Summer Monsoon Onset Research Articles

Constrained Projections Indicate Less Delay in Onset of Summer Monsoon over the Bay of Bengal and South China Sea

AbstractThe 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%.

Impacts of early spring soil moisture over the Greater Mekong Subregion on the interannual variation of South China Sea summer monsoon onset

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.

Structure, Propagation of the 10–25-Day and 30–60-Day Intraseasonal Oscillations and Their Role in Triggering the South China Sea Summer Monsoon Onset

Abstract The onset of the South China Sea (SCS) summer monsoon (SCSSM) is a complex process that involves multiscale variabilities, including the 10–25-day high-frequency intraseasonal oscillation (HISO) and the 30–60-day low-frequency intraseasonal oscillation (LISO) along with the seasonal variation. Using the reanalysis and satellite data during the period of 1979–2020, this study comprehensively investigates how the HISO and LISO trigger the SCSSM onset, which improves the understanding of the abrupt SCSSM onset from the point of intraseasonal view. It is clearly documented that in most years, the abrupt SCSSM onsets are triggered by the first branch of HISO and/or LISO propagating into the northern SCS along their distinctive pathways. The HISOs (LISO) originating from the northwestern Pacific Ocean (tropical eastern Indian Ocean) propagates westward (northeastward) toward central-to-northern SCS. The diagnosis based on the moist static energy (MSE) budget reveals the different governing mechanisms of HISO and LISO propagations. HISO’s westward propagation is favored by the zonal advection of the anomalous MSE by the mean easterly wind. In contrast, the surface turbulent heating by sea surface temperature (SST) is the dominant term for LISO’s northward propagation, followed by the meridional advection of the anomalous MSE by the mean southerly wind. Significance Statement This study provides the comprehensive intraseasonal view of the SCSSM onset, where the respective roles of 10–20-day HISO and 30–60-day LISO in triggering the SCSSM onset are identified and their propagating mechanisms are revealed. The prevailing easterlies in the northwestern Pacific and SST-induced turbulent heat flux in SCS are confirmed as the respective dominant processes governing their distinct propagations. These results improve the understanding of the SCSSM onset and hence have significant implication to the monitoring, simulation, and prediction of SCSSM.

Generation of the Quasi‐Biweekly Oscillation in the Surface Potential Vorticity Over the Tibetan Plateau During Boreal Summer: A Case Study of 2014

AbstractThe atmospheric circulation around the Tibetan Plateau (TP) exhibits a substantial 10–20‐day quasi–biweekly oscillation (QBWO), profoundly impacting weather and climate locally and remotely. Understanding the factors influencing the generation of QBWO over the TP (QBWOTP) and its physical mechanism is crucial. This study has investigated the influence of multi–timescale and land–atmosphere interactions on the generation of the QBWOTP in surface potential vorticity (SPV), a valuable tool for characterizing the mechanical and thermal variabilities in mountain forcing, based on a 2014 case study. Results indicate that in the free atmosphere, the summer monsoon onset over the Bay of Bengal induces a northward shift in the westerly jet toward the TP, manifested as an increase in low‐frequency zonal winds. This shift facilitates the propagation of wave trains, leading to atmospheric quasi–biweekly potential temperature anomalies (θa) over the TP through a multi‐timescale interaction. Additionally, the TP's surface thermal forcing and arrival of wave trains trigger anomalous upward motion and increase cloud cover. The resultant decrease in net short‐wave radiations and increase in net long‐wave radiations contribute to variations in surface potential temperature (θs) over the TP. As θa and θs evolve, the difference between them enlarges, resulting in the generation of the SPV QBWOTP. Given the relationship between the QBWOTP and downstream rainfall, this study could provide novel insights into understanding and predicting downstream rainfall QBWO.

What caused the lag between oxygen-18 and deuterium excess in atmospheric vapor and precipitation during the earlier summer season in southwest China?

A combined interpretation of oxygen-18 (δ18O) and deuterium excess (d-excess) in various archives in the Indian monsoon-dominated region may add value to the isotopic climate significance. Yet, coupling water δ18O and d-excess together is rarely discussed. Here, we present measurements for both δ18O and δ2H in water vapor and precipitation samples collected in the Nujiang River valley, an important moisture corridor for the Indian monsoon in southwestern China, aiming for an improved understanding of the hydrological cycle on seasonal scales. The observation found a significant drop occurred in the earlier summer season for δ18O and d-excess in vapor and precipitation. However, an approximately one-month lag between vapor δ18O and vapor d-excess is observed in seasonal variations, revealing different drivers for the seasonal patterns of vapor d-excess and δ18O. Spatial correlation analysis found that d-excess is mainly modulated by the relative humidity over the core moisture source over the Bay of Bengal (BOB), while the sharp decrease of vapor δ18O corresponds to the onset of the Indian summer monsoon associated with large-scale convective activities, which significantly depleted the vapor heavy isotopes. The finding of the phase difference in the seasonal isotope signals and the underlying mechanism will benefit the study in particularly using both δ18O and d-excess for an improved understanding of the hydrological processes in the Indian monsoon-dominated region, and will provide new insight into studies of paleoclimate rebuilding by using isotope-based proxy as well.

Thermal effect of the Southeast Asian low-latitude highlands on interannual variability in the date of the Bay of Bengal summer monsoon onset

The timing of the Bay of Bengal summer monsoon (BOBSM) onset has implications for the evolution of the Asian summer monsoon and associated precipitation. This study employs Japanese 55-year Reanalysis and NOAA's Gridded Precipitation Reconstruction over Land data to explore the spatiotemporal variation in the thermal influence of the Southeast Asian low-latitude highlands (SEALLH) and its effect on the BOBSM onset date. There is a significant correlation between pre-monsoon season (February–April) shortwave radiative heating (SWH) in the SEALLH and BOBSM onset in the interannual time scale. During the enhanced SWH, an anomalous vertical circulation, which converges and ascends in the lower troposphere over the SEALLH and converges and descends in the upper troposphere north of the Bay of Bengal (BOB), develops over the SEALLH–north of BOB during the pre-monsoon season with anticlockwise rotation. The warmer air mass resulting from the adiabatic heating is accumulated around the north of BOB by the insulation effect related to the anomalous vertical circulation. By facilitating the reversal of the land-sea thermal contrast, the warmer upper troposphere over the north of the BOB causes a 16-day earlier onset of the BOBSM than the case of weakened SWH. The findings shed light on subsequent research into the thermal effects of the SEALLH on Asian summer monsoon variability.

Diagnosis of Atmospheric Processes from a Local Perspective for the Western North Pacific Summer Monsoon Onset

Abstract The climatological western North Pacific summer monsoon onset, a so-called convection jump, occurs around the 41st pentad, corresponding to an abrupt northeastward extension of strong convection. This study investigates the process of convection jump from a local perspective. Composite analyses are performed based on the onset dates that are identified in individual years. The results show that the convective inhibition (CIN) decreases dramatically around the onset dates, while the convective available potential energy (CAPE) reaches its maximum long before the onset, suggesting that the CIN, rather than CAPE, plays a dominant role in triggering convection. Further analysis indicates that the reduction of CIN is induced by the increased low-level relative humidity, which is the result of enhanced water vapor convergence. The moisture transportation is primarily contributed by the wind transfer from easterlies to southeasterlies or southerlies along the southern boundary of the convection jump region, in accordance with the monsoon trough establishment. The present observational results may be used to evaluate climate models in simulating the stepwise evolution of summer monsoon. Significance Statement The convection jump refers to rapid convection extension to the northeastern part of the Philippines Sea in late July and corresponds to the deepening of the monsoon trough and northward shift of the western North Pacific subtropical high. Therefore, the convection jump has important impacts on the East Asian summer monsoon. The crucial role of transient atmospheric adjustment in triggering the convection jump has been shown in previous studies. However, the reason for the temporal preference of convection jump remains unknown. This study further specifies the process of atmospheric adjustments around the convection jump from a local perspective and reveals that the transition from the easterlies to southeasterlies or southerlies decreases convective inhibition and thus favors the convection jump. The present results can be used to diagnose the ability of climate models to simulate the stepwise evolution of Asian–Pacific summer monsoon.

Delayed Summer Monsoon Onset in Response to the Cold Tongue in the South China Sea

Delayed Summer Monsoon Onset in Response to the Cold Tongue in the South China Sea

Weakening of La Niña Impact on Negative Indian Ocean Dipole Under Global Warming

AbstractAs global warming intensifies, the coupling relationship between negative Indian Ocean Dipole (nIOD) and La Niña has substantially changed. However, the characteristics and mechanisms of these changes are not yet fully understood. Here, we find that the impact of La Niña on nIOD has considerably weakened since 1999, with the frequency of nIOD occurrences during La Niña years plummeting to a mere one‐third of the pre‐1999 levels. This is primarily attributed to the early onset of Indian summer monsoon and the decrease in La Niña intensity, while the effect of variations in Bjerknes feedback is relatively minor. Model simulations suggest that the influence of La Niña on nIOD will continue to weaken under future global warming through similar mechanisms as in the observations, increasing the complexity of air‐sea coupling in the Indian Ocean.

Isotopic characteristics of extreme “dragon-boat water” rainfall between mid-May and mid-June in 2022 in Fuzhou, southeastern China

With global warming, the surge in torrential “dragon-boat water” (DBW) events during mid-May and mid-June, following the onset of the South China Sea summer monsoon (SCSSM), poses escalating threats to southern China through severe flooding and impacts on human livelihoods and socio-economic development. To better understand their fundamental mechanisms, we investigated extreme DBW rain events that occurred in southeastern China in 2022. Analysis of stable isotopes in precipitation samples collected in Fuzhou before and after SCSSM onset uncovered a significant decrease in δ18O values following monsoon onset, indicating intensified regional convective activity. This decrease is attributed to the seasonal variation in moisture sources, with water vapor originating from distant oceans after SCSSM onset contributing to increased Rayleigh distillation and subsequent δ18O reduction. A negative correlation between upstream rainfall and δ18O values, particularly at 96 h along the backward trajectory, indicates the influences of accumulative convective activities on precipitation isotopes along water vapor transport paths. Individual rain events typically exhibit characteristic “V” or “W” shapes in δ18O, with the lowest values mostly observed in stratiform zone, governed by microphysical processes, such as mesoscale subsidence, vapor deposition of ice particles, and rain re-evaporation below cloud-base. Notably, events with low δ18O values often coincide with low initial values, suggesting decreased vapor δ18O before precipitation due to accumulative convection or upstream rainout. Our findings elucidate the mechanisms driving the long-term changes in precipitation isotopes and their evolution during DBW events, enriching our understanding of the interplay between precipitation isotopes, large-scale convection, and microphysical processes.

Role of Strong Sea Surface Temperature Diurnal Variation in Triggering the Summer Monsoon Onset Over the Bay of Bengal in a Climate Model

AbstractThe 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.

Dynamics of May ‘onset’ of Indian summer monsoon over Northeast India

AbstractAcknowledging the prolonged duration of the rainy season in Northeast India (NEI) compared to Central India, the official onset of the Indian summer monsoon over NEI is traditionally marked around 5 June, with May rainfall categorized as ‘pre‐monsoon’. However, our study reveals that May rainfall in NEI occurs in active/break spells driven by persistent synoptic‐scale systems, contributing to a significant monsoon heat source during this period. Through an objective analysis, we determine that the climatological ‘onset’ in NEI actually occurs around 18 May, with withdrawal around 14 October, resulting in an extended rainy season of approximately 150 days. The enigma of the May onset, while the Intertropical Convergence Zone remains proximate to the equator, is addressed by identifying a conducive climate in May. This climate is characterized by low‐level cyclonic vorticity over the region, influenced by the interannual strengthening (weakening) by the Atlantic Niño (El Niño). The introduction of potential vorticity from extratropical transient Rossby waves in May significantly amplifies low‐level cyclonic vorticity by 3–4 times, acting as a catalyst for the monsoon onset in NEI. Furthermore, a 5–6 times intensification of northward moisture transport from the Bay of Bengal sustains the monsoon heat source post onset. The May onset is made feasible by the uplifting of low‐level cyclonic winds, facilitated by the horseshoe‐shaped orography around the Brahmaputra valley. This process is complemented by an increased local moisture content resulting from evapotranspiration. Our findings challenge conventional notions by demonstrating that a component of Indian summer monsoon rainfall in NEI is not directly related to the Intertropical Convergence Zone. This challenges the fundamental definition of the South Asian Monsoon and calls for a reassessment of the prevalent belief that the Indian monsoon season is confined to June–September in NEI.

Triggering effects of spring thermal contrast between Yun-Gui Plateau and Beibu Gulf on the interannual variations of South China Sea summer monsoon onset dates

Triggering effects of spring thermal contrast between Yun-Gui Plateau and Beibu Gulf on the interannual variations of South China Sea summer monsoon onset dates

The relationship between the South China Sea summer monsoon onset and North Pacific meridional sea surface temperature anomalies

AbstractThis 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.

Stagnation of Eddy Kinetic Energy After Summer Monsoon Onset in the South China Sea

AbstractThe monsoon is the primary force behind the upper‐layer circulation system and regulates eddy activity in the South China Sea (SCS). Using satellite observations and a state‐of‐the‐art oceanic reanalysis, this study examines transferring processes of eddy kinetic energy (EKE) during the SCS Summer Monsoon (SCSSM) onset (20th May on average). The SCSSM onset transforms surface winds from easterly to southwesterly, increasing EKE in the southwestern SCS by surface wind work. Despite the persistent energy input from wind work, EKE growth ceases during the first 20 days after SCSSM onset. An EKE budget analysis suggests that most energy input dissipates in the mixed layer due to the high wind speed, which explains the EKE stagnation. Additionally, eddy‐mean flow interactions transfer kinetic energy from eddies to mean flows during the SCSSM onset. The Vietnam coastal upwelling system provides available potential energy to EKE on the offshore side through buoyancy work. Our study reveals energy‐transferring processes from the summer monsoon to eddy activity and dissipation in the SCS, with implications for understanding multiscale dynamic evolutions during the monsoon transition.

The contrasting interannual variation and mechanism of the first rainy season precipitation before and after the South China Sea summer monsoon onset

The first rainy season (FRS) is usually considered as a whole when its precipitation variability and the underlying mechanism are discussed. This strategy does not consider the South China Sea summer monsoon (SCSSM) onset occurring within the FRS, which implies an abrupt change in nature and, thereby, the possible characteristics and mechanism of precipitation. To overcome this deficiency, this study divides the FRS into two stages before and after the SCSSM onset, revisits their interannual precipitation variability, and finds remarkable differences in the two FRS stages. During the FRS stage before the SCSSM onset, enhanced precipitation is accompanied by an anticyclone centered on the Indian Peninsula and extending further eastward to the Philippines, active synoptic-scale disturbances in the upstream regions of South China and an anomalous anticyclone in the mid-latitudes of East Asia. This configuration reinforces the warm-moisture air transportation from the Bay of Bengal and the South China Sea and cold air invasion from the mid-latitude regions to South China. It shapes the pattern of initial ascending motions over South China by both dynamical and thermal forcing. During the FRS stage after the SCSSM onset, in contrast, enhanced precipitation is accompanied by an anomalous anticyclone in the western North Pacific and the mid-latitude Silk Road pattern. This configuration reinforces warm-moisture air transportation from the tropical Western Pacific and cold air from the northwest direction to South China and shapes the initial ascending motion mainly by thermal forcing. The interannual precipitation variability is tied to a tropical Pacific sea surface temperature (SST) dipole for the FRS period before the SCSSM onset but not after. Such a difference is possibly due to the changes in the basic thermal-dynamical state of the atmosphere near the Maritime Continent and the tropical Northwestern Pacific and their different responses to the SST forcing before and after the SCSSM onset.

Comparative analysis of indices in capturing the onset and withdrawal of the South Asian Summer Monsoon

The South Asian summer monsoon (SASM) exerts a profound influence on South Asia and the southern Tibetan Plateau. The timing of its onset and withdrawal significantly impacts regional rainfall, a critical water source for agriculture and the economy. Various SASM indices, employing different variables, have been employed to gauge monsoon onset and withdrawal, each demonstrating distinct characteristics. This study provides a comprehensive comparison of prominent SASM indices related to onset and withdrawal, revealing similar timing but varying magnitudes. Across nearly all indices, a consistent pattern emerges, indicating a trend towards earlier onset and delayed retreat during 1979–2018, marked by pronounced interdecadal variability, with a notable turning point around 1997. The earlier onset and later retreat are likely to enhance rainfall and potential water resources for South Asia and the Tibetan Plateau. Delving into the mechanisms revealed a delayed connection between the SASM onset to the large-scale sea surface temperature (SST) anomalies characterized by the Oceanic Niño Index (ONI) in the preceding spring, but a simultaneous connection between SASM withdrawl and ONI in autumn. Current index definitions, relying on single or dual variables, may fall short in accurately depicting monsoon onset and withdrawal. To address this, we introduce a novel monsoon index derived from multiple large-scale circulation variables, offering improved trend capture and enhanced representation of interannual variability in SASM onset and withdrawal. This study advances our understanding of SASM indices and their correlation with monsoon rainfall, providing insights into the dynamics of SASM onset and withdrawal.

Enhanced impacts of the North Pacific Victoria mode on the Indian summer monsoon onset in recent decades

AbstractVictoria 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.

Enhanced Late Spring Ozone in Southern China by Early Onset of the South China Sea Summer Monsoon

AbstractThe onset of the South China Sea summer monsoon (SCSSM) has profound impacts on meteorological conditions over East Asia. However, whether the interannual variability in monsoon onset date impacts ozone (O3) pollution remains unclear. Here, we investigate the relationship between early onset of SCSSM and late spring O3 in southern China. Our results show notable differences in surface O3 concentrations before and after SCSSM onset during early onset events in southern China. The enhanced O3 of 11.1 μg m−3 is supported by increased air temperature and solar radiation of 1.1 K and 30.9 W m−2 and reduced relative humidity of −5.7%. Both observation and model simulations confirm that O3‐favorable meteorological conditions modulated by early SCSSM onset can be found in May. It increases the boundary layer height and biogenic emissions of volatile organic compounds, enhancing O3 by 10 μg m−3 over southern China. Chemical processes dominate such increases in O3 with enhanced chemical production of 0.27 Tg month−1. Descending motion in southern China vertically transports O3 to surface by 0.10 Tg month−1, whereas horizontal advection reduces O3 concentration by 0.12 Tg month−1. The meteorological responses to colder sea surface temperature (SST) in the central equatorial Pacific are pronounced, leading to higher O3 concentrations over the Yangtze River Delta, while warmer SST in the Philippine Sea contributes O3 over the Pearl River Delta and eastern China. This study highlights the importance of SCSSM onset with respect to O3 in southern China, with promising applications in management of air pollution and agriculture.

Climatological Madden-Julian Oscillation during boreal spring leads to abrupt Australian monsoon retreat and Asian monsoon onsets

Abrupt monsoon onsets/retreats are indispensable targets for climate prediction and future projection, but the origins of their abruptness remain elusive. This study establishes the existence of three climatological Madden-Julian Oscillation (CMJO) episodes contributing to the rapid Australian summer monsoon retreat in mid-March, the South China Sea (or East Asian) summer monsoon onset in mid-May, and the Indian summer monsoon onset in early June. The CMJO displays a dynamically coherent convection-circulation structure resembling its transitionary counterpart, demonstrating its robustness as a convectively coupled circulation system and the tendency of the transient MJOs’ phase-lock to the annual cycle. The CMJO is inactive during the boreal winter due to destructive year-to-year modulations of El Niño-Southern Oscillation. We hypothesize that the interaction between atmospheric internal variability (MJO) and the insolation-forced slow annual cycle generates the sudden monsoon withdrawal/onset during the boreal spring. Understanding the factors determining the timing and location of the MJO’s phase-locking and its variability is vital for monsoon forecasting and climate projection.