East Asia Western North Pacific Research Articles

Coherent amplification of the Okhotsk high, Korean trough, and northwestern Pacific subtropical high during heavy rainfall over Japan in August 2021

In August 2021, rain front stagnation in Japan resulted in prolonged and disastrous rainfall across the entire country. During the heavy rainfall period, the large-scale atmospheric field over the East Asian–western North Pacific region was characterized by meridional tripolar circulation anomalies: the Okhotsk high (OH), the trough over the Korean Peninsula (Korean trough), and the northwestern Pacific subtropical high (NWPSH). Simultaneously, tropical convective activity was enhanced over the eastern Indian Ocean and suppressed over the tropical western–central Pacific. This study investigates the dynamic mechanism of linkage of the extratropical tripolar anomalies and the effects of tropical convective modulation using a reanalysis dataset, a cutoff low detection scheme, the potential vorticity inversion method, and numerical experiments. Upper-tropospheric blocking over eastern Siberia connected to the surface OH is conducive to the stagnation of synoptic depressions, including cutoff lows and troughs, over the Korean Peninsula, contributing to the development and maintenance of the quasi-stationary Korean trough. Rossby waves emanating from the Korean trough excite an anticyclonic anomaly over the northwestern Pacific. This upper-level anomalous anticyclone acts to enhance the surface NWPSH through zonal heat transport, accompanied by a northward tilting structure with height. Simultaneously, the tropical intraseasonal oscillation is amplified over the Indo–western Pacific Ocean sector under the negative-phase Indian Ocean dipole and multi-year La Niña conditions. The combination of enhanced convection over the eastern Indian Ocean and suppressed convection across the tropical western–central Pacific reinforces the NWPSH. The anomalous circulation associated with the extratropical tripolar pattern and concurrent tropical heat forcing causes more moisture transport, convergence, and anomalous ascent, which contribute to heavy rainfall in Japan. These results suggest that the dynamically correlated amplification of tropical and extratropical circulation anomalies plays a crucial role in precipitation variability in East Asia.

Possible Impact of Early Spring Arctic Sea Ice on Meiyu Cessation over the Yangtze–Huaihe River Basin

The timing of the cessation of Meiyu is closely connected to the amount of Meiyu rainfall and the commencement of the rainy season in North China. Accurately forecasting the Meiyu withdrawal date (MWD) over the Yangtze–Huaihe River basin (YHRB) has significant implications for the prevention and mitigation of flooding in eastern China. This study observed an intimate out-of-phase relationship between MWD variations and early spring (March and April) Arctic Sea ice area (SIA) anomalies to the north of the Chukchi and Beaufort Seas, as well as SIA anomalies to the north of the Queen Elizabeth Islands (75° N–82° N, 170° E–130° W and 82° N–86° N, 130° W–80° W, respectively) on the interannual timescale. As such, these can be considered key Arctic Sea ice domains connected to Meiyu cessation in the YHRB. The Arctic SIA anomalies in the key domains persist from early spring to early summer (May and June), thus exerting a lag modulation effect on year-to-year changes in Meiyu cessation, which can be demonstrated through observational analysis and results from the Community Earth System Model Large Ensemble Numerical Simulation (CESM-LENS) project. Specifically, the preceding negative SIA anomalies in the key domains are linked to a planetary-scale Rossby wave-like pattern emanating over areas to the northwest of the Chukchi Sea. This tele-connected wave-like pattern is conducive to the generation and maintenance of a quasi-barotropic “north-low–south-high” meridional see-saw pattern over the East Asian–Western North Pacific sector in July, which is a pivotal circulation pattern responsible for delayed Meiyu termination. Furthermore, the situation is the opposite in response to increased sea ice in these key domains within the Arctic. This study proposes a significant cryospheric forcing indicator for Meiyu cessation over the YHRB, which may provide helpful information for operational forecasting of the withdrawal timing of the Meiyu over the YHRB.

Impacts of the East Asian winter monsoon on winter precipitation variability over East Asia-western North Pacific

Impacts of the East Asian winter monsoon on winter precipitation variability over East Asia-western North Pacific

Inter‐model spreading of changes in East Asian winter monsoon circulation under 1.5 and 2.0°C global warming targets

AbstractThis study investigates inter‐model spreading of changes in East Asian winter monsoon circulation and the underlying reasons under 1.5 and 2.0°C global warming for Representative Concentration Pathway 8.5. Results show that the spatial patterns of multi‐model ensemble and inter‐model spreading of changes in 850‐hPa zonal and meridional winds over East Asia are similar under the two global warming targets. The multi‐model ensemble changes in 850‐hPa meridional wind (V850) over southern China are near zero under the two targets. Meanwhile, the V850 changes over southern China exhibit large inter‐model spreading, which is closely related to those of regional differences in sea surface temperature changes (DSSTC) in the tropical Pacific. Models with positive V850 anomalies over southern China tend to project weak (strong) surface warming in the western North Pacific (South China Sea and the equatorial central and eastern Pacific). Such a configuration results in decreased precipitation over the east of the Philippines and hence an anomalous lower‐level high to its northwest. Accordingly, southwesterly anomalies prevail over southern China. The southwesterly anomalies are further enhanced by related positive feedback processes. Models with negative V850 anomalies tend to project strong (weak) surface warming in the western North Pacific (South China Sea). The projected surface warming in the equatorial central and eastern Pacific is also strong, but weaker than that of models with positive V850 anomalies. Therefore, the induced precipitation and circulation changes over the East Asia–western North Pacific are basically opposite to those described above. Besides, the inter‐model spreading of V850 changes over southern China decrease by about 36.2% from 1.5 to 2.0°C global warming, which can be partly attributed to the decrease in the inter‐model spreading of DSSTC in the tropical Pacific. Numerical experiments confirm the role of the sea surface temperature changes in the tropical Pacific.

Summertime Moisture Sources and Transportation Pathways for China and Associated Atmospheric Circulation Patterns

Based on the Lagrangian particle dispersion model, HYSPLIT 4.9, this study analyzed the summertime atmospheric moisture sources and transportation pathways affecting six subregions across China. The sources were: Midlatitude Westerly (MLW), Siberian-Arctic regions (SibArc), Okhotsk Sea (OKS), Indian Ocean (IO), South China Sea (SCS), Pacific Ocean (PO), and China Mainland (CN). Furthermore, the relative contributions of these seven moisture sources to summertime precipitation in China were quantitatively assessed. Results showed that the CN precipitation source dominates the interannual and interdecadal variation of precipitation in most subregions, except Southwest and South China. The Northeast China vortex and Pacific–Japan (PJ) teleconnection, which transport water vapor from the MLW, OKS and PO sources, are crucial atmospheric systems and patterns for the variation of precipitation in Northeast China. The interannual variation of precipitation in Northwest and North China is mainly dominated by mid–high-latitude Eurasian wave trains, which provide the necessary dynamical conditions and associated moisture transport from the MLW and SibArc sources. In addition, an enhanced western North Pacific subtropical high (WNPSH) accompanied by the East Asian–western North Pacific summer monsoon and PJ teleconnection, transports extra moisture to North China from the SCS and PO sources, as well to the Yangtze River Valley and South China. The Indian summer monsoon (ISM) is also critically important for the interdecadal change in precipitation over the Yangtze River Valley and South China, via the southwesterly branch of moisture transport from the IO source. The interdecadal changes in precipitation over Southwest China are determined by the IO and SCS sources, via enhanced WNPSH coupling with a weakened ISM. These results suggest that the interdecadal and interannual variations of moisture sources contribute to the attendant variation of summertime precipitation in China via large-scale circulation regimes in both the mid–high and lower latitudes.

Impact of orbitally-driven seasonal insolation changes on Afro-Asian summer monsoons through the Holocene

Understanding what drives a shift of the Afro–Asian summer monsoons from the continents to oceanic regions provides valuable insight into climate dynamics, changes, and modeling. Here we use data–model synthesis to focus on the differential seasonal responses of solar insolation and monsoons to orbital changes in the Holocene. We observe coordinated and stepwise seasonal evolution of summer monsoons across the mid-Holocene, suggesting the strengthening of the midlatitude jet stream as a bridge in the upper troposphere. Prior to the mid-Holocene, insolation had decreased considerably in early summer; the continental monsoons migrated southeastward, which corresponded to a more pronounced rainy season in coastal East Asia. In late summer, insolation did not decrease until the mid-Holocene. The continued weakening of the continental monsoons, combined with weakened insolation, rapidly enhanced the intrinsic dynamics over East Asia–Western North Pacific and accelerated a large-scale migration of the monsoon, suggesting orbital control of seasonal diversity.

Influence of Tibetan Plateau autumn snow cover on interannual variations in spring precipitation over southern China

Using observational data from 1979–2015 and a linear baroclinic model (LBM), the present study examined the impact of autumn snow cover (ASC) over the western Tibetan Plateau (WTP) on the subsequent spring precipitation over southern China (SPSC). Focus was placed on the interannual time scale of the snow-precipitation relationship. The ASC over the WTP is positively correlated with the SPSC, and this snow-precipitation relationship is independent of sea surface temperature anomalies over the tropical Indo-Pacific regions. When the ASC over the WTP is more extensive than normal, in the following spring, anomalous southerly winds over eastern China occur and transport water vapor northward from the tropical ocean to the interior of the continent, which is favorable for more SPSC. Further analysis of the energy budget shows that the anomalous ASC over the WTP persists to the following spring because of local positive snow-air feedback. Higher-than-normal ASC over the WTP can cool the above atmosphere and is associated with pronounced negative geopotential height anomalies that dominate in the East Asian-western North Pacific region. These negative height anomalies lead to a weakened and northward-shifted spring East Asian subtropical jet (EASJ). The changes in the spring EASJ lead to divergent anomalies at the upper level over the upstream as well as the central areas of the EASJ; therefore, anomalous low-level convergence and ascent motion dominate over coastal East Asia and in nearby regions, providing a favorable environment for positive SPSC anomalies.

Obliquity-driven changes in East Asian seasonality

Abstract Obliquity, as a factor controlling seasonality, has been considered a modulator of paleomonsoon evolution. However, in contrast to the clearly identified contribution of precession, East Asian proxy records rarely provide a robust signal of obliquity. By investigating climate simulations with extremely low versus high obliquity, this study examined the seasonal contribution of obliquity, finding that obliquity has substantial effects on the diversity of precipitation stages over East Asia–Western North Pacific (WNP). The sensitivity of greenhouse gases (GHG) to the effects of obliquity also suggests that the contribution of obliquity could depend on GHG concentrations. When GHG forcing is weak, summer precipitation migrates following the obliquity-driven meridional insolation changes over Africa, South Asia, and East Asia. In contrast to a condition of high obliquity with a continental origin of the monsoon, summer monsoon precipitation is confined to the subtropical area including the existence of the WNP monsoon trough in low obliquity. The intrinsic dynamic mode over East Asia–WNP, characterized by a meridional dipole pattern in circulation and precipitation, disappears when obliquity is extremely high. In winter, the strength of synoptic scale (the midlevel trough, Siberian high, and Aleutian low over and around East Asia) and larger scale (midlatitude westerly jet stream and Hadley circulation) circulation is coincidently weaker when compared with that in high obliquity. Obliquity-driven changes over East Asia–WNP are also partly sensitive to GHG concentrations. Furthermore, some major obliquity-driven changes weaken and even disappear when GHG concentrations increase.

Long-lead ENSO control of the boreal summer intraseasonal oscillation in the East Asian-western North Pacific region

The boreal summer 30–60-day intraseasonal oscillation (ISO) in the East Asian-western North Pacific (EAWNP) region strongly influences persistent heavy rainfall in East Asia and tropical cyclone activities in the tropical western Pacific. In this study, we show that there exists a significant interannual south-north shift of the EAWNP-ISO activity between the equatorial eastern Indian Ocean—Maritime continent region and the subtropical South China Sea—western Pacific. This is reflected by the year-to-year changes in the occurrence frequency of individual ISO phases that correspond to different latitudinal locations of the convection anomaly. This interannual shift is largely controlled by the El Niño—Southern Oscillation (ENSO) in the preceding winter and spring. Following an El Niño event, the ISO convective activity tends to occur in the equatorial eastern Indian Ocean—Maritime continents, whereas after a La Niña, frequent ISO-related precipitation occurs in the region of south China and the subtropical South China Sea—western Pacific. This result implies long-lead predictability for the occurrence of the summertime EAWNP-ISO activity and its associated weather.

Decadal Transition of the Leading Mode of Interannual Moisture Circulation over East Asia–Western North Pacific: Bonding to Different Evolution of ENSO

AbstractThe El Niño–Southern Oscillation (ENSO) cycle has a great impact on the summer moisture circulation over East Asia (EA) and the western North Pacific [WNP (EA-WNP)] on an interannual time scale, and its modulation is mainly embedded in the leading mode. In contrast to the stable influence of the mature phase of ENSO, the impact of synchronous eastern Pacific sea surface temperature anomalies (SSTAs) on summer moisture circulation is negligible during the 1970s–80s, while it intensifies after 1991. In response, the interannual variation of moisture circulation exhibits a much more widespread anticyclonic/cyclonic pattern over the subtropical WNP and a weaker counterpart to the north after 1991. Abnormal moisture moves farther northward with the enhanced moisture convergence, and thus precipitation shifts from the Yangtze River to the Huai River valley. The decadal shift in the modulation of ENSO on moisture circulation arises from a more rapid evolution of the bonding ENSO cycle and its stronger coupling with circulation over the Indian Ocean after 1991. The rapid development of cooling SSTAs over the central-eastern Pacific, and warming SSTAs to the west over the eastern Indian Ocean–Maritime Continent (EIO-MC) in summer, stimulates abnormal descending motion over the western-central Pacific and ascending motion over the EIO-MC. The former excites an anticyclone over the WNP as a Rossby wave response, sustaining and intensifying the WNP anticyclone; the latter helps anchor the anticyclone over the tropical–subtropical WNP via an abnormal southwest–northeast vertical circulation between EIO-MC and WNP.

Impact of the intra‐seasonal oscillation on tropical cyclone genesis over the western North Pacific

This study investigates the impact of the East Asian–western North Pacific (EAWNP) intra‐seasonal oscillation (ISO) on tropical cyclone (TC) genesis over the South China Sea (SCS) and the western North Pacific (WNP). The result indicates that the 20–70‐day oscillation (EAWNP‐ISO1) mainly modulates the frequency of TC genesis, while the 10–20‐day quasi‐biweekly oscillation (EAWNP‐ISO2) usually influences the genesis location. The impacts of ISO are not only on the individual TC genesis but also on the intra‐seasonal variation of TC activity.In order to better monitor and predict TC genesis, we group the ISO intensity into three categories (wet, normal, and dry) using the EAWNP ISO‐1 index and classify the ISO location into three categories (south, middle, and north) using the EAWNP ISO‐2 index. By combining these six categories, nine combinations are defined to further investigate how the ISO impact on TC genesis. It was found that through affecting the strength of monsoon trough and the location of the western Pacific subtropical high, the ISO combinations modulate the thermodynamic conditions and lead to changes in frequency and location of TC genesis.The relative contributions of four variables of the genesis potential index (GPI) tend to be dependent on ISO conditions. When the ISO conditions are favourable (unfavourable) to TC genesis, the mid‐level humidity and low‐level vorticity contribute positively (negatively) to the GPI anomalies over the SCS and WNP region, while the vertical wind shear, except for in the north SCS, and the potential intensity contribute negatively (positively). During the normal ISO conditions, the contributions of these four variables are generally weaker than those under the other two backgrounds.

Time-varying structure of the wintertime Eurasian pattern: role of the North Atlantic sea surface temperature and atmospheric mean flow

The Eurasian (EU) pattern was shown to be closely linked to the North Atlantic sea surface temperature (SST) in boreal winter, and this study suggest that such linkage varies with time and that the structure of the EU pattern also changes accordingly. During the period with strong EU-SST linkage (1961–1977 and 1993–2013, denoted as HIGH epoch), the EU pattern is a well-defined barotropic wave train stretching from the North Atlantic towards East Asia-western North Pacific, and it facilitates cooling and slight reduction of precipitation over East Asia and enhanced precipitation over Europe in its positive phase. During the period with weak EU-SST linkage (1948–1960 and 1978–1992, denoted as LOW epoch), in contrast, its centre over Northern Europe weakens sharply, and the wave-like structure of the EU pattern is not well established. In this case, its influences on precipitation over Europe almost vanish, but its influences on both the surface air temperature and precipitation over East Asia amplify significantly. The Atlantic centre of the EU pattern forms from different mechanism during the two epochs. It can be excited by the underlying North Atlantic SST via eddy-mediated processes during the HIGH epoch, while it is likely generated by an incoming Rossby wave train from upstream region during the LOW epoch. Nevertheless, its location and intensity remain similar during the two epochs. Numerical experiments with a barotropic model suggest that the contrasting structure of the EU pattern is mainly attributed to the changes of the atmospheric mean flow.

Characteristics and mechanisms of the diurnal variation of winter precipitation in Taiwan

Using gridded precipitation data remapping from rain‐gauge observations of the time period of 2001–2015 winter months (December, January and February), this study examines the spatio‐temporal characteristics of the climatological diurnal winter precipitation in Taiwan. Our results show that the timings of the maximum values of the diurnal precipitation in Taiwan exhibit clear east–west regional differences, with night‐time maxima over eastern Taiwan but early morning maxima over western Taiwan. Analyses also show that the climatological characteristics of diurnal winter precipitation documented in this study are not controlled by some unusual, heavy precipitation events and can be seen as the common features. By examining the changes in meteorological variables (including winds, relative humidity and temperature) extracted from the in situ observations and the MERRA (Modern‐Era Retrospective Analysis for Research and Applications) reanalysis, this study further suggests that (a) the radiative cooling effect and (b) the surface wind convergence induced by the interaction between the local orography, the local land–sea breeze and the large‐scale diurnal circulation changes over the East Asian–western North Pacific (EAWNP) region are two of the possible formation mechanisms responsible for the spatio‐temporal differences of the climatological diurnal winter precipitation in Taiwan. This finding sheds light on the importance of understanding the role of the large‐scale diurnal circulation changes over the EAWNP region in modulating local diurnal precipitation properties during the winter months.

Large-scale control of the Arabian Sea monsoon inversion in August

The summer monsoon inversion in the Arabian Sea is characterized by a large amount of low clouds and August as the peak season. Atmospheric stratification associated with the monsoon inversion has been considered a local system influenced by the advancement of the India–Pakistan monsoon. Empirical and numerical evidence from this study suggests that the Arabian Sea monsoon inversion is linked to a broader-scale monsoon evolution across the African Sahel, South Asia, and East Asia–Western North Pacific (WNP), rather than being a mere byproduct of the India–Pakistan monsoon progression. In August, the upper-tropospheric anticyclone in South Asia extends sideways corresponding with the enhanced precipitation in the subtropical WNP, equatorial Indian Ocean, and African Sahel while the middle part of this anticyclone weakens over the Arabian Sea. The increased heating in the adjacent monsoon systems creates a suppression effect on the Arabian Sea, suggesting an apparent competition among the Africa–Asia–WNP monsoon subsystems. The peak Sahel rainfall in August, together with enhanced heating in the equatorial Indian Ocean, produces a critical effect on strengthening the Arabian Sea thermal inversion. By contrast, the WNP monsoon onset which signifies the eastward expansion of the subtropical Asian monsoon heating might play a secondary or opposite role in the Arabian Sea monsoon inversion.

The multidecadal variations of the interannual relationship between the East Asian summer monsoon and ENSO in a coupled model

This study investigates the multidecadal variations of the interannual relationship between the East Asian summer monsoon (EASM) and El Nino-Southern Oscillation (ENSO) in 1000-year simulation of a coupled climate model. The interannual relationship between ENSO and EASM has experienced pronounced changes throughout the 1000-year simulation. During the periods with significant ENSO-EASM relationship, the ENSO-related circulation anomalies show a Pacific-Japan (PJ)-like pattern with significant wave-activity flux propagating from the tropics to the north in lower troposphere and from the mid-latitudes to the south in upper troposphere. The resultant ENSO-related precipitation anomalies are more (less) than normal over the East Asia (western North Pacific) in the decaying summers of El Nino events. In contrast, the circulation and precipitation anomalies are weak over East Asia-western North Pacific during the periods with weak ENSO-EASM relationship. Based on the energy conversion analysis, the related anomalies achieve barotropic and baroclinic energy from the mean flow during the periods with strong ENSO-EASM relationship. On the contrary, during the low-correlation periods, the energy conversion is too weak to form the link between the tropics and mid-latitudes. The main reason for the multidecadal variations of ENSO-EASM relationship is the amplitude discrepancy of SST anomalies over the Indo-western Pacific Ocean which, in turn, leads to the intensity difference of the western North Pacific anomalous anticyclone (WPAC) and related climate anomalies.

A Robustness Analysis of CMIP5 Models over the East Asia-Western North Pacific Domain

Abstract The Coupled Model Intercomparison Project (CMIP) is an international community-based infrastructure that supports climate model intercomparison, climate variability, climate prediction, and climate projection. Improving the performance of climate models over East Asia and the western North Pacific has been a challenge for the climate-modeling community. In this paper, we provide a synthesis robustness analysis of the climate models participating in CMIP-Phase 5 (CMIP5). The strengths and weaknesses of the CMIP5 models are assessed from the perspective of climate mean state, interannual variability, past climate change during the mid-Pliocene (MP) and the last millennium, and climate projection. The added values of regional climate models relative to the driving global climate models are also assessed. Although an encouraging increase in credibility and an improvement in the simulation of mean states, interannual variability, and past climate changes are visible in the progression from CMIP3 to CMIP5, some previously noticed biases such as the ridge position of the western North Pacific subtropical high and the associated rainfall bias are still evident in CMIP5 models. Weaknesses are also evident in simulations of the interannual amplitude, such as El Nino-Southern Oscillation (ENSO)-monsoon relationships. Coupled models generally show better results than standalone atmospheric models in simulating both mean states and interannual variability. Multi-model intercomparison indicates significant uncertainties in the future projection of climate change, although precipitation increases consistently across models constrained by the Clausius-Clapeyron relation. Regional ocean-atmosphere coupled models are recommended for the dynamical downscaling of climate change projections over the East Asia-western North Pacific domain.

The dependence on atmospheric resolution of ENSO and related East Asian-western North Pacific summer climate variability in a coupled model

The authors present results for El Nino-Southern Oscillation (ENSO) and East Asian-western North Pacific climate variability simulated in a new version high-resolution coupled model (ICM.V2) developed at the Center for Monsoon System Research of the Institute of Atmospheric Physics (CMSR, IAP), Chinese Academy of Sciences. The analyses are based on the last 100-year output of a 1000-year simulation. Results are compared to an earlier version of the same coupled model (ICM.V1), reanalysis, and observations. The two versions of ICM have similar physics but different atmospheric resolution. The simulated climatological mean states show marked improvement over many regions, especially the tropics in ICM.V2 compared to those in ICM.V1. The common bias in the cold tongue has reduced, and the warm biases along the ocean boundaries have improved as well. With improved simulation of ENSO, including its period and strength, the ENSO-related western North Pacific summer climate variability becomes more realistic compared to the observations. The simulated East Asian summer monsoon anomalies in the El Nino decaying summer are substantially more realistic in ICM.V2, which might be related to a better simulation of the Indo-Pacific Ocean capacitor (IPOC) effect and Pacific decadal oscillation (PDO).

Role of the Indochina Peninsula Narrow Mountains in Modulating the East Asian–Western North Pacific Summer Monsoon

Abstract Unrealistic topographic effects are generally incorporated in global climate simulations and may contribute significantly to model biases in the Asian monsoon region. By artificially implementing the Arakan Yoma and Annamese Cordillera—two south–north-oriented high mountain ranges on the coasts of the Indochina Peninsula—in a 1° global climate model, it is demonstrated that the proper representation of mesoscale topography over the Indochina Peninsula is crucial for realistically simulating the seasonality of the East Asian–western North Pacific (EAWNP) summer monsoon. Presence of the Arakan Yoma and Annamese Cordillera helps simulate the vertical coupling of atmospheric circulation over the mountain regions. In late May, the existence of the Arakan Yoma enhances the vertically deep southwesterly flow originating from the trough over the Bay of Bengal. The ascending southwesterly flow converges with the midlatitude jet stream downstream in the southeast of the Tibetan Plateau and transports moisture across the Indochina Peninsula to East Asia. The existence of the Annamese Cordillera helps the northward lower-tropospheric moisture transport over the South China Sea into the mei-yu–baiu system, and the leeside troughing effect of the mountains likely contributes to the enhancement of the subtropical high to the east. Moreover, the eastward propagation of wave energy from central Asia to the EAWNP suggests a dynamical connection between the midlatitude westerly perturbation and mei-yu–baiu. Including the Annamese Cordillera also strengthens a Pacific–Japan (PJ) pattern–like perturbation in late July by enhancing the cyclonic circulation (i.e., monsoon trough) in the lower-tropospheric western North Pacific. This suggests the contribution of the mountain effects to the intrinsic variability of the summer monsoon in the EAWNP.

Interdecadal variability of early summer monsoon rainfall over South China in association with the Pacific Decadal Oscillation

ABSTRACTThe interdecadal variability of early summer South China (SC) monsoon rainfall (SCMR) is investigated based on several long‐term rainfall data sets from 1926 to 2013. Both correlation and composite analyses suggest a significant out‐of‐phase relationship between interdecadal fluctuations of SCMR and the preceding wintertime Pacific Decadal Oscillation (PDO), with negative (positive) PDO epochs favouring above‐normal (below‐normal) SCMR. The physical mechanism relating the wintertime PDO to the interdecadal variability of SCMR appears to take place via atmospheric and oceanic teleconnections. The below‐normal SCMR is characterized by a meridionally arranged vortex (MAV) pattern across the East Asia–western North Pacific (WNP) sector, with an anomalous cyclone, an anomalous anticyclone and an anomalous cyclone occurring alternately over the South China Sea (SCS)–WNP, Yangtze Basin and Lake Baikal in the early summer. This may respond to the PDO‐related positive sea surface temperature (SST) anomalies locally over the SCS–WNP and remotely over the tropical eastern Pacific, with the latter forcing anomalous Hadley and Walker circulations. SC is located directly under the divergent moisture environment of anomalous northeasterlies between the anomalous cyclone over the SCS–WNP and the anticyclone over the Yangtze Basin. Again, the below‐normal SCMR is preceded by negative (positive) SST anomalies in the midlatitude North Pacific (tropical eastern Pacific) associated with positive PDO in wintertime. Such an anomalous SST pattern subsequently induces a strengthened and northward‐shifted upper‐tropospheric jet in early summer, leading to significant upper‐tropospheric convergence over SC. The out‐of‐phase relationship between SCMR and wintertime PDO as well as related physical processes are validated by coupled model simulations.

Monitoring early-flood season intraseasonal oscillations and persistent heavy rainfall in South China

Rainfall variability during the early-flood season (April–June) in South China is largely controlled by both the 10–20 and 20–70-day intraseasonal oscillations (ISO). In this study, a method is described to monitor the ISO and persistent heavy rainfall in South China. Three existing daily real-time 20–70-day ISO indices are compared. It is found that the regional East Asia–western North Pacific (EAWNP) ISO index best represents the early-flood season 20–70-day ISO in South China. A new bivariate boreal summer ISO index is designed to describe the 10–20-day ISO in the EAWNP region. Composite analysis shows that the rainfall anomaly in South China is well captured by the northward propagation of both the 10–20 and 20–70-day ISO. With different phase combinations of the 10–20 and 20–70-day EAWNP ISO, nine conditions are defined ranging from those favorable to those unfavorable to heavy rainfall in South China that can be used to effectively monitor the early-flood season ISO and persistent heavy rainfall in South China.