South China Sea Summer Monsoon Onset Research Articles

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.

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.

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.

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.

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.

Prediction of South China Sea summer monsoon onset in the CAMS‐CSM climate forecast system

AbstractBased on a set of hindcast experiments from 2011 to 2020, the prediction skills of the Chinese Academy of Meteorological Science Climate System Model (CAMS‐CSM) climate forecast system (CFS) on the abrupt changes and characteristic processes associated with the South China Sea summer monsoon (SCSSM) establishment are evaluated. We studied predictions for three different lead times, that is, LT1‐30, LT31‐60 and LT61‐90. The CAMS‐CSM CFS captures the climatological SCSSM onset date at all lead times, while the prediction skill of the SCSSM onset index decreases with the increasing lead times. The features of abrupt changes and characteristic processes related to the SCSSM establishment can be generally reproduced in the predictions, but different biases exist at three lead times. Besides, the CAMS‐CSM CFS provides higher skill on the transition of the zonal wind shear (ZWS) than that of meridional temperature difference (MTG). The transition date of MTG can only be accurately predicted at LT1‐30, indicating the limited ability of predicting temperature. Further results confirm that the CFS cannot well capture the eastward withdrawal of the western North Pacific subtropical high on late onset years, thereby leading a poorer prediction skill than early onset years. In addition, skillful prediction on the eastward extension of westerlies from the equatorial Indian Ocean to SCS also contributes to increase predictability of SCSSM onset.

The first observational evidence of a mixed Rossby–gravity wave contribution to triggering the onset process of the South China Sea summer monsoon

Abstract Convectively coupled equatorial waves (Kelvin waves, equatorial Rossby waves, the Madden–Julian Oscillation, tropical depression–type waves, and mixed Rossby–gravity (MRG) waves) are important components of the tropical atmosphere. It is already known that the first four kinds of equatorial waves can trigger the onset of the South China Sea summer monsoon (SCSSM). This study provides observational evidence that an MRG wave contributes to triggering the onset process of the SCSSM in 2023, which took place very early (advanced by two weeks). Specifically, anomalous upper-tropospheric southerly winds first appeared in the equatorial central Pacific at the end of April. These southerly winds propagated westwards and downwards to the equatorial western Pacific and became an MRG wave. As this MRG wave continued to propagate westwards, the associated southwesterly winds contributed to the SCSSM onset. In addition to the MRG wave, the westerly phase of the Madden–Julian Oscillation is located around the Maritime Continent, which also creates a favorable environment for the SCSSM onset in 2023. After addressing the important role of the MRG wave, this study completes the last piece of the puzzle of the impacts of equatorial waves on the monsoon onset.

A statistical study on promoting effects of tropical cyclones over the Bay of Bengal on the South China Sea summer monsoon onset

AbstractThe 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.

Mechanism of the late summer monsoon onset in Bay of Bengal and South China Sea during 2021: Impacts of intraseasonal oscillation and local thermal contrast

The process of Bay of Bengal (BOB) summer monsoon (BOBSM)- and South China Sea (SCS) summer monsoon (SCSSM)-onset in 2021 is investigated by using the NCEP/NCAR reanalysis, NOAA OLR and ERA5 SST data from 1981 to 2021. Results show that BOBSM and SCSSM are built up two pentads later than climatology, on May 18 (pentad 28) and May 29 (pentad 30), respectively. For the BOBSM, the late onset is attributed to the late-originating 10–30-day intraseasonal oscillation (ISO), of which the wet phase strengthens the vortex and local convection over BOB in pentad 28, providing dynamical and thermodynamical conditions for the establishment of BOBSM. For the SCSSM, however, the onset is impeded by the anomalously weak Indochina Peninsula (ICP)-SCS thermal contrast before pentad 30 and the lack of the mid-latitude trough (a trigger of SCSSM confirmed in previous studies) in pentad 29. We propose a possible trigger of SCSSM in 2021, i.e., land wind effect, based on the sudden strengthening of ICP-SCS thermal contrast, which connects BOBSM convection and SCSSM onset. Specifically, a sub-vortex is formed over the ICP in pentad 29 following the vigorous BOBSM convection. This sub-vortex structure results in a drastic surface temperature decrease in ICP, causing fairly significant thermal contrast between ICP and SCS. Then, the dynamical response to the low-level baroclinicity, i.e., land wind effect, triggers the onset of SCSSM in pentad 30.

Response of sea surface heat fluxes to the South China Sea summer monsoon onset in 2021

By using the ship-based data collected over the South China Sea during the South China Sea summer monsoon (SCSSM) in 2021, the characteristics of near-surface meteorological conditions, surface heat budget, and atmospheric boundary layer structure are analyzed. We found that near-surface air temperature (relative humidity) decreased significantly during the developed monsoon period, while near-surface wind speed showed an obvious increase trend from the pre-monsoon period to the developed monsoon period. The significant reduction of solar radiation due to cloud cover led to the reverse of oceanic heat flux direction from downward to upward. The sea surface sensible and latent heat fluxes were restrained after the SCSSM onset. The atmospheric boundary layer height presented a positive (negative) correlation with near-surface air temperature (humidity). In addition, the vertical profiles of wind speed, virtual potential temperature, and air humidity revealed that: after monsoon onset, (i) the atmospheric circulation within the atmospheric boundary layer accelerated, with a strong low-level jet located below 500-m height; (ii) there was a strong inversion layer below 500-m height, especially at 1300 and 1900 local time (LT); (iii) the atmospheric humidity in low-level raised significantly.

Close Linkage of the South China Sea Summer Monsoon Onset and Extreme Rainfall in May over Southeast Asia: Role of the Synoptic-Scale Systems

Abstract Based on multiple datasets and methods, this study reveals a close relation between the South China Sea summer monsoon (SCSSM) onset and the occurrence frequency of extreme rainfall over Southeast Asia (including the Indochina Peninsula, Hainan Island, and the Philippines) in May. Specifically, an early SCSSM onset tends to be accompanied by a higher chance of extreme rainfall over Southeast Asia, while a late SCSSM onset is likely to coincide with less heavy rainfall in May. It is suggested that the amplitude of tropical synoptic-scale systems may play an important role in the linkage between the SCSSM onset and the extreme rainfall over Southeast Asia. Accompanying an early SCSSM onset, anomalous cyclonic circulation appears over the SCS and Philippine Sea, which tends to increase the quantity of midtroposphere moisture via moisture advection and Ekman pumping. Besides, the barotropic energy conversion (from mean kinetic to eddy kinetic energy) associated with this anomalous cyclone will facilitate the development of the synoptic-scale systems. The vertical easterly wind shear accompanying an early SCSSM onset will confine the synoptic-scale wave train to the lower troposphere, which is also favorable for its growth. Once these synoptic-scale systems develop, the baroclinic process (from eddy available potential energy to eddy kinetic energy) will further promote their development. As such, the intensity of synoptic-scale systems is much stronger in early than late SCSSM onset years. The increased moisture and vigorous synoptic-scale systems further lead to an increase in the occurrence of extreme rainfall over Southeast Asia.

The 1–31-Day Predictions of the South China Sea Summer Monsoon in the CAMS-CSM Climate Forecast System

The South China Sea summer monsoon (SCSSM) is crucial for the East Asian monsoon system, which has been detected from plenty of aspects, while its prediction has been relatively less investigated on the subseasonal timescale. The 1–31-day predictions of SCSSM, including fundamental dynamic and thermodynamic characteristics, indices, onset date and associated circulations, are examined and diagnosed for different climate systems, i.e., T106 and T106 × T255 (with a nudging process added) in the Chinese Academy of Meteorological Sciences climate system model (CAMS-CSM). The results indicate the general decreasing prediction skills of the model with the growing lead times. For lead times of 1–10 days, zonal winds at the lower (850 hPa) and higher (200 hPa) levels can be reasonably predicted, as well as the pseudo-equivalent potential temperatures at 850 hPa. Meanwhile, the prediction skill for the higher level generally shows a better performance than that for the lower level. The prediction capability is relatively weak during the circulation adjustment period before the monsoon onset, while a significant enhancement occurs after that. During the analyzed period of 2011–2020, the prediction of SCSSM onset date is mainly skillful in most years, while the year of 2015 shows a prediction result with at least six pentads earlier than the observation, which is subsequently taken as a failure case for further investigation. At the lower level, the model could not effectively predict the weakening and eastward withdrawal of the Western Pacific subtropical high and the shift in wind field during the SCSSM onset. As for the upper level, the rapid northward movement of the South Asia high and its establishment in the Indochina Peninsula are neither well captured. In addition, the models of T106 and T106 × T255 do not show significant differences in most cases, but the latter tends to be more skillful on the continent.

Interannual Variation in Moisture Sources for the First Rainy Season in South China Estimated by the FLEXPART Model

Abstract The first rainy season (April–June) of South China includes the phases before and after the onset of the South China Sea summer monsoon (SCSSM). Abundant moisture supply is the key dynamic process for precipitation formation. Thus, we employ the FLEXPART model to explore the corresponding moisture sources for the two phases. Before the onset of SCSSM, land regions contribute more moisture to the precipitation over South China than the ocean sources. The main source regions are Southeast Asia (27.01%), the South China Sea (25.96%), South China (11.12%), and the southern part of the northwestern Pacific (10.23%). Land sources (66.87%) play a more important role than ocean sources (33.13%) in the interannual variations, with the contributions mainly from Southeast Asia (47.56%) and the South China Sea (28.79%). After the onset of SCSSM, the climatological contribution of ocean sources is larger than that of land regions, and the main source regions are the South China Sea (20.78%), Southeast Asia (17.51%), the Bay of Bengal (13.76%), and South China (11.21%). For the interannual variations, the contributions of land sources and ocean regions are comparable, and mainly from Southeast Asia (33.53%) and the Bay of Bengal (32.26%). The moisture transports for the interannual variations in first rainy season precipitation over South China before and after the onset of SCSSM are significantly correlated with the east–west contrast of sea surface temperature anomalies over northern part of North Pacific and the uniform warming over Indian Ocean, respectively. This study provides important guidance in improving the regional precipitation predictions and understanding the water resources changes.

Subseasonal Predictability of South China Sea Summer Monsoon Onset With the ECMWF S2S Forecasting System

AbstractAccurate forecasting of the South China Sea summer monsoon (SCSSM) onset is critical for water resource management in the rainy season across East Asia. The present work evaluates the skill of the ECMWF subseasonal‐to‐seasonal (S2S) forecasting system in predicting SCSSM onset dates using a hindcast experiment. The model provides accurate predictions of the SCSSM onset date 10 days in advance. The predictability mainly arises from the sea surface temperature anomaly (SSTA) in the northern Indian Ocean and central‐eastern tropical Pacific, which usually coincides with moderate and extreme decaying El Niño, thus creating windows of opportunity for SCSSM onset forecasting. The model can reasonably reproduce the SCSSM‐related SSTA and its induced atmospheric teleconnections. However, when anomalous Ural blocking influences the mid‐high latitudes, the ECMWF model forecasts of the SCSSM onset are trapped in the synoptic timescale because of the limited ability of the model to consider blocking in forecasting.

Roles of synoptic‐scale waves and intraseasonal oscillations in the onset of the South China Sea summer monsoon

AbstractThe maintenance period (20 days after the onset day) of the South China Sea (SCS) summer monsoon (SCSSM) onset is investigated in this paper, the main focus of which is the roles of synoptic‐scale (2–10‐day period) waves (SSWs) relative to those of 10–20‐day and 30–60‐day intraseasonal oscillations (ISOs). The results reveal that the eddy kinetic energy (EKE)‐based SSW intensity displays a greater amplitude and closer relationship with the SCSSM onset date (correlation coefficient: −0.50) than either ISO during the SCSSM onset maintenance period, indicating that frequent synoptic‐scale processes play an important role in the SCSSM onset. Regression analyses based on the original EKE‐based wave intensity further reveal that SSW‐related horizontal wind anomalies show significant cyclonic anomalies over the SCS, with westerlies occupying the central and southern SCS. Moreover, SSW‐induced low‐pressure systems, including negative outgoing longwave radiation (OLR) anomalies, low‐level convergence and weakening of the 500 hPa height field, are observed over the SCS and maintain the SCSSM onset. A three‐dimensional EKE diagnosis is conducted to investigate the relative contributions of SSWs and both ISOs. The results demonstrate that the SSW‐related horizontal barotropic EKE conversion and EKE conversion from eddy available potential energy cooperate to contribute more than the two ISOs to the dynamic processes during the maintenance stage of the SCSSM onset.

Impact of Pacific Decadal Oscillation on interannual relationship between El Niño and South China Sea summer monsoon onset

AbstractIn this study, we investigate impact of the Pacific Decadal Oscillation (PDO) on the interannual relationship between the El Niño and South China Sea summer monsoon onset (SCSSMO) using observations and community atmosphere model (CAM) simulations. We find that the interannual relationship between El Niño and SCSSMO during 1948–2019 has significant decadal variation, namely, this variation is consistent with PDO phase change. Specifically, a high El Niño‐SCSSMO correlation appears during the positive PDO phase from the mid‐1970s to the mid‐1990s; in contrast, no significant correlation exists during the negative PDO phase from the mid‐1950s to the mid‐1970s and also from the 2000s to the 2010s. Further analyses suggest that the PDO‐related sea surface temperature anomalies (SSTAs) in the western North Pacific (WNP), rather than in the North Pacific, modulate the El Niño‐SCSSMO relationship. The SSTAs in the WNP can prolong the existence of the anomalous WNP anticyclone (WNPAC) from the El Niño mature winter to the following May, and the corresponding western Pacific subtropical high (WPSH) is stronger and extends further westward, thus causing a late SCSSMO during the positive PDO phase. In contrast, the WNPAC disappears and the WPSH withdraws earlier in May during the negative PDO phase, inducing an early onset of the SCSSM. The observed PDO's impact on the interannual relationship between the El Niño and SCSSMO is confirmed by CAM simulations.

Impacts of the Wave Train along the Asian Jet on the South China Sea Summer Monsoon Onset

The South China Sea (SCS) summer monsoon (SCSSM) onset signifies the commencement of large-scale summer monsoon over East Asia and the western North Pacific (WNP). Previous studies on the influencing factors of the SCSSM onset mainly focus on the tropical systems, such as El Niño-Southern Oscillation (ENSO). This study reveals that the wave train along the Asian jet could act as an extratropical factor to modulate the SCSSM onset, and it is largely independent of ENSO. The SCSSM onset tends to be earlier during the positive phase of the wave train (featured by northerly anomalies over Central Iran plateau and eastern China, southerly anomalies over Arabian Peninsula, eastern Indian subcontinent, and eastern Bonin islands). The wave train affects the SCSSM onset mainly via modulating the WNP subtropical high. The wave train during the positive phase can induce negative geopotential height anomalies in the mid-troposphere and anomalous cyclones in the lower-troposphere over the SCS and the Philippine Sea, leading to the weakening of the WNP subtropical high. Specifically, the anomalous ascending motions associated with the low-level cyclone are favorable for the increased rainfall over the SCS, and the anomalous westerly on the south of the anomalous cyclone is conducive to the transition of the zonal wind (from easterly to westerly). The above circulation anomalies associated with the positive phase of the wave train provide a favorable environment for the advanced SCSSM onset.

Subseasonal-to-Seasonal Predictability of Onset Dates of South China Sea Summer Monsoon: A Perspective of Meridional Temperature Gradient

AbstractThe onset of the South China Sea summer monsoon (SCSSM) has traditionally been ascribed to the El Niño–Southern Oscillation (ENSO) on an interannual timescale, but the two do not correspond in some years. The present study applies harmonic analysis on the meridional temperature gradient (MTG) in mid–upper troposphere over South China Sea (SCS) and decomposes the onset process to be a slow-varying seasonal cycle and transient subseasonal component. The ENSO-related air temperature anomaly in the southern SCS provides seasonal predictability of SCSSM onset by a stable and robust relationship between ENSO and MTG seasonal cycle. However, in the northern SCS, the MTG is regulated by an intraseasonal oscillation (ISO) of extratropical air temperature with a significant 10–30-day period. This ISO originates over the western TP, then propagates eastward and gets enhanced by anomalous diabatic heating due to spring rainfall anomaly over South China, as a result of subseasonal thermal forcing of TP. When the ISO arrives to the north of the SCS, it directly changes the tropospheric temperature to modulate the MTG. Meanwhile, the upper-level circulation associated with the ISO alters the meridional potential vorticity advection and pumping effect, followed by the anomalous low-level westerly wind and monsoon convection over the SCS. The SCSSM onset is evidently disrupted from its seasonal cycle when this ISO is more active. Since the independence of its intensity from ENSO, this extratropical ISO over TP and South China provides additional subseasonal predictability of the onset dates of the SCSSM.

Interdecadal Change of the Winter-Spring Tropospheric Temperature Over Asia and its Impact on the South China Sea Summer Monsoon Onset

The interdecadal change of the winter-spring tropospheric temperature over Asia and its impact on the South China Sea summer monsoon (SCSSM) onset are investigated in this study. The SCSSM onset experiences a significant advance around the mid-1990s, which is attributed to the positive interdecadal anomaly of meridional temperature gradient in the mid-upper troposphere in May over the South China Sea monsoon region. This positive interdecadal anomaly can be traced back to the previous winter and spring, mainly associated with the mid-upper tropospheric warming in the East Asian subtropics. During the interdecadal anomalous seasonal evolution of the East Asian subtropical mid-upper tropospheric temperature, advection of the warm temperature anomaly by climatological mean wind overtakes the effects of anomalous adiabatic cooling and diabatic cooling, leading to a net interdecadal seasonal warming in winter. In spring, the adiabatic heating caused by the interdecadal anomalous subsidence flow cannot offset the effects of interdecadal anomalous cold advection and diabatic cooling, resulting in a net interdecadal seasonal cooling. However, the interdecadal seasonal cooling in spring is not strong enough to offset the interdecadal seasonal warming in winter, preserving an interdecadal mid-upper tropospheric warming over the subtropical East Asia in late spring. This interdecadal warming provides a favorable condition for the interdecadal advance of the SCSSM onset. Both observational analyses and numerical experiments suggest that the interdecadal change of atmospheric thermodynamic processes in winter−spring is related to the interdecadal warming in the tropical western Pacific. The enhanced convection and condensation heating over the regions from the South China Sea to the Philippines, forced by the tropical western Pacific warming, stimulates an anomalous anticyclone in the upper troposphere and warming in the whole troposphere over the subtropical East Asia.