Anom a lous Upwelling in Nan Wan : July 2008

Tid ally in duced, sud den tem per a ture drops in Nan Wan, par tic u larly pro nounced dur ing the spring tide, of ten bear eco log i cal con se quences. We use an Ocean Nowcast/Fore cast Sys tem (ONFS) at the US Na val Re search Lab o ra tory (NRL) to ex am ine a widely pub li cized cold-wa ter in tru sion event in July 2008, and com pare it with what we per ceived as a nor mal in tru sion event in June 2007. For a nor mal cold-wa ter in tru sion, cold anom a lies even tu ally prop a gate away in forms sim i lar to in ter nal Kel vin waves. For early July of 2008, the NRL ONFS sug gested an anom a lous, subtidal and es sen tially east ward cur rent near Nan Wan. The subtidal cur rent, su per im posed on the ebb cur rent, blocks the prop a ga tion of cold anom a lies in the di rec tion of in ter nal Kel vin waves, in duc ing anom a lous and in ef fi cient cold-wa ter dis persal. The NRL ONFS also sug gested the anom a lous subtidal current as a post-typhoon response to the passage of Typhoon Fengshen.


IN TRO DUC TION
Nan Wan, or the south bay of Tai wan, is a semi-en closed ba sin open to the Luzon Strait (Fig. 1).The north ward flowing Kuroshio fol lows the west ern ex treme of the Phil ip pine Sea from the east coast of Luzon to the east coast of Tai wan.In sum mer, the Luzon Strait is more or less free from Kuroshio en croach ment and, in con se quence, the Kuroshio often stays some dis tance away from Nan Wan and its vicin ity.In win ter, the en croach ing Kuroshio of ten en ters the north ern South China Sea, loop ing anticyclonically be fore ex it ing the South China Sea along the south ern tip of Tai wan (e.g., Centurioni et al. 2004;Wu and Chiang 2007;Liang et al. 2008).Con ceiv ably, the en croach ment breeds eddy shedding and moves the Kuroshio much closer to Nan Wan and its vi cin ity.
Tidal cur rents to the south of Nan Wan are es sen tially east ward dur ing ebb and west ward dur ing flood.Subtidal cur rents, es pe cially those re lated to the Kuroshio, could conceiv ably mod ify the strength of the flood and ebb cur rents.Di ur nal tides are, more of ten than not, stron ger than semi -di ur nal tides in the Luzon Strait and Nan Wan.The anomalous am pli fi ca tion of di ur nal tides over semidiurnal tides is due to the ba sin-wide res o nance that in volves the en tire ba sin of the South China Sea (Jan et al. 2007).In Nan Wan and its vi cin ity, the di ur nal tide, mod u lated by a semidiurnal beat, of ten in duces upwelling or cold-wa ter intru sion that oc curs daily (Lee et al. 1997).The upwelling is par tic u larly strong and wide spread dur ing the spring tide (Chen et al. 2004).On the east side of Nan Wan, the temper a ture drop lasts for a few hours each and usu ally oc curs once ev ery di ur nal tidal cy cle.In west and cen tral parts of the ba sin, sim i lar tem per a ture drop usu ally oc curs twice in one di ur nal tidal cy cle (Lee et al. 1999).The tem per a ture drop could be sud den and dras tic.On 24 No vem ber 1988, for ex am ple, a sud den tem per a ture drop of more than 10°C re sulted in a mass kill of fish (Su et al. 1989).
The o ret i cally, both capes (Mou-Bi-Tou and O-Luan-Bi in Fig. 1b) and their south ward ex ten sion ridges are ca pa ble of pro duc ing upwelling.A ridge in duces upwelling by lifting the in com ing tidal cur rent.A cape can achieve a sim i lar ef fect.It blocks the in com ing tidal cur rent, which needs to de tour around the cape to sup ply wa ters to the lee side.The in ef fi ciency of wa ter sup ply cre ates a rel a tively low sea level anom aly in the lee and bot tom wa ters nor mally upwell to fill the void.This is, in es sence, the con clu sion from the previous nu mer i cal model (Lee et al. 1999).Given the complex tidal con stit u ents, bathymetry and coast line ori en ta tion in and around Nan Wan, it is dif fi cult to as sess the im portance of each cape and ridge in pro duc ing upwelling with out re al is tic nu mer i cal sim u la tions.
In the af ter noon of 3 July 2008 lo cal time (early morn ing of July 3 in GMT), fish er men spot ted a large school of fish float ing in the west ern reaches of Nan Wan.The col lec tive di ag no sis af ter wards pointed to hy po ther mia as the likely cause.A bot tom tem per a ture sen sor at sta tion T lent fur ther sup port to this idea.The event mo ti vated us to in ves ti gate fur ther us ing an Ocean Nowcast/Fore cast Sys tem (ONFS) of the US Na val Re search Lab o ra tory (NRL) (Ko et al. 2008).Not sur pris ingly, the NRL ONFS iden ti fied this pe riod as a spring tide that gen er ated strong upwelling in Nan Wan.When com par ing the July 2008 upwelling event with previous ones, we also iden ti fied two types of path ways for the dis persal of cold anom a lies: one dis perses in the prop a ga tion di rec tion of in ter nal Kel vin waves and the other in the op posite di rec tion.In the for mal case or nor mal upwelling event, cold anom a lies seem to dis perse eas ily and do not overaccumulate in Nan Wan.In the lat ter case or anom a lous upwelling event, the dis per sion seems slow and the tem per ature drop in Nan Wan be comes more se vere.For the particular cold event in July 2008, the NRL ONFS sug gests that typhoon Fengshen in the pre ced ing month cre ated the pre con di tion ing for the anom a lous upwelling.

OCEAN NOWCAST/FORE CAST SYS TEM
We use a real-time Ocean Nowcast/Fore cast Sys tem developed at the US Na val Re search Lab o ra tory to fa cil itate the pres ent study.The NRL ONFS pro duces a daily, short-term (un der a week) fore cast of: mesoscale ocean currents, tem per a ture, sa lin ity, and sea level vari a tion.The system can be re lo cated to dif fer ent re gions and, once set up for a par tic u lar re gion, op er ates au to mat i cally.The sys tem is an in te gra tion of: a data-as sim i lat ing, dy nam i cal ocean model, a sta tis ti cal data-anal y sis model, and var i ous data streams for ocean bathymetry, cli ma to log i cal data, sur face forc ing, open bound ary forc ing, and ob ser va tions for data as sim i lation (Ko et al. 2008).The NRL Mod u lar Ocean Data As sim ila tion Sys tem (MODAS; Carnes et al. 1996;Fox et al. 2002) is used within ONFS as the data-anal y sis model.MODAS uses sat el lite data, in-situ ob ser va tions and his tor i cal sta tistics to gen er ate three-di men sional ocean tem per a ture and salinity anal y ses.The anal y ses are then as sim i lated into the dynamic model to pro duce an ocean nowcast.From the nowcast, the fore cast is con ducted with out data as sim i la tion using a me te o ro log i cal fore cast.This pa per only uses the nowcast por tion of the re sults for anal y sis.This pa per pri mar ily uses re sults from a high res o lu tion Luzon Strait Nowcast/Fore cast Sys tem (LZSNFS), an ap plica tion of NRL ONFS, to dis cuss upwelling in and around Nan Wan.The sys tem do main ex tends from 116°E to 125°E and from 17°N to 24°N, cov er ing the north ern South China Sea, the Luzon Strait, and a por tion of the Phil ip pine Sea with a hor i zon tal res o lu tion of 2.3 km.In terms of ver ti cal res o lu tion, there are 11 s-lay ers in the top 147 m and 29 z-lay ers be low.The sys tem de rives its ocean bot tom topography from the Dig i tal Bathymetry Data Base with 2minute res o lu tion (DBDB2).Open bound ary con di tions are de rived from a larger scale ONFS for the en tire East Asian Seas (EASNFS).The Or e gon State Uni ver sity (OSU) tidal forc ing drives the tidal cur rents.A global weather fore cast model (Navy Op er a tional Global At mo spheric Pre dic tion Sys tem or NOGAPS) and a re gional weather fore cast model (Cou pled Ocean/At mo sphere Mesoscale Pre dic tion System or COAMPS) pro vide the ocean sur face forc ing.The LZSNFS also as sim i lates sat el lite al tim e ter data and Multi-Chan nel Sea Sur face Tem per a ture (MCSST) from sat el litede rived AVHRR to im prove ac cu racy fur ther.
The sys tem is able to pro duce rather ac cu rate tidal current in the Luzon Strait and north ern South China Sea.For ex am ple, tidal cur rents flow ing over the ridges in the Luzon Strait gen er ate large-am pli tude non lin ear in ter nal waves in the north ern South China Sea, viv idly seen in sat el lite images.To re pro duce the tim ing and lo ca tion of these in ter nal waves re quires ac cu rate spa tial and tem po ral dis tri bu tion of tidal cur rents and LZSNFS was able to achieve this (Chao et al. 2007).
In ad di tion to LZSNFS, we also use re sults from the larger scale EASNFS, an other ap pli ca tion of NRL ONFS, to di ag nose the or i gin of anom a lous Nan Wan upwelling in July 2008.The EASNFS cov ers the west ern Pa cific and all the Asian mar ginal seas from 17.3°S to 52.2°N and from 99.2°E to 158.2°E with a res o lu tion of 1/16 de gree or 9.2 km at lat i tude of Luzon Strait.The sys tem de rives its open ocean bound ary con di tions from NRL global NCOM (Navy Coastal Ocean Model) (Rhodes et al. 2002) that op er ates daily.Con trary to fine-scale LZSNFS, the EASNFS does not simulate tides.The ver ti cal res o lu tion, forc ing and met hodology of EASNFS are oth er wise sim i lar to that in LZSNFS.The EASNFS has been ap plied for stud ies of cur rent at Korea/Tsushima Strait (Teague et al. 2006), trans port in the Gulf of Pa pua (Keen et al. 2006), ty phoon-ocean in ter ac tion at the West Pa cific Ocean (Lin et al. 2008) and air-sea in terac tion at the Ja pan/East Sea (Mooers et al. 2009).

NOR MAL UPWELLING DUR ING SPRING TIDES
Fig ure 2 il lus trates the agree ment be tween the ob served sea level and LZSNFS-pro duced coun ter part at sta tion T from 15 May to 15 June 2007.Be ing a mix ture of di ur nal and semidiurnal tides, the sea level vari a tion also con tains a pro nounced spring-neap cy cle.We choose June 2 to il lustrate Nan Wan upwelling event dur ing the spring tide.Tidal range in this day ex ceeds 1.6 m.
Fig ure 3    The right-bounded move ment of the two cold-wa ter masses ex plains why the tem per a ture drop oc curs once daily on the east side of Nan Wan, but twice daily in the west and cen tral parts of the ba sin (Lee et al. 1999).Leav ing Nan Wan upwelling aside, the ebb cur rent also gen er ates a cold cy clonic eddy on the lee (east) side of O-Luan-Bi.It be gins to form at hour 4, ex pands and in ten si fies un til hour 12, and de cays slowly there af ter.
In coastal oceans with con tin u ous strat i fi ca tion and vari able bot tom slope, coastal-trapped waves be come a hybrid of in ter nal Kel vin waves, con ti nen tal shelf waves and top o graphic Rossby waves (Wang and Mooers 1976).It is dif fi cult to sep a rate them cleanly even if the ocean o graphic set ting is strictly uni form along shore.The ir reg u lar coast line and bathymetry around Nan Wan com pli cate the set ting even fur ther.Nev er the less, fur ther re sults point to the close re sem blance of cold-anom aly prop a ga tion to in ter nal Kel vin waves.In the LZSNFS, siz able cold anom a lies and their right-bounded prop a ga tion show up to gether dur ing the spring tide but di min ish there af ter.The pre dom i nance of den sity forc ing as so ci ates the right-bounded prop a ga tion mostly to in ter nal Kel vin waves, which owe their ex is tence to den sity anom a lies, rather than con ti nen tal shelf waves or top o graphic Rossby waves, which arise from the bot tom slope.Judg ing em pir i cally, one can es ti mate the phase speed from the move ment of the cold anom aly that ini tially appears off Mou-Bi-Tou.From hour 8 to hour 14, the prop a gation speed is about 45 km per day or ~0.5 m s -1 .Am bi ent currents around Nan Wan, tidal and subtidal alike do not seem able to stop the right-bounded prop a ga tion.To the contrary, the right-bounded prop a ga tion thrives in the "shadow zone" of the ad verse am bi ent cur rents.For ex am ple, from hour 6 to hour 10, the west ward ex pan sion of the cold anomaly from O-Luan-Bi is re stricted mostly to Nan Wan and its im me di ate south, where the ad verse am bi ent cur rent is weak or re versed.From hour 12 to hour 16, the rightbounded prop a ga tion from Mao-Bi-Tou to the west coast of Tai wan oc curs at the ex pense of di min ish ing ad verse ambient cur rent.
The right-bounded prop a ga tion of cold anom a lies in Fig. 3 is against the pre vail ing tidal cur rent from hour 6 to hour 10.Tidal cur rent is there fore not re spon si ble for it.To as so ci ate it with the earth's ro ta tional ef fect, we have ter minated the Coriolis force in Fig. 4 af ter hour 6, the on set of right-bounded prop a ga tion.By re mov ing the earth ro ta tion, tidal cur rents also change grad u ally in time.Nev er the less, the right-bounded prop a ga tion di min ishes quickly be fore the tidal cur rent changes sig nif i cantly.For ex am ple, the tidal cur rent in Fig. 4 (with out the earth ro ta tion) es sen tially re -mains the same as in Fig. 3 (with the earth ro ta tion) from hour 6 to hour 12, but the right-bounded prop a ga tion es sentially van ishes in Fig. 4 within only six hours af ter the ter mina tion of Coriolis force.This is plau si ble, be cause the earth ro ta tion sup ports the prop a ga tion of in ter nal Kel vin waves.
If ide al ized to the ex treme, in ter nal Kel vin waves propagate along a ver ti cal coast and over a flat bot tom.With a seaward deep en ing bot tom in the pres ent set ting, where is the ef fec tive coastal wall and ap pro pri ate wa ter depths to sup port the cold-wa ter prop a ga tion in forms sim i lar to an inter nal Kel vin wave?To ex am ine along this line, we use LZSNFS out puts to com pute the daily av er age of strat i fi cation mesh-by-mesh for 2 June 2007, and de rive the phase speed and Rossby ra dius of the first baroclinic mode around Nan Wan from it (Fig. 5).Both quan ti ties in crease with the wa ter depth and there fore vary mark edly in the area.Nev erthe less, rel a tive to the prop a ga tion speed along the ax ial route of cold anom aly prop a ga tion as seen in Fig. 3, Fig. 5 sug gests a com pa ra ble phase speed of about 0.5 m s -1 and a com pa ra ble Rossby ra dius rang ing from 5 to 10 km.It is difficult to quan tify fur ther be yond this point be cause, in addition to com plex i ties men tioned above, the os cil lat ing tidal cur rent also mod u lates the prop a ga tion.
For lack of a better term, we use the term "in ter nal Kelvin wave" loosely herein to in clude also its non lin ear and vari able bot tom ex ten sions.In the ory, the ef fect of am bi ent cur rent on the in ter nal Kel vin wave prop a ga tion could be non lin ear.Even with out the am bi ent cur rent, non lin ear steep en ing of in ter nal waves could en hance their prop a gation speed up to about 10 per cent.How ever, the com plex envi ron ment around Nan Wan, as we il lus trated in Fig. 4, prevents us from iso lat ing the non lin ear ef fect.

ANOM A LOUS UPWELLING IN JULY 2008
Fig ure 6a shows the LZSNFS-pro duced sea level vari ation (in blue) at sta tion T from 27 June to 8 July 2008.The ob served coun ter part (in red), dis con tin ued for ser vice af ter July 2, is also su per im posed to il lus trate the agree ment.Enter ing July, the tides be gin to en ter the spring tidal pe riod.On July 3, for ex am ple, the tidal range is about 1.8 m, comparable to the tidal range that pro duces nor mal upwelling dur ing spring tides (Fig. 2), but the Nan Wan upwelling, as we dem on strate be low, is much stron ger.
Fig ure 6b shows the ob served bot tom tem per a ture at sta tion T for the same pe riod (in red) and cor re spond ing bottom tem per a ture pro duced by LZSNFS (in blue).The Nowcast/Fore cast Sys tem re pro duces the tim ing of each tem per a ture drop rather well.How ever, the LZSNFS gen erally un der es ti mates the am pli tude of each tem per a ture drop.In par tic u lar, af ter each drop, the tem per a ture of ten does not re bound to the ob served level in LZSNFS.Two rea sons can ac count for the dis crep ancy.First, sta tion T is less than 1 km south west of the ther mal dis charge from a nu clear power plant.Con ceiv ably, the ther mal dis charge is able to raise the tem per a ture, but its scale is too small to be in cluded in LZSNFS.Sec ond, the hor i zon tal res o lu tion of LZSNFS (2.3 km), though fine enough from the mod el ing point of view, will likely gen er ate hor i zon tal tem per a ture dif fu sion that is higher than re al ity, thus re duc ing the am pli tude of tem per a ture drops and re bounds.
Fig ure 7 shows bi hour ly cur rents and tem per a ture at 25 m depth for 2 July 2008.Dur ing the time span, there is a strong subtidal out flow from the north ern South China Sea to the Phil ip pine Sea.When su per im posed on the tidal current, the ebb cur rent of ten ex ceeds 3 m s -1 .The out flow, tidal plus non-tidal, is strong enough to block the in ter nal Kel vin wave prop a ga tion, the nat u ral con duit for the dis persal of cold anom a lies.This must be the rea son for the larger-thannor mal tem per a ture drops.general, cold anom a lies off both capes fail to dis perse follow ing the prop a ga tion of in ter nal Kel vin waves dur ing the time span; block ing by the strong subtidal out flow ap pears to be re spon si ble.Be cause of the block ing, tem per a ture drops are gen er ally larger than nor mal.The anom a lous upwelling pat tern as seen from LZSNFS per sists from 1 July to about 5 July 2008.This is the pe riod of al leged hy po ther mia in the fish pop u la tion.Fig ure 8, sim i lar to Fig. 5, shows the LZSNFS-pro duced phase speed and Rossby ra dius of the first baroclinic mode around Nan Wan based on the daily av er aged strat i fi ca tion of 2 July 2008.Rel a tive to the nor mal upwelling con di tion on 2 June 2007, a slightly stron ger strat i fi ca tion on 2 July 2008 in creases the phase speed and Rossby ra dius mar ginally near Nan Wan.With such a mod est phase speed of about 0.5 m s -1 just out side the per im e ter of Nan Wan, the strong, east ward subtidal flow, as we il lus trated in Fig. 7, could easily block the right-bounded prop a ga tion of cold anomalies in forms sim i lar to in ter nal Kel vin waves.

SUG GESTED OR I GIN OF ANOM A LOUS SUBTIDAL EAST WARD FLOW OFF NAN WAN
Ty phoon Fengshen moved through the north ern South China Sea from 22 to 25 June 2008, lead ing us to sus pect it as the main cause of the anom a lous subtidal east ward flow off Nan Wan.We now turn to the larger-scale EASNFS, which ex cludes tides, to ex am ine the ef fect of this ty phoon.Fig ure 9 con trasts subtidal sur face cir cu la tion pat terns between nor mal upwelling pe riod (2 June 2007) and anom alous upwelling pe riod (2 July 2008).The dif fer ence is more than sub tle.For 2 June 2007 (Fig. 9a), the Kuroshio more or less stays away from the South China Sea.This is the typ i cal summerlike pat tern, in which the Kuroshio Cur rent stays some dis tance away from Nan Wan.For 2 July 2008 (Fig. 9c), shortly af ter the pas sage of ty phoon Fengshen, the Kuroshio Cur rent be comes rather in tru sive; the en croach ment produces two an ti cy clonic me an ders mov ing into the north ern South China Sea.The north ern me an der pro duces strong   To the right of Figs.9a and c are con tem po ra ne ous optimal in ter po la tions of sea sur face height fields de rived from all avail able sat el lite al tim e ter mea sure ments, in cluding JA SON, GFO, and ENVISAT (Jacobs et al. 2002).In par tic u lar, this re sult en ters EASNFS through data as sim i lation, which slowly in jects the op ti mal in ter po la tion into the dy nam i cally ac tive sys tem (Chap man et al. 2004).The sim ilar ity be tween EASNFS-de rived and al tim e ter-de rived prod ucts emerges as ex pected be cause of the data as sim i la tion.The dis crep ancy is also un der stand able be cause of the difference in meth od ol ogy.Sim i lar i ties and dis sim i lar i ties aside, one also sees anom a lous Kuroshio en croach ment in July 2008 from the sat el lite-de rived prod uct (Fig. 9d).
The en croach ing Kuroshio af ter Ty phoon Fengshen is counterintuitive be cause it is a dis tinc tively winterlike behav ior.Though dif fer ent in scale, the en croach ment is a miniature form of win ter in tru sion.In win ter, the pre vail ing north east mon soon moves wa ters from the north ern part to the south ern part of the South China Sea.The con se quent low sea level anom aly in the north ern South China Sea, in turn, in vites Kuroshio to in trude (e.g., Chern and Wang 2003).In other words, the low sea level in the north ern South China Sea is the re quired pre con di tion ing for the Kuroshio in tru sion in win ter.Ex trap o lat ing the win ter scenario fur ther, did Ty phoon Fengshen cre ate a sim i lar but weaker, win ter-like low sea level anom aly to in vite the Kuroshio en croach ment?
Fig ure 10, again pro duced by EASNFS, shows se lected snap shots of wind stress field, ocean sur face cur rents and sea sur face height from 1800 GMT 22 June to 1500 GMT 1 July 2008.Ty phoon Fengshen moved rather slowly from Luzon to south ern China in three days (from June 22 to 25).A sta tion ary ty phoon over the north ern South China Sea would pro pel wa ters away from its cen ter through sur face Ekman trans port and gen er ate a sea level de pres sion un derneath it.Most of the ex cess wa ters should exit the Luzon Strait, the only ma jor deep open ing of the South China Sea.Since Fengshen was mov ing, one ex pects a de layed response in its wake.A mov ing ty phoon will also gen er ate iner tial cur rents ra di at ing away from its wake with a pe riod of Fig ure 10 clearly shows ra di at ing in er tial cur rents from the wake of Fengshen.Of more rel e vance to Nan Wan is the strong east ward subtidal flow in its vi cin ity on June 26 (middle right panel).The east ward flow does not in duce strong upwelling in Nan Wan or hy po ther mia in the fish pop u la tion, be cause June 26 is in the mid dle of neap tide pe riod.Af ter Fengshen land ing on south ern China, the sea level de pression in the north ern South China Sea be comes rather ex tensive, ap par ently squeez ing a sub stan tial amount of sur face wa ter away from the South China Sea into the Phil ip pine Sea.The con se quent pres sure def i cit in the north ern South China Sea in vites Kuroshio en croach ment, in duc ing strong, east ward subtidal flow near Nan Wan on July 1 (bot tom right panel).Sim ply put, the ra di at ing in er tial cur rent from the wake of Fengshen first ini ti ated the east ward subtidal flow near Nan Wan, and the Kuroshio encroachment continued it later on.
Ob vi ously, the low sea level anom aly in the north ern South China Sea must be close enough to the Kuroshio to in duce the Kuroshio en croach ment.Fig ure 11 (left panel), again from EASNFS, shows the zonal dis tri bu tion of threehourly sea sur face height, av er aged from 20°N to 22°N, be fore, dur ing and af ter Fengshen pas sage.Fengshen moved th rough the north ern South China Sea from June 23 to 25.Shortly af ter, the de layed for ma tion of low sea level anom aly ex pands, with a mi nor pause in be tween, to the Luzon Strait (121°E) on July 1.There af ter the Kuroshio, char ac ter ized by high sea level, be gins to en croach as antic i pated.
Fig ure 11, right panel, shows a con cur rent time se ries of sea sur face height anom aly at 21.5°N and 120°E from AVISO NRT (near real time) prod uct.AVISO de rives the sea sur face height from near real time al tim e ter data with al go rithms that are com pletely in de pend ent from the treatment of al tim e ter data in EASNFS.It there fore pro vides a some what in de pend ent ver i fi ca tion of EASNFS's per formance.Spe ci f i cally, AVISO NRT prod uct blends alongtrack mea sure ments of JA SON, ERS-2, Topex-Po sei don, GFO, and EVISAT with an op ti mal in ter po la tion.At any given time, the op ti mal in ter po la tion also weighs mea surements in a time win dow that spans from 43 days be fore to 3 days af ter.Judg ing from the re sult, AVISO seems to overweigh the past and un der-weigh the fu ture.In con se quence, the Kuroshio en croach ment in AVISO oc curs ap prox i mately one day ear lier than in EASNFS.The slight dis crep ancy, how ever, does not up set the dy namic pro cess we in tend to es tab lish herein.
To quan tify fur ther, Fig. 12 shows the 3-hourly vol ume trans port across the en tire cross-sec tion of Luzon Strait from 15 June to 10 July 2008, pro duced by the EASNFS.Also shown is a smoother curve af ter ap ply ing a 2-day low-pass fil ter.In com put ing them, the depth av er age is not ideal because it also con tains con tri bu tions from greater depths that are not rel e vant to the Kuroshio en croach ment.It nev er theless tells a lit tle more.Leav ing high-fre quency fluc tu a tions aside, the in flow from the Phil ip pine Sea to the South China Sea gen er ally in creases prior to the ar rival of Ty phoon Fengshen (June 22).Af ter Fengshen im pacts fully on the north ern South China Sea (June 24), the in flow from the Phil ip pine Sea de creases con tin u ously and even tu ally becomes an out flow around June 26.Ap par ently, Fengshen cre ates a ra di at ing in er tial cur rent in its wake and pro pels north ern South China wa ters out of the Luzon Strait in this time span.The out flow re verses to be come a pro nounced inflow on June 29; this pre con di tions the Kuroshio en croachment in re sponse to the low sea level of the north ern South China Sea.The in flow re verses back to a weak outflow shortly be fore July 1 and grad u ally be comes a mod est in flow again there af ter.Fig ure 13 shows power spec trum den si ties for the Luzon Strait trans port.The red spec trum, cov er ing the pe riod from 2008/06/01 00 GMT to 2008/07/18 18 GMT, con tains a substantial, rel a tively qui es cent, pre-ty phoon time span.Two peaks stand out counterintuitively near di ur nal and semidiurnal pe ri ods, even though the EASNFS does not sim ulate tides.The near-di ur nal os cil la tions arise from res o nance (quar ter-wave seiche) that in volves the en tire South China Sea (Jan et al. 2007).In the Luzon Strait, the peak-to-peak zonal dis tance be tween the two north-south run ning ridges (Fig. 1a) is about 120 km.It takes about 12 hours for long (non-dispersive) in ter nal waves to travel from one ridge to the other (Chao et al. 2007), pro duc ing the near-semidiurnal spec tral peak.
Re strict ing at ten tion to the post-ty phoon pe riod, the blue spec trum in Fig. 13 cov ers the time span from 0000 GMT 2008/06/25 to 1800 GMT 2008/07/09.In ad di tion to the two peaks near di ur nal and semidiurnal pe ri ods, a third peak near the in er tial pe riod (35 hours) emerges.The in ertial cur rent ra di at ing away from the wake of the ty phooncov ered area pro duces these os cil la tions in the Luzon Strait.

CON CLU SIONS
Nan Wan upwelling in ten si fies dur ing the spring tide.Both capes and their south ward ex ten sion ridges are responsible for the upwelling.Us ing NRL Luzon Strait Ocean Nowcast/Fore cast Sys tem, we have iden ti fied and contrasted two modes of dis persal for the upwelled cold anomalies.Un der nor mal con di tions, cold-wa ter masses propag ate away from Nan Wan in forms sim i lar to in ter nal Kelvin waves.Since this is the nat u ral way of dis per sion, our results, al though lim ited so far, in di cate that the cold anom a lies are less likely to over-ac cu mu late in Nan Wan under this nor mal mode of dis per sion.For early July of 2008, the Nowcast/Fore cast Sys tem sug gested the ex is tence  of an anom a lous, subtidal and es sen tially east ward flow near Nan Wan, which, when su per im posed on the ebb tide, blocks the cold wa ter dis per sion in the prop a ga tion di rec tion of inter nal Kel vin waves.We sug gested Ty phoon Fengshen as the main cause of the anom a lous subtidal flow.Un der this anom a lous con di tion, the Nowcast/Fore cast Sys tem suggested over-ac cu mu la tion of cold anom a lies in Nan Wan.Not sur pris ingly, the al leged hy po ther mia in the fish pop u lation oc curred dur ing this pe riod.
Our re sults, though a step for ward in un der stand ing the Nan Wan upwelling, also open a se ries of ques tions wait ing for an swers.For ex am ple, are there still other modes of upwelling in Nan Wan? Will ty phoons with sub stan tially differ ent tracks also in duce the anom a lous upwelling in Nan Wan? Dur ing the least ob served but ex ten sive Kuroshio intru sion pe riod in win ter, is Nan Wan upwelling mostly as anom a lous as in July 2008?Our in ves ti ga tions along this line are ongoing.
shows bi hour ly cur rents and tem per a ture at 25 m depth for 2 June 2007.At hour 0 (GMT), the start of ebb cur rent, upwelling be gins to ap pear south west of O-Luan-Bi.The south ward ex ten sion ridge from O-Luan-Bi ap par ently lifts the ebb cur rent to pro duce the upwelling.It in ten si fies and ex pands un til hour 8. Upwelling off the tip of Mou-Bi-Tou starts at hour 4, and there af ter in ten si fies and ex pands un til hour 8.At hour 10, as the tidal cur rent recesses, the cold anom aly off Mou-Bi-Tou be gins to move to the west ern coast of Tai wan, while the cor re spond ing anomaly off O-Luan-Bi moves to the cen tral ba sin of Nan Wan.Both move ments are in keep ing with the prop a ga tion di rection of coastal-trapped waves.From hour 10 to hour 14, the two cold anom a lies con tinue to prop a gate away in the pro -

Fig. 1 .
Fig. 1.Bathymetry in the vi cin ity of (a) Luzon Strait and (b) Nan Wan.Two capes bound ing Nan Wan are Mou-Bi-Tou (MBT) and O-Luan-Bi (OLB).Sta tion T in (b) re cords bot tom wa ter tem per a ture at 7 m depth and sea level vari a tions.

Fig. 2 .
Fig. 2. The ob served sea level vari a tion (in red) and model-pro duced coun ter part (in blue) at sta tion T from 15 May to 15 June 2007.

Fig. 3 .
Fig. 3. Bi hour ly cur rents and tem per a ture at 25 m depth for 2 June 2007, show ing nor mal cold-wa ter in tru sion dur ing the spring tide.Num bers on land in di cate time (GMT) in hours.

Fig. 4 .
Fig. 4. Same as Fig. 3 ex cept with the re moval of Coriolis force af ter 2 June 2007 at 0600 GMT.
At hour 2, the south ward ex ten sion ridge off O-Luan-Bi lifts the east ward flow to pro duce upwelling.The cold anom aly from O-Luan-Bi in ten si fies, ex pands and moves toward the cen tral ba sin of Nan Wan from hour 2 to hour 6, and dis si pates in Nan Wan at hour 8.The ad verse pre vail ing outflow is ap par ently too strong to al low this cold anom aly mov ing to the west coast of Tai wan.At hour 10, the gen erally west ward cur rent from east of O-Luan-Bi gen er ates another size able cold anom aly on the lee (west) side of O-Luan-Bi.It dis si pates in cen tral Nan Wan from hour 10 to hour 18, un able to move to the west side of Tai wan.The cold anom aly gen er ated off Mou-Bi-Tou is even more vul ner able.At hour 4, a small re gion of cold anom aly first ap pears to the south east of Mou-Bi-Tou.It in ten si fies and ex pands a little from hour 4 to hour 8, and dis si pates there af ter.In

Fig. 5 .
Fig. 5.The phase speed (a) and Rossby ra dius (b) for the first baroclinic mode around Nan Wan on 2 June 2007.

Fig. 6 .
Fig. 6.(a) The ob served sea level vari a tion (in red) from 27 June to 2 July 2008 at sta tion T and LZSNFS-pro duced coun ter part (in blue) from 27 June to 8 July 2008.(b) Cor re spond ing time se ries of bot tom tem per a ture from ob ser va tion (red) and LZSNFS (blue).The time is in GMT, which is 8 hours ahead of lo cal time.
from the north ern South China Sea to the Phil ip pine Sea near Nan Wan.

Fig. 9 .
Fig. 9. Subtidal sur face cir cu la tion and sea sur face height (in color) for (a) nor mal upwelling at 1200 GMT on 2 June 2007 and (c) anom a lous upwelling at 1200 GMT on 2 July 2008, pro duced by the East Asian Seas Nowcast/Fore cast Sys tem (EASNFS).Pan els (b) and (d) show op ti mal in terpo la tions of cor re spond ing sea sur face height fields from all avail able, con tem po ra ne ous al tim e ter data.

Fig. 11 .
Fig. 11.Zonal dis tri bu tion of sea sur face height (av er aged from 20°N to 22°N) be fore, dur ing, and af ter Fengshen pas sage, pro duced by 3-hourly outputs of EASNFS (left), and sat el lite-de rived sea sur face height at 21.5°N and 120°E from AVISO (right).

Fig. 12 .
Fig. 12. Three-hourly vol ume trans port across the Luzon Strait (120.8°E)be fore, dur ing and af ter the pas sage of ty phoon Fengshen, pro duced by East Asian Seas Nowcast/Fore cast Sys tem (EASNFS).Also shown is the low-fre quency re sponse af ter ap ply ing a 2-day low-pass fil ter.Pos i tive (neg ative) trans ports are east ward (west ward).

Fig. 13 .
Fig. 13.Power spec trum den si ties for the 3-hourly Luzon Strait transport.The red line uses a time se ries from 0000 GMT 2008/06/01 to 1800 GMT 2008/07/18, in clud ing a sub stan tial qui es cent pe riod be fore the ar rival of Ty phoon Fengshen.The blue line uses a shorter, posttyphoon time se ries from 0000 GMT 2008/06/25 to 1800 GMT 2008/07/09, ac cen tu at ing the post-ty phoon in er tial os cil la tions.