Feng XUEand Fangxing FANInternationalCenterforClimate and EnvironmentSciences,Institute ofAtmospheric Physics, Chinese Academy ofSciences,Beijing 100029

AnomalousWestern Pacif c SubtropicalHigh during Late Summer in Weak La Ni˜na Years:Contrastbetween 1981 and 2013

Feng XUE∗and Fangxing FAN
InternationalCenterforClimate and EnvironmentSciences,Institute ofAtmospheric Physics, Chinese Academy ofSciences,Beijing 100029

Both 1981 and 2013 were weak La Ni˜na yearsw ith a similar sea surface temperature(SST)anomaly in the tropical Pacific,yet thewestern Pacific subtropical high(WPSH)during Augustexhibited an opposite anomaly in the two years.A comparison indicates that,in the absence of a strong SST anomaly in the tropics,the cold advection from Eurasian high latitudesand the convection of thew estern Pacificw arm pool play im portant roles in influencing the strength and position of theWPSH in August.In August1981,the spatial pattern of 500 hPa geopotential heightwas characterized by ameridional circulation w ith a strong ridge in the Ural Mountains and a deep trough in Siberia,which provided favorable conditions for cold air invading into the lower latitudes.Accordingly,the geopotential height to the north of theWPSH was reduced by the cold advection anomaly from high latitudes,resulting in an eastward retreatof theWPSH.Moreover,an anomalous cyclonic circulation in the subtropicalwestern Pacific,excited by enhanced warm pool convection,also contributed to the eastw ard retreatof theWPSH.By contrast,the influence from high latitudeswas relativelyw eak in August2013 due to azonal circulation patternoverEurasia,and theanomalousanticyclonic circulation induced by suppressedwarm poolconvectionalso facilitated thewestward extensionof theWPSH.Therefore,the combined e ff ectsof thehigh latitudeand tropical circulations may contributea persistentanomaly of theWPSH in latesummer,despite the tropical SST anomaly beingweak.

w estern Pacific subtropicalhigh,latesummer,tropical circulation,high latitude circulation,w arm pool convection

1.Introduction

As a major circulation system of the East Asian summermonsoon(EASM),thewestern Pacific subtropical high (WPSH)exhibits distinct intraseasonal variation during boreal summer.Usually,itmigrates northward in a stepw ise manner,w ith two distinctnorthward jumps,occurring in the middle of June and late July,respectively.After the second jump,the EASM region enters into the late summer period,characterized by high temperature and high hum idity. Compared w ith the early summ er period,the WPSH weakens sharply.In themeantim e,itmoves to itsmost northern position and retreats eastward to the south of Japan.Consequently,the EASM circulation patternsexhibitcontrasting characteristicsbetween early and late summer(Su and Xue, 2010;Xueetal.,2015).If theWPSH extendswestward w ith strong intensity in late summer,persistenthigh temperatures and severedroughtoccur in southern China,as in the caseof August2013 examined in this study.

Besides intraseasonal variability,theWPSH also shows significant interannual variability.It is generally recognized that tropical circulation largely regulated by El Ni˜no and Southern Oscillation(ENSO)p lays an im portant role in the interannualvariability of theWPSH.In particular,theWPSH tends to extend southwestward w ith stronger intensity during ElNi˜no decayingsummers(Fu and Teng,1988).Nitta(1987) found that an anomalousWPSH ismore directly related to warm poolconvection in thewestern Pacific.A Rossbywave train isgeneratedbyanomalousconvectionand propagates to theextratropics,thus influencing thenorthwardmovementof theWPSH.In addition,Lu(2001a)indicated that the zonal displacement of the WPSH is also a ff ected by warm pool convection.A low-level cyclonic(anticyclonic)anomaly in the subtropical western Pacific induced by enhanced(suppressed)convection leads to eastward(westward)movement of theWPSH.

The high latitude circulation system over the Eurasian continentisconsideredasanother important factor forWPSH variation,especially in late summerwhen theWPSH arrives at itsmostnorthern position.Ithasbeen demonstrated that the high pressure ridgeof theWPSH in Augustnear Japan isgenerated by a stationary Rossby wave propagating to Japan, and its intensity is regulated by the Asian jet(Enomoto et al.,2003;Enomoto,2004).Tao and Wei(2006)found that the stationary Rossby wave in the Eurasian continentpropagates eastward along the subtropical jet and excites a longwave ridge along the eastern coastof China.The developmentof the longwave ridge results in thewestward extension of the WPSH.Even in strong La Ni˜na years,there can be a greatdi ff erence in theWPSH(e.g.,between 1989 and 1999), especially in August,due to the di ff erent circulation at high latitudes(Xue,2008).

It is worth noting that the intraseasonal variation—especially the di ff erence between early and late summer—can further a ff ect the interannual variation.Kawataniet al. (2008)comparedWPSH variation in June,July and August and found that the interannualand submonthly variability are smallest in June and largest in August.Xue and Liu(2008) also showed that the influence of El Ni˜no on the WPSH is w eakest in June and strongest in August,suggesting that the influence of El Ni˜no on theWPSH ism ore significant in late summer.

Fig.1.SST anomaly during summer(a)1981 and(b)2013(units:°C).

Despite theessential rolesofmany factors inWPSH variation,mostpreviousstudieshave focused on the influenceof one factor,such asElNi˜no,on theWPSH,w ith thecombined influences of all possible factors having rarely been examined.For the purpose of predicting the position and strength of theWPSH more precisely,all possible factors should be taken into consideration.Besides,the role of ENSO should not be overemphasized,although it is the strongest interannual signal.In fact,theWPSH may also exhibita persistent and significantanomaly,especially in August,under the condition of aweak tropical SST anomaly.Because it is di ffi cult to perform conventionalnumerical sim ulations using a general circulationmodelw ithoutstrong SST forcing,this issue concerning the WPSH anomaly associated w ith weak SST anomaliesisnotyetwellunderstood.

The objective of this study is to explore the primary factors responsible forapersistentWPSH anomaly underaweak tropicalSSTanomaly condition.Wepursue thisinvestigation by comparing the anomalousWPSH patterns in two particular years(1981 and 2013).Despite the fact that both 1981 and 2013 wereweak La Ni˜na years,and theWPSH in June and July was close to norm al,the WPSH anomalies in Augustw ereopposite,w ith aweakening in 1981 and a strengthening in 2013.During late summer 2013,the unusualwestward extension of theWPSH broughtaboutaprolongedheatwaveand severedroughtin southernChina(Sun,2014;Peng, 2014).By comparing theWPSH in these two years,we intend to reveal themajor impacting factorsand their combined e ff ects on the WPSH anomaly in late summerw ith a weak tropicalSST anomaly,and providenew clues forWPSH prediction.

2.Data description and WPSH in 1981 and 2013

The daily data ofw ind and geopotential heightwere obtained from NCEP–DOEReanalysis-2 on a 2.5°×2.5°grid from 1979 to 2013(Kanam itsu etal.,2002).Outgoing longwave radiation(OLR)w ith thesame resolution,used to represent the convection intensity in the tropics,wasderived from NOAA satellite observations(Liebmann and Smith,1996). Themonthlymean SST datawereobtained from NOAA on a 2°×2°grid(Sm ith etal.,2008).Thedaily datawere further processed into pentadmeans to facilitateanalysis.

TheWPSH is representedby the5880gpm contourat500 hPa over EastAsia and thewestern Pacific.Thewest point, which is defined as thewesternmost point of the 5880 gpm contour,isused to describe the zonalposition of theWPSH (Zhao,1999).For simplicity,wealso take Augustas the late summer period,which usually begins from late July(Su and Xue,2010).

Fig.2.The WPSH represented by the 5880 gpm contour in June,July and August(a–c)1981 and(b–f)2013.The climatologicalmean is represented by the dashed contour for comparison(units:gpm).

The WPSH in 1981 was selected for com parison w ith that in 2013 because both 1981 and 2013 were weak La Ni˜nayearsw ith asimilar tropicalSST anomaly.Asshown in Fig.1,the SST anomaly during summer(June–July–August mean)in the tropical Pacific had a typical La Ni˜na pattern, i.e.,negativeanomalies in the centraland eastern Pacific and positive anomalies in thewestern Pacific.The SST anomaly in 2013wasslightly stronger than that in 1981,w ith amaximum negativeanomalyof-1°Co ff thePeru coastand amax-imum positive anomaly of 0.5°C in thewarm pool region of thewestern Pacific.

Figure2 illustrates themonthlymeanWPSH represented by the 5880 gpm contour in June,July and August,together w ith the climatologicalmean for comparison.While the WPSH was close to the climatology in June and July,there was a significant discrepancy in August.In August 1981 (Fig.2c),theWPSH was locatedmore southeastward,w ith the west point at 155°E and the ridge line at about 17.5°N. M eanwhile,theWPSH becam e very weak.In sharp contrast, theWPSH in August2013 extendedwestward to 115°Ew ith much stronger intensity(Fig.2f).Di ff erent from the pattern in a typicalLaNi˜nayear,theanomalouspattern of theWPSH in 2013was sim ilar to that in astrong ElNi˜no year(Xue and Liu,2008).

3.The infuence of circulation in high-latitude Eurasia

Since the response of the WPSH to a La Ni˜na signal is generally weak and the intensity of La Ni˜na in the two yearswasalsoweak(Xue,2008),the largediscrepancyof the WPSH in Augustbetween the two yearscannotbeattributed to the tropical SST forcing.In addition,therewere strong SST anomalies in the North Pacific and North Atlantic in 2013(Fig.1b),but the anomaly in them idlatitude oceans is driven by theatmospheric circulation and the responseof the atmosphere to the SST anomaly is generally believed to be weak(Frankignoul,1985;Park and Schubert,1997).Therefore,the SST anom aly in them id latitude oceans isnotamajor factor.

In contrastto thesim ilarSSTanomaly in the tropics,there was a large discrepancy of 500 hPa geopotential height in high-latitude Eurasia during August in the two years(Fig. 3).In August1981,the circulation patternwascharacterized by two ridgesand two troughsover Eurasia.With a strong ridgein the UralMountainsandadeep trough in Siberia,this patternwas favorable for the southward invasion of cold air from high latitudes,thereby influencing theWPSH.By comparison,the circulation in August 2013 was a zonal pattern superimposed w ith small ridgesand troughs.

Fig.3.The500 hPageopotential height in August(a)1981 and(b)2013(units:gpm).

The anomalous geopotential height in Fig.4 is consistentw ith that in Fig.3,w ith a positive anomaly in the ridgeregion and a negative anom aly in the trough region.Over the Northeast Asian region,therewas a negative anom aly in both years,but the intensity in 2013wasonly half of that in 1981.In addition,the negative anomaly in 1981 extended southward into the deep tropics,indicating that the circulation athigh latitudes played an important role in weakening theWPSH in August1981.

Fig.4.The500 hPageopotentialheightanomaly in August(a)1981 and(b)2013(units:gpm).

To further reveal theinfluenceofhigh-latitudecirculation on theWPSH,Figs.5 and 6 show the longitude–pentad cross section of the850 hPameridionalw ind anomaly along 40°N (north of the WPSH)and the 500 hPa geopotential height anomaly along 30°N(the WPSH area).In 1981(Fig.5a), a northerly anomaly appeared to the northwestof theWPSH (near 120°E)from Julian pentad 42(hereafter P42;25–29 July)and reached itsmaximum during P44–P45(4–18 August).Corresponding to the northerly anomaly,there was a negative geopotential height anomaly east of 130°E,w ith amaximum over-40 gpm in late summer(Fig.5b).It is also noted that themaximum geopotential height anomaly waspreceded by themaximum northerly anomaly aboutone pentad,indicating thatthenortherlyanomaly athigh latitudes p laysa leading role in theanom alousWPSH.In contrastw ith 1981,a southerly anom aly in late summer 2013 was associated w ith a positive geopotential height anom aly(Figs.6a and b).Both themaximum southerly anomaly and geopotential height anomaly emerged simultaneously in P43(30 July to 3August).Therefore,thenortherly anomaly from the high latitudesplaysamoreactive role in reducing geopotential height,whereas the southerly anomaly helps sustain the positiveanomaly in theWPSH area to a certain degree.

As an example,the eastward retreat at the beginning of August1981 is used to explain how the circulation at high latitudesa ff ects theWPSH(Fig.7).In P43(30 July to 3 August),theWPSH extendedmorenorthwestwardw ith thewest point at 95°E.In P44(4–8 August),however,theWPSH retreated rapidly to the east of 140°E w ith sharply weakened intensity.

The eastward retreatbetween P43 and P44was related to the circulation athigh latitudes.Corresponding to the strong ridge in the UralMountains(Fig.3a),therewasan anomalous anticyclone in high-latitude Eurasia(Fig.8a).In East Siberia,therewasastrong northerlyanomaly justto thenorth of theWPSH.Due to the cold advection anomaly,thegeopotential height in Northeast Asia was largely reduced,w ith am axim um over 200 gpm near the Sea of Japan(Fig.8b).As a result,the WPSH split into two parts,and them ain body retreated rapidly to theeastof 140°E in P44(Fig.7b).

Fig.5.Longitude–pentad cross section of the(a)850 hPameridionalw ind anomaly along 40°N(units:m s-1)and (b)500 hPageopotentialheightanomaly along 30°N(units:gpm)during summer 1981.Thenumberson the ordinate represent the Julian pentad.

Fig.6.Asin Fig.5,except for2013.

Since theWPSH in August is located ata higher latitude, it iseasily influenced by the circulation athigh latitudes.The above example is notunique and can frequently be seen in other years,such as August1989(Xue,2008).By contrast, the zonal circulation pattern in August2013 could not result in asimilar retreatof theWPSH as in 1981(Fig.3b).Instead, theWPSH tended to be locatedm orewestward(Fig.2f).

Fig.7.TheWPSH in(a)P43(30 July to 3 August)and(b)P44 (4–8 August)1981.The climatologicalmean is represented by the solid and dashed contours,respectively(units:gpm).

Fig.8.The(a)850 hPa w ind anomaly in P43 in 1981(units: m s-1)and(b)the di ff erence in 500 hPa geopotential height between P43 and P44 in 1981(units:gpm).

4.The infuence of tropicalcirculation

Besides the circulation athigh latitudes,theWPSH isalso influenced by tropical circulation.In particular,enhanced (suppressed)warm pool convection can excitean anomalous cyclone(anticyclone)over thesubtropicalwestern Pacific in the lower troposphere,inducing eastward(westward)movementof theWPSH(Lu,2001a).Thewarm pool convection is related to the SST anomaly aswell asatmospheric perturbations,especially cross-equatorial flow(Lu,2001b;Su and Xue,2010).

Figure 9 show s the longitude–pentad cross section of the cross-equatorial flow anomaly and OLR anomaly at 15°N in 1981.Note that the latter isused asa surrogate for thewarm poolconvection anomaly,w ith anegative(positive)anomaly corresponding to enhanced(suppressed)convection.The cross-equatorialflow near145°Eduring July–Augustwasapparently intensified,and the anomaly exceeded 2m s-1from P40(15–19 July).Afterwards,thewarm poolconvection began to enhance gradually,w ith OLR anomalies lower than -40W m-2between P43 and P45(9–13August).Thenegative OLR anom aly lasted for onemonth.It is also noted that the cross-equatorial flow near 130°Ewas largely reduced,as indicated by a negative anomaly,and itwas less related w ith thewarm pool convection.

In contrast,the cross-equatorial flow in 2013wasgenerally weak(Fig.10a).After P39(10–14 July),itchanged to a negativeanomaly.Correspondingly,therewerepositiveOLR anomalies(suppressed convection)during late summer(Fig. 10b).The OLR anomaly reachedmaximum intensity in P42 (25–29 July)andweakened slightly in P44(4–8August),before beginning to intensify again up until the end of August. The positive OLR anomaly lasted as long as one and a half months.

Itisevident that thewarm poolconvection anom aly is related to the seasonalmarch of atmospheric circulation in the western Pacific during summer.Compared w ith early summer,the anomaly of warm pool convection in late summer ismuchmoresignificantand lasts for a relatively long time. As noted previously(Ueda et al.,1995;Xiang et al.,2013), warm pool convection tends to be enhanced during the seasonalmarch from July to August,corresponding to theonset of thewestern Pacific summermonsoon.Asa result,warm pool convection ism uch m ore sensitive to cross-equatorial flow.

Fig.9.Longitude–pentad cross section of the 850 hPa(a)cross-equatorial flow anomaly(units:m s-1)and(b)OLR anomaly along 15°N(units:W m-2)during summer 1981.Thenumberson theordinate represent the Julian pentad.

Fig.10.As in Fig.9,except for2013.

An anomalous cyclonic circulation in August 1981 appeared in the subtropical western Pacific due to enhancedwarm pool convection(Fig.11a),further inducing lower geopotential height and an eastward retreat of the WPSH (Fig.4a).By contrast,therewasan anomalousanticyclonic circulationdue tosuppressed convectionin August2013(Fig. 11b).With highergeopotentialheight in the subtropics(Fig. 4b),theWPSH tended to extend westward w ith stronger intensity(Fig.2f).Thisanomalouspatternpersisted throughout late summer due to the lack of the influenceof cold advection from high latitudesas in August1981.It is also important to note that the anomalous cyclonic circulation(Fig.11a)was locatedmorenortheastward than the anomalousanticyclonic circulation(Fig.11b),because the subtropical circulation in August1981wasalso a ff ected by the circulation athigh latitudes.

It should be emphasized that the warm pool SST was higher during summer in both years,and waseven higher in 2013 than in 1981(Fig.1).Hence,the contrast inwarm pool convection between the two yearsdid notmainly result from the SST di ff erence in the tropics.Instead,the cross-equatorial flow played amore im portant role in thewarm pool convection.

5.Summary and discussion

A lthough both 1981 and 2013wereweak La Ni˜na years w ith a typical La Ni˜na pattern(i.e.,negative SST anomalies in the central and eastern Pacific and positive SST anomalies in thewestern Pacific),theWPSH exhibited an opposite anomaly in August in the two years.While the WPSH in August 1981 retreated eastward w ith weak intensity,the WPSH in August 2013 extended westward w ith strong intensity,despite thenormalconditions in Juneand July.

Fig.11.The850 hPawind anomaly in August(a)1981 and(b) 2013(units:m s-1).

The contrast between the two years indicates that there wasa significantdiscrepancy in the high latitude circulation over the Eurasian continentduring August.In August1981, there was ameridional circulation pattern in Eurasia,w ith a strong ridge in the Ural Mountains and a deep trough in Siberia.In particular,the anomalous northerly ahead of the trough induced a lower geopotentialheightin NortheastAsia through a cold advection anomaly.As a result,theWPSH tended to retreat eastward.In August2013,however,there was a zonal circulation system in Eurasia,w ith weak ridges and shallow troughs.Accordingly,the WPSH was less affected by the circulation athigh latitudes.

Besides the circulation athigh latitudes,therewasa large di ff erence in tropical circulation between the two years.In late summer 1981,an anomalous cyclonic circulation appeared in the subtropicalwestern Pacific,excited by the enhanced warm poolconvection due to the perturbation from a strong cross-equatorial flow,thereby leading to the weakening of theWPSH.By contrast,therewasan anomalous anticyclonic circulation in late summer2013,corresponding to suppressed convection over thewarm poolassociated w ith a weak cross-equatorial flow,resulting in thewestward extension of theWPSH.In themeantime,theweak influence of cold advection from high latitudes helped sustain the strong intensity of theWPSH.

Evenw ith aweak tropical SST anomaly,theWPSHmay also exhibita persistentanomaly in late summer ow ing to the combined e ff ectsof the di ff erence in tropical circulation and the circulation athigh latitudes.W hen predicting theWPSH, wemust pay special attention to the in-phase condition of these two factors.In latesummer2013,for instance,both the zonal circulation athigh latitudesand suppressed warm pool convection contributed to the westward extension and prolongedmaintenance of theWPSH,resulting in a heatwave and droughtin southernChina.If therehadbeen ameridional circulation system athigh latitudes in August2013 as in August1981,theanomalousWPSH couldnothavepersisted for a long tim e.Instead,the WPSH would have retreated eastward due to the influence of the high-latitude circulation.

Our findings concerning the roles of tropical and highlatitude circulation in the developmentand persistence of an anomalousWPSH during late summer are sim ilar to those presented in some previous studies.Ogasawara and Kawamura(2007)found thatanomaloussummerweather in Japan is a ff ected by two teleconnection patterns:the West Asia–Japan and Pacific–Japan patterns.The combination of these two patterns is favorable for the establishment of a zonally elongated anticyclonic anomaly in Japan,resulting in hot weather there.In agreementw ith our results,they noted that the com binede ff ectof the two patternson anomalousweather in Japan ismuchmore significant than those of each single pattern.

The contrastbetween 1981 and 2013 also indicates that theWPSH in late summer isvery di ff erent from that in earlysummer.Even though the WPSH is close to the climatology in June and July,itmay exhibit a significant anomaly in Augustw ith the seasonalmarch of the circulation in East Asia and the western Pacific.In the seasonalmarch from July to August,enhanced warm pool convectionmakes the tropicalcirculationmoresensitive to perturbationslike crossequatorial flow,thereby influencing theWPSH anomaly.In addition,due to the fact that theWPSH is located ata higher latitude in late summer,it is easily a ff ected by the circulation athigh latitudes.Therefore,forWPSH prediction in late summer,a combination of these two factorsmust be considered comprehensively,especially when the tropical SST anomaly isweak.

It isalso important to note that theWPSH exhibitsa consistentwestward extension throughout thewhole summer in strong El Ni˜no years(Xue,2008).However,when the tropical SST anomaly isweak,as in 1981 and 2013,theWPSH anomaly cannotpersistthroughoutthewholesummer.In this case,the seasonal forecast is largely lim ited due to the lack of strong tropical forcing.Instead of relyingmerely on longrange forecasting,we should place emphasis on m onthly forecasting in order to further improve the forecastskill.

Acknow ledgements.The authors appreciated the comments and suggestions from the two anonymous reviewers.This studyw as supported by theNational Science Foundation of China(GrantNos. 41475052 and 41405058).

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(Received 29December2015;revised 10 June2016;accepted 22 June2016)

∗Corresponding author:Feng XUE

Email:fxue@lasg.iap.ac.cn

©Institute ofAtmospheric Physics/Chinese Academy of Sciences,and Science Press and Springer-Verlag Berlin Heidelberg 2016