A Study of East Asian Cold Surges dur ing the 2004 / 05 Win ter : Im pact of East Asian Jet Stream and Sub trop i cal Up per-Level Rossby Wave Trains

Cold surges were un usu ally ac tive in sub trop i cal East Asia dur ing Jan u ary Feb ru ary 2005. These cold surges were pre ceded by up stream wave trains, which orig i nated in the Med i ter ra nean-Sa hara re gion and prop a gated east ward along the sub trop i cal jet stream over the Eur asian con ti nent. The north erly of the up per-level cy clonic anom aly in East Asia cou pled with the low-level north erly upon the ar rival of wave ac tiv ity, and this was fol lowed by a quick south ward pen e tra tion of cold air mass and sur face an ti cy clone. Di ag nos tic and nu mer i cal re sults sug gest that the an oma lously ac tive wave ac tiv ity af fect ing the East Asian cold surges may be at trib uted to an an oma lously en hanced jet stream over the Mid dle East and an an oma lously west ward ex ten sion of the East Asian Jet Stream. The con fig u ra tion of these two sub trop i cal jet streams es tab lished a strong wave guide through which wave ac tiv ity forced in the Med i ter ra nean-Sa hara re gion could ef fi ciently prop a gate to East Asia, re sult ing in above av er age cold surge events in sub trop i cal East Asia. Wave-like per tur ba tion tended to be amplified at the entrance to the East Asian jet through barotropic energy conversion from the mean flow.


IN TRO DUC TION
The East Asian cold surge is one of the most prom i nent phe nom ena in the East Asian win ter mon soon. The cold surge, ac com pa nied by strong north erly winds, causes not only nat u ral di sas ters in East Asia, but also brings mon soon rain fall to South east Asia (Boyle and Chen 1987). There are two types of cold surges over East Asia, namely the east erly surge and the north erly surge (Wu and Chan 1995). Cold air and sur face an ti cy clone in a cold surge event of ten move south ward along the east ern pe riph ery of the moun tains east of the Ti betan Pla teau from Si be ria to South ern China, Taiwan, and even the South China Sea in some strong events (e.g., Lau and Lau 1984;Hsu 1987;Compo et al. 1999). Taiwan, an is land off the south east ern coast of China, was affected by this south ward-pen e trat ing cold surge 2.5 times per win ter from 1981 -2000 (Wu et al. 2007). How ever, in the 2004/2005 win ter, four cold surges reached Tai wan. This un usu ally strong cold surge ac tiv ity broke a 40-year re cord and caused sig nif i cant ag ri cul ture loss in Tai wan.
Pre vi ous stud ies have re vealed that East Asian cold surges are of ten pre ceded by up stream up per-level dis turbances orig i nat ing in the west ern Eur asian con ti nent (Joung and Hitchman 1982;Wu and Chan 1997;Chen et al 2002). Sim i larly, the four cold surge events in the 2004/ 05 win ter were pre ceded by wave-like dis tur bances propagating east ward from Eu rope to East Asia along the subtrop i cal jet stream, which has been iden ti fied as a Rossby wave guide in pre vi ous stud ies (e.g., Hsu and Lin 1992;Hoskins and Ambrizzi 1993). Through this jet-stream wave guide Asia weather can be af fected by, forc ing or per tur ba tion oc cur ring as far up stream as Eu rope in just a few days. Terr. Atmos. Ocean. Sci., Vol. 20, No. 2, 333-343, April 2009doi: 10.3319/TAO.2008 De spite this knowl edge of up stream wave-like dis turbances, the mech a nism guid ing the wave train has not been fully ex plored. In view of strong cold surges and up stream wave ac tiv ity in the 2004/2005 win ter, this study aims to ex plore the wave guide ef fect on wave-like dis tur bances in the up per tro po sphere and the pos si ble ef fect of up per-level wave ac tiv ity on the ini ti a tion of cold surges in the 2004/05 win ter. In this study, we will dem on strate that up per-level Rossby wave trains orig i nat ing in the Med i ter ra nean-Sahara re gion and prop a gat ing through the jet-stream waveguide helped trig ger the East Asian cold surge. It is also sug gested that the East Asian Jet Stream (EASJ) plays an im por tant role in am pli fy ing the wave-like dis tur bances through wave-mean flow in ter ac tion. Nu mer i cal sim u lation will also de m on strate the sen si tiv ity of the wave propagation to the de tailed struc ture of the sub trop i cal jet stream.

DATA AND MODEL
Daily NCEP/NCAR reanalysis wind, geopotential hei ght (Kalnay et al. 1996) dur ing the 2004/05 win ter are used in this study. Cold surges were iden ti fied based on cri te ria de fined by the Cen tral Weather Bu reau, the of fi cial me te o ro log i cal agency of Tai wan. The cri te ria are: (i) the daily max i mum tem per a ture of Tai pei (lo cated in north ern Tai wan) drops at least 4°C within 24 -48 hours; and (ii) the daily min i mum tem per a ture is less than 10°C. To make sure the cold surge is not sta tion de pend ent, the data of Peng chiayu, an is land to the north east of Tai pei, is also used to iden tify cold surges. Four cold surge events listed in Ta ble 1 were iden ti fied. The cor re spond ing time se ries of daily min i mum tem per a ture is also shown in Fig. 1. The abrupt tem per a ture drop ranges from 4.8 to 9.9°C and the min i mum tem per a ture fell to 7.3 -9.6°C. It is also in ter est ing to note that the cold surges oc curred regu larly ev ery 16 -18 days af ter a warm De cem ber.
The model used to ex plore the wave guide effect on Rossby wave-like dis tur bances is a linearized per tur ba tion barotropic vorticity equ ation. It is out lined as: ( 1) where, prime rep re sents the anom aly de vi a tion from monthly cli ma tol ogy, over bar de notes the cli ma to log i cal mean vari able, z the 200-hPa rel a tive vorticity, V two di mensional wind, f the Coriolis pa ram e ter, kz ¢ Ray leigh (lin ear) drag term, rÑ 4 z ¢ biharmonic dif fu sion, and s ¢ vorticity forcing. The co ef fi cients for Ray leigh fric tion and bihamonic dif fu sion are 3.935 ´ 10 -7 s -1 (equiv a lent to a 10-day damp ing) and 2 ´ 10 16 m 4 s -1 , re spec tively. The model is a nondivergent global spec trum model with a T42 hor i zon tal res o lu tion (128 ´ 64). Pre vi ous tests show that this model sim u lates well Rossby wave prop a ga tion ei ther for the initial or forced prob lem (Hsu 1994). In the fol low ing ex per iments, a pre scribed el lip ti cal forc ing (s ¢ ), the Rossby wave source (RWS, Sardeshmukh and Hoskins 1988), cal cu lated from ob ser va tion is added to Eq. (1). The RWS was mul tiplied by 10 -5 to make the sim u la tion es sen tially lin ear. Three sets of mean flows at 200 hPa, i.e., the cli ma to log i cal zonal mean flow (here af ter climz_exp), cli ma to log i cal time mean flow (here af ter clim_exp) and the mean flow in Jan u ary -Feb ru ary 2005 (here af ter surge_exp), were used as ba sic flow in the nu mer i cal ex per i ments to ex plore the wave guide ef fect of the ba sic flow on the per tur ba tion.

COLD SURGE AND UP STREAM WAVE AC TIV ITY
Fig ure 2 shows the 200-hPa wind and the 850-hPa temper a ture anom a lies on the first day of cold surge de fined by Ta ble 1. An up per wave-like wind anom aly ap pear ing in the sub trop i cal jet is ob served for all four events. The distri bu tions of neg a tive tem per a ture anom a lies (shad ing) seem to in di cate that the south ward pen e tra tion of cold air mass along the lee side of Ti betan Pla teau is ac com pa nied Im pact of Sub trop i cal Jet Stream on East Asian Cold Surges 335 Fig. 2. 200-hPa wind and 850-hPa tem per a ture anom a lies (con tour) on the first day of four cold surge events de fined in Ta ble 1. Con tour in ter val is 2°C and neg a tive tem per a ture anom aly is shaded.
by the ar rival of the north erly anom a lies aloft. To fur ther dem on strate the re la tion ship be tween the up per and lowlevel wind anom a lies, a lon gi tude-height cross sec tion of me rid i o nal wind anom aly av er aged over 20 -30°N is shown in Fig. 3. Zonal wind is also plot ted to in di cate the ex is tence of two jet streams: the EASJ and an other one over the Mid dle East. The fig ure re veals a wave-like structure with zonal scale equiv a lent to zonal wavenumber 6 in all cold surge events (Fig. 3). The wave-like pat terns exhibit equiv a lent baro tropic ver ti cal struc ture with the largest am pli tude in the up per tro po sphere. Par tic u larly in terest ing is the re gion of north erly anom aly near the East Asian coast. The north erly anom aly ex ists from the up per tro po sphere to near the sur face and co in cides with the southward pen e tra tion of cold air and the low-level north erly em bed ded in the cold surges. An ex am i na tion of the tem poral evo lu tion of each cold surge event in di cates that the region of the near-sur face north erly started mov ing southward upon the ar rival of the wave-like dis tur bance. This seems to sug gest that the north erly in the wave-like dis turbance helped trig ger the south ward ad vance of cold surge. This is con sis tent with the cou pling of the up per trough with low-level cir cu la tion re ported by Chen et al. (2002).
One ex am ple will be shown and dis cussed in Fig. 5. The tem po ral evo lu tion of these four wave-like dis turbances is shown in Fig. 4, which pres ents the Hovmüller di agram of 200-hPa me rid i o nal wind anom aly av er aged over 20 -30°N. There are many wave-like pat terns, which are char ac ter ized by the in ter mit tently pos i tive and neg a tive meridional wind anom a lies from 0 to 120°E and by the sequential de vel op ment of a new anom aly cen ter in the down stream. But only four events, in di cated by the dashed ar rows, ex hibit north erly anom aly at the most east ern end of these wave-like struc tures. Also plot ted in con tours is the me rid i o nal wind anom aly at 850 hPa be tween 80 and 120°E. It is in ter est ing that the north erly anom aly at 850 hPa is also ob served only dur ing the four cold surge events. Note that the north erly anom aly in early De cem ber is not ac com panied by a cold surge be cause of the warm De cem ber ( Fig. 1) and the weak Si be rian High. The above fea tures also sug gest that the cold surges oc curred upon the ar rival of wave activity, which prop a gates east ward from north ern Af rica (Sahara) to East Asia.
To un der stand how wave ac tiv ity prop a gates, the wave ac tiv ity flux (WAF) de fined by Takaya and Naka mura (2001) was com puted. Here only the WAF and the cor re -336 Hong et al. Fig. 3. Same as in Fig. 2, but for the lon gi tude-height cross sec tion of anom a lous me rid i o nal wind (av er aged over 20 -30°N). Dark bar and gray shading de note to pog ra phy and to tal zonal wind, re spec tively. Con tour rep re sents me rid i o nal wind anom aly with 5 m s -1 in ter val.
spond ing stream func tion anom aly at 200 hPa for the first cold surge are shown in Fig. 5. The other cases ex hibit sim i lar char ac ter is tics. Fig ure 5a re veals sig nif i cant wave ac tiv ity flux from the Med i ter ra nean-Sa hara to the Mid dle East 5 days be fore the on set of cold surge in Tai wan. Contin u ous east ward prop a ga tion of wave ac tiv ity and de velop ment of new cen ters of ac tion along the sub trop i cal jet are ev i dent at Day -3 (Fig. 5b). By Day -1, a cy clonic anom aly ap peared over east ern China while the WAF had fur ther ex tended into the west ern North Pa cific. This series in di cates the east ward en ergy dis per sion of the Rossby wave-like dis tur bances through the sub trop i cal jet-stream wave guide from the Med i ter ra nean-Sa hara to East Asia in only four days. The right pan els of Fig. 5 repre sent the cor re spond ing sea level pres sure and 850-hPa tem per a ture at Day -5, -3, and -1. The cold air mass and the an ti cy clone did not move quickly south ward to ward Taiwan un til Day -3 to -1 when the cy clonic anom aly over east -ern China de vel oped and the WAF ar rived at East Asia. This se quence dem on strates again that this cold surge was closely re lated to the up per-tro po spheric wave ac tiv ity orig i nat ing in the Med i ter ra nean-Sa hara re gion. A close ex am i na tion of Fig. 5 re veals that the dis turbances tend to am plify at the en trance of the EAJS, e.g., the am pli tudes of the pos i tive anom aly over north ern India and the neg a tive anom aly over east ern China increased from Day -3 to -1. As noted above, the north erly over eastern China was en hanced in the whole tro po sphere and cold air mass started mov ing south ward upon the ar rival of the wave-like dis tur bances. Am pli fi ca tion of the dis turbances may pro vide the ex tra push to the oc cur rence of cold surge.

BAROTROPIC KI NETIC EN ERGY CON VER SION
Both barotropic and baroclinic in sta bil ity could contribute to the am pli fi ca tion of dis tur bances through wavemean flow in ter ac tion. Con sid er ing the barotropic na ture of the dis tur bances, the fol low ing dis cus sion will fo cus on the barotropic part, al though the con tri bu tion of the baroclinic com po nent may not be neg li gi ble. Fol low ing Simmons et al. (1983), we di ag nose barotropic ki netic en ergy con ver sion of dis tur bances to un der stand how the EAJS in flu ences the amplitude of dis tur bances.
The barotropic per tur ba tion ki netic en ergy equa tion can be writ ten as: ( 2) where, KE = (u¢ 2 + v¢ 2 ) / 2 rep re sents the per tur ba tion kinetic en ergy, (u¢, v¢) de notes hor i zon tal wind per tur ba tion, the over bar in di cates the cli ma to log i cal mean. As the clima to log i cal-mean wind is nearly non-di ver gent and the zonal wind shear ( ¶ ¶ u y ) is much larger than me rid i o nal wind shear ( ¶ ¶ v y ), the per tur ba tion ki netic en ergy equation can be ap prox i mated by: (3) The first term (-¢ ¢ u v u y ¶ ¶ ) in the right hand side corresponds to the me rid i o nal shear in sta bil ity of zonal mean flow, and the sec ond term ( ) ¢ -¢ v u u x 2 2 ¶ ¶ cor re sponds to the zon ally asym met ric in sta bil ity of zonal mean wind, which be comes im por tant when the dis tur bance passes through jet stream.
Both terms were cal cu lated and the val ues av er aged over 20 -30°N. They are shown in the Hovmüller di a gram of Fig. 6. It shows that the four ma jor events of en ergy con version oc curred con cur rently with the four cold surges af ter late De cem ber and the ki netic en ergy con ver sion, mainly con trib uted by the term -uv u y ¶ ¶ / , was pos i tive over east ern China (100 -120°E) dur ing the four cold surge events. Although the rel a tive con tri bu tion of the two en ergy conversion terms var ied from case to case due to the vari ance of the sub trop i cal jet, for ex am ple, the mag ni tude of the term ( ) / ¢ -¢ v u u x 2 2 ¶ ¶ in case 4 was smaller than the other three cases, the dis tur bances in all four cases grew at the ex panse of the ki netic en ergy of the mean flow, in di cat ing the contribution of barotropic in sta bil ity to cold surges.
It is also in ter est ing to note that en ergy con ver sion peaks (~400 m 2 s -2 day -1 on av er age) near 110°E. This sug gests that the dis tur bance can be sig nif i cantly am pli fied near the entrance of EAJS, and it may ex plain why the mag ni tude of the up per north erly anom aly has a lo cal max i mum in south ern China as shown in Figs. 2 and 3.

WAVE GUIDE EF FECT OF SUB TROP I CAL JET STREAM
The re sults shown above sug gest that the Rossby wavelike dis tur bances orig i nat ing in the Med i ter ra nean-Sa hara help trig ger the East Asia cold surges dur ing the 2004/05 Fig. 5. Tem po ral evo lu tion of 200-hPa wave ac tiv ity flux and stream func tion anom aly (left panel), and 850-hPa tem per a ture and sur face pres sure (right panel) at Day -5, -3, and -1 of cold surge case 1. The num ber ing of "Day" de notes the lead ing days rel a tive to cold surge on set. Shad ing in the left and right pan els rep re sents the 200-hPa jet stream and sur face pres sure, re spec tively. The rect an gu lar box in Fig. 5a in di cates the EAJS and the entrance of EAJS is marked by the thick ar row.
winter, and the EAJS is linked to the cold surges by acting as a waveguide and reinforcing the amplitude of the disturbances. This waveguide effect was simulated using the barotropic model described in section 2.
To show the waveguide effect of the basic flow on wave propagation, numerical experiments with the same forcing in three different basic flows, i.e., climz_exp, clim_exp, and surge_exp, were carried out. Note that December 2004 exhibited very different climatic characteristics from January -February 2005. Therefore, this study focuses on the latter period. The three basic flows are shown in Fig. 7 for comparison. It shows that the EAJS is significantly enhanced (zonal mean wind exceeds 55 m s -1 ) in the surge_exp. The larger peak wind is accompanied by a larger meridional gradient, implying larger wind shear instability.
An examination of the upper-tropospheric divergence in January -February 2004/05 revealed a strong divergence anomaly over the Sahara (Fig. 8a); it was the strongest divergence anomaly in the Northern Hemisphere. Interestingly, the January -February meridional wind anomaly exhibits a clear wave-like pattern (Fig. 4) similar to the one shown in Fig. 5.

Impact of Subtropical Jet Stream on East Asian Cold Surges 339
Fig. 6. Same as in Fig. 4, but for the barotropic kinetic energy conversion term, (a) (v ¢ 2 -u ¢ 2 ) ¶ ¶ u x / , and (b) -¢ ¢ u v u y ¶ ¶ / . Only energy conversion larger than 100 m 2 s -2 day -1 is plotted. Contour interval is 100 and 200 m 2 s -2 day -1 for Figs. 6a and b respectively. Gray shading indicates the four cold surge events. The RWS associated with the divergence over the Mediterranean-Sahara region was calculated (Fig. 8a) and a similar pattern was prescribed as forcing in the barotropic model (Fig. 8b). The steady responses of stream function to the prescribed RWS are shown in Figs. 9a -c. In the experiment of climatological zonal-mean flow, major disturbances forced near the Sahara propagate northeastward along a great-circle like path to Siberia and East Asia (Fig. 9a). Although disturbances also appear in the subtropical waveguide, their magnitude is very weak. In the climatological mean flow experiment (clim_exp), the northeastward propagation becomes much weaker, while the major disturbances propagate mostly eastward along the subtropical jet stream (Fig. 9b). Although the waveguide effect of the subtropical jet stream is much clearer in this experiment, the disturbance is still weak to the north of India, where the weak absolute vorticity gradient leads to energy leakage to the north. In the experiment of the January -February 2005 mean flow (surge_exp), there is almost no northeastward propagation (Fig. 9c). Almost all disturbances propagate eastward along the subtropical waveguide. Note the great similarity in the simulated wave-like pattern to the observed pattern shown in Fig. 5c. This contrast between surge_exp and clim_exp can be attributed to the westward extension of the EAJS during January -February 2005 to form a better defined waveguide. It is also interesting to note the enhancement of the anticyclonic circulation to the north of India, which results in a stronger northerly over eastern China. The distribution of barotropic kinetic energy conversion in the surge_exp experiment is similar to that observed (Fig. 10a). The local maximum of energy conversion located near 110°E reveals the amplification of disturbance in East Asia (Figs. 2, 3) by extracting kinetic energy from the basic flow, which is consistent with the Hovmüller diagrams of the observed barotropic kinetic energy conversion (Fig. 6). 340 Hong et al.

CON CLU SION
This study ex plores the pos si ble ef fect of up stream wave ac tiv ity on East Asian cold surges that af fected Tai the Eur asian con ti nent. The north erly of the up per-level cy clonic anom aly in East Asia cou pled with the low-level north erly upon the ar rival of wave ac tiv ity, fol lowed by a quick south ward pen e tra tion of cold air mass and sur face an ti cy clone. b. The re sults from the di ag no sis and nu mer i cal ex per i ments sug gest that the an oma lously ac tive wave ac tiv ity af fecting the East Asian cold surges may be at trib uted to the anomalously en hanced jet stream over the Mid dle East and the an oma lously west ward ex tended EAJS. The config u ra tion of these two sub trop i cal jet streams es tab lished a strong wave guide through which the wave ac tiv ity forced in the Med i ter ra nean-Sa hara re gion ef fi ciently prop a gated to East Asia and re sulted in more-thanaverage cold surge events in sub trop i cal East Asia. Watanabe (2004) pointed out that the Med i ter ra nean-Sa hara re gion is a key re gion for cli ma tic fluc tu a tion in the At lan tic, in flu enc ing the East Asian win ter cli mate through the wave guide ef fect of the sub trop i cal jet stream. This study dem on strates sim i lar phe nom e non on a syn op tic time scale dur ing a par tic u lar win ter. Al though it is not clear what fac tors lead to the anom a lous sub trop i cal jet stream in Jan u ary -Feb ru ary 2005, the an oma lously strong waveguide had ap par ently re sulted in a shift of the pre ferred path for the Rossby wave en ergy dis per sion that led to the anoma lous be hav ior of the syn op tic dis tur bances in East Asia dur ing Jan u ary -Feb ru ary 2005. How of ten and ro bust this mech a nism af fects cold surge ac tiv ity in East Asia is an open ques tion and is be ing in ves ti gated based on multiyear datasets.
Ac knowl edge ments This study was sup ported by NSC Fig. 10. (a) Ob served and (b) sim u lated barotropic ki netic en ergy con ver sion. The shad ing in di cates the 200-hPa zonal wind larger than 40 m s -1 . Contour in ter val is 100 and 10 -8 m 2 s -2 day -1 for (a) and (b), re spec tively. Since the RWS is mul ti plied by 10 -5 , the sim u lated num ber is much smaller than the ob served.