East Asian Jet Stream Research Articles

Summer High Temperature Extremes over China Linked to the Pacific Meridional Mode

AbstractThis study investigates the association between summer high temperature extremes (HTEs) over China and the Pacific meridional mode (PMM) that is characterized by an anomalous north–south sea surface temperature gradient and an anomalous surface circulation over the northeastern subtropical Pacific. It is found that the HTE activities over most parts of southern China (particularly eastern China) are prominently intensified during the positive PMM phase and weakened during the negative phase. Further examinations suggest that the PMM is linked with HTEs in China through processes that entail both eastward and westward development of signals emanating from the PMM site. The westward development is associated with the formation of an anomalous low-level cyclone over the western North Pacific (WNP), which may be viewed as a Matsuno–Gill-type response to the off-equatorial heating in the eastern Pacific. This circulation change is accompanied by anomalous ascent over WNP and northern China, and subsidence over eastern China. On the other hand, the eastward development process is linked to the PMM-induced displacement of the East Asian jet stream and the generation of a midlatitude Rossby wave train. In the positive PMM phase, the above circulation changes are accompanied by anomalous air subsidence and enhanced adiabatic heating, reduced precipitation, anomalous lower-level anticyclone, and rising surface pressure over the eastern part of China. Moreover, the land surface of that region receives more solar radiation. Opposite changes are discernible over northern China. These changes are favorable for the occurrence and persistence of HTEs over eastern China and tend to suppress HTEs over northern China.

Spatially Distinct Effects of Two El Niño Types on Summer Heat Extremes in China

AbstractChanges in the activity of summer high‐temperature extremes (HTEs) over many regions are linked with the El Niño‐Southern Oscillation. However, how the HTEs respond to different flavors of El Niño remains unclear. Using observational analyses, we show that summer HTEs in China during eastern Pacific (EP) and central Pacific (CP) types of El Niño exhibit different spatial patterns. EP (CP) El Niño is significantly correlated with summer HTEs over South China (Yangtze River valley). These relationships can be attributed to the modulation of the western North Pacific subtropical high and the East Asian jet stream. EP (CP) El Niño events are accompanied by westward (eastward) extension of the western North Pacific subtropical high and equatorward (poleward) movement of the jet stream over East Asia (Pacific). During EP (CP) El Niño episodes, the westward (eastward) and equatorward (poleward) displacements of upper‐tropospheric anticyclone over the western North Pacific coincide with the subsidence anomaly over South China (Yangtze River valley).

Decadal variations of the East Asian winter monsoon in recent decades

AbstractObservational studies indicated that the East Asian winter monsoon (EAWM) showed significant decadal variations and experienced an interdecadal weakening in the mid‐1980s. How did the EAWM evolve thereafter? In this study, we investigate the decadal variations of the EAWM in the past three decades using five reanalysis datasets and one observational dataset. In total, five members of EAWM system are examined. In the lower troposphere, the Siberian high intensity becomes stronger around 2005 in all five reanalysis datasets, whereas the Aleutian low strengthens in the mid‐1990s and weakens in the mid‐2000s. The two subsystems show opposing changes in the past two decades. In the middle troposphere, the intensity of the East Asian trough remains weakening in recent decades only with some short‐time strengthening around the years of 1995 and 2010. In the upper troposphere, the EAWM index based on the meridional shear of the East Asian jet stream at the 300‐hPa level shows a weak positive trend since the 1990s. It has decadal variations similar to those of the East Asian trough. In the context of these members' changes, the low‐level northerly wind index exhibits two cycles since the year 1980. This index increases in the mid‐1980s, then decreases since the mid‐1990s, and finally increases in the mid‐2000s. Further analysis on the East Asian surface air temperature also reflects similar decadal variations. In summary, unlike the interdecadal shift in the mid‐1980s, the EAWM members (i.e., the Siberian high, the Aleutian low, the low‐level northwesterly wind, the East Asian trough and the East Asian jet stream) do not show consistent decadal variations over the past 30 years. The low‐level members show more significant changes.

Rainy season onset over Northeast China and the related atmospheric circulations

AbstractNortheast China is one of the major commodity crop bases in East Asia, thus, it plays an important role in regional and global food security. The transition from a dry to a wet season, that is, the beginning of the rainy season, is critical for agricultural planning in the region. In this study, the rainy season onset (RSO) dates in Northeast China are first determined based on the “anomalous accumulation” (AA) method, and the RSO‐related atmospheric circulations are then explored. The results show that the mean RSO date in Northeast China is May 13, with a standard deviation of 13 days. The RSO is related to the sudden changes in atmospheric circulations, including the weakening of the East Asian trough and the East Asian jet stream, and the northward movement of the convergence zone. The RSO dates for Northeast China show strong year‐to‐year variability. The composite difference in the atmospheric circulations between earlier and later RSO years implies that a meridional dipole pattern reflected by anomalous 500 hPa geopotential height and zonal wind over East Asia is closely related to the RSO interannual variability. This dipole pattern is accompanied by southeasterly (northwesterly) wind anomalies in the middle and low troposphere over Northeast China during the earlier (later) RSO years, consequently enhancing (weakening) moisture transportation and moist static energy over the region and thereby favouring an advanced (delayed) seasonal transition from the dry to wet season. The RSO dates revealed in this study are valuable for guiding spring sowing practices in the region.

More robust changes in the East Asian winter monsoon from 1.5 to 2.0°C global warming targets

AbstractThis study investigates changes in the East Asian winter monsoon (EAWM) under 1.5 and 2.0°C global warming (1.5 and 2.0GW) targets using multi‐model results from the Coupled Model Intercomparison Project phase 5 (CMIP5). It is found that the surface air temperature increases over the Asian continent and Pacific, and the precipitation increases over the northern Asian continent in winter during the 1.5 and 2.0GW periods. For the 1.5GW period, large uncertainties can be found in the projection of EAWM system members (i.e., Siberian high, Aleutian low, low‐level winds over eastern Asia, East Asian trough [EAT] and East Asian jet stream [EAJS]). For the 2.0GW period, more than two‐thirds of the models show that the Siberian high, the EAT and the meridional shear of the EAJS are weakened, but the Aleutian low is intensified. In addition, the intensified Aleutian low enhances low‐level northwesterly wind along the coast of northeastern Asia. Compared to those in the 1.5GW period, the EAWM system members show more robust changes during the 2.0GW period, except for the low‐level winds at mid‐latitude eastern Asia. These conclusions are suitable for both the RCP4.5 and RCP8.5 scenarios.

Interannual variation of the East Asia Jet Stream and its impact on the horizontal distribution of aerosol in boreal spring

Interannual variation of the aerosol optical depth (AOD) in East Asia has been investigated using Moderate Resolution Imaging Spectroradiometer (MODIS) data and Modern Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) data for 2000–2018. The data analysis focuses on boreal spring when Siberian biomass burning is at its seasonal maximum. The results indicate that the significant increase in organic and black carbon is primarily caused by emissions from biomass burning in East Asia, which leads to significant interannual variations in aerosol loading and pan-Pacific transport. The anomalous large-scale climate variability associated with the East Asia Jet Stream (EAJS) provides favorable conditions for increasing the AOD of organic and black carbon in Northeast Asia and may represent an underlying physical mechanism. When the EAJS shows greater weakening than normal, abnormal high-pressure anomalies are maintained in East Asia, which tend to drive warm advection over Northeast Asia. This warm advection expedites the melting of the Eurasian snow cover, which helps increase surface dryness in late spring and provides favorable conditions for biomass burning. The EAJS index can be predictable with statistical significance up to lead 1 month by the dynamical ensemble seasonal forecasts, suggesting a possible implementation of the empirical AOD forecasts using climate forecast models.

Interdecadal Variations of the East Asian Winter Monsoon in CMIP5 Preindustrial Simulations

AbstractIn this study, focusing on the interdecadal time scale, we investigate the internal variability of the East Asian winter monsoon (EAWM) using output from 19 coupled models’ long-term preindustrial control (piControl) simulations within phase 5 of the Coupled Model Intercomparison Program (CMIP5). In total, we identify 53 cases of significant interdecadal weakening of the EAWM from these 19 piControl simulations. In most weakening cases, both the Siberian high and the East Asian trough are significantly weakened. The East Asian jet stream in the upper troposphere shifts poleward. Southerly wind anomalies are evident over East Asia in the lower troposphere. At the same time, both the Arctic Oscillation (AO) and the North Pacific Oscillation are in their positive phases. Associated anomalous anticyclonic circulation can be found over the North Pacific. Additionally, the North Pacific shows negative Pacific decadal oscillation (PDO)-like sea surface temperature (SST) anomalies. In contrast, we also analyzed 49 cases of significant strengthening of the EAWM, and the atmospheric and oceanic anomalies show opposite signals with the weakening cases. This suggests that internal variabilities of the climate system can also cause interdecadal variations of the EAWM. In addition, the phase shifting of the AO is likely the main reason for the EAWM’s interdecadal variations in the unforced long-term simulations. Further numerical experiments using the Community Atmosphere Model, version 4 (CAM4), deny the causal relationship between the interdecadal variations of EAWM and PDO-like SST anomalies. This study also implies that the internal variabilities of the climate system could contribute to the observed interdecadal weakening of the EAWM around the mid-1980s.

Intensifying effects of El Niño events on winter precipitation extremes in southeastern China

This paper studies the impacts of El Nino-Southern Oscillation (ENSO) on wintertime extreme precipitation in China from 1961 to 2017, and possibly different influences of two El Nino types are also examined. We find that ENSO poses profound influences on extreme precipitation in many portions of China. El Nino (La Nina) intensifies (weakens) the winter precipitation extremes in southeastern China (SEC) and slightly weakens (intensifies) those in central-north China, mainly by changing the frequency rather than the intensity of extreme precipitation. Further analysis suggests that the East Asian winter monsoon tends to be weaker during El Nino winters, suppressing the southward invasion of cold dry air. Via the weakened Walker circulation, El Nino also triggers anomalously descending motion and anticyclone over the western North Pacific (WNP), which exhibits southwesterly anomalies over SEC and is thus conducive to the transport of sufficient moisture into that area. The changes over WNP are also accompanied by ascending motions over East Asia via a local meridional circulation alike the Pacific–Japan pattern. Meanwhile, El Nino induces a southward-displaced East Asian jet stream, as characterized by intensified westerly over southern China, corresponding to anomalously ascending motion there. Together, these changes facilitate the updrafts of circulations and the condensation of water vapor, thus increasing the occurrence of extreme precipitation events in SEC. Moreover, we find that different Nino types exhibit distinct subregional influences. In particular, the canonical eastern Pacific El Nino significantly increases the frequency of extreme precipitation in South China, whereas the Modoki central Pacific El Nino mainly impacts that in East China.

Linkage between the Arctic Oscillation and summer climate extreme events over the middle reaches of Yangtze River Valley

The Arctic Oscillation (AO) is commonly recognized as a dominant large-scale mode influencing climate over the Northern Hemisphere. Here, the influences of May AO on summer (JJA) extreme precipitation events and summer extreme warm days over the middle reaches of Yangtze River Valley for the period 1961-2014 are investigated. Following a positive May AO, there are usually fewer summer extreme precipitation events but more summer extreme warm days over the middle reaches of Yangtze River Valley. Composite analyses show that positive May AO induces the northward displacement of the East Asian jet stream and northeastward displacement of the western Pacific subtropical high (WPSH), and causes a stronger, more northwestern subtropical northwest Pacific cyclone/anticyclone anomaly, as well as an anticyclonic circulation anomaly on the north side of the South China Sea, resulting in a northward shift of the rainfall belt and an enhancement of the East Asia summer monsoon. Therefore, the cumulative distribution probability of daily precipitation values shift significantly to a lower precipitation value, indicating lower probabilities of summer extreme precipitation events following positive May AO. A weakening of WPSH induces an anomalous sinking motion over the middle reaches of the Yangtze River Valley. The 850 hPa wind field shows southerly wind anomalies over the Jiang-Huai River Basin, which cause a decrease in total cloud cover, resulting in an increase in solar radiation flux. A significant shift of the daily maximum temperature probability distribution towards to higher values indicates higher probabilities of summer extreme warm day occurrences following positive May AO. This study will provide useful insights to help improve the understanding of the dynamics and projections of future regional extreme precipitation changes over the middle reaches of Yangtze River Valley.

Maize Drought Hazard in the Northeast Farming Region of China: Unprecedented Events in the Current Climate

AbstractThe Northeast Farming Region (NFR) of China is a critically important area of maize cultivation accounting for ~30% of national production. It is predominantly rain fed, meaning that adverse climate conditions such as drought can significantly affect productivity. Forewarning of such events, to improve contingency planning, could therefore be highly beneficial to the agricultural sector. For this, an improved estimate of drought exposure, and the associated large-scale circulation patterns, is of critical importance. We address these important questions by employing a large ensemble of initialized climate model simulations. These simulations provide 80 times as many summers as the equivalent observational dataset and highlight several limitations of the recent observational record. For example, the chance of a drought greater in area than any current observed event is approximately 5% per year, suggesting the risk of a major drought is significantly underestimated if based solely on recent events. The combination of a weakened East Asian jet stream and intensified subpolar jet are found to be associated with severe NFR drought through enhanced upper-level convergence and anomalous descent, reducing moisture and suppressing precipitation. We identify a strong 500-hPa geopotential height anomaly dipole pattern as a useful metric to identify this mechanism for relevance to seasonal predictability. This work can inform policy planning and decision-making through an improved understanding of the near-term climate exposure and form the basis of new climate services.

Precursor in Arctic oscillation for the East Asian January temperature and its relationship with stationary planetary waves: Results from CMIP5 models

AbstractThe emerging precursor in December Arctic oscillation (AO) for the following January temperature over East Asia (TEA) has been noticed by recent researches. This study evaluates the precursor in December AO for the following January TEA using 35 climate models from the Coupled Model Intercomparison Project Phase 5. Results indicate that, 24 (multi‐model ensemble mean; MME‐GOOD) out of total 35 models reproduce the statistically significant positive correlation between the December AO and the January TEA identified in the observation, but the remaining 11 models (MME‐BAD) fail. Further, the investigation focuses on these models' simulations for the December AO‐associated atmospheric circulation in January. Eighteen (MME‐GOOD‐18) out of the above 24 models reproduce the persistence of December AO‐related atmospheric circulation, including the seesaw structure of the sea level pressure anomalies, the sweeping southerly 850‐hPa wind anomalies over the East Asia–Northwest Pacific region, the weakening of the East Asian trough, and the weakened meridional shear of the East Asian jet stream in January following an enhanced December AO. It is found that the propagation of stationary planetary waves plays a crucial role in connecting the December AO and the January TEA. Therefore, the models' ability to simulate the stratosphere–troposphere interaction is essential for the successful simulations on the persistence of December AO. Additionally, the future projections indicate that the precursor in December AO for the following January TEA is still robust during the 21st century.

Change in Relationship between the East Asian Winter Monsoon and the East Asian Jet Stream during the 1998–99 Regime Shift

Abstract A composite analysis was conducted on the reanalysis dataset for 1979–2016, along with an idealized model experiment to show that the relationship between the East Asian jet stream (EAJS) and the East Asian winter monsoon (EAWM) is nonstationary. The relationship between EAWM and the EAJS weakened during the late 1990s. This decadal change in the EAJS–EAWM relationship was mainly due to a change in the secondary circulation across the EAJS between two contrasting periods, induced by the northward shift of the EAJS. A possible mechanism associated with the decadal change in meridional displacement of the EAJS is proposed. The enhanced convective activity in the western tropical Pacific after the late 1990s results in stronger Hadley circulation that could have contributed to the northward displacement of the Hadley circulation boundary latitude. Subsequently, this leads to the northward shift of the EAJS. Therefore, it is necessary to define a new EAJS index to account for the EAWM variability based on the change in the oceanic and atmospheric mean state across the late 1990s.

Influence of December snow cover over North America on January surface air temperature over the midlatitude Asia

AbstractThe impact of North American (NA) snow cover on the Northern Hemisphere climate has drawn increasing attention in recent years. This study reveals a negative relationship between NA snow cover in December and surface air temperature (SAT) over the midlatitude Asia in the following January. The results show that, in December, the extensive snow cover decreases SAT over NA via local diabatic cooling, which further intensifies the Atlantic jet stream by enlarging the meridional gradient of the 1,000‐ to 300‐hPa layer thickness, especially along 40°N. Meanwhile, we found negative sea surface temperature (SST) anomalies over the Western North Atlantic and enhanced transient eddy activity over the Eastern North Atlantic due to anomalous net energy conversion from the mean flow. In January, a zonally oriented dipole of transient eddy anomalies straddles the North Atlantic, with the negative–positive (west–east) anomaly centres attributing to local oceanic cooling and the persistence of the eddy–mean flow interaction, respectively. Such dipole of transient eddy activity in turn favours the downstream development of the Atlantic jet stream. Besides, the changes in surface turbulent heat flux over the North Atlantic trigger a Rossby wave train that propagates across Europe and towards the Far East. Coincident with the strengthened Siberian High and the accelerated East Asian jet stream in January, SAT decreases over the midlatitude Asia. Hence, our analysis indicates that December NA snow cover could potentially be exploited for subseasonal predictability over Asia.

Strengthened linkage between midlatitudes and Arctic in boreal winter

Early studies have suggested a linkage between the surface warming over the Barents-Kara Seas and the strength of the Siberian high in boreal winter. Here, we show that the linkage is not stable, and with an apparent interdecadal change in the late-1990s. Coinciding with Arctic surface warm anomalies in recent decades (1997–2017), the Siberian high has been significantly intensified, the East Asian jet stream has expanded westward, and an apparent Rossby wave has propagated from the Arctic to East Asia, suggesting an atmospheric teleconnection between midlatitudes and Arctic. In contrast, midlatitude atmospheric circulation anomalies coinciding with Arctic surface warm anomalies were barely statistically significant during 1979–1996. The associated strong anomalous ascending/descending motions and divergent/convergent upper troposphere air masses over the Arctic-Eurasian sector seem to have favored the midlatitude-Arctic linkage during 1997–2017. We further hypothesize that Arctic mid-tropospheric warming plays a crucial role for the linkage between midlatitudes and Arctic in boreal winter. Multi-model simulations support this, and also point to internal atmospheric variability as the cause for the interdecadal shift in the strength of the midlatitude-Arctic linkage.

The decadal shift of the long persistent rainfall over the northern part of China and the associated ocean conditions

The long persistent rainfall (LPR) over the northern part of China has a great impact on its society and economics. This study investigates the decadal shift of the regional‐mean LPR frequency over the northern part of China (LPR frequency) from 1971 to 2014 and the associated ocean conditions. The LPR frequency has experienced a significant decadal shift around 1996, with an enhanced (reduced) anomaly before (after) 1996, which is closely associated with a meridional tri‐pole sea surface temperature (SST) anomaly over the central Pacific, a La Niña‐like anomaly for the pre‐1996 epoch (1971–1996) and an obvious warming over the south Indian Ocean and the western part of south Pacific Ocean in the post‐1996 epoch (1997–2014). In the pre‐1996 epoch, associated with the typical SSTs, the ascent motion with increasing humidity anomalies are over the northern part of China. Meanwhile, high (low) pressure anomalies are over western Asia and East Asia (Europe and central Asia), which resembles as the positive phase of the Silk Road pattern (SRP). It would be associated with the polewards shift of the East Asian jet stream (EAJS) and therefore result in the northwards shift of the rain belt and benefit the occurrence of the LPR. In the post‐1996 epoch, associated with the warming over the specific south oceans, a descend anomaly dominates over the northern part of China, as well as the resembled negative phase of the SRP over mid‐high latitudes and equatorwards shift of the EAJS. These would suppress the rainfall over the northern part of China and reduce the LPR frequency over there after 1996.

Enlightenment from heavy autumn rain of West China in 2017: synergic role of atmospheric circulation at mid-high latitudes and oceanic background

Autumn rain of West China (ARWC), a typical climate phenomenon characterized by continuous rainfall, is prone to flooding and secondary disasters. The ARWC in 2017, the greatest one since the last 30 years, resulted in an economic loss of 1.9 billion dollars and serious social impacts. It is thus urgent to understand the cause for this anomaly. In this article, the atmospheric circulations affecting the 2017’s ARWC are identified. They are (1) a dipole pattern with the blocking over Europe and the trough over Lake Balkhash, which favors the southward outbreak of cold airs into West China, (2) increased water vapor transportation toward West China from the Pacific and Indian Oceans, and (3) a strengthening and northward displacement of the East Asian jet stream. The cold sea surface temperature anomaly (SSTA) in the equatorial central Pacific provides a superimposed effect. Further analysis reveals a synergic role in the interannual variability of the ARWC from the anomalies of the atmospheric circulation over Lake Balkhash and the SST in the equatorial central Pacific. The combination of anomalous trough over Lake Balkhash with cold SSTA in the equatorial central Pacific is most favorable for the increase of ARWC, while that of anomalous ridge with warm SSTA generally results in a decrease of ARWC. The combination of anomalous ridge with cold SSTA or that of anomalous trough with warm SSTA has no specific indication for ARWC anomalies.

Response of early winter haze in the North China Plain to autumn Beaufort sea ice

Abstract. Recently, early winter haze pollution in the North China Plain has been serious and disastrous, dramatically damaging human health and the social economy. In this study, we emphasized the close connection between the number of haze days in early winter in the North China Plain and the September–October sea ice in the west of the Beaufort Sea (R=0.51) via both observational analyses and numerical experiments. Due to efficient radiative cooling, the responses of atmospheric circulations partially manifested as reductions of surface wind speed over the Beaufort Sea and Gulf of Alaska, resulting in a warmer sea surface in the subsequent November. The sea surface temperature anomalies over the Bering Sea and Gulf of Alaska acted as a bridge. The warmer sea surface efficiently heated the above air and led to suitable atmospheric backgrounds to enhance the potential of haze weather (e.g., a weaker East Asian jet stream and a Rossby wave-like train propagated from North China and the Sea of Japan, through the Bering Sea and Gulf of Alaska, to the Cordillera Mountains). Near the surface, the weakening sea level pressure gradient stimulated anomalous southerlies over the coastal area of China and brought about a calm and moist environment for haze formation. The thermal inversion was also enhanced to restrict the downward transportation of clear and dry upper air. Thus, the horizontal and vertical dispersion were both limited, and the fine particles were apt to accumulate and cause haze pollution.

Interdecadal Weakening of the East Asian Winter Monsoon in the Mid-1980s: The Roles of External Forcings

Observations show that the East Asian winter monsoon (EAWM) experienced an interdecadal weakening in the mid-1980s. This is evident for all members of the EAWM system (i.e., East Asian trough, upper-tropospheric jet stream, and lower-tropospheric monsoon circulation). Here, we investigate the relative contributions of natural (volcanic aerosols and solar variability) and anthropogenic [greenhouse gases (GHGs) and anthropogenic aerosols] forcings to this interdecadal weakening using multiple coupled models within phase 5 of the Coupled Model Intercomparison Project (CMIP5). The results indicate that in the midtroposphere, the increased GHG concentrations play an important role in weakening the East Asian trough (EAT) by increasing the sea surface temperatures (SSTs) over the North Pacific. In the upper troposphere, natural external forcings contribute to the observed weakening of the meridional shear of the East Asian jet stream (EAJS) by regulating the meridional temperature gradient (MTG) over the East Asian region. In the lower troposphere, both anthropogenic and natural forcings can weaken the Siberian high during this period. Overall, based on the present analysis of the CMIP5 output, GHGs and natural forcings play key roles in shaping the observed interdecadal weakening of the EAWM during the mid-1980s. Additionally, contributions from internal variability cannot be neglected and require further investigation.

Afro-Eurasian Intermediate-Frequency Teleconnection and Modulation by ENSO

The Afro-Eurasian intermediate-frequency atmospheric teleconnection conveys meteorological signals zonally, leads to various atmospheric variations, and causes extreme events along its path. This study, aimed at demonstrating the characteristics of the teleconnection, reveals that the teleconnection accounts for nearly half of the atmospheric variability and significantly influences different meteorological fields. With the propagation of signals associated with the teleconnection, local weather varies from prolonged dry and warm days to extended wet and cold days. El Niño–Southern Oscillation (ENSO) modulates the interannual variation of the teleconnection: it becomes more active and its downstream pattern shifts southward during El Niño events. Two responsible mechanisms are proposed for the ENSO modulation: the eddy-to-eddy interaction that leads to the change in the activeness of the teleconnection and the waveguide effect that accounts for the shift of the teleconnection. First, the El Niño–related Atlantic anomalies of the Rossby wave train and storm track amplify the Atlantic disturbances of the intermediate frequency and thus the activeness of the teleconnection. Second, during El Niño years, the East Asian jet stream shifts southward, resulting in the southward shifts of the downstream waveguide effect and thus the downstream pattern. This study also demonstrates that when investigating an atmospheric mode or its impacts, the signals of different time scales should be separated and the cross-frequency interactive systems necessitate examinations.

Interannual variation in the number and severity of autumnal haze days in the Beijing–Tianjin–Hebei region and associated atmospheric circulation anomalies

In contrast with the considerable number of studies on the variability of winter haze pollution over eastern China, there have been few studies of autumnal haze in this region. The interannual variability of autumnal (September–November) haze days in the Beijing–Tianjin–Hebei region (AHDBTH) is significantly correlated with the Scandinavia–central Siberia–Western Pacific (SCSWP) teleconnection pattern, a quasi-barotropic atmospheric circulation anomaly. The SCSWP teleconnection pattern may therefore be a useful indicator for forecasting AHDBTH. The positive phase of the SCSWP teleconnection pattern enhances the anomalous anticyclonic circulation centered over the Western Pacific via the southeastward propagating Rossby wave train emanating from Scandinavia. The positive SCSWP teleconnection pattern, together with the northward shift of the East Asian jet stream induced by low-level atmospheric baroclinicity, could provide the favorable local meteorological circumstances (i.e., suppressed cold air activity, higher surface temperatures and low-level atmospheric inversion, and lower planetary boundary layer height and decreased surface wind speed) required to explain the recent increased incidence of AHDBTH.