Search Results (19)

Midlatitude westerly and East Asian summer monsoon (EASM) are crucial circulation systems in the upper and lower troposphere of East Asia that significantly influence mid-summer precipitation pattern. However, their synergistic effect on mid-summer precipitation in North China (NC) remains unclear. In this study, the concurrent variations of mid-summer westerly and EASM are categorized into two configurations: strong westerly–strong EASM (SS) and weak westerly–weak EASM (WW). At the synoptic timescale, the SS configuration significantly enhances precipitation in NC, whereas the WW configuration suppresses mid-summer rainfall. The underlying mechanism is that the SS pattern stimulates an anomalous quasi-barotropic cyclone–anticyclone pair over the Mongolian Plateau–Yellow Sea region. Two anomalous water vapor channels (westerly-driven and EASM-driven water vapor transport) are established in the southern and western peripheries of this cyclone–anticyclone pair, ensuring abundant moisture supply over NC. Meanwhile, frequently occurring westerly jet cores in northern NC form a jet entrance region, favoring strong upper-level divergent pumping and deep accents in its southern flank. This synergy between strong westerlies and EASM enhances both the moisture transports and ascending movements, thereby increasing precipitation over NC. Conversely, the atmospheric circulation associated with the WW pattern exhibits opposite characteristics, resulting in decreased NC rainfall. Our findings elucidate the synoptic-scale influences of westerly–monsoon synergy on mid-summer rainfall, through regulating moisture transports and westerly jet-induced dynamic uplift, potentially improving predictive capabilities for mid-summer precipitation forecasting. Full article

Based on statistical analyses of long-term reanalysis data, we have investigated the interdecadal variations of autumn precipitation in central China (APC-d) and the associated atmospheric teleconnection. It reveals that the increased autumn rainfall in central China during the last decade is a portion of the APC-d, which exhibits a high correlation coefficient of 0.7 with the interdecadal variations of the Europe–China pattern (EC-d pattern) teleconnection. The EC-d pattern teleconnection presents in a “+-+” structure over Eurasia, putting central China into the periphery of a quasi-barotropic anticyclonic high-pressure anomaly. Driven by positive vorticity advection and the inflow of warmer and moist air from the south, central China experiences enhanced ascending motion and abundant water vapor supply, resulting in increased rainfall. Further analysis suggests that the EC-d pattern originates from the exit of the North Atlantic jet and propagates eastward. It is captured by the Asian westerly jet stream and proceeds towards East Asia through the wave–mean flow interaction. The wave train acquires effective potential energy from the mean flow by the baroclinic energy conversion and simultaneously obtains kinetic energy from the basic westerly jet zones across the North Atlantic and the East Asian coasts. The interdecadal variation of the mid-latitude North Atlantic sea surface temperature (MAT-d) exhibits a significant negative relationship with EC-d, serving as a modulating factor for the EC-d pattern teleconnection. Experiments with CMIP6 models predict that the interdecadal variations in APC-d, EC-d, and MAT-d will maintain stable high correlations for the rest of the 21st century. These findings may contribute to forecasting the interdecadal autumn dry–wet conditions in central China. Full article

A major challenge for climate system models in simulating the impacts of El Niño–Southern Oscillation (ENSO) on the interannual variations of East Asian rainfall anomalies is the wide inter-model spread of outputs, which causes considerable uncertainty in physical mechanism understanding and short-term climate prediction. This study investigates the fidelity of 40 models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) in representing the impacts of ENSO on South China Spring Rainfall (SCSR) during the ENSO decaying spring. The response of SCSR to ENSO, as well as the sea surface temperature anomalies (SSTAs) over the tropical Indian Ocean (TIO), is quite different among the models; some models even simulate opposite SCSR anomalies compared to the observations. However, the models capturing the ENSO-related warm SSTAs over TIO tend to simulate a better SCSR-ENSO relationship, which is much closer to observation. Therefore, models are grouped based on the simulated TIO SSTAs to explore the modulating processes of the TIO SSTAs in ENSO affecting SCSR anomalies. Comparing analysis suggests that the warm TIO SSTA can force the equatorial north–south antisymmetric circulation in the lower troposphere, which is conducive to the westward extension and maintenance of the western North Pacific anticyclone (WNPAC). In addition, the TIO SSTA enhances the upper tropospheric East Asian subtropical westerly jet, leading to anomalous divergence over South China. Thus, the westward extension and strengthening of WNPAC can transport sufficient water vapor for South China, which is associated with the ascending motion caused by the upper tropospheric divergence, leading to the abnormal SCSR. Full article

The jet stream is a primary factor contributing to turbulence, especially for upper-level aircraft. This study utilized pilot reports and ERA5 data from 2023 to investigate the relationship between upper-level turbulence and the East Asian westerly jet (EAJ). The results indicate that approximately 45.9% of upper-level aircraft turbulence occurs within the jet stream, with the lowest proportion in August and the highest in January. Additionally, the strongest vertical wind shear (VMS) is found concentrated in the lower part of the jet stream core, particularly in the South–Down part of the jet stream, where upper-level aircraft turbulence occurs most frequently (27.1%). The most turbulent area is located between 30–40° N and 110–120° E, with the main air routes experiencing turbulence being the Henan sections of G212 and B208. From a seasonal perspective, there is less frequent occurrence of upper-level aircraft turbulence in summer and autumn but more in winter and spring. The EAJ volume increases with the strengthening of the jet core wind speed, with the jet core regions being most distinct at altitudes of 200~300 hPa. Meanwhile, the jet stream intensity index peaks at 70.6 m/s in January and reaches its lowest value of 7.1 m/s in August. The jet stream axis shifts southward in winter and northward in summer, reaching the southernmost position in December at 32.2° N and the northernmost position in August at 43.5° N. Furthermore, the VMS at turbulence points within the jet stream is higher than that at the turbulence points outside the jet stream, and the Richardson number (RI) is lower. Moreover, the temporal distribution of upper-level aircraft turbulence is primarily determined by the location and intensity of the jet stream, of which the jet stream intensity index provides guidance and thus serves as a reliable indicator. Full article

The summer Asian westerly jet (AWJ)’s shifting in latitudes is one important characteristic of its variability and has great impact on the East Asian summer climate. Based on the observed and reanalyzed datasets from the Hadley Center Sea Ice and Sea Surface Temperature dataset (HadISST), the Japanese 55-year reanalysis (JRA-55), and the fifth generation of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5), this study investigates the relationship between the spring tripole North Atlantic SST (TNAT) anomalies and the summer meridional shift of the AWJ (MSJ) for the period of 1958–2020. Through the method of correlation analysis and regression analysis, we show that the ‘+ - +’ TNAT anomalies in spring could induce a northward shift of the AWJ in the following summer. However, such a climatic effect of the spring TNAT anomalies on the MSJ is unstable, exhibiting an evident interdecadal strengthening since the early 1990s. Further analysis reveals that this is related to a strengthened intensity of the spring TNAT anomalies in the most recent three decades. Compared to the early epoch (1958–1993), the stronger spring TNAT anomalies in the post epoch (1994–2020) could cause a stronger pan-tropical climate response until the following summer through a series of ocean–atmosphere interactions. Through Gill responses, the resultant more prominent cooling in the central Pacific in response to the ‘+ - +’ TNAT anomalies induces a pan-tropical cooling in the upper troposphere, which weakens the poleward gradient of the tropospheric temperature over subtropical Asia. As a result, the AWJ shifts northward via a thermal wind effect. By contrast, in the early epoch, the spring TNAT anomalies are relatively weaker, inducing weaker pan-tropical ocean–atmosphere interactions and thus less change in the meridional shit of the summer AWJ. Our results highlight a strengthened lagged effect of the spring TNAT anomalies on the following summer MSJ and have important implications for the seasonal climate predictability over Asia. Full article

A remarkable intraseasonal reversal of temperature anomaly is witnessed in eastern China in early 2022, characterized by a warm January and a cold February. ERA5 daily reanalysis data, multiple regression and the Linear Baroclinic Model (LBM) are employed to investigate the characteristics and causes of this abnormal temperature Pattern. The findings indicate that: (1) The two Rossby wave trains along the south and north westerly jets over Eurasia have synergistic impacts on middle and high latitudes. In January, the south branch Rossby wave train exhibited a positive phase, coinciding with a negative phase in the north branch wave train. As a result, the south trough strengthens, while the north trough weakens, leading to anomalous warm advection that warms eastern China. In February, the phases of these two Rossby waves are reversed, causing anomalous cold advection as the southern trough diminishes and the northern trough intensifies, resulting in colder conditions in eastern China. (2) Tropical convection activity weakens in January, whereas it intensifies in February in the northeast Indian Ocean. The weakening of the East Asian trough as a result of the convective latent heat anomalies caused an anticyclonic circulation over the Korean Peninsula in January through the Pacific-Japan teleconnection-like pattern, which is necessary for the maintenance of warm anomalies. Conversely, increased convective activity in February induces cyclonic circulation, deepening the East Asian trough over the Korean Peninsula and contributing to the persistence of cold anomalies. (3) The Rossby wave trains along the two westerly jets and the tropical convective activity in the northeastern Indian Ocean work in tandem, simultaneously strengthening or weakening the East Asian trough. Consequently, the East Asian trough weakens in January and strengthens in February. Full article

The spring sensible heating (SSH) over the Tibetan Plateau (TP), which can significantly affect the precipitation in China, has experienced three different stages of change, showing significant increasing (1961–1979, Stage I), decreasing (1980–2002, Stage II), and increasing (2003–2014, Stage III) trends. In this study, the impact of these different trends in TP SSH on spring precipitation (SPR) in China and their possible mechanisms are investigated, based on observations and the reanalysis product. In Stage I, the SPR represents a contrasting north-south pattern associated with the increasing TP SSH, showing increasing trends over southern China and decreasing trends over central and northern China. Further, the spatial distribution of SPR trends shows a contrasting east-west pattern in Stage II. That is, persistent weakening TP SSH plays a more crucial role in increasing and decreasing SPR over southwestern and southern China, respectively. However, compared with the significant trend in SPR in Stage III, the regulation of TP SSH on SPR weakens significantly. Dynamically, the increasing TP SSH in Stage I can strengthen the subtropical westerly jet in the upper layer, simultaneously configured with an anomalous cyclone in northeastern China, which deepens the East Asian trough. Thus, anomalous convergence in the upper layer occurs over central and northern China, favoring the downdraft. It then causes more cold and dry air to move southward in the lower troposphere, which then encounters the warm and wet southwest airflows, boosting the updraft over southern China. In Stage II, regression analysis shows that if the TP SSH increases, an anomalous cyclone in the middle and upper troposphere occurs over the western TP, causing the downdraft and less precipitation over southwestern China, while a cyclone in the lower troposphere occurs over the western North Pacific and extends to southern China, promoting the ascending motions and more precipitation in southern China. However, in this stage, TP SSH actually weakens, thus contributing to more precipitation over southwestern China and less precipitation over southern China. Full article

The uneven water resource distribution between different areas across the globe has been worsening. The area where the eastern margin of the Asian westerly jet area meets the low-altitude area of the transition zone (which has a temperate continental monsoon climate) is delicate and sensitive to climate change. An urgent issue is to determine the climate change pattern of this area in the past. On the basis of core samples of four tree species in four typical regions of this joint area, we built a standardized chronological table according to tree-ring width and reconstructed the cumulative precipitation from March to August series in the above area in the past 203–343 years. Since the reconstructed results corresponded well to climate events and drought and flooding periods in historical records, the reconstructed model was stable and reliable. The results were as follows: The precipitation in the study area from east to the west in the growth season has changed dramatically, and the period has gradually shortened. In the 19th century, there was a wet period in the high-latitude area. From the 19th century to the 1950s, the entire study area experienced a significant dry period that lasted for 20–45 years; the starting time of the dry period was gradually delayed from the west to the middle, and the wet period gradually grew shorter at an increasing rate. In the past half-century, this area experienced a significant drought period, and the drying rate was higher in the west and east regions than in the central region. In the past two centuries, the precipitation varied significantly in the study area, and the wet period played a dominant role, growing gradually shorter. The middle and west regions of the Asian westerly jet area and the high-latitude regions of the transition zone all experienced significant wetting or drying processes in the first half of the 19th century, and since then, all areas experienced significant drying processes under the influence of global warming, which may be intensified by the westerly circulation. Full article

The middle reaches of the Yellow River (MRYR) are an important base for agricultural and husbandry production and coal and coal-based power and chemical industries. Understanding the variability of autumn (especially September) precipitation over the MRYR region and the associated atmospheric circulation anomalies and precursory signals is of great importance for the prevention and mitigation of meteorological disasters during autumn rainy season. This study primarily explored precursory signals for September precipitation over the MRYR from the perspectives of sea surface temperature (SST) and soil moisture (SM) anomalies. The results reveal that the northward-shifted East Asian westerly jet (EAWJ) and the strengthened and westward-extended western Pacific subtropical high (WPSH) are responsible for more precipitation over the MRYR region. Further analyses show that the September MRYR precipitation is significantly related to the preceding July–August southern Pacific SST pattern (SPSP) and Yunnan-Guizhou Plateau (YGP) SM. The preceding SPSP anomaly, which reflects the La Niña/El Niño-like SST anomalies, can be maintained until September and plays an important role in modulating the September MRYR precipitation. Moreover, the above SST anomalies may adjust the SM anomalies in the YGP during July–August. The SM anomalies in The YGP persist from July–August to September and eventually affect the MRYR precipitation through exciting an anomalous vertical motion during September. The effect of the preceding SPSP anomaly on the September MRYR precipitation decreases when the SM effect is absent, which suggests that the YGP SM anomalies act as a bridge linking the preceding Pacific SST anomalies and the ensuing September MRYR precipitation. This study discloses the joint contribution of the preceding Pacific SST and YGP SM anomalies to the September MRYR precipitation and may shed new light on the short-term prediction of autumn precipitation over the MRYR. Full article

Central Asia is one of the most important sources of mineral saline dust worldwide. A comprehensive understanding of Central Asian dust transport is essential for evaluating its impacts on human health, ecological safety, weather and climate. This study first puts forward an observation-based climatology of Central Asian dust transport flux by using the 3-D dust detection of Cloud-Aerosol LiDAR with Orthogonal Polarization (CALIOP). The seasonal difference of transport flux and downstream contribution are evaluated and compared with those of the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). Central Asian dust can be transported not only southward in summer under the effect of the South Asian summer monsoon, but also eastward in other seasons under the control of the westerly jet. Additionally, the transport of Central Asian dust across the Pamir Plateau to the Tibetan Plateau is also non-negligible, especially during spring (with a transport flux rate of 150 kg m⁻¹ day⁻¹). The annual CALIOP-based downstream contribution of Central Asian dust to South Asian (164.01 Tg) is 2.1 times that to East Asia (78.36 Tg). This can be attributed to the blocking effect of the higher terrain between Central and East Asia. Additionally, the downstream contributions to South and East Asia from MERRA-2 are only 0.36 and 0.84 times that of CALIOP, respectively. This difference implies the overestimation of the wet and dry depositions of the model, especially in the low latitude zone. The quantification of the Central Asian dust transport allows a better understanding of the Central Asian dust cycle, and supports the calibration/validation of aerosol-related modules of regional and global climate models. Full article

The six-hourly ERA-interim reanalysis data were used to analyze the intraseasonal oscillation (ISO) characteristics of the zonal wind in the exit region of the East Asian subtropical westerly jet (EAJ) during the winter (November to April). The results indicate that from East Asia to the North Pacific, the zonal wind in the upper troposphere shows significant 10–40-day oscillations, propagating eastward toward the jet exit region. The strength of the intraseasonal zonal wind anomaly increases from the lower troposphere to the upper troposphere, reaching a peak between 300 and 200 hPa. The zonal wind ISO in the jet exit area is closely related to the intraseasonal inverse temperature tendency between the north and south of the jet exit in the troposphere. In the acceleration (deceleration) phase of the intraseasonal west wind, the air temperature decreases (increases) in the north of the exit and increases (decreases) in the south of the exit. The intraseasonal temperature tendency is stronger in the north of the EAJ exit than that in the south. In the north of the EAJ exit, the intraseasonal temperature tendency is decided by the temperature advection, where the whole troposphere is controlled by the north wind in the west wind acceleration phase and controlled by the south wind in the west wind deceleration phase, so the intensity of temperature advection is strong. However, adiabatic heating plays a decisive role in affecting the temperature evolution in the south of the jet exit area, and the intraseasonal meridional wind is the opposite between the mid-upper troposphere and the lower troposphere, resulting in weak temperature advection and the weak temperature tendency. Therefore, although the zonal wind ISO in the jet exit area is the result of the joint action of the ISOs in different latitudes, the influence of mid-high latitudes is particularly important. Full article

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. Full article

Based on daily precipitation data from 115 climate stations, seasonal and annual precipitation and their extremes over the Tibetan Plateau and its surroundings (TPS) in 1963–2015 are investigated. There exists a clear southeast-northwest gradient in precipitation and extreme daily precipitation but an opposite pattern for the consecutive dry days (CDDs). The wet southeast is trending dry while the dry center and northwest are trending wet in 1963–2015. Correspondingly, there is a drying tendency over the wet basins in the southeast and a wetting tendency over the dry and semi-dry basins in the center and northwest in summer, which will affect the water resources in the corresponding areas. The increase (decrease) in precipitation tends to correspond to the increase (decrease) in maximum daily precipitation but the decrease (increase) in CDDs. Extreme precipitation events with 20-year, 50-year, 100-year, and 200-year recurrence occurred frequently in the past decades especially in the 1980s. The greatest extreme precipitation events tend to occur after the late 1990s and in the southeastern TPS. The ERA5 reanalysis and climate system indices reveal that (1) decreased moisture transports to the southeast in summer due to the weakening of the summer monsoons and the East Asian westerly jet; (2) increased moisture transports to the center in winter due to the strengthening of the winter westerly jet and north Atlantic oscillation; and (3) decreased instability over the southeast thus suppressing precipitation and increased instability over the northwest thus promoting precipitation. All these are conducive to the drying trends in the southeast and the wetting trends in the center. Full article

The increasing anthropogenic aerosols (AAs) over East Asia have caused significant regional climate responses, but the role of urban land-use changes which occur simultaneously, in altering these AA-induced changes, is not well understood. Here, the modulation of the AAs’ effect on the East Asian winter (November–January) climate by the urban cover in eastern China was investigated using the Community Atmosphere Model version 5.1 coupled with the Community Land Model version 4. Results show that the winter sulfate aerosol burden is higher from central eastern China to southern Japan in the case with the presence of urban cover than in the case without it, resulting from urban-induced circulation changes. Such aerosol changes markedly increase the cloud fraction and precipitation over northern China and the adjacent ocean to the east, especially convection activities around southern Japan. This leads to a cooling effect near the surface over northern China and in the mid-upper troposphere to the east due to aerosol direct and indirect effects. The resulting circulation responses act to shift the mid-tropospheric East Asian trough southward and the upper-level East Asian westerly jet-stream as well, further supporting the surface changes. These winter climate responses to the urban-modulated aerosols can largely offset or even reverse those to the AAs forcing without the urban cover in the model, especially in northern East Asia. This study highlights the need to consider the modulating role of urban land-use changes in assessing the AAs’ climatic effect over East Asia and other regions. Full article

We obtained a 15 m drill core from Deukryang Bay on the southwest coast of Korea, which is now an area of reclaimed land used for agriculture. We investigated changes in the depositional environment and hydrological climate responses to sea level changes using sedimentary facies, radiocarbon ages, grain-size analysis, total organic carbon (TOC), total sulfur (TS), and stable carbon isotopes (δ¹³C). Sediment deposition began at 12,000 cal yr BP and was divided into four stages based on changes from fluvial to intertidal environments related to Holocene marine transgression events. Stage 1 (>10,000 cal yr BP) is represented by fluvial sediments; Stage 2 (10,000–7080 cal yr BP) is represented by the deposition of mud facies in an intertidal zone in response to sea level rise; Stage 3 (7080–3300 cal yr BP) was a period of gradually descending sea level following the Holocene maximum sea level and is characterized by gradual changes in TOC, TS, and C/S ratios compared with the mud facies of Stage 2. Stage 4 (3300 to present) was deposited in a supratidal zone and contains low TS and an abundance of TOC. Based on our TS and C/S ratio results, the south coast of Korea was mainly affected by sea level rise between 7000 and 3000 cal yr BP, during the middle Holocene. At 3000 cal yr BP, sea level began to stabilize or gradually decrease. In addition, changes in δ¹³C values are clearly observed since ca. 5000 cal yr BP, in particular, large hydrological changes via freshwater input are confirmed in 4000–3000 cal yr BP. We consider these shifts in freshwater input indicators of an increased influence of El Niño and La Niña conditions, related to the weakening of the East Asian Summer Monsoon (EASM) and changes in sea surface temperature (SST) of the Western Pacific Ocean during the middle Holocene climatic optimum (between 7800 and 5000 cal yr BP). The cooling periods of SST in East Asia between 8400 and 6600 cal yr BP reported from the west coast of Korea are related closely to changes in vegetation (as evidenced by δ¹³C) from 7700 cal yrs BP to the present in the southwest coast of Korea. We interpret the freshwater input events at 4000–3000 cal yr BP to be related to changes in SST in response to the weakening of the EASM on the southwest coast of Korea. However, additional research is needed to study the southward migration effect of the westerly jet related to SST and atmospheric circulation controlling terrestrial climate in the middle Holocene. Full article