East Asian Trough Research Articles

The North China/Northeastern Asia Severe Summer Drought in 2014

Abstract In summer 2014, north China and large areas of northeastern Asia (NCNEA) suffered from the most severe drought of the past 60 years. This study indicates that the East Asian summer precipitation in 2014 exhibited a tripole anomaly, with severe negative anomalies in NCNEA, strong positive anomalies in south China, South Korea, and Japan, and intense negative anomalies in the western North Pacific. Along with the severe tripole precipitation anomalies, there were strong intensities of the Silk Road pattern, the Pacific–Japan pattern, and the Eurasian teleconnection pattern, which were responsible for the strong precipitation anomaly in 2014 through changes to the western Pacific subtropical high (WPSH) and the East Asian trough. Further analysis indicates that the sea surface temperature (SST) in the North Pacific was nearly the warmest in the past 60 years and, together with the strong SST warming in the warm pool region, thus caused the strong Pacific–Japan teleconnection pattern, southward positioning of the WPSH, and weakened East Asian summer monsoon. Additionally, the summertime sea ice cover in the Arctic Ocean was anomalous, resulting in high SST in the Laptev–Kara Sea and, hence, triggering a strong Eurasian teleconnection pattern and contributing to the severe drought of NCNEA. Furthermore, the intense warming over the European Continent and Caspian Sea favored the Silk Road pattern, also contributing to the southward positioning of the WPSH and the NCNEA drought. The NCNEA severe drought was therefore the joint result of Pacific SST anomalies, Arctic sea ice anomalies, and warming over the European Continent and Caspian Sea.

Role of the North Pacific sea surface temperature in the East Asian winter monsoon decadal variability

In this study, a possible mechanism for the decadal variability in the East Asian winter monsoon (EAWM) is proposed. Specifically, the North Pacific sea surface temperature (SST) may play an important role. An analysis of the observations shows that the North Pacific SST has a remarkable decadal pattern whose phase shifted around the mid-1980s. This North Pacific SST decadal pattern can weaken the East Asian trough and enhance the North Pacific Oscillation through changing air–sea interactions over the North Pacific. The weak East Asian trough enhances the zonal circulation and weakens the meridional circulation over East Asia, consequently leading to a weaker southward cold surge and East Asia warming around the mid-1980s. The numerical experiment further confirms the pronounced physical processes. In addition, over the longer period of 1871–2012, the indices of the EAWM and North Pacific SST decadal pattern are also highly consistent on the decadal timescale, which further confirms the impact of the North Pacific SST decadal pattern on the EAWM decadal variability.

Observation of a summer tropopause fold by ozonesonde at Changchun, China: Comparison with reanalysis and model simulation

Tropopause folds are one of the key mechanisms of stratosphere–troposphere exchange (STE) in extratropical regions, transporting ozone-rich stratospheric air into the middle and lower troposphere. Although there have been many studies of tropopause folds that have occurred over Europe and North America, a very limited amount of work has been carried out over northeastern Asia. Ozonesondes produced by the Institute of Atmospheric Physics were launched in Changchun (43.9°N, 125.2°E), Northeast China, in June 2013, and observed an ozone-enriched layer with thickness of 3 km and an ozone peak of 180 ppbv at 6 km in the troposphere. The circulation field from the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim) dataset shows that this ozone peak was caused by a tropopause fold associated with a jet stream at the eastern flank of the East Asian trough. By analyzing the ozone data from the ozone monitoring instrument and Weather Research and Forecasting model with Chemistry (WRF-Chem) simulations, it was found that a high ozone concentration tongue originating from the lower stratosphere at high latitude (near central Siberia) intruded into the middle troposphere over Changchun between 5 and 8 km on 12 June 2013. The high-resolution WRF-Chem simulation was capable of describing events such as the tropopause fold that occurred on the cyclonic shear side of the jet stream. In addition, the TRAJ3D trajectory model was used to trace the origin of measured secondary ozone peaks in the middle troposphere back, for example, to stratospheric intrusion through the tropopause fold.

Combined effect of the Arctic Oscillation and the Western Pacific pattern on East Asia winter temperature

The combined effect of the Arctic Oscillation (AO) and Western Pacific (WP) teleconnection pattern on the temperature variation during the winter in the northern hemisphere and East Asia over the last 56 years (1958/1959–2013/2014) was investigated using NCEP/NCAR reanalysis data. The study results revealed that the effect of the AO on winter temperature in East Asia could be changed depending on the phase of the WP pattern in the North Pacific. The negative relationship between the temperature of East Asia (25–45°N, 110–145°E) and the AO increased when the AO and WP were in-phase with each other. Hence, when winter negative (positive) AO was accompanied by negative (positive) WP, negative (positive) temperature anomalies were dominant across the entire East Asia region. Conversely, when the AO and WP were out-of-phase, the winter temperature anomaly in East Asia did not show distinct changes. Furthermore, from the perspective of stationary planetary waves, the zonal wavenumber-2 patterns of sea level pressure and geopotential height at 500 hPa related to the East Asian winter monsoon (EAWM) circulation strengthened when the AO and WP were in-phase but were not significant for the out-of-phase condition. An index considering the effect of both AO and WP on East Asia winter temperature was proposed. The correlation between the index and the East Asia winter temperature was statistically significant at the 99 % confidence level. The index was correlated with synoptic characteristics of the EAWM, including the Siberian High, East Asian trough, East Asian jet stream and surface air temperature.

Two Opposite Extreme Events in Seasonal Mean Winter Rainfall over East China during the Past Three Decades

In this study, the extremes of winter seasonal mean precipitation have been investigated by using daily precipitation data from 91 stations in East China, the Na- tional Centers for Environmental Prediction/the National Center for Atmospheric Research (NCEP/NCAR) month- ly reanalysis, and sea surface temperature data from the Hadley Centre for 1979-2007. The largest anomalous rainfall amount was observed in regions south of the Yangtze River. In the most recent three decades, extreme events in the seasonal mean winter precipitation occurred in 1985 and 1997. Because it was influenced mainly by a La Nina event, the precipitation in 1985 showed a deficit following a stronger winter monsoon. The rainfall amount in 1997 was influenced by El Nino and was significantly larger than normal with a weaker winter monsoon. Both the circulation anomalies and wave energy dispersions during the winters of 1985 and 1997 differed significantly. In 1985, the North Atlantic Oscillation anomalously ex- cited the Eurasian-Pacific teleconnection and circum- global teleconnection phenomena. Consequently, Rossby wave energy propagated along the north and south branches of the westerlies, strengthening the East Asian trough along with a stronger winter monsoon, which fa- cilitated the wintertime dry extreme in East China. In 1997, however, Rossby wave energy propagated from low latitudes northeastward into the southern part of China, resulting in a weaker winter monsoon and the wettest winter. The results of this study will be helpful for future monitoring and prediction of extreme winter rainfall events in East China.

Stationary Wave Activity Associated with the East Asian Winter Monsoon Pathway

The pathway of the East Asian winter mon- soon (EAWM) that usually leads to the out-of-phase pat- tern of surface air temperature between northern and southern East Asia is an important feature in the variabil- ity of the EAWM besides its strength. Using the European Centre for Medium-Range Weather Forecasts 40-year (ERA40) reanalysis dataset, this study investigates the pathway-related stationary wave activity to explore the mechanism of the interannual variations in the EAWM pathway. It reveals that when the southern pathway of the EAWM is strong, the phase of the climatological station- ary wave tends to be shifted westward significantly in both the horizontal and vertical directions by an anoma- lous wavenumber 2 pattern at mid-latitudes, whereas the changes are relatively small in the subtropics. The hori- zontal changes in the stationary wave phase facilitate a north-south-oriented East Asian trough in the middle tro- posphere that eventually produces the strong southern pathway of the EAWM. The vertical changes in the sta- tionary wave, in contrast, feature a westward-tilted phase line with height over the North Pacific, indicating en- hanced upward propagation of waves into the strato- sphere. This result suggests that the phase of stationary waves at mid-latitudes dominate the interannual variations in the EAWM pathway. Moreover, it supports our previ- ous interpretation of the possible role of the North Pacific sea surface temperature (SST) in the EAWM pathway variability. It also implies that the excitation of anomalous mid-latitude stationary waves may be the key in the re- sponse of the EAWM pathway to the North Pacific SST.  Keywords: East Asian winter monsoon, pathway, sta- tionary wave, phase Citation: Wang, L., 2014: Stationary wave activity asso- ciated with the East Asian winter monsoon pathway, At- mos. Oceanic Sci. Lett., 7, 7-10, doi:10.3878/j.issn.1674- 2834.13.0043.

Haze Days in North China and the associated atmospheric circulations based on daily visibility data from 1960 to 2012

AbstractHaze is a severe hazard that greatly influences traffic and daily life with great economic losses and threats to human health. To enhance understanding of the haze occurrences, this study examined the haze variations over North China and their associated atmospheric circulations for the period of 1960–2012 using daily visibility data. Results indicate that the haze events over this region primarily occur in boreal winter of year and mainly in the morning of day. The results of the analysis of the long‐term variations indicate that the annual haze days were relatively few in the 1960s but increased steeply in the 1970s and have remained stable to the present. Some differences are obvious among seasons. A stably increasing trend is discernable in summer and autumn, relatively low in the 1960s and the 1990s–2000s and relatively high in the 1970s–1980s in spring and winter. Haze variations in urban regions are quite similar to haze variations in rural regions but with more haze days in urban regions because of the high aerosol emissions. Further analyses indicate that the occurrences of severe haze events in boreal winter generally correlate with the weakened northerly winds and the development of inversion anomalies in the lower troposphere, the weakened East Asian trough in the midtroposphere, and the northward East Asian jet in the high troposphere. All of these factors provide a favorable atmospheric background for the maintenance and development of haze events in this region.

Eddy contributions at multiple timescales to the evolution of persistent anomalous East Asian trough

Persistent strong and weak East Asian trough (EAT) cases during boreal winter are captured by projecting 12 UTC geostrophic vorticity onto that of a seasonal strong EAT. The persistent cases are investigated in terms of seasonal, intraseasonal, and synoptic temporal scales. The evolution of persistent strong and weak EAT cases displays characteristics of intraseasonal temporal variation. The onset and decay of strong (weak) cases is associated with the passage of an intraseasonal negative (positive) 500 hPa height anomaly from Northeast Asia to the midlatitude central Pacific, through the region of the EAT. The onset and decay stages of persistent strong (weak) EAT cases occur in conjunction with a seasonal stronger-than-normal (weaker-than-normal) EAT. In addition, it is noted that the difference between the number of strong and weak cases exhibits significant covariability with the strength of the seasonal EAT. Meanwhile the seasonal EAT also plays a role in the development and decay of persistent cases. The contributions of dynamic and thermodynamic processes during the evolution of persistent strong and weak EAT cases are investigated. Since the forcing of horizontal temperature advection and that of adiabatic and diabatic processes are likely to offset each other, dynamic processes make a more important contribution to the evolution of the EAT than do thermodynamic processes. Furthermore, horizontal absolute vorticity advection is dominant in the dynamic processes and is caused mainly by the advection of synoptic and intraseasonal vorticity by seasonal wind. The seasonal wind transports positive vorticity to (away from) the EAT, which results in a remarkable strengthening (weakening) of the EAT during the onset (decay) stage for persistent strong EAT cases.

Comparative Analysis of Sustained Drought Processes in Jiangsu during 1971-2011

By using the monthly precipitation data of Jiangsu province and the NCEP / NCAR monthly reanalysis data,combining with the abnormal precipitation mechanism analysis in Jiangsu province in recently 40 years,the differences betw een the Autumn-Winter-Spring Drought(AWSD) in 2010 /2011 and other three sustained drought processes in recent 40 years in Jiangsu have been studied,respectively. The results show that the drought has maintained for the longest time in 1980 /1981 and been most serious in 2010 /2011. From the perspective of climatic factors,the drought grade of the troposphere and the intensity of sinking movement have been strong and sustained for a long time. In 1980 /1981 and 2010 /2011,potential height field presented high pressure in the west and low pressure in the east on 500 h Pa,which is beneficial to East Asia trough enhancement and maintenance. Meanw hile,consistent with the blocking situation in the west Baikal Lake and the stronger winter monsoon,cold air moved southw ard frequently,which blocked the transmission of the south warm air,and made air more drier. SST anomaly distribution displayed as La Nina type. SST anomalies were negative in the east central Pacific around the equator and near the eastern coast in China,while positive in west central Pacific. Such abnormal distribution tended to form ‘east low and west high'in potential height. Compared with the other two sustained drought processes,the drought intensity in 1998 /1999 and 2008 /2009 were slightly weaker,so there were also some differences in atmosphere background field and SST anomaly distribution.

Direct impact of El Niño on East Asian summer precipitation in the observation

This study investigates the direct impact of El Nino in the tropical Pacific on the East Asian summer precipitation . Generalized equilibrium feedback assessment is used to isolate this direct impact from interrelated ocean forcings in the observations. Results indicate that the El Nino can directly influence the summer precipitation in East China significantly. The precipitation response presents a tri-pole pattern, with anomalous wet in the Southeast and the Northeast China and anomalous dry in the northern China. Amplitude of the precipitation response is around 20 % of the total precipitation for 1 °C El Nino forcing in most area of the East China, with maximal response up to 30 %/°C. The tri-pole precipitation response is attributed to an El Nino-induced cyclonic anomaly in the Northeast Asia and an anticyclonic anomaly in the western North Pacific (WNP). The anomalous cyclone deepens the East Asian trough southwestward, favoring an air ascending in front of the trough in the Southeast and the Northeast China, and an air descending at the rear of the trough in the northern China. The anomalous anticyclone in the WNP strengthens the WNP Subtropical High northeastward, providing adequate water vapor to the Southeast China. The anomalous cyclone and anomalous anticyclone work together to generate the tri-pole precipitation response pattern in the East China. Further investigation suggests that these two key anomalous circulations are part of a northwestward propagating Rossby wave, which is excited by the El Nino warming-induced convection over the subtropical west-central Pacific. This study can serve as a reference for the prediction of the East Asian precipitation in both the developing and decaying summer of El Nino.

A new circulation index to describe variations in winter temperature in Southwest China

A new circulation index (I SW) that can realistically describe winter temperature variations over Southwest China is defined based on analysis of the NCEP/NCAR reanalysis data (version 1) and the observations at 585 stations in China. The study period is from January 1961 to February 2011. The relationship between I SW and general circulation patterns in East Asia is also analyzed. Results show that I SW successfully captures the variations in winter temperature over Southwest China. High I SW values correspond to the intensified Mongolian high, the weakened Aleutian low, increases in the strength of the Middle East westerly jet stream over the south of the Tibetan Plateau (TP), and decreases in the strength of the subtropical westerly jet over the north of the TP. Meanwhile, the East Asian trough deepens and extends southwestward, making it easier for the cold air mass from the north to intrude Southwest China along the trough. These circulation patterns lead to a decrease in winter temperature over Southwest China (and vice versa). In addition to the East Asian winter monsoon, the two westerly jets that dominate the upper level circulation over East Asia also exert important influences on winter temperature in Southwest China, especially the Middle East westerly jet to the south of the TP.

Implications of Ural Blocking for East Asian Winter Climate in CMIP5 GCMs. Part I: Biases in the Historical Scenario

Abstract This study assesses the ability of the 25 GCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to simulate Ural blocking (UB) and its linkage with the East Asian winter climate [December–February (DJF)] in a historical run (1950/51–2004/05). A Ural blocking index (UBI) is defined as the DJF-mean blocking frequency over 45°–90°E for each winter. Regression analyses suggest that the long-term mean bias of UBI is caused by the long-term mean circulation bias over the North Atlantic. On seasonal time scales, the GCMs simulating a positive bias of UBI are associated with a stronger Atlantic jet stream, as well as stronger westerly momentum fluxes from the North Atlantic to Europe. On synoptic time scales, however, these GCMs tend to be associated with a weaker Siberian high and East Asian trough during the evolution of a UB event. Altogether, there is no apparent linkage between the long-term mean bias of UB and the East Asian winter climate. Further studies are needed to explore the teleconnection between UB and the East Asian winter climate in the GCMs.

Variation of circulation and East Asian climate associated with anomalous strength and displacement of the East Asian trough

Variations of the East Asian trough (EAT) and its relationship with the East Asian climate are documented in this study. The variations investigated include the trough’s strength and its meridional and zonal displacements. Anomalous cold (warmth) in Southeast Asia is concurrent with the southward (northward) displacement of the EAT. The southward displacement of the EAT is likely associated with an anomalous strong Siberian high and Aleutian low, which manipulate anomalous northerly wind between them. On the other hand, the temperature in northeast Asia is influenced by both the strength and the zonal displacement of the trough. These properties of the EAT are closely linked to temperatures in the northern and southern portions of East Asia. The relation between variations of the EAT and the East Asian winter monsoon is also examined. In addition, the relation between the EAT and transient eddies is studied. Anomalous transient eddies concurrent with variations of the EAT are contributed by a localized anomalous meridional gradient due to the variation of stationary eddies and zonal symmetric structure. Variations in the polar vortex are significant for variations in the strength of the EAT. To investigate the role of eddies in the anomalous evolution of the EAT, we also study stationary and transient eddy forcing on the evolution of anomalous EAT and zonal symmetric structures using wave activity flux and extended E-vectors.

A connection between the tropical Pacific Ocean and the winter climate in the Asian‐Pacific region

AbstractThe impact of the tropical Pacific sea surface temperature (SST) anomaly on the winter mean surface air temperature (SAT) in the Asian‐Pacific region is investigated during the period from 1948 to 2008 using both observations and a linear baroclinic model (LBM). A singular value decomposition (SVD) analysis is conducted between the 500 hPa geopotential height (Z500) over the Northern Hemisphere and the SST over the tropical Pacific Ocean to obtain the large‐scale atmospheric patterns related to tropical Pacific SST. Focus is given to the second pair of SVD mode (SVD2) which bears some similarities in the Z500 field to the Arctic Oscillation over the North Atlantic sector and can impact the SAT over a larger area of Asian‐Pacific. In the winter of a positive SVD2 the SAT over the midlatitude to high‐latitude Asian continent, the Arctic Ocean, the Indian Ocean, and the western subtropical Pacific Ocean tends to be warmer than normal, while the North Pacific Ocean around the Bering Strait is abnormally cold, and vice versa. Examination of the associated surface general circulation shows that a positive SVD2 tends to shift the Siberian High southward and the Aleutian Low eastward resulting in anomalous weak pressure gradient between the Asian continent the North Pacific. Anomalous positive sea level pressure anomalies around Japan and southerly wind along the east coast of the Asian continent are observed. At the same time, the East Asian trough at midtroposphere becomes weaker than normal and the East Asian westerly jet stream is increased in magnitudes and shifted northward. The analysis of the wave activity flux and result of idealized numerical experiments show a possible influence of the western tropical Pacific SST forcing on the SVD2.

Impact of Stratospheric Sudden Warming on East Asian Winter Monsoons

Fifty-two Stratospheric sudden warming (SSW) events that occurred from 1957 to 2002 were analyzed based on the 40-year European Centre for Medium-Range Weather Forecasts Reanalysis dataset. Those that could descent to the troposphere were composited to investigate their impacts on the East Asian winter monsoon (EAWM). It reveals that when the SSW occurs, the Arctic Oscillation (AO) and the North Pacific Oscillation (NPO) are both in the negative phase and that the tropospheric circulation is quite wave-like. The Siberian high and the Aleutian low are both strengthened, leading to an increased gradient between the Asian continent and the North Pacific. Hence, a strong EAWM is observed with widespread cooling over inland and coastal East Asia. After the peak of the SSW, in contrast, the tropospheric circulation is quite zonally symmetric with negative phases of AO and NPO. The mid-tropospheric East Asian trough deepens and shifts eastward. This configuration facilitates warming over the East Asian inland and cooling over the coastal East Asia centered over Japan. The activities of planetary waves during the lifecycle of the SSW were analyzed. The anomalous propagation and the attendant altered amplitude of the planetary waves can well explain the observed circulation and the EAWM.

Spatiotemporal Variability and Change of the South China Spring Precipitation during 1961–2012

We analyze precipitation data from 47 meteorological stations spanning between 1961 and 2012 and NCEP/NCAR reanalysis to understand spatiotemporal variability and change of spring precipitation of South China and their relations to atmospheric circulations. Empirical orthogonal function (EOF) analysis and rotated EOF (REOF) are used to reveal dominant spatial structures of precipitation anomaly and Mann-Kendall testing method to determine the temporal locations of abrupt changes during the analyzed time span. We find that the first spatial mode of the spring precipitation of the South China has a domain uniform structure; the second is dominated by a spatial dipole; and the third contains six variability centers. 1980s was the decade of the largest amount of precipitation while 1960s the decade of the smallest amount of precipitation. The spring precipitation also appeared to have a decreasing trend since 2000. We also find that spring precipitation of the South China has experienced a few abrupt changes: sudden increment at 1964, sudden decrement at 2002, and sudden increment at 1995. In addition to these abrupt changes, the precipitation could also be characterized by variability of multiple temporal scales, with dominant periodicities of 4 years, 8 years, and 14 years. The South China spring precipitation is also closely tied to the atmospheric circulations: when Aleutian Low strengthens, westerly weakens, and the center of the Western Pacific subtropical high shifts southeastward in the early spring; and the South China precipitation tends to be abundant (positive anomaly). In contrast, when Ural ridge strengthens, the southern branch of the East Asian trough weakens and the Western Pacific subtropical high shifts northwestward in the early spring, the South China precipitation tends to be reduced (negative anomaly).

Extreme cold and warm events over China in wintertime

The characteristics of extreme cold and warm events (ECE; EWE) over the whole China in December, January and February are investigated using the observed daily surface air temperature for 1961–2012. Among the 3 months, both the EWE and ECE in February are most frequent and strongest and least frequent in December and weakest in January. More than 50% of the EWE and ECE in February and less than 40% of the EWE and ECE in January exhibit persistent feature (lasting at least 5 days). Generally, the persistent ECE are more frequent than the persistent EWE. The EWE (ECE) in February exhibit intensive increasing (decreasing) tendency and show more (less) persistent feature in 1961–1986 than that in 1987–2012. The persistent feature of the EWE (ECE) in December and January is more (less) obvious in 1961–1986 than that in 1987–2012 although the EWE (ECE) in 1961–1986 is less (more) frequent than that in 1987–2012. The significant weakening (strengthening) Siberia high and East Asia trough are the two main key factors for the EWE (ECE) in January and February and they can be found up to 5 days before the occurrence of the EWE (ECE). The ECE (EWE) in December are associated with the three-wave structure anomalies in troposphere, but these anomalies cannot be found before the occurrence of the ECE (EWE). The warm North Atlantic and tropical central-eastern Pacific in previous autumn (SON, September–October–November) trigger significant weakening of Siberia high and East Asia trough in February and thereby cause the occurrence of the EWE. On decadal time scale, the increasing EWE in February seem to be the consequence of the warming North Atlantic. The cold SON Indian Ocean corresponds to significant atmospheric circulation anomalies in December similar to those associated with the ECE in December, indicating its importance in the ECE in December.

Vertical structure, physical properties, and energy exchange of the East Asian trough in boreal winter

The relation between the intensity of the East Asian trough and the strength of the East Asian winter monsoon is documented. A prominent dipole of the available potential energy of the zonal mean flow can be observed over the midlatitudes and polar regions and is related to temperature anomalies. Variation in the intensity of the East Asian trough is caused by variation in the energy conversion from the available potential energy of the zonal mean flow to the East Asian trough. The in-phase relation between the intensities of the East Asian jet and East Asian trough is illustrated by the relation between the potential energy and kinetic energy of stationary eddies. The dipole of height anomalies over the midlatitude and polar regions develops in late autumn and decays in early spring. We investigate the effect of both stationary and transient eddy forcing on the evolution of the dipole. The combined forcing of the two types of eddy makes an important contribution to the development, maintenance, and decay of the dipole.

Regime-Dependent Nonstationary Relationship between the East Asian Winter Monsoon and North Pacific Oscillation

Abstract The East Asian winter monsoon (EAWM) and the North Pacific Oscillation (NPO) constitute two outstanding surface atmospheric circulation patterns affecting the winter sea surface temperature (SST) variability in the western North Pacific. The present analyses show the relationship between the EAWM and NPO and their impact on the SST are nonstationary and regime-dependent with a sudden change around 1988. These surface circulation patterns are tightly linked to the upper-level Ural and Kamchatka blockings, respectively. During the 1973–87 strong winter monsoon epoch, the EAWM and NPO were significantly correlated to each other, but their correlation practically vanishes during the 1988–2002 weak winter monsoon epoch. This nonstationary relationship is related to the pronounced decadal weakening of the Siberian high system over the Eurasian continent after the 1988 regime shift as well as the concomitant positive NPO-like dipole change and its eastward migration in tropospheric circulation over the North Pacific. There is a tight tropical–extratropical teleconnection in the western North Pacific in the strong monsoon epoch, which disappears in the weak monsoon epoch when there is a significant eastward shift of tropical influence and enhanced storm tracks into the eastern North Pacific. A tentative mechanism of the nonstationary relationship between the EAWM and NPO is proposed, stressing the pivotal role played in the above teleconnection by a decadal shift of the East Asian trough resulting from the abrupt decline of the EAWM since the late 1980s.

Overview of the major 2012–2013 Northern Hemisphere stratospheric sudden warming: Evolution and its association with surface weather

In this study, we analyzed the dynamical evolution of the major 2012–2013 Northern Hemisphere (NH) stratospheric sudden warming (SSW) on the basis of ERA-Interim reanalysis data provided by the ECMWF. The intermittent upward-propagating planetary wave activities beginning in late November 2012 led to a prominent wavenumber-2 disturbance of the polar vortex in early December 2012. However, no major SSW occurred. In mid December 2012, when the polar vortex had not fully recovered, a mixture of persistent wavenumber-1 and -2 planetary waves led to gradual weakening of the polar vortex before the vortex split on 7 January 2013. Evolution of the geopotential height and Eliassen-Palm flux between 500 and 5 hPa indicates that the frequent occurrence of tropospheric ridges over North Pacific and the west coast of North America contributed to the pronounced upward planetary wave activities throughout the troposphere and stratosphere. After mid January 2013, the wavenumber-2 planetary waves became enhanced again within the troposphere, with a deepened trough over East Asia and North America and two ridges between the troughs. The enhanced tropospheric planetary waves may contribute to the long-lasting splitting of the polar vortex in the lower stratosphere. The 2012–2013 SSW shows combined features of both vortex displacement and vortex splitting. Therefore, the anomalies of tropospheric circulation and surface temperature after the 2012–2013 SSW resemble neither vortex-displaced nor vortex-split SSWs, but the combination of all SSWs. The remarkable tropospheric ridge extending from the Bering Sea into the Arctic Ocean together with the resulting deepened East Asian trough may play important roles in bringing cold air from the high Arctic to central North America and northern Eurasia at the surface.