South China Sea Summer Monsoon Research Articles

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.

Connection between Barents Sea Ice in May and Early Summer Monsoon Rainfall in the South China Sea and Its Possible Mechanism

The impacts of Arctic sea ice on climate in middle and high latitudes have been extensively studied. However, its effects on climate in low latitudes, particularly on summer monsoon rainfall in the South China Sea (SCS), have received limited attention. Thus, this study investigates the connection between the Arctic sea ice concentration (SIC) anomaly and the early summer monsoon rainfall (ESMR) in the SCS and its underlying physical mechanism. The results reveal a significant positive correlation between the Barents Sea (BS) SIC in May and the ESMR in the SCS. When there is more (less) SIC in the Barents Sea (BS) during May, this results in a positive (negative) anomaly of the local turbulent heat flux, which lasts until June. This, in turn, excites an upward (downward) air motion anomaly in the vicinity of the BS, causing a corresponding downward (upward) motion anomaly over the Black Sea. Consequently, this triggers a wave train similar to the Eurasian (SEU) teleconnection, propagating eastward towards East Asia. The SEU further leads to an (a) upward (downward) motion anomaly and weakens (strengthens) the western Pacific subtropical high (WPSH) over the SCS, which is accompanied by a southwest adequate (scarce) water vapor anomaly transporting from the Indian Ocean, resulting in more (less) precipitation in the SCS. Furthermore, the response of ESMR in the SCS to the SIC in the BS is further verified by using the Community Atmosphere Model version 5.3 (CAM5.3). This study introduces novel precursor factors that influence the South China Sea summer monsoon (SCSSM), presenting a new insight for climate prediction in this region, which holds significant implications.

Plausible impact of ENSO on the enhanced correlation of the onset date between the Bay of Bengal and South China sea summer monsoon

Plausible impact of ENSO on the enhanced correlation of the onset date between the Bay of Bengal and South China sea summer monsoon

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.

A study of reanalysis characteristics and evaluation of interdecadal variation of the intensity of South China Sea Summer Monsoon in the early 1990s

A study of reanalysis characteristics and evaluation of interdecadal variation of the intensity of South China Sea Summer Monsoon in the early 1990s

Interdecadal delay of the South China Sea summer monsoon onset after 2010

Interdecadal delay of the South China Sea summer monsoon onset after 2010

Effect of quasi‐biennial oscillation on the intensity of the South China Sea summer monsoon

AbstractThe interannual relationship between the quasi‐biennial oscillation (QBO) and the South China Sea (SCS) summer monsoon intensity (SCSSMI) is investigated using the ERA5 data set in the period of 1953–2020. The QBO–SCSSMI connection is weak in El Niño‐Southern Oscillation (ENSO)‐related summer, but significant in neutral ENSO summer. In neutral ENSO summer, there is significant positive correlation between QBO and SCSSMI, that is, the SCSSMI is enhanced in the QBO westerly phase, while it is weakened in the QBO easterly phase with larger amplitude of the SCSSMI anomaly. QBO may modulate the SCSSMI in three possible ways. The QBO‐related temperature anomalies downward to the lower stratosphere directly affect the SCS monsoon convection by modulating the upper‐tropospheric stability and tropopause height over the SCS region. Moreover, the QBO‐related anomalous zonal wind downward to the lower stratosphere may regulate cross‐tropopause vertical wind shear and then modulate the convection development. Besides, the QBO‐related zonal wind and temperature signals propagating downward into the upper troposphere can affect the horizontal divergence by influencing the vertical vorticity and the geopotential via thermal forcing. Through those three processes, QBO directly affects the deep convection activities and then modulates the SCSSMI via internal dynamic adjustment. In the southern SCS, cross‐tropopause vertical wind shear makes a main contribution to deep convections, while upper‐tropospheric static stability makes a major contribution to deep convection in the northern SCS. The conclusions are also generally verified by the CMIP6 model simulation.

Reconciling roles of the South China Sea summer monsoon and ENSO in prediction of the Indian Ocean dipole

The Indian Ocean dipole (IOD) is a remarkable interannual variability in the tropical Indian Ocean. The improved prediction of IOD is of a great value because of its large socioeconomic impacts. Previous studies reported that both El Niño-Southern Oscillation (ENSO) and South China Sea summer monsoon (SM) play a dominant role in the western and eastern pole of the IOD, respectively. They can be used as predictors of the IOD at 3 month lead beyond self-persistence. Here, we develop an empirical model of multi-factors in which the western pole is predicted by ENSO and persistence and the eastern pole is predicted by SM and persistence. This new empirical model outperforms largely the average level of the dynamical models from the North American multi-model ensemble (NMME) project in predicting the peak IOD in boreal autumn, with a correlation coefficient of ∼0.86 and a root mean square error of ∼0.24 °C. Furthermore, the hit rate of positive culminated IOD in this new empirical model is equivalent to that in current NMME models (above 65%), much higher than that for negative culminated IOD. This improvement of skill using the empirical model suggests a perspective for better understanding and predicting the IOD.

Quantifying the dynamic and synergistic effects of low and mid-latitudes signals on regional extreme climate events

Unlike previous studies solely focused on the influences of low or mid-latitudes signals on regional extreme events, this paper used the three-pattern decomposition of global atmospheric circulation (3P-DGAC), taking the 2018 hot summer in East Asia for example, to quantitatively reveal the dynamic and synergistic effects of low and mid-latitudes signals on regional extreme events. By analyzing the anomalous three-dimensional structure of atmospheric circulations in 2018 hot summer, we discovered that the local meridional circulation at low latitudes and zonal circulation at mid-latitudes both produced abnormal downdrafts in East Asia, which were consistent with the anomalous anticyclone in this region. Then, we investigated the relative vorticity advection combined with 3P-DGAC to uncover the dominant circulation patterns, and found the horizontal circulation at mid-latitudes and local meridional circulation at low latitudes were the major contributor to the anomalous anticyclone, accounting for 56% and 44% respectively. This result indicated the 2018 hot summer might be influenced by the synergistic effect of low and mid-latitudes signals. Finally, by investigating sea surface temperature anomaly (SSTA) and mid-latitudes wave train, we considered the tripole-like SSTA pattern in the North Atlantic and positive SSTA in Kuroshio and its extension area (K-KE) as the main causes of horizontal circulation anomalies, and the anomalous meridional circulation was related to the extremely late onset of South China Sea summer monsoon (SCSSM). This study provided a novel way to quantitatively study synergistic effects of low and mid-latitudes signals on regional extreme events.

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.

Observed Air–Sea Turbulent Heat Flux Anomalies during the Onset of the South China Sea Summer Monsoon in 2021

Abstract This study presents observational findings of air–sea turbulent heat flux anomalies during the onset of the South China Sea summer monsoon (SCSSM) in 2021 and explains the mechanism for high-resolution heat flux variations. Turbulent heat flux discrepancies are not uniform throughout the basin but indicate a significant regional disparity in the South China Sea (SCS), which also experiences evident year-to-year variability. Based on buoy- and cruise-based air–sea measurements, high-temporal-resolution (less than hourly) anomalies in the latent heat flux during the SCSSM burst are unexpectedly determined by sea–air humidity differences instead of wind effects under near-neutral and mixed marine atmospheric boundary layer (MABL) stability conditions. However, latent heat anomalies are mainly induced by wind speed under changing MABL conditions. The sensible heat flux is much weaker, with its anomalies dominated by sea–air temperature differences regardless of the boundary layer condition. The observational results are used to examine the discrepancies in turbulent heat fluxes and associated air–sea variables in reanalysis products. The comparisons indicate that latent and sensible heat fluxes in the reanalysis are overestimated by approximately 55 and 3 W m−2, respectively. These overestimations are mainly induced by higher estimates of sea–air humidity/temperature differences. The relative humidity is underestimated by approximately 4.2% in the two high-resolution reanalysis products. The higher SST (near-surface specific humidity) and lower air temperature (specific air humidity) eventually lead to higher estimates of sea–air humidity/temperature differences (1.75 g kg−1/0.25°C), which are the dominant factors controlling the variations in the air–sea turbulent heat fluxes. Significance Statement Air–sea interactions are significant in predicting the onset of East Asian monsoon systems, including the SCSSM. During the SCSSM in 2021, four buoys and cruise observations are used to investigate anomalies in the latent and sensible heat fluxes. The physical mechanism of the variations in turbulent heat fluxes under different MABL stability conditions is explored in this study. The humidity and wind speed anomalies play roles under mixed boundary conditions in determining the high-resolution variations in latent heat fluxes. Based on these observational results, the heat fluxes and associated air–sea variables from reanalysis products are compared to identify the differences in the operational systems. These comparison results can help improve the reanalysis to obtain better monsoon predictions.

Interdecadal Change in the Interannual Variability of South China Sea Summer Monsoon Intensity

Interdecadal Change in the Interannual Variability of South China Sea Summer Monsoon Intensity

Interannual Migration of the Summertime Eastward Jet in the South China Sea Associated With the Upper Layer Thickness Distribution

AbstractThe summertime eastward jet (SEJ) in the South China Sea (SCS) can be regarded as the boundary between the southern and northern upper circulation of the SCS in climatology. The dynamic process of SEJ migration in response to the variation of SCS summer monsoon (SCSSM) strength on the interannual scale is investigated, based on the Simple Ocean Data Assimilation (SODA) dataset and by a 1.5‐layer reduced gravity model. Analysis shows that SEJ position is closely related to SCSSM strength, with southward/northward migration of SEJ coinciding with stronger/weaker SCSSM. Besides migration of wind stress curl (WSC) zero‐line and north‐south difference of WSC variation, the north‐south difference of the SCS upper layer thickness (ULT) in summer, that is, thick in the southern SCS and thin in the northern SCS, also plays a significant role in the dynamic process of SEJ migration. It is the joint effect of wind strength and ULT distribution. With the variation of SCSSM strength, the thinner ULT in the northern SCS corresponds to the larger magnitude of potential vorticity (PV) variation inputted by wind stress, which leads to more pronounced strengthening/weakening of the western boundary current (WBC) in the northern SCS than that in the southern SCS. Then, the north and south WBCs compete with each other until the upper layer circulation reaches a new equilibrium state, and SEJ migrates southward/northward accordingly.

Response of Future Summer Marine Heatwaves in the South China Sea to Enhanced Western Pacific Subtropical High

AbstractMarine heatwaves (MHWs) are prevalent in the South China Sea (SCS) and are primarily caused by the anomalous western Pacific subtropical high (WPSH) which suppresses the SCS summer monsoon. Our study investigates future summer MHWs in the SCS in response to the enhanced WPSH. The results show that the WPSH will be strengthened in most models under the highest emission scenario. Total days of summer SCS MHWs will significantly increase with the enhancement of the WPSH and peak at the end of the 21st century. The high‐value area of total days of summer MHWs (>50 days) appears in the south‐central SCS. The intensity of MHWs exhibits little response to the strengthening WPSH. In addition, the weakening of the SCS cold filament due to the anomalous WPSH also favors summer MHWs. Furthermore, increased future severe summer SCS MHWs will create a higher bleaching risk for coral reefs.

Regional Characteristics of Summer Precipitation Anomalies in the Northeastern Maritime Continent

Based on the monthly mean reanalysis data from NCEP/NCAR (National Centers for Environmental Prediction/ National Center for Atmospheric Research) and GPCP (Global Precipitation Climatology Project) (1979–2020), the regional characteristics of precipitation in the warm pool side of the Maritime Continent (MC) and the relationships between different precipitation patterns and atmospheric circulations are studied. The results show that there are significant correlations as well as differences between the precipitation in the east of the Philippines (area A) and that in the Pacific Ocean near the Northern Mariana Islands (area B). Precipitation in area A is closely related to the eastern Pacific ENSO (El Nino-Southern Oscillation) and EAP/PJ (East Asia-Pacific/Pacific-Japan) teleconnection pattern, while precipitation in area B is linked to the Indian Ocean basin-wide and the South China Sea summer monsoon. When the precipitation anomaly in area A is positive, the East Asian summer monsoon is weak. A cyclone appears to the northwest of area A at 850 hPa with convergence airflow. After filtering out the effects of precipitation in area B, the cyclone retreats to the west, and an anticyclone appears to the southeast of area A. When the precipitation is above normal in area B, the circulation and water vapor transportation are similar to that in area A but more to the east. The updraft and downdrafts to both north and south sides of area B form two closed meridional vertical circulations. When the influence of area A is moved out, the circulation center in the warm pool area moves eastward. This research contributes to a better understanding of the regional characteristics of the Maritime Continent and the East Asian summer monsoon.

Seasonal–interannual–decadal variations of sea level in the South China Sea and connections with the tropical–subtropical Pacific

AbstractDifferent timescale contributions to the sea‐level variation in the South China Sea (SCS) are evaluated in this paper. On seasonal timescale, the large variance of the sea‐level anomaly (SLA) could account for more than 70% of the monthly sea‐level variation in the western shallow continental shelf and the area of Luzon cold eddy. Besides, the seasonal explained variance percentage (skill) has 2 weak centres (50%–70%) off eastern Vietnam, which is related to the summertime eddy pair in the SCS. On interannual‐to‐decadal (low‐frequency) timescale, the skill has two large‐value regions (Zone A and Zone B). Zone A is characterized by double centres off eastern Vietnam, but the meridional position of the double centres is farther north than that of the eddy pair. The skill in Zone A is in the range of 35%–55%, and about 50%–70% of the low‐frequency variation comes from the steric effect. Zone B is off the west coast of the Philippines. The skill in Zone B increases southward, and it is dominated by the wind‐induced dynamic process which is attributed to the remote Rossby wave from the tropical Pacific. Based on the S‐EOF analysis, the SEOF‐1 mode mainly portrays the decadal variation of the SLA, while the SEOF‐2 mode reflects the interannual variation of the SLA. In Zone B, almost all of the low‐frequency variation could be attributed to ENSO and PDO, especially to PDO which has a larger contribution. However, the double centres in Zone A are governed by the SCS summer monsoon, instead of ENSO and PDO.

Recent Advances in Understanding Multi-scale Climate Variability of the Asian Monsoon.

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.

Relationship between South China Sea Summer Monsoon and Western North Pacific Tropical Cyclones Linkages with the Interaction of Indo-Pacific Pattern

The South China Sea (SCS) summer monsoon (SCSSM) and Western North Pacific tropical cyclones (TCs) are both tropical systems that interact with each other on multiple scales. This study examines the differences in TCs activity characteristics between anomalous strong and weak SCSSM years, and explores the possible mechanisms behind these differences through the coupling relationship between tropical atmospheric circulation and oceanic surface conditions. Results show that the destructiveness of TCs over the Western North Pacific is stronger during weak SCSSM years than in strong years, whereas the opposite occurs for TCs over the SCS. The interaction between the tropical Indo-Pacific ocean and atmosphere plays a key role in the relationship between SCSSM intensity and TCs activity. In strong (weak) SCSSM years, the sea surface temperature anomaly (SSTA) in the tropical Pacific Ocean tends to correspond to a La Niña-like (El Niño-like) distribution, whereas the tropical Indian Ocean shows an Indian Ocean dipole-negative (positive) phase distribution. Moreover, Walker circulations in both the Indian and Pacific Oceans are coupled during these years, which creates a seesaw-like relationship in the conditions for TCs formation between the SCS and the Western Pacific Ocean. During weak SCSSM years, the formation and activity of TCs over the SCS are suppressed due to the weakened water vapor transport caused by abnormal easterly winds from the eastern Indian Ocean to the southern SCS. Meanwhile, the higher SSTA in the Western Pacific Ocean enhances the TCs activity. In strong SCSSM years, the enhanced monsoon drives a stronger monsoon trough, improving the convective environment over the SCS, whereas in contrast, the Western Pacific Ocean is covered by colder water, resulting in poorer conditions for TCs genesis.