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Using ERA5 reanalysis data, this study explores the interannual variability of the occurrence frequency of the stronger Kunming quasi‐stationary front during in winter over the period 1980–2021 and the main causes of the variability. The results indicate that the occurrence frequency is significantly modulated on interannual timescales by southwesterly anomalies over the warmer region west of 103° E and northeasterly anomalies over the colder region east of 106° E. When the stronger southwesterly anomalies over the warmer region are comparable with stronger northeasterly anomalies over the colder region, the inhomogeneities of horizontal convergence and diabatic heating are strengthened near the frontal zone. This anomalous circulation causes a higher‐than‐normal occurrence frequency of the stronger Kunming quasi‐stationary front in winter. The East Asian winter monsoon (EAWM) and circumglobal teleconnection (CGT) are the two most important controls on the interannual variability of the occurrence frequency of the stronger Kunming quasi‐stationary front. Both the CGT and EAWM modulate the inhomogeneities of horizontal convergence and diabatic heating on the west flank of the front, and the EAWM further regulates diabatic heating over the east flank of the front. [ABSTRACT FROM AUTHOR]

The influence of winter circumglobal teleconnection （CGT） on the interannual variability of winter precipitation in the Southeast Asian low-latitude highlands were statistically analyzed using ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts （ECMWF）， monthly mean precipitation data from the Climate Hazards Group Infrared Precipitation with Station data （CHIRPS） developed by the United States Geological Survey and the University of California and the monthly mean precipitation data of the Global Precipitation Climatology Project （GPCP） provided by the Global Precipitation Climatology Center.Results of correlation analysis show that the CGT presents two main patterns with approximately the same variance contribution rate.On the vertical direction， the CGT shows an equivalent barotropic structure with four anomalous centers.During the CGT positive phase， the negative anomalous centers are located near the Mediterranean Sea and the Indian Peninsula， and the positive anomalous centers near the Arabian Peninsula and Southeast Asian low-latitude highlands.The first mode of CGT （CGT1） significantly correlates with winter precipitation over the Southeast Asian low-latitude highlands on the interannual time scale.When the CGT1 is in the positive phase in winter， the anomalous "anticyclone， cyclone， anticyclone， cyclone" control the Western Europe， the northwestern Arabian Peninsula， the Arabian Sea and Southeast Asian low-latitude highlands， respectively.The anomalous southwesterly wind on the east side of the anomalous cyclone will increase the water vapor from the Bay of Bengal and the South China Sea to the Southeast Asian low-latitude highlands.The more water vapor converged and condensed in Southeast Asian low-latitude highlands finally results in heavier winter precipitation over the Southeast Asian low-latitude highlands.On the contrary， when the CGT1 is in the negative phase in winter， the Western Europe， the northwestern Arabian Peninsula， the Arabian Sea and the Southeast Asian low-latitude highlands are controlled by the anomalous "cyclone， anticyclone， cyclone， anticyclone".The anomalous northeast wind on the east flank of the anomalous anticyclone will reduce the water vapor from the Bay of Bengal and the South China Sea to the Southeast Asian low-latitude highlands.The anomalous divergence and descending motion further lead to less precipitation in winter over the Southeast Asian low-latitude highlands.The significant positive correlation between CGT and precipitation over the Southeast Asian low-latitude highlands， sharing almost the same key physical process as winter， can be observed in December， January and February.Results of typical case further confirm the key physical process through which the CGT modulates the interannual variability of winter precipitation over the Southeast Asian low-latitude highlands.

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This study leverages the relationships between the Great Plains low-level jet (GP-LLJ) and the circumglobal teleconnection (CGT) to assess the enhancement of 30-day rainfall forecast in the Northern Great Plains (NGP). The assessment of 30-day simulated precipitation using the Climate Forecast System (CFS) is contrasted with the North American Regional Reanalysis, searching for sources of precipitation predictability associated with extended wet and drought events. We analyze the 30-day sources of precipitation predictability using (1) the characterization of dominant statistical modes of variability of 900 mb winds associated with the GP-LLJ, (2) the large-scale atmospheric patterns based on 200 mb geopotential height (HGT), and (3) the use of GP-LLJ and CGT conditional probability distributions using a continuous correlation threshold approach to identify when and where the forecast of NGP precipitation occurs. Two factors contributing to the predictability of precipitation in the NGP are documented. We found that the association between GP-LLJ and CGT occurs at two different scales—the interdiurnal and the sub-seasonal, respectively. The CFS reforecast suggests that the ability to forecast sub-seasonal precipitation improves in response to the enhanced simulation of the GP-LLJ and CGT. Using these modes of climate variability could improve predictive frameworks for water resources management, governance, and water supply for agriculture. [ABSTRACT FROM AUTHOR]

A record-setting extreme heatwave occurred over western North America (WNA) in the summer of 2021, which was associated with an extreme atmospheric Rossby wave ridge (ARR) over WNA and a minimum record event of the preceding winter Barents Sea ice concentration. We identify the remote effect and investigate how the Barents Sea ice loss (BSIL) in the preceding winter relates to the intensity of the ARR and extreme heatwaves in the following June and July (JJ) by analyzing the reanalysis data. Our results suggest that the atmospheric wave activity flux associated with the BSIL transferred wave energy towards the circumglobal teleconnection (CGT)-like wave trains, increasing the wave amplitude and enhancing the ARR over WNA. The anomalous sea ice melting in JJ, originating from the BSIL in the preceding winter through ocean-air-sea ice positive feedback processes, also induces a northward shift of jet stream by meridional temperature gradient change. The weakening of the local jet from the original jet axis area drives anticyclonic anomalies which increases atmospheric subsidence and shortwave radiation. The energy is transferred from the weakened jet to synoptic-scale disturbances via wave-current interaction. The northward jet as a waveguide causes the CGT to shift northward and receive signals from the higher latitude. Both the CGT-like wave trains and the shifted jet streams synergistically contribute to the increase of ARR intensity and the frequency of extreme heatwaves. Overall, the BSIL will serve as an early predictor for properly forecasting the extreme heatwaves over WNA six months in advance, which potentially improves the prediction skills. [ABSTRACT FROM AUTHOR]

The intraseasonal Pacific–Japan (PJ) pattern, characterized by a pronounced quasi-biweekly oscillation, is triggered by deep convection around the western North Pacific. Three possible dynamical mechanisms on multiple timescales responsible for the growth and decay of the quasi-biweekly PJ pattern are proposed in this study based on daily reanalysis data from the Japanese 55 year Reanalysis for the 1958‒2021 period. First, the eastward-propagating wave energy associated with the quasi-biweekly circumglobal teleconnection in the upstream region enters the mid-latitude North Pacific and induces the wavelike barotropic geopotential height anomalies, amplifying the magnitudes of three mid-latitude centers of the PJ pattern by about 40% through their linear constructive interference. Secondly, the barotropic feedback forcing of both high-frequency and low-frequency transient eddies triggered by the pronounced meridional SST gradient over the mid-latitude Pacific is beneficial to the development and persistence of the PJ-related centers to the east of Japan and around the Bering Strait, whereas it damps the PJ-related center in the Gulf of Alaska, increasing the amplitude difference between the former two centers and the latter center. Such feedback forcing also leads to the asymmetry of the positive and negative PJ events. Thirdly, the dry energy conversion from the background atmospheric circulation and the moist process due to the convective heating over the western North Pacific are both efficient enough to energize the PJ pattern in the developing and mature stages, indicating that the quasi-biweekly PJ pattern can be viewed as a convectively coupled dynamical mode. [ABSTRACT FROM AUTHOR]

This study leverages the relationships between the Great Plains low-level jet (GP-LLJ) and the circumglobal teleconnection (CGT) to assess the enhancement of 30-day rainfall forecast in the Northern Great Plains (NGP). The assessment of 30-day simulated precipitation using the Climate Forecast System (CFS) is contrasted with the North American Regional Reanalysis, searching for sources of precipitation predictability associated with extended wet and drought events. We analyze the 30-day sources of precipitation predictability using (1) the characterization of dominant statistical modes of variability of 900 mb winds associated with the GP-LLJ, (2) the large-scale atmospheric patterns based on 200 mb geopotential height (HGT), and (3) the use of GP-LLJ and CGT conditional probability distributions using a continuous correlation threshold approach to identify when and where the forecast of NGP precipitation occurs. Two factors contributing to the predictability of precipitation in the NGP are documented. We found that the association between GP-LLJ and CGT occurs at two different scales—the interdiurnal and the sub-seasonal, respectively. The CFS reforecast suggests that the ability to forecast sub-seasonal precipitation improves in response to the enhanced simulation of the GP-LLJ and CGT. Using these modes of climate variability could improve predictive frameworks for water resources management, governance, and water supply for agriculture.

Based on the fifth-generation reanalysis dataset from the European Centre for Medium-Range Weather Forecasts for 1979–2019, we investigated the effects of the circumglobal teleconnection (CGT) on the interdecadal variation of the March atmospheric heat source (AHS) over the Southeast Asian low-latitude highlands (SEALLH). The dominant mode of the March AHS over the SEALLH features a monopole structure with an 8–11-year period. Decadal variations in the AHS make an important contribution to the 11-year low-pass filtered component of the AHS index, whichexplains 54.3% of the total variance. The CGT shows a clear interdecadal variation, which explains 59.3% of the total variance. The March AHS over the SEALLH is significantly related to the CGT on interdecadal timescales. When the CGT is optimally excited by a significant cyclonic vorticity source near northern Africa (i.e., in its positive phase), the SEALLH is dominated by anomalous southerly winds and ascending motions on the east of the anomalous cyclone. The enhanced advection and upward transfer result in a high-enthalpy air mass that converges into and condenses over the SEALLH, leading to a larger-than-average March AHS over this region. The key physical processes revealed by this diagnostic analysis are supported by numerical experiments. [ABSTRACT FROM AUTHOR]

Tropical-extratropical teleconnection during boreal summer is characterized by a circumglobal teleconnection (CGT) pattern and is often considered as one of the major predictability sources of extratropical atmospheric circulation. The extent to which the summer circumglobal teleconnection pattern is remotely forced by tropical sea surface temperature (SST) variability remains to be elucidated. Here, we use a suite of pacemaker simulations from the Community Earth System Model (CESM1) to investigate the contribution of tropical sea surface temperature variability in the Eastern Tropical Pacific, Northern Atlantic, and Indo-Western Pacific to the summer circumglobal teleconnection circulation. We find that the variability of circumglobal teleconnection in the summer is dominated by the sea surface temperature variability in the Indo-Western Pacific and its remotely forced ENSO-like anomaly. As a result, the Indo-Western Pacific pacemaker experiment exhibits precipitation and surface temperature teleconnection patterns closest to the observed patterns in the Northern Hemisphere. A better understanding of such tropical-extratropical interactions can inform better seasonal forecasts and model development.

Extremely hot days in Japan are known to persist for a week or more, and they are measured by the temperature anomaly at 850 hPa averaged over Japan derived from the JRA-55 reanalysis data, denoted as T850JP. Severe high-temperature anomalies are often accompanied by multiple teleconnection patterns that affect the weather in Japan, but their relative contribution to individual heat wave events has not yet been quantified. In this study, we examined the effects of three major teleconnection patterns, namely, the Pacific-Japan (PJ), circumglobal teleconnection (CGT), and Siberian patterns, on T850JP in July and August from 1958 - 2019 using daily low-pass-filtered anomalies with 8 days cutoff timescale derived from the reanalysis. A linear regression analysis demonstrated that T850JP tended to show a large positive anomaly one or two days after the peak of these patterns. On the basis of this relationship, we reconstructed a daily T850JP time series using a multivariate statistical model wherein the parameters were estimated using regression analyses between T850JP and the indices of the three teleconnection patterns. The reconstructed T850JP showed that the three teleconnection patterns together accounted for 50 % of the total variance of T850JP for extremely hot summers, to which each of the three teleconnection patterns were found to have a similar degree of contribution. The statistical model reproduces the interannual variability along with the long-term T850JP trend. The PJ pattern has the largest effect on the interannual variability of T850JP, probably due to the PJ teleconnection occurring over a longer timescale compared with the other two patterns. The reconstructed T850JP also displays a warming trend associated with an increasing trend in the CGT index, which may be a factor, along with the direct thermodynamic effects due to global warming, to explain the long-term increase in the heat wave frequency in Japan. [ABSTRACT FROM AUTHOR]

In this work, we examine the impacts of wintertime western disturbances (WDs) on precipitation over the southeastern Tibetan Plateau (SETP) during 1979–2017. Most WDs initially formed within the central Himalayas and dissipated over southeastern China and its neighboring seas. All extreme precipitation days over the SETP region during this period were linked to WDs, often when a WD center entered the region about 10° west of the SETP. Significant warm temperature anomalies over the SETP region preceding WD arrival favored moisture accumulation there. During the passage of WDs, both strong updrafts along the eastern edge of the WD and rapid decreases in temperature over the SETP (linked to WD‐associated cold anomalies below 300 hPa) favored the occurrence of wintertime precipitation over the SETP region. WD strength and moist static energy variations over the SETP region were key factors affecting the intensity of WD‐induced precipitation, while the size and latitudinal position of the WD played minor roles. The relatively strong WDs that produced extreme precipitation days over the SETP mainly originated above the eastern Mediterranean Sea, northern Arabian Peninsula, and Iranian Plateau. A distinct transition of the circumglobal teleconnection pattern modulated the generation of strong‐intensity WDs within these major genesis regions, as well as their eastward propagation and the generation of extreme precipitation over the SETP region. Key Points: All wintertime extreme precipitation days over the southeastern Tibetan Plateau (SETP) occurred in tandem with western disturbances (WDs)Most WDs with extreme precipitation originated in a band extending from the Eastern Mediterranean to the Iranian PlateauThe circumglobal teleconnection modulated the effects of WDs on precipitation over the SETP [ABSTRACT FROM AUTHOR]

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.

The thermal effect of the Tibetan Plateau (TP) on the northern hemisphere climate has long been a hot topic of scientific research. However, the global effects of the TP heat source are still unclear. We investigate the teleconnection patterns coincident with the TP heat source in boreal summer using both observational data and numerical models including a linearized baroclinic model and an atmospheric general circulation model. The western TP shows the most intense variability in atmospheric heating and the most active connection to atmospheric circulations. The surface sensible heating component of the western TP heat source is associated with a high-latitude wave train propagating from North Japan to central North America through the Bering Sea and Canada. The radiative heating component is accompanied by a wavenumber-4 wave train over Eurasia. We focus on the global zonally-oriented pattern that is connected with the latent heat release from the western TP, referred to here as the TP–circumglobal teleconnection (TP-CGT). The TP-CGT pattern is triggered by the western TP latent heating in two parts starting from the TP: an eastward-propagating wave train trapped in the westerly jet stream and a westward Rossby wave response. The TP-CGT accounts for above 18% of the total variance of the circumglobal teleconnection pattern and modulates mid-latitude precipitation by superimposition. The western TP is the key region in which diabatic heating can initiate the two atmospheric responses concurrently, and the heating over northeastern Asia or the Indian Peninsula is unable to induce the circumglobal pattern directly. The unique geographical location and strong tropospheric heating also make the western TP as a "transit area" of transferring the indirect impact of the Indian summer monsoon (ISM) to the TP-CGT. These results enhance our understanding of the relationship between the circumglobal teleconnection and the ISM and is helpful for improving the prediction of the circumglobal teleconnection variability. [ABSTRACT FROM AUTHOR]

The circumglobal teleconnection (CGT) is an important mode of circulation variability, with an influence across many parts of the northern hemisphere. Here, we examine the excitation mechanisms of the CGT in the ECMWF seasonal forecast model, and the relationship between the Indian summer monsoon (ISM), the CGT and the extratropical northern hemisphere circulation. Results from relaxation experiments, in which the model is corrected to reanalysis in specific regions, suggest that errors over northwest Europe are more important in inhibiting the model skill at representing the CGT, in addition to northern hemisphere skill more widely, than west-central Asia and the ISM region, although the link between ISM precipitation and the extratropical circulation is weak in all experiments. Thermal forcing experiments in the ECMWF model, in which a heating is applied over India, suggest that the ISM does force an extratropical Rossby wave train, with upper tropospheric anticyclonic anomalies over east Asia, the North Pacific and North America associated with increased ISM heating. However, this eastward-propagating branch of the wave train does not project into Europe, and the response there occurs largely through westward-propagating Rossby waves. Results from barotropic model experiments show a response that is highly consistent with the seasonal forecast model, with similar eastward- and westward-propagating Rossby waves. This westward-propagating response is shown to be important in the downstream reinforcement of the wave train between Asia and North America. [ABSTRACT FROM AUTHOR]

Recently there has been an increase in interest in hemispheric-scale teleconnections. The circumglobal teleconnection pattern (CGT) is a special zonal phase of the jet whereby disturbances originating near the South Asian jet entrance, or within the jet, remain trapped and can propagate around the globe. Recent studies have shown that this CGT pattern affects climatic conditions across the entire hemisphere, from North America to Europe to South Asia. The CGT is generally thought of as an internal mode of atmospheric variability, but this study aims to examine whether the CGT can be forced during the boreal winter when the jet is at its peak amplitude. To accomplish this, we make use of a simple general circulation model to test whether an imposed diabatic heating anomaly in the tropics can force the CGT. Results from the simple model show that heating over the equatorial Indian Ocean and cooling over the western Tropical Pacific Ocean can drive a CGT-like pattern. Lagged regression using pentad reanalysis data supports these results, showing that a dipole convection pattern in these same regions precede the CGT. [ABSTRACT FROM AUTHOR]

An extreme drought occurred from 1876 to 1878 in most regions of northern China, leading to a series of social impacts, including harvest failures, price inflation, and population migration. Most regions of the Northern Hemisphere concurrently experienced extreme drought. Here, we use reconstructed high-resolution hydroclimatic (Palmer Drought Severity Index/precipitation) datasets and investigate the seasonal–annual hydroclimatic spatial patterns and drought intensity in North China from 1876 to 1878. Furthermore, we select combined sea surface temperature (SST) modes with positive Indian Ocean Dipole (IOD)/Pacific Decadal Oscillation (PDO) and El Niño conditions from the 1200-year control run simulations of HadCM3 to determine the possible causes of this severe drought. The extent and intensity of the selected SST modes are similar to those in the Pacific and Indian oceans during the 1876–1878 period from the National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed Sea Surface Temperature (ERSST) dataset. The results show that the large-scale drought of 1876–1878 is mainly driven by El Niño and a positive PDO, while the effect of IOD is not significant. El Niño may trigger the circumglobal teleconnection of the Northern Hemisphere. And, the meridional disturbance of the eastward Rossby wave train at mid-latitudes may change the intensity of the troughs and ridges and further block water vapor transport from ocean to land. [ABSTRACT FROM AUTHOR]

The impact of sea surface temperature anomalies (SSTAs) over the North Pacific (32°–45° N, 140° E–150° W) and North Atlantic (north: 52°–68° N, 60°–20° W; south: 0°–30° N, 100°–40° W) on the summer surface air temperature (SAT) over the Mongolian Plateau (MP) is studied using NCEP/NCAR and Climatic Research Unit (CRU) reanalysis data. The results show that the circumglobal teleconnection (CGT) wave train related to the SSTAs in the North Atlantic propagates along with the westerly jet, resulting in a geopotential height anomaly at 200 hPa that favors an SAT anomalies (SATAs) on the MP. In addition, an anticyclonic (cyclonic) circulation anomaly with the characteristics of an atmospheric ultralong wave at 200 hPa over the Eurasia–Pacific region associated with the SSTAs in the North Pacific is also responsible for the positive (negative) SATAs on the MP. The results further reveal that the SSTAs in the North Pacific and the northern North Atlantic act synergistically on the SATAs on the MP. If there are strongly positive (negative) SSTAs in the North Pacific and the northern North Atlantic at the same time, the probability of a strongly positive (negative) SATAs on the MP is 67% (100%) for the research period of this paper. However, if the strongly positive (negative) SSTAs only appear in the North Pacific or the northern North Atlantic, the probability of a strongly positive (negative) SATAs on the MP is less than 34% (31%). The synergism of the SSTAs in the North Pacific and the northern North Atlantic enhances Rossby wave energy in Eurasia, with more Rossby wave energy spreading to the MP that in turn causes the circulation anomaly on the MP. Meanwhile, baroclinic conditions at 850 hPa north of the North Pacific, the northern and southern North Atlantic favor a northward shift of the westerly jet that guides the Rossby wave downstream so that the circulation anomaly that produces the SATAs on the MP is located in the mid–high latitudes. [ABSTRACT FROM AUTHOR]

The Silk Road Pattern (SRP) is the dominant teleconnection in the upper troposphere and is closely linked to the Asian summer monsoon in both India and subtropical East Asia. This study reveals a close interannual relationship between the SRP in September and the South China Sea (SCS) summer monsoon (SCSSM) withdrawal. During positive phases of the September SRP (characterized by negative geopotential height anomalies over eastern Europe and central China and positive geopotential height anomalies over western Europe, western and central Asia, and Northeast Asia), withdrawal of the SCSSM tends to be later than normal. The September SRP could impact the SCSSM withdrawal via modulating atmospheric circulation anomalies over the SCS. In particular, the notable low‐level cyclonic anomaly over central China related to the positive phase of the September SRP induces significant southwesterly wind anomalies over the northern SCS, which favour a late withdrawal of the SCSSM. This study suggests that besides the tropical factors that have been identified by previous studies, the climate systems over mid‐latitudes may also play an important role in contributing to interannual variability of the SCSSM withdrawal. [ABSTRACT FROM AUTHOR]

Modern observations have demonstrated that the Indian Summer Monsoon (ISM) rainfall shows a west-east dipole pattern influenced by the Circumglobal Teleconnection (CGT). In this study, we investigated the temporal evolution of the ISM rainfall and the dynamic mechanisms using a set of transient simulations (TraCE-21 ka, simulation of Transient Climate Evolution over the past 21,000 years) during the Last Deglaciation. The results show that, the responses of ISM rainfall always present west-east dipole patterns under different forcing factors and such responses are closely related to the CGT-like wave trains in the Northern Hemisphere summer. During the Last Deglaciation, the ISM is strengthened and summer precipitation is increased over northwestern India but decreased over northeastern India. With increased solar insolation and atmospheric CO 2, as well as reduced global ice volume, the positive-phase CGT-like wave trains, characterized by an anticyclonic anomaly over west-central Asia (WCA) and a cyclonic anomaly over the Tibetan Plateau (TP), tends to shift the South Asian High (SAH) westward and strengthens (weakens) the divergence in the upper-troposphere over northwestern (northeastern) India. Simultaneously, the intensification of the monsoon trough (MT) due to the local baroclinic atmospheric circulations over Arabian Peninsula-Iranian Plateau leads to the moisture convergence (divergence) over northwestern (northeastern) India in the lower troposphere. For the Younger Dryas (YD) cooling event, the pattern of CGT-like wave trains and Indian summer rainfall do the reverse. Theses rainfall responses highlight the spatial complexity of multi-scale Indian monsoon evolution and it should be cautious when explaining geological proxies. • A west-east dipole pattern of Indian summer monsoon (ISM) precipitation on orbital and millennial timescales is presented. • The dipole precipitation pattern is strongly modulated by the circumglobal teleconnection (CGT). • ISM-CGT relationship is realized via dynamic coupling of upper-troposphere circulation and low-level moisture fluxes. [ABSTRACT FROM AUTHOR]

This study examines the formation of circulation patterns favorable to wintertime extreme precipitation events over South China between 1979 and 2013. During these extreme precipitation events, a barotropic wave train having seven centers of action was observed to extend from the Arabian Sea to the west coast of the North American continent with a maximum amplitude at 300 hPa. A center of action located over South China, comprised of cyclonic anomalies, favored powerful updrafts and large-scale moisture convergence over South China. About 77% of wintertime extreme precipitation events in South China were preceded by European blocking events. The formation mechanism for the 141 precipitation events with pre-existing European blocking highs is presented. Rossby wave energy propagation associated with the positive phase of circumglobal teleconnection enabled the European blocking event with a lead time of ∼10 days to generate cyclonic anomalies over South China. Moreover, significant warm anomalies were present over South China before the onset of these blocking-related extreme precipitation events. Increases in atmospheric moisture holding capacity associated with these warm anomalies enabled a buildup of precipitable water via moisture fluxes into South China through the western and southern boundaries. Onset of the extreme precipitation events was then triggered by the intrusion of cold temperature anomalies from the north, which lifted warm moist air upward from the surface and lowered the moisture holding capacity, producing large amounts of precipitation. [ABSTRACT FROM AUTHOR]

The favorable circulation pattern and moisture sources for wintertime extreme snowfall events over East China during 1979–2017 are explored. A coupling between a Rossby wave train along the subtropical jet stream (i.e., the circumglobal teleconnection) and central Siberian blocking activities plays a leading role in triggering these events. This coupling leads to the formation of cyclonic anomalies, strong southwesterly winds, large‐scale moisture convergence, and powerful updrafts over East China, all of which prompt the occurrence of precipitation. The central Siberian blocking activities increase the fraction of the precipitation that falls as snowfall through enhancing cold air intrusion into East China. An Eulerian moisture budget analysis reveals that moisture fluxes across the southern and western boundaries of the East China region provide the moisture for the precipitation during the extreme snowfall events, while the increase in the precipitable water helps extend the duration of these events. The geographic distributions of the evaporative moisture sources for both the snowfall and rainfall during these extreme events are estimated using a Lagrangian approach. This analysis reveals that the major moisture sources for snowfall originate from land areas, while those for the rainfall originate from sea areas. In addition, local moisture recycling over East China provides a remarkably higher relative contribution to snowfall than it does to rainfall. The identification of the synoptic conditions and moisture sources not only improves our understanding of the formation of extreme snowfall events over East China but also provides forecast guidance for extreme snow disasters. Key Points: The coupling of the circumglobal teleconnection and central Siberian blocking activities is critical to East China extreme snowfall eventsCentral Siberian blocking activities favor a considerable part of the precipitation over East China occurring as snowfallLocal moisture recycling plays a leading role in providing the evaporative moisture sources for extreme snowfall events [ABSTRACT FROM AUTHOR]

Understanding the variations of extreme weather/climate events is important to improve the seasonal forecast skill of such harmful events. Previous studies have linked boreal summer hot extremes to decaying El Niño-Southern Oscillation (ENSO) events at the interannual scale, but how these hot extreme episodes respond to developing ENSO events remains unclear. Using observational analyses, we demonstrate strong linkages between developing ENSO and extreme heat events in northern continents. In particular, heat extremes in North America, Eastern Europe–Central Asia and Northeast Asia tend to be more frequent during La Niña developing summers and less frequent during El Niño developing phases. Associated atmospheric changes reveal that developing ENSO events feature a circumglobal teleconnection (CGT) pattern over the mid-latitudes. In the La Niña developing summer, this CGT pattern exhibits enhanced geopotential height and anomalous anticyclones over North Pacific, North America, Eastern Europe–Central Asia and Northeastern Asia, and the jet stream generally shifts northward. The atmospheric circulation changes lead to more persistent weather conditions that favor extreme heat events in mid-latitudes. Conversely, opposite changes associated with developing El Niño can inhibit heat extremes in the above locations. The responses of heat extremes to different types (i.e., conventional Eastern Pacific and Modoki Central Pacific) and durations (1 and 2 year) of ENSO events are also discussed.

Abstract Due to the lack of reliable climatic proxies, the variability of precipitation in "Westerlies-dominated" arid central Asia (ACA) during the Holocene is debated. Here, we present a high-resolution (ca. 100 years per sample) organic carbon isotopic composition (δ13C org) record from bulk samples from the Lujiaowan10 section in the northern piedmont of the Tienshan Mountains, Xinjiang Province, northwestern China. A robust chronology is provided by K-feldspar pIRIR dating. During the early to middle Holocene, from 12 ka to 6 ka (1 ka = 1000 years ago), the δ13C org values are relatively invariant with an average of around −22.5‰. After about 6 ka, the δ13C org values exhibit an overall negative trend with the most negative value of −25.2‰ occurring in the uppermost part of the section, within the interval of Holocene soil formation. Our investigation of the relationship between modern climatic variables and surface soil δ13C org values from the region reveals a significant negative correlation between surface soil δ13C org and summer precipitation amount (June to August). Therefore, we used the calibration model between summer precipitation amount and surface soil δ13C org to quantitatively reconstruct the Holocene summer precipitation history of the study area. Our results indicate that summer precipitation in the region was the lowest (ca. 85 mm) during the early to middle Holocene (12–6 ka); however, after 6 ka, the precipitation increased continuously to about 137 mm per year at the present. Our reconstruction is generally consistent with climate simulation results and with regional effective moisture records. Thus, we conclude that the highest summer precipitation, and therefore the wettest climate, occurred in the late Holocene. Decreasing Northern Hemisphere summer insolation may have effected a change in the CGT (circumglobal teleconnection) from a positive to negative phase and, together with more negative trends in AO (Arctic Oscillation) or NAO (North Atlantic Oscillation), may have been responsible for the observed pattern of summer precipitation evolution. Highlights • Loess δ13C org is an effective summer precipitation proxy in the 'westerlies-dominated' arid central Asia (ACA). • Summer precipitation was low during 12–6 ka, and it increased consistently after ~6 ka in the core area of ACA. • Decreasing of NH summer insolation and turning negative of AO/NAO caused the increase of summer precipitation since ~6 ka. [ABSTRACT FROM AUTHOR]

In this study, authors investigated both year-to-year variation and long-term changes in precipitation in Northwest China. Based on statistical analysis, anomalous regional rising motion and convergence of moisture flux are identified as the causes of year-to-year variation of precipitation in Northwest China. Aside from the year-to-year variation, long-term changes in these regional circulations also led to a significant enhancement of precipitation in Northwest China after the mid-1980 s. In addition, the changes in these regional circulations are controlled by anomalous eddies in the upper troposphere. The anomalous regional rising motion and the convergence of moisture flux are a result of the anomalous divergence of wind in the upper troposphere and southeasterly wind in the mid to lower troposphere. Both are induced by a negative center of eddy height in the upper troposphere to the west of Northwest China. It is also noted that the year-to-year variation of the negative center is concurrent with the circumglobal teleconnection. On the other hand, the long-term change in the negative center is concurrent with the weakening of midlatitude eddies in the mid-1980 s. [ABSTRACT FROM AUTHOR]

Considering the possible lag of the impact of the winter Arctic oscillation (AO) on the subsequent summer climate anomalies over East Asia and that the relationship between them may differ during different periods, the non-stationary relationship between these systems and the corresponding mechanism of connection are investigated in this study. A regime shift of the winter AO index was detected around 1988, with mean AO indices of −0.69 and 0.10 for 1958-1987 and 1988-2014, respectively, and their differences passed the 99% confidence level. After the regime shift and especially since the early 1990s, negative (positive) winter AO indices corresponds with positive (negative) height anomalies over central East Asia (CEA) during the following summer. Meanwhile, the westerly jet stream tends to be northward (southward) and the East Asian summer monsoon (EASM) is strong (weak), which contributes to the warm (cold) anomalies over central China and the significantly enhanced (reduced) rainfall over northeast China. After the early 1990s, the enhanced connection of the winter AO and the following summer height anomalies over CEA is largely due to the enhanced connection of the winter AO and the following Indian summer monsoon (ISM) rainfall during the same period. The negative (positive) winter AO favors increasing (decreasing) rainfall over the ISM region, which triggers a positive (negative) circumglobal teleconnection (CGT), and results in significantly positive (negative) height anomalies over CEA. One way the winter AO could influence the following ISM rainfall is that the negative (positive) winter AO memories will be maintained by the positive (negative) sea surface temperature (SST) anomalies over the tropical Atlantic of the following summer, which are responsible for the La Niña (El Niño) developing phase and increases (decreases) the ISM rainfall. [ABSTRACT FROM AUTHOR]

ABSTRACT: A quasi‐stationary circumglobal teleconnection (CGT) in the Northern Hemisphere in summer is associated with summer droughts and floods and exhibits some vorticity anomalies. The CGT anomaly and its forcing sources were explored based on reanalysis data and the EI Nino 3.4 index. The term ∂v′/∂x was used to describe the multi‐scale variability of vorticity and the CGT anomaly, which had a 7‐wavenumber and a small interannual amplitude of ∂v′/∂x from 1987 to 2000 but a large amplitude from 2000 to 2010. The interannual CGT intensity was enhanced by an EI Nino sea surface temperature pattern in spring via key forcing sources that led to an enduring effect on the summer circulation; it was also a response to anomalies in Tibetan Plateau (TP) heating in spring–summer in two ways. First, increasing TP heating in the mid‐eastern TP caused an eastward shift and northerly extension of the South Asian High (SAH), which exhibited a Tibetan‐mode SAH as well as a 7‐wavenumber CGT anomaly. This enhanced the CGT over Eurasia and extended the wave pattern of the EI Nino contribution over the North Pacific and Eurasia. Second, increased snow cover in the west of the TP could have offset the TP heating effect and adjusted the position and intensity of the Tibetan‐mode SAH. This phenomenon weakened vorticity and wave amplitude and finally led to a weak CGT anomaly. This finding indicates a coupled effect of double‐mode TP heating (i.e. both heating and TP snow cover) on the CGT anomaly, which reveals the linkage of TP heating with weather over Eurasia and North America that could be predicted by TP heating and other factors. [ABSTRACT FROM AUTHOR]

The summer circumglobal teleconnection (CGT), which crucially affects mid-latitude climate in the Northern Hemisphere, is generally characterized by a mid-latitude wave train along the strong upper-tropospheric westerly jet. In this study a significant change is identified in the spatial pattern of the early-summer CGT after the late 1970s: An additional high-latitude wave train has occurred over northern Eurasia. Based on observational evidences and simulation results with a linear baroclinic model, it is proposed that the post-1970s CGT change is induced by enhanced impact of rainfall over southern Europe (SE) after the late 1970s. Specifically, the mid-latitude wave train of CGT in early summer is dominated by Indian rainfall before the late 1970s but by both rainfall over India and SE after the late 1970s; the high-latitude wave train of CGT occurring after the late 1970s, however, is induced only by the SE rainfall. The coupled Indian and SE rainfall after the late 1970s, which is probably due to the basic flow change over the North Atlantic, induce both mid-latitude and high-latitude wave trains of the CGT. [ABSTRACT FROM AUTHOR]

Using ERA5 reanalysis data, this study reveals the relationship between variations of winter surface solar radiation (SSR) over the Southeast Asian low-latitude highlands (SEALLH) and the circumglobal teleconnection (CGT). The results indicate that the winter SSR shows significant interannual variability with a monopole pattern over the SEALLH. The winter CGT significantly modifies the interannual variability of the simultaneous SSR over the SEALLH. When the CGT is in its positive phase, a wave train appears over the central-eastern Mediterranean, Arabian Peninsula, Indian Peninsula, and East Asia with anomalous "northerly, southerly, northerly, southerly" nondivergent meridional wind. The associated southerly wind anomalies around the SEALLH favor greater transport of water vapor from the Bay of Bengal that converges over the SEALLH. The consequent increase of cloud cover in the lower–middle troposphere and aerosol optical depth (AOD) reduce the winter SSR over the SEALLH. When the CGT is in its negative phase, the anomalous northerly wind with anomalous divergence increases the winter SSR over the SEALLH by decreasing both cloud cover in the lower–middle troposphere and the AOD. • The interannual variability of winter SSR shows a monopole pattern over the SEALLH. • CGT impacts the interannual variability of winter SEALLH SSR via exciting a gyre. • Winter SEALLH SSR is regulated by local cloud cover and AOD induced by the gyre. [ABSTRACT FROM AUTHOR]

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In this study, a significant link is found between summer rainfall over southern-central Europe ( SCE) and the circumglobal teleconnection ( CGT). It is proposed that the SCE rainfall drives the CGT. The downstream-extended CGT-like wave train is reproduced with a linearized barotropic vorticity equation model forced by an upper-tropospheric divergence over SCE. This study provides an alternative mechanism for the formation and maintenance of the CGT. [ABSTRACT FROM AUTHOR]

This paper provides another look at the response of the Asian summer monsoon (ASM) to insolation forcing and oceanic feedback during the Holocene, using a fully coupled general circulation ocean–atmosphere model forced by Earth’s orbital variations. The model results revealed a recurrent circumglobal teleconnection (CGT) pattern in the summertime (June–July–August) mid-latitude circulation of the Northern Hemisphere during the Holocene. The CGT index showed a decreasing trend before ~5 ka BP and a slight increasing trend afterwards, affected by the combined effects of summer insolation, Indian summer monsoon (ISM), North Atlantic and Indian Ocean–western Pacific Ocean sea surface temperature (SST). The CGT showed a close relationship with ASM precipitation and surface air temperature during the Holocene and, therefore, could act as a bridge linking the ASM to insolation, high-latitude forcing (North Atlantic SST), and low-latitude forcing (tropical Ocean SST). We emphasize that the mid-latitude atmospheric circulation has been a key factor for the evolution of the ASM during the Holocene. In addition, the CGT provides a viable explanation for the out-of-phase relationship in the moisture evolution but an in-phase relationship in the speleothem δ18O between arid central Asia and the ISM region during the Holocene. [ABSTRACT FROM AUTHOR]

Using the latest daily observational rainfall datasets for the period 1961-2008, the present study investigates the interannual variability of June-September (JJAS) mean rainfall in northern China. The regional characteristics of JJAS mean rainfall are revealed by a rotated empirical orthogonal function (REOF) analysis. The analysis identifies three regions of large interannual variability of JJAS rainfall: North China (NC), Northeast China (NEC), and the Taklimakan Desert in Northwest China (TDNWC). Summer rainfall over NC is shown to have displayed a remarkable dry period from the late 1990s; while over NEC, decadal-scale variation with a significant decreasing trend in the last two decades is found, and over TDNWC, evidence of large interannual variability is revealed. Results also show that the interannual variability of JJAS rainfall in northern China is closely associated with the Northern Hemisphere circumglobal teleconnection (CGT). Correlation coefficients between the CGT index and regional-averaged JJAS mean rainfall over NC and NEC were calculated, revealing values of up to 0.50 and 0.53, respectively, both of which exceeded the 99% confidence level. [ABSTRACT FROM AUTHOR]

The interannual variability of Indian summer monsoon rainfall (ISMR) have been examined in association with the variability of 2-metre surface temperature (2mST), and lower- and upper-tropospheric pressure and wind patterns over the globe using ERA-40 reanalysis dataset. The analysis of 44-years (1958–2001) data reveals a recurrent negative phase of ENSO (La Niña) pattern. The increase in 2mST over tropical southwest Pacific and intensification of subtropical westerly jet stream over Indian Ocean due to the consequence of thermal wind balance. The intensification of jet forms anticyclonic anomaly which intensifies Mascarene High as the atmospheric response is equivalent barotropic over there. The Mascarene High intensifies the cross equatorial flow and hence Indian summer monsoon (ISM) circulation. While, during non-ENSO years data reveals a recurrent positive relationship of ISMR with 2mST anomaly over northwest of North Atlantic and intensification and shift of North Atlantic westerly jet stream to higher latitudes. The jet stream intensifies the meridional vorticity dipole, formed due to Azores High and the entrance of Asian subtropical westerly jet stream over North Atlantic and west Mediterranean Sea. A wave activity flux for stationary Rossby waves diverges out of the vorticity dipole toward downstream along the Asian jet stream over the Eurasian region. The jet stream acts as a wave guide and the successive troughs and ridges of the Rossby waves travel along the jet and influence upper-tropospheric Tibetan High and hence ISM circulation. [ABSTRACT FROM AUTHOR]

Cool and wet weather conditions hit northern Central Asia, East Asia and central North America during the 2009 summer in concert with a strong jet stream and a prominent meandering upper-level circulation in the Northern Hemisphere mid-latitudes despite the fact that the year 2009 is the fifth warmest year globally in the modern record. It is found that the conspicuous atmospheric variability in the entire Northern Hemisphere mid-latitudes during the summer of 2009 was caused by a combination of teleconnections associated with significant tropical thermal forcings, strong polar forcing, and interaction between high-frequency weather events and climate anomalies. The strong negative circumglobal teleconnection pattern associated with the deficient Indian summer monsoon rainfall and developing El Niño condition was the major contributor to the cool and wet summer in June. On the other hand, the July weather conditions were attributable to the high-latitude impact of the unprecedented negative Arctic Oscillation, together with the Rossby wave response to the subtropical heating generated by convective activities over the Western North Pacific summer monsoon region. It is also noted that enhanced storm track activity and frequent cold surges from high-latitudes may have played a role in the cool and wet summer over the regions of interest.

The formation mechanism for wintertime extreme precipitation events over the southeastern Tibetan Plateau (SETP) is explored. The crucial step in the development of these events was the emergence of a cyclonic anomaly above the Tibetan Plateau. Wave activity fluxes along a Rossby wave train embedded in the subtropical jet stream (i.e., the circumglobal teleconnection) played the dominant role in producing this cyclonic anomaly, supported by weaker wave activity fluxes along a second Rossby wave train originating over Scandinavia. The cyclonic anomaly then moved over the SETP, favored strong updrafts, large‐scale moisture convergence, and intense precipitation. Extreme precipitation was more likely to ensue when the arrival of the cyclonic anomaly was preceded by persistent warm anomalies over the SETP, which favored moisture accumulation there. Temperatures above the SETP dropped sharply around the onset of the extreme precipitation. A heat budget analysis indicates that adiabatic cooling associated with convective ascent along the downstream edge of the cyclone played a leading role in this temperature drop, while a cold air intrusion associated with an anticyclonic anomaly over western Siberia (one center of action along the second wave train) played a complementary role. An Eulerian moisture budget analysis shows that variations in precipitable water delayed the onset and enhanced the intensity of these events, with moisture for precipitation delivered to the SETP mainly through the western and southern boundaries. A companion Lagrangian moisture source analysis reveals that the land areas south of the Tibetan Plateau typically contributed 78.7% of the moisture supply for these events. Key Points: The circumglobal teleconnection pattern played a key role in the development of extreme precipitation over the southeastern Tibetan PlateauThe evolution of precipitable water delayed the timing and increased the intensity of extreme precipitation eventsLand areas to the south of the Tibetan Plateau contributed 78.7% of the moisture for the extreme precipitation [ABSTRACT FROM AUTHOR]