterlies and induced enhanced low‐level divergence over Luzon Island. The westward extension of the WNPSH may be facilitated by the phase change of the boreal summer intraseasonal oscillation (BSISO). About 59% (23/39) of the monsoon break cases occurred when suppressed convection, associated with the dry phases of the BSISO, is apparent over the western North Pacific. This suppressed convection favours the westward expansion of the WNPSH. With the occurrence of the monsoon break in early summer, the seasonal march of the early summer monsoon over the Philippines can be divided into three phases: (1) the monsoon onset phase, which occurs between mid to late May under the influence of the westerly/southwesterly low‐level winds, (2) the monsoon break phase, when rainfall decreases over Luzon Island in early June, and (3) the monsoon revival phase, when rainfall increases again due to the intrusion of monsoon southwesterlies over the Philippines. This study highlights the complex features of the summer monsoon onset and the impact of the WNPSH on the local climate of the Philippines in early summer. [ABSTRACT FROM AUTHOR]

A local and flexible definition of the monsoon season based on hydrological evidence is important for the understanding and management of Himalayan water resources. Here, we present an objective statistical method to retrieve seasonal hydrometeorological transitions. Applied to daily rainfall data (1951–2015), this method shows an average longitudinal delay of ~15 days, with later monsoon onset and earlier withdrawal in the western Himalaya, consistent with the continental progression of wet air masses. This delay leads to seasons of different length along the Himalaya and biased precipitation amounts when using uniform calendric monsoon boundaries. In the Central Himalaya annual precipitation has increased, due primarily to an increase of premonsoon precipitation. These findings highlight issues associated with a static definition of monsoon boundaries and call for a deeper understanding of nonmonsoonal precipitation over the Himalayan water tower. Plain Language Summary: Precipitation in the Himalayas determines water availability for the Indian foreland with large socioeconomic implications. Despite its importance, spatial and temporal patterns of precipitation are poorly understood. Here, we estimate the long‐term average and trends of seasonal precipitation at the scale of individual catchments draining the Himalayas. We apply a statistical method to detect the timing of hydrometeorological seasons from local precipitation measurements, focusing on monsoon onset and withdrawal. We identify longitudinal and latitudinal delays, resulting in seasons of different length along and across the Himalayas. These spatial patterns and the annual variability of the monsoon boundaries mean that oft‐used, fixed calendric dates, for example, 1 June to 30 September, may be inadequate for retrieving monsoon rainfall totals. Moreover, we find that, despite its prominent contribution to annual rainfall totals, the Indian summer monsoon cannot explain the increase of the annual precipitation over the Central Himalayas. Instead, this appears to be mostly driven by changes in premonsoon and winter rainfall. So far, little attention has been paid to premonsoon precipitation, but governed by evaporative processes and surface water availability, it may be enhanced by irrigation and changed land use in the Gangetic foreland. Key Points: We propose a new statistical definition of monsoon onset and withdrawal applicable a posteriori to daily precipitation time seriesMonsoon bounds show longitudinal delays and strong year‐to‐year variations, crossing commonly used seasonal calendric boundariesThe long‐term increase of annual precipitation in Nepal seems to be driven primarily by higher premonsoon rainfall [ABSTRACT FROM AUTHOR]

A lesser known relationship between boreal spring El Niño state and the Indian summer monsoon (ISM) is explored, considering the differences in the evolution of sea surface temperature and convection over the Pacific. Rossby wave tracing analysis reveals that the boreal spring El Niño convective state permits two distinct eastward waveguides from the central Pacific to the ISM domain. The first waveguide reaches the northern Indian region roughly along the African‐Asian jet, and the second waveguide crosses over to the Southern Hemisphere over the eastern Pacific and extends up to the south Indian Ocean region. The net result is an altered free tropospheric temperature distribution over the ISM domain in the month of May leading to a delayed monsoon onset. The existence of the two waveguides and the modulation of monsoon onset makes the boreal spring El Niño state an important component to be accounted for in ISM prediction. Plain Language Summary: Atmospheric heating associated with the El Niño state over the Pacific Ocean is a critical factor to consider when studying the El Niño's impact on the Indian summer monsoon (ISM). In this study using a dynamical framework, we trace the pathways of energy dispersion from the Pacific to the ISM domain. Boreal spring El Niño state over the Pacific is found to induce temperature perturbations over the north Indian and south Indian Ocean regions through two distinct pathways of energy dispersion. This leads to a change in the tropospheric temperature distribution over the ISM domain in the month of May, which in turn leads to a delay in the onset of monsoon. The finding calls for considering the boreal spring El Niño state as a critical factor for predicting the onset of monsoon. Key Points: Monsoon domain is influenced by two waveguides supported by the boreal spring El Niño convective stateA premonsoon temperature modulation is observed over the northern Indian and south Indian Ocean regionsBoreal spring El Niño delays the monsoon onset [ABSTRACT FROM AUTHOR]

Besides local emissions, biomass burning (BB) emissions in peninsula Southeast Asia (PSEA) and domestic anthropogenic emissions in North China (NC) are also significant black carbon (BC) sources over South China (SC) in spring. Meanwhile, the East Asian summer monsoon (EASM) is established with the wind field reversal, influencing the region‐based contributions to BC over SC. Herein, BC sources for SC were tracked by region and by sector using the Community Earth System Model with a BC‐tagging technique. During the spring of 2000–2014, 27% of BC surface concentration (BCS) and 64% of BC column burden (BCC) over SC stems from nonlocal sources. BC from NC is mainly transported below 850 hPa. It is the dominant nonlocal contribution to BCS (17%) and largely composed of residential and industrial sectors. Nonlocal emissions inside and outside China contribute 28% and 36% to BCC, respectively. Generally transported above 850 hPa, BC from PSEA is the largest nonlocal contributor (20%) to BCC and contributes 80% of BCC in BB sector. Additionally, the interannual variation in EASM onset times bring a maximum of −5% to +7%/−2% to +7% variation in BCC/BCS. The BC outflow/inflow contributed from NC dominates the BC decrease/increase over SC with southerly/northerly wind anomaly induced by early/late EASM onset, yet regional transport from PSEA contributes minor BC changes. The simulated BC is significantly positively correlated with the varying EASM onset times, but not with emissions, indicating the decisive role of meteorology in the interannual variation of BC over SC during springtime. Key Points: Biomass burning from peninsula Southeast Asia dominates the black carbon column burden in biomass burning sector (80%) over South ChinaBC outflow/inflow from North China dominates the BC decrease/increase over South China induced by early/late East Asian summer monsoon onsetVariation in East Asian summer monsoon onset times dominates the interannual variation of BC over South China in spring [ABSTRACT FROM AUTHOR]

This study investigates the climatology of the wet and dry conditions in the pre‐summer monsoon season of the Philippines that are less emphasized in previous works. Wet cases (Type W), which account for about 23% of the total pre‐summer monsoon days from 1979 to 2015, are identified from April 1 to the monsoon onset defined by the Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA). The differences between the Type W and dry cases (Type D) are presented. The synoptic conditions associated with the Type W and Type D cases are sub‐divided into two sub‐types based on the wind conditions, the (a) westerly winds at 925 hPa (WINDS925hPa) intrusion over Luzon Island (120°–122.5°E, 12.5°–22°N) (Type WW and Type DW), and (b) enhanced easterly WINDS925hPa (Type WE and Type DE). The Type WE (Type WW) cases account for about 63.9% (36.1%) of the total Type W cases, while the Type DE (Type DW) cases account for about 88.1% (11.9%) of the total Type D cases. A subset of Type WE cases (Type WEDef), which features an eastward propagating cold front to the north of Luzon Island and an intensifying anticyclone to its west originating from the east of the Tibetan Plateau and their differences are also examined. These two opposing circulations interact with the subtropical high over the northern Pacific and the easterlies over Luzon Island, and form a deformation zone. The confluence region of this deformation zone, where the cold front interacts with the warm and humid air brought by the easterly WINDS925hPa, lies over Luzon Island and contributes to the convective activities during the pre‐summer monsoon over this region. This is the first attempt to clarify and document the role of deformation zones as another synoptic‐scale convective system during the pre‐summer monsoon season of the country. [ABSTRACT FROM AUTHOR]

Pakistan receives huge amount of rainfall during summer monsoon season that provides water replenishment for transition periods, helps in maintaining natural and anthropogenic ecosystems, and increased crop productivity. In this changing world, shifts in summer monsoon onset in Pakistan have been observed that seems to affect the society in general. Therefore, it is vital to address these summer monsoon onset shifts to help policy makings and implementation. The study was carried out to analyse the spatio-temporal variability in summer monsoon onset in four objectively defined regions covering all Pakistan. A total of 35 meteorological stations spreading over four regions (i.e., northern, central east, central west, and southern) were taken in to account and shifts in summer monsoon onset have been calculated for the period of 1971–2010. The analysis is based on the observational data of daily precipitation from 20th Jun-20th July for 40 years. The onset for each year and mean onset for each decade has been calculated for all stations. The data was analysed for homogeneity, spatial and temporal variability of monsoon rainfall has been calculated for all four regions, and station wise monsoon onset has been discussed in detail. The temporal analysis shows that the onset of monsoon has shift towards earlier onset from first week of July to last week of June at most of the stations in which the investigation was carried out during the studied period. The spatial analysis shows that the amount of monsoon precipitation during the onset period has decreased after 1970's in almost all regions. This variability in monsoon onset can have major impacts on rain fed agriculture and cultivation of crops like maize, soybean, rice and sugarcane etc. and will have to revisit the cropping calendar. [ABSTRACT FROM AUTHOR]

The onset of the Indian summer monsoon (ISM) marks the arrival of the rainy season and affects billions of people. Therefore, exploring the factors influencing ISM onset is essential, especially sea surface temperature (SST) anomalies. In addition to the well-known El Niño-Southern Oscillation in the tropical Pacific, in this study, we find that the SST anomalies in the extratropical Pacific (Victoria mode, VM) can significantly impact ISM onset. The positive phase of the May VM causes monsoon onset delay, and two distinct mechanisms linking these phenomena are identified. The tropical pathway involves divergent circulation and equatorial Rossby waves. The VM-associated SST gradient causes low-level divergence and suppressed rainfall around the Indo-China Peninsula. Such suppressed rainfall induces anomalous anticyclone circulation by exciting the equatorial Rossby wave response, which prevents the establishment of monsoonal southwesterly winds and moisture transport, therefore delaying ISM onset. The extratropical pathway involves the Rossby wave train and land–sea thermal contrast. The VM triggers an eastward Rossby wave train via upper-level divergence around the Gulf of Alaska. This wave train propagates downstream and generates anomalous negative geopotential heights and low air temperatures to the northwest of India, which reduces the land–sea thermal contrast and delays ISM onset. Our result suggest that the ISM onset predictions should consider the role of May VM. • The North Pacific Victoria Mode delays the onset of the Indian summer monsoon via the tropical and extratropical pathways. • The tropical pathway includes the large-scale divergent circulation and the equatorial Rossby wave response. • The extratropical pathway includes the stationary Rossby wave train and the land-sea thermal contrast. [ABSTRACT FROM AUTHOR]

Advancing the leading time for onset prediction of the Indian summer monsoon (ISM) onset is an imperative task; however, it has remained a challenging subject. In particular, the land‐atmosphere coupling associated with monsoon onset prediction is poorly understood. Here we attempt to investigate the land factor as the ISM onset precursor through studying the internal mechanism of atmospheric heating, which is distinguished by monsoon onset. The low (high) soil moisture in the Iranian desert during March and April advances (delays) ISM onset by enhancing (disturbing) the vertical easterly wind shear. In addition, midtropospheric heating is affected by soil moisture in the Iranian desert. By investigating the internal atmospheric heating process and suggesting the relationship between low soil moisture and ISM onset, these findings clarify the monsoon onset mechanism in terms of the vertical atmospheric profile and land‐atmosphere interaction, which eventually extend the lead time for the onset prediction. Key Points: We interpret the atmospheric vertical structure of the Indian summer monsoon (ISM) onset in the context of the apparent heat sourceThe spring soil moisture over the Iranian desert that plays a role as a precursor of ISM onset is identifiedThese results advance the leading time of ISM onset as resulting from the land‐atmosphere interaction [ABSTRACT FROM AUTHOR]

Unlike in other parts of the Asian Monsoon region, the summer monsoon in the Northwestern Maritime Continent (NwMC) is relatively dry. Hence, an objective definition is needed in order to determine the arrival of the dry season in this particular sub‐region. Criteria for the objective definition of summer monsoon onset in the NwMC were defined based on reanalysis data from the National Centers for Environment Prediction (NCEP) and Global Precipitation Climatology Project (GPCP) databases. Considering the Matsuno‐Gill theoretical relationship between tropical precipitation and low‐level winds, the summer monsoon onset date can be determined objectively based on the commencement of the sustained 850‐hPa zonal winds averaged over the area from 5° to 10°N and from 100° to 115°E. The establishment of steady westerly/southwesterly winds over the NwMC region is consistent with the abrupt shift of rainfall from the equatorial region (5°S–5°N) to areas north of the equator. The onset is also associated with the northeastward propagation of anomalous convection from the western equatorial Indian Ocean on an intraseasonal time scale. In view of its representation of large‐scale circulation and objectiveness, this new monsoon onset index will provide a useful metric with which to predict NwMC summer monsoon onset and for studying the interannual variation in monsoon onset. A simple lead–lag correlation shows the monsoon onset in NwMC has a close relationship with ENSO. Generally an early (late) monsoon onset can be expected when cold (warm) ENSO event occurred in the preceding summer. [ABSTRACT FROM AUTHOR]

Water stable isotope ratios of tropical precipitation predominantly reflect moisture source and precipitation intensity. Trees can incorporate the isotopic signals into annual tree-ring cellulose records, permitting reconstruction of the temporal changes of hydroclimate over decades to millennia. This is especially valuable in the Himalayas where the understanding of monsoon dynamics is limited by the lack of a dense and representative observational network. We have analyzed tree ring δ 18 O records from two distinct physiographic sites along the upper Kali Gandaki valley in the central Nepal Himalayas, representing the wet High-Himalayas and the Trans-Himalayan dryland to the north. Empirical correlations and regression analyses were compared to an in-situ calibrated oxygen isotope fractionation model, exploring the relationships between tree ring δ 18 O and seasonal-mean variability of hydroclimatic forcing at the different locations. For this purpose, gridded precipitation data from the Asian rain gauge dataset APHRODITE, as well as high resolution onsite observations (relative humidity, air temperature, δ 18 O of precipitation and radial tree growth) were used. We found that two distinct sets of meteorological values, reflecting pre-monsoon and monsoon conditions, are needed to reproduce the measured tree ring δ 18 O values from the High-Himalayan site, but that a single set of monsoonal values performs best for the Trans-Himalayan site. We conclude that Trans-Himalayan trees capture long-term changes in strength of the Indian summer monsoon. In contrast, High-Himalayan tree ring δ 18 O records a more complex hydro-climatic signal reflecting both pre-monsoon and monsoon seasons with very contrasting isotopic signatures of precipitation. This difference in the two hydroclimatic proxy records offers an opportunity to reconstruct first-order hydroclimate conditions, such as local precipitation rates, and to gain new insights into monsoon timing and seasonal water source determination across the Himalayan orographic region. • Hydroclimate seasonality is preserved in tree ring δ 18 O from the Central Himalayas. • A fractionation model confirms relationships from empirical correlation analyses. • High-Himalayan tree ring δ 18 O compounds pre- and monsoon signals • Trans-Himalayan tree ring δ 18 O only contains monsoon signal. • At different locations across the Himalayas, different climate features are recorded. [ABSTRACT FROM AUTHOR]

Accurate and precise forecasting of the Indian monsoon is important for the socio-economic security of India, with improvements in agriculture and associated sectors from prediction of the monsoon onset. In this study we establish the skill of the UK Met Office coupled initialized global seasonal forecasting system, GloSea5-GC2, in forecasting Indian monsoon onset. We build on previous work that has demonstrated the good skill of GloSea5 at forecasting interannual variations of the seasonal mean Indian monsoon using measures of large-scale circulation and local precipitation. We analyze the summer hindcasts from a set of three springtime start-dates in late April/early May for the 20-year hindcast period (1992–2011). The hindcast set features at least fifteen ensemble members for each year and is analyzed using five different objective monsoon indices. These indices are designed to examine large and local-scale measures of the monsoon circulation, hydrological changes, tropospheric temperature gradient, or rainfall for single value (area-averaged) or grid-point measures of the Indian monsoon onset. There is significant correlation between onset dates in the model and those found in reanalysis. Indices based on large-scale dynamic and thermodynamic indices are better at estimating monsoon onset in the model rather than local-scale dynamical and hydrological indices. This can be attributed to the model's better representation of large-scale dynamics compared to local-scale features. GloSea5 may not be able to predict the exact date of monsoon onset over India, but this study shows that the model has a good ability at predicting category-wise monsoon onset, using early, normal or late tercile categories. Using a grid-point local rainfall onset index, we note that the forecast skill is highest over parts of central India, the Gangetic plains, and parts of coastal India—all zones of extensive agriculture in India. El Niño Southern Oscillation (ENSO) forcing in the model improves the forecast skill of monsoon onset when using a large-scale circulation index, with late monsoon onset coinciding with El Niño conditions and early monsoon onset more common in La Niña years. The results of this study suggest that GloSea5's ensemble-mean forecast may be used for reliable Indian monsoon onset prediction a month in advance despite systematic model errors. [ABSTRACT FROM AUTHOR]

Using a high‐resolution global model with explicit representation of convection, the physical processes involved in the abrupt onset of South Asian summer monsoon are investigated within a moist entropy budget framework. The monsoon onset is a two‐stage transition. During the first stage, which starts around 2 months before onset, the source terms of column‐integrated moist entropy gradually increase, while the export by the large‐scale circulation slowly strengthens. The second stage is marked by a sudden increase of radiative heating and surface latent heat flux 10 days prior to the onset, followed by abrupt strengthening of large‐scale export of moist entropy. When either cloud‐radiative or wind‐evaporation feedback is disabled in numerical experiments, the monsoon experiences much smoother and weaker onset. The evolution of the system in a gross moist stability plane demonstrates that these positive feedbacks destabilize the system and are responsible for the abruptness of the transition. Key Points: The evolution of moist entropy through monsoon onset features two distinct stages: gradual accumulation and abrupt increaseThe onset of the South Asian summer monsoon is marked by a sudden increase in radiative heating and surface latent fluxWithout cloud‐radiative and wind‐evaporation feedbacks, the monsoon experiences much smoother onset [ABSTRACT FROM AUTHOR]