Subseasonal mode of cold and wet climate in South China during the cold season:a climatological view

YU Minjie,ZHU Congwen and JIANGNing

Institute of Climate System,Chinese Academy of Meteorological Sciences,Beijing,China

ABSTRACT The authors investigate the dominant mode of climatological intraseasonal oscillation(CISO)of surface air temperature(SAT)and rainfall in China,and discuss the linkage of cold and wet climate in South China(SC)with the Arctic circulation regime during the cold season(from November to March).Results show that a positive CISOdisplays a cold-dry climate in North China,whereas a cold-wet pattern prevails in SCwith a quasi-30-day oscillation during the peak winter season.In SC,the intraseasonal variability of SAT plays a leading role,altering the cold-wet climate by the southward shift of a cold front.Evidence shows that the circulation regime related to the cold and wet climate in SCis mainly regulated by a pair of propagating ISO modes at the 500-hPa geopotential height in the negative phase of Arctic Oscillation.It is demonstrated that the local cyclonic wave activity enhances the southward movement of the Siberian high,favoring an unstable atmosphere and resulting in the cold-wet climate over SC.Therefore,the cold-air activity acts as a precursor for subseasonal rainfall forecasting in SC.

KEYWORDS Climatological intraseasonal oscillation;cold and wet climate;South China;Arctic circulation impact

1.Introduction

The intraseasonal oscillation(ISO)of the East Asian winter monsoon(EAWM)can alter the seasonal evolution of surfaceair temperature(SAT)and rainfall,and causecold and wet extremeeventsin Chinaduring thecold season(from November to March).For instance,unexpected extreme cold spells with snowstorms attacked South China(SC)during January 2008,causing severe icing conditionsand catastrophic damage for the local economy and society(Wen,Yang,and Kumar et al.2009;Hong and Li 2009;Zhou et al.2009;Shi,Xu,and Lu 2010).During January 2016,unexpected cold spells with low SAT repeatedly affected East Asia and,during 21-25 2016,broke the record in SCsince observations began in 1961(Cheung et al.2016;Ma et al.2018).Evidence shows that cold and wet events are mainly caused by an enhanced EAWM,with an intensi f i ed Siberian high in terms of sea level pressure(SLP)and East Asian trough(EAT)at 500 hPa,as well as a southward shift of the East Asian jet stream at 200 hPa(Boyle and Chen 1987;Chen,Graf,and Huang 2000;Wen et al.2009;Song et al.2015;Ma et al.2018).

A cold and wet extreme event usually occurs during the cold season,displaying a seasonal phase-locking with enhanced atmospheric ISOactivity(Wen et al.2009;Song et al.2015;Ma et al.2018),although it varies with the interannual variations of Arctic sea-ice cover(Wu and Wang 2002;Cheung et al.2018;Ma et al.2018)or tropical sea surface temperature(Xu et al.2018a).The climatological ISOcomponents of SATand rainfall re f l ect the seasonal phase-locking characteristics,which provides us with possible clues for the subseasonal forecasting of cold and wet events in China.In the present study,we investigate the climatological ISO(CISO)mode of SATand rainfall during the cold season in China,and reveal its related circulation regime,with respect to the subseasonal variations of Arctic circulation from a climatological view.

2.Data and methods

Thedatautilized in thisstudy includedailyin-situ observed rainfallin Chinafrom 722stations,provided by the National Meteorological Information Center of the China Meteorological Administration,and NCEP-DOEAMIP-IIreanalysis atmospheric data,with a horizontal resolution of 2.5°×2.5°(Kanamitsu et al.2002).The climatology is de f i ned as the 30-yr arithmetic average between 1981 and 2010,and the CISO is de f i ned as the 10-90-day harmonic componentsrelative to the annual cycle by harmonic analysis(Wang and Xu 1997;Wheeler and Hendon 2004;Song et al.2016).

We apply the method of multivariate empirical orthogonal function(MV-EOF)analysis(Wang 1992)to reveal the subseasonal mode of the SATand rainfall anomalies over China during the cold season(November to March).The degrees of freedom(DOF)for testing the statistical signi f i cance of the correlation are calculated according to the following equation(Yan,Zhong,and Zhu 2003):

whereΔT isthesampling interval;theperiodic rangeof the harmonic analysisis T1,T2(T1＜T2);and N isthe number of independent samples.Besides,we use local cyclonic wave activity(LCWA,based on the 500-hPa geopotential height(GPH,z500))to describe the quantities of cold-air activity(Chen et al.2015;Huang and Nakamura 2015).Firstly,we select a contour value z0500and de f i ne an equivalent latitudeφesuch that the area S bounded by z0500is

while

where a is the radius of Earth,λis longitude,andφis latitude.De f i ning^z=z500-z0500,we can obtain the LCWA As(Xue et al.2017):

Moreover,non-dimensional eigenvectors are used to standardize the combined meteorological f i elds.

3.The CISO mode and its related circulation regime

Figure 1 shows the f i rst MV-EOFmode(MV-EOF1)of the ISOcomponentsof SATand rainfallduring thecold season.Thismodeaccountsfor 32.2%of thetotal ISOvariance,and statistically passes the criterion of North et al.(1982).Corresponding to the positive phase of MV-EOF1,cold SAT prevails over most areas,centered over the lower reaches of the Yangtze River,while the Tibetan Plateau(TP)is characterized by warm SAT.Also,enhanced rainfall isobserved in SCand the TP,with respect to cold and warm SAT,respectively(Figure 1(a)).Therefore,a cold-dry and cold-wet climate often prevails in North China and SC,respectively,in the area east of 115°E.The time series of the f i rst principal component(PC1)is de f i ned as the CISO index.It is found that cold SAT always occurs during December to February with respect to the cold period of the annual cycle of SAT in SC.The CISO index displays a seasonal phase-locking with the cold-wet climate in December and late-January,which most likely causes the low temperature and snowstorms(Figure 1(b)).Wavelet analysis(Torrence and Compo 1998)shows that the CISO index exhibitsa qusi-30-day periodicity(f i gure not shown).Because cold and wet extreme events usually occur in SC with enhanced atmospheric ISO activity(Wen et al.2009;Song et al.2015),we mainly focuson the ISOcomponents of SATand rainfallin SC,to discusstheir relativeimportance for the CISOmode.The correlation coefficient of the CISO index with the ISO of SATin SCis-0.90,which is greater than that of the ISO of precipitation(+0.45),suggesting a dominant role of SAT in SC on the intraseasonal time scale during the cold season.

Figure 1.The f i rst MV-EOFmode of the CISOof daily in-situ SAT and rainfall over China:(a)SAT(color-shading)and rainfall(contours);(b)time series of the standardized annual cycle of SAT(gray area),CISO index(black line),and the ISO components of SAT(red line)and rainfall(blue line)in SC(green dashed box in(a))during the cold season.

The atmospheric circulation regime,correlated with the CISOindex,exhibitsa very distinct vertical structure in the GPH and potential temperature(PT)f i elds(Figure 2).Corresponding to a positive CISO index,an enhanced Siberian high at 850 hPa dominates East Asia,with strong northeasterly winds prevailing over most areas in China(Figure2(a)).Acold anomalouscycloneat 500 hPacontrols the lower reachesof the Yellow River,with signi f i cant cold advection(Figure 2(b)),conducive to a vertical southward shift of the cold front.In contrast,a low-level air f l ow convergenceof meridionalwindsisobserved between 20°and 30°N below 500 hPa with an unstable atmospheric air temperature over SC,corresponding to enhanced rainfall over SC(Figure 2(c)).Therefore,the CISOmode of the SAT and rainfall in China re f l ects a typical cold front-dominant circulation regime during the analysisperiod.

To explore the origin of the cold-air activity,we calculate the lead-lag correlations of the CISO index with the ISO components of GPH and PT at 850 hPa,500 hPa,and 200 hPa along 115°E(Figure 2(d-f)).The leading correlation coefficient of PT and GPH suggests that the signal of cold-air activity with higher pressure at 850 hPa can be tracked at 65°N before 15 days.It moves southward,accompanied by a warm low south of 30°N,before a cold and wet climate occurs in SC 15 days later(Figure 2(d)).At 500 hPa,it exhibits a southward movement of cold and cyclonic circulation with an enhanced warm anticyclone south of 30°N(Figure 2(e)).The correlation pattern of GPH at 200 hPa displays a dipole mode,with negative and positive centers in the polar and midlatitude regions,respectively(Figure 2(f)),and resembles the negative phase of the Arctic Oscillation(AO)(Thompson and Wallace 1998).Therefore,the cold-air activity is closely correlated with the ISO of Arctic circulation.

4.Impact of Arctic circulation

Figure 2.Spatial distribution of the correlations of the CISOindex with the winds(vectors)and PT(color shading)during the cold season at(a)850 hPa and(b)500 hPa.The red and blue arrows indicate the regions of the convergence and divergence f i eld corresponding to the active region of the Siberian high and EATat 850 hPa and 500 hPa.(c)Latitude-height correlation of the CISO index with zonal-mean(110°-120°E)anomalies of GPH(contours),winds(vectors),and PT(color shading).Lead-lag correlations between the CISOindex and GPH(contours)and PT(color shading)at(d)850 hPa,(e)500 hPa,and(f)200 hPa,along 115°E.The number represents the lead-lag days,and the gray dotted areasrepresents the 90%con f i dence level.The green dashed box is SC.

Figure 3.EOFanalysis of the ISOcomponents of daily climatological GPHat 500 hPa during the cold season:(a-c)the three EOF leading modes;(d)their corresponding PCs.(e)Lag correlations between PC2 and PC3,and with itself.The two black dashed lines represent the 90%con f i dence level,based on the Student’s t-test,and the number represents the lead-lag days.

Figure 4.Lead-lag correlations of the CISOindex with the reconstructed GPH(color shading)at 500 hPa,along 115°E,based on(a)EOF1,(b)EOF1+EOF2,(c)EOF2+EOF3,and(d)EOF1+EOF2+EOF3.The number represents the lag days,and the gray dotted areas represents the 90%con f i dence level.

The AO is de f i ned by the f i rst EOFof SLPwith a vertical baroclinic pattern below the midtroposphere of 50 hPa(Thompson and Wallace 1998).It has been considered as the dominant regulator of the cold-air activity affecting the climate over Eurasia(e.g.Wu and Wang 2002;Cheung et al.2018).Figure 3 shows the f i rst three leading EOF modes of the ISO components of 500-hPa GPH,accounting for 20.7%,12.5%,and 9.7%of the total variance,respectively.The f i rst EOF mode shows a seesaw pattern,with positive and negative loading over the polar and midlatitude regions,respectively,consistent with the negative phase of the AO(Figure 3(a)).EOF2 is characterized by meridional dipole anomalies,but the positive and negative centers are southward in position,away from the polar region(Figure 3(b)).EOF3 shows a zonal dipole variation of GPH between the Icelandic and Aleutian Islands region(Figure 3(c)).The lead-lag correlations of PC1 with PC2 and PC3 are not signi f i cant,but the correlation reaches a maximum when PC2 leads the variation of PC3 by seven days(Figure 3(d-e)).Therefore,EOF3 can be considered as the morphological change of EOF2,manifesting the spatial ISO propagation of the Arctic circulation.

The CISOcomponentsof SATand rainfall in China can be ascribed to the joint impact of the f i rst three modesof Arctic circulation.To verify this,we reconstruct the f i rst three EOFmodesof 500-hPa GPH,and calculate the leadlag correlations of the CISOindex with the reconstructed GPH anomalies based on the f i rst three EOF modes(Figure 4).The impact of the AO alone exhibits a simultaneous response of the GPH anomalies over Eurasia,characterized by a tripole correlation pattern of the CISO index at 115°E(Figure 4(a)).The 500h-Pa GPH reconstructed bythe f i rst two EOFmodesbasically re f l ects the 10-20-day leading circulation anomaly of the CISO(Figure4(b)).Thesum of EOF2 and EOF3 can duplicatethe southward movement of cold-air activity originating at 40°N(Figure 4(c)),but against the observed positive correlations at the latitudes of SC during the same time period(Figure 2(e)).When we reconstruct the three EOF modes together,the lead-lag correlations of the CISO index basically produce the southward propagation of cold-air activity(Figure 4(d)).Therefore,the negative phase of the AO,together with a pair of propagating modes of Arctic circulation,is jointly responsible for the cold and wet climate in SC.

5.Summary and discussion

The subseasonal variation of SAT and rainfall in China exhibitsaquasi-30-day oscillation during the cold season,climatologically.Corresponding to the positive phase of the CISO mode,a cold and dry climate prevails in North China,but a cold and wet climate appears in SC.This modeisdominated by SATanomaliesin SC,and iscaused by the cold front-related regime over East Asia.The coldair activity originates from the Arctic region,and the negative phase of the AOtogether with a pair of propagating ISOmodes can jointly enhance the Siberian high,which is conducive to the cold front's southward movement.The cold front causesan unstable atmosphere and results in cold and wet events over SC,before then triggering subseasonal rainfall as a precursor.

A greater value of LCWA represents stronger cold-air activity.The subseasonal cold-air activity is nested in a cyclonic circulation at 500 hPa,which can be veri f i ed by diagnostic analysisof LCWA(Figure 5).Besidesthe considerable contribution of Arctic circulation,winter climate variations over China can also be affected by movement of the East Asian trough,India-Burma trough,and Urals-Siberiablocking(Cheung et al.2012;Leung and Zhou 2016;Leung,Cheung,and Zhou 2017;Li,Chen,and Zhou 2017),aswellasthevariabilityof the Madden-Julian Oscillation,El Ni?o,and pan-Arctic sea-ice concentration(Jia et al.2001;Zhang et al.2015;Cheung et al.2018;Xu et al.2018b).Our work revealsapossible source of thefrequency of cold-wet eventsin SC,aswellastheir seasonalphase-locking.In fact,thecold and wet climateover SChaschanged,which most likely causes the high frequency of extreme events seen after 2000,consistent with the results of Wei,Chen,and Zhou(2011)and Cheung et al.(2016).The climate mean background iswarming,with alarger enhanced amplitude of the ISOcomponent,while the effect of the annual cycle of warming on the cold and wet climate needs further investigation.In addition,the interannual variation of cold and wet extremeeventsiscontinually complex,so it would also be worthwhile paying more attention to other impact factorsover China in future work.

Figure 5.(a)Spatial distribution of the correlation of the CISO index with LCWA.(b)Lead-lag correlation of the CISO index with LCWAat 115°E.The number represents the lag days,and the gray dotted areas represent the 90%con f i dence level.

Acknowledgments

We are gratefulto Dr.XUEDaokaifor providing the LCWAcoding.

Disclosure statement

No potential con f l ict of interest was reported by the authors.

Funding

This work was jointly supported by the National Natural Science Foundation of China[grant numbers 41475057,41775052,and 41505049],the Special Fund for Public Welfare Industry[grant number GYHY20140619],the Basic Scienti f ci Research and Operation Foundation of CAMS[grant numbers 2018Z006 and 2017R001],and the Jiangsu Collaborative Innovation Center for Climate Change.