East Asian Summer Monsoon Climate Research Articles

The joint impact of PM2.5, O3, and CO2 on the East Asian Summer Monsoon in 2013 and 2018 due to contrasting emission reduction

With the rapid urbanization and the implementation of a series of air pollution control measures, significant changes have occurred in the emission of fine particulate matter (PM2.5), ozone (O3) precursors, and CO2 in China. In this study, we comprehensively assess the impact of emission variations on the East Asian Summer Monsoon (EASM) climate from 2008 to 2018 using a regional-chemistry-ecology model RegCM-Chem-YIBs. The investigations indicated a strengthening of the EASM from 2008 to 2018, with all four EASM Indices exhibiting an increase trend ranging from 0.08 to 1.09 per year based on the simulations. Since 2008, alterations in the emissions of PM2.5 and O3 precursors in 2013 and 2018 led to a rise in near-surface temperatures in China by 0.5–1.5 K, thereby enhancing the EASM. Conversely, the changed CO2 emissions, through its influence on radiative balance and altering BVOCs (Biogenic Volatile Organic Compounds) emissions via its effect on vegetation growth, decreased temperatures by 0.5–1.5 K, thus weakening the EASM. However, when considering the combined effects of total emissions changes in PM2.5, O3 precursors, and CO2, surface temperatures increased by 0.5–2 K, ultimately strengthening the EASM. In addition, compared to 2013, EASM was strongly enhanced in 2018 due to the notable increase in CO2 and significant reduction of PM2.5 and O3 precursors. Our study underscores the significant influence of variations in PM2.5, O3 precursors, and CO2 emissions on EASM climate and emphasizes the importance of coordinated governance of air pollution and climate change.

Features of Radial Growth Rate of Trees in Agro-Pastoral Transition Zone, Northern China

The Vaganov–Shashkin process-based model was used to explore the variation characteristic of the radial growth rate of Pinus tabulaeformis in the agro-pastoral transition zone in northern China. The tree-ring width chronologies of the four sampling sites were significantly positively correlated with the simulated series (p < 0.01), and the simulated onset and end dates of tree radial growth indicated that April to October was the main growing season. Temperature affects the radial growth rate of tree stems at the start and end of the growing season, while soil moisture availability affects the radial growth rate in the main growing season. Despite the differences in amplitude, the integral growth rate showed a bimodal pattern, which to some extent responded to the hydrothermal configuration of the East Asian summer monsoon climate. Compared with the peak changes in the summer monsoon fringe area in Northwest China, the highest peak of the integral growth rate in this study area appeared around August in the late growing season, reflecting the adaptability of trees to the local climatic environment. The average values of the integral growth rate and rate due to soil moisture, inferred from extreme wide-ring and narrow-ring years, were significantly different (p < 0.01), while the average growth rates due to temperature were not significant (p > 0.05). The analysis results indicate that moisture availability is a key limiting factor for the radial growth of Pinus tabulaeformis. Our study provides valuable knowledge about the growth processes of the main tree species related to the hydroclimatic variables in northern China and offers a new perspective on mitigating the adverse effects of a warmer climate on the forest in the semi-arid region in the future.

Response of East Asian summer monsoon climate to North Atlantic meltwater during the Younger Dryas

The Younger Dryas (YD) event, recognized as one of the most typical abrupt climate changes on the millennial time scale, results in striking cooling in most regions of the North Atlantic. The most acceptable hypothesis believes that this event is related to a large volume of meltwater fluxes injected into the North Atlantic. In remote Asia, various paleoclimate reconstructions have revealed that the East Asian summer monsoon (EASM) is significantly depressed during the cold YD episode. However, the effect of North Atlantic meltwater-induced cooling on the whole downstream Eurasian regions and its potential dynamics remains been not fully explored till now. In this study, the responses of Asian climate characteristics during the YD episode, especially the EASM, are evaluated based on modeling data from the Simulation of the Transient Climate of the Last 21,000 years (TraCE 21ka). The results show that the cooling signal during the YD, which is mainly caused by meltwater flux, spreads from the North Atlantic to the whole Eurasia. In agreement with the paleoclimatic proxies, the simulated EASM is obviously weakened. The summer precipitation is also suppressed over East, South, and Central Asia. Dynamically, the North Atlantic cooling produces an eastward propagated wave train across the mid-latitude Eurasia, which facilitates weaker EASM circulation. The weakened land-sea thermal contrast over East Asia also contributes to the monsoon decrease during YD cooling.

Sediment provenance and paleoenvironmental change in the Ulleung Basin of the East (Japan) Sea during the last 21 kyr

Despite the well-reconstructed paleoceanography of the late Quaternary deposits in the East (Japan) Sea (ES), provenances of terrigenous sediments and a comprehensive interpretation of their variations since the Last Glacial Maximum remain unclear, especially in regard to the mainland China- and Taiwan-derived matter. Grain size, conservative trace elements, rare earth elements (REEs), Nd isotope, and clay mineralogy of core sediments from the Ulleung Basin of the ES were investigated for better understanding of detrital sediment provenance and transport process and then their forcing mechanisms over the last 21kyr. Geochemical–mineralogical multi-indices in this study showed notable three-phase changes in sediment provenance, which can be closely related to sea-level fluctuation, development of the Tsushima Warm Current as well as intermittent influence from the East Asian summer monsoon climate evolution. During Unit 1 (21.1–8.4kyr BP) when the Tsushima Warm Current was absent in the ES, the paleo-Changjiang and the paleo-Huanghe mouth might be situated close to the study area and thus mainland China could have played major role in the southwestern Ulleung Basin sedimentation. In particular, the formation of rhythmic dark-colored alternation in Unit 1 is possibly attributable to the pulse of paleo-Huanghe discharge associated with centurial- to millennial-scale variability in the East Asian summer monsoon. In Unit 2 (8.4–7.2kyr BP), these river mouths gradually retreated to their present positions with global sea-level rise, leading to decreasing terrigenous sediment supply from mainland China to the study area. Since 7.2kyr BP (Unit 3), sedimentation in the Ulleung Basin should still be mainly derived from mainland China. Meanwhile, increasing chlorite/kaolinite ratio but decreasing εNd value revealed that terrigenous matter from Taiwan might have been limitedly transported northward to the study area by the fully evolved Tsushima Warm Current. A prominent decline in chlorite/kaolinite ratio, together with contrast change on εNd value, centered at around 2.9kyr BP (3.3–2.4kyr BP) may have been caused by the Tsushima Warm Current suppression, possibly related to the Pulleniatina minimum event popularly found in the study area and nearby seas.

Construction of the OKJ teleconnection index

This study focuses on the construction of an index to measure Okhotsk–Japan (OKJ) teleconnection pattern that mainly occurs in early summer. There are three major results presented here. The first is that four indices were developed to describe the OKJ pattern occurring in the monthly mean height field by using one point correlation and empirical orthogonal function (EOF) analysis. Although there is no substantial difference among them, the third OKJ index (OKJ-III) gives a better result for the OKJ pattern when correlated with height field at 500 hPa in June. Second, there exists a significant positive correlation between the East Asian monsoon index and the OKJ-III in June. The positive OKJ-III is associated with the positive anomalies of precipitation rate to the south of the Yangtze River and Northeast China in June and negative anomalies of the surface air temperature over Japan in June through August and vice versa. All of these coincide well with the features of Rossby wave propagation in OKJ pattern that were summarized in previous studies. This index that simply represents the strength of the OKJ propagation can be used as a diagnostic tool to assess East Asian summer monsoon climate, especially for the month of June. Third, the OKJ pattern can be detected by using the first principal component of the EOF analysis in June 500 hPa height field, and it accounts for large portion of the total variance. Such a strong OKJ signal was not found in the rest of the summer months, i.e., July and August. These results strongly suggest the importance of the OKJ pattern in the East Asian circulation of early summer.

Impact of air‐sea interaction on East Asian summer monsoon climate in WRF

This study investigates the effects of air‐sea interaction on the simulated East Asian summer monsoon (EASM) climate in a regional climate model. An ocean mixed layer model with a revised surface roughness length formulation that was originally designed for tropical cyclone simulation and a prognostic sea surface skin temperature scheme that considers the heat budget at the water surface are systematically evaluated on the monsoonal climate over East Asia for July 2006 in the regional Weather Research and Forecasting (WRF) model. Also, 9‐year (2000–2008) June–August simulations are performed to evaluate the overall impacts of these three components on the simulated EASM climatology. The 1 month simulation for July 2006 reveals that the inclusion of the ocean mixed layer model cools the water surface due to enhanced mixing, in particular, when winds are strong. Such cooling is largely compensated by the inclusion of prognostic skin temperature since solar heating in daytime overwhelms the cooling in nighttime. The revised surface roughness length effectively reduces the surface heat flux by reducing the exchange coefficients, against the conventional Charnock formula. Consideration of the three components together results in the reduction of systemic biases of excessive precipitation and weakening of the North Pacific high in the summer climate from 2000 to 2008. It is concluded that the methodology designed in this study can be an efficient way to represent the air‐sea interaction in regional atmospheric models for numerical weather prediction and climate simulation.

Evolution and variability of the East Asian summer monsoon during the Pliocene: Evidence from clay mineral records of the South China Sea

The Late Pliocene is thought to be characterized by the simultaneous intensification of both the East Asian winter monsoon (EAWM) and East Asian summer monsoon (EASM). However, the evolution of the EASM during the Pliocene remains still controversial and only little is known about the dynamics of the EASM during the Pliocene on orbital time scales. Here we use clay mineral assemblages in sediments from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS) to obtain proxy records of past changes in the EASM climate during the Pliocene. Provenance analysis suggests that illite, chlorite and kaolinite originated mainly from the Mekong River drainage area. Smectite was derived mainly from the Indonesian islands. The kaolinite/illite ratio and the chemical index of alteration (CIA) of siliciclastic sediments allowed us to reconstruct the history of chemical weathering and physical erosion of the Mekong River drainage area and thus, the evolution of the EASM during the Pliocene. Our clay minerals proxy data suggests a stronger EASM during the Early Pliocene than during the Late Pliocene. We propose that the long-term evolution of the EASM has been driven by global cooling rather than the uplift of the Tibetan Plateau. Spectral analysis of kaolinite/illite ratio displays a set of strong periodicities at 100 ka, 30 ka, 28 ka, 25 ka, and 22 ka, with no clear obliquity-related signal. Our study suggests that the Pliocene EASM intensity on orbital time scales is not only controlled by the Northern Hemisphere summer insolation, but also strongly influenced by equatorial Pacific ENSO-like ocean–atmosphere dynamics.

East Asian summer monsoon simulation by a 20-km mesh AGCM

East Asian summer monsoon climate simulated by a global 20-km mesh atmospheric general circulation model (AGCM) forced by the global sea surface temperature during the period 1979–1998 is investigated. In comparison with a lower resolution (180-km mesh) model experiment, it is revealed that the 20-km mesh AGCM shows the superiority in simulating orographic rainfall not only its location but also its amount. The Baiu frontal structure is also better simulated in the higher resolution model, which leads to stronger Baiu rainfall. The 20-km model also shows more intense extremes in precipitation. Interannual variability of June–August mean precipitation and seasonal march of the monsoon rain band are also investigated.

South and East Asian Summer Monsoon Climate and Variation in the MRI Coupled Model (MRI-CGCM2)

Simulations of the major characteristics of the summer monsoon climate over South Asia, East Asia, and the western North Pacific by the new version of the Meteorological Research Institute coupled GCM (MRI-CGCM2) are analyzed. In addition to assessing the simulation of mean summer monsoon rainfall and its association with SST and basic circulation parameters, the model's performance in reproducing the seasonal variation, climatological onset, peak, withdrawal, and subseasonal variation of the monsoon is also studied. The mean rainfall distribution and the maximum rainfall centers over South Asia and the western North Pacific, including the monsoon rainbelt over central India, though shifted southward of its observed position by about 5°, and the baiu rainband across Japan, are well simulated. However, the model underestimates the monsoon rainfall over southeast China. The biases in model mean rainfall are found to be associated with biases in wind, moisture convergence, and vertical stability of the atmosphere. The relationship between mean SST and organized convection in the model is close to the observations, with a propensity for deep convection increasing with an increase in SST above the threshold value. The simulated summer anomalies of SST and surface fluxes imply the dominance of the SST–wind–evaporation feedback system over most of the Indian and western Pacific Oceans on intraseasonal time scales. In addition, in the model, equatorial eastern Pacific SST anomalies strongly influence the Indian summer monsoon predominantly on biennial time scales. The model captures the basic monsoon seasonal cycle, onset, peak, and withdrawal over India and the western North Pacific across Japan. However, the simulation shows an early onset of the monsoon over mainland China and the early occurrence of peak rainfall over southeast China. The active–break spells in Indian monsoon rainfall and meridional propagations of rainbelts over India and the western North Pacific are realistically simulated. But, over southeastern China, the model is unable to simulate northward propagation of rainbelts, which contributes to the poor simulation of seasonal mean rainfall.

Molecular fossils in a Pleistocene river terrace in southern China related to paleoclimate variation

The red paleosols (Oxisols) in southern China provide a record of global climate oscillation during at least the last 0.80 Ma. Here we present a correlation between the marine oxygen isotope record (stage 11–21) and the paleosol molecular fossil signature in the fourth Xiushui river terrace in southern China. Temporal variations seen in two molecular ratios—C15–21/C22–33n-alkanes and pristane to phytane (Pr/Ph)—down the paleosol profile are postulated to have arisen from variations in biological input and subsequent pedogenesis in response to the changing strength of the East Asian summer monsoon climate. The preliminary findings suggest strong couplings among the soil, vegetation and climate, and provide a basis for a correlation between the terrestrial and oceanic records.