Silk Road Pattern Research Articles

Different mechanisms for the extremely hot central-eastern China in July–August 2022 from a Eurasian large-scale circulation perspective

In July and August of 2022, unprecedented and long-lasting heatwaves attacked central and eastern China (CEC); and the most affected area was in the Yangtze River (YR) basin. The extreme heatwaves and associated drought and wildfire had significant social impacts, but the underlying mechanisms remain unknown. Observational analysis indicates that the heatwaves were regulated by anomalous anticyclone in the mid-upper troposphere over northern CEC. Specifically, the easterly anomalies at the southern flank of the anticyclone caused air isentropic sliding and transported low moist enthalpy (cold and dry) air to the YR basin, contributing to anomalous sinking motions and extreme heatwaves. In comparison, heatwaves were more serious in August than in July due to stronger upper-level anomalous anticyclone and associated easterlies. Importantly, different mechanisms were responsible for the heatwaves in the two months. In July, the relatively weaker anticyclonic anomaly over northern CEC was dominated by the forcing of diabatic heating over northwestern South Asia (NWSA), corresponding with the record-breaking rainfall in and around Pakistan. In August, a powerful anticyclonic condition for the CEC heatwaves originated from an extreme silk road pattern (SRP), superposing the effect of NWSA diabatic heating due to persistent downpour. We notice that another upstream anticyclonic node in the SRP also created heatwaves in Europe. Therefore, the CEC extreme heat was actually associated with other concurrent extremes over the Eurasian continent through large-scale atmospheric teleconnections in 2022.

Why has the summer rainfall increased prominently in the West Tarim Basin of Northwest China since 2010?

Based on the monthly rainfall data at 32 weather stations located in the periphery of the Tarim Basin in Northwest China, which can mostly represent the rainfall variations in the whole basin, provided by the China Meteorological Data Network and the reanalysis data provided by the NCEP/NCAR during 1961–2021, the changes in summer rainfall in the Tarim Basin and associated underlying mechanism have been investigated. Results show that there are two distinct wet periods (1988–2009: Wet Period I; 2010–2021: Wet Period II) of summer rainfall in the Tarim Basin during 1961–2021. The northeast basin has received more summer rainfall during Wet Period I, but during Wet Period II more summer rainfall is concentrated in the west basin, corresponding to different large scale circulation and moisture transport conditions. After the linear trend is removed, the correlation coefficient between the Tarim Basin summer rainfall and the Silk Road pattern index (SRPI) during 2010-2021can reach 0.73, which suggests that the summer rainfall in the west part of the basin is closely related to the Silk Road pattern in this period. In 2010–2021, the meridional wind anomaly center over central Asia in the Silk Road Pattern is located more southward and westward. On the one hand, it causes the location of the anomalous cyclone at 500 hPa over central Asia to stay southward and westward, matching with the anomalous vertical upward motion near 70°E, on the other hand, it also strengthens the anomalous transport of water vapor from the tropical Indian Ocean and Pacific Ocean. The above two provide favorable large scale circulation and abundant water vapor for the increased summer rainfall in the west part of Tarim Basin during 2010–2021.

What Percentage of Silk-Road Pattern Triggers Pacific–Japan Pattern through Rossby Wave Breaking?

In this study, we investigate the rate at which the Silk Road pattern (SRP) with Rossby wave breaking (RWB) near the Asian jet exit causes the Pacific–Japan (PJ) pattern in boreal summer. In this study, the SRP case is detected using the two principal components of upper-tropospheric meridional winds over Eurasia and characterized by the presence of an upper-level anticyclonic anomaly over the Yellow Sea or near Japan. They are further classified into cases with and without RWBs.In the SRP case with RWB, the upper-level anticyclonic anomaly near the Asian jet exit has a more extended shape in the zonal direction and larger amplitude than in the case without RWB. In the composite, a wave train associated with the SRP appears over Eurasia, which is accompanied by the RWB near the Asian jet exit. The occurrence of RWB is associated with strong deceleration and diffluence in the basic state there. The RWB promotes enhanced convection on its southern side due to the intrusion of upper-level high potential vorticity toward the southwest, resulting in the formation of the PJ pattern. The excited PJ pattern in the composite has a dipole structure with cyclonic anomalies to the south and anticyclonic anomalies to the north. Approximately 60–70 % of the SRP case with RWB is accompanied by the PJ patterns.Conversely, in the case of the SRP without RWB, the composite represents a wave train structure over Eurasia but indicates neither enhanced convection south of the RWB nor PJ patterns. Approximately 40–50 % of the SRP case without RWBs is accompanied by the PJ patterns. Hence, the presence of RWBs increases the percentage of the formation of positive PJ patterns by a factor of 1.2–1.7, indicating that the RWB plays an important role in the excitation of PJ patterns.

Intraseasonal oscillations of the Silk Road pattern lead to predictability in East Asian precipitation patterns and the Mei Yu front

The Silk Road pattern (SRP) is analysed on intraseasonal timescales over summer using empirical orthogonal functions (EOFs) of the meridional wind at 200 hPa. The first two principal components explain almost equal amounts of variance, hence both are required to represent the intraseasonal SRP. The associated spatial loadings are 90° out of phase with each other, providing evidence that propagating oscillations are a natural mode of variability of the intraseasonal SRP. This is supported by Hovmöller diagrams of the meridional wind at 200 hPa and by phase diagrams of the first two EOFs, which both show a predominantly eastward-propagating oscillation. The oscillations are identified as plausibly being Rossby waves by means of waveguide theory. The subtropical westerly jet and East Asian rainfall patterns are found to be dependent on the phase of the oscillation: wet anomalies occur to the east of troughs in the jet, which are also regions where local jet entrances cause upper-level divergence via an ageostrophic circulation. Dry anomalies occur to the west of troughs, which are regions of upper-level convergence. The time-delayed location of the summer Mei Yu front relative to its climatology is dependent on the phase of the oscillation: when there is an upper-level trough located over the Korean Peninsula, the Mei Yu front is likely to be located further north than normal 3–8 days later, before returning to its climatological position. This suggests that the phase of the intraseasonal SRP acts as a potential source of predictability of the location of the Mei Yu front, which might allow for better prediction of the associated rains.

Joint effect of the Indian Ocean Dipole and the Silk Road Pattern on Indian rainfall during summer to autumn transition

Variations in Indian summer rainfall pose a threat to water security that can cause widespread socio-economic impacts. Summer rainfall is affected by both tropical and extratropical circulation anomalies owing to India's particular geographical location. Using observational, reanalysis, and CMIP6 model datasets, this study reveals the individual and joint effects of the Indian Ocean Dipole (IOD) and Silk Road Pattern (SRP) on the interannual variability of Indian rainfall during the summer to autumn transition (i.e., September). The results indicate that there is a consistent positive correlation between the IOD and SRP and the second mode of the Indian rainfall anomaly, which features a north–south inverse dipole pattern. During the positive phase of the IOD, the zonal sea surface temperature anomaly (SSTA) gradient of warm west and cold east in the equatorial Indian Ocean drives a Gill-type response, with an anticyclonic anomaly over southern India. This suppresses local convection and results in precipitation deficit across southern India. The SRP exhibits a baroclinic structure over west-central Asia, with an upper (lower) tropospheric anticyclonic (cyclonic) anomaly over west-central Asia (northern India) during its positive phase. This lower-level cyclonic anomaly is drastically amplified under the easterly vertical shear condition, favoring stronger moisture convergence and increased precipitation over northern India. The joint effect of the IOD and SRP explains nearly 29% of the Indian rainfall anomaly, higher than the individual contributions of the IOD (17%) or SRP (12%). These results highlight the need to consider the joint effects of tropical and mid-latitude anomalous drivers on precipitation patterns and provide the basis for more accurate simulation and prediction of Indian rainfall.

Maximum July-August temperatures for the middle of the southern Tien Shan inferred from tree-ring latewood maximum densities.

Long-term temperature reconstructions are urgently needed to prolong meteorological climatic data, which are too short to evaluate the anthropogenic effect on climate change since the Industrial Revolution. The maximum latewood chronology (MXD) of Picea schrenkiana in the middle of the southern Tien Shan was established, and it showed a strong correlation with the mean maximum temperature of the current July to August (TmaxJA), with r = 0.773 (p < 0.001, 1959-2016), which implies that a high temperature in the late growing season could increase the cell wall thickness and lead to high latewood density. Then, the TmaxJA of the middle of the southern Tien Shan was reconstructed over the period of 1720-2018. Three MXD chronologies from Kyrgyzstan significantly correlated with our TmaxJA reconstruction at the interannual scale, and they also showed similar variations on decadal scales. None of these MXD series showed a warming trend in the past century, which was also found in several MXD series from different regions of the world. Spatial correlation analysis revealed that our TmaxJA reconstruction showed significant correlations with that in eastern Asia, southern Europe, and north-western Africa, forming a teleconnection called the Silk Road Pattern. However, moving correlation analysis between our TmaxJA reconstruction and Hokkaido temperature series indicated that this teleconnection was unstable in the past 3 centuries. The volcanic eruptions from the mid-high latitudes in the Northern Hemisphere showed a stronger cooling effect than those from the Southern Hemisphere and the low latitudes of the Northern Hemisphere. The summer North Atlantic Oscillation was also shown to affect the temperature in the Tien Shan to a certain extent.

An Interdecadal Enhancement of the Impact of ENSO on the Summer Northeast Asian Circulation around 1999/2000 through the Silk Road Pattern

Abstract In this study, we find that the negative relationship between El Niño–Southern Oscillation (ENSO) and summer Northeast Asia (NEA; 30°–50°N, 110°–140°E) circulation, especially the geopotential height anomalies in the upper troposphere (H200), is weakened from the early 1970s, remains stable since the middle 1980s, and is strengthened dramatically after 1999/2000. The recent transitions of the ENSO–NEA H200 relationship are closely connected with the variation of circumglobal teleconnection (CGT)/Silk Road pattern (SRP), which is further modified by the interdecadal shift of the ENSO–Indian summer monsoon rainfall (ISMR) relationship and ENSO evolution. During 1980–99 when the continuing ENSOs dominate, the weakened ENSO–ISMR relationship leads to an inactive ENSO-related CGT/SRP wave train in the upper-level troposphere and then a weakened connection between ENSO and H200 over the NEA. On the other hand, when the emerging ENSOs dominate after 1999/2000, the restored ENSO–ISMR relationship reinforces the ENSO-related CGT/SRP wave train and then enhances ENSO–NEA H200 relationship. This mechanism is well simulated in the Atmospheric Model Intercomparison Project models (AMIP) and Pacific Ocean–Global Atmosphere (POGA) experiments.

The Relationship between the North Atlantic Oscillation and the Silk Road Pattern in Summer

Abstract The Silk Road pattern (SRP), which is the leading mode of upper-tropospheric meridional wind anomalies over midlatitude Eurasia, has been widely used to explain the impacts of the summer North Atlantic Oscillation (SNAO) on East Asian climate. However, the relationship between the SNAO and SRP has not been fully elaborated yet. This study classifies the SNAO into two categories according to whether it is closely associated with the SRP or not: the strongly linked category and weakly linked category, on the interannual time scale. The SNAO of the strongly linked category features a concentrated and significant southern pole over the northwestern Europe, and corresponding significant negative (positive) precipitation and upper-tropospheric wind convergence (divergence) anomalies over the northwestern Europe. The wind convergence (divergence) anomalies directly induce the positive (negative) planetary vortex stretching anomalies, which contribute overwhelmingly to positive (negative) Rossby wave source anomalies of the northwestern Europe. These Rossby wave source anomalies, acting as disturbances, further inspire circulation anomalies of surrounding regions, including meridional wind anomalies over the Caspian Sea, which are crucial for the SRP formation. As a result, the downstream SRP is triggered. All these essential features responsible for a strong SNAO–SRP linkage are weak for the weakly linked category. The SNAO–SRP correspondence on the interdecadal time scale is also discussed, and generally similar results are found. Results suggest the importance of shapes for the SNAO southern pole (including the location, the space extent, and the intensity) in determining whether the SNAO can closely link the SRP. Therefore, the shape of the SNAO southern pole should be involved in the discussion of the SNAO’s remote impacts.

The British–Okhotsk Corridor Pattern and Its Linkage to the Silk Road Pattern

Abstract Based on observation and reanalysis datasets, numerical experiments with a simple dynamical model, and climate model outputs, this study investigates the second leading waveguide teleconnection along the summer polar front jet (PFJ) over Eurasia on the interannual time scale, the British–Okhotsk Corridor (BOC) pattern. The BOC pattern explains 20.8% of the total variance over northern Eurasia, which is only slightly lower than the first leading teleconnection, the British–Baikal Corridor pattern. It consists of four zonally oriented action centers over the British Isles, western Russia, northern Siberia, and the Sea of Okhotsk. It is primarily confined to northern Eurasia and leads to wavelike temperature and precipitation anomalies along its routine. Besides, it is occasionally coupled to the dominant waveguide teleconnection along the subtropical jet (STJ), the Silk Road pattern (SRP). A bifurcated wavelike pattern appears over Eurasia when the coupling is strong, with two branches of waves over the PFJ and the STJ, respectively. The fluctuations of the BOC–SRP linkage play a profound role in shaping the dominant climate variability modes over Eurasia. Numerical experiments with a simple dynamical model suggest that the basic flow cannot directly influence the BOC–SRP linkage by affecting the propagation condition of Rossby waves. Nevertheless, the basic flow can indirectly influence the linkage by changing the exciting locations of the BOC pattern through modulating the wave–mean flow interaction at the exit of the Atlantic jet stream. The climate model INMCM4.0 can reproduce the observed BOC–SRP linkage and its time-varying characteristics, supporting the observation and the proposed mechanism. Significance Statement Over Eurasia, extreme summer weather events are often related to long-lasting stagnant atmospheric circulation anomalies in the upper troposphere, which can be well determined by a few recurring modes called atmospheric teleconnections. Atmospheric teleconnections over Eurasia usually propagate along the subtropical jet (STJ) and the polar front jet (PFJ). Although the teleconnections along the STJ have been well understood, the teleconnections along the PFJ are currently not fully understood. This paper investigates the British–Okhotsk Corridor (BOC) pattern, a newly defined major summer teleconnection along the PFJ. The BOC pattern can be occasionally coupled to a dominant teleconnection along the STJ. Fluctuations of this linkage play a profound role in shaping the dominant surface climate variability modes over Eurasia. Observation and reanalysis datasets, numerical experiments with a simple dynamical model, and climate model output are used to understand this linkage in the study.

Summer Precipitation Extremes over the Yellow River Loop Valley and Its link to European Blocking

Characteristics of extreme precipitation over Yellow River Loop Valley (YRLV) and links to European blocking are investigated in this study. Spatial and temporal analysis of extreme precipitation shows that it contributes more than 30% of the total summer precipitation in the YRLV and is characterized by a strong and short period of local rainfall. Most of the extreme rains in the YRLV occur in July and August. Two typical circulation patterns were identified using a k-means clustering method. The extreme precipitation results from the combined actions of intensified high pressure over northeast China (NECH) and the westward extension of the western Pacific subtropical high (WPSH). The intensified southerly flow of the amplified NECH strengthens the water vapor transport induced by the westward extension of the WPSH from the northwest Pacific or Bay of Bengal into the YRLV. The NECH is amplified by the wave energy propagating from European blocking via the Silk Road pattern (SRP). This is the subseasonal cause of extreme precipitation over the YRLV. The composited July and August mean 500 hPa geopotential anomaly pattern for extreme precipitation years shows a high-pressure anomaly over the European continent and a negative phase of the SRP. The former provides a background for the occurrence of European blocking, and the latter explains the preexistence of the NECH and provides a linkage between the activity of European blocking and the subseasonal evolution of the NECH. Thus, the interannual variation in the extreme precipitation over the YRLV is mainly reflected by the phase of the SRP and the stationary waves over Europe.

Impacts of mid-high latitude atmospheric teleconnection patterns on interannual variation of the Tibetan Plateau summer monsoon

The current study investigates the interannual variation of the Tibetan Plateau summer monsoon (TPSM) and its response to atmospheric teleconnection patterns over Eurasia is examined in the present study. The interannual variation of TPSM is characterized by anomalous low pressure and cyclonic convergence over the lower atmosphere of the Tibetan Plateau (TP), as well as increased (decreased) precipitation over the eastern (western) TP. The associated circulation anomalies manifest as an eastward propagating wave train from the North Atlantic to North and East Asia, resembling the North Atlantic-East and North Asia (NAENA) pattern. NAENA pattern is the second leading mode of 200-hPa meridional wind anomalies over the Eurasian continent, which can partly contribute to the variation of TPSM with the explained variance of 30%. When the NAENA pattern is in a positive phase, low pressure and cyclonic convergence over the TP tend to be above normal, together with enhanced (decreased) vertical motion and precipitation over the eastern (western) TP, resulting in a stronger TPSM. Furthermore, it has been discovered that the Silk Road pattern (SRP) also plays a vital role in TPSM. The anomalies associated with SRP are characterized by anomalous low pressure over the lower level atmosphere of western TP. Analysis of the combined effects of SRP and NAENA on TPSM suggests that when the two waves are in-phase configuration, they can significantly change the lower-level low pressure and cyclonic circulation of the TPSM. On the contrary, when the two waves are out of phase, TPSM responds weakly.

Possible Causes of Anomalous Glacier Mass Balance in the Western Kunlun Mountains

AbstractUnder current global warming, most glaciers on the Tibetan Plateau have shown significant mass loss during 2000–2019. However, the average glacier mass balance in the western Kunlun Mountains (WKM) was positive during this period. The causes of these opposing average status and the temporal glacier mass changes over the last several decades in the WKM continue to be the subject of active debate. The long‐term mass balance of the Guliya Ice Cap (Guliya) in the WKM during 1970–2019 was reconstructed using an energy and mass balance model and calibrated ERA5 data, which was calibrated and validated by in situ measurements and geodetic mass balances. Guliya showed a positive mean mass balance during 2000–2019 because annual snowfall was larger than low ablation‐season meltwater caused by low air temperature (T a ). At interannual scales, annual mass balances were equally controlled by ablation‐season precipitation and T a during 1970–2019. From 1970s–1990s to 1990s–2010s, most Tibetan glaciers showed decreased mass balance due to increased ablation season T a , while the WKM glaciers experienced increased mass balance due to increased ablation‐season precipitation related to changes in the Silk Road Pattern. However, during 2000–2019, the annual mass balance of Guliya followed a decreasing trend due to increasing ablation‐season T a . The positive mass balance of the WKM glaciers will cease under future warming due to annual snowfall decreasing below ablation‐season meltwater. These changes will threaten lives and livelihoods in the Tarim Basin, whilst also jeopardizing ice core‐derived climate histories from the WKM glaciers.

Modulation of Coupled Modes of Tibetan Plateau Heating and Indian Summer Monsoon on Summer Rainfall over Central Asia

Abstract It has been suggested that summer rainfall over central Asia (CA) is significantly correlated with the summer thermal distribution of the Tibetan Plateau (TP) and the Indian summer monsoon (ISM). However, relatively few studies have investigated their synergistic effects of different distribution. This study documents the significant correlations between precipitation in CA and the diabatic heating of TP and the ISM in summer based on the results of statistical analysis and numerical simulation. Summer precipitation in CA is dominated by two water vapor transport branches from the south that are related to the two primary modes of anomalous diabatic heating distribution contributed by the TP and ISM precipitation, that is, the “+−” dipole mode in the southeastern TP and the Indian subcontinent (IS), and the “+−+” tripole mode in the southeastern TP, the IS, and southern India. Both modes exhibit obvious midlatitude Silk Road pattern (SRP) wave trains with cyclone anomalies over CA, but with different transient and stationary eddies over South Asia. The different locations of anomalous anticyclones over India govern two water vapor transport branches to CA, which are from the Arabian Sea and the Bay of Bengal. The water vapor flux climbs while being transported northward and can be transported to CA with the cooperation of cyclonic circulation. The convergent water vapor and ascending motion caused by cyclonic anomalies favor the precipitation in CA. Further analysis corroborates the negative south Indian Ocean dipole in February could affect the tripole mode distribution of TP heating and ISM via the atmospheric circulation, water vapor transport, and anomalous Hadley cell circulation. The results indicate a reliable prediction reference for summer precipitation in CA.

Dominant Modes of Tibetan Plateau Summer Surface Sensible Heating and Associated Atmospheric Circulation Anomalies

Based on empirical orthogonal function (EOF) analysis, the dominant modes of variations in summer surface sensible heating (SH) over the Tibetan Plateau (TP), as well as the associated atmospheric circulation anomalies, were investigated in this study. The results show that the first dominant mode of summer SH presents a feature of decadal reduction over the whole TP on an interdecadal time scale, and the second dominant mode is characterized by a zonally asymmetric pattern with positive (negative) SH anomalies in the western (eastern) TP on an interannual time scale. The variations of summer SH are dominated by anomalies in downwelling surface shortwave radiation (DSWR), which are associated with atmospheric circulation changes. The first dominant mode of variation in SH is connected to the interdecadal variation of the Silk Road Pattern (SRP). Further analysis reveals that the interdecadal phase shift of the SRP induces anticyclone circulation to the northeast of the TP, leading to enhanced water vapor supply and convergence over the TP. This can lead to an increase in the total cloud cover, and a reduction in DSWR, contributing to the decadal reduction in SH over the TP. The second dominant mode of variation in SH is related to a stationary teleconnection pattern over the Eurasian continent named the North Atlantic-East and North Asia pattern (NAENA). Corresponding to the positive phase of the NAENA, there is a cyclone anomaly to the west TP, leading to anomalous water vapor convergence (divergence) over the eastern (western) TP. This can result in enhanced (decreased) cloud cover, reduced (increased) DSWR, and therefore, an anomalous decrease (enhancement) in SH over the east (west) of the TP. Furthermore, the southwesterly wind anomaly, which is accompanied by the anomalous cyclone to the west TP, leads to positive SH in the western TP.

Modulation of Atlantic Multidecadal Oscillation on the Interdecadal Variation of South Asian High and Somali Jet in Summer

As two important components of the Asian summer monsoon system, the intensities of South Asian High (SAH) and Somali jet (SMJ) in summer exhibit both interannual and decadal variabilities. On the interdecadal timescale, the temporal evolution of the SAH intensity is in phase with that of the SMJ intensity. By comparison, we find that both of them evolve synchronously with the Atlantic Multidecadal Oscillation (AMO), with AMO cold/warm phases corresponding to the weakening/strengthening of SAH and SMJ. Further diagnoses indicate that the interdecadal variabilities of the SAH and SMJ intensities in summer may be modulated by the AMO phase. Mechanistically, this modulation appears to be achieved via an interdecadal Silk Road pattern (SRP)-like wave train along the Asian westerly jet and Matsuno–Gill tropical atmospheric response. The cold SST anomaly over extratropical North Atlantic related to the AMO firstly induces an anomalous high over Western Europe and produces a well-organized wave train between 30°N and 60°N. The anomalous Iranian Plateau low along with the wave train path leads to a weakened SAH. Besides, the AMO-related cold SST anomalies over tropical North Atlantic cool the tropical tropospheric atmosphere through the moist adjustment process and produce a Matsuno–Gill-like atmospheric response covering the tropical Indian Ocean. Due to the Matsuno–Gill response, subsidence motion anomalies over the central tropical Indian Ocean corresponding to a result in increased lower-level divergence and upper-level convergence are excited over the tropical Indian Ocean. Finally, the tropical Indian Ocean divergence in the lower troposphere leads to the weakened summer SMJ, and the tropical Indian Ocean convergence in the upper troposphere results in the decrease and northward displacement of SAH in summer.

Reduction of mid-summer rainfall in northern India after the late-1990s induced by the decadal change of the Silk Road pattern

Variation of summer rainfall can pose substantial challenges to water resources and agriculture in northern India, which includes the most fertile land in India, i.e. the Ganges Plain. This study identifies a significant rainfall reduction in northern India during its peak rainy season (July and August) after the late-1990s, based on observational and reanalysis data since 1958. This rainfall reduction is about 0.86 mm d−1, equivalent to 8.3% of the climatological mean. We suggest that the decadal reduction of northern Indian rainfall is induced by the decadal change of the Silk Road pattern (SRP), which is an atmospheric teleconnection pattern along the upper-tropospheric Asian westerly jet. After the late-1990s, corresponding with the decadal phase shift of the SRP, there is a cyclonic anomaly over West Asia, which can reduce rainfall in northern India. Furthermore, this rainfall reduction is unprecedented since the beginning of the 20th century, when observational data are available. Possible roles of the lower-tropospheric circulation changes and the Atlantic Multidecadal Oscillation are discussed.

Interdecadal Variations of Different Types of Summer Heat Waves in Northeast China Associated with AMO and PDO

AbstractThe summer heat waves (HWs) in Northeast China (NEC) during 1961–2016 can be classified into two types, namely, wave train HWs and blocking HWs based on the hierarchical clustering algorithm by using ERA-Interim daily datasets. Wave train HWs occurred accompanied by eastward-moving wave trains with a “− + − +” structure formed over Eurasia, while the blocking HWs occurred with blocking circulation anomalies over Eurasia. In general, the blocking HWs could cause the positive temperature anomalies in NEC to last longer than wave train HWs. During the period from 1961 to 2016, the wave train HWs experienced an interdecadal variation from less to more, while the blocking HWs experienced interdecadal variations of less–more–less. Regression analysis and information flow indicate that the interdecadal variation of the wave train HWs is associated with Atlantic multidecadal oscillation (AMO) and Pacific decadal oscillation (PDO), while the interdecadal variation of the blocking HWs is more likely associated with PDO. The positive phase of AMO (negative phase of PDO) could increase the wave train (blocking) HWs by strengthening the zonal wave train similar to the Silk Road pattern (the arched wave train like the polar–Eurasian pattern). The observed results are in agreement with the numerical experiments with the NCAR Community Atmosphere Model, version 5.3.

Comparison of Atmospheric Circulation Anomalies between Dry and Wet Extreme High-Temperature Days in the Middle and Lower Reaches of the Yellow River

Many previous studies have reported that atmospheric circulation anomalies are generally the direct cause of extreme high-temperature (EHT). However, the atmospheric circulation anomalies of EHT days with different humidity and the differences between them are less often discussed, while humidity plays an important role in how people feel in a high-temperature environment. Therefore, this study uses 1961–2016 CN05.1 daily observational data and NCEP/NCAR reanalysis data to classify summer EHT days in China into dry and wet. Furthermore, we investigate the atmospheric circulation anomalies associated with the dry and wet EHT days in the middle and lower reaches of the Yellow River (MLRYR). The results reveal that dry EHT days are likely to be caused by adiabatic heating from anomalous subsidence, while wet EHT days are more likely caused by the low-latitude water vapor and heat anomalies brought by the Western Pacific Subtropical High (WPSH). This may be due to a remarkable westward/southward/narrowed extension of the Continental High (CH)/WPSH/South Asian High (SAH) accompanied by an occurrence of dry EHT day. The opposite pattern is observed for wet EHT days. Moreover, a wave train like the Silk Road pattern from the midlatitudes could affect the dry EHT days, while wet EHT days are more likely to be affected by a wave train from high latitudes. Knowing the specific characteristics of dry and wet EHT days and their associated atmospheric circulations could offer new insights into disaster risk prevention and reduction.

The 2019 Autumn Hot Drought Over the Middle‐Lower Reaches of the Yangtze River in China: Early Propagation, Process Evolution, and Concurrence

AbstractHot droughts (i.e., concurrent droughts and hot anomalies) have drawn much attention recently, but current knowledge about early propagation and concurrence remains limited. The 2019 autumn severe hot drought (2019AHD) over the middle‐lower reaches of the Yangtze River (MLYR) is employed for process‐based analysis, while water vapor transport, heating budgets, and atmospheric dynamics are comprehensively investigated. The main achievements are as follows: (a) The 2019AHD over MLYR is not isolated and is related to early propagation from adjacent North China (NC) two months earlier. (b) Direct physical causes are explored. Drought evolution is related to anomalous water vapor transport dominated by low‐level cyclonic anomalies over the East China Sea. Concurrently, local low‐level hot anomalies over MLYR are related to amplified horizontal advection heating, while near‐surface warming is favored by decreased low cloud cover and enhanced downward shortwave radiation. (c) Regional‐scale concurrence of droughts and hot extremes might be understood by the combined effects of amplified subsidence (suppressing precipitation formation) and intensified low‐level divergence (reducing low cloud cover). (d) From a large‐scale dynamic perspective, early propagation and development of the 2019AHD are related to the southward shift and enhancement of the subsidence branch of meridional circulations. Meanwhile, intensity variation in 200 hPa cyclonic anomalies over MLYR corresponds well with local drought development, and the enhanced intensity at the time of drought occurrence is probably related to Silk Road Pattern–like wave train propagation over Eurasia. These insights can help us understand the mechanisms and detect early signals of hot droughts.

Distinct Holocene precipitation trends over arid Central Asia and linkages to westerlies and Asian monsoon

The response of precipitation over arid Central Asia since the mid Holocene is long considered to be controlled by astronomical insolation-induced changes in the westerlies. However, it is still controversial in proxy and modeling studies whether the trend of Central Asian precipitation is in accordance with Asian monsoon or not. Compared to the monsoon, the westerlies-associated precipitation changes are complicated and also rely on regional mountain complex. In this study, we conducted a high-resolution transient experiment by a coupled climate model to evaluate the response of Central Asian precipitation. The simulated precipitation changes are distinctly different over regions although they are mainly associated with the westerlies. Since mid Holocene, the annual precipitation increases over Balkhash-Altai region but decreases over Turan Plain, Mongolia-Baikal and Tarim Basin, in all of which the summer precipitation contributes most. Following the summer insolation, the southward shift of westerly jet is accompanied by wave-like anomalies in the upper-tropospheric meridional winds across mid-latitude Eurasia (the Silk Road pattern), which results in distinct summer precipitation changes. The decreased precipitation over Tarim is mainly explained by weakened low-level easterly wind anomaly and less moisture supply from East Asia. In contrast, the responses of winter precipitation are limited even in the winter-rainfall-dominated regime due to the small insolation change. Such simulated complicated precipitation trends during the Holocene are in qualitative agreement with existing proxies, highlighting the modulation of Silk Road pattern on Central Asian rainfall on the orbital timescale.