Download Interdecadal Linkages Between the Pacific Decadal Oscillation and

CHINESE JOURNAL OF GEOPHYSICS Vol.56, No.2, 2013, pp: 117–128
INTERDECADAL LINKAGES BETWEEN THE PACIFIC DECADAL
OSCILLATION AND INTERHEMISPHERIC AIR MASS OSCILLATION
AND THEIR POSSIBLE CONNECTIONS WITH EAST ASIAN MONSOON
LU Chu-Han1 , GUAN Zhao-Yong1∗ , LI Yong-Hua2 , BAI Ying-Ying3
1 Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of
Information Science and Technology, Nanjing 210044, China
2 Chongqing Climate Center, Chongqing 401147, China
3 Chongqing Institute of Meteorological Sciences, Chongqing 401147, China
Abstract The Pacific decadal oscillation (PDO) recently emerged in the literature as a robust signal in the
Northern Hemisphere climate variability. Many studies reported that the relationships between PDO and East
Asian monsoon (EAM) and climate variability in China are significant. Their possible mechanisms are, however,
still unclear. Using the observational NCEP/NCAR reanalysis and Chinese station data during the period of
1969–2008, this study investigates the interdecadal relationship between Pacific decadal oscillation (PDO) and
interhemispheric air mass imbalance or oscillation (IHO) between the Northern and Southern Hemispheres. The
possible connection of PDO and IHO with both East Asian monsoon and climate variability in China are also
assessed. It is found that the interdecadal components (11∼38 years) of PDO, IHO, and EAM contribute large
variance to low frequency variations, and they are well-matched with each other on (inter) decadal timescale. In
particular, their negative phases mainly appeared in the 1970 s and late 1990 s, while positive phase in period
from 1980 s to mid 1990 s. Decadal change of global mean air columnar temperature may be the key factor for
the notable difference between PDO and IHO from mid 1970 s to mid 1990 s. The spatial distributions of PDO
and IHO associated surface air temperature and surface pressure anomalies exhibit high similarity and large scale
characteristics, indicative of their intimate linkage with air mass redistribution over the global domain especially
over 30◦ S–50◦ N. The PDO associated columnar integral of velocity potential anomalies that maintain the air
mass redistribution show a dipole pattern with air mass flux emanating mainly from the eastern hemisphere
to the Pacific regions in positive PDO phase. This contributes to hemispherical and land-sea mass exchange
and redistribution, and also leads to the decadal displacement of both upward and downward branch of Walker
circulation. In positive phase of PDO, an anomalous anticyclone is found in the Mongolian region in both
boreal summer and winter seasons, inducing significant anomalous northerlies in the eastern China, and hence
intensifying (weakening) the east Asian winter (summer) monsoon. Consequently, the interdecadal components
of temperature and precipitation at most stations in east China are simultaneously correlated with the Pacific
decadal oscillation index significantly.
Key words Pacific decadal oscillation (PDO), Interhemispheric oscillation (IHO), East Asian monsoon, Decadal
change, Climate variability in east China
1 INTRODUCTION
The Pacific decadal oscillation (PDO) is a long-lived ENSO-like pattern of climate variability proposed
first by Mantua et al.[1] , featured by a pronounced oscillation period on a decadal basis. Studies suggest that the
tropical Pacific decadal variability has a direct effect on the 20∼30 a oscillatory period of the PDO[2−3] . Hence,
the PDO is viewed as an important mode that influences climate variation over the North Hemisphere, especially
the Pacific and its thereabouts, expressed mainly by the innegligible modulation of the interannual variability
and in particular, ENSO-induced variation of climate. For example, the PDO mode exerts salient impacts on
the predictability of US winter climate[4−5] , and on China’s climate variability and PDO also modulates greatly
the different-phase ENSO effect on China’s winter/summer climate anomalies[6] .
E-mail: [email protected]
*Corresponding author: [email protected]
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Differing from ENSO-related interannual signals, the PDO has its preferred region at mid latitudes of
the North Pacific, and it is in intimate relation to the interdecadal variation of the PNA teleconnection[7] .
Consequently, the PDO-caused anomalies of weather and climate in North America downstream have attracted
widespread attention[8] . Besides, analysis of canonical correlation of atmospheric circulations at northern midlatitudes with Pacific SST arrives at a leading PDO-like mode, showing that PDO has its impacts on atmospheric
circulations at those latitudes[9] . Both on an interannual and a decadal basis the PDO change well corresponds
to the winter Aleutian low and Mongolian high evolving in a synchronous see-saw fashion[10−11] , thus leading to
related variation in zonal land-sea pressure difference so as to associate closely the PDO with the change in the
strength of upstream winter EAM[10−12] . In addition, the PDO can cause decadal variation in summer EAM
rainfall via lower-troposphere atmospheric forcing[13] . The stability of the correlativity between ENSO and the
summer EAM experiences similar interdecadal variation as well[14] .
In association with this, precipitation over China, especially the eastern part of North China and the JiangHuai valley as well as South China experience remarkable rainfall variability on a decadal basis[15] . North China,
for example, has undergone a transition from a wet to dry phase in the recent 50 years, in good agreement with
the transition of PDO from a cold to a warm phase in the mid-late 1970 s[16] . To the contrary, Zhou[17] noted
that South China winter precipitation was considerably higher in 1978–2002 compared to 1960–1977, in relation
to interdecadal variation of Pacific SST. Also, Li et al.[18] present their study of the rainfall characteristics in
March 1951–2005 over southern China, revealing a pronounced correlation between rainfall and PDO. Using
reconstructed data of eastern summer rainfall (dryness indices), Shen et al.[19] derived successfully a temporal
sequence of PDO indices since 1470 AD.
The studies aforementioned show that PDO has strong effects on winter/summer EAM and eastern-China
precipitation on an interdecadal scale. However, few researches are conducted of specific physical processes
and influencing way of upstream effects of PDO, especially on EAM and rainfall. Recent researches indicate
that there is an interdecadal passage (IP) for the Pacific SST change, which is associated so closely with
PDO and winter EAM as to be regarded as a tie for air-sea interaction between the Pacific ocean at mid and
tropical latitudes[12,20] . In essence, monsoon is one of the features for inter-hemispheric interactions, driven by
planetary-scale thermal convection and air-sea thermal contrast[21] while the active strength of low-frequency
oscillations in inter-hemispheric interplay is determined by the difference in air mass between the southern and
northern hemispheres. A study is made accordingly by Guan and Yamagata[22] , noting that air mass oscillations
occur between both the hemispheres, denoted as Interhemispheric Oscillation (IHO). Lu et al.[23] discovered
that IHO makes higher variance contribution to the interannual anomalies of atmospheric mass in the eastern
hemispheric active monsoon region as robust signals related closely to Asian monsoon. It can be inferred
accordingly that PDO has its effect on Asian monsoon possibly unlimited to the teleconnection forcing between
Asia and the Pacific but likely associated with bihemispheric atmospheric circulation anomalies, particularly
inter-hemispheric air mass exchange and its redistribution, thereby exerting impacts on the anomalies of Asian
monsoon. In consequence, this work attempts to explore the PDO-IHO interrelation at a decadal scale, their
impacts on Asian monsoon activities and, further, on China’s climate anomaly. Evidently, the results to be
obtained will shed light on understanding anomalies of the EAM climate.
2 DATA AND METHODOLOGY
This study is based on the NCEP/NCAR reanalysis datasets, consisting mainly of surface pressures,
surface temperatures and high-level winds with resolution of 2.5◦ × 2.5◦ long./lat., seasonally averaging the
data of December, January and February (winter), March, April and May (spring), June, July and August
(summer), and September, October and November (autumn). The PDO index was taken from the website
http://jisao.Washington.edu/pdo/PDO.latest. This index comes from the standardized EOF1 time coefficients
for Pacific SSTA north of 20◦ N. Because of the possible coding errors in NCEP surface and sea-level pressures
prior to 1968, this work used the data only from 1969 to 2008, together with 1969–2008 monthly mean station
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temperatures and rainfall from 160 stations of China provided by National Climate Center, China Meteorological
Administration (CMA). Referring to Guan and Yamagata[22] , we constructed IHO index by means of the
differences between the northern areally weighted mean surface pressures, (from which the seasonality has been
deducted), and the related southern equivalents, and this resulting index represents the active strength of interhemispheric interactions caused by imbalance in air mass between the two hemispheres. Again, referring to
Shi and Yang[24] , the differences in sea-level pressures between 110◦ E and 160◦ E were used to construct EAM
index. To investigate the decadal variability, a wavelet analysis was used to extract the 11∼38 a components
related to PDO, IHO and EAM indices, denoted, respectively, by Pdec , Idec and Edec .
For examining relations of these decadal indices with circulations and precipitation, both the linear regression and linear correlation techniques were applied, and in view of lowered freedom of the decadal component
indices (Pdec , Idec ), the Monte Carlo simulation method was applied for the significance test to verify the regressions and correlations. Specifically, the orders in Pdec and Idec time series were disrupted 1000 times, followed by
regression and correlation analysis of the element fields, leading to F statistics of the sequences and the statistics
of absolute values of correlation coefficients (each totaling 1000 points) for a related station/grid. Then, these
two kinds of 1000 values were put into series in decreasing order, from which the 51st (11th ) statistic was taken
as its critical value at significance of 0.05 (0.01) on a statistical basis. When the regressed F statistics and the
absolute values of correlation coefficients exceed the critical values, the F statistics and correlation coefficients
at the point can be judged to pass significance tests at 0.05 (0.01) level.
3 RESULTS
3.1 Relationship Between PDO and IHO
To investigate PDO and IHO variations on an interdecadal scale, Fig. 1 presents the separate time series
of normalized season-to-season PDO and IHO indices, the former experiencing remarkable decadal variation in
the period, with prominent abrupt shift in the 1920 s, 1940 s and 1970 s, in good correspondence to the sudden
change in the northern winter atmospheric center of action occurring three times for the recent 100 years[25] .
This illustrates that there is a possibility of PDO in relation to the abnormal distribution of inter-hemispheric
Fig. 1 Standardized seasonal PDO index (upper panel) and IHO index (bottom panel). Thick lines denote
5-year running means. The simultaneous correlation coefficient between two indices is 0.364 (above
significance level of 0.01 by 1000 Monte Carlo simulations)
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air mass on a decadal scale. It is worth noting that a
shift of PDO phase took place once more after 2007,
ushering probably in its another shift. On the other
hand, the temporal evolution of IHO index is almost
similar to that of PDO equivalents, both in high positive correlation, arriving at the simultaneous correlation coefficient of 0.364 that passed the stochastic test
of 1000 Monte Carlo simulations at 0.01 significance.
Further analysis shows the decadal oscillation of IHO
to be in general agreement with that of PDO, which
implies the increase in the imbalance of air mass between the hemispheres. The imbalance is produced
dominantly by their difference in dry air mass despite
of the small contribution of interannual variability of
hemispherically averaged vapor mass[26] .
The two types of oscillation indices indicate the
consistent decadal oscillations at periods >10 years
Fig. 2 Time series of interdecadal component of PDO, (Fig. 1), the PDO amplitude being particularly noticeIHO, and EAM index from 1969 to 2008. The solid, able. A wavelet analysis was adopted accordingly to exdashed, and dash-dotted lines in the upper panel denote tract 11∼38 a components of PDO and IHO, as shown
Pdec , Idec , and Edec respectively. The variance contributions in Fig. 2 by Pdec and Idec . The two interdecadal comof low frequency components in Pdec , Idec , and Edec to ponents explain 32.9% and 20.1%, respectively, of total
total variance of the three indices are 32.9%, 20.1%, variance of oscillations above 1 year periods, indicat38.4%. The solid, dashed, and dash-dotted lines in the ing that decadal oscillations are really the innegligible
bottom panel denote Pdec , global mean columnar air components of PDO and IHO variation. Fig. 2 depicts
temperature integration (T), and T-removed Idec (IHO-T), that Pdec are in a negative (positive) stage, viz., cold
respectively
(warm) phase in the 1970 s and late 1990 s (during the
1980 s to mid-late 1990 s), wherein occurred a remarkable decadal oscillation in 1985–1997. The evolution
of Idec component is in consistent with that of Pdec equivalent except for some lag in the former peak-value
phase in the early 1980 s, and somewhat weaker decadal oscillation in 1985–1997. The consistency of Idec with
Pdec in pattern suggests that on a decadal scale, as PDO exhibits striking oscillations, not only are Pacific airsea interactions regulated to great degree but also the inter-hemispheric mass imbalance increases accordingly,
thereby leading to the fact that inter-hemispheric atmospheric interplay is intensified. In particular, when PDO
index is in a positive (warm) phase the SST decadal anomaly increases over the tropical mid-eastern Pacific,
Kuroshio current with its passage areas and the central part of the boreal Pacific suffer cool anomalies, with air
mass anomalies piling up over the North Hemisphere and v.v.
To investigate the effect of atmospheric mass imbalance upon EAM, Fig. 2 also presents the related EAM
decadal component (Edec ), which accounts for as much as 38.4% of the total oscillation variance. The timedependent Edec shows its interdecadal oscillation to be in good agreement with those of Idec and especially Pdec ,
indicating that there is likely intimate association of PDO and IHO with winter/summer EAM anomalies, and
the linkage will be examined in the following.
It deserves attention that Idec varied in good harmony with Pdec prior to 1977 and subsequent to 1995,
with great difference in between, possibly in relation to the effect of global temperature change. It is noted that
the global mean SST has been deducted in constructing Pdec index in order to eliminate the impacts of global
warming (see http://jisao.Washington.edu/pdo/PDO.latest). Change of atmospheric mass in an air column
follows the impacts of temperature inside. To remove the effects of changed global temperature on IHO, the
decadal component of globally-averaged total columnar temperature (T) is found, followed by subtracting from
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Idec the regression series of the original Idec on T (IHO-T), as shown in the lower part of Fig. 2. The agreement of
IHO decadal component after removing T signal with PDO equivalent (Pdec ) in phase is significantly improved,
and we can infer accordingly that global mean columnar temperature exhibited a bimodal structure from the
mid 1970 s to mid 1990 s, which may act as the dominant factor of the discrepancy between Idec and Pdec
components.
To further examine the circulation mode associated with PDO and IHO decadal variability, regression is
made of surface temperature and pressure anomaly upon Idec and Pdec , respectively. It is obvious that the
spatial patterns resulting from the two above index regressions show higher similarity, as shown in Fig. 3, where
one can see that the surface temperature regression coefficients reveal prominent PDO/IHO impacts on the
Pacific SST decadal variation, showing positive air temperature anomalies over the tropical mid-eastern Pacific
in contrast to considerably negative anomalies of air temperature at midlatitudes of both hemispheres, which is
in agreement with the PDO-driven SSTA classical space pattern[1] . The significant air temperature anomalies
consist of the Indian Ocean, Mediterranean Sea, mid East, central Siberia, Aleutian Islands and the Iceland
region (Figs. 3(a,b)), indicating that the decadal anomalies of PDO/IHO induced surface air temperature are
characteristic of the globe.
Fig. 3 The regression of (a, b) surface air temperature anomalies and (c, d) surface pressure anomalies onto Pdec and
Idec . Left panels are for Pdec , and the right ones for Idec . The units are in ◦ C in the upper panel and in hPa in the
bottom panel. Shaded denote values exceeding the 0.05 significance level in 1000 times of Monte Carlo simulations
Despite the fact that PDO-/IHO-related surface pressure anomalies show that both south and north Pacific
midlatitudes are marked by a pair of remarkable abnormal low-value centers, it deserves attention that the
positive-anomaly expanses of the eastern hemisphere in 30◦ S–50◦ N in contrast to the negative-anomaly region
in the austral polar region (Figs. 3(c,d)). The two large opposite-sign anomalous regions denote mass oscillatory
relation inter-hemispherically, further showing a close association of PDO with IHO on a decadal basis and
the resulting inter-hemispheric pressure differences especially in their eastern parts will strengthen interaction
between the hemispheres, producing impact on monsoons’ activity anomalies. Meanwhile, the Eurasian positivevalue pressure center is situated the place where the winter Mongolian high resides, and each of the two striking
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high-pressure centers is in southern and northern America, thus influencing markedly pressure gradients from
the Pacific to coasts, which also pose impacts on EAM activities. In addition, the redistribution of air mass on
a decadal basis is likewise able to explain the causes of the linkage of the winter boreal atmospheric center of
action to PDO-related decadal variation[25] .
Because there exists high similarity of the Pdec -related space pattern to the Idec equivalent (Fig. 3), the
related analysis of decadal variability in the following is based on Pdec . As shown in the above analysis, PDO
shows intimate association with decadal IHO. How is the interrelation established? Change of atmospheric
mass depends on mass flux by wind transportation. Hence, the whole-extent integrated winds are used to
calculate Pdec -related anomalous velocity potential functions and regression coefficients of divergent winds on
Pdec (Fig. 4a). One can see that PDO-related whole-extent air mass transport follows chiefly a dipole pattern,
in association with manifest air mass convergence over the Pacific, an anomalous center being in the tropical
South Pacific at 8×108 kg·s−1 . To the contrary, anomalous air mass is transported from the eastern hemisphere,
with the divergent center around the Bay of Bengal at –14×108 kg·s−1 . Since the eastern hemisphere is covered
mainly by land surface, the anomalous air mass distribution shows dominantly the air-mass redistribution
between land and sea. With PDO in a positive phase the atmosphere takes the tropical Indian Ocean as its
center and, air mass is generally transferred from Europe, Asia and Africa into the Pacific and Atlantic Ocean,
particularly the zonally movement eastward from the maritime continent and western North Pacific “warm
pool”, and meridionally, mass flows are prevalent from tropics into high latitudes in the eastern hemisphere and
v.v. for the western hemisphere. In association with this, there occurs salient air-mass exchange between tropics
and extratropics, leading to increasing difference in mass between the northern and southern hemispheres, and
thus to the establishment of the linkage of PDO to the IHO equivalent.
Fig. 4 Regressions of (a) velocity potential anomalies (vertically integrated) and (b) those at 200 hPa onto Pdec .
Superimposed vectors are for their correspondingly regressed winds. units are in 108 kg·s−1 (contour) and in
102 kg·m−1 ·s−1 (vector) in (a), and in 106 m2 ·s−1 (contour) and in m·s−1 (vector) in (b)
The association of PDO with air mass movement on a decadal basis also shows its effect on Walker cell
anomalies. Fig. 4b depicts that decadal anomalies of PDO-related 200-hPa velocity potential produce a largevalued divergent region over the tropical eastern Pacific, with a convergent core residing in the western North
Pacific to the east of Australia, thereby resulting in an opposite Walker circulation anomaly to the Climatological
mode. And the anomaly, to the contrary, strengthens the Walker circulation over the Indian Ocean and Africa.
This situation together with positive anomaly temperature at lower levels above the mid-eastern tropical Pacific
and tropical Indian Ocean of Figs. 3(a,b), we see that the temperature increasing at low levels corresponds to
a divergent center in the higher troposphere, and the resulting anomalous vertical motion may probably exert
effects on monsoon rainfall anomalies. Kumar et al.[27] noted likewise that the relationship between Indian
monsoon rainfall and ENSO has been weakened greatly since the late 1970 s, in association with the decadal
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shift in position of the rising and sinking branches of the Walker cell. Because the cell is of symmetrical structure
featured by overturning, its role in redistributing air mass needs further examination.
The interhemispheric linkage of both PDO and IHO can be inferred from air mass transport through the
equatorial vertical cross section to IHO in particular and PDO. We calculate the seasonal cross-equatorial mass
flow index (VQ) by vertically integrated air mass of meridional winds averaged across 5◦ S–5◦ N (Fig. 5). The
figure shows VQs exhibit their significant linear growth with IHO and PDO, with the correlation coefficients
0.427 and 0.374, respectively, at 0.01 significance for 1000 times of Monte Carlo simulations. This shows that
when the convoyed air mass increases northward through the equatorial vertical section, so does the VQ index,
i.e. strengthened mass shifting into the northern hemisphere, thus creating a pattern of “more mass in the
boreal than in the austral hemisphere,” accompanied by corresponding increase in PDO and IHO indices. Since
IHO index is expressed by the boreal pressure anomaly minus the austral equivalent[22] , the linear growth of
VQ index in good agreement with that of IHO demonstrates physically the relation of inter-hemispheric mass
shift and PDO index to winds.
Fig. 5 Scatter plot of (a) standardized IHO with standardized cross-equator air mass flux (VQ), and (b)
standardized PDO with VQ. Solid line represents the linear-fitting of IHO with VQ in (a), and PDO with
VQ in (b). The correlation coefficients of IHO and PDO respectively with VQ are 0.427 and 0.374, which
are significant above 0.01 level of significance in 1000 times of Monte Carlo simulations
3.2 PDO - EAM Linkage
Foregoing analysis shows that PDO is in close relation to the redistribution of inter-hemispheric air mass,
with PDO giving rise to the changes in pressure gradients therein, intensifying the inter-hemispheric interactions,
and causing the alteration of pressure anomalies of the surface atmospheric center of action and changes in
land-sea pressure gradients, thereby leading to good agreement of EAM decadal oscillation in strength with
Pdec and Idec , as delineated in Fig. 2. For further examining PDO impacts on winter and summer EAM, the
related seasonal components of Pdec are employed, against which regression is made of surface pressures and
850-hPa winds in the same period, separately (Figs. 6(a,d) in order). In summer, surface pressure positiveanomaly high-valued belts concentrate in the eastern active monsoon areas in 30◦ S–50◦ N and southern South
America, with negative-anomaly large-value bands in the Antarctic polar region, indicating the bihemispheric
difference in air mass distribution (northern high mass and southern lower mass, Fig. 6a). The 850-hPa wind
pattern shows greatly anomalous anticyclones to be over Mongolia, western Asia and eastern Indian Ocean,
with the anomalous northerlies on the southeastern side of the Mongolian anomalous high stretching southward
deeply into southeastern Asia and northern India (Fig. 6c). This anomalous pattern deters summer EAM
from northward advance carrying rich vapor. Lu et al.[23] made research into the linkage of summer IHO
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to interannual variability of the eastern monsoon activity arriving at a similar pattern of wind and pressure
anomalies. Therefore, corresponding to the decadal oscillation in PDO- and IHO-driven atmospheric mass interhemispherically, the summer EAM northern boundary and north- and southward movement exhibit matching
decadal oscillation[28−29] .
Fig. 6 Regression of anomalies of surface pressure (a, b) and 850hPa horizontal winds (c, d) in boreal summer
and winter onto simultaneous Pdec . Left panels are for summer, and right panels for winter. The units are in
hPa in upper panels and in m·s−1 in bottom panels. Shaded in upper panels and bold vectors in bottom panels
denote values exceeding the 0.05 significance level in 1000 Monte Carlo simulations
The winter pattern of surface pressure anomalies has some changes, i.e., in the northern hemisphere the
principal features are anomalous Mongolian high, Aleutian low and western North-American high in contrast to
the dominance of negative anomalies at extratropics of the southern hemisphere (Fig. 6b). And in the associated
850-hPa abnormal winds, an anomalous anticyclone appears that is centered in Mongolia and more pronounced
in intensity compared to that in summer, accompanied by an abnormally increased Aleutian low, an anomalous
cyclone around the Caspian Sea, and the continued persistence of strong northerly anomalies in eastern China
leading to intensified winter EAM, which is in harmony with the variations in winter EAM and ENSO on a
decadal basis[12] .
On the basis of a close relation between PDO and winter/summer EAM, we constructed a plot of simultaneous correlations of Pdec index with station temperatures and rainfall over eastern China (Fig. 7). In view
of possible discrepancy in the impacts of interannual and decadal circulation background upon eastern-China
climate[30] , a 5-year moving averaging scheme was employed to remove the interannual components considerably from the elements of related stations. Fig. 7a shows a pattern of salient “+ – +”for Pdec index related to
eastern-China summer temperatures on a decadal scale from south to north, viz., intensified Pdec correspond to
higher summer temperature in North and South China, with lower temperature over the Yangtze basin while
the pattern of Pdec -rainfall correlations is basically opposite to that of Pdec -temperature correlativity, indicative
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of summer eastern climate featured by “dry/hot-wet/cool”[31] . We also notice that with PDO index in a positive phase the rainfall increases greatly over the Yangtze basin and particularly its mid-upper valley in sharp
contrast to northern China, accompanied by intensified precipitation in Northeast China, a trend in agreement
with the decadal rainfall over eastern China[13,16,30] .
In winter, Pdec are correlated, on the whole, with eastern temperatures in a pattern of negative in the
south and positive correlations in the north except relatively weaker correlativity over north China (Fig. 7b),
suggesting that PDO-induced winter EAM decadal fluctuation has weaker effects on the northern portion of
China, that is, a loose relation of intensified monsoon to temperature fall in that region. Winter Pdec are, in
sharp contrast, associated closely with eastern precipitation (Fig. 7d), with high negative correlation in northern
China and the Yangtze-Huaihe river valley, highlighting the prevention of enhanced winter monsoon from the
coming of water vapor as rain or snowflake into these regions. At the same time significant positive correlations
are present in southern China and Sichuan region, suggesting great impacts of reduced temperature on increase
of rainfall, with their decadal variations under the significant influence of PDO and IHO, corresponding to the
decadal variability of winter precipitation increased significantly from the late 1970 s over southern China[17] .
Fig. 7 Coeval correlations of station temperatute (a, b) and precipitation (c, d) in boreal summer and winter
with Pdec . Left panels are for summer, and right panels for winter. Circles denote values exceeding the 0.05
significance level in 1000 Monte Carlo simulations
4 CONCLUDING REMARKS
This work is devoted to analysis of the PDO and IHO with their EAM impacts on a decadal scale,
indicating that PDO/IHO are related to large-scale globally uniform temperature and pressure anomalies and by
changing columnar temperature and density so as to make the column elongated and air mass inside redistributed
there occur the convergence (divergence) of large-scale anomalous winds and related cross-equatorial air mass
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shift between two hemispheres, thereby establishing their interrelations. In addition, further discussion is
conducted of their possible effects upon the eastern China climate anomalies in winter and summer. The
primary conclusions are as follows:
(1) There are considerable decadal fluctuations of PDO, IHO and EAM strength that are well related with
each other, which were in a negative phase during 1970 s and late 1990 s and positive phase in the 1980 s to
mid-late 1990 s. The interdecadal variability of globally averaged columnar temperature is the main cause for
great decadal discrepancy in PDO and IHO from the mid 1970 s to mid 1990 s. PDO and IHO exert significant
and spatially consistent impacts in making large-scale lower-level temperature anomalies and air mass migration.
Especially as the Pdec and Idec are augmented, the decadal pressure rises uniformly over the active monsoon
region in 30◦ S–50◦ N in the eastern hemisphere, with the negative extratropical pressure anomalies in the austral
hemisphere.
(2) With PDO in a positive phase, there occurs a dipole pattern-like shift of whole-extent decadal air mass
anomaly from the eastern hemisphere into the western Pacific convergence zone, accompanied by northward
anomalous displacement of cross-equatorial mass flows, resulting in a pattern with positive (negative) mass
anomalies in the northern (southern) hemisphere and v.v. During this period there exist accordingly the
abnormal surface active centers of semi-permanent anticyclones and cyclones in the northern hemisphere and
change in pressure gradient between land and sea, particularly between Asia and Pacific, leading simultaneously
to decadal change of the position of the Walker circulation and related vertical motion. Thereby the interrelations
are established among PDO, IHO and EAM.
(3) PDO anomalous variation bears an intimate relationship with the change in the surface pressure over
the Mongolian region. As PDO index is enhanced, 850-hPa strong anticyclonic winds follow in winter and
summer, resulting in northerly anomalies over eastern China and thereby leading to pronounced impacts on
winter/summer EAM. In association with this, PDO has an innegligible effect on the decadal components of
station temperature and rainfall over much of eastern China. Specifically, the Pdec are in high positive correlation
as a “– + –” pattern to summer precipitation over southern China, the Yangzte-Huaihe River valley and North
China. In contrast, it bears significant correlation with summer temperature in a pattern almost opposite to that
of the rainfall. In winter, the rise of PDO index matches the decadal variability trend of prominent reduction of
rainfall over much of the northern region of East China, during which more precipitation happens in southern
China and Sichuan of southwest China. In addition, PDO index exhibits pronounced negative correlation to
winter temperature over southern China.
The findings presented here indicate that PDO and IHO are closely associated with the pressure fluctuation
over monsoon active regions in the eastern hemisphere, and, correspondingly, the inter-hemisphere difference
in air mass increases, thus intensifying inter-hemispheric interactions. That is shown by saliently enhanced
cross-equatorial flows over Somali and northern Australia (Fig. 6c). With no change in vapor amount, the local
change in surface pressure is directly proportional to the flux of atmospheric mass divergence and the resulting
divergent circulation is responsible for the variation in inter-regional monsoons cooperating with each other[32] .
For example, the summer EAM bears a connection to its North African summer monsoon on a decadal basis[33] .
It is possible that on a decadal basis, summer EAM are related to Indian summer monsoon via teleconnection
between high-level meridional winds over North Africa and Eurasia[34] . As a result, decadal PDO-associated
IHO is probably responsible for the linkage between anomalous monsoon activity of other regions. Therefore,
the interrelations among PDO, IHO and global monsoon anomalies need further studies.
ACKNOWLEDGMENTS
The authors are grateful to the two anonymous reviewers for their constructive suggestions. The NCEP/
NCAR data were taken from NEP/CIRES Climate Diagnostics Center, whose website is http://www.cde.noaa.
gov/cde/reanalysis/reanalysis.shtml, with graphs made by the software GrADS. This work was supported by
National Natural Science Foundation of China (41005046, 41175062, 40975058).
Lu C H et al.: Interdecadal Linkages Between the Pacific Decadal Oscillation and Interhemispheric Air · · ·
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