South China Sea Summer Monsoon Onset 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.

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.

Seasonal Forecast of South China Sea Summer Monsoon Onset Disturbed by Cold Tongue La Niña in the Past Decade

It has been suggested that a warm (cold) ENSO event in winter is mostly followed by a late (early) onset of the South China Sea (SCS) summer monsoon (SCSSM) in spring. Our results show this positive relationship, which is mainly determined by their phase correlation, has been broken under recent rapid global warming since 2011, due to the disturbance of cold tongue (CT) La Nina events. Different from its canonical counterpart, a CT La Nina event is characterized by surface meridional wind divergences in the central-eastern equatorial Pacific, which can delay the SCSSM onset by enhanced convections in the warming Indian Ocean and the western subtropical Pacific. Owing to the increased Indian-western Pacific warming and the prevalent CT La Nina events, empirical seasonal forecasting of SCSSM onset based on ENSO may be challenged in the future.

Regional Characteristics of Cloud Radiative Effects before and after the South China Sea Summer Monsoon Onset

The South China Sea summer monsoon (SCSSM) onset is characterized by rapid thermodynamical changes in the atmosphere that are critical to regional weather and climate processes. So far, few studies have focused on the changes in the associated cloud and radiative features. This study investigates spatiotemporal characteristics of top-of-atmosphere (TOA) cloud radiative effects (CREs) before and after the SCSSM onset over the South China Sea (SCS) and South China (SC), based on the 2001–2016 Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) satellite data and ERA-Interim reanalysis data. Before the SCSSM onset, strong net CRE (NCRE) dominated by its cooling shortwave component occurs over SC, while descending motion and weak NCRE prevail over the SCS. In the SCSSM onset pentad, convection, high clouds, and longwave and shortwave CREs (LWCRE and SWCRE) abruptly increase over the southern and central SCS, and their high-value centers subsequently move northeastward and are strongly affected by the western Pacific subtropical high. The strong offset between LWCRE and SWCRE enables the NCRE intensity (TOA radiation budget) to be quite small (large) between the SCS and the western North Pacific after the SCSSM onset. In contrast, low-middle-level clouds and strong cooling SWCRE remain over SC after the SCSSM onset, but the increasing high clouds and LWCRE weaken (intensify) the regional NCRE (TOA radiation budget) intensity. These marked latitudinal differences in CREs between the SCS and SC primarily arise from their respective dominant cloud types and circulation conditions, which manifest the differences between the tropical SCSSM and subtropical East Asian monsoon processes. The results indicate that regional cloud fractions and CREs before and after the SCSSM onset are strongly modulated by quickly changed large-scale circulation over the East Asian monsoon regions, and the spatiotemporal variation of CREs is a response to the monsoonal circulation adjustment to a large extent.

Revisiting the relationship between Indo‐Pacific heat content and South China Sea summer monsoon onset during 1980–2020

AbstractThe relationship of the tropical Indo‐Pacific upper ocean heat content (HC) and the South China Sea (SCS) summer monsoon (SCSSM) onset during 1980–2020 was investigated. It was found that when HC anomalies in the preceding winter–spring are positive (negative) in the eastern Indian–western Pacific Ocean or negative (positive) in the southwestern Indian Ocean and eastern Pacific Ocean, SCSSM onset is usually early (late). Empirical orthogonal function (EOF) analysis suggested that the primary EOF modes of HC anomaly in both tropical Pacific and Indian Oceans are associated with El Niño–Southern Oscillation (ENSO). It is through modulating the westerly over the north Indian Ocean–SCS and the western north Pacific subtropical high (WNPSH) via the Walker circulation over the Indo‐Pacific basin, monsoon circulation in the Indian Ocean that the ENSO related Indo‐Pacific HC variation connects with the SCSSM onset. Both the second EOF modes for the tropical Pacific and Indian Ocean also show significant correlations with the SCSSM onset. The former related vertical motion over the SCS–far northwestern Pacific can influence the WNPSH and convection, thus occurrence of the SCSSM. The latter represents the intrinsic Indian Ocean HC variability; the air temperature variation associated with it can modulate the atmospheric circulation in the Indian Ocean–SCS through thermal wind balance, and thus the SCSSM onset by acting on the westerly over the SCS and the WNPSH. A western Pacific index and an Indian Ocean dipole index were defined to represent the leading HC modes in the tropical Pacific and Indian Ocean, respectively. A binary linear regression model was constructed using these two indices to improve SCSSM onset prediction.

Impact of the March Arctic Oscillation on the South China Sea summer monsoon onset

AbstractThis study reveals that the South China Sea summer monsoon (SCSSM) onset is closely linked to the preceding March Arctic oscillation (AO), the dominant mode of atmospheric circulation over extratropical Northern Hemisphere. The physical processes for the impact of the March AO on the SCSSM onset are further examined. March AO leads to an anomalous low‐level cyclone over subtropical western North Pacific (WNP) via eddy–mean flow interaction. The southwesterly wind anomalies to the southeastern side of the anomalous cyclone weaken the climatological trade winds, which contribute to warm sea surface temperature (SST) anomalies over tropical western‐central North Pacific via reduction of surface evaporation and latent heat flux. These warm SST anomalies excite an atmospheric Rossby wave response and reinforce the anomalous cyclone over the subtropical WNP. Via the above positive atmosphere–ocean interaction, the anomalous cyclone induced by the March AO could persist through the whole spring and shift equatorward. This anomalous cyclone creates favourable environment for the SCSSM onset via weakening the WNP subtropical high and is conducive to the transition of the low‐level zonal wind. Further analysis indicates that the March AO‐SCSSM onset relationship is independent of El Niño‐Southern Oscillation (ENSO). This result suggests that, besides the dominant tropical system (i.e., ENSO), interannual variation of the SCSSM onset also has a close relation with the dominant extratropical atmospheric system (i.e., AO), which may provide additional source for the seasonal prediction of the monsoon onset.