J. Meteor. Res.  2015, Vol. 29 Issue (6): 917-934   PDF    
http://dx.doi.org/10.1007/s13351-015-5095-6
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TANG Weiya, GUAN Zhaoyong. 2015.
ENSO-Independent Contemporaneous Variations of Anomalous Circulations in the Northern and Southern Hemispheres:The Polar-Tropical Seesaw Mode
J. Meteor. Res., 29(6): 917-934
http://dx.doi.org/10.1007/s13351-015-5095-6

Article History

Received May 19, 2015;
in final form October 15, 2015
ENSO-Independent Contemporaneous Variations of Anomalous Circulations in the Northern and Southern Hemispheres:The Polar-Tropical Seesaw Mode
TANG Weiya1,2,3, GUAN Zhaoyong1,3,4     
1 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044;
2 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081;
3 Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science & Technology, Nanjing 210044;
4 Polar Climate System and Global Change Laboratory, Nanjing University of Information Science & Technology, Nanjing 210044
ABSTRACT: Using the NCEP/NCAR reanalysis and the ENSO indices from the Climate Prediction Center over the period 1978-2014, we have investigated the contemporaneous circulation variations in the Northern and Southern Hemispheres by performing the singular value decomposition analysis of sea level pressure anomalies (SLPA) after the ENSO signal is regressed out. It is found that there exists a polar-tropical seesaw mode (PTSM) that characterizes with the out of phase fluctuations of SLPA between the polar and tropical regions in the Northern and Southern Hemispheres in boreal winter. This PTSM explains 47.74% of the total covariance of SLPA and is almost independent of ENSO. It demonstrates a long-term trend and oscillation cycles of 2-3 and 4-6 yr. The long-term trend in PTSM indicates that the sea level pressure gradually decreases in the tropics and increases in the polar region with time. This PTSM looks roughly symmetric about the equator besides the seesaw pattern of SLPA between the tropics and polar region in each hemisphere. The disturbances in the geopotential height field in association with the PTSM shows baroclinic features in the tropics whereas equivalent barotropic features in the mid and high latitudes in the troposphere. The anomalous thermal forcing in the tropical region is possibly one of the factors facilitating the formation of this PTSM. Significant global precipitation and temperature anomalies related to the PTSM are observed. In the positive PTSM phase, precipitation and temperature are higher than normal in southern Europe and the Mediterranean and surrounding areas, but lower than normal in northern Europe and Siberia. Precipitation is higher than normal while temperature is lower than normal in Northeast Asia. Significant temperature and precipitation anomalies possibly occur in the regions of western China, northern India, parts of North America, parts of subtropical Africa, Maritime Continent, and Antarctic. These results are helpful for better understanding of the circulation variations and the mechanisms behind the interactions between the Northern and Southern Hemispheres and the related winter climate anomalies over globe.
Keywords: Northern and Southern Hemispheres     polar-tropical seesaw mode     climate anomaly     boreal winter    
1. Introduction

In recent decades,researchers have paid more attention to the atmospheric circulations on hemispheric and global scales because of increases of global observations and rapid development of global climate models. Our underst and ings of the variations of global atmospheric circulation and regional climate along withthe mechanisms behind have been hence deepened,based on the research progresses in the global circulation changes(e.g.,Christy et al., 1989; Kusunoki et al., 2006; Xiao et al., 2010; Weng et al., 2011; Bai et al., 2012),climate simulations using various globalclimate system models(e.g.,Giorgi,1990; Thompson and Pollard, 1995; Man and Zhou, 2011; Zhang,2012;Chen et al., 2014,Zhou et al., 2014),simulated globalcirculation variations in the context of AtmosphericModel Intercomparison Project(AMIP),Ocean ModelIntercomparison Project(OMIP), and Coupled ModelIntercomparison Project(CMIPS)programs(Meehl et al., 2007; Wang et al., 2008; Taylor et al., 2012; Li et al., 2013; Shao et al., 2013; Zhang et al., 2013), and interactions between the Northern Hemisphere(NH) and Southern Hemisphere(SH)(Findlater,1969; Carleton,1992; Guan and Yamagata, 2001; Zeng and Li, 2002; Huang et al., 2003). Though significant progresses have been made,it is still far from a fullyunderst and ing of the complicated variations of atmospheric circulations and the mechanisms behind.

It is well known that the atmospheric circulationsvary differently between the NH and SH. The majorpatterns of the complicated atmospheric circulationvariations in the NH are considered to be independentof those in the SH. This can be attributed to the lateral boundary effect of equator where the Coriolis forceis zero,the different l and -sea contrasts,the asymmetry of solar radiation except in the spring and autumnequinoxes, and the different ocean-atmosphere interactions between the two hemispheres. This is manifestedby some strong variabilities in different hemispheres.These are,for example,the North Atlantic Oscillation(NAO)/Arctic Oscillation(AO)(Thompson and Wallace, 1998; Wallace,2000; Watanabe,2004)inthe mid and high latitude regions in the NH,the Pacific Decadal Oscillation(PDO; Mantua et al., 1997),Antarctic Oscillation(AAO)in the SH(Gong and Wang, 1999), and various other teleconnection patterns in each hemisphere(Wallace and Gutzler, 1981;Hoskins et al., 1983; Shi and Zhu, 1993; Ding et al., 2014; Tan and Chen, 2014),indicating the independent circulation variations in the two hemispheres.

However,there exist some contemporaneous variations of circulations in the two hemispheres, and eventhese variations are highly correlated. This is becauseof some factors including the earth’s rotation and otherexternal forcing such as the solar radiation and theENSO,as well as the cross-equator flow and its subsequent lateral coupling,which provide the possibility for the atmospheric interaction between the twohemispheres(Wang et al., 2013). From the perspective of global dry atmospheric mass conservation,theinter-hemispheric oscillation(IHO; Guan and Yamagata, 2001; Guan et al., 2010)is,to a certain degree,another linkage of the circulation variations betweenthe two hemispheres.

As the strongest interannual variability in theequatorial Pacific region,ENSO can induce anomalous changes of atmospheric circulation not only in thetropics,but also in the mid and high latitudes(e.g.,Huang et al., 2003; Ashok et al., 2007; Gushchina and Dewitte, 2012). For example,the Pacific North America(PNA)teleconnection is triggered by ENSO events(Wallace and Gutzler, 1981)in the NH whereas the Pacific South America(PSA)pattern(Carleton,1992)inthe SH. Such contemporaneous variations of anomalous circulations in the two hemispheres are considered to be an important mode in the global domainon the interannual timescales. It is found that theENSO-related anomalies of the near surface circulation that are roughly symmetric about the equatorshow an east-west distribution with dipole or triplestructure in the low latitudes of Indo-Pacific sector.However,some asymmetric components about equator are also observed. These asymmetric circulationanomalies about equator vary with time,which are affected by, and affect in turn the global weather and climate variations and even the global changes. Nonlinear interactions between the symmetric and antisymmetric components of the atmospheric circulationvariations may also play an important role in the symmetry of the atmospheric circulation anomalies. Without any doubt,both the symmetric and asymmetriccomponents of circulation anomalies are very important in shaping our climate in global domain(Guan et al., 1994; Wang et al., 1994). However,until now themajor circulation variation patterns in the two hemispheres that are highly correlated and change contemporaneously,symmetric or not,have not been fullyrevealed yet.

Since the AO and AAO are two strong signals independent of each other,it seems that,besides IHO,the anomalous circulations respectively in the NH and SH must change contemporaneously as a result of theENSO-related thermal forcing in tropics. However,contributions of ENSO only accounts for a part of thetotal covariance of the contemporaneous changes inthe two hemispheres(Chen,2002; Yuan et al., 2012,2014; Chen et al., 2013). Thus,is it possible to findthe physical mode of circulation anomalies that is independent of ENSO varies contemporaneously in theNH and SH? To answer this question,we reveal thismode in the present study by performing the singularvalue decomposition(SVD)for the sea level pressureanomalies(SLPA)after the ENSO signal is filteredout. The results will be beneficial for us to better underst and the mechanisms of circulation co-variationsbetween the SH and NH and their possible influenceson global climate anomalies.

This paper is organized as follows. A brief description of data sources and methodology employedin this study are given in Section 2 after a brief introduction to study purpose. The polar-tropical seesawmode(PTSM)is explored in Section 3. In Section 4,the structure of this seesaw mode along with its dynamics is investigated. In Section 5,the possible influences of this PTSM on boreal winter climate includingsurface temperature and precipitation are examined.Section 6 gives the conclusions and discussion.

2. Data and methodology

The data used in this study include(1)theNCAR/NCEP monthly mean reanalysis with a resolution 2.5°(lon.)× 2.5°(lat.)in horizontal and 17 isobaric levels in vertical in global domain(Kalnay et al., 1996). The quantities are surface variables includingsea level pressure(SLP),surface pressure, and surfacetemperature, and isobaric level variables including thegeopotential height,specific humidity,winds, and airtemperature;(2)NOAA global reconstructed sea surface temperature(SST)with a resolution of 2.0°(lon.)× 2.0°(lat.)on a grid mesh; and (3)CMAP precipitation dataset(Xie and Arkin, 1997),AO indices,AAO indices, and SST indices over various Nino ˜ regions from NOAA Climate Prediction Center(CPC).All the datasets cover the 37-yr period from 1978 to2014,except that the CMAP precipitation dataset isfor the period 1979–2011. Winter is defined to be December,January, and February(DJF)in the NH. Wintertime average refers to the average over DJF.

Methods used in this study are the SVD analysis,the wavelet analysis, and correlation and regressionanalysis.

3. Contemporaneous circulation changes independent of ENSO3.1 The polar-tropical seesaw mode(PTSM)

The SLPA is a good indicator that describes thevariations of circulations and even the related climate anomalies. The SLP anomalies are employed toget the major features of contemporaneous circulationchanges in the two hemispheres independent of ENSO.The ENSO signal is removed from SLPA by regressingSLPA onto the Nino3.4 SSTA(sea surface temperature anomalies)index for 36 winters from 1978/1979to 2013/2014. Let INino3.4 be the normalized timeseries of Nino3.4 SSTA index and PSLa be the SLPA.Then we have

where µ is the regression coefficient, and PSLar(denoted by SLPAr hereafter)is the remainder part ofSLPA after having the ENSO-related component being removed. Apparently,SLPAr is simultaneously independent of ENSO in statistical sense.

The SVD is performed of the variations of SLPAr.The SLPAr to the north(south)of 2.5°N in the NH(SH)is taken as the left(right)field for the SVD analysis. Statistics of SVD modes are presented in Table 1.The two leading modes of the SVD account for 47.74% and 15.07% of the total of squares of covariance respectively, and the cumulative covariance proportionis up to 62.81%. They represent the spatial modes that correspond to the most significant contemporaneous SLPAr changes in the two hemispheres duringthe wintertime.

Table 1. Statistics of the two leading SVD modes for the SLPAr in the two hemispheres

The contemporaneous SLPAr changes in the twohemispheres that are independent of ENSO are highlycorrelated; the correlation coefficient is 0.73(P <0.05)for the leading SVD mode 1. The spatial distribution of SLPAr in each hemisphere(Figs. 1a and 1b)demonstrates an anti-phase feature in the low and high latitude regions, and the SLPAr in the twohemispheres are roughly symmetric about the equator. This is an interesting phenomenon. This mode asrevealed by SVD1 is referred to as the PTSM that ischaracterized by the contemporaneous SLPAr changesin the NH and SH with the seesaw structure of SLPArbetween the low and high latitude regions.

The spatial distribution of SVD1(Figs. 1a and 1b)displays negative SLP disturbances in the lowerlatitudes whereas positive in the mid and high latitudes,indicating a seesaw structure in meridional.Large negative disturbance centers are located inNorth Africa,South Asia,the Maritime Continent(MC), and the equatorial East Pacific. In the highlatitudes,the SLP anomalies are largely positive; thepositive SLPA centers are located near the polar circle or in the polar region. For the convenience in thefollowing sections,the positive phase(negative phase)of PTSM is defined as the phase when the time coefficient of SVD1 is positive(negative)along with thepositive(negative)SLPAr in the polar region whilenegative(positive)SLPAr in the tropics.

Despite the overall symmetric distribution ofPTSM about equator,there still exists large differences in its spatial distribution between the two hemispheres(Figs. 1a and 1b). Comparing the SLPArbetween the two hemispheres,it can be found thatthe SLPAr is more zonally symmetric in the NH. Areas where the larger anomalies of SLPAr appear inthe NH are broader than those in the SH; in the SH,variations of SLPAr look to be more localized. Such adifference in the spatial distribution of SLPAr mightbe related to the different l and -sea contrasts betweenthe two hemispheres. It may also be relevant to thedifference in seasons,i.e.,the NH winter correspondsto the SH summer.

Fig. 1 Heterogeneous correlations of SLPAr for mode SVD1 in (a) the NH (left field) and (b) the SH (right field) along with (c) the corresponding time series of coefficients,and (d) the zonal averages of SVD1.Contour intervals are 0.1 in (a) and (b).The critical value at the 95% level of confidence is found to be 0.32 by using a t-test.

Note that the spatial distribution of the PTSMrevealed in the present study is quite different fromthe results of previous studies about the IHO(Guan and Yamagata, 2001; Lu et al., 2008; Guan et al., 2010; Lu et al., 2013). The IHO is a physical modethat describes the simultaneous variations of circulation anomalies in the NH and SH from the perspectiveof hemispherical-scale atmospheric disturbance(Guan et al., 2010). The physical basis for the IHO is the redistribution of the atmospheric mass in the two hemispheres under the constraint of global dry atmosphericmass conservation(more in the SH and less in theNH, and vice versa). Without any doubt,this IHOmode is important for us to better underst and the circulation interactions between the NH and SH. However,the IHO is actually the third mode out of all theEOF(empirical orthogonal function)modes that weobtained from EOF decomposition of the surface pressure anomalies. Considering the total variance of theatmospheric disturbance,we need to find the dominantmode of the contemporaneous circulation changes inthe two hemispheres. The PTSM seems like this dominant mode that accounts for 47.74% of total covarianceof SLPAr. Explicitly,the PTSM is definitely differentfrom the IHO that the results from the cross-equatorialatmospheric mass exchange between the two hemispheres. Instead,the PTSM in the present study reveals a component of circulation anomalies that variescontemporaneously in the two hemispheres when theENSO effects have been regressed out.

The correlation coefficient between the time coefficients of the right and left fields of SVD1 is 0.73,suggesting that the SLPAr changes in the NH and SH are still synergetic even after the ENSO signal is removed. Figure 1c shows clearly that changes in SLPArare dominated by interannual variability in both theNH and SH. Wavelet analysis of the time series ofSLPAr in both hemispheres displays that the majorperiod has changed from 2–3 to 4–6 yr at the beginning of the 21st century. A long-term trend is foundin the time series of the time coefficient(Fig. 1c)forthe most recent 30 years since 1978,indicating thatSLP is decreasing in the tropics while increasing inthe polar region during the boreal winter in the past30 years. This result is clearly consistent with the factthat the global warming is more distinct in the polarregion than in other regions(IPCC,2014; Li et al., 2014).

The meridional structure of PTSM looks seesawlike. Figure 1d depicts a clear meridional seesaw between the SLPAr in the polar region and in the tropics,which is distinct in both hemispheres. The seesawbetween the polar and near-equator regions in bothhemispheres looks clear,but in general,it is strongerin the NH than in the SH. The reversal zone for theSLPAr from positive to negative is located between 50° and 55°N in the NH whereas between 55° and 60°S inthe SH.

In order to reconfirm the PTSM mode as a physical entity,here we(1)analyze the pattern correlations,i.e.,the pattern similarity correlation coefficients,between the SVD1 and SLPA to examine whether theSLPA displays a pattern similar to SVD1 in certainyears; and (2)obtain the interannual variation of regionally averaged SLPAr to examine whether thereexists an anti-phase relationship between the SLPAchanges in the lower and higher latitudes.

The spatial pattern correlations between theSVD1 and SLPA in the NH and SH are calculated,respectively. If the correlation coefficient is large(|R| > 0.55),then the spatial patterns of SLPA and SVD1 left(right)field are considered to be quite similar. Figure 2a shows the normalized time series ofthe similarity coefficient between the spatial patternsof SVD1 and SLPA. It is seen from Fig. 2a that twotime series for the NH and SH respectively are quitesimilar; the correlation of the time series of similaritycoefficients in the NH with that in the SH is foundto be 0.57. As what we expected,the correlations oftime series of similarity coefficients are highly correlated with the time series of coefficients of SVD1 inboth hemispheres; both the correlation coefficients arelarger than 0.93(P < 0.05). These results indicatethat the SVD1 is indeed a mode describing the contemporaneous changes in SLPA in the NH and SH. Table 2lists the years when the spatial patterns of SLPA and SVD1 are similar. Further examination verifies thatthe SLPA distribution in these years(figure omitted)is exactly like that shown in Figs. 1a and 1b. Notethat those years listed in Table 2 include years whenENSO is in its cold phase,warm phase, and neutralphase,indicating that the PTSM mode is independentof ENSO.

Fig. 2 Normalized time series of (a) the pattern correlations between mode SVD1 and SLPAr with red (blue) lines for the NH (SH),and (b) DJF mean SLPAr averaged over zonal regions of 55°-90°N (red),30°S-30°N (black),and 60°-90°S (blue),respectively.

Table 2. The years when the spatial patterns of PTSM and SLPA are similar to each other as identified by pattern correlation coefficient with its absolute value larger than 0.55

Based on the spatial patterns shown in Figs. 1a and 1b,three regions,i.e.,55°–90°N,30°S–30°N, and 60°–90°S,are roughly selected,over which the arealweighted means of SLPAr are computed. The resultsare shown in Fig. 2b. It is found that when the positive SLPA occurs in the low latitudes,the negativeSLPA will simultaneously appear in high latitudes ofboth hemispheres. That is to say,the SLP changein low-latitudes is highly negatively correlated with theSLPA in high-latitude regions of both hemispheres. In fact,it is found that the correlation coefficients arefound to be –0.77 and –0.41(P < 0.05)in the NH and SH,respectively. These results again suggestthat the SLP in high-latitudes will adjust correspondingly when it changes in low-latitude region. Moreover,these suggest that the variations of SLPA in lowlatitudes work as a physical linkage that links the variations of the anomalous circulations in high latitudesbetween the two hemispheres.

3.2 Independence of PTSM to ENSO

In order to reconfirm the independence of PTSMto ENSO,we analyze first the contribution of thePTSM and ENSO to SLPA. Figure 3 shows the contributions of SVD1-related component of SLPA and those of the Nino3.4 index-related component of theSLPA to the total variance of the SLPA. It is seenfrom Fig. 3 that the large contributions of PTSM tothe variance of SLPA are found in high-latitude regionsof the two hemispheres,while the large contributionsof ENSO-related variability are found in the tropics.Zonally averaged contributions of SLPA component inassociation with the PTSM and ENSO to the totalSLPA variance are listed in Table 3. Again,large contributions of the PTSM to total variance of SLPA appear in high latitudes of the two hemispheres whereasthe contributions of ENSO are mainly observed in thetropics. Specifically in the NH,the PTSM-relatedSLPA variability accounts for up to 70% of the total SLPA variance in regions around the Barents Sea and Novaya Zemlya in high latitudes and 60% in thewestern Greenl and . It also accounts for up to 70% ofthe total SLPA variance in North Africa,the Mediterranean region, and southern Europe(Fig. 3a). However,only about 30% of the total variance of SLPAcan be explained by PTSM-related SLPA variability in East Asia. In the SH,the large contributions ofPTSM-related SLPA are found in high latitudes to thesouth of 60°S(Fig. 3b).

Fig. 3 Percentages of the total SLPA variance explained by the Nino3.4 index-related variability (shadings) and those by PTSM-related variability as obtained by regressing SLPA onto the time series of coefficients of SVD1 (contours) for (a) the NH and (b) the SH.

Table 3. Zonal means of the percentage contribution as shown in Fig. 3

The above shows that the PTSM can explain40%–70% of the total variance of SLPA in high latitudes,but only 10%–40% of the total variance inlow latitudes. Apparently,in addition to ENSO,thePTSM is one important mode that describes the SLPvariations; it reveals that the tropical SLPA oscillationinherently links the circulation variations in mid and high latitudes of the two hemispheres via redistribution of the atmospheric mass between lower and higherlatitudes,the meridional vertical circulations, and thepropagation of Rossby waves excited by anomalousforcing in tropical region(Sardeshmukh and Hoskins, 1988). In this way,the close relationship between variations of circulation anomalies in high latitudes of thetwo hemispheres is hence established. In other words,the PTSM actually explores the seesaw-like anti-phasevariations of SLP between tropical low air pressure region and polar higher air pressure region.

The anomalous thermal forcing in tropical regionmay play a crucial role in the formation of PSTM.It is found that the correlation of time coefficient ofthe SVD1 with the time series of surface air temperature(surface air pressure)anomalies averaged over20°S–20°N is 0.46(–0.86),suggesting that the aircolumn anomalously exp and s and air pressure correspondingly decreases in the tropics. However,in themidlatitudes,anomalous surface air temperature is simultaneously anti-correlated with the time coefficientof the SVD1 with the correlation coefficient of –0.44.More than this,the anti-correlation gradually intensifies poleward.

Based on the aforementioned,the physical meaning of the PTSM is that,when the low air pressure inthe tropics changes due to abnormal adiabatic forcingof ocean-atmosphere interaction and /or cloud radiative effects,pressure in the mid and high latitudes willadjust correspondingly for at least the changes in theatmospheric mass will remain balanced on the hemispheric scale. Of course,the reversed situation mayalso happen,i.e.,air pressure changes in the mid and high latitudes induced by some anomalous internal/external forcing such surface albedo anomaliesmay result in changes of air pressure in low latitudes.

It is known that the atmosphere interacts withthe oceans in both tropics and higher latitudes. Whenthe significant SSTA occurs in middle and east equatorial Pacific,the atmospheric circulation will respondto this SSTA forcing with a time lag by at least onemonth. Particularly,the tropical Indian Ocean is usually believed to be a slave to the Pacific,whose SSTAchanges are a response to the SSTA changes in eastequatorial Pacific with a lag of about six months(e.g.,Ashok et al., 2003). In this way,the variations of atmospheric circulation anomalies may have a significantlag correlation with Nino3.4 index. Then,is the PTSMstill independent of ENSO? To answer this question,here we calculate the correlation coefficients betweenvarious ENSO indices and SVD1 time series of coefficients(Table 4). It shows in Table 4 that no matterwhether the PTSM advances ENSO by 1–3 seasons,oris of the same period of ENSO,or lags ENSO by 1–3seasons,its correlation coefficients with all the ENSOindices are very small. Since ENSO has a cycle of 3–7yr,we also examine the correlations of wintertimePTSM with wintertime ENSO indices that are either leading or lagging 1–3 yr(Table 5). No large correlations are found in Table 5. Thereby,it is reasonableto deduce that the PTSM and ENSO are independentof each other from the perspective of statistics. In addition,the correlation between the PTSM and IHOis also very low(Table 4). These are in agreementwith what we have discussed previously in the presentstudy,reconfirming the PTSM independent of ENSO.

Table 4. Lag correlations of the time series of coefficients of SVD1 with various indices of ENSO, AO, AAO, and IHO

Table 5. Lag correlations of the time series of coefficients of SVD1 with various wintertime ENSO indices that are either leading or lagging 1–3 yr

It is worth noting that,despite the different spatial distributions of extreme values of SLPAr and thedifferent reversal latitudes where the SLPAr changesfrom positive to negative,the PTSM is still highly correlated with AO and AAO and the correlation is relatively high(Table 4). This implies that there exist certain relationships between the PTSM and AO/AAO.However,if both ENSO and AO/AAO signals are filtered out by using the regression analysis,the PTSMmode can still be obtained by using the SVD analysisof abnormal SLP,except that the anomalies of SLP inthe SH are much weaker than those in the NH.

4. Spatial structure of the PTSM4.1 Anomalous horizontal circulations

The horizontal pattern of the ENSO-independentPTSM in SLPA as displayed in Fig. 1 reveals the major mode of contemporaneous changes of circulationanomalies in the NH and SH. However,the whole features of this PTSM at different isobaric levels have notexhibited till now. As the SLPAs vary as a result ofboth the variations of anomalous atmospheric circulations above the earth surface and the abnormal surfacethermal forcing,the structure of PTSM above sea levelmay show some interesting features. The spatial features of PTSM at different heights can be examinedby using the regression analysis. In this way,we haveobtained the anomalous circulations at levels 850,500, and 200 hPa(Fig. 4).

Fig. 4 Anomalous winds and geopotential heights as obtained by regressing these quantities onto the time series of coefficients of SVD1.(a) Rotational (streamlines) and divergent (vectors;m s−1) wind components at 850 hPa with shades in blue for anomalous convergence.(b) Anomalous wind (vectors;m s−1) and geopotential height (contour;gpm) at 500 hPa.Bold arrows and grey shaded areas are for values at/above the 95% confidence level by using an F-test.(c) Anomalous rotational (streamlines) and divergent (vectors;m s−1) wind components at 200 hPa with shadings in red for divergence anomalies.

Similar to the aforementioned,the abnormal circulation patterns at various levels of the tropospherelook also roughly symmetric about the equator,particularly in the mid and high latitudes. This can beseen from Figs. 4a and 4c in places including the areaeast of the Meridian,the area nearby 60°E,the AsianAustralian region,the western Pacific,area east of thedateline, and the American region. Over the AsianAustralian region(90°–120°E),the anomalous anticyclone,cyclone, and anticyclone are found from theequator to the polar region in both hemispheres, and they are roughly symmetric about the equator. Theseanomalous circulations intensify with height with anequivalent barotropic structure. In the region nearby 150°W to the east of the dateline,the anomalous cyclone,anticyclone, and cyclone are distributed alternatively in the meridional direction and also roughlysymmetric about the equator in the NH and SH. Similarly,these systems are also equivalent barotropic inthe vertical and intensify with height.

Relatively large differences are observed in theanomalous circulations related to the PTSM betweenthe SH and NH although the overall appearance ofanomalous circulations is symmetric about the equator. The wintertime abnormal circulation in the mid and high latitudes is stronger in the NH than that inthe SH. And disturbances in the mid and high latitudes in the NH are more zonal than those in the SH.In the high latitudes in the SH,zonal disturbances arerelatively weak.

The anomalous circulations over the midlatitudeEurasia,Pacific, and Atlantic are probably associatedwith some teleconnection patterns in the NH. For example,the disturbances occurring in West Europe,the Ural Mountains, and the coastal region in EastAsia are probably associated with the Eurasian(EU)teleconnection pattern; disturbances occurring in theNorth Atlantic including its subtropical region exhibitan East Atlantic(EA)teleconnection pattern. It isinteresting to note that even after the Nino3.4 signals are filtered out,a horizontal wave train similarto the PNA teleconnection pattern still exists fromthe central-eastern Pacific to North Pacific and thento North America and finally to the Gulf of Mexico(Wallace and Gutzler, 1981). In the SH,a wave trainwith the structure similar to the PSA pattern also occurs over the southern Pacific and Atlantic(Carleton,1992).

Furthermore,when the PTSM is in its positivephase,the anomalous easterly winds occur in the mid and lower troposphere over the equatorial central Pacific(Figs. 4a and 4b)whereas anomalous westerlywinds occur at 200 hPa in the upper troposphere(Fig. 4c). Meanwhile,over the Indian Ocean,westerly windanomalies are dominant in the mid and lower troposphere while easterly anomalies occur in the upper troposphere. These anomalous winds induce convergencein the mid and lower troposphere and divergence in theupper troposphere above the MC,which is actually theascending branch of the abnormal Walker circulation.Anomalous convective activities in this MC region actas an “atmospheric bridge” when the equatorial Pacific interacts with the tropical Indian Ocean(Wu and Meng, 1998; Alexander et al., 2002). The descendingbranch of the anomalous Walker circulation is locatedin the equatorial central-eastern Pacific and its centeris around 150°W nearby the equator. This means thatthe climatological mean Walker circulation is intensified when the PTSM is in its positive phase. Note thatthe situations are opposite when PTSM in its negativephase.

4.2 Anomalous vertical circulations

In order to further reveal the vertical structure ofthe PTSM,we have performed regressions of geopotential height anomaly in the troposphere and verticalvelocity anomaly onto the time series of coefficients ofSVD1. The results are shown in Fig. 5.

Fig. 5 Regressions of anomalous vertical circulation (streamlines) and geopotential height (shaded contours;gpm) onto time-series of coefficients of the SVD1 with that (a) for EQ,(b)0°,(c)30°E,(d)100°E,(e)150°E,(f)150°W,and (g)60°W,respectively.

Above the equator(Fig. 5a),the sign-reverse ofgeopotential height anomalies from positive to negative are found at around 700 hPa over 90°W–45°E, and at about 500 hPa over 175°E–90°W. Interestingly,thesign-reverse height at around 90°E is at about 450 hPa,higher than in other regions over the equator. Thelargest negative anomalies of geopotential height overequatorial region are observed in layer from the earthsurface to 850 hPa while the largest positive anomaliesat about 200 hPa.

In the mid and high latitudes of the NH,it is seenfrom Figs. 5b–g that the disturbances of the geopotential height anomaly in troposphere are the equivalentbarotropic although the axis of maximum anomalies ofgeopotential height in the mid and lower tropospheretilts slightly poleward with height. In the SH,disturbances of anomalous geopotential heights largelydemonstrate an equivalent barotropic structure.

The ascending branch of the anomalous Walkercirculation(Fig. 5a)is located above the MC,while the two descending branches are found over thecentral-western Indian Ocean and the central-easternPacific to the east of the dateline,respectively. A couple of vertical circulations can also be found from the equatorial eastern Pacific to the equatorial Atlantic and the equatorial Africa. The vertical extension ofthese two circulations is lower than that of the Walkercirculation over Indo-Pacific sector. During positivephase of PTSM,these anomalous vertical circulationsare favorable for the formation and maintenance ofnegative SLPAr over the MC region, and possibly resulting in negative SLPAr in eastern Pacific due toeastward propagation of Kelvin waves in equatorialregion.

It is noticed that the geopotential height anomalies in the NH and SH distribute in a way of roughlysymmetric about the equator(Figs. 5b–g). However,the anomalies in the NH are distinctly larger thanthose in the SH. Moreover,the abnormal meridionalcirculations look,to a certain extent,different in different meridional planes.

In general,the PTSM-related circulation changeshows a seesaw feature between the polar region and the tropics. Its spatial structure is roughly symmetricabout the equator. The circulation anomaly shown inthe PTSM mode is stronger and with more distinctzonal disturbances in the NH than in the SH. The related disturbances of anomalous geopotential heightsintensify with height with equivalent barotropic structure in mid and high latitudes in both hemispheres.

5. Relationship between PTSM and the globalclimate anomaly

The correlations of PTSM with global precipitation and temperature anomalies are calculated byusing time series of coefficients of SVD1 to investigate the linkage between the PTSM mode and climateanomalies at various regions of the world. It is foundthat anomalous global precipitation and temperaturechanges are closely related to the PTSM. The precipitation and temperature anomalies during borealwinter are highly correlated with PTSM not only inmid and high latitudes but also in tropical regions inboth hemispheres,suggesting that the PTSM has possible impacts on surface climate in wide areas of theglobe(Fig. 6). These impacts on climate conditionsin two hemispheres are contemporaneously. Note thatcorrelations in the tropics are also significant thoughrelatively weaker than those in the extra-tropics.

Fig. 6 Correlations of time series of coefficients of SVD1 with precipitation anomalies (shaded contours) along with superimposed anomalous water vapor fluxes (vectors;kg m−1s−1) as obtained by regressing the anomalous vapor fluxes integrated vertically from the earth surface up to 300 hPa onto the time series of coefficients of SVD1.(a) Bold arrows indicate the fluxes are significant at/above the 95% confidence level by using an F-test.(b) Correlations of temperature anomalies with time series of coefficients of SVD1.The critical value at the 95% confidence level is found to be 0.32 by using a t-test.
5.1 Precipitation anomalies related to PTSM

In the mid and high latitudes of the NH,whenthe PTSM is in its positive phase,precipitation is lessthan normal from Baffin Isl and in North America tothe eastern Siberia; these dry climate conditions arespecifically observed in the area from North Atlanticto central-northern Europe and Siberia,in region westof the Alaska of US, and in southwestern part of China.However,several regions in the extra-tropics receivemore than normal precipitation. These regions arethe northeastern part of North America,Azores incentral Atlantic,southern Europe,the Mediterranean and its surrounding areas,Mongolia,the northern partof China,easternmost of Russia,the northern part ofJapan, and part of Northeast Pacific.

In the tropical regions,significant positive correlations are sparsely found in the Bay of Bengal,theMC region,the eastern equatorial Pacific,the Amazon region in South America, and eastern equatorialAfrica along with the western equatorial Atlantic,indicating that more precipitation during boreal winteris received in these regions when PTSM is in its positive phase. Nevertheless,significant correlations areobserved in the equatorial Indian Ocean,implying therainfall is less than normal there.

In the extra-tropics of the SH,high positive correlations are found in the northern Australia,the westcoast of Australia, and the west coasts of both SouthAmerica and South Africa in midlatitudes. Strongpositive correlations are also found in both South Pacific and South Atlantic south of 45°S,indicating thatthere will be more than normal rainfall in these regions when PSTM is in its positive phase(Fig. 6a).Simultaneously,negative correlations are found in theAntarctica and its vicinity,the scattered regions insubtropical part of the southern Pacific,the southernAtlantic, and the southern Indian Ocean,demonstrating that less rainfall will possibly occur in these regions. The situations will be opposite when the PTSMis in its negative phase.These precipitation anomalies that possibly occurin global domain are intrinsically related to anomalouscirculation patterns(Fig. 4) and atmospheric mois ture transport(Fig. 6a). It is found from Fig. 6athat where there significantly northward(southward)transports of water vapor in the NH(SH),or the water vapor transported from oceanic region into the l and area,there will possibly occur more than normal rainfall. On the other h and ,less than normal rainfall tendsto occur in regions where the water vapor fluxes pointsouthward(northward)in the NH(SH), and in regionswhere the vapor fluxes explicitly diverge.

5.2 Temperature anomalies related to PTSM

The surface air temperatures in boreal winter arepossibly influenced by the PTSM. In the mid and highlatitudes of the NH,significantly positive correlationsare found over regions around Greenl and and Aleutianisl and s,indicating that in these regions the surface airtemperature is higher when the PTSM is in positivephase. However,it is seen in Fig. 6b that significantly negative correlations are found in a geographical b and from the southeastern part of the United States northeastward to Eurasian continents and then to Japanisl and s,suggesting that the climate conditions are significantly colder than normal.

In the tropical and subtropical regions,significant positive correlations of time series of coefficientsof SVD1 with temperature anomalies are observedover tropical Atlantic, and regions around the Saharadesert and Saudi Arabia Peninsula,Maldives Isl and ,the southwestern part of China,the northern part ofIndian subcontinent,the MC,the western part of Australia, and the tropical western Pacific. This indicatesthat the higher than normal surface air temperaturespossibly occur in these tropical and subtropical regions. The warmer surface conditions can in returndrive the atmosphere to ascend anomalously in tropical and subtropical regions.

In the mid and high latitudes of the SH,the largepositive correlations exist in Antarctic whereas significant negative correlations are found over oceans southof South America and south of Australia(Fig. 6b).This means that when the PTSM in positive phase,thewarmer than normal condition will occur over Antarctic and colder than normal temperatures will possiblyappear over Southern Ocean.

6. Conclusions and discussion

By performing the SVD analysis on SLPA,wehave investigated the leading SVD mode(SVD1) and its related circulation variations as well as the climateanomalies in both the NH and SH in boreal winter.Major conclusions are as follows.

There indeed exists the polar-tropical seesawmode(PTSM)that describes the contemporaneouscirculation variations in the NH and SH. This modecan be identified by using the SVD analysis on SLPAfrom which the ENSO signal is regressed out. ThisPTSM can explain 47.74% of the total SLPA covariance of circulation anomalies in the two hemispheres.It is found that the PTSM in the NH is in overallstronger than that in the SH. This mode varies mainlyon interannual timescale with periodicities of 2–3 and 4–6 yr, and has a long-term trend with decreases inSLP in the tropics and increases in SLP in the polarregion in the recent 30 years.

The PTSM looks roughly symmetric about theequator, and has a horizontal structure with lowerSLPA in tropical region while higher SLPA in mid and high latitudes in both hemispheres when PTSMis in its positive phase. The PTSM-related anomalous circulations over Eurasia,Pacific, and Atlantic inthe NH are associated with atmospheric teleconnections including EU and PNA patterns(Wallace and Gutzler, 1981). In the SH,a wave train similar to thePSA teleconnection pattern is found over the southernPacific and Atlantic. Moreover,in the tropical region,disturbances of geopotential height in the lower troposphere are in opposite phase to those in the uppertroposphere,showing that the anomalous circulationsare baroclinic correspondingly. In the mid and highlatitudes,the anomalous circulations are equivalentbarotropic.

Significant wintertime temperature and precipitation anomalies correlated with the PTSM can be foundglobally. In the positive phase of PTSM,positivetemperature and precipitation anomalies occur in theAzores Isl and s in the central Atlantic,southern Europe, and the Mediterranean and nearby areas whilenegative precipitation and temperature anomalies occur in North Atlantic,central and northern Europe, and Siberian region. Precipitation is higher than normal but temperature is lower than normal in Northeast Asia. In western China,precipitation is lowerthan normal but temperature is higher than normal.Over the northeastern part of North America,bothprecipitation and temperature are higher than normal.Negative temperature anomaly is found in southeastAmerica. Meanwhile,precipitation is higher than normal in southwest Africa and northern Australia whilepositive temperature anomaly can be found in centraleastern South America,North and Southeast Africa, and southern Australia.

It is worth noting that AO is correlated with AAOwith a value of 0.33(P < 0.05),indicating that circulation changes in the mid and high latitudes of thetwo hemispheres are not highly correlated. Both AO and AAO show an anti-phase SLPA oscillation between the mid–high latitudes and the polar region, and the corresponding SLPA changes are basicallysymmetric in zonal(Gong and Wang, 1999; Thompson and Wallace, 2000). The correlation of the timecoefficient of SVD1 with AO is found to be –0.86 and that with AAO is –0.42(P < 0.05)(Table 4),but the spatial structure of PTSM is different fromthose of both AO and AAO. This difference can befound at least in two aspects:(1)the zonal mean ofSVD1 mode(Fig. 1d)shows larger anomalies in boththe tropics and the polar region with opposite signswhereas both the AO and AAO show their stronganomalies in midlatitudes and polar regions, and (2)the latitudes where signs of anomalies of SLPA reversefrom positive to negative are also different from thoseof both AO and AAO. These suggest that the tropicalvariability in PTSM may be more important than thatin AO/AAO,being possibly a result of response of theSLPA in mid and high latitudes to SLPA changesin the tropics. Certainly,a reversed situation mightexist if SLPA change in low latitudes as a responseto SLPA changes in mid and high latitudes. In thissense,despite the high correlation between the AO and PTSM,AO actually represents the SLP oscillation between the polar region and the midlatitudeswhile the PTSM depicts a seesaw like variation between the polar region and the tropics.

In addition,the NH annular mode is physicallyconsistent with AO,which to a certain degree is aresult of diabatic surface heating in polar or midlatitudes. The internal dynamic processes in the atmosphere may also play an important role in formingthis annular mode. However,it is well known thatthe tropical region works as a source region for atmospheric energy supply with a very strong forcing ofdiabatic heating. The PTSM is most likely a result ofatmospheric response to anomalous tropical thermalforcing over both oceans and continents(Figs. 5 and 6b). Therefore,despite the high correlation betweenAO and PTSM,they are different due to possible different causes. Anyway,in-depth study is necessary tofurther explore the mechanism for the PTSM formation and the dynamic and thermo-dynamic processesinvolved in the interaction between mid–high latitudes and low latitudes, and hence to clarify whether thePTSM is or not AO.

Moreover,note that there occurred a severe seaice change event in region to the west of 144.6°E,66.56°–67.6°S in late December 2013,making two polar vessels,Akademik Shokalskiy and Xuelong trappedin the Antarctic coastal(Wang et al., 2014). It wasfound that a cyclone that occurred in edge area of Antactic induced the rapid creation of extreme Antarcticsea ice conditions there,causing this vessel trap event(Wang et al., 2014). As seen in Fig. 1,in 2013,thePTSM was in its positive phase. It is speculated thathigher than normal temperature anomalies were possibly observed in the Antarctic region(Fig. 6b)in2013. Whether there are some relationships of thePTSM with this kind of extreme sea ice conditions ornot deserves more investigations in the future.

Acknowledgments. The NCEP/NCAR reanalysis data used here are obtained from theNOAA-CIRES Climate Diagnosis Center accessible athttp://www.esrl.noaa.gov. The CMAP rainfall data,oscillation index, and the Nino indices are downloadedfrom http://www.cpc.ncep.noaa.gov. All graphs inthis paper are plotted by using GrADS.

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