Asian Summer M onsoon Onset in Simulations and CM IP5 Projections Using Four Chinese Climate M odels

ZOU Liweiand ZHOU Tianjun

State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

Asian Summer M onsoon Onset in Simulations and CM IP5 Projections Using Four Chinese Climate M odels

ZOU Liwei∗and ZHOU Tianjun

State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

The reproducibility and future changes of the onsetof the Asian summermonsoon were analyzed based on the simulations and projections under the Representative Concentration Pathways(RCP)scenario in which anthropogenic em issions continue to rise throughoutthe 21stcentury(i.e.RCP8.5)by all realizations from four Chinese models thatparticipated in the Coupled Model Intercomparison Project Phase 5(CM IP5).Delayed onset of the monsoon over the Arabian Sea was evident in all simulations for present-day climate,which was associated w ith a too weak simulation of the low-level Somali jet in May. A consistent advanced onsetof the monsoon was found only over the Arabian Sea in the projections,where the advanced onset of the monsoon was accompanied by an increase of rainfall and an anomalous anticyclone over the northern Indian Ocean.In all the models exceptFGOALS-g2,the enhanced low-levelSomali jet transported more watervapor to the Arabian Sea,whereas in FGOALS-g2 the enhanced rainfallwas determined more by the increased w ind convergence.Furthermore, and again in allmodels except FGOALS-g2,the equatorial SST warm ing,w ith maximum increase over the eastern Pacific, enhanced convection in the centralWestPacific and reduced convection over the eastern Indian Ocean and Maritime Continent region,which drove the anomalous anticyclonic circulation over the western Indian Ocean.In contrast,in FGOALS-g2,there was m inimal(near-zero)warm ing of projected SST in the centralequatorialPacific,w ith decreased convection in the central West Pacific and enhanced convection over the Maritime Continent.The broader-scale differences among the models across the Pacific were related to both the differences in the projected SST pattern and in the present-day simulations.

Asian summer monsoon,onset,climate projection,Chinese climate models

1.Introduction

The Asian monsoon system is one of the mostpowerfulin the world.Its evolution and variability affectnotonly the localagriculture,ecosystems,economics and society,butalso the climate on a global scale(e.g.Ding and Wang,2005). The Asian monsoon is characterized by a strong annual cycle w ith a rainy summer and dry w inter.Understanding and forecasting changes of the monsoon’s onset is of great importance,since a late orearly onsetof the monsoon may have significant impacts on agriculture(Websteretal.,1998).

Recentstudieson the monsoon’schangeshave shown that the Asian summermonsoon exhibits a salientadvance by 2–4 pentads after 1993/94,extending from the Bay of the Bengal to the South China Sea(Kajikawa etal.,2012).Advances in monsoon onsetover the Arabian Sea and Bay of the Bengal have also been revealed(Wang etal.,2012;Yu etal.,2012). The causesofsuch advancesin Asian monsoon onsetare still under debate.Kajikawa etal.(2012)and Wang etal.(2012) ascribed itto enhanced heatcontrastbetween the Asian continentand the tropical Indian Ocean,while Kajikawa and Wang (2012)suggested that SST warm ing over the western Pacific is the key factor that favors advanced monsoon onsetover the South China Sea.A recentstudy argues that the advances in Asian summer monsoon onset during the last three decades are closely tied to the decadal mean state change in the Pacific,characterized by a distinct La Ni˜na-like pattern(Xiang and Wang,2013).

Climate system modelshave been w idely employed to reproduce the annual cycle of the Asian monsoon.For example,the climatological intraseasonal oscillation of precipitation over the Asian–western Pacific region was reasonably reproduced in a multi-model ensemble(MME)of 10 atmospheric general circulation models(AGCMs)(Kang et al., 2002).Sperber et al.(2013)compared the performance of 25 Coupled Model Intercomparison ProjectPhase 5(CM IP5) models and 22 CM IP Phase 3(CM IP3)models in simulating the Asian summer monsoon at various time scales.Overall, the CM IP5 MME mean showed better performance in simu-lating the onsetof the monsoon than the CM IP3 MME mean in terms of the pattern correlation,but the onsetof the monsoon over India was typically too late in both MMEs.

Information regarding future changes of monsoon onset under global warm ing scenarios is helpful for decisionmaking and impactstudies.Although the observed onsetof the Asian summer monsoon has exhibited an advance in recentdecades,projected future changes do notshow the same features.For example,based on projections under the SpecialReporton Em issions Scenarios(SRES)A1B scenario derived from 19 CM IP3 models,Inoue and Ueda(2011)found that the onsetofmonsoon over the Bay of the Bengal,the Indochina Peninsula and the South China Sea could be delayed by 5 to 10 days by the end of the 21st century,associated w ith a delay in the reversalof upper-tropospheric meridional thermalcontrast.Zhang etal.(2012)found thatmonsoon onset tends to delay over the tropical Indian Ocean,Maritime Continent and Indochina Peninsula in the majority of models under warm ing scenarios.Importantly,the projection of changes in monsoon onsetis sensitive to the way in which the monsoon onsetis defined.Kitoh etal.(2013)analyzed the future projections under RCP scenarios of 29 CM IP5 models, and the resultssuggested thatmonsoon retreatw illbe delayed butmonsoon onsetw illeither advance or show no change at the end of 21stcentury.

Recognizing the importance of climate models in climate predictions and projections,the Chinese climate research community has been involved in the developmentof climate models since the late 1970s.Zhou et al.(2014a)reviewed the development of coupled earth/climate system models in China during approximately the past20 years from the CM IP perspective.There was only one Chinese climate model that participated in CM IP1;however,in the latest phase of the project(CM IP5)there are five models developed in China, and there w illbe even more climate models from China participating in CM IP6 in the near future(Zhou et al.,2014a). To comprehensively evaluate the performance of currentChinese earth/climate system models,several climate modeling centersin China have jointly initiated a collaborative research project to further encourage collaboration in future model developments.The general performances of Chinese models have been systematically assessed using some observational metrics in Zhou et al.(2014b).The models broadly show reasonable performance in simulating the SST mean state,seasonalcycle,the spatialpattern ofMadden–Julian oscillation amplitude and tropical Genesis Potential Index,the globalmonsoon precipitation pattern,and the SST anomalies related to the ElNi˜no–Southern Oscillation(ENSO)and Pacific DecadalOscillation(Zhou etal.,2014b).However,the simulations and projections of Asian summermonsoon onset in these Chinese models have yet to be evaluated and compared.We address this know ledge gap in the current paper, and also include an example of moisture budget analysis to examine the monsoon onsetchanges overa specific region.

The remainderof the paper is organized as follows.Section 2 describes the data and methods.The results,including historical simulations of monsoon onset and future projections,are presented in section 3.Section 4 summarizes the major findingsof the study and providessome furtherdiscussion.

2.Data and methods

2.1.Modeldata and observational data

Historicalsimulations from 1981–2005 derived from four Chinese models that participated in CM IP5 are used.These fourmodels are BCC-CSM 1-1 from the Beijing Climate Center,China Meteorology Administration(Wu et al.,2010, 2014),BNU-ESM from Beijing Normal University(Wu et al.,2013),and two versions of the FGOALS2 model,viz. FGOALS-g2(Li et al.,2013)and FGOALS-s2(Bao et al., 2013),from the Institute of Atmospheric Physics(IAP), Chinese Academy of Sciences.These four models have comparable spatial resolutions in their atmospheric components,w ith FGOALS2 having a slightly higher resolution. FGOALS-g2 and FGOALS-s2 share the same oceanic component,while BCC-CSM and BNU-ESM employ different versions of the same oceanic model.More detailed model information can be found in Table 1.

The future projections by these four models under the Representative Concentration Pathways(RCP)scenario in which anthropogenic em issions continue to rise throughout the 21st century,resulting in a radiative forcing of 8.5 W m-2(i.e.RCP8.5),are employed to investigate the possible future changes in the Asian summermonsoon.We focus on the last25 years of the 21stcentury(2075–2099)and use all the available realizations of these models.For FGOALS-s2,three realizations are employed,while for the other three models one realization is available.The three realizations of FGOALS-s2 are referred to as“FGOALS-s2 r1”,“FGOALS-s2 r2”,and“FGOALS-s2 r3”,respectively.

The follow ing datasets are used to evaluate the models’performances in terms of the present-day climate:(1)pentad precipitation data from the Global Precipitation Climatology Project(GPCP;Adler et al.,2003);(2)pentad precipitation derived from the Climate Prediction Center(CPC) Merged Analysis of Precipitation(CMAP)(Xie and Arkin, 1997);(3)observational SST derived from the Hadley Centre Global Sea Ice and Sea Surface Temperature(Had ISST) dataset(Rayner et al.,2003);(4)monthly mean circulation fields[e.g.,zonalw ind(u),meridionalw ind(v),specific humidity(q)]derived from the Interim European Centre for Medium-Range Weather Forecasts(ECMWF)Re-Analysis (ERAIM)(Dee et al.,2011).For simplicity,the rainfall datasetand the reanalysis-derived circulation datasetare referred to as“observation”in the follow ing discussion.

2.2.Methods

Monsoon onset is defined follow ing the method of Wang and Lin(2002).The pentad time series of precipitation are fi rstly smoothed w ith a five-pentad running mean,and then the January mean rainfall is subtracted from each pentad to obtain a relative rainfall rate series.Monsoon onsetis definedas the fi rst pentad in which the relative rainfall rate exceeds 5 mm d-1during May–September.This definition has been w idely used in studies on monsoon annualcycles(e.g.Xiang and Wang,2013;Sperber etal.,2013;Kitoh etal.,2013).

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3.Results

3.1.Evaluation of simulated Asian monsoon onset for present-day climate

Figure 1 shows the spatial distributions of pentads of Asian monsoon onsetderived from simulations and observation.In the observation,the monsoon is fi rstestablished over Southeast Asia(Matsumoto,1997),and then over the South China Sea and to the southwest of the Indian subcontinent. Later,the South China Sea monsoon gradually progresses northward to North China.The finalestablished monsoon is the western North Pacific summer monsoon.These characteristics are discussed at length in Wang and Lin(2002).The featuresare consistentbetween GPCP and CMAP,w ith a spatial pattern correlation coefficient(SCC)of 0.76.However, compared w ith GPCP,the extension of the western North Pacific monsoon in CMAP is farther eastward and the associated onsetoccurs earlier.

In all six experiments,the simulated monsoon onset fi rstly(finally)occurs over Southeast Asia(western North Pacific).These features are consistentw ith the observations. FGOALS-g2 has the highestmodelskillamong these experimentsin termsof SCC(Fig.1).However,FGOALS-g2 lacks any onsetovermuch of India,SoutheastAsia,and the South China Sea.The simulated monsoon onsets by BNU-ESM, FGOALS-g2 and FGOALS-s2 are later than in the observation over the western North Pacific and SoutheastAsia,while they are earlier in BCC-CSM.Some common biases are also evident in the four modes,i.e.the underestimated(overestimated)extension ofmonsoon overeastern China,Korea,and Japan(western Pacific Ocean).These biases are also found in mostother CM IP5 models(Sperber etal.,2013).

It is found that the monsoon onset over the Arabian Sea (5◦–15◦N,65◦–75◦E)occurs later than thatobserved in allsix experiments.The regional averaged onset over the Arabian Sea is pentad 29.6(28.9)in GPCP(CMAP),while it is later than pentad 31 in all six experiments(33.6 in BCC-CSM, 36.2 in BNU-ESM,31.6 in FGOALS-g2,31.4 in FGOALS-s2 r1,31.9 in FGOALS-s2 r2,and 31.3 in FGOALS-s2 r3). Since the observed monsoon onsetmainly occursin late-May, examining the simulated mean circulation field in May is helpful for understanding why all the models tend to simulate a delayed monsoon onset.Figure 2 shows the spatialdistributions of 850-hPa low-levelw ind and rainfall in May derived from observation and the corresponding biases derived from the simulations.In the observation,the low-level Somalicross-equatorial flows(CEFs)reach north of 10◦N(Fig. 2a),while the simulated Somali CEFs mainly locate south of 5◦N and the associated intensities are too weak compared w ith the observation(figures notshown).Among the six experiments,BNU-ESM has the weakest SomaliCEF,and theevident northeasterly biases are found over the northern Indian Ocean(Fig.2c).Since the Somali CEF plays a dominant role in moisture transport,the weakly simulated Somali CEFs lead to dry biases over the Arabian Sea in all six experiments(Figs.2b–g),leading to a delayed monsoon onset over the Arabian Sea.

Fig.1.Spatialdistributions of the Asian summer monsoon onsetpentad derived from(a)GPCP,(b)CMAP,(c) BCC-CSM,(d)BNU-ESM,(e)FGOALS-g2,(f)FGOALS-s2 r1,(g)FGOALS-s2 r2,and(h)FGOALS-s2 r3.“r1”,“r2”,and“r3”denote the three realizations of FGOALS-s2,respectively.The data of coupled models are derived from the 20th century historical climate simulations during 1981–2005.The results are presented w ith the grid of individual model/observation.Note that pentad 30 is the end of May.The numbers shown in the upper-right cornerof each plotare the spatialpattern correlation coefficients w ith GPCP data and w ith CMAP data,the latter being the number in brackets.

The weak SomaliCEFs in the simulations are associated w ith Arabian Sea cold SST biases.Figure 3 shows the spatial distributions ofsimulated SST biases in borealspring w ith respect to HadISST.The cold SST biases over the Arabian Sea are evident in allsix experiments.Levine etal.(2013)examined the extentand impactof cold SST biases in the northern Arabian Sea in the CM IP5 MME.The results showed that such cold SST biases are common in CM IP5 models,and a clear relationship exists between Arabian Sea cold SST biases in boreal spring and weak monsoon rainfall.The Arabian Sea cold SST biases reduce the moisture fluxes over the Arabian Sea(Levine etal.,2013),which w ill lead to decreases of rainfalland favordelayed monsoon onsets.

Fig.2.(a)Spatial distributions of rainfall(shading,mm d-1)and 850-hPa low-level w ind in May averaged during 1981–2005 derived from GPCP and ERA-interim,and the corresponding simulated biases(simulations m inus the observations)derived from(b)BCC-CSM,(c)BNU-ESM,(d)FGOALS-g2,(e)FGOALS-s2 r1,(f) FGOALS-s2 r2,and(g)FGOALS-s2 r3.

3.2.Projected changes of monsoon onset under RCP 8.5

Figure 4 shows the projected changes of Asian monsoon onset for the period 2075–2099 under the RCP8.5 scenario relative to 1981–2005.There are some changes in the monsoon domain based on the definition of monsoon onset.At fi rstsight,consistency among allsix experiments w ith regard to the projected changes of monsoon onset is low.For instance,all realizations of FGOALS-s2 project an advanced monsoon onset over the western North Pacific,while the other three models overall exhibit a delayed monsoon onset.For the Indian subcontinent,advanced monsoon onset is found in BCC-CSM,FGOALS-g2,and FGOALS-s2 r1, while a delayed monsoon onset is prom inent in BNU-ESM, FGOALS-s2 r2 and r3.For the region of Vietnam/Cambodia,delayed onset is found in FGOALS-g2 and the three realizations of FGOALS-s2,while such a signal is not evident in BCC-CSM and BNU-ESM.

Fig.3.Spatialdistributions ofsimulated SST biases(◦C)in borealspring w ith respect to Had ISST derived from (a)BCC-CSM,(b)BNU-ESM,(c)FGOALS-g2,(d)FGOALS-s2 r1,(e)FGOALS-s2 r2,and(f)FGOALS-s2 r3.

Based on the results shown in Fig.4,Fig.5 further depicts the regional averaged changes in monsoon onset over the Arabian Sea(5◦–15◦N,65◦–75◦E),Bay of Bengal(10◦–20◦N,85◦–95◦E),Southeast Asia(8◦–20◦N,100◦–110◦E), and the western North Pacific(10◦–20◦N,125◦–160◦E)in the results of the six experiments.The monsoon over the Arabian Sea tends to be established one to two pentads earlier than at present,which is a consistent signal among the six experiments.For the monsoon over the Bay of Bengal,delayed onset is found in BNU-ESM and FGOALS-s2 r2,while advanced onset is found in the other experiments, w ith the largestadvance found in FGOALS-g2.For Southeast Asia,the monsoon onset is projected by FGOALS-g2 and FGOALS-s2 to become delayed.Delayed monsoon onset over the Indochina Peninsula has also been found in previous studies based on CM IP3 projections(Inoue and Ueda, 2011;Zhang etal.,2012).In addition,the western North Pacific monsoon tends to be established earlier in FGOALS-s2, while it tends to be later in FGOALS-g2.In the follow ing discussion,we exam ine why the models tend to project an advanced monsoon onsetover the Arabian Sea.

3.3.Moisture diagnosis of rainfall changes over the Arabian Sea

Since the monsoon onset over the Arabian Sea occurs in early-June(early-July)in BCC-CSM,FGOALS-g2, FGOALS-s2(BNU-ESM)for present-day climate,we focus on the projected changes of mean fields in May(June)in BCC-CSM,FGOALS-g2,FGOALS-s2(BNU-ESM)to exam ine the changes of large-scale circulations that favor an advanced monsoon onsetunder the RCP8.5 scenario.Figure 6 shows the projected changes of the 850-hPa w ind field and precipitation in May(June)from BCC-CSM,FGOALS-g2, FGOALS-s2(BNU-ESM)at the end of21stcentury.The results are also presented for the three realizations of FGOALS-s2.A pairofanomalous anticyclones is found over the Indian Ocean,w ith much greater strength over the northern Indian Ocean.The westerly anomalies associated w ith the northern branch of the anomalous anticyclone over the northern IndianOcean are accompanied by increased precipitation over the Arabian Sea,which favors advanced monsoon onsets since the definition of monsoon onset is solely based on precipitation.Note that the anomalous anticyclone is much weaker in FGOALS-g2 than in the othermodels.

Fig.4.Spatial patterns of projected Asian summer monsoon onset changes under the RCP8.5 scenario for the period 2075–2099 relative to 1981–2005(onsetpentad for 2075–2099 m inus onsetpentad for 1981–2005)derived from(a)BCC-CSM,(b)BNU-ESM,(c)FGOALS-g2,(d)FGOALS-s2 r1,(e)FGOALS-s2 r2,and(f) FGOALS-s2 r3.Negative values indicate earlier onset.

Fig.5.Regional averaged changes in monsoon onset over the(a)Arabian Sea(5◦–15◦N,65◦–75◦E), (b)Bay of Bengal(10◦–20◦N,85◦–95◦E),(c)Southeast Asia(8◦–20◦N,100◦–110◦E),and(d)western North Pacific(10◦–20◦N,125◦–160◦E)under the RCP 8.5 scenario derived from differentexperiments. The numbers on the x-axis correspond to differentmodels or different realizations:(1)BCC-CSM;(2) BNU-ESM;(3)FGOALS-g2;(4)FGOALS-s2 r1;(5)FGOALS-s2 r2;(6)FGOALS-s2 r3.The time axis is pentads.Negative values indicate earlieronset.

Fig.6.Projected changes of rainfall(mm d-1)and 850-hPa low-level w ind(m s-1)in May(June)for the period 2075–2099 under the RCP 8.5 scenario relative to 1981–2005 derived from(a)BCC-CSM,(b)BNU-ESM,(c)FGOALS-g2,(d) FGOALS-s2 r1,(e)FGOALS-s2 r2,and(f)FGOALS-s2 r3.

To reveal the physical linkages between the rainfall changes and the circulation changes over the Arabian Sea (5◦–15◦N,65◦–75◦E),we exam ine the vertically integrated moisture budgetequation shown below:

wherePis precipitation,VVVis the horizontal w ind vector,qis the specific hum idity,Eis evaporation,∆represents the difference between the future and present-day climate simulations(RCP8.5 m inus present-day),〈〉indicates vertical integration from 1000 to 100 hPa,and∇is the horizontal gradientoperator.According to Eq.(1),the rainfall changes may be attributed to changes in horizontal moisture advection,moisture convergence associated w ith mass convergence and surface evaporation.

The diagnosis result(Fig.7a)shows that the increase of horizontalmoisture advection contributes the most to the increase of rainfallover the Arabian Sea,except in FGOALS-g2.In FGOALS-g2,the most important factor is the increase ofmoisture convergence,while this factor is the second most important factor in the other experiments.The smaller contribution from the moisture advection change in FGOALS-g2 is related to the weaker circulation changesshown in Fig.6c. The large easterlies biases in FGOALS-g2 over the Arabian Sea in present-day climate(Fig.2d)may also favor weaker moisture advection changes.The surface evaporation change also contributes positively to the rainfall increase but the amplitude is much smaller than for the other two terms.

As both the changes in atmospheric moisture and circulation affect the moisture advection and moisture convergence, we furtherdecompose these two termsinto three components to reveal the relative contributions.The changesofhorizontal moisture advection and moisture convergence can be decomposed into three terms,as shown below:

The fi rst term on the right-hand side of Eqs.(2)and(3)is associated w ith the change of water vapor,while the second term is associated w ith circulation change.The third term is a nonlinear term including the contribution of both moisture and circulation changes.The subscript“pd”denotes the present-day climate.The diagnosis results(Figs.7b and c) indicate that the increase of moisture advection due to the change of circulation plays a crucial role in enhancing rainfall over the Arabian Sea.In FGOALS-g2,the increase of moisture convergence due to the increase of w ind convergence plays an important role.

In summary,the above moisture budgetanalyses demonstrate that the anomalous circulation changes over northern Indian Ocean in these four models enhance either the w ind convergence(in FGOALS-g2)or the low-level Somali CEF (in the other three models).Either of these effects increases the rainfallover the Arabian Sea,favoring an advance in monsoon onsetover the Arabian Sea under the RCP8.5 scenario.

Fig.7.(a)Moisture processes responsible for the rainfall increase(June for BNU-ESM,May for the othermodels)over the Arabian Sea(5◦–15◦N,65◦–75◦E)from the relative contributions of moisture advection,moisture convergence and surface evaporation.(b)Contribution to the moisture advection by the moisture change,circulation change,and nonlinear product of the two changes.(c)Contribution to the moisture convergence by the circulation change,moisture change,and the nonlinear productof the two changes.Units:mm d-1.

3.4.Possible causes of the circulation changes

The change in 200-hPa velocity potential matches w ith the change in convection over the equatorial region.Figure 8 shows the projected changes in 200-hPa velocity potential and the corresponding divergentw ind field for May(June)in the results of BCC-CSM,FGOALS-g2,FGOALS-s2(BNUESM)at the end of 21st century.The positive anomalies (upper-level anomalous convergences)over the eastern Indian Ocean and Maritime Continentare evident in allmodels w ith different strengths apart from in FGOALS-g2.In FGOALS-g2,the positive anomalies are found over the centraland eastern equatorial Pacific.

The upper-level anomalous convergences are the indicators of the reduced convection.This is also supported by the rainfall change.In all the models except FGOALS-g2, the reduced rainfall is mainly found over the tropics between 100◦E and 140◦E(Fig.6).However,in FGOALS-g2,a reduction in rainfall is evident over the central equatorial Pacific near the date line(Fig.6c).The reduced convection in the equatorial region tends to induce anomalous local descending motion and low-level easterly flow to the west of the region,along w ith off-equatorial anticyclonic low-level flow farther west through the Matsuno–Gilleffect(Matsuno, 1966;Gill,1980).So,in all the models except FGOALS-g2,it is likely that the reduction in convection over the eastern Indian Ocean and Maritime Continent region drives the anomalous anticyclonic circulations over the western Indian Ocean.The much stronger low-levelanomalous anticyclone over the northern Indian Ocean is ascribed to the strong easterly shear to the north of the equator(Wang and Xie,1996; Xiang and Wang,2013).The low-level anomalous anticyclonic circulation provides the increase in moisture advection into the Arabian Sea region.By contrast,in FGOALS-g2 the suppressed convection is farthereast,such thatthe anomalous anticyclonic circulation is notas well defined(evidenced by the weak circulation changes over the Indian Ocean),so the increased rainfall comes more from increased moisture convergence.The low-level w ind anomalous convergence over the Arabian Sea in FGOALS-g2 may be in part contributed to by the strong enhanced rainfallover the western North Indian Ocean(Fig.6c).

The SST warming pattern is the main cause of the change in zonalatmospheric overturning circulation over the tropical Pacific Ocean(Tokinaga et al.,2012),and the precipitation changes over the tropical Pacific through the“warmer-getwetter”mechanism(Xie etal.,2010).Figure 9 shows the future changes in SST projected by the four models,including the three realizations of FGOALS-s2(June for BNU-ESM, butMay for the other three models).The weakestwarm ing of projected SST is found over the central equatorial Pacifi c in FGOALS-g2,while the strongestwarming is found over the eastern equatorial Pacific in the other three models.In all the models except FGOALS-g2,the strongestwarming over the eastern equatorialPacific features a reduced zonalgradientof SST,which favors the eastward shiftof atmospheric convection,w ith reduced convection found over the eastern Indian Ocean and Maritime Continent region,as seen in Figs.6 and 8.In FGOALS-g2,the weakestwarming over the central Pacific favors the reduction of local convection,as evidenced by reduced rainfall and upper-level anomalous convergence (Figs.6 and 8).

Fig.8.As in Fig.6 except for the 200-hPa velocity potential change(107m2s-1,shading)and corresponding divergent w ind change(m s-1).

Fig.9.As in Fig.6 except for the SST change.

Although the models(except FGOALS-g2)behave similarly in the Maritime Continent and western Indian Ocean region,they are quite different across the rest of the Pacific,even among the different members of FGOALS-s2.In BCC-CSM and FGOALS-s2 r1,the weak upper-levelanomalousdivergences(negative anomalies)are found over the central equatorial Pacific and southeastern Pacific.In BNUESM,the whole eastern Pacific is occupied by the upperlevel anomalous divergences,w ith the center being north of the equatorat150◦W.However,in FGOALS-s2 r2 and r3 the upper-levelanomalous divergences are only evidentover the fareastern Pacific.

The broader-scale differences(shown in Fig.8)may be related to the differences in the projected SST pattern and in the present-day simulations.Figure 10 shows the spatialdistributions of the 200-hPa velocity potentialand corresponding divergentw ind for May(June)from ERA-interim,BCCCSM,FGOALS-g2,FGOALS-s2(BNU-ESM)in presentday climate.The observation is characterized by a deep convection around the Maritime Continent region.There are two kinds of models in terms of the location of the deep convection.In BCC-CSM and BNU-ESM the deep convections are located around the Maritime Continent region,w ith greater strength in BNU-ESM;whereas in FGOALS-g2 and FGOALS-s2 the deep convections are centered over the central Pacific.Consequently,compared w ith BNU-ESM,the eastward displacement of the projected upper-level anomalous divergences over the Pacific in FGOALS-s2(in partic-ular the r2 and r3 realizations)are mainly due to the corresponding eastward shiftof the deep convection underpresentday conditions,given the rather sim ilar projected SST warming pattern.The weaker projected responses in BCC-CSM are caused by both the weaker projected SST warm ing and the weaker deep convection around the Maritime Continent in the present-day simulation.Compared w ith FGOALS-s2 r1 and r2,the larger amplitude in projected changes in r3 is related to the stronger deep convection in the present-day simulation.

Fig.10.Spatialdistributions of the 200-hPa velocity potential(107m2s-1,shading)and corresponding divergentw ind(m s-1)in May(June for BNU-ESM)averaged from 1981–2005 derived from(a)ERA-interim,(b)BCC-CSM,(c)BNU-ESM, (d)FGOALS-g2,(e)FGOALS-s1 r1,(f)FGOALS-s2 r2,and(g)FGOALS-s2 r3.

4.Summary and discussion

4.1.Summary

In this study,the reproducibility of the onsetof the Asian summer monsoon by four Chinese models that participated in CM IP5 was evaluated.Three available realizations of FGOALS-s2 were also employed.The future projections under the RCP8.5 scenario derived from these fourmodels were then analyzed.The major conclusions can be summarized as follows:

(1)The general characteristics of the onsetof the Asian summer monsoon for present-day climate were reasonably captured in the four models,albeit w ith several differences also evident.Although FGOALS-g2 showed better skill in terms of the spatial pattern correlation coefficient,it lacked any onsetover much of India,Southeast Asia and the South China Sea.A ll the models failed to reproduce the monsoon region over EastChina,Korea and Japan,and tended to simulate a delayed onsetof monsoon over the Arabian Sea. The simulated much weaker low-level Somali CEFs in May weakened the moisture fluxes,resulting in dry biases over the Arabian Sea,which led to a delayed onset of the monsoon there.The biases of simulated SomaliCEFs were associated w ith the cold biases of SST in borealspring.

(2)The four models including the three realizations of FGOALS-s2 all projected an advanced onset of monsoon over the Arabian Sea at the end of 21st century under the RCP8.5 scenario.An increase of rainfall over the Arabian Sea,prior to the present-day onsetdate,favored the advanced monsoon onset in allmodels.The increased rainfallwas accompanied by intensified local low-levelwesterly anomalies. The moisture budgetanalyses demonstrated that the increase of moisture convergence due to the increase of w ind convergence played an important role in enhancing the rainfallover the Arabian Sea in FGOALS-g2,while the increase ofmoisture advection due to the anomalous circulation was crucial in the other three models.

(3)In all the models except FGOALS-g2,the intensifi ed low-levelwesterly anomaliesover the Arabian Sea were associated with the low-levelanomalous anticyclone over north-ern Indian Ocean,which was driven by the reduction in convection over the eastern Indian Ocean and Maritime continent region.By contrast,in FGOALS-g2 the anomalous anticyclonic circulation was not as well defined since the suppressed convection was farther east over the central equatorial Pacific.The low-levelw ind convergence anomaly over the Arabian Sea in FGOALS-g2 may have been in part contributed to by the strong enhanced rainfall over the western North Indian Ocean.

(4)In all the models except FGOALS-g2,the equatorial maximum in projected SST warming over the Pacific favored the eastward shiftof atmospheric convection and then led to the reduction in convection over the eastern Indian Ocean and Maritime Continent region.However,in FGOALS-g2 the weakest warm ing of projected SST over the central equatorial Pacific reduced the local convection.The broader-scale differences in the projected changes by the models may have been related to the differences in the projected SST pattern and in the present-day simulations.

4.2.Discussion

Our suggested mechanism for the models(except FGOALS-g2)in this study is to some extent similar to the mechanisms proposed for the observed advance of the Asian summer monsoon onsetsince the m id-to-late 1990s.The observed onset advance occurs over a large region,extending from the Bay of Bengal to the South China Sea.Westerly w ind anomalies are found from the Arabian Sea to the equatorialwestern North Pacific(Xiang and Wang,2013).Modeled projected westerly anomalies also favoran advanced onset of monsoon over the Arabian Sea.The observed westerly trend is the Rossby wave response to the intensified convection to the east of the Philippines due to both the SST warming over the western Pacific and the SST cooling over the central eastern Pacific(Xiang and Wang,2013).Modeled projected westerly anomalies are also the Rossby wave response,but to the suppressed convection over the eastern Indian Ocean and Maritime Continent region.

In addition,Wang etal.(2012)speculate thatthe observed early onsetof monsoon over the Arabian Sea since 1997 may be caused by enhanced land–ocean thermal contrastbetween the Asian landmass and the equatorial Indian Ocean.Consistentincreased land–ocean thermalcontrastunder the RCP8.5 scenario isevidentin the fourmodelsexamined in the present study(figure notshown)due to the large heatcapacity of the oceans,butthe signalw ith such a large scale may notbe a key factor for the advanced monsoon onsetover a lim ited region.

Finally,we only employed the model resultsderived from four Chinese models in this study,and gave an example of moisture budgetanalysis on the onsetchanges over the Arabian Sea.In future work we intend to use model results from more CM IP5 models to testthe robustness ofour findings and the suggested mechanism.

Acknow ledgements.We thank the two anonymous reviewers and the editor M ike DAVEY for constructive comments thathelped greatly to improve the original manuscript.This work was jointly supported by the National Natural Science Foundation of China (GrantNos.41330423,41205080,and 41023002),the Carbon Budgetand Related Issues projectof the Chinese Academy of Sciences (GrantNo.XDA05110301),and the JointCenter for GlobalChange Studies(Project No.105019),Beijing,China.

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10.1007/s00376-014-4053-z.

(Received 17 March 2014;revised 10 October 2014;accepted 24 October 2014)

∗Corresponding author:ZOU Liwei

Email:zoulw@mail.iap.ac.cn

©Institute of Atm ospheric Physics/Chinese Academ y of Sciences,and Science Press and Springer-Verlag Berlin Heidelberg 2015