Air–Sea Coupling Enhances the East Asian Winter Climate Response to the Atlantic Multidecadal Oscillation

ZHOU Xiaomin,LI Shuanglin,LUO Feifei,GAO Yongqi,and Tore FUREVIK

1Nansen-Zhu International Research Centre and Climate Change Research Center,Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing 100029

2Joint Center for Global Change Studies,Beijing 100875

3University of Chinese Academy of Sciences,Beijing 100049

4Geophysical Institute and Bjerknes Center for Climate Research,University of Bergen,Bergen,Norway

Air–Sea Coupling Enhances the East Asian Winter Climate Response to the Atlantic Multidecadal Oscillation

ZHOU Xiaomin1,2,3,LI Shuanglin?1,2,LUO Feifei1,2,GAO Yongqi1,and Tore FUREVIK4

1Nansen-Zhu International Research Centre and Climate Change Research Center,Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing 100029

2Joint Center for Global Change Studies,Beijing 100875

3University of Chinese Academy of Sciences,Beijing 100049

4Geophysical Institute and Bjerknes Center for Climate Research,University of Bergen,Bergen,Norway

A simple air–sea coupled model,the atmospheric general circulation model(AGCM)of the National Centers for Environmental Prediction coupled to a mixed-layer slab ocean model,is employed to investigate the impact of air–sea coupling on the signals of the Atlantic Multidecadal Oscillation(AMO).A regional coupling strategy is applied,in which coupling is switched off in the extratropical North Atlantic Ocean but switched on in the open oceans elsewhere.The coupled model is forced with warm-phase AMO SST anomalies,and the modeled responses are compared with those from parallel uncoupled AGCM experiments with the same SST forcing.The results suggest that the regionally coupled responses not only resemble the AGCM simulation,but also have a stronger intensity.In comparison,the coupled responses bear greater similarity to the observational composite anomaly.Thus,air–sea coupling enhances the responses of the East Asian winter climate to the AMO.To determine the mechanism responsible for the coupling amplif i cation,an additional set of AGCM experiments, forced with the AMO-induced tropical SST anomalies,is conducted.The SST anomalies are extracted from the simulated AMO-induced SST response in the regionally coupled model.The results suggest that the SST anomalies contribute to the coupling amplif i cation.Thus,tropical air–sea coupling feedback tends to enhance the responses of the East Asian winter climate to the AMO.

amplif i cation,Atlantic Multidecadal Oscillation,air–sea coupling feedback,East Asian winter climate

1.Introduction

As a type of internal variability of the natural climate system linked to the thermohaline circulation(Delworth and Mann,2000;Knight et al.,2006),the leading decadal variability mode of the Atlantic basin-scale SST,the Atlantic Multidecadal Oscillation(AMO)has been found to have signifi cant impact on global or regional climate(Kerr,2000). It is associated with North American and European summer climate(Sutton and Hodson,2005,2007),Atlantic hurricanes (Goldenberg et al.,2001;McCabe et al.,2004),African Sahel rainfall(Zhang and Delworth,2006),and even Asian summer rainfall(Goswami et al.,2006;Lu et al.,2006;Li et al.,2008; Wang et al.,2009;Luo et al.,2011).

Whether and how the AMO inf l uences East Asian winter climate is the key to understanding the contribution rate of internal natural variability to the substantial warming during recent decades,since the warming is strongest in winter and the AMO accounts for the largest fraction of observed winter surface air temperature Luo and Li(2014). It is diff icult to address this issue by observational analyses alone,due to the insuff i cient lengths of instrumental records.Thus,sensitivity experiments with atmospheric general circulation models(AGCMs)are often used.For example,Sutton and Hodson(2005)used an AGCM to study the AMO’s impact on Northern Hemispheric climate.Li and Bates(2007)(LB07 hereafter)conducted experiments with three AGCMs and found that positive-phase AMO causes warmer winters in East Asia.Wang et al.(2009)extended LB07 and showed a similar effect in the three other seasons. Although the simulated East Asian winter responses in these AGCMs are qualitatively consistent with the observed composite anomaly,further scrutiny suggests evident differences. For example,in observations,the strongest surface air temperature(Ts)anomaly is located in central China(～100?E),while the strongestTsresponse in models is located in East China[～115?E.cf.Figs.2a and 4e with Figs.9a,e and i in Wang et al.(2009)].Also,the simulated warming amplitude in China is about 0.35?C[a response of 0.7?C to the doubled AMO SST anomaly(SSTA);Fig.9 in Wang et al.(2009)], which is about one half of the observedTsanomaly linked to the AMO warm phase[～0.6?C;see Fig.4 in Wang et al. (2009)].What causes the gap between AGCM results and the observed anomalies is unclear.One candidate factor may be the exclusion in AGCMs of air–sea coupling processes, although other internal factors of the climate system like the Arctic Oscillation(Gong et al.,2001)and the Pacific Decadal Oscillation(Yu et al.,2014),or external forcing like volcanic eruptions(Wang et al.,2013b),may also play a role.

The air–sea interaction within the adjacent oceans of the Asian–Australian–Pacific monsoonal region(the western subtropical Pacific and the tropical Indian Ocean)is of importance in modulating the monsoonal variability(Wang et al.,2005;Wu et al.,2006).AGCMs or coupled models failing to capture realistic air–sea interaction tend to yield a weakened or distorted monsoon response to forcing.Thus, air–sea coupled models capable of capturing the observational air–sea variability may be more useful for simulating the Asian climate responses to the AMO.This consideration is the primary motivation behind the present study.Here, we use a simple air–sea coupled model,the AGCM of the National Centers for Environmental Prediction(NCEP)coupled to a mixed-layer slab ocean model,to re-examine the AMO’s impact.It is found that the coupled model reproduces the observed intrinsic variability better than the AGCM,although the latter exhibits high skill.We emphasize the results from the coupled model,since the results from the uncoupled model and observations have been documented in detail by LB07 and Wang et al.(2009).

The paper is organized as follows.Section 2 describes the models and the experimental design.Section 3 analyzes the model’s intrinsic variability and reports the modeled responses in the coupled model.The results are compared with those from the parallel AGCM experiments,and also with observational composites.Because the coupled responses are evidently stronger than the uncoupled AGCM simulation, an additional set of diagnostic AGCM experiments are conducted to understand the reasons for the coupling amplif i cation,the results of which are presented in section 4.Finally, a summary and discussion is provided in section 5.

2.Models and experimental design

2.1.Air–sea coupled model and experiments

The air–sea coupled model is an AGCM coupled to a mixed-layer slab ocean model with a f i xed 50 m mixed-layer depth based on the flux-correction scheme(Peng et al.,2005; Li et al.,2006).A regional coupling scheme is adopted in which coupling is switched off within the extratropical North Atlantic Ocean but switched on in the open oceans elsewhere.In other words,air–sea coupling is permitted only in the open oceans beyond the extratropical North Atlantic.The aim of this coupling strategy is to isolate the impact of the atmosphere–ocean interaction,outside the North Atlantic,on the AMO-forced direct atmospheric signals.A similar coupling strategy has been used in previous studies(Lu et al., 2006;Shukla and Kinter III,2014).The AGCM,consisting of the atmospheric component of the coupled model,is an earlier version of the NCEP’s Global Forecast System for seasonal prediction.It is confi gured with 28 vertical levels and has a horizontal resolution corresponding to a T42 spectral truncation(Peng et al.,2002,2003).

We perform two sets of coupled ensembles,each with 60 members.Ensemble one,referred to as the“control ensemble”,is formed from the runs forced with the climatological SST seasonal cycle in the North Atlantic.Ensemble two,referred to as the“AMO ensemble”,is from the runs with the warm-phase AMO SST anomaly pattern added on to the SST climatology.The AMO SST anomaly pattern is extracted from the difference of the averaged annual SST during the warm AMO phase(1935–55)minus that during the cold phase(1970–90)in the North Atlantic basin(0?–60?N,75?–7.5?W)(Fig.1).As in LB07,the Kaplan Extended SST dataset(version 2)from 1870–2014 is used to obtain the AMO SSTA.

All members in the above ensembles start from different initial fields and are integrated for 12 months from September to the following August.The 60 initial fields are from the NCEP–(NCAR:National Center for Atmospheric Research) reanalysis of 0000 UTC,1–3 September 1980–99(Kalnay et al.,1996).Thus,a total of 60 model years are available for analysisin each ensemble.Forone single variable,likeTs,the modeled response is expressed as the difference of the winter mean in the AMO ensemble minus that in the control ensemble.The Student’st-test is used to check the significance of the response.

2.2.Parallel AGCM experiments

The coupled simulation results from the above experiments,including both the intrinsic variability and the SST-forced response,are compared with corresponding parallel uncoupled AGCM runs.Here,the AGCM runs are adapted from Wang et al.(2009).The response in the AGCM is determined in the same way as described above.Since the only difference of the coupled model ensembles relative to the uncoupled is the utilization of coupling,such a comparison can isolate the air–sea coupling impact on the AMO-forced direct signals.

3.Results

3.1.Modeled intrinsic variability

Previous studies suggest that a model’s intrinsic variability modulates atmospheric responses to forcing.Whether a model realistically reproduces the observed atmospheric intrinsic variability is a vitalfactorforthe reliability ofmodeled results.Hence,we first examine and compare the models’intrinsic variabilities in SST and one large-scale atmospheric circulation variable,500 hPa geopotentialheight(Z500),with the observed.Here,the observational monthly SST data are from the Met Off i ce Hadley Centre(HadISST;Rayner et al., 2003),and the observational monthly Z500 data are from the NCEP/NCAR reanalysis(Kalnay et al.,1996).

Figure 2 compares the standard deviation of winter (December–January–February,DJF)averaged monthly SST in the regionally coupled model’s control runs with the observed.Overall,the regionally coupled model captures the general features of the observed SST variability north of 10?N.This can be seen clearly from the two maximum centers situated in the Kuroshio extension of the North Pacific and the Gulf Stream extension of the western North Atlantic, although the model’s maxima are slightly stronger and the model overstates the maximum in the northeastern Pacific and shifts it to the coast of Alaska.

Figure 3 displays the standard deviation of winteraveraged monthly Z500 and its leading EOF modes.First, both the models reproduce the observed standard deviation generally well,including the two maxima in the North Atlantic and the North Pacific,respectively.In comparison,the value in the regionally coupled model is relatively larger and thus closer to the observed than the uncoupled model.Second,the simulated leading EOFs of Z500 in both the models resemble the observed well,which can be seen from their substantial projection onto the Northern Hemisphere annular pattern.In comparison,the explained variance rate in the coupled model(～30%)is closerto the observed(～30%)than the uncoupled model(～33%).Thus,both the uncoupled and regionally coupled model reproduces the observed variability reasonably,and the regionally coupled model is relatively better than the uncoupled.

3.2.Coupled response and its comparison with the uncoupled and the observational composite

3.2.1.Tsand precipitation

Figures 4a and b show the responses ofTsand precipitation to the AMO SSTA.There are significantly warmerTsresponses over the whole of East Asia,especially mainland China,with an amplitude of about 1?C.In comparison,theTsresponse in east China is larger than that in western China. The warmer response in East Asia extends southwestward to India and eastward to the adjacent seas of the western North Pacific and Japan(Fig.4a).

By contrast,the precipitation responses in East Asia are relatively weaker(Fig.4b).There is precipitation intensif i cation in North China along with suppression in South China. The suppression signal is signifi cant in southwestern China and the northern Indo-China Peninsula.Besides,there is sub-stantial suppression over the western North Pacific and the tropical Indian Ocean,possibly linked to the AMO-induced SST responses there,which is discussed later.

In the uncoupled AGCM simulation(Fig.4c),a significantly warmer response is seen in East China and the Mongolian region,which bears an overall resemblance to the above coupled results.However,in comparison with the coupled model,the difference is still clear.First,the area with a warmerTsresponse does not extend as widely as the coupled result(cf.Figs.4a and c).Second,the response amplitude is weaker in the AGCM.The weaker response is clearly seen from the averagedTsvalue over the region(20?–50?N,100?–120?E).The value in the uncoupled model is 0.53?C,while it is 0.95?C in the coupled model—the latter almost double the former.Thissuggeststhatcoupling ampli fi estheTsresponses to the AMO by a factor close to two.

Consistent with theTsresponse,the precipitation response pattern in the uncoupled model bears an overall resemblance to the coupled model,including the south-driernorth-wetter dipolar structure in East China and the intensifi cation in the southern Tibetan Plateau(Figs.4b and d).In comparison,the response in the uncoupled model is weaker. The domain size with intensi fi cation in East China shrinks obviously,and the response strength in the tropical IndianOcean weakens greatly.The weaker precipitation response in the AGCM again suggests that the coupling amplif i es the atmospheric responses to the AMO.Despite an overall resemblance over the Asian continent,a substantial difference is seen over the open oceanic regions.This illustrates the importance of air–sea coupling in shaping the AMO’s signals there.

For further analysis,the modeled results are compared with observational composites.Slightly different from LB07, in which the observational composite is derived from the differences of the AMO warm period(1935/36 to 1955/56)minus the cold period(1970/71 to 1990/91),here,one more recent warm period(1995/96 to 2012/13)is additionally used. From Fig.5,the compositeTsor precipitation in this recent warm period is consistent with the earlier warm period.For example,Tsin both the warm periods has a large-scale southwarmer-north-colder dipolar pattern.This suggests that the AMO’s signals in the observation may be robust.

From the comparison of the modeledTsresponses with the observational composites(cf.Figs.4a and c with Figs.5a and d),an overall resemblance to each other is apparent.In comparison,theTsresponse in the regionally coupled model is closer to the observation,including its spatial pattern and amplitude.From the comparison of precipitation(cf.Figs.4b and d with Figs.5b and e),the dipolar pattern in southeastern China seen in both the coupled and uncoupled responses also emerges in the observational composites,albeit with a southward shift of its nodal line.Thus,the more evident responses in East China in the regionally coupled model are closer to the observation,and are perhaps more realistic relative to the uncoupled model.

3.2.2.Large-scale atmospheric circulation

The large-scale atmospheric circulation responses in the regionally coupled and uncoupled models are analyzed in this subsection.Figure 6 compares the Z500 and 1000 hPa geopotential height(Z1000)responses.The latter is used to ref l ect surface pressure characteristics.In the regionally coupled model,there is a zonally extended positive Z500 response across the subtropics,with the maxima located over the eastern subtropical North Atlantic and eastern Eurasia (Fig.5a).The latter weakens the climatological East Asian grand trough—an essential component of the East Asian winter monsoon system.Besides,there is a substantial negative Z1000 response over northern Eurasia(Fig.5b),which weakens the Siberia/Mongolian cold high—another core member of the winter monsoon system.Thus,the atmospheric circulation responses can explain the weakened East Asian winter monsoon and warmerTsin China.

The uncoupled response exhibits an overall resemblance to the coupled response.For example,the Z500 response also projects onto a positive-phase North Atlantic Oscillation(NAO).In spite of the similarity,certain differences are still identif i able.The uncoupled Z500 response has a smaller projection coeff i cient onto the NAO.Although the positive height responses over Eurasia are evident,their maximumshifts to the west,away from the climatological East Asian grand trough(Fig.6c).Similar to Z500,there are negative Z1000 responses extending from the northeastern Atlantic to northern Eurasia,albeit the Eurasian responses are less signifi cant(Fig.6d).

The modeled circulation responses are also compared with observationalcomposites.Figure 5(lowerpanels)shows the observational sea level pressure(SLP)anomaly composite between the aforementioned two AMO periods.There are signifi cant negative SLP anomalies extending from the northeastern North Atlantic to midlatitudinal East Asia in both the composites.This agreement suggests robustness in the observational signals.Besides,these negative SLP anomalies are accompanied by warmerTs.This connection appears physically reasonable,because the negative SLP anomalies over the Asian continent weaken the winter Mongolian cold high and cause warmerTsin China.Comparing the modeled and observed Z1000(Fig.6)reveals similarities;however, the coupled response is relatively closer to the observational composites.

Figures 7a and c compare the 850 hPa wind responses.In the regionally coupled model,a positive AMO SSTA leads to a substantial westerly response over the tropical western Pacific,but an easterly response in the western North Pacific(Fig.7a).Over coastal East Asia,it causes an evident southerly response in East China,which corresponds to a weakened winter monsoon.On a larger scale,there is an anomalous cyclone over the subtropical western Pacific along with an anticyclone in the northern Pacific.The easterly in the northern f l ank of the cyclone transports more moisture to the west,enhancing precipitation over eastern coastal China.

The above circulation response is clearer in the 850 hPa stream function(Fig.7b).In the subtropical western Pacific, there is an anomalous cyclone corresponding to the above cyclonic wind anomalies.In the north,there is an anomalous anticyclone that extends from the Sea of Japan to the northern North Pacific.Such an anticyclonic anomaly along coastal northern East Asia may lead to westward extension of the climatological western Pacific subtropical anticyclone (Lu,2001).This is conducive to a weakened East Asian winter monsoon and contributes to more precipitation in eastern coastal China(Fig.4b).

In the AGCM,the easterly response over the tropical western Pacific expands toward the southeastern coastal area of Asia(Fig.7c),but is much weaker in comparison with the coupled response.The southerly response over East China is less clear.Corresponding to the wind response,the stream function response over the western Pacific weakens and becomes less signifi cant(Fig.7d).The anomalous anticyclonic center along coastal northern East Asia disappears,and the cyclonic anomaly over the western North Pacific shrinks.All the differences are in agreement with the weakened response in the uncoupled model.

4.AGCM-simulated responses to the AMO-induced tropical SSTA:impact of air–sea interaction

The above response difference between the regionally coupled and uncoupled simulations should originate from the air–sea feedback,because the only difference between the simulations is the switching off of coupling in the uncoupled experiment.The surface flux response can provide clues as to the existence of such processes.Figures 8a and b compare the simulated AMO-induced SST response with the observed SST composite anomaly.There are large-scale warmer SST responses in the central North Pacific meridionally extending to the central tropical-southern Pacific(Fig. 8a).The warmer SST response is zonally sandwiched by two large cooling centers:one in the subtropical western Pacific expanding to the northern Indian Ocean,and the other in the tropical eastern Pacific extending from the northeastern to southeastern Pacific.In general,the basin-scale SST response pattern resembles the observational composite SST anomalies(cf.Figs.8a and b).

Figure 8c displays the response of surface heat flux, which may provide indications about the formation of the above SST responses.First,the heat flux responses over the northern Atlantic are upward(positive),which is understandable because the AMO SSTA is prescribed there and provides an inf i nite heat source to the atmosphere above.The downward flux over the western North Pacific suggests that the warmer SST response therein is forced by the atmosphere above.Corresponding to their individual SST responses in the western and central tropical Pacific,there are overall opposite-sign heat fluxes,indicating the existence of atmospheric forcing.Thus,the warm AMO results in these Indo-Pacific SST responses through atmosphere–ocean interaction processes.This is in agreement with Dong et al.(2006),who revealed the criticalrole ofair–sea coupling feedback in caus-ing the remote response of wind and SST to a warm North Atlantic SST pattern.

Considering the primary importance of the tropical SST in inducing the atmospheric responses,we investigate the role of AMO-induced tropical Indo-Pacific SST anomalies in forcing the coupled responses.We conduct an additional set of ensemble runs in the AGCM with the AMO-induced SST forcing.Similar to the AGCM experiments,the monthly SSTresponses over the tropical Indo-Pacific(30?S–30?N)(Fig.9) are overlapped onto the seasonally climatological SST cycle. With the 20 differentinitialfields from 1 September1980–99, a total of 20 runs are conducted.

Figure 10 shows the AGCM-simulatedTsand precipitation responses.TheTsresponse pattern exhibits warming in much of China,which is consistentoverallwith the difference of the regionally coupled run minus the uncoupled run(cf. Figs.10a and c).This is further seen from their substantial spatial correlation(～0.49).In addition toTs,the enhanced precipitation response over northern East China is also consistent with the difference of the model responses(cf.Figs. 10b and d),with a spatial correlation coeff i cient of 0.55.This is even clearer from the rainfall suppression in the southern coastal area of China and the Indian–western Pacific Ocean.

For the atmospheric circulation,Z500 possesses wave train–like characteristics over the northern Atlantic(Fig. 11a),with a negative response extending to the polar region. Over the East Asian coastal area,there are positive responses, which weaken the climatological East Asian grand trough. The Z1000 response shows a similar wave-train structure over the Atlantic,along with negative anomalies over Eurasia.These responses also resemble the difference between the coupled and uncoupled responses.

From the 850 hPa wind and stream function(Fig.12), over coastal East Asia and the western Pacific,the SSTA leads to a response pattern similar to the modeled response difference to the AMO in the regionally coupled and uncoupled models.For example,there is a southerly response from the South China Sea to East China(Fig.12a),corresponding to the southerly anomaly in the difference(Fig.12c).In response,the easterly over the subtropical western Pacific (～30?N)and the westerly in the tropical western Pacific(0?–5?N)also have counterparts in the difference.This similarity is even clearer in the stream function.There is a signifi cant cyclone in the subtropical central-western Pacific along withan anticyclone in the western boundary seas,which is apparent in both Figs.12b and d.Thus,air–sea coupling feedback may indeed have contributed to the amplif i ed response in the coupled model.

5.Summary and discussion

A simple air–sea coupled model,the AGCM of the NCEP coupled to a mixed-layer slab ocean model,is employed to investigate the impact of air–sea coupling on the signals of the AMO.A regional coupling strategy is applied,in which coupling is switched off in the extratropical North Atlantic Ocean but switched on in the open oceans elsewhere.Two sets of ensemble experiments are conducted by forcing the model with the climatological SST seasonal cycle or with the warm-phase AMO SST anomalies overlapping onto the climatological SST seasonal cycle.Parallel uncoupled AGCM experiments conducted previously are adapted for comparison.

First,the model’s atmospheric intrinsic variability is compared with the observed,and the results suggest that the both the coupled and uncoupled models can reproduce the observed atmospheric intrinsic variability well.In comparison,the coupled model performs slightly better than the uncoupled model.Then,the modeled responses in the regionally coupled model are compared with those in the parallel AGCM.The resultssuggestthatthe regionally coupled model not only yields qualitatively similar atmospheric responses to the uncoupled model,butalso enhances the responses’amplitude.A comparison with the observational composites from two different AMO warm periods suggests that the regionally coupled response is closer to the observation.This implies that the air–sea interaction amplif i es the pure atmospheric responses to the AMO.The amplif i cation may be realistic,considering the improved performance of the coupled model in simulating the observed atmospheric intrinsic variability.The amplif i cation is also verif i ed by an additional set of AGCM experiments,in which the AGCM is forced with the AMO-induced SSTA over the tropical Indian–Pacific Ocean.Thus the air–sea coupling feedback over the tropical ocean may play the key role in enhancing the atmospheric response to the AMO.

Previous multiple-AGCM studies have shown that the AMO SSTA induces a 0.35?CTswarming response in China [0.7?C response to the double enhanced AMO;Fig.9 in Wang et al.(2009)].Such a magnitude is about one half of the observedTsanomaly linked to the AMO warm phase in instrumental records[～0.6?C;see Fig.4 in Wang et al. (2009)].A recent observational and modeling study demonstrated that the AMO can explain most of the observedTsdecadal variance in the past century(Luo and Li,2014).This illustrates that previous AGCM experiments may have underestimated theTsresponse in the actual climate system.The factthatair–sea coupling intensif i esthe atmospheric response provides one possible explanation for the gap between the observational composite and the AGCM simulated responses. These results further confi rm the substantial inf l uence of the AMO on East Asia,as suggested by instrumental records and recent proxy data studies(LB07;Wang et al.,2013a,2014).

In previous studies(LB07;Wang et al.,2009),a mechanism was proposed to explain the AMO’s impact on East Asia.The warm-phase AMO directly heats the atmosphere above.The heat received by the overlying atmosphere is advected and transported downstream by midlatitudinal westerly f l ow.The AMO-induced heating also results in an atmospheric dynamical response,which may act as a Rossby Wave source to activate energy to propagate downstream. Both these two inf l uences heat the atmosphere downstream, which will result in a warmer mid-upper tropospheric atmosphere over Eurasia,causing a weakened land–sea thermal contrast in winter and thus a weakened East Asian winter monsoon.Because only atmospheric thermodynamical processes are involved,this mechanism may be referred to as an“atmospheric bridge”.In the present study,another mechanism,which emphasizes the role of air–sea coupling,and can thus be referred to as an“air–sea coupling bridge”,is revealed.It provides a supplemental explanation of the AMO’s impact on East Asia.

Since the coupled model used here is a simple one based on the flux-correction scheme,implicitly,it only includes a fraction of the oceanic dynamical processes.Previous studies have shown that ocean dynamics also play a role in extending the AMO’s climate impact.For example, Zhang and Delworth(2005)used the global coupled ocean–atmosphere model of the Geophysical Fluid Dynamics Laboratory(CM2.0)and found that the AMO-related Atlantic thermohaline circulation can cause a southward shift of the Intertropical Convergence Zone over the Atlantic and Pacific, an El Ni～no–like pattern in the southeastern tropical Pacific, and weakened Indian and Asian summer monsoons.Wu et al.(2005),Dong et al.(2006)and Chen et al.(2010)utilized a coupled general circulation model and revealed that anomalous Atlantic SSTs can modulate ENSO.These studies illustrate that ocean dynamics may indeed inf l uence the AMO’s signals in the East Asian winter climate.The relative importance of the air–sea flux exchange and detailed oceanic dynamics in enhancing the AMO’s signals is,however,unclear and requires further study.

Acknowledgements.This study was jointly supported by the strategic project of the Chinese Academy of Sciences(Grant No. XDA11010406)and the National Natural Science Foundation of China(Grant Nos.41375085 and 41421004).

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10.1007/s00376-015-5030-x.

26 January 2015;revised 28 May 2015;accepted 4 June 2015)?

LI Shuanglin Email:shuanglin.li@mail.iap.ac.cn