Arctic Oscillation and Antarctic Oscillation in Internal Atmospheric Variability with an Ensemble AGCM Simulation

In this study, we investigated the features of Arctic Oscillation （AO） and Antarctic Oscillation （AAO）, that is, the annular modes in the extratropics, in the internal atmospheric variability attained through an ensemble of integrations by an atmospheric general circulation model （AGCM） forced with the global observed SSTs. We focused on the interannual variability of AO/AAO, which is dominated by internal atmospheric variability. In comparison with previous observed results, the AO/AAO in internal atmospheric variability bear some similar characteristics, but exhibit a much clearer spatial structure： significant correlation between the North Pacific and North Atlantic centers of action, much stronger and more significant associated precipitation anomalies, and the meridional displacement of upper-tropospheric westerly jet streams in the Northern/Southern Hemisphere. In addition, we examined the relationship between the North Atlantic Oscillation （NAO）/AO and East Asian winter monsoon （EAWM）. It has been shown that in the internal atmospheric variability, the EAWM variation is significantly related to the NAO through upper-tropospheric atmospheric teleconnection patterns.

Arctic oscillation and the antarcic oscillation modes in the atmospheric stratosphere

On the basis of the EOF analysis of global geopotential height anomaly (GHA)field at 10 hPa level, the arctic oscillation (AO) and the means antarctic oscillation (AAO) can bedetected more obviously at the upper level of atmosphere than the AO or the AAO in surface layer.Unlike the hemisphere pattern of the AO and the AAO in the surface lager given by previous authors,the AO or the AAO in the stratosphere has its global features. The zonal oscillations—the SouthernOscillation (SO) and the north oscillation (NO) in atmospheric surface layer become less clear inthe upper air. The first mode (AO mode, abbreviated to AOM hereafter) and the second mode (AAO mode,abbreviated to AAOM hereafter) respectively have 41.47% and 27.04% of the total variancecontribution. The cumulative variance contribution of the first two modes reaches 68.51%. These twomodes are the main components for the interdecadal or decadal oscillation in the stratosphere. Inaddition, there still exist two kinds of oscillation patterns with less probability, namely, thesymmetric pattern at mid-high latitudes in the Southern Hemisphere and the asymmetric pattern.Spectral analysis shows that the AOM and the AAOM all have a spectral peak for 22 a period, beingconsistent with the periodic variations of the solar magnetic field, and a peak for 11 a period,being consistent with the period of the numbers of sunspots. Step filter analysis shows that theinfluencing factor for the upper atmospheric oscillation is the solar activity. The fluctuation ofthe solar magnetic field is the more influencing factor than the variation of the sunspot number.

Possible Impacts of the Arctic Oscillation on the Interdecadal Variation of Summer Monsoon Rainfall in East Asia

The influences of the wintertime AO (Arctic Oscillation) on the interdecadal variation of summer monsoon rainfall in East Asia were examined. An interdecadal abrupt change was found by the end of the 1970s in the variation of the AO index and the leading principal component time series of the summer rainfall in East Asia. The rainfall anomaly changed from below normal to above normal in central China, the southern part of northeastern China and the Korean peninsula around 1978. However, the opposite interdecadal variation was found in the rainfall anomaly in North China and South China. The interdecadal variation of summer rainfall is associated with the weakening of the East Asia summer monsoon circulation. It is indicated that the interdecadal variation of the AO exerts an influence on the weakening of the monsoon circulation. The recent trend in the AO toward its high-index polarity during the past two decades plays important roles in the land-sea contrast anomalies and wintertime precipitation anomaly. The mid- and high-latitude regions of the Asian continent are warming, while the low-latitude regions are cooling in winter and spring along with the AO entering its high-index polarity after the late 1970s. In the meantime, the precipitation over the Tibetan Plateau and South China is excessive, implying an increase of soil moisture. The cooling tendency of the land in the southern part of Asia will persist until summer because of the memory of soil moisture. So the warming of the Asian continent is relatively slow in summer. Moreover, the Indian Ocean and Pacific Ocean, which are located southward and eastward of the Asian land, are warming from winter to summer. This suggests that the contrast between the land and sea is decreased in summer. The interdecadal decrease of the land-sea heat contrast finally leads to the weakening of the East Asia summer monsoon circulation.

Influence of Arctic Oscillation on winter climate over China

In this study the relationship between the Arctic Oscillation (AO) and climate in China in boreal winter are investigated. Correlation analysis for the last 41 years shows that the winter temperature and precipitation in China change in phase with AO. High positive correlation (>0.4) between temperature and AO appears in the northern China. High correlation coefficients between precipitation and AO cover the southern China (close to the South China Sea) and the central China (between 30 o -40 o N and east of ~100 o E), with the values varying between +0.3 and +0.4. It is found that during the past several decades the precipitation was strongly affected by AO, but for the temperature the Siberian High plays a more important role. At the interdecadal time scale the AO has significant influence on both temperature and precipitation. Multivariate regression analysis demonstrates that AO and the Siberian High related variance in temperature and precipitation is 35% and 11% respectively. For precipitation, however the portion is rather low, implying that some other factors may be responsible for the changes in precipitation, in addition to AO and the Siberian High.

Teleconnections of Inter-Annual Streamflow Fluctuation in Slovakia with Arctic Oscillation,North Atlantic Oscillation,Southern Oscillation,and Quasi-Biennial Oscillation Phenomena

The aim of the paper is to analyze a possible teleconnection of Quasi-Biennial Oscillation （QBO）, Southern Oscillation （SO）, North Atlantic Oscillation （NAO）, and Arctic Oscillation （AO） phenomena with longterm streamflow fluctuation of the Bela River （1895-2004） and Cierny Hron River （1931-2004） （central Slovakia）. Homogeneity, long-term trends, as well as inter-annual dry and wet cycles were analyzed for the entire 1895-2004 time series of the Bela River and for the 1931-2004 time series of the Cierny Hron River. Inter-annual fluctuation of the wet and dry periods was identified using spectral analysis. The most significant period is that of 3.6 years. Other significant periods are those of 2.35 years, 13.5 years, and 21 years. Since these periods were found in other rivers of the world, as well as in SO, NAO, and AO phenomena, they can be considered as relating to the general regularity of the Earth.

On the Weakened Relationship between Spring Arctic Oscillation and Following Summer Tropical Cyclone Frequency over the Western North Pacific:A Comparison between 1968–1986 and 1989–2007

This study documents a weakening of the relationship between the spring Arctic Oscillation （AO） and the following summer tropical cyclone （TC） formation frequency over the eastern part （150°-180°E） of the western North Pacific （WNP）. The relationship is strong and statistically significant during 1968-1986, but becomes weak during 1989-2007. The spring AO- related SST, atmospheric dynamic, and thermodynamic conditions are compared between the two epochs to understand the possible reasons for the change in the relationship. Results indicate that the spring AO leads to an E1 Nifio-like SST anomaly, lower-level anomalous cyclonic circulation, upper-level anomalous anticyclonic circulation, enhanced ascending motion, and a positive midlevel relative humidity anomaly in the tropical western-central Pacific during 1968-1986, whereas the AOrelated anomalies in the above quantities are weak during 1989-2007. Hence, the large-scale dynamic and thermodynamic anomalies are more favorable for TC formation over the eastern WNP during 1968-1986 than during 1989-2007.

The Significant Relationship between the Arctic Oscillation (AO) in December and the January Climate over South China

Using NCEP/NCAR reanalysis data, the China rainfall and surface temperature data of the China Meteorological Administration, and the Arctic Oscillation （AO） indices of NOAA, the author investigates relationships between the AO and the precipitation and temperature over China. There exists a good relationship between the AO index in December and the succeeding January precipitation over South China, indicating that when the December AO index is positive （negative）, the January precipitation over South China increases （decreases）. A remarkable negative correlation between the December AO index and the January surface temperature also exists over South China, indicating that when the December AO index is positive （negative）, the January temperature over South China drops （rises）. The occurrence of this anomalous climate is related to the anomalies of the atmospheric circulation systems. The December AO greatly influences circulation anomalies in January. A positive phase of the AO is found to lead to a stronger subtropical jet over the south side from the Iran Plateau to the Tibetan Plateau. Consequently, it results in a deepening pressure trough around the Bay of Bengal, which transports the warm and wet air to South China continuously. The Siberian High in January is stronger and extends farther southeastward. It results in continual cold air at 1000 hPa pouring into South China, inducing low temperature. Cooperating with the trough of the Bay of Bengal, anomalous precipitation occurs over South China. For the negative phase of the December AO, the opposite situation is observed.

Spatio-temporal variations of Arctic amplification and their linkage with the Arctic oscillation

The Arctic near-surface air temperatures are increasing more than twice as fast as the global average-a feature known as Arctic amplification （AA）. A modified AA index is constructed in this paper to emphasize the contrast of warming rate between polar and mid-latitude regions, as well as the spatial and temporal characteristics of AA and their influence on atmospheric circulation over the Northern Hemisphere. Results show that AA has a pronounced annual cycle. The positive or negative phase activities are the strongest in autumn and winter, the weakest in summer. After experiencing a remarkable decadal shift from negative to positive phase in the early global warming hiatus period, the AA has entered into a state of being enlarged continuously, and the decadal regime shift of AA in about 2002 is affected mainly by decadal shift in autumn. In terms of spatial distribution, AA has maximum warming near the surface in almost all seasons except in summer. Poleward of 20~N, AA in autumn has a significant influence on the atmospheric circulation in the following winter. The reason may be that the autumn AA increases the amplitude of planetary waves, slows the wave speeds and weakens upper-level zonal winds through the thermal wind relation, thus influencing surface air temperature in the following winter. The AA correlates to negative phase of the Arctic oscillation （AO） and leads AO by 0-3 months within the period 1979-2002. However, weaker relationship between them is indistinctive after the decadal shift of AA.

Monthly Changes in the Influence of the Arctic Oscillation on Surface Air Temperature over China

Partial Least Squares Regression （PLSR） is used to study monthly changes in the influence of the Arctic Oscillation （AO） on spring, summer and autumn air temperature over China with the January 500 hPa geopotential height data from 1951 to 2004 and monthly temperature data from January to November at 160 stations in China. Several AO indices have been defined with the 500-hPa geopotential data and the index defined as the first principal component of the normalized geopotential data is best to be used to study the influence of the AO on SAT （surface air temperature） in China. There are three modes through which the AO in winter influences SAT in China. The influence of the AO on SAT in China changes monthly and is stronger in spring and summer than in autumn. The main influenced regions are Northeast China and the Changjiang River drainage area.

A Preliminary Study on the Relationship Between Arctic Oscillation and Daily SLP Variance in the Northern Hemisphere During Wintertime

In the present study, the authors investigated the relationship between the Arctic Oscillation (AO) and the high-frequency variability of daily sea level pressures in the Northern Hemisphere in winter (November through March), using NCEP/NCAR reanalysis datasets for the time period of 1948/49-2000/01. High-frequency signals are defined as those with timescales shorter than three weeks and measured in terms of variance, for each winter for each grid. The correlations between monthly mean AO index and high-frequency variance are conducted. A predominant feature is that several regional centers with high correlation show up in the middle to high latitudes. Significant areas include mid- to high-latitude Asia centered at Siberia, northern Europe and the middle-latitude North Atlantic east of northern Africa. Their strong correlations can also be confirmed by the singular value decomposition analysis of covariance between mean SLP and high-frequency variance. This indicates that the relationship of AO with daily Sea Level Pressure (SLP) is confined to some specific regions in association with the inherent atmospheric dynamics. In middle-latitude Asia, there is a significant (at the 95% level) trend of variance of-2.26% (10 yr)-1. Another region that displays a strong trend is the northwestern Pacific with a significant rate of change of 0.80% (10 yr)-1. If the winter of 1948/49, an apparent outlier, is excluded, a steady linear trend of +1.51% (10 yr)-1 shows up in northern Europe. The variance probability density functions (PDFs) are found to change in association with different AO phases. The changes corresponding to high and low AO phases, however, are asymmetric in these regions. Some regions such as northern Europe display much stronger changes in high AO years, whereas some other regions such as Siberia show a stronger connection to low AO conditions. These features are supported by ECMWF reanalysis data. However, the dynamical mechanisms involved in the AO-high frequency SLP variance connection have not been well understood, and this needs further study.

Three-Dimensional Dynamic Features of Two Arctic Oscillation Types

We investigated the differences between stratospheric （S-type） and tropospheric （T-type） Arctic Oscillation （AO） events on the intraseasonal time scale, in terms of their influences on surface air temperature （SAT） over the Northern Henfisphere and the dynamic features associated with their spatial structures. S-type AO events showed a stratosphere-troposphere coupled structure, while T-type events exhibited a stratosphere-troposphere uncoupled structure. The annular SAT anomalies over the Northern Hemisphere were found to be associated with S-type AO events, whereas such an annular feature was substantially de- structed in T-type AO events. The different horizontal structures in the troposphere of the two types could mainly be attributed to transient eddy feedback forcing. As for the vertically uncoupled structure of T- type events, the underlying dynamical features that differentiate them from S-type events lie in the vertical propagation of zonally confined Rossby waves. In T-type events, the zonally confined Rossby wave packets can emanate from the significant height anomalies over Northeast Asia, where one vertical waveguide exists, and then propagate upward into the stratosphere. In contrast, such a vertical propagation was not evident for S-type events. The stratospheric anomalies associated with the upward injection of the zonally confined Rossby waves from the troposphere in T-type events can further induce the anomalous vertical propagation of planetary waves （PWs） through the interference between the climatological-mean PWs and anomalous PWs, leading to the final stratosphere troposphere uncoupled structure of T-type events.

Impact of Arctic Oscillation on cloud radiative forcing and September sea ice retreat

The Arctic Oscillation(AO)has important effects on the sea ice change in terms of the dynamic and thermodynamic processes.However,while the dynamic processes of AO have been widely explored,the thermodynamic processes of AO need to be further discussed.In this paper,we use the fifth state-of-the-art reanalysis at European Centre for Medium-Range Weather Forecasts(ERA5)from 1979 to 2020 to investigate the relationship between AO and the surface springtime longwave(LW)cloud radiative forcing(CRF),summertime shortwave(SW)CRF in the Arctic region(65°-90°N).In addition,the contribution of CRF induced by AO to the sea ice change is also discussed.Results indicate that the positive(negative)anomalies of springtime LW CRF and summertime SW CRF are generally detected over the Arctic Ocean during the enhanced positive(negative)AO phase in spring and summer,respectively.Meanwhile,while the LW(SW)CRF generally has a positive correlation with AO index(AOI)in spring(summer)over the entire Arctic Ocean,this correlation is statistically significant over 70°-85°N and 120°W-90°E(i.e.,region of interest(ROI))in both seasons.Moreover,the response of CRF to the atmospheric conditions varies in spring and summer.We also find that the positive springtime(summertime)AOI tends to decrease(increase)the sea ice in September,and this phenomenon is especially prominent over the ROI.The sensitivity study among sea ice extent,CRF and AOI further reveals that decreases(increases)in September sea ice over the ROI are partly attributed to the springtime LW(summertime SW)CRF during the positive AOI.The present study provides a new pattern of AO affecting sea ice change via cloud radiative effects,which might benefit the sea ice forecast improvement.

Relationship between Temperature Variation in Winter of China and Arctic Oscillation

[Objective] The study aimed to discuss the relationship between temperature variation in winter of China and arctic oscillation. [Method] By using the NCEP/NCAR reanalysis data, AO index series and monthly temperature data from 160 stations of China, the variation characteris- tics of temperature in winter and AO index series as well as their relationship from 1951 to 2007 were studied. [Result] Temperature in winter of Chi- na correlated positively with AO index series. Both arctic oscillation and winter temperature in China had two major periods of 8 and 18 years. ⅠWTr and ⅠAO series, which had obvious inter-annual and inter-decadal variations, presented the almost same rising trend and had close relationship on an inter-decadal scale. Compared with AO, the Siberia high had more distinguished influences on inter-annual variations of temperature in winter of China. AO might affect changes in the Siberian high and thereby influence temperature in winter of China. AO had a greater influence on tempera- ture in winter of China than the Siberia high on an inter-decadal scale. When the East Asian through was weak, temperature in winter of China and AO index were low, while they were high as it was strong. The impact of AO index on an inter-decadal scale was more significant than the impact on an inter-annual scale. [ Condusien] The research could make the effects of AO on winter temperature of China clear.

The Arctic and Antarctic Oscillations in the IPCC AR4 Coupled Models

This study evaluates the fidelity of Arctic and Antarctic oscillations（AO and AAO for short,respectively） in the coupled general circulation models participating in the Fourth Assessment Report of Intergovernmental Panel on Climate Change（IPCC AR4）.The AO and AAO during 1970-1999 in 24 models are analyzed and compared with that in ERA-40 and NCEP-1.Models＇ performance is seasonally dependent,with best reproducibility of both spatial structure and trend in winter.In most models,the spatial pattern and temporal trend of AAO during this period are more delicately simulated than AO.After picking out models with better performance according to the Taylor diagram,we find that their ensemble mean can obviously improve models＇ reproducibility.The AO and AAO in the Special Report on Emission Scenarios（SRES） A1B Projection during the 21st century are also briefly analyzed.The results reveal that both the AO and AAO indices keep increasing during 1970-2099,with a steadier pace of AO than AAO.The spatial difference of sea level pressure between 2060-2089 and 1970-1999 shows decreased values in polar regions,and increased values in midlatitudes.The results manifest that the ozone recovery during the mid 21st century may not weaken such a trend.

The Hindcast of Winter and Spring Arctic and Antarctic Oscillation with the Coupled Climate Models

This study evaluates the ability of the global coupled climate models in hindcasting the Arctic Oscillation （AO） and Antarctic Oscillation （AAO）. The results show that the models can well simulate the spatial distribution of AO with better results in winter than in spring. In the troposphere in spring, the simulation of AO on the whole is still relatively good with a comparatively high correlation with the NCEP/NCAR reanalysis. The models can also well reproduce the spatial distribution of AAO throughout the year at all levels of the troposphere, and the spatial simulation is better at 850 hPa than at the surface. Although the simulation is better in winter than in other seasons, the seasonal variation is not so significant and the differences among different models are relatively smaU. In addition, the capability of the models for ＂predicting＂ the AO and the AAO index time series is limited, because only a few models can capture their observed interannual variability at the 95% significance level.

Possible influence of Arctic Oscillation on dust storm frequency in North China

This study has investigated the influence of Arctic Oscillation （AO） on dust storm frequency in North China in spring seasons during 1961-2007. There is a significant linkage between dust storm frequency and AO; a negative （positive） AO phase is related to an increased （decreased） dust storm frequency in North China. This relationship is closely related to changes in the cold air activity in Mongolia. The cold air activity exerts large impacts on the dust storm frequency; the frequency of cold air activity over Mongolia not only positively correlates with the dust storm frequency in North China, but also shows a long-term decreasing trend that is an important reason for the long-term decreasing of dust storm frequency in North China. The AO has large influence on the frequency of cold air activity over Mongolia; a negative （positive） AO phase is highly related to an increased （decreased） frequency of cold air activity over Mongolia, which results in an increased （decreased） dust storm frequency in North China.

The extremely cold 2009-2010 winter and its relationship with the Arctic oscillation

The Northern-Hemisphere high-latitude continents experienced extremely cold weathers in winter 2009 2010. In the present paper, we show that the cold winter was associated with the activity of tile Arctic oscillation （AO）, which demonstrated the strongest negative polarity over the past six decades and persisted from December, 2009 to March, 2010. It is found that variations of the surface AO was closely linked to stratospheric polar vortex anomalies, and that the surface AO phases followed downward propagation of stratospheric Northern-Hemisphere Annular mode （NAM） anomalies during the winter. The case of 2009-2010 winter provides us with a typical example that anomalous stratospheric signals can be used to improve skills of long-range weather forecast and intra-seasonal climate prediction in winter time. We also show that the E1 Nifio event, which started developing from May 2009, might contribute the formation of exceptionally negative and persistent AO and stratospheric NAM, particularly over North Pacific and North America.

DYNAMICS OF THE ZONAL ASYMMETRIC STRUCTURE OF ARCTIC OSCILLATION

The Arctic Oscillation (AO), though basically is a zonal symmetricphenomenon, also shows zonal asymmetric variation. The dynamics of this zonal variation isconsidered here from the point of view of the planetary propagation on the earth sphere. Based onthe linear barotropic vorticity e-quation, the planetary wave ray path is calculated in the winterJanuary. It shows that NAO( North Atlantic Oscillation), AO and North Pacific could be linkedthrough the planetary wave propagation, which acts as the atmospheric bridge. The zonal symmetricand asymmetric structures both have association with these Rossby wave activities.

Skillful prediction of winter Arctic Oscillation from previous summer in a linear empirical model

The winter Arctic Oscillation(WAO),as a primary atmospheric variability mode in the Northern Hemisphere,plays a key role in influencing mid-high-latitude climate variations.However,current dynamical seasonal forecasting systems have limited skills in predicting WAO with lead time longer than two months.In this study,we design a linear empirical model using two effective precursors from anomalies of the Arctic sea ice concentration(SIC)and the tropical sea surface temperature(SST)initiated in preceding late summer(August)which are both significantly correlated with WAO in recent four decades.This model can provide a skillful prediction of WAO at about half-year lead started from previous summer and perform much better than the dynamical models.Such a significantly prolonged lead time is owed to the stable precursor signals extracted from the SIC and SST anomalies over specific areas,which can persist from previous August and be further enhanced through autumn months.Validation results show that this model can produce a 20-year independent-validated prediction skill of 0.45 for 1999–2018 and a 39-year cross-validated skill of 0.67 for 1980–2018,providing a potentially effective tool for earlier predictions of winter climate variations at mid-high latitudes.

Decadal Shift in the Relationship between Winter Arctic Oscillation and Central Indian Ocean Precipitation during the Early 2000s

The present study investigated the long-term change in the interannual relationship between the boreal winter Arctic Oscillation(AO)and tropical Indian Ocean(TIO)climate during 1979-2019 and found that their linkage experienced a decadal change in 2001/2002.The 19-yr sliding correlation coefficient between the January-February-March(JFM)AO index and central TIO(0-10°S,65°-80°E)precipitation was significant,with values of approximately 0.50-0.75 during 1979-2001,but abruptly decreased to 0.35 in 2002 and 0.10 in 2010.Meanwhile,the spatial patterns of the AO-related atmospheric circulation anomalies also displayed different features before and after 2001.During 2002-2019,the anomalous anticyclone in the middle troposphere over the Arabian Sea moved northwestward and strengthened,and the JFM AO was more closely correlated to the anticyclone,with correlation coefficient changed from-0.38 before 2001 to-0.63 after 2001;correspondingly,strong cross-equator air flows were observed over the western TIO(40°-50°E),but no significant anomalies of precipitation in the central TIO were observed.During 1979-2001,however,significant southward cross-equator air flows appeared over the central TIO(65°-75°E),which enhanced the intertropical convergence zone and upward air motions,leading to more precipitation in central TIO.An analysis shows that the AO may modulate the Arabian anticyclone through two Rossby wave paths in the upper troposphere:a midlatitude(50°-60°N)path during 1979-2001 from North Atlantic southeastward to the Middle East and the neighboring Arabian Sea;and a subtropical(20°-30°N)path during 2002-2019 from North Atlantic eastward to the Middle East and Arabian Sea.Large wave activity fluxes induced by AO were concentrated along the two paths before and after 2001,and the location of the cross-equator flows depends on the location of the anticyclone.Causes of the decadal changes in the AO-associated wave trains need further investigation.