RESEARCH ON INTERANNUAL CHANGE ABOUT TELECONNECTION OF GENERAL CIRCULATION IN SUMMER DURING 1980s OVER THE NORTHERN HEMISPHERE

In the paper the 5°×10°latitude-longitude grid point data of daily 500 hPa geopotential height over the NorthernHemisphere(NH)in summer(June—August)during 1980s are used.The base point(20°N,120°E)is selected to calcu-late point correlation between the base point and other grid points.We find that the summer heat source anomaly of thetropical western Pacific causes anomaly of summer general circulation over NH and teleconnection of general circula-tion similar to PNA pattern forms from East Asia to North America.The teleconnections show great interannualchanges.     

ANALYSES OF THE DYNAMIC EFFECTS ON WINTER CIRCULATION OF THE TWO MAIN MOUNTAINS IN THE NORTHERN HEMISPHERE——Ⅱ.VERTICAL PROPAGATION OF PLANETARY WAVES

A linear,hemispheric and stationary spectral model with multilayers in the vertical was employed to simulate thevertical propagation of waves triggered by mountains.Results show that,in cooperation with the East Asia zonal meanflow,Tibetan Plateau can excite a strong wavenumber 1 perturbation in the stratosphere with its ridge and trough lo-cated over the Pacific and Atlantic Oceans respectively.On the other hand,the stratospheric wavenumber 1 perturbationcaused by the mechanical forcing of the Rocky Mountains in cooperation with the North America zonal mean flow isvery weak.Calculations from observational data of the vertical profile of critical wavenumber for vertically propagatingwaves imply that the tropospheric wavenumber 1 perturbation can hardly penetrate the North America tropopause up-wards,whereas it can freely propagate through the East Asia tropopause into the stratosphere.Two-dimensional E-Pcross-sections obtained from both observational data and simulated results also demonstrate that waves excited by theRocky Mountains are refracted towards low latitudes in the troposphere during their upward propagation:whereas,inaddition to the above mentioned equatorward leaning branch,the wavenumber 1 and 2 planetary waves excited by theTibetan Plateau possess another branch which is refracted to high latitudes during upward propagation and penetratesthe tropopause into the stratosphere.It is therefore concluded that the difference in the horizontal and vertical wavepropagations in the two hemispheres is a result of the different dynamical forcing induced by the two main mountains inthe Northern Hemisphere.     

TELECONNECTIONS OF THE SEA SURFACE TEMPERATURE IN THE INDIAN OCEAN WITH SEA SURFACE TEMPERATURE IN THE EASTERN EQUATORIAL PACIFIC, AND WITH THE 500 hPa GEOPOTENTIAL HEIGHT FIELD IN THE NORTHERN HEMISPHERE

The observational data of global sea surface temperature and the 500 hPa geopotential height field in the Northern Hemisphere are used to get two teleconnection patterns. The time interval of the data is from January 1950 to December 1979. The results show that the sea surface temperature anomalies in the Indian Ocean have evident positive correlation with the SSTA in the eastern Pacific and obvious teleconnection with the 500 hPa geopotential height field in [he Northern Hemisphere.     

Joint Effect of East Asia–Pacific and Eurasian Teleconnections on the Summer Precipitation in North Asia

The East Asia-Pacific(EAP) and Eurasian(EU) teleconnections are independent of each other on the seasonal timescale(with a correlation coefficient of only 0.03).But they may occur concurrently with consistent or opposite phases.This paper investigates their synergistic effect on the summer precipitation in North Asia.Based on the signs/phases of EAP and EU indices,the EAP and EU teleconnection anomalies occur in four cases:(Ⅰ) positive EAP+positive EU,(Ⅱ) negative EAP+negative EU,(Ⅲ) positive EAP+negative EU,and(Ⅳ) negative EAP+positive EU.Further analyses show that these four configurations of EAP and EU anomalies are coherently related to different atmospheric circulations over the midlatitude Eurasian continent,leading to different summer precipitation modes in North Asia.Category Ⅰ(Ⅱ) corresponds to a zonal tripole structure of the geopotential height at 500 hPa over eastern Europe and the Sea of Japan,leading to less(more) than normal precipitation in eastern Europe,Japan,and the surrounding areas,and more(less) precipitation from central China to Lake Baikal and eastern Russia.Category Ⅲ(Ⅳ) corresponds to a meridional dipole structure of the geopotential height at 500 hPa over North Asia,leading to more(less) precipitation in the northern North Asia and less(more) precipitation in most of the southern North Asia.Independent analysis reveals that the EAP teleconnection itself is positively correlated with the precipitation in the region between the eastern part of Lake Baikal and Okhotsk Sea,and negatively correlated with the precipitation in the region between Northeast China and Japan.Coincidently,the EU pattern and precipitation have negative correlations in Ural Mountain and Okhotsk Sea areas and positive correlations in the Lake Baikal area.The respective relations of EAP and EU with the summer precipitation in North Asia suggest that the EAP northern lobe overlapped with the EU central and eastern lobes could extend the geopotential anomalies over Lake Baikal to Russian Far East,creating an EAP-EU synergistic effect on the summer precipitation in North Asia.     

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 the2012–2013 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.     

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 ma jor 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.     

EFFECTS OF VARIATION OF WINTER SEA-ICE AREA IN KARA AND BARENTS SEAS ON EAST ASIA WINTER MONSOON

By analyzing the observation data and performing the numerical simulation tests,it is shownthat the Kara and the Barents Sea area is a key region to influence climate variation over theNorthern Hemisphere.The variation of winter sea-ice area in the key region is closely associatedwith that of the EU teleconnection pattern at 500 hPa and East Asia winter monsoon(EAWM)intensity.When a heavy sea-ice prevails in the key region,the EU teleconnection pattern at 500hPa is excited easily(there are positive 500 hPa height anomalies over around Japan and WestEurope),and winter Siberia high is weakened,meanwhile,sea level pressure(SLP)has positiveanomalies over the Northern Pacific.Therefore,EAWM will be weakened,winter temperatureover East Asia is above normal and the frequency of cold-air activity in February in China will bedecreased.When the light sea-ice occurs in the key region,the results will be opposite.     

THE EFFECTS OF THE QINGHAI-XIZANG PLATEAU ON THE MEAN SUMMER CIRCULATION OVER EAST ASIA

Four numerical experiments of simulation have been conducted in this paper by the use of a five-layer primitive equation numerical model with incorporated pressure-sigma vertical coordinate system. The initial fields are taken from the July zonal mean data of many years, while the heat sources and sinks are ideally specified according to the mean heating field over the East Asia calculated from the real data of July, 1979. On the basis of simulated results of temperature and geopctential height patterns we emphatically discuss the effects of the topography and the heating of the Qinghai-Xizang Plateau. From the analyses in this paper, it appears that the heating over the Bengal region makes a larger contribution to the middle and the south branches of the monsoon cell and is also the main cause for the existence of the southerly channel to the east of the Plateau, for the break of the subtropical anticyclcne belt below the 500 hPa level and for the formation of the summer Asian anticyclone at the 300 hP     

ON THE MECHANISM OF TELECONNECTIONS BETWEEN EL NINO AND EAST ASIAN ATMOSPHERIC CIRCULATION

Based on the linear planetary wave theory and observed data,the teleconnections between El Nino andatmospheric circulation in the Northern Hemisphere(NH)are studied.It is shown that the zonal groupvelocity for stationary Rossby waves in the atmosphere is always positive.Thus,the energy of thesedisturbances can not propagate westward.In certain conditions,however,the westward propagation ofnonstationary waves is possible.Going further into this question,we prove that the low-frequency distur-bances over the eastern equatorial Pacific,which are closely related to the large-scale air-sea interactionin low-latitudes,can influence the extratropical atmospheric circulation in the NH along two typical energypaths;i.e.,the waves with wavenumber one(n=1)propagate through the western North Pacific into EastAsia and form the“East Asian Path”,which plays an important part in the teleconnections between ElNino and atmospheric anomalies over East Asia,besides,the waves with n≥2 propagate along the“PNAPath”into the middle and eastern North Pacific as well as North America,leading to the relevant atmos-pheric anomalies to be excited over there.     

A MULTI-STATISTICAL ANALYSIS OF THE SOUTHERN OSCILLATION (SO) AND ITS RELATION TO THE MEAN MONTHLY ATMOSPHERIC CIRCULATION AT 500 hPa IN THE NORTHERN HEMISPHERE

In this paper the correlation analysis, factor analysis, fuzzy classification, and principal component analysis (PCA) are performed for the southern oscillation index (SOI) from the Climate Analysis Center (CAC) at the NOAA. It is shown that the 12-month SOI can be classified into two groups: one from January through April and the other from May through December. They differ in persistency and correlation. It is also found that the year of strong or weak SO can be defined by the first principal component of the SOI. The 11 years of weak SO thus defined contain 9 El Nino events.In addition, the relations between the SOI and 500 hPa geopotential height, mean monthly zonal height, mean monthly interzonal height differences, centers of atmospheric activities, characteristics of the atmospheric circulation (the intensity index of the north polar vortex, the area index of the subtropical West Pacific high, mean monthly zonal and meridional circulation indexes in Asia and Eurasia) in the period of 24 months fr     

Characteristics of Two General Circulation Patterns During Floods over the Changjiang-Huaihe River Valley

Characteristics of the atmospheric general circulation during the catastrophic floods over the Changjiang- Huaihe River Valley(CHRV)are investigated.There are two precipitation patterns over China in the CHRV flood years:the CHRV flood-whole country-wet(P1)pattern and the CHRV flood-south(north)side-dry (P2)pattern.The circulation analysis results show that there are obvious differences between the NH 500- hPa geopotential height fields of P1 and P2 precipitation patterns.The establishment of East Asia-Atlantic (EAA)correlation chain(the South China Sea(SCS)high-the Meiyu trough-the Okhotsk Sea high over East Asia)is a critical condition for excessive summer precipitation over the CHRV,while the European blocking high plays an important role in determining the precipitation pattern over China in the CHRV flood years.Besides,the relation between the EAA correlation chain and the sea surface temperature anomaly (SSTA)in the North Pacific is also studied.     

THE NUMERICAL SIMULATION OF THE THREE-DIMEN-SIONAL TELECONNECTIONS IN THE SUMMER CIRCULATION OVER THE NORTHERN HEMISPHERE

In this paper, a quasi-geostrophic, 34-level spherical coordinate model with Rayleigh friction, Newtonian cooling and the horizontal eddy thermal diffusivity is used to simulate the three-dimensional telecon nection in the summer circulation over the Northern Hemisphere.The computed results show that the change of the heat source over the Tibetan Plateau may cause the change of the atmospheric circulation over the middle and high latitudes in the Northern Hemisphere. When the heal source over the Tibetan Plateau is enhanced, it may cause the Tibetan high to enhance over South Asia and cause the change of the atmospheric circulation over East Asia and North America, i. e., Northeast China and North Japan will be controlled by a trough, which brings about a cold summer in this area. In the same way, an anticyclone will be enhanced over the Okhotsk sea. Moreover, another trough will be formed over Alaska, while another ridge will develop to the northeast of North America. Besides, the Pacific subtropical high will be weakened. These results are in good agreement with those obtained from the observed data.     

THE EAST ASIA/PACIFIC PATTERN TELECONNECTION OF SUMMER CIRCULATION AND CLIMATE ANOMALY IN EAST ASIA

In this paper,many observations show that the thermal states including the SST,the convective activities in the west-ern Pacific warm pool largely influence the interannual and intraseasonal variations of summer circulation and the cli-mate anomalies in East Asia.Moreover,it is pointed out that there is a teleconnection pattern of summer circulationanomalies in the Northern Hemisphere,the so-called East Asia/Pacific pattern.The cause of the teleconnection pattern is studied by using the theory of quasi-stationary planetary wave propaga-tion,and it may be due to the propagation of quasi-stationary planetary waves forced by heat source around thePhilippines.Moreover,this pattern is well simulated by using a quasi-geostrophic,linear,spherical model and theIAP-GCM,respectively.     

SYNERGISTIC EFFECTS ON ATMOSPHERIC ANOMALY OF THE TROPICAL INDIAN AND PACIFIC OCEANS IN NORTHERN WINTER

In terms of observational analysis and numerical simulations,study is performed of thesynergistic effects of the tropical Indian Ocean and Pacific SSTA on the atmosphere in northern win-ter.It turns out that the SSTA features show in-phase variation for the Indian Ocean and the central/eastern Pacific at low latitudes and anti-phase change compared with that in the neighborhood of thewestern Pacific“warming pool”.When negative SSTA in the western Pacific and positive SSTA inthe central/eastern Pacific and the Indian Ocean are available,the 500 hPa height anomaly exhibits awave train of a positive-negative-positive anomaly following a great-circle path emanating from thesubtropical western Pacific via the NW portion to North America,with enhanced westerly jet overmidlatitude East Asia and the NW Pacific,and v.v.The comparison to the results from the numeri-cal modeling of the tropical western Pacific SSTA reveals the importance of the synergistic effects ofthe oceans on atmospheric circulations.     

Numerical Simulation of the Relationship between the Anomaly of Subtropical High over East Asia and the Convective Activities in the Western Tropical Pacific

In this paper, a close relationship between the intraseasonal variation of subtropical high over East Asia and the convective activities around the South China Sea and the Philippines is analysed from OLR data.This relationship is studied by using the theory of wave propagating in a slowly varying medium and by using a quasi-geoslrophic, linear, spherical model and the IAP-GCM, respectively. The results show that when the SST is warming around the western tropical Pacific or the Philippines, the convective activities are intensified around the Philippines. As a consequence, the subtropical high will be intensified over East Asia. The computed results also show that when the anomaly of convective activities are caused around the Philippines, a teleconnection pattern of circulation anomalies will be caused over South Asia, East Asia and North America.     

Analysis of Stationary-Wave Nonstationarity in the Northern Hemisphere 500-hPa Height Field

In this paper, the concept of stationary-wave nonstationarity is presented and elucidated in the framework of the Lorenz circulation decomposition. This concept indicates the relative magnitude of the zonal nonuniform abnormity to the intensity of stationary waves on the monthly mean scale. Based on the Lorenz circulation decomposition, the nonstationarity degree Ius(Ilus) of the global (local) stationary waves is defined, and then used to analyze the stationary-wave nonstationarity at 30° 60°N, where the intensity of stationary waves at 500 hPa in the Northern Hemisphere, as is well known, is very high. The following findings are obtained: (1) There exist seasonal southward and northward movements in the position of the nonstationarity zones of the global stationary waves. The steady stationary waves occur in midlatitudes (35°-55°N) in winter and in the subtropical region (south of 35°N) in summer, associated with the major troughs over East Asia and North America and the weak European trough in winter, and with the relatively steady subtropical high system in summer. A high value center of Ius is at 35°N in spring and 50°N in summer, which might be caused by the seasonal variation of stationary-wave intensity, particularly in association with the interannual variability of trough ridge positions of stationary waves on the monthly mean maps. (2) There exists obvious asymmetry in Ilus, with the steady zones always located in the areas controlled by strong troughs/ridges and the unsteady ones in the areas where the stationary-wave intensity is low. The Ilus in the subtropics (south of 35°N) is larger in winter than in summer, and vice versa in the midlatitude region (north of 35°N). The summertime distribution of Ilus on the whole shows a rather complicated structure. However, North Europe is the most unsteady area for local stationary waves, as represented by high values of Ilus in both summer and winter, while over the North American continent (about 120°E-60°W), the °Ilus is slightly less than 1 in summer, indicating that the stationary waves in this region are more steady than those over other mid and high latitude regions. (3) From North China to Northwest Pacific, there is a high value zone of Ilus in summer, with its center (45°N, 130°E) located in the east of Heilongjiang Province. This influences the summer climate of northern China, including Northeast, North, and Northwest China. It is obvious that the nonstationarity is an intrinsic attribute of stationary waves, and can be regarded as being of the same importance as the intensity and energy-spectrum structure of stationary waves in the studies of the general circulation system.     

INTERDECADAL VARIATIONS OF INTERACTION BETWEEN NORTH PACIFIC SSTA AND EAST ASIAN SUMMER MONSOON

Identification of key SST zones is essential in predicting the weather / climate systems in East Asia. With the SST data by the U.K. Meteorological Office and 40-year geopotential height and wind fields by NCAR / NCEP, the relationship between the East Asian summer monsoon and north Pacific SSTA is studied, which reveals their interactions are of interdecadal variation. Before mid-1970's, the north Pacific SSTA acts upon the summer monsoon in East Asia through a great circle wavetrain and results in more rainfall in the summer of the northern part of China. After 1976, the SSTA weakens the wavetrain and no longer influences the precipitation in North China due to loosened links with the East Asian summer monsoon. It can be drawn that the key SST zones having potential effects on the weather / climate systems in East Asia do not stay in one particular area of the ocean but rather shift elsewhere as governed by the interdecadal variations of the air-sea interactions. It is hoped that the study would help shed light on the prediction of drought / flood spans in China.     

INVESTIGATION OF LAGGED CORRELATION BETWEEN THE TEMPERATURE OF CHINA AND THE GLOBAL OUTGOING LONGWAVE RADIATION IN 1974-1985

1-12 month lagged correlations between winter temperatures at 22 stations in China and the global dis-tribution of the Outgoing Longwave Radiation(OLR)are investigated.The basic results are:(1)Themonthly averaged temperatures of Dec.,Jan.and Feb.of China are correlated to the global distribution ofOLR of Oct.,Nov.and Dec.,respectively.It is consistent with the response period(about 50 days)of 500hPa geopotential height field in extratropical latitudes to the thermal forcing in tropical latitudes,and also consis-tent with the Walker cell of a 40-60 day oscillation.(2)There is a significant positive correlation betweenthe winter temperature of the most part of China and the OLR in the Gulf of Mexico.It shows that whenthe thermal forcing of the Gulf of Mexico is stronger,the excited barotropic instability of westerlies in northernAtlantic could influence the East Asian circulation and cause the temperature of China to be below normal.(3)The temperature around the Tibetan(Xizang)Plateau is negatively correlated to the OLR in western equa-torial Pacific,showing that when the Walker cell is stronger,the local Hadley cell in Southeast Asia isstronger and it causes the temperature around the Tibetan Plateau to be higher than normal.     

Simple metrics for representing East Asian winter monsoon variability: Urals blocking and western Pacific teleconnection patterns

Instead of conventional East Asian winter monsoon indices(EAWMIs), we simply use two large-scale teleconnection patterns to represent long-term variations in the EAWM. First, the Urals blocking pattern index(UBI) is closely related to cold air advection from the high latitudes towards western Siberia, such that it shows an implicit linkage with the Siberian high intensity and the surface air temperature(SAT) variations north of 40?N in the EAWM region. Second, the well-known western Pacific teleconnection index(WPI) is connected with the meridional displacement of the East Asian jet stream and the East Asian trough. This is strongly related to the SAT variations in the coastal area south of 40?N in the EAWM region.The temperature variation in the EAWM region is also represented by the two dominant temperature modes, which are called the northern temperature mode(NTM) and the southern temperature mode(STM). Compared to 19 existing EAWMIs and other well-known teleconnection patterns, the UBI shows the strongest correlation with the NTM, while the WPI shows an equally strong correlation with the STM as four EAWMIs. The UBI–NTM and WPI–STM relationships are robust when the correlation analysis is repeated by(1) the 31-year running correlation and(2) the 8-year high-pass and low-pass filter. Hence,these results are useful for analyzing the large-scale teleconnections of the EAWM and for evaluating this issue in climate models. In particular, more studies should focus on the teleconnection patterns over extratropical Eurasia.     

Tropical Cyclones over the Western North Pacific Strengthen the East Asia–Pacific Pattern during Summer

The contribution of tropical cyclones(TCs)to the East Asia–Pacific(EAP)teleconnection pattern during summer was investigated using the best track data of the Joint Typhoon Warning Center and NCEP-2 reanalysis datasets from 1979 to2018.The results showed that the TCs over the western North Pacific(WNP)correspond to a strengthened EAP pattern:During the summers of strong convection over the tropical WNP,TC days correspond to a stronger cyclonic circulation anomaly over the WNP in the lower troposphere,an enhanced seesaw pattern of negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia in the middle troposphere,and a more northward shift of the East Asian westerly jet in the upper troposphere.Further analyses indicated that two types of TCs with distinctly different tracks,i.e.,westward-moving TCs and northward-moving TCs,both favor the EAP pattern.The present results imply that TCs over the WNP,as extreme weather,can contribute significantly to summer-mean climate anomalies over the WNP and East Asia.