Circumglobal Teleconnection Pattern Research Articles

Atmospheric Teleconnections Responsible for the Dominant Patterns of Interannual Variability in Summer Drought over Northern Asia

Abstract Summer drought over northern Asia (NA) seriously threatens the local fragile ecological environment and social economy development. In this study, using the standardized precipitation evapotranspiration index (SPEI), we first identify the dominant modes of interannual variability in summer drought condition over NA and then explore the atmospheric patterns responsible for the formation of the modes. The results show that the first empirical orthogonal function (EOF1) mode of summer SPEI over NA exhibits a meridional dipole pattern, which is influenced primarily by the polar–Eurasian teleconnection (POL) and circumglobal teleconnection (CGT) patterns. Under the influence of negative POL and positive CGT patterns, there is an anomalous anticyclone (cyclone) over northwestern Siberia (Lake Baikal to Northeast China). Such atmospheric circulations lead to meridional dipole patterns in air temperature, moisture condition, vertical motion, and cloud cover over NA, favoring decreased (increased) precipitation and increased (decreased) potential evapotranspiration over northern (southern) NA, finally contributing to the formation of EOF1. The second EOF (EOF2) shows an approximate zonal dipole pattern, which is influenced by the British–Baikal Corridor (BBC) and Scandinavia (SCA) teleconnection patterns. The positive BBC and SCA patterns can lead to an anomalous anticyclone over the Ural Mountains and a cyclone over Lake Baikal. Such atmospheric circulations result in a zonal dipole pattern in precipitation and potential evapotranspiration over NA through changing the local moisture condition, air temperature, and radiation, consequently favoring the formation of EOF2. Fitting analysis indicates that the aforementioned atmospheric factors can explain 76% (55%) of the interannual variability of EOF1 (EOF2).

Synergistic impacts of wintertime regional snow anomalies in the Northern Hemisphere on the summer rainfall pattern in China

Changes in winter snow cover in the Northern Hemisphere (NH) could have a profound impact on mid-latitude weather. Previous studies have focused on the role of regional, e.g. Eurasian or Tibetan, snow cover in summer precipitation anomaly, without considering the synergistic impacts of hemispheric wintertime snow. In this study, we find that the dominant pattern of the NH winter overall snow cover anomaly with a synergistic impact, has a stronger cross-seasonal association with the China’s summer rainfall pattern than regional snow cover anomaly. We summarize three synergistic impact paths of regional snow cover. One is extratropical path, that is the westerlies are affected by less snow in Europe through the snow-soil moisture-atmospheric feedback, and the influence is strengthened by less snow in Mongolia through enhanced temperature anomalies. The second is subtropical path, that is the meridional thermal difference anomaly caused by more snow anomaly on the Tibetan Plateau is strengthened by less Mongolian snow and then impacts the behavior of the upper-tropospheric westerly jet. Third, concurrently, more North American snow enhances the above two synergistic influence paths via the Circumglobal Teleconnection pattern. These three paths can be simultaneously reflected in the associated circulations of the first mode of NH snow cover. Their synergistic impacts eventually influence the meridional East Asia-Pacific pattern circulation anomalies in summer, leading to increased precipitation in the Yangtze River Basin. The cross-seasonal influences of synergistic effects of multiple regional snow anomalies can be identified by CMIP6 multi-model ensembles, particularly the impact of European snow cover.

Relationship between the Silk Road and Circumglobal Teleconnection Patterns on the Interannual and Interdecadal Timescales

The Silk Road pattern (SRP) and circumglobal teleconnection pattern (CGT) are two well-known teleconnection patterns, representing the summer circulation variations of the Northern Hemisphere mid-latitudes, which have different definitions but are often regarded as one teleconnection pattern. In view of the distinct features of the SRP/CGT on the interannual (IA) and interdecadal (ID) timescales, the present study investigates the linkages and differences between the SRP and CGT on the two timescales, respectively. On the IA timescale, both the SRP and CGT feature a similar circumglobal wave train structure with strong and significant centers over Eurasia but show clear independence. Specifically, the SRP and CGT illustrate largely the mid-/high-latitude-related and tropics-related parts of the Northern Hemisphere upper tropospheric circulation variations, respectively. Also, the CGT shows a stronger connection to the Indian summer monsoon (ISM) heating and El Niño–Southern Oscillation than the SRP, which makes the CGT more like a tropical forcing-driven atmospheric mode and the SRP more like an internal atmospheric mode. The linkages and differences between them are associated with their asymmetrical relationship during their positive and negative phases, which are attributed mainly to the asymmetrical impact of the ISM heating/cooling on the Eurasian circulations. On the ID timescale, the SRP and CGT are characterized by a coherent two-wave train structure over Eurasia and feature a similar teleconnection pattern over Eurasia, which is associated with the Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation. The present findings on their linkages and differences are helpful in understanding the variability and prediction of the SRP and CGT.

Predictability of the unprecedented 2022 late summer Yangtze River Valley and Tibetan Plateau heatwaves by the NCEP CFSv2

By using the one-month‑lead predictions from the Climate Forecast System version 2 (CFSv2), this study evaluates the predictability of the unprecedented heatwaves hitting the Yangtze River Valley (YRV) and Tibetan Plateau (TP) in 2022 late summer. CFSv2 successfully reproduces the central China heatwaves, but shows distinct performances between the YRV and TP. Specifically, CFSv2 exhibits stronger ability to capture TP heatwaves that is close to the highest historical prediction record, whereas the predicted extremity of the YRV heatwaves is underestimated compared to the observations. The accurate simulation of the intense upper-level circumglobal teleconnection (CGT) pattern, particularly for the East Asia CGT center (CGT EA), is essential for the success predictions of high temperature anomalies, due to the strong and accurate linkage between CGT EA and YRV/TP heatwaves in CFSv2. The intense CGT pattern is associated with a stronger model response to developing La Niña in CFSv2, quite different from the observed leading contribution from the unprecedented Pakistan rainfall. The impacts of developing La Niña and Pakistan rainfall on the intense CGT pattern are significantly overestimated and underestimated by CFSv2, respectively. Additionally, the La Niña-forced enhancement of lower-level western Pacific subtropical high (WPSH) plays another important role in YRV heatwaves, but the reproduction of WPSH intensity is unable to reflect YRV heatwaves in CFSv2 due to the southeastward shift of the predicted WPSH location, which should be responsible for the underestimated YRV heatwaves. The model improvements in aspect of the Pakistan rainfall, El Niño-Southern Oscillation–CGT linkage and lower-level WPSH location should help effectively predict the central China heatwaves in the future.

The influence of natural variability on extreme monsoons in Pakistan

The monsoons in Pakistan have been exceptionally harsh in recent decades, resulting in extraordinary drought conditions and record flooding events. The changing characteristics of extreme events are widely attributed to climate change. However, given this region’s long history of floods and droughts, the role of natural climate variability cannot be rejected without a careful diagnosis. Here, we examine how oceanic and atmospheric variability has contributed to unusual precipitation distributions in West South Asia. Variations in sea surface temperatures in the tropical Pacific and northern Arabian Sea, and internal atmospheric variability related to the circumglobal teleconnection pattern and the subtropical westerly jet stream, explain more than 70% of monthly summer precipitation variability in the 21st century. Several of these forcings have co-occurred with record strength during episodes of extreme monsoons, which have exacerbated the overall effect. Climate change may have contributed to increased variability and the in-phase co-occurrences of the identified mechanisms, but further research is required to confirm any such connection.

Validation of boreal summer tropical–extratropical causal links in seasonal forecasts

Abstract. Much of the forecast skill in the mid-latitudes on seasonal timescales originates from deep convection in the tropical belt. For boreal summer, such tropical–extratropical teleconnections are less well understood compared to winter. Here we validate the representation of boreal summer tropical–extratropical teleconnections in a general circulation model in comparison with observational data. To characterise variability between tropical convective activity and mid-latitude circulation, we identify the South Asian monsoon (SAM)–circumglobal teleconnection (CGT) pattern and the western North Pacific summer monsoon (WNPSM)–North Pacific high (NPH) pairs as the leading modes of tropical–extratropical coupled variability in both reanalysis (ERA5) and seasonal forecast (SEAS5) data. We calculate causal maps based on the Peter and Clark momentary conditional independence (PCMCI) causal discovery algorithm, which identifies causal links in a 2D field, to show the causal effect of each of these patterns on circulation and convection in the Northern Hemisphere. The spatial patterns and signs of the causal links in SEAS5 closely resemble those seen in ERA5, independent of the initialisation date of SEAS5. By performing a subsampling experiment (over time), we analyse the strengths of causal links in SEAS5 and show that they are qualitatively weaker than those in ERA5. We identify those regions for which SEAS5 data well reproduce ERA5 values, e.g. the southeastern USA, and highlight those where the bias is more prominent, e.g. North Africa and in general tropical regions. We demonstrate that different El Niño–Southern Oscillation phases have only a marginal effect on the strength of these links. Finally, we discuss the potential role of model mean-state biases in explaining differences between SEAS5 and ERA5 causal links.

Do heat waves worsen air quality? A 21-year observational study in Seoul, South Korea

Heat waves are generally known to deteriorate air quality. However, the impacts of heat waves on air quality can substantially vary depending on the characteristics of heat waves. In this study, we examine air quality changes in Seoul during heat waves and their associations with large-scale atmospheric patterns. For this, air quality data from 25 stations and meteorological data from 23 weather stations and reanalysis datasets during July and August of 2001–2021 are used. Under heat waves, the mean daily PM10, NO2, and CO concentrations decrease by 7.9 %, 6.1 %, and 4.6 %, respectively, whereas the mean daily PM2.5, O3, and SO2 concentrations increase by 4.1 %, 17.2 %, and 2.9 %, respectively. The atmospheric circulation under heat waves is less favorable for long-range transport of air pollutants to Seoul. The PM2.5/PM10 ratio increases under heat waves, indicating that the secondary formation of aerosols becomes more important under heat waves. 37 % of the heat wave days are accompanied by severe O3 pollution exceeding the O3 concentration standard in South Korea. There is a significant variability of air quality in Seoul within heat waves. The heat wave days with higher concentrations of PM2.5, PM10, O3, NO2, and CO than their non-heat wave means exhibit a prominent difference in large-scale atmospheric pattern from the heat wave days with lower concentrations. This difference is characterized by a zonal wave-like pattern of geopotential height, which is similar to the circumglobal teleconnection pattern known as one of the major patterns for heat waves in South Korea. This zonal wave-like pattern produces more stagnant conditions over Seoul.

Taylor expansion of the correlation metric for an individual forecast evaluation and its application to East Asian sub-seasonal forecasts

This study develops a skill evaluation metric for an individual forecast by applying a Taylor expansion to the commonly-used temporal correlation skill. In contrast to other individual forecast evaluation metrics, which depend on the amplitude of forecasted and observed anomalies, the so-called “association strength (AS) skill” is less affected by the anomaly amplitude and mainly depends on the degree of similarity between the forecasted and the observed values. Based on this newly developed index, the forecast skill is evaluated for an individual case, then, a group is categorized with respect to the AS skill. The cases with the highest AS skill exhibit the highest correlation skill than any group randomly selected, indicating that the AS skill is a powerful metric to evaluate the non-dimensionalized forecast skill. This strategy is adopted for the subseasonal East Asian summer precipitation forecasts produced by the UK Met Office’s ensemble Global Seasonal forecast system version 5 (GloSea5). In the group with the highest AS skill of the East Asian summer precipitation index (i.e., highest AS cases), the geopotential height anomalies showed quasi-stationary Rossby waves from the North Atlantic to East Asia. The spatial distribution of the dominant subseasonal anomalies for cases with the highest AS is distinct from the cases or groups with the lowest AS skill. Furthermore, the dominant pattern with the highest AS is not solely explained by any well-known typical subseasonal climate patterns, such as the Madden–Julian Oscillation, circumglobal teleconnection pattern, Pacific-Japan pattern, or the Summer North Atlantic Oscillation. This implies that the excitation of well-known climate patterns only partly contributes to increasing the mid-latitude climate predictability in the GloSea5.

The impact of tropical SST variability on the northern hemisphere circumglobal teleconnection pattern

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.

Impacts of Western Disturbances on Wintertime Precipitation Over the Southeastern Tibetan Plateau

AbstractIn 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.

Three leading coupled modes of summer rainfall with atmospheric circulations over northern East Asia

AbstractNorthern East Asia (NEA) is characterized by wet monsoon region in the east and mid‐latitude westerly‐controlled dry inland in the west in summer, where ecosystem and environment are vulnerable to rainfall variability. Previous studies found close relationship between summer rainfall in NEA and local large‐scale atmospheric circulation systems including East Asian upper‐tropospheric westerly jet (EAJ) and middle‐tropospheric trough (EAT), and lower‐tropospheric low over NEA (NEAL). However, major features of rainfall–circulation coupling over NEA are still not fully understood. This study addresses this issue using multivariate empirical orthogonal functions method based on the ERA‐Interim reanalysis dataset and the GPCP precipitation data for 1979–2018. Three leading coupled modes are identified, together accounting for 30–50% of summer rainfall year‐to‐year variance in most of NEA. The first mode features consistent decrease of rainfall in NEA related to the decelerated EAJ, southward shifted EAT, and weakened NEAL, with a significant interdecadal change in late 1990s. The second mode exhibits a meridional dipole rainfall pattern, with less rainfall along the East Asian subtropical rainy belt and more rainfall in southern Russian Far East, coupled with the northward shifted EAJ, weakened EAT, and intensified NEAL. The third mode presents a zonal dipole rainfall pattern, with suppressed rainfall in southern Russia east of northeast China and enhanced rainfall around the Lake Baikal related to the decelerated EAJ, and westward shifted EAT and NEAL. Further investigation shows that the first mode is remotely modulated by summer rainfall anomalies in the Sahel region and Atlantic Multidecadal Oscillation (AMO) on the interdecadal timescales, and by the Polar/Eurasia teleconnection on the interannual timescales. The second and third modes are, respectively, modulated by summer rainfall in the western North Pacific and north central‐eastern India through the East Asia‐Pacific and circumglobal teleconnection patterns.

Dominant Anomalous Circulation Patterns of Tibetan Plateau Summer Climate Generated by ENSO-Forced and ENSO-Independent Teleconnections

Abstract The interannual variability of Tibetan Plateau (TP) summer climate has tremendous impacts on both regional hydrological cycles and global climate. In this study, we extract four dominant modes of the summertime large-scale circulation over the TP and surrounding areas from both the observation and simulations by a coupled general circulation model, CAS-FGOALS-f3-L. Based on the 10-member tropical Pacific pacemaker experiments, the ENSO-forced and ENSO-independent signals are isolated, each of which is represented by two dominant modes. The two ENSO-forced modes correspond to ENSO developing and decaying summer, respectively. The positive phase of the developing (decaying) ENSO-related mode is characterized by an anomalous baroclinic cyclone (anticyclone) over the western TP excited by the variations of the tropical summer monsoon rainfall. During the El Niño developing summer, the Indian monsoon rainfall variation is driven by an eastward shift of the Walker circulation due to warm anomalies in the equatorial central-eastern Pacific, whereas during the El Niño decaying summer it is caused by the basinwide warming in the tropical Indian Ocean. The two ENSO-independent modes are associated with the summer North Atlantic Oscillation (SNAO) and the circumglobal teleconnection (CGT) pattern, respectively. The positive phases of the SNAO- and CGT-related modes are characterized by an anomalous anticyclone over the western TP and zonal cyclone–anticyclone dipole pattern over the TP, respectively, both of which are associated with mid- and high-latitude stationary Rossby wave trains.

The Role of Latent Heating Anomalies in Exciting the Summertime Eurasian Circulation Trend Pattern and High Surface Temperature

AbstractDynamical mechanisms for the summer Eurasian circulation trend pattern are investigated by analyzing reanalysis data and conducting numerical model simulations. The daily circulations that resemble the Eurasian circulation trend pattern are identified and categorized into two groups based on surface warming signal over central and eastern Europe. In the group with large warm anomaly, the upper-level circulation takes on a wave packet form over Eurasia, and there are enhanced latent heating anomalies centered over the North Sea and suppressed latent heating anomalies over the Caspian Sea. The numerical model calculations indicate that these latent heating anomalies can excite an upper-level circulation response that resembles the Eurasian circulation trend pattern. Additional analysis indicates that trends of these two latent heating centers contribute to the long-term circulation trend. In the weak warm anomaly group, the circulation pattern takes on a circumglobal teleconnection (CGT) pattern, and there is no heating signal that reinforces the circulation. These results indicate that not all CGT-like patterns excite temperature anomalies that are persistent and in phase with the trend pattern, and that quasi-stationary forcings, such as the latent heating anomalies, play an important role in driving the boreal summer circulation anomaly that accompanies the strong and persistent surface temperature signal.

Precursors and Formation Mechanisms of Event-Based Extreme Precipitation during Springtime in Central-Eastern China

AbstractThis study investigates the precursors and formation mechanisms of spring (April–May) event-based extreme precipitation (EEP) during 1961–2014 in central-eastern China. The EEP events during springtime are primarily characterized by extreme precipitation that occurs at the first half of an event. During early stages of spring EEP events, a Rossby wave grows over western Europe and the North Atlantic Ocean. The wave propagates eastward toward East Asia, exhibiting a circumglobal teleconnection (CGT) pattern. A strong anticyclone related to the CGT pattern is formed over the islands of Japan in the upper troposphere, enhancing the divergence anomalies and bringing more water vapor anomalies from the Sea of Japan into central-eastern China. Meanwhile, the westerly jet jumps northward and anomalous southwesterly water vapor flux is significantly prevalent, both associated with the onset of the Bay of Bengal summer monsoon (BOBSM). When the anomalous southwesterly and northeasterly moisture fluxes into central-eastern China combine, strong convergence is formed, providing abundant water vapor for extreme precipitation. The moisture budget analysis further suggests that the dynamic processes associated with horizontal wind anomalies play a crucial role in the moisture convergence for the spring EEP events. The advection of zonal and meridional moisture is strongly related to the anomalous winds of the CGT waves and BOBSM, respectively; the horizontal thermodynamic processes related to specific humidity and vertical advection contribute much less. The results indicate the preceding signals in the midlatitudes and subtropics for the spring EEP events, enabling extreme precipitation forecasting and hydrological prediction.

Qualifying Contributions of Teleconnection Patterns to Extremely Hot Summers in Japan

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.

The quasi-biweekly oscillation of eastern China PM2.5 in response to different Rossby wave trains over the Eurasian continent

This study investigates the wintertime quasi-biweekly oscillation characteristic of PM2.5 in eastern China and its relationship with Rossby wave trains propagating over the Eurasian continent. The Empirical orthogonal function (EOF) analysis extracts the three leading modes of PM2.5, including one monopole pattern (Mcon), a north-south dipole pattern (Msn), and a west-east dipole pattern (Mew). The three modes exhibit distinct quasi-biweekly oscillation features (QBWO) under the coupling effects of two Rossby wave trains within the polar front jet and the subtropical westerly jet. The quasi-biweekly evolutions of the Eurasian teleconnection pattern (EU) and Circumglobal teleconnection pattern (CGT) are responsible for the propagation of the two wave trains. The QBWO typical combination cases of the two wave trains are the decay of the EU negative phase and development of the CGT negative phase for Mcon, the development of the EU negative phase and decay of the CGT positive phase for Msn, and the decay of the EU negative phase and development of the CGT positive phase for Mew. A consistent change in surface southerly wind anomalies forms in the north and south when the decay of the EU negative phase works cooperatively with the development of the CGT negative phase, resulting in the PM2.5 monopole pattern (Mcon). The combination case with the development of the EU negative phase and decay of the CGT positive phase favors strong stratification stability in the north and vigorous vertical convection in the south, causing opposing north-south changes in pollutants (Msn). When the decay of the EU negative phase couples with development of the CGT positive phase, surface southerly wind anomalies prevail in the Fenwei Plain, and surface northeasterly wind anomalies from the sea trigger convection in the Yellow River Basin and Huaihe River Basin, forming opposing west-east changes in PM2.5 (Mew).

Interannual Variability of Springtime Extreme Heat Events over the Southeastern Edge of the Tibetan Plateau: Role of A Spring-type Circum-global Teleconnection Pattern

AbstractDue to the high mountains to the west and north of the plateau, and the control by westerly mean flow in spring, hot and dry conditions are often observed over the southeastern edge of the Tibetan Plateau (SETP), and hence favoring occurrences of extreme heat events there. Indeed, maximum centers and remarkable increasing trends of extreme heat (EH) days in spring are found over the region. Springtime EH events over the SETP also exhibit strong interannual variability and are closely linked to a spring-type circum-global teleconnection (SCGT) pattern, which is the second leading mode of 200-hPa meridional wind over the North Hemisphere in spring. This SCGT shows distinctive features from the traditional circum-global teleconnection patterns found in boreal summer and winter. It is revealed by a circum-globally navigated Rossby wave train along the mid-high latitudes, which splits to a north branch along the polar jet and a south branch along the subtropical jet over Eurasia after propagating through the North Atlantic. The two branches eventually reach the SETP, forming an anomalous anticyclonic circulation over the region. Hence, conditions in the SETP are controlled by significant anomalous subsidence and a clearer sky, resulting in below-normal rainfall and above-normal air temperature, in favor of more EH events in the region. The SETP EH events are also closely linked to the spring-type CGT-like pattern in April and May, but not in March. In addition, the influence of the foehn effect on the SETP EH is discussed.

Occurrence of heatwave in Korea by the displacement of South Asian high

The South Asian high (SAH) index was defined using the 200 hPa geopotential height for 1973–2019. Of the movements of the SAH center in the north–south, east–west, northwest-southeast, and southwest-northeast directions, the movements in the northwest-southeast direction showed the highest positive correlation with heatwave days (HWDs) in South Korea. Thirteen years with the highest values in the northwestward shift of the SAH (positive SAH years) and 13 years with the highest values in the southeastward shift of the SAH (negative SAH years) were selected from a time series of SAH indices from which the linear trend was removed, and the differences between these two groups were analyzed. An analysis of vertical meridional circulation averaged along 120°–130° E showed that in the latitude zones containing Korea, anomalous downward flows with anomalous high pressures formed in the entire troposphere and coincided with a positive anomaly of air temperature and specific humidity. An analysis of stream flows and geopotential heights showed that in the positive SAH years, anomalous anticyclones developed in Korea, the North Pacific, North America, Western Europe, and the Iranian Plateau. These anticyclones had the wavenumber-5 pattern and showed more distinct barotropic vertical structures at higher altitudes, which resembled the circumglobal teleconnection (CGT) pattern. The maintenance of CGT depends on the interaction between the CGT circulation and the Indian summer monsoon (ISM), which has a major influence on the mid-latitude atmosphere. Strengthening of the ISM results in the formation of upper-level anomalous anticyclones in the northwestern Iranian Plateau and produces continuous downstream cells along the waveguide due to the Rossby wave dispersion. When diabatic heating by Indian summer monsoon precipitation is strengthened, the SAH is strengthened to the northwest of India, and a positive CGT pattern is formed. As a result, anomalous anticyclones formed in all layers of the Korean troposphere, resulting in heatwaves, tropical nights, and droughts exacerbated in South Korea.

Central Asian Precipitation Shaped by the Tropical Pacific Decadal Variability and the Atlantic Multidecadal Variability

AbstractKnown as one of the largest semiarid to arid regions in the world, central Asia and its economy and ecosystem are highly sensitivity to the changes in precipitation. The observed precipitation and related hydrographic characteristics have exhibited robust decadal variations in the past decades, but the reason remains unknown. Using the pacemaker experiments of the Community Earth System Model (CESM1.2), we find that the tropical Pacific decadal variability (TPDV) and the Atlantic multidecadal variability (AMV) are the main drivers of the interdecadal variations in central Asian precipitation during 1955–2004. Both the decadal-scale warming of the tropical Pacific and North Atlantic are favorable for wetter conditions over central Asia. The positive TPDV is accompanied with high sea level pressure (SLP) over the Indo–western Pacific warm pool. Southwesterly winds along the northwestern flank of the high SLP can transport more moisture to southeastern central Asia. The warm AMV can excite a circumglobal teleconnection (CGT) pattern. A trough node of the CGT to the west of central Asia drives an anomalous ascending motion and increased precipitation over this region. The results based on the CESM model are further demonstrated by the pacemaker experiments of MRI-ESM2-0. Based on the observational TPDV and AMV indices, we reasonably reconstruct the historical precipitation over central Asia. Our results provide hints for the decadal prediction of precipitation over central Asia.

A zonally-oriented teleconnection pattern induced by heating of the western Tibetan Plateau in boreal summer

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.