Combined Impact of El Niño–Southern Oscillation and Pacific

atmosphere

Article Combined Impact of El Niño–Southern Oscillation and Paciﬁc Decadal Oscillation on the Northern Winter Stratosphere

Jian Rao 1,2,3,* , Rongcai Ren 1,2, Xin Xia 2, Chunhua Shi 1 and Dong Guo 1

1 Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environment Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science & Technology, Nanjing 210044, China; [email protected] (R.R.); [email protected] (C.S.); [email protected] (D.G.) 2 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; [email protected] 3 Fredy & Nadine Herrmann Institute of Earth Sciences, Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram Jerusalem 91904, Israel * Correspondence: [email protected] or [email protected]; Tel.: +86-025-58699805

Received: 8 March 2019; Accepted: 18 April 2019; Published: 22 April 2019

Abstract: Using reanalysis and the sea surface temperature (SST) analysis, the combined impact of El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on the northern winter stratosphere is investigated. The warm and weak stratospheric polar vortex response to El Niño simply appears during positive PDO, whereas the cold and strong stratospheric polar vortex response to La Niña is preferable during negative PDO in the reanalysis. Two mechanisms may account for the enhanced stratospheric response when ENSO and PDO are in phase. First, the asymmetries of the intensity and frequency between El Niño and La Niña can be identified for the two PDO phases. Second, the extratropical SST anomalies in the North Pacific may also play a role in the varying extratropical response to ENSO. The North Pacific SST anomalies related to PDO superimpose ENSO SST anomalies when they are in phase but undermine them when they are out of phase. The superimposed North Pacific SST anomalies help to increase SST meridional gradient anomalies between tropical and extratropics, as well as to lock the local height response to ENSO. Therefore, the passages for the upward propagation of waves from the troposphere is more unimpeded when positive PDO is configured with El Niño, and vice versa when negative PDO is configured with La Niña.

Keywords: northern winter stratosphere; El Niño-Southern Oscillation (ENSO); Paciﬁc Decadal Oscillation (PDO); North Paciﬁc

1. Introduction The significant response of the northern winter stratosphere to El Niño–Southern Oscillation (ENSO) has been widely reported in a large number of observational [1–5] and modeling [6–14] studies. However, recent studies showed that the enhanced planetary wave response to El Niño and the suppressed planetary wave response to La Niña is asymmetric [15–17]. Specifically, due to the nonlinear relationship between sea surface temperature (SST) and precipitation, the positive Pacific–North America (PNA)-like height pattern induced by El Niño is usually stronger than the negative PNA pattern induced by La Niña. The positive PNA teleconnection corresponds to a low anomaly center over the North Pacific region (i.e., the Aleutian low), which is in phase with the

Atmosphere 2019, 10, 211; doi:10.3390/atmos10040211 www.mdpi.com/journal/atmosphere Atmosphere 2019, 10, 211 2 of 16 climatological stationary trough and favors enhancement of the amplitude of climatological waves that can propagate upward into the stratosphere [2,3,18,19]. The intensity and spatial pattern of the PNA teleconnection have been reported to be modulated by the diabatic heating forcing over different tropical ocean basins [14,18,20,21]. The sign of the PNA response is sensitive to the zonal position of the diabatic heating profile. In fact, the precipitation response to tropical SST anomalies in the Pacific is not only determined by the SST anomaly intensity, but also depends on the background SST mean state [15–17]. For example, in warm ocean regions, a small SST anomaly can induce strong adiabatic heating, whereas in cold ocean regions, a large SST anomaly can only limitedly disturb the adiabatic heating. Evidence has indicated that the structure and intensity of the ENSO teleconnection also exhibit profound decadal variations. Since the late 1970s, the PNA teleconnection has shifted eastward, which is intimately associated with the decadal change of the convective center over the equatorial Pacific during El Niño winter [22,23]. The tropospheric ENSO teleconnection pattern shift may be related to the natural variabilities of ENSO [24,25] and the global warming [26,27]. As reported in Zhou et al. [23], different future scenarios in CMIP5 (Phase 5 of the Coupled Model Intercomparison Project [28]) models project an eastward shift of the positive rainfall anomaly center over the equatorial Pacific, which further induces a pattern shift for the forced PNA. It is also documented that with the ENSO intensity increasing, the height anomaly centers of the PNA moves eastward. The configuration between the climatological stationary waves and the projected wave anomalies changes as the PNA shifts [16,29]. In the Pacific sector, a decadal mode known as the Pacific Decadal Oscillation (PDO) can changes the SST mean state and even the ENSO regimes. It was also reported that the PDO can be forced by the tropical Pacific SST [30–33]. As the first empirical orthogonal function of SST variability in the northern extratropical Pacific [34,35], the PDO is related to change in the occurrence ratio of El Niño and La Niña events. In the positive PDO phase, El Niño occurs more frequently, while La Niña appears less frequently; and vice versa [36]. The modulation of ENSO’s impact on regional climate by PDO is also confirmed in recent studies. For example, the impact of ENSO on Alaskan wintertime temperature, Rocky Mountain fire, European, Mexican, and the Pacific–American climate, the northern Andean glaciers, and the East Asian winter monsoon may depend on the PDO state [37–43]. It was demonstrated that the European wintertime rainfall response to ENSO is stronger and more significant during the positive PDO than during the negative PDO [44]. Another example is the modulation of the ENSO–China precipitation relationship by PDO [45,46]. The positive correlations between ENSO and South China spring rainfall are largely enhanced when El Niño coincides with the positive PDO and La Niña coincides with the negative PDO [46]. Considering that the forced PNA teleconnection in the troposphere can bridge the ENSO SST and the stratospheric response in the extratropics, the decadal changes in its structure and intensity may have led to the unstable relationship between the stratospheric response and ENSO [47,48]. In this study, we will provide evidence that the unstable stratospheric response to ENSO is mainly controlled by the PDO. We will prove that the stratospheric response to ENSO is more significant when ENSO and PDO are in phase. This paper consists of five sections. Following the introduction in Section1, Section2 describes the data and methodology. Section3 briefly presents the ENSO SST patterns during di fferent PDO phases. The observed tropospheric responses to El Niño and La Niña under different PDO phases are discussed in Section4, and the stratospheric responses to di fferent ENSO/PDO configurations are also provided. Finally, the summary and discussion are presented in Section5. Atmosphere 2019, 10, 211 3 of 16

2. Data and Methodology

2.1. Atmospheric Reanalysis and SST The National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis I, covering the period 1948–2017 [49], is employed as the atmospheric observations. As reported in Rao et al. [50], the NCEP/NCAR reanalysis has the longest timespan among several reanalyses that have reliable stratospheric climatology and interannual variations. More detailed descriptions about this reanalysis can be found in Kalnay et al. [49]. The SST used to obtain the spatial pattern and time series of PDO and El Niño is extracted from the Hadley Centre sea-ice and sea surface temperature dataset (HadISST) [51].

2.2. Data Processing and Methods For any variable from the atmospheric reanalysis and SST analysis, its anomaly is obtained by subtracting its long-term trend and seasonal cycle from its monthly mean raw fields during 1948–2017. The observed El Niño signal in the stratosphere may be mixed with the equatorial stratospheric quasi-biennial oscillation (QBO) [13,14,52–55]. As in previous studies [13–15,19], we regressed the wintertime stratospheric circulation and temperature anomalies against the 50 hPa QBO time series. The QBO is defined as the equatorial (5◦ S–5◦ N) zonal-mean zonal wind anomalies. Then, the QBO-related circulation and temperature anomalies (products of the regression patterns and the QBO values) are subtracted from the raw anomaly fields above 100 hPa. Then, the QBO-like signals can be filtered out from the stratosphere before the composite is performed, preventing the possible mixture of QBO and ENSO signals. After subtracting the long-term trend and seasonal cycle of the SST raw field, the Niño3 index was computed as the area-averaged SST anomalies in the Niño3 region (5◦ S–5◦ N, 150–90◦ W). The El Niño events are selected once the winter (December–January–February, DJF) mean Niño3 index exceeds its half standard deviation in the observations (~0.5 K). The PDO mode [30,31,34,56] is the first empirical orthogonal function (EOF) of SST anomalies in the extratropical Pacific sector (poleward of 20◦ N), and the corresponding normalized time series is defined as the PDO index. We regressed the SST anomalies against the PDO index time series to represent the PDO SST spatial pattern. The positive (high) and negative (low) PDO phases refer to its index sign.

3. ENSO SST Patterns during Positive and Negative PDO Phases Figure1 shows the regressed PDO SST pattern and the normalized PDO time series from HadISST. It can be seen that the Northern Hemisphere midlatitude Pacific is covered with anomalous cold water and the western coast of North America and the tropical–subtropical Pacific regions are dominated by warm water during the positive PDO (Figure1a). The tropical East Pacific cold tongue region is warmer during the positive PDO phase than during the negative PDO phase. The phase conversion of PDO usually leads to decadal change of the stratosphere. For example, Hu et al. [35] reported in their modeling study that the decreasing trend of the PDO (warming of the North Pacific) is responsible for the strengthening of the northern winter stratospheric polar vortex in the past two decades. In observations, short warm PDO phases interrupted the long cold PDO phases from 1940s to 1970s, and short cold PDO phases also broke in the long warm PDO phases from late-1970s to mid-1990s (Figure1b). Atmosphere 2019, 10, 211 4 of 16 Atmosphere 2019, 10, x FOR PEER REVIEW 4 of 15

FigureFigure 1.1. ((aa)) RegressedRegressed seasea surfacesurface temperaturetemperature (SST)(SST) anomaliesanomalies againstagainst thethe normalizednormalized PDOPDO indexindex defineddefined asas thethe firstfirst empiricalempirical orthogonalorthogonal functionfunction (EOF)(EOF) timetime seriesseries ofof thethe PacificPacific SSTSST anomaliesanomalies polewardpoleward ofof 2020°◦ N. N. ((bb)) TheThe normalizednormalized PDOPDO indexindex time time series. series.

TableTable1 1lists lists the the di differentfferent configurationsconfigurations betweenbetween thethe 1919 (29)(29) observedobserved ElEl NiñoNiño (La(La Niña)Niña) eventsevents andand thethe PDOPDO phases.phases. To easilyeasily distinguishdistinguish thethe fourfour configurationsconfigurations betweenbetween ENSOENSO andand PDO,PDO, wewe markmark thethe ElEl NiñoNiño/positive/positive PDOPDO configurationconfiguration with ElEl/P./P. Similarly,Similarly, thethe ElEl/N,/N, LaLa/P,/P, andand LaLa/N/N referrefer toto thethe ElEl NiñoNiño/negative/negative PDO,PDO, LaLa NiñaNiña/positive/positive PDO,PDO, andand La NiñaNiña/negative/negative PDO configurations,configurations, respectively. ItIt cancan bebe found that that during during the the positive positive PDO PDO phase, phase, the the number number of ofEl ElNiño Niño and and La LaNiña Niña events events are arelargely largely asymmetric; asymmetric; namely, namely, El Niño El Niñoevents events are twice are as twice likely as to likely happen to as happen La Niña as events, La Niña consistent events, consistentwith the finding with the in Verdon finding and in Verdon Franks and [36] Franksthat po [sitive36] that PDO positive favors PDOthe occu favorsrrence the of occurrence El Niño events. of El NiñoIn contrast, events. La In Niña contrast, events La are Niña more events likely are to moreoccur likelythan El to Niño occur during than El the Niño negative during PDO the phase. negative On PDOaverage, phase. the OnEl Niño average, intensity the El is Niño observed intensity to enha is observednce in the to enhancepositive inPDO the (El/P: positive 1.3 PDO K; El/N: (El/P: 0.76 1.3 K). K; ElOn/N: contrary, 0.76 K). the On averaged contrary, theLa Niña averaged intensity La Niña is obse intensityrved to is increase observed in to the increase negative in thePDO negative phase (La/P: PDO phase (La/P: 0.7 K; La/N: 0.9 K). –0.7 K; La/N:− –0.9 K). − Table 1. The El Niño–Southern Oscillation (ENSO) events classified by the Pacific Decadal Oscillation Table 1. The El Niño–Southern Oscillation (ENSO) events classified by the Pacific Decadal Oscillation (PDO) phases in observations. El/La denotes El Niño/La Niña events in the first column. P/N denotes (PDO) phases in observations. El/La denotes El Niño/La Niña events in the first column. P/N denotes the positive/negative phases of the PDO. the positive/negative phases of the PDO. ENSO/PDOENSO/PDO Configuration Winter CompositeComposite Niño3 Winter Configuration 13 events (1957/58, 1965/66, 1969/70, 1976/77, Niño3 El/P 1982/83, 1986/87, 1987/88, 1991/92, 1997/98, 1.30 K 13 events (1957/58, 1965/66, 1969/70, 1976/77, 1982/83, 1986/87, El/P 2002/03, 2009/10, 2014/15, 2015/16) 1.30 K 1987/88, 1991/92,6 events 1997/98, (1951/52, 2002/03, 1963/64, 19682009/10,/69, 1972 2014/15,/73, 2015/16) El/N 0.76 K El/N 6 events (1951/52, 1963/64,1994 1968/69,/95, 2006/07) 1972/73, 1994/95, 2006/07) 0.76 K La/P 5 events (1983/84, 1984/85, 1985/86, 1995/96, 1996/97) −0.70 K 24 events (1949/50, 1950/51, 1954/55, 1955/56, 1956/57, 1962/63, La/N −0.91 K 1964/65, 1966/67, 1967/68, 1970/71, 1971/72, 1973/74, 1974/75,

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Table 1. Cont.

ENSO/PDO Configuration Winter Composite Niño3 5 events (1983/84, 1984/85, 1985/86, La/P 0.70 K 1995/96, 1996/97) − 24 events (1949/50, 1950/51, 1954/55, 1955/56, 1956/57, 1962/63, 1964/65, 1966/67, 1967/68, La/N 1970/71, 1971/72, 1973/74, 1974/75, 1975/76, 0.91 K Atmosphere 2019, 10, x FOR PEER REVIEW − 5 of 15 1988/89, 1998/99, 1999/00, 2000/01, 2005/06, 2007/08, 2008/09, 2010/11, 2011/12, 2012/13) 1975/76, 1988/89, 1998/99, 1999/00, 2000/01, 2005/06, 2007/08, 2008/09, 2010/11, 2011/12, 2012/13) TheThe composite composite SST SST anomaly anomaly distributionsdistributions for di fferentfferent configurations configurations between between ENSO ENSO and and PDO PDO andand their their di differencesfferences are are shown shown inin FigureFigure2 2 for for HadISST.HadISST. TheThe distribution of of the the Pacific Pacific ENSO ENSO SST SST anomaliesanomalies is is modulated modulated by by the the PDO. PDO. PositivePositive PDO (Figure 2a)2a) enhances warm warm SST SST anomalies anomalies in inthe the middlemiddle and and eastern eastern Pacific Pacific during during El El Niño Niño winters winters (Figure (Figure2c) 2c) with with respect respect to to negative negative PDO PDO (Figure (Figure2b). The2b). observed The observed amplification amplification of warm of SST warm anomalies SST inanomalies the equatorial in the eastern equatorial Pacific eastern is accompanied Pacific is by theaccompanied strengthening by ofthe cold strengthening SST anomalies of cold in theSST North anomal Pacific.ies in the The North North Pacific. Pacific The cold North SST Pacific anomalies cold do notSST form anomalies in the El do/N winters,not form whichin the areEl/N cancelled winters, out which by theare localcancelled warm out SST by anomalies the local relatedwarm SST to the negativeanomalies PDO. related to the negative PDO.

FigureFigure 2. 2.Composite Composite SST SST anomalies anomalies duringduring El Niño winters winters when when the the PDO PDO is isin inits its (a) ( apositive) positive and and (b)(b negative) negative phases. phases. ( c()c) The The di differencefference betweenbetween ( a)) and and ( (bb).). (d (d––f)f )As As in in (a (–ac–)c but) but for for La La Niña Niña winters. winters. (g,(hg),h The) The half half di ffdifferenceerence between between El NiñoEl Niño and and La La Niña Niñ duringa during the the positive positive/negative/negative phases phases of theof the PDO. (i)PDO. The half(i) The diff halference difference between between (a) and (a ()e and). The (e). blackThe black contours contours mark mark the the SST SST anomalies anomalies at at the the 90% confidence90% confidence level. level.

Similarly,Similarly, the the La NiñaLa Niña cold cold SST anomaliesSST anomalies in the in equatorial the equatorial middle middle and eastern and eastern Pacific arePacific amplified are duringamplified the negative during the PDO negative phase (FigurePDO phase2e), while(Figure the 2e), cold while SST the anomaly cold SST band anomaly becomes band narrow becomes in the equatorialnarrow in middle the equatorial and eastern middle Pacific and during eastern the Pacifi positivec during PDO the phase positive (Figure PDO2d). phase Comparing (Figure the2d). El NiñoComparing and La Niña the El SST Niño patterns, and La coldNiña SST SST anomaliespatterns, cold dominated SST anomalies in the Northdominated Pacific in duringthe North the Pacific positive PDOduring phase, the although positive PDO the tropical phase, although SST anomalies the tropical in El NiñoSST anomalies and La Niña in El winters Niño and are La oppositely Niña winters signed duringare oppositely the positive signed PDO during (Figure the2a,d). positive It is PDO also noticed(Figures that 2a,d). the It SSTis also anomaly noticed patterns that the inSST the anomaly El /P and Lapatterns/N winters in the are El/P nearly and identical La/N winters but theare signnearly is reversedidentical but (Figure the sign2a,e). is reversed It is expected (Figures the 2a,e). ENSO It SSTis anomaliesexpected werethe ENSO filtered SST out anomalies in Figure were2c,f, filtered because out the in SST Figures di ff erence1c and between1f, because El the/P and SST Eldifference/N, as well asbetween the difference El/P and between El/N, as La well/P and as the La /differenceN, shows between a PDO SST La/P pattern and La/N, (Figure shows2c,f; a alsoPDO see SST Figure pattern1a). The(Figures difference 2c,f; also between see Figure El/P and1a). The La/ Pdifference or the di befferencetween El/P between and La/P El/N or and the difference La/N filters between out the El/N PDO and La/N filters out the PDO SST anomalies (Figures 2g,h). The “pure” ENSO patterns are very similar, but the central SST intensity in the equatorial eastern Pacific during the positive PDO is stronger than during the negative PDO. The similar SST pattern in El/P and La/N winters can also be verified by their half difference (Figure 2i).

4. Modulation of the ENSO Teleconnection by PDO

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SST anomalies (Figure2g,h). The “pure” ENSO patterns are very similar, but the central SST intensity in the equatorial eastern Pacific during the positive PDO is stronger than during the negative PDO. The similar SST pattern in El/P and La/N winters can also be verified by their half difference (Figure2i).

4. Modulation of the ENSO Teleconnection by PDO Atmosphere 2019, 10, x FOR PEER REVIEW 6 of 15 4.1. Sensitivity of Tropospheric ENSO Teleconnections to PDO Phases 4.1. Sensitivity of Tropospheric ENSO Teleconnections to PDO Phases Figure3 presents the composite precipitation and 200-hPa geopotential height responses to differentFigure configurations 3 presents of the ENSO composite and PDO. precipitation Warm SST and anomalies 200-hPa in geopotential the equatorial height middle responses and eastern to different configurations of ENSO and PDO. Warm SST anomalies in the equatorial middle and Pacific increase the atmospheric instability and enhance the convection, leading to more rainfall eastern Pacific increase the atmospheric instability and enhance the convection, leading to more (Figure3a,b). It can be seen that the El Niño-induced rainfall anomalies in the equatorial middle Pacific rainfall (Figures 3a,b). It can be seen that the El Niño-induced rainfall anomalies in the equatorial are larger during the positive PDO than during the negative PDO, which can be explained by the middle Pacific are larger during the positive PDO than during the negative PDO, which can be larger warm SST anomalies in El/P winters than in El/N winters (c.f., Figure2a,b). The general PNA explained by the larger warm SST anomalies in El/P winters than in El/N winters (c.f., Figures 2a,b). responseThe general is also PNA stronger response in El is/P also winters stronger than in in El/P El/N winters winters. than For in example, El/N winters. the PNA For lobes,example, including the thePNA anomalous lobes, including tropical Pacificthe anomalous high and tropical North Pacific Pacific high low areand also North stronger Pacific in low El /areP winters. also stronger It can in also beEl/P found winters. that the It can anomalous also be found North that Pacific the anomal low thatous responds North Pacific to El Niñolow that is largely responds weakened to El Niño during is thelargely negative weakened PDO. The during cold the SST negative anomalies PDO. that The usually cold SST accompany anomalies Elthat Niño usually can accompany be neutralized El Niño by the negativecan be PDO,neutralized which by results the negative in a weaker PDO, SST which meridional results in gradient a weaker anomaly SST meridional in the North gradient Pacific anomaly and less significantin the North height Pacific response and less at significant 200 hPa. Hu height et al. response [57] also at found 200 hPa. that Hu the et globalal. [57] warming also found SST that forcing the decreasingglobal warming from theSST Equator forcing decreasing to the Poles from induces the Equator a stronger to the Poles extratropical induces a wave stronger and extratropical stratospheric temperaturewave and stratospheric responses than temperature the uniform responses warm SSTthan anomalythe uniform forcing warm does. SST anomaly forcing does.

1 FigureFigure 3. 3.Composite Composite precipitation precipitation anomalies anomalies (shadings,(shadings, units: mm mm d d–1−) and) and 200-hPa 200-hPa height height anomalies anomalies (contours;(contours; units: units: gpm; gpm; interval: interval: 10)10) duringduring El Niño winters winters when when the the PDO PDO is isin in its its (a) ( apositive) positive and and (b) (b) negativenegative phases phases and and those those during during La La NiñaNiña winterswinters when the PDO PDO is is in in its its (c ()c positive) positive and and (d) ( dnegative) negative phases.phases. Purple Purple contours contours mark mark the the heightheight anomaliesanomalies at 90% confidence conﬁdence level. level.

InIn contrast, contrast, the the cold cold SST SST anomalies anomalies inin thethe NorthNorth Pacific Pacific related related to to the the positive positive PDO PDO overwhelm overwhelm thethe La La Niña Niña SST SST anomalies, anomalies, and and the the coldcold SSTSST anomaly band band in in the the tropical tropical Pacific Pacific is isalso also fairly fairly narrow narrow (Figure(Figure2d), 2d), so theso negativethe negative PNA PNA response response is not is formed not formed (Figure (Figure3c). Instead, 3c). Instead, an anomalous an anomalous low appears low overappears the North over Pacific,the North although Pacific, negative although (albeit negative weak) (albeit height weak) response height alsoresponse appears also over appears the tropicalover Pacific.the tropical The La Pacific. Niña The teleconnection La Niña teleconnection during the during negative the PDOnegative (Figure PDO3 d)(Figure is largely 3d) is similarlargely similar to the El Niñoto the teleconnection El Niño teleconnection during the during positive the PDO positive (Figure PDO3a) (Figure but with 3a) the but sign with reversed. the sign Anreversed. anomalous An highanomalous appears overhigh theappears North over Pacific, the North and two Pacific, anomalous and two lows anomalous appear lows over theappear tropical over middlethe tropical Pacific andmiddle Arctic Pacific Canada, and respectively. Arctic Canada, Therefore, respectively. the localTherefore, SST associated the local SST with associated the PDO with state the should PDO be consideredstate should for be the considered extratropical for the Pacific extratropical response Pacific to ENSO. response to ENSO. To well understand the height responses in Figure 3, the 200-hPa wave activity flux [58] anomalies during the four different ENSO/PDO configurations are shown in Figure 4. As the convection over the tropical Central and East Pacific is enhanced, a wave-like response forms over the extratropical Pacific. The wave train emanates from the tropical Central Pacific, propagates along an arc, and bridges tropical thermal forcing with local circulation variations in the extratropical Pacific and North America (Figures 4a,b). The Pacific ENSO acts as a wave source through changing

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To well understand the height responses in Figure3, the 200-hPa wave activity flux [ 58] anomalies during the four different ENSO/PDO configurations are shown in Figure4. As the convection over the tropical Central and East Pacific is enhanced, a wave-like response forms over the extratropical Pacific. The wave train emanates from the tropical Central Pacific, propagates along an arc, and bridges tropicalAtmosphere thermal 2019, 10 forcing, x FOR withPEER REVIEW local circulation variations in the extratropical Pacific and North America7 of 15 (Figure4a,b). The Pacific ENSO acts as a wave source through changing the tropical convections and the tropical convections and excites a PNA-like response. The amplitude and direction of the excites a PNA-like response. The amplitude and direction of the horizontal wave flux components are horizontal wave flux components are nearly identical over the North Pacific between El/P and El/N nearly identical over the North Pacific between El/P and El/N configurations. The vertical wave flux configurations. The vertical wave flux components, however, is contrastingly different in El Niño components, however, is contrastingly different in El Niño winters between the two phases of PDO, winters between the two phases of PDO, implying different vertical passages for upward wave implying different vertical passages for upward wave propagation. The dominant vertical passage is propagation. The dominant vertical passage is situated over East Asia and the western coast of North situatedAmerica over for East the Asiaanomalous and the upward western propagating coast of North waves America during for the the positive anomalous PDO, upward while this propagating passage wavesis biased during poleward the positive to the PDO, Arctic while Ocean this during passage the negative is biased PDO. poleward In contrast, to the the Arctic response Ocean of duringthe wave the negativeactivity PDO. flux at In 200 contrast, hPa in theLa/P response winters is of mainly the wave controlled activity by flux the atcold 200 SST hPa anomalies in La/P wintersin North is Pacific mainly controlledassociated by with the cold the SSTpositive anomalies PDO (Figure in North 4c), Pacific which associated largely resembles with the the positive response PDO in (Figure the El/P4c), whichwinters. largely Specifically, resembles the the enhanced response waves in the in Elthe/P troposphere winters. Specifically, propagate upward the enhanced from the waves East Asia in the troposphereand the western propagate coast upwardof Canada from but propagate the East Asia down andward the over western the North coast Atlantic. of Canada The wave but propagate activity downwardresponse overto SST the anomalies North Atlantic. during the The La/N wave winters activity is generally response symmetric to SST anomalies with that during the the El/P La/ N winterswinters is generally(Figures 4a,d). symmetric The positi withve height that during anomalies the El over/P winters the Nort (Figureh Pacific4a,d). during The the positive La/N winters height anomaliesweaken overthe climatological the North Pacific stationary during trough the La there/N winters (Figure weaken 4d). Furthermore, the climatological negative stationary vertical wave trough thereactivity (Figure flux4d). appears Furthermore, in the East negative Asia and vertical the western wave activitycoast of fluxCanada, appears where in theplanetary East Asia waves and are the westernsuppressed coast ofto Canada,propagate where upward. planetary waves are suppressed to propagate upward.

FigureFigure 4. 4.Composite Composite 200-hPa200-hPa height anomalies anomalies (contours; (contours; units: units: gpm; gpm; interval: interval: 10) and 10) 200-hPa and 200-hPa wave 2 2 waveactivity activity flux ﬂux anomalies anomalies (vectors (vectors for the for horizontal the horizontal component, component, units: m units:2 s–2; shadings m s− ; shadingsfor the vertical for the 2 2 2 verticalcomponent, component, units: units:10–2 m2 10 s–2−) duringm s− )El during Niño winters El Niño when winters the whenPDO is the in PDO its (a is) positive in its (a )and positive (b) andnegative (b) negative phases phases and those and during those duringLa Niña La winters Niña winterswhen the when PDO theis in PDO its (c) is positive in its (c and) positive (d) negative and (d ) negativephases. phases.

DisturbanceDisturbance of of the Arctic Arctic stratospheric stratospheric polar polar vortex vortex results from results the frominterference the interferenceof anomalous of anomalouswaves propagating waves propagating upward from upward the fromtroposphere the troposphere with the withclimatological the climatological troughs and troughs ridges and ridges[1,3,9,19,29,59]. [1,3,9,19,29 ,59To] .better To better understand understand the disturba the disturbancence of polar of polarvortex vortex by the by interference the interference of the of thetropospheric tropospheric wave wave sources sources with with the climatological the climatological zonal waves, zonal waves,the vertical the cross vertical section cross for section the eddy for theheight eddy heightresponses responses averaged averaged in the inmid-to-high the mid-to-high latitudes latitudes (45–75° (45–75 N) is ◦shownN) is shownin Figure in Figure5 in the5 in thepressure-longitude pressure-longitude plane plane for for the the four four ENSO/PDO ENSO/PDO co configurations.nfigurations. It Itcan can be be seen seen that that a anegative negative tropospherictropospheric height height anomaly anomaly center center forms forms overover thethe NorthNorth PacificPacific near near the the da datete line line during during El El Niño Niño winters,winters, but but stronger stronger during during the the positive positive PDO PDO than th duringan during the negativethe negative PDO PDO (Figure (Figures5a,b). 5a,b). The NorthThe PacificNorth height Pacific anomaly height centeranomaly is projected center is ontoprojected the negative onto the lobe negative of the lobe PNA-like of the responsePNA-like (Figure response3a,b (Figures 3a,b and 4a,b), and it is located near the climatological trough. The projected wavenumber– 1 response is in phase with the climatological wavenumber–1 (shadings) to enhance the total waves during El Niño winters. In contrast, the wavenumber–1 response is stronger in El/P winters than in El/N winters (Figures 5e,f). Both the eddy height response and its zonal wavenumber–1 tilt westward, indicating an enhanced upward propagation of planetary waves. The extratropical height response

Atmosphere 2019, 10, 211 8 of 16 and Figure4a,b), and it is located near the climatological trough. The projected wavenumber–1 response is in phase with the climatological wavenumber–1 (shadings) to enhance the total waves during El Niño winters. In contrast, the wavenumber–1 response is stronger in El/P winters than in

El/NAtmosphere winters 2019 (Figure, 10, x FOR5e,f). PEER Both REVIEW the eddy height response and its zonal wavenumber–1 tilt westward,8 of 15 indicating an enhanced upward propagation of planetary waves. The extratropical height response to Lato Niña La Niña during during the positive the positive PDO PDO is fairly is fairly diﬀerent different from from that that during during the the negative negative PDO PDO (Figure (Figures5c,d). The5c,d). extratropical The extratropical eddy height eddy height response response lags the lags climatological the climatological waves waves inLa in/ La/PP winters, winters, whereas whereas the wavethe responsewave response in La/ Nin wintersLa/N winters is nearly is nearly a mirror a mirror image image of the of wave the wave response response in El /inP winters.El/P winters. It is alsoIt seenis also that seen the wavenumber–1 that the wavenumber–1 component component and its climatologyand its climatology are in quadratureare in quadrature in La/ Pin winters, La/P winters, but the wavebut responsethe wave isresponse out of phase is out withof phase its climatology with its climatology in La/N in winters La/N winters (Figure (Figures5g,h). 5g,h).

Figure 5. Pressure–longitude cross-sections of composite eddy height anomalies averaged in the Figure 5. Pressure–longitude cross-sections of composite eddy height anomalies averaged in the 45– 45–7575°◦ NN latitude latitude band band during during El Niño El Niño winters winters when when the PDO the is PDO in its is (a in) positive its (a) positive and (b) negative and (b) negativephases phasesand andthose those during during La Niña La Niña winter winterss when when the thePDO PDO is in is its in its(c) (cpositive) positive and and (d ()d negative) negative phases phases (contours;(contours; units: units: gpm; gpm; interval: interval: 10).10). ((ee––h)) AsAs in (a–d)) but but for for the the zonal zonal wavenumber-1 wavenumber-1 components. components. ShadingsShadings are are the the corresponding corresponding eddy eddy height height climatology climatology (units:(units: gp gpm)m) in in the the 45–75° 45–75 ◦NN latitude latitude band. band.

4.2. Upward Propagation of Planetary Waves and Stratospheric Response Figure 6 presents the composite Eliassen–Palm (E–P) flux anomalies and the E–P flux divergence [60] to demonstrate the upward propagation and dissipation of planetary waves from the troposphere to stratosphere. Consistent with the positive PNA response in the troposphere in El Niño winters, the planetary wave activity is intensified, and its upward propagation is strengthened in the mid-to-high latitudes (Figures 6a,b). The upward propagation of waves in mid-to-high latitudes is

Atmosphere 2019, 10, 211 9 of 16

4.2. Upward Propagation of Planetary Waves and Stratospheric Response Figure6 presents the composite Eliassen–Palm (E–P) flux anomalies and the E–P flux divergence [60] to demonstrate the upward propagation and dissipation of planetary waves from the troposphereAtmosphere 2019 to, stratosphere. 10, x FOR PEER REVIEW Consistent with the positive PNA response in the troposphere in El9 of Niño 15 winters, the planetary wave activity is intensified, and its upward propagation is strengthened in the more uniform and more intense in El/P winters than in El/N winters (Figures 6a,b), consistent with mid-to-high latitudes (Figure6a,b). The upward propagation of waves in mid-to-high latitudes is more the larger easterly anomalies in the circumpolar region in El/P winters. The stratospheric polar vortex uniform and more intense in El/P winters than in El/N winters (Figure6a,b), consistent with the larger response to El Niño is sensitive to the PDO phase: When the cold mean state appears in the North easterlyPacific anomalies as during in the the circumpolarpositive PDO, region the inenhanced El/P winters. SST meridional The stratospheric gradient polar can vortex strengthen response the to El Niñoextratropical is sensitive response to the PDOto tropical phase: SST When anomalies. the cold meanFor example, state appears Hu et in al. the [57] North reported Pacific that as during the thenorthern positive winter PDO, thestratospheric enhanced SSTresponse meridional to an gradientocean forcing can strengthenwith maximum the extratropical SST warming response in the to tropicaltropics SST that anomalies. decreases with For example,latitude in Hu both et al.hemisp [57]heres reported is usually that the stronger northern than winter the response stratospheric to a responseuniform to SST an oceanwarming forcing pattern. with maximum SST warming in the tropics that decreases with latitude in both hemispheresSimilarly, the is usuallysuppressed stronger upward than propagation the response of toplanetary a uniform waves SST warmingand the positive pattern. E–P flux divergenceSimilarly, anomalies the suppressed in the circumpolar upward propagation region is more of planetary significant waves in La/N and winters the positive than in E–P La/P flux divergencewinters (cf. anomalies Figures in6c,d). the The circumpolar strengthened region polar is more jet response significant in inthe La circumpolar/N winters thanregion in is La found/P winters to (cf.appear Figure in6c,d). La/N The winters strengthened and in La/P polar winters. jet response We find inthe the polar circumpolar night jet in region La/P winters is found seemingly to appear in Laexhibits/N winters a southward and in shift La/P and winters. the response We find is themuch polar stronger night than jet inother La/ Pconfigurations winters seemingly of ENSO exhibits and a southwardPDO (Figure shift 6c). andThe theLa/P response case number is much is relatively stronger small than than other other configurations configurations, of ENSOand the and strong PDO (Figurezonal6 c).wind The response La /P case in the number composite is relatively might reflect small the than large other internal configurations, variation of and the the stratospheric strong zonal windpolar response vortex. inAs the the composite North Pacific might height reflect response the large is generally internal symmetric variation ofbetween the stratospheric El/P and La/N polar vortex.winters, As thethe Northstratospheric Pacific heightpolar vort responseex response is generally pattern symmetric is largely similar between but El /withP and the La sign/N winters, reversed the (Figures 6a,d). stratospheric polar vortex response pattern is largely similar but with the sign reversed (Figure6a,d).

Figure 6. Pressure–latitude cross-sections of composite zonal mean zonal wind anomalies (contours; Figure 6. Pressure–latitude cross-sections of composite zonal mean zonal wind anomalies (contours; units: m s 1; interval: 0.5), composite E–P ﬂux anomalies (vectors; units: m3 s 2; normalized by local units: m− s–1; interval: 0.5), composite E–P flux anomalies (vectors; units: m3 s–2; normalized− by local air 1 1 airdensity density for for clarity), clarity), and and composite composite E–P E–P flux ﬂuxdivergence divergence anomalies anomalies (shadings; (shadings; units: m units: s–1 d–1 m) during s− d− ) duringEl Niño El Niño winters winters when when the PDO the is PDO in its is ( ina) itspositive (a) positive and (b and) negative (b) negative phases phasesand those and during those La during Niña La Niñawinters winters when when the thePDO PDO is in isits in (c its) positive (c) positive and ( andd) negative (d) negative phases. phases.

ToTo easily easily compare compare the the disturbances disturbances ofof thethe stratosphericstratospheric circulation circulation related related to to the the anomalous anomalous upwardupward propagation propagation of of waves waves originating originating from from thethe troposphere, Figure Figure 7 showsshows the the composite composite height height responseresponse and and its zonalits zonal wavenumber–1 wavenumber–1 component component at 10 at hPa. 10 hPa. Consistent Consistent with with Figure Figures6a,b, the 6a,b, positive the positive height response in the Arctic stratosphere is stronger in El/P winters than in El/N winters (Figures 7a,b), and so is the wavenumber–1 response (Figures 7e,f). This infers once again that the stratospheric polar vortex response to El Niño varies with the PDO phase. The wavenumber–1 response is in phase with its climatology and favors the displacement of the stratospheric polar vortex

Atmosphere 2019, 10, 211 10 of 16 height response in the Arctic stratosphere is stronger in El/P winters than in El/N winters (Figure7a,b), and so is the wavenumber–1 response (Figure7e,f). This infers once again that the stratospheric polar vortex response to El Niño varies with the PDO phase. The wavenumber–1 response is in phase with itsAtmosphere climatology 2019, 10 and, x FOR favors PEER theREVIEW displacement of the stratospheric polar vortex toward the10 Atlantic of 15 sector where the negative height anomaly and negative lobe of the wavenumber–1 prevail. However, toward the Atlantic sector where the negative height anomaly and negative lobe of the wavenumber– the stratospheric polar vortex responses to La Niña during the positive and negative PDO phases 1 prevail. However, the stratospheric polar vortex responses to La Niña during the positive and show little similarity. The orthogonal distribution of wavenumber–1 with its climatology observed negative PDO phases show little similarity. The orthogonal distribution of wavenumber–1 with its in the extratropical eddy height response (Figure5c,g) also appears in the height response at 10 hPa climatology observed in the extratropical eddy height response (Figures 5c,g) also appears in the (Figureheight7c,g). response In contrast, at 10 hPa the (Figures height response 7c,g). In patterncontrast, in the La height/N winters response is generally pattern in symmetric La/N winters to that is in El/generallyP winters symmetric (Figure7a,d). to that The in wavenumber–1 El/P winters (Figures response 7a,d). in La The/N wavenumber–1 winters is nearly response out of phasein La/N with thatwinters in El/ Pis andnearly El/ Nout winters. of phase with that in El/P and El/N winters.

FigureFigure 7. 7.Composite Composite height height anomalies anomalies atat 10 hPa (top row) row) and and the the zonal zonal wavenumber–1 wavenumber–1 components components (bottom(bottom row) row) during during El El Niño Niño winters winters when when the the PDO PDO is inis in its its (a,e (a,e) positive) positive and and (b,f (b,f) negative) negative phases phases and thoseand during those during La Niña La winters Niña winter whens the when PDO the is PDO in its is (c,g in) its positive (c,g) positive and (d,h and) negative (d,h) negative phases (contours;phases units:(contours; gpm; interval:units: gpm; 20; zerointerval: lines 20; skipped zero lines for clarity).skipped Shadingsfor clarity) are. Shadings the corresponding are the corresponding winter height climatologywinter height (units: climatolo gpm)gy at (units: 10 hPa. gpm) at 10hPa.

4.3.4.3. A A Comprehensive Comprehensive View View

TheThe scatterplots scatterplots of of the the ENSO ENSO index index versus versus the the stratospheric stratospheric polar polar cap cap temperature temperature (Tpole (Tpole; 70–20; 70–20 hPa, pole 60–90hPa,◦ N),60–90° stratospheric N), stratospheric circumpolar circumpolar westerly westerly jet (Upole jet; ( 50–10U ; 50–10 hPa, 65–75 hPa, ◦65–75°N), PNA N), index,PNA index, and Western and PaciﬁcWestern (WP) Pacific index (WP) are shownindex are in shown Figure 8in during Figure all8 during winters all fromwinters the from observations. the observations. The PNA The and PNA WP indicesand WP were indices also were adopted also toadopted study to the study predictability the predictability of sudden of sudden stratospheric stratospheric warming warming events events and its troposphericand its tropospheric precursors precursors [61,62]. The[61,62]. linear The relationship linear relationship between between each indicator each indicator and Niño3 and Niño3 index is index is regressed respectively for warm (Niño3 > 0) and cold (Niño3 < 0) ENSO states and for positive regressed respectively for warm (Niño3 > 0) and cold (Niño3 < 0) ENSO states and for positive (red) and (red) and negative (blue) PDO phases, respectively. Because super El Niño events might have a negative (blue) PDO phases, respectively. Because super El Niño events might have a diﬀerent impact different impact on the northern winter stratosphere from that of moderate El Niño events [13,15,16], on the northern winter stratosphere from that of moderate El Niño events [13,15,16], the three super El the three super El Niño events (Niño3 > 2.5 °C) in the 1982/83, 1997/98, and 2015/16 winters as extreme Niño events (Niño3 > 2.5 C) in the 1982/83, 1997/98, and 2015/16 winters as extreme outliers were outliers were excluded to◦ construct the linear relationship between ENSO and the four indicators. excluded to construct the linear relationship between ENSO and the four indicators. The stratospheric The stratospheric polar cap temperature anomaly tends to be warm in most moderate warm ENSO polarwinters cap temperatureespecially when anomaly ENSO tendsand PDO to be are warm in phase. in most On moderate the contrary, warm it tends ENSO to winters be cold especiallyin cold whenENSO ENSO winters and when PDO ENSO are in phase.and PDO On are the in contrary, phase (Figure it tends 8a). to The be coldlinkage in coldbetween ENSO super winters El Niño when ENSOevents and and PDO Tpole are is inweak, phase and (Figure the polar8a). cap The temperature linkage between is anomalously super El Niño weak events in two and of theTpole superis weak, El andNiño the polarwinters. cap The temperature regressed is relationship anomalously between weak in warm two of (cold) the super ENSO El Niñoand T winters.pole during The positive regressed (negative) PDO shows a positive slope. It is also noticed that most points for Upole is negative especially when both PDO and ENSO are positive, and vice versa, when the PDO and ENSO indices are negative (Figure 8b). In contrast, the points are more scattered and the linear correlation between ENSO and Upole is weak when the PDO and ENSO indices are out of phase.

Atmosphere 2019, 10, 211 11 of 16

relationship between warm (cold) ENSO and Tpole during positive (negative) PDO shows a positive slope. It is also noticed that most points for Upole is negative especially when both PDO and ENSO are positive, and vice versa, when the PDO and ENSO indices are negative (Figure8b). In contrast, the points are more scattered and the linear correlation between ENSO and U is weak when the PDO Atmosphere 2019, 10, x FOR PEER REVIEW pole 11 of 15 and ENSO indices are out of phase. As previous studies [[3,14,21]3,14,21] reported,reported, one of the ENSO teleconnections, the PNA, can bridge the stratospheric polar vortex response to ENSO. Most points for PNA are mainly scattered above (below) the zero line when both ENSO and PDO areare positive (negative), while the PNA coordinates are homogeneously distributed above and below the zero line and the ENSO–PNA correlation is also weak when ENSO and PDO are out of phase (Figure8 8c).c). TheThe otherother ENSOENSO teleconnection,teleconnection, thethe WPWP isis less sensitivesensitive toto thethe phase phase of of ENSO ENSO and and PDO. PDO. For For example, example, during during the the positive positive PDO, PDO, the regressedthe regressed line lineslopes slopes for warm for warm and coldand cold ENSO ENSO states states are nearly are nearly the same the same (red lines(red lines in Figure in Figure8d). The 8d). regression The regression lines linesfor cold for ENSOcold ENSO states states during during positive positive and negativeand negative PDO PDO are nearly are nearly parallel parallel (blue (blue lines lines in Figure in Figure8d). 8d).It may It bemay inferred be inferred that the that unstable the relationshipunstable rela betweentionship ENSO between and theENSO northern and winterthe northern stratosphere winter is stratospheremainly explained is mainly by the explained PNA response. by the The PNA consistent response. dynamic The consistent response betweendynamic the response troposphere between and thestratosphere troposphere provides and stratosphere evidence for provides the possible evidence contribution for the possible of the PDO contribution to the unstable of the relationshipPDO to the unstablebetween ENSOrelationship and the between northern ENSO winter and stratosphere. the northern winter stratosphere.

Figure 8. ScatterplotsScatterplots of of the the normalized normalized winter Niño3 index versus four indicators: ( a) the the normalized normalized pole pole winter Tpole, ,((bb)) the the normalized normalized winter winter U pole, ,((cc) )the the normalized normalized winter winter PNA PNA index, index, and and ( (d)) the normalized winterwinter WP WP index index when when the the PDO PDO is in is its in positive its positive (red) and (red) negative and negative (blue) phases. (blue) Blue phases./Red Blue/Redlines present lines the present linear the relationships linear relationships between the between winter the Niño3 winter index Niño3 and index each of and the each four of indicators the four whenindicators the PDO when is the in its PDO positive is in/ negativeits positive/negative phase. Note phase. that the Note warm that and the cold warm ENSO and states cold are ENSO separately states arechecked. separately The checked. percentage The of percentage variance explained of variance by explained Niño3 (R by2) Niño3 is printed (R2) inis printed each plot in each for di plotfferent for differentENSO/PDO ENSO/PDO configurations. configurations The sample. The sizessample for sizes the four for the configurations four configurations are 20 (El are/P, 20R2 (El/P,0.13 R for2 ≥ ≥ 0.13α 0.1for), α 7 ≤ (El 0.1),/N, 7R (El/N,2 0.38 R2 for≥ 0.38α for0.1), α ≤ 11 0.1), (La 11/P, (La/P,R2 0.25 R2 ≥ for0.25α for0.1), α ≤ 0.1), and 31and (La 31/ N,(La/N,R2 R0.092 ≥ 0.09 for ≤ ≥ ≤ ≥ ≤ ≥ forα α0.1). ≤ 0.1). ≤ 5. Summary and Discussion Recent studies have indicated the unstable relationship between ENSO and the norther winter stratospheric polar vortex. The northern winter stratospheric response to El Niño has been weakening in the past decades. Using the NCEP/NCAR reanalysis and HadISST analysis data, we explore in this study the possible modulation of the ENSO–northern winter stratosphere relationship by the PDO, which exerts an impact on SST mean state in the North Pacific on an interdecadal timescale. El Niño and La Niña events show large asymmetry in the positive and negative PDO phases, respectively. In general, El Niño events usually occur more frequently than La Niña events in the positive PDO,

Atmosphere 2019, 10, 211 12 of 16

5. Summary and Discussion Recent studies have indicated the unstable relationship between ENSO and the norther winter stratospheric polar vortex. The northern winter stratospheric response to El Niño has been weakening in the past decades. Using the NCEP/NCAR reanalysis and HadISST analysis data, we explore in this study the possible modulation of the ENSO–northern winter stratosphere relationship by the PDO, which exerts an impact on SST mean state in the North Pacific on an interdecadal timescale. El Niño and La Niña events show large asymmetry in the positive and negative PDO phases, respectively. In general, El Niño events usually occur more frequently than La Niña events in the positive PDO, whereas more La Niña events appear in the negative PDO than El Niño events. Furthermore, the average El Niño intensity is larger in the positive PDO than in the negative PDO, whereas the average La Niña intensity is larger in the negative PDO than in the negative PDO. As the composite El Niño is stronger in the positive PDO than in the negative PDO, the tropical precipitation response is stronger in El/P winters than in El/N winters. The positive PNA response to El Niño and the negative height anomalies over the North Pacific are much larger and more significant in El/P winters than in El/N winters. The asymmetric behaviors of El Niño events between the positive and negative PDO phases may partially explain the unstable stratospheric response to El Niño. The extratropical SST anomalies in El Niño winters that can be superimposed by the positive PDO or undermined by the negative PDO dominate more in locking the height response over the North Pacific and therefore the positive PNA response. Therefore, the passages for the upward propagation of waves from the troposphere is more unimpeded over the western coast of the North American continents and East Asia when the PDO is in its positive phase than in its negative phase. However, it is also revealed that the negative height response over the North Pacific also appears in La Niña winters when the PDO is positive, which may imply that the extratropical cold SST anomalies in the North Pacific associated with the positive PDO overwhelm the tropical cold SST anomalies associated with La Niña in determining the height anomaly pattern. In contrast, the negative PNA response forms and the upward propagation of waves in the western coast of North America and East Asia is suppressed in La/N winters, which is nearly symmetric to the response in El/P winters. The negative height response over the North Pacific is mainly projected to the wavenumber–1 in El/P winters, which shows a westward tilt from the troposphere to the stratosphere and favors its upward propagation. The enhanced upward-propagating waves dissipate in the mid-to-high latitude stratosphere, weakening the stratospheric polar vortex. As the positive PNA response to El Niño is diminished in the negative PDO phase, and the enhanced upward propagation of the extratropical waves is limited. In comparison, the weakening of the stratospheric polar vortex in El/N winter is less significant than in El/P winters. It is not found that the extratropical height response to La Niña should weaken the climatological waves from the troposphere to the stratosphere: the wavenumber–1 response in La/N winters seems to be in quadrature with its climatology. In contrast, consistent with the negative PNA response, a weakened wavenumber–1 pattern forms in La/N winters, and the upward propagation of waves is largely suppressed in the mid-to-high latitudes, leading to a stronger polar vortex response. The composite stratospheric El Niño signal may have been entangled with other external factors like the QBO and solar cycle from the relatively short observations. Because limited ENSO cases can be selected from observations, model evidence is required to further confirm the modulation of the stratospheric ENSO teleconnection by PDO. The PDO can change the mean SST state in tropical Pacific where ENSO occurs, leading to the asymmetry of ENSO’s intensity and frequency in different PDO phase. Recent studies also found an asymmetric response of the stratospheric temperature to ENSO [15–17]. It was reported that El Niño has a stronger impact on the northern winter stratosphere than La Niña does. Our result also shows a stronger stratospheric response to El Niño than to La Niña, but this conclusion may be only true when the PDO index is high. This asymmetry can be possibly reversed if the PDO index is low. Moreover, the modulation of PDO on the stratospheric ENSO pathway may explain the weakening of the stratospheric El Niño signal in recent decades [47]. Atmosphere 2019, 10, 211 13 of 16

Actually, the PDO is not the only SST mode that can modulate the interdecadal change of ENSO teleconnection. The Atlantic multidecadal oscillation can also affect the tropospheric ENSO’s impact on the global climate, so its modulation on the stratospheric ENSO teleconnection also deserves further investigation in the future study. Although we use the NCEP/NCAR reanalysis, which is longer than other reanalyses, the sample size for different ENSO/PDO configurations is still small, especially for El/N and La/P configurations. However, our results are highly consistent with a previous modeling study by Kren et al. [63]. Using a 200-year preindustrial simulation by WACCM [63], a relationship between the positive PDO and the weak stratospheric polar vortex is established. Based on a 2000-year simulation dataset, the modulation of PDO on the stratospheric ENSO teleconnection might be confirmed in our multi-model study (Rao et al., submitted to Journal of Climate).

Author Contributions: Conceptualization, J.R. and R.R.; methodology, J.R.; software, J.R. and X.X.; validation, C.S. and D.G.; formal analysis, J.R. and R.R.; investigation, J.R.; writing—original draft preparation, J.R.; writing—review and editing, R.R.; visualization, X.X.; supervision, R.R.; project administration, R.R. and C.S. Funding: This work was jointly supported by grants from the National Natural Science Foundation of China (41705024, 41875048), the Strategic Priority Research Program of Chinese Academy of Sciences (XDA17010105), the National Key R&D Program of China (2016YFA0602104), and the Startup Foundation for Introducing Talent of NUIST (2016r060). Acknowledgments: We acknowledge the NCEP/NCAR (https://www.esrl.noaa.gov/psd/data/gridded/data.ncep. reanalysis.pressure.html) and Hadley Centre (https://www.metoffice.gov.uk/hadobs/hadisst/data/download.html) for providing the reanalysis and SST, respectively. Conflicts of Interest: The authors declare no conflict of interest.

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