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.

Verification and Improvement of the Ability of CFSv2 to Predict the Antarctic Oscillation in Boreal Spring

The boreal spring Antarctic Oscillation(AAO)has a significant impact on the spring and summer climate in China.This study evaluates the capability of the NCEP's Climate Forecast System,version 2(CFSv2),in predicting the boreal spring AAO for the period 1983-2015.The results indicate that CFSv2 has poor skill in predicting the spring AAO,failing to predict the zonally symmetric spatial pattern of the AAO,with an insignificant correlation of 0.02 between the predicted and observed AAO Index(AAOI).Considering the interannual increment approach can amplify the prediction signals,we firstly establish a dynamical-statistical model to improve the interannual increment of the AAOI(DY AAOI),with two predictors of CFSv2-forecasted concurrent spring sea surface temperatures and observed preceding autumn sea ice.This dynamical-statistical model demonstrates good capability in predicting DY AAOI,with a significant correlation coeffcient of 0.58 between the observation and prediction during 1983-2015 in the two-year-out cross-validation.Then,we obtain an improved AAOI by adding the improved DY AAOI to the preceding observed AAOI.The improved AAOI shows a significant correlation coeffcient of 0.45 with the observed AAOI during 1983-2015.Moreover,the unrealistic atmospheric response to March-April-May sea ice in CFSv2 may be the possible cause for the failure of CFSv2 to predict the AAO.This study gives new clues regarding AAO prediction and short-term climate prediction.

Possible Impact of the Boreal Spring Antarctic Oscillation on the North American Summer Monsoon

This study examined the relationship between the boreal spring(April?May) Antarctic Oscillation(AAO) and the North American summer monsoon(NASM)(July?September) for the period of 1979?2008.The results show that these two systems are closely related.When the spring AAO was stronger than normal,the NASM tended to be weaker,and there was less rainfall over the monsoon region.The opposite NASM situation corresponded to a weaker spring AAO.Further analysis explored the possible mechanism for the delayed impact of the boreal spring AAO on the NASM.It was found that the tropical Atlantic sea surface temperature(SST) plays an important role in the connection between the two phenomena.The variability of the boreal spring AAO can produce anomalous SSTs over the tropical Atlantic.These SST anomalies can persist from spring to summer and can influence the Bermuda High,affecting water vapor transportation to the monsoon region.Through these processes,the boreal spring AAO exerts a significantly delayed impact on the amount of NASM precipitation.Thus,information about the boreal spring AAO is valuable for the prediction of the NASM.

IMPACTS OF ANTARCTIC OSCILLATION ON SUMMER MOISTURE TRANSPORT AND PRECIPITATION IN EASTERN CHINA

Using NCEP/NCAR reanalysis data and monthly precipitation over 160 conventional stations in China, analyses of moisture transport characteristics and corresponding precipitation variation in the east part of China in summer are made, and studies are carried out on possible influence on moisture transport and precipitation in summer by the variation of Antarctic Oscillation (AAO). The results show that the abnormal variation of the AAO affected the summer precipitation in China significantly. The variation of AAO can cause the variation of intension and location of Northwestern Pacific High, which in turn cause the variation of summer monsoon rainfall in the eastern China.

The Antarctic Oscillation-East Asian Summer Monsoon Connections in NCEP-1 and ERA-40

Connections between the spring Antarctic Oscillation （AAO） and the East Asian summer monsoon （EASM） in two reanalysis datasets-NCEP-1 （NCEP/NCAR Reanalysis 1） and ERA-40 （ECMWF 40- year Reanalysis）-are investigated in this study. Both show significant correlation between AAO and EASM rainfall over the Yangtze River valley, especially after about 1985. Though ERA-40 shows weaker anomalous signals connecting AAO and EASM over southern high latitudes than NCEP-1, both datasets reveal similar connecting patterns between them. A wave-train-like pattern appears in the upper levels, from southern high latitudes through east of Australia and from the Maritime Continent to East Asia. In positive AAO years, the cross equatorial southeasterly flow over the Maritime Continent in the lower levels is strengthened, the specific humidity of the whole atmosphere over East Asia increases, and convective activity is enhanced; thus the summer rainfall over East Asia increases. The spring AAO-EASM connection may be better represented in ERA-40.

Linkage between the Atlantic Tropical Hurricane Frequency and the Antarctic Oscillation in the Western Hemisphere

To examine the zonal asymmetry of the Antarctic oscillation (AAO), different portions of the AAO from June to October (JJASO) in the interannual variability of the Atlantic tropical hurricanes number (ATHN) are documented in this research. It follows that the AAO in the Western Hemisphere (AAOWH) is positively correlated with the ATHN, at 0.36 during the period of 1871-1998 and 0.42 during the period of 1949-98. After removing the linear regressions on the Southern Oscillation Index (SOI) in all time series, the above correlation coefficients are 0.25 and 0.30, respectively. The underlying mechanisms are studied through analyses of the atmospheric general circulation variability associated with the AAOWH. It turns out that the positive (negative) phase of JJASO AAOWH corresponds with several factors: decreased (increased) vertical zonal wind shear magnitude, low-level anomalous convergence (divergence), high-level anomalous divergence (convergence), and warmed (cooled) sea surface temperature in the tropical Atlantic. Therefore, the positive (negative) phase of JJASO AAOWH is favorable (unfavorable) to the tropical hurricane genesis.

Strengthened Relationship between the Antarctic Oscillation and ENSO After the Mid-1990s during Austral Spring

This paper documents a decadal strengthened co-variability of the Antarctic Oscillation （AAO） and ENSO in austral spring after the mid-1990s. During the period 1979-93, the ENSO （AAO） spatial signatures are restricted to the tropicsmidlatitudes （Antarctic-midlatitudes） of the Southern Hemisphere （SH）, with a weak connection between the two oscillations. Comparatively, after the mid-1990s, the E1 Nifio-related atmospheric anomalies project on a negative AAO pattern with a barotropic structure in the mid-high latitudes of the SH. The expansion of E1 Nifio-related air temperature anomalies have a heightened impact on the meridional thermal structure of the SH, contributing to a weakened circumpolar westerly and strengthened subtropical jet. Meanwhile, the ENSO-related southern three-cell circulations expand poleward and then strongly couple the Antarctic and the tropics. Numerical simulation results suggest that the intensified connection between ENSO and SST in the South Pacific since the mid-1990s is responsible for the strengthened AAO-ENSO relationship.

The relationship between the canonical ENSO and the phase transition of the Antarctic oscillation at the quasi-quadrennial timescale

A correlation analysis is performed to investigate the relationship between El Nino-Southern Oscillation （ENSO） and the Antarctic oscillation （AAO） at the quasi-quadrennial （QQ） timescale.It is found that the cold tongue index （CTI） and the AAO index （AAOI） are negatively correlated with about a 7-month lead-time,while they are positively correlated with about a 15-month lag-time.To further explore this relationship,complex empirical orthogonal function analysis is employed in the QQ sea level pressure （SLP） anomalies from 1951 to 2002.The results indicate that,during the ENSO cycle,there exists one kind of global tropical wave of wavenumber 1 （GTW1） propagating eastward.With the traveling of GTW1,the tropical SLP anomaly tends to intrude into the southern mid-latitudes.Accordingly,three strong signals travel synchronously along the circumSouth-Pacific path,and a relatively weak signal extends eastward and poleward over the South Ocean in the Atlantic-Indian Ocean sector.Following the propagation of these signals,the AAO phase tends to be reversed progressively.As a result,there exists an evident lead-lag correlation between CTI and AAOI.It can be concluded that ENSO plays a key role in the phase transition of AAO at the QQ timescale.It is also noticed that this regular relationship is only evident in the canonical ENSO events,for which sea surface temperature （SST） anomalies extend westward from the tropical eastern Pacific.On the other hand,the similar relationships are not found among those atypical ENSO events for which SST anomalies spread eastward from the central Pacific,such as the 1982-1983 ENSO event.

Teleconnected Influence of the Boreal Winter Antarctic Oscillation on the Somali Jet： Bridging Role of Sea Surface Temperature in Southern High and Middle Latitudes

The teleconnection impact of the boreal winter Antarctic Oscillation（AAO） on the Somali Jet（SMJ） intensity in the following spring and summer is examined in this paper.The variability of the boreal winter AAO is positively related to the SMJ intensity in both spring and summer.The analyses show that the SST in southern high and middle latitudes seems to serve as a bridge linking these two systems.When the AAO is in strong positive phase,SST over the Southern Ocean cools in the high latitudes and warms in the middle latitudes,which persists into summer;however,the variability of SST in southern high and middle latitudes is also closely correlated to SMJ intensity.A possible mechanism that links SST variability with the AAO-SMJ relationship is also discussed.The AAO in boreal winter produces an SST anomaly pattern in southern high and middle latitudes through the air-sea coupling.This AAOrelated SST anomaly pattern modulates the local Ferrel cell anomaly in summer,followed by the regional Hadley cell anomaly in tropics.The anomalous vertical motion in tropics then changes the land-sea thermal contrast between the tropical Indian Ocean and the Asian continent through the variability of low cloud cover and downward surface longwave radiation flux.Finally,the land-sea thermal contrast anomaly between the tropical Indian Ocean and the Asian continent changes the SMJ intensity.The results from Community Atmosphere Model experiments forced by the SST anomaly in southern high and middle latitudes also confirm this diagnostic physical process to some extent.

Interdecadal and Interannnual Variabilities of the Antarctic Oscillation Simulated by CAM3

Based on four sets of numerical simulations prescribed with atmospheric radiative forcing and sea surface temperature(SST) forcing in the Community Atmospheric Model version 3(CAM3), the interannual and interdecadal variabilities of the Antarctic oscillation(AAO) during austral summer were studied. It was found that the interannual variability is mainly driven by SST forcing. On the other hand, atmospheric radiative forcing plays a major role in the interdecadal variability. A cooling trend was found in the high latitudes of the Southern Hemisphere(SH) when atmospheric radiative forcing was specified in the model. This cooling trend tended to enhance the temperature gradient between the mid and high latitudes in the SH, inducing a transition of the AAO from a negative to a positive phase on the interdecadal timescale. The cooling trend was also partly weakened by the SST forcing, leading to a better simulation compared with the purely atmospheric radiative forcing run. Therefore, SST forcing cannot be ignored, although it is not as important as atmospheric radiative forcing.

Statistical Reconstruction of the Antarctic Oscillation Index Based on Multiple Proxies

Based on multiple proxies from the Southern Hemisphere, an austral summer (December-January-February: DJF) Antarctic Oscillation Index (AAO) since 1500 A.D. was reconstructed with a focus on interannual to interdecadal variability (<50 a). By applying a multivariate regression method, the observational AAO-proxy relations were calibrated and cross-validated for the period of 1957 89. The regressions were employed to compute the DJF-AAO index for 1500 1956. To verify the results, the authors checked the explained variance (r 2 ), the reduction of error (RE), and the standard error (SE). Cross-validation was performed by applying a leave-one-out validation method. Over the entire reconstruction period, the mean values of r 2 , RE, and SE are 59.9%, 0.47, and 0.67, respectively. These statistics indicate that the DJF-AAO reconstruction is relatively skillful and reliable for the last ～460 years. The reconstructed AAO variations on the interannual and interdecadal timescales compare favorably with those of several shorter sea level pressure (SLP)-based AAO indices. The leading periods of the DJF-AAO index over the last 500 years are ～2.4, ～2.6, ～6.3, ～24.1, and ～37.6 years, all of which are significant at the 95% level as estimated by power spectral analysis.

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.

IMPACTS OF FEBRUARY ANTARCTIC OSCILLATIONS ON SUMMER PRECIPITATION IN EASTERN CHINA

Impacts of February Antarctic oscillations (AAO) on the subsequent summer precipitation in the eastern part of China are analyzed in this paper in the context of the NCEP/NCAR reanalysis data and the monthly precipitation data at 160 weather stations.Results show that on interannual time-scale,the correlation coefficient between February AAO index and July-August rainfall in North China is 0.53,which exceeds the confidence level of 99.9%.When February AAO is strong,then the subsequent summer precipitation in North China is more-than-average,and meanwhile the summer precipitation in the Changjiang River Valley is less-than-average:and vice versa.Changes of AAO affect the subsequent intensity and position of the Northwest Pacific subtropical high,thus resulting in the south-north direction shift of the summer monsoon rain belt in China.

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.

Low-Frequency Coupled Atmosphere-Ocean Variability in the Southern Indian Ocean

The low-frequency atmosphere ocean coupled vestigated using observation data over 1958-2010 variability of the southern Indian Ocean （SIO） was in- These data were obtained from ECMWF for sea level pressure （SLP） and wind, from NCEP/NCAR for heat fluxes, and from the Hadley Center for SST. To obtain the coupled air-sea variability, we performed SVD analyses on SST and SLP. The primary coupled mode represents 43% of the total square covariance and is featured by weak westerly winds along 45~ 30~S. This weakened subtropical anticyclone forces fluctuations in a well-known subtropical dipole structure in the SST via wind-induced processes. The SST changes in response to atmosphere forcing and is predictable with a lead-time of 1 2 months. Atmosphere ocean coupling of this mode is strongest during the austral summer. Its principle component is characterized by mixed interannual and interdeeadal fluctuations. Titere is a strong relationship between the first mode and Antarctic Oscillation （AAO）. The AAO can influence tile coupled processes in the SIO by modulating the subtropical high. The second mode, accounting for 30% of the total square covariance, represents a 25-year period interdecadal oscillation in tile strength of the subtropical anticyclone that is accompanied by fluctuations of a monopole structure in the SST along the 35~ 25~S band. It is caused by subsidence of the atmosphere. The present study also shows that physical processes of both local thermodynamic and ocean circulation in the SIO have a crucial role in the fornmtion of the atmosphere-ocean eovariability.

Multi-scale variation characteristics of polar vortex at 10 hPa in the Southern Hemisphere

By use of 1948-2007 NCEP/NCAR reanalysis monthly geopotential data, a set of circulation indices are defined to characterize the polar vortex at 10 hPa in the Southern Hemisphere, including area-（S）, intensity-（P） and centre position-（λc , φc） indices. Sea-sonal variation, interannual anomalies and their possible causes of 10 hPa polar vortex in the Southern Hemisphere are analyzed by using these indices, the relationship between 10 hPa polar vortex strength and the Antarctic Oscillation are analyzed as well. The results show that： （1） the polar region at 10 hPa in the Southern Hemisphere is controlled by anticyclone （cyclone） from Dec. to Jan. （from Mar. to Oct.）, Feb. and Nov. are circulation transition seasons. （2） Intensity index （P） and area index （S） of anticy-clone （cyclone） in Jan. （Jul.） show a significant spike in the late 1970s, the anticyclone （cyclone） enhances （weakens） from ex-tremely weak （strong） oscillation to near the climatic mean before a spike, anticyclone tends to the mean state from very strong oscillation and cyclone oscillates in the weaker state after the spike. （3） There is significant interdecadal change for the anticyclone center in Jan., while markedly interannual variation for cyclone center in July. （4） The ozone anomalies can cause the interannual anomaly of the polar anticyclone at 10 hPa in the Southern Hemisphere in Jan. （positive correlation between them）, but it is not related to the polar cyclone anomalies. （5） There is notable negative correlation between the polar vortex intensity index P and the Antarctic Oscillation index （AAOI）, thus AAOI can be represented by P.

Trends in Latent and Sensible Heat Fluxes over the Southern Ocean

In this study, the trends in latent and sensible heat fluxes (LHF and SHF) over the Southern Ocean (oceans south of 35?S) and the contributions of the Antarctic Oscillation (AAO), the Pacific-South America teleconnection patterns (PSA1 and PSA2) and The El Ni?o-Southern Oscillation (ENSO) to these heat fluxes were investigated using the Objectively Analyzed Air-Sea Fluxes (OAFlux) dataset from 1979 to 2008. Significant positive annual trends in LHF occur over the Agulhas Current, the Brazil Current, the oceans in the vicinity of New Zealand and southern Australia, and the eastern Pacific Ocean near between 35?S and 40?S. Significant negative seasonal trends occur in LHF which differ among the four seasons. The spatial pattern and seasonal variation of the trends in SHF over the Southern Ocean are similar to those of LHF. The spatial patterns of the trends in LHF and SHF caused by the AAO, PSA1, PSA2 and Southern Oscillation Index (SOI) indices show a wave-like feature, varying with different seasons, that can be explained by the anomalous meridional wind associated with the four indices. The above four indices account for a small portion of the trend in LHF and SHF. The residual trends in LHF over the Southern Ocean may be explained by a climate shift in the late 1990s for the four seasons. But the residual trends in SHF over the Southern Ocean are not associated with the climate shift.