the nondeterministic nature of kuroshio penetration and ...the ﬁrst one enters luzon strait and...

1712 VOLUME 31J O U R N A L O F P H Y S I C A L O C E A N O G R A P H Y

The Nondeterministic Nature of Kuroshio Penetration and Eddy Shedding in theSouth China Sea*

E. JOSEPH METZGER AND HARLEY E. HURLBURT

Naval Research Laboratory, Stennis Space Center, Mississippi

(Manuscript received 6 October 1999, in final form 1 September 2000)

ABSTRACT

A ⅛8, 6-layer Pacific version of the Naval Research Laboratory Layered Ocean Model is used to investigatethe nondeterministic nature of Kuroshio intrusion and eddy shedding into the South China Sea (SCS) on annualand interannual timescales. Four simulations, which only differ in the initial state, are forced with 1979–93European Centre for Medium-Range Weather Forecasts reanalysis 1000 hectopascal (hPa) winds and then con-tinued in 1994–97 with ECMWF operational 1000-hPa winds. The model shows differing amounts of Kuroshiopenetration across all four simulations for the yearly means, indicating a large degree of nondeterminism at thistimescale. This nondeterminism is quantified by a technique that separates the variability of a model variableinto deterministic (caused by direct atmospheric forcing) and nondeterministic (caused by mesoscale flow in-stabilities) components. Analysis indicates substantial nondeterministic sea surface height and upper-layer ve-locity variability in the vicinity of Luzon Strait. A quantitative measure of Kuroshio intrusion into the SCS ispresented that allows interexperiment comparisons and investigation of interannual variability, and attempts aremade to positively correlate it with the oceanic and atmospheric environment. Yearly mean Kuroshio intrusionis not strongly linked to Luzon Strait transports or to changes in the North Equatorial Current bifurcation latitude(which is related to the northward Kuroshio transport east of Luzon). Likewise, no relationship could be foundthat linked interannual variability of yearly mean Kuroshio intrusion or monsoon season mean Luzon Straittransport with the corresponding zonal or meridional wind components, wind stress magnitude, or wind stresscurl. However, there was a close relationship between the mean seasonal cycles of the Luzon Strait transportand the northeast–southwest monsoon. Eddy shedding and deep Kuroshio intrusion are rare events during theperiod of ECMWF reanalysis forcing, but are persistent features during the ECMWF operational time frame.While the wind stress is consistent across the reanalysis/operational time boundary, large differences exist inthe wind stress curl pattern over the Luzon Strait and interior of the SCS basin. For contemporaneous years,the ECMWF operational winds produce higher curl extrema (by a factor of 2) and a much sharper north–southgradient in Luzon Strait. The net effect is to produce more Ekman pumping, a deepening of the thermocline,and a more deeply penetrating Kuroshio during the 1994–97 ECMWF operational forcing time frame. Thus,while normal interannual variations of the wind curl did not produce a deterministic response of simulatedKuroshio intrusion, the marked differences in curl between the two atmospheric products did have a substantialimpact.

1. Introduction

The South China Sea (SCS) is the largest semien-closed marginal sea in the western tropical Pacific Ocean(Fig. 1). Its main connection with the Pacific Ocean isvia Luzon Strait, which is very wide and has a sill depthof approximately 2400 m. The connections to the southare either very narrow (Sibutu Passage in the southernSulu Sea) or shallow (a 29-m sill depth at KarimataStrait, which connects the Sunda Shelf and the Java

* Naval Research Laboratory Contribution Number JA/7323-99-0038.

Corresponding author address: Mr. E. Joseph Metzger, Naval Re-search Laboratory, Code 7323, Stennis Space Center, MS 39529-5004.E-mail: [email protected]

Sea). To the north, Taiwan Strait connects the SCS withthe East China Sea but it is generally believed that theTaiwan Warm Current flows northward throughout theyear (Chuang 1986; Zhao and Fang 1991). Thus, themain interaction of the SCS with the surrounding watersis through Luzon Strait.

Metzger and Hurlburt (1996, hereafter MH) and thepresent study have considered three major componentsof upper ocean flow through Luzon Strait. The first oneenters Luzon Strait and exits through another strait. Itappears as a branch off the Kuroshio into the SCS justnorth of the Philippine Islands and is the only one ofthese that contributes to the long-term mean LuzonStrait transport. The second component is due to vari-ations in the volume of water above the thermoclinewithin the SCS. This one has a strong seasonal cycleassociated with the northeast–southwest monsoon. De-

JULY 2001 1713M E T Z G E R A N D H U R L B U R T

FIG. 1. Geometry of the South China Sea region plotted on the ⅛8 NRL Layered Ocean Modelgrid. The model land–sea boundary is the 200-m isobath, except for Taiwan Strait and the twointerior Philippines passages.

pending upon the direction of the flow, it can appear asa branch off the Kuroshio into the SCS on the southernside of Luzon Strait or as a current feeding into theKuroshio on the northern side of the strait. The thirdcomponent appears as a Kuroshio loop or intrusion intothe SCS that enters through the southern part of LuzonStrait and exits through the northern part. In relation tothe flow through Luzon Strait, MH concentrated on thefirst two components, while the present study focusesprimarily on the third. However, it is not always clearhow to sort out observations in relation to these threecomponents as well as locally driven flows. Thus, attimes this will not be clear-cut in the subsequent dis-cussions of observational evidence.

The upper ocean circulation within the SCS is dom-inated by the seasonally reversing monsoon winds,which typically blow from the northeast during boreal

winter and from the southwest during boreal summer.Shaw et al. (1999) suggest that the wind stress curl isthe main driving force of the circulation in the deepbasin of the SCS except near Luzon Strait. Huang et al.(1994) and Fang et al. (1998) indicate a central basincyclonic (anticyclonic) circulation during the winter(summer) monsoon. In the northern SCS along the shelfbreak at a depth of 200–400 m, and most likely orig-inating near Hainan Island, the South China Sea WarmCurrent (SCSWC) flows northeastward, even during theperiods of the northeast monsoon (Guan 1978, 1985).However, the existence of the SCSWC is somewhat con-troversial. Repeated measurements have yet to confirmthat this warm current is a permanent feature. Imme-diately offshore of the SCSWC, Guo et al. (1985) havefound an equally strong southwestward flowing currentduring the winter. The coastal current along Vietnam


also responds to the seasonally changing monsoonwinds; it flows to the north during the southwest mon-soon and to the south during the northeast monsoon(Wyrtki 1961).

a. Hydrographic evidence for Kuroshio intrusion intothe SCS

The Kuroshio (Black Stream) originates where theNorth Equatorial Current (NEC) bifurcates along theeastern coast of the Philippines at approximately13.58N; the southward flowing Mindanao Current has avolume transport of approximately 27 Sv (Sv [ 106 m3

s21) at 88N, while the northward-flowing Kuroshiotransport is approximately 14 Sv at 188N (Qu et al.1998). As the Kuroshio flows northward and enters theregion of Luzon Strait, it has been observed to bypassthe strait (Wang and Chern 1996) as well as penetratedeeply into the SCS and shed eddies. When penetrating,the inflow is typically northwestward in the southernLuzon Strait (Balintang Channel). It turns anticycloni-cally and exits as a northeastward-flowing current in thenorthern Luzon Strait (Bashi Channel) (Nitani 1972;Huang et al. 1994).

Hydrographic data exist for the northern SCS/LuzonStrait region, but repeated cruises during all seasons arelacking and, thus, the seasonal climatology is not fullyknown. A few studies are described here, but it shouldbe noted that cruises taken in the same season do notindicate the same degree of Kuroshio intrusion. Thus,they provide evidence that the interaction between thisPacific western boundary current and the SCS is gov-erned by deterministic interannual variability, nonde-terministic variability, or a combination of both. Spring(March) cruise data were analyzed by Xu et al. (1995),who found the Kuroshio penetrating to approximately1198E and mostly in the northern half of the strait, whilethe autumn (September) cruise data of Guo and Fang(1988) indicated intrusion to about 1208E. As opposedto these one-time cruises, Shaw (1991) looked at his-torical hydrographic data across all seasons and over awide region of the northern SCS and found evidence ofan intrusion current over the entire continental marginof the northern SCS in late autumn and winter. Thatstudy, along with evidence gathered by Shaw (1989),describes the seasonal cycle as follows: (i) from Mayto September the Kuroshio front meanders into LuzonStrait (an intrusion current may not exist), (ii) fromOctober to January an intrusion current may begin toform and penetrate along the continental margins, and(iii) from February to May the intrusion current decays.Shaw (1991) also suggests that it is unlikely that aninertial current flows hundreds of kilometers into theSCS, but the intrusion current is part of the generalcirculation of the northern SCS. A more recent studypublished by Fang et al. (1998) provides a concise de-scription of observational studies related to the Kuroshiointrusion, many of which come from the Chinese lit-

erature. These again indicate the variability of the Ku-roshio’s interaction with the SCS.

Evidence of warm core (anticyclonic) eddies in thenorthern SCS have been found by Wang and Chern(1987a) and Li et al. (1998). The former study reportsan eddy diameter of approximately 200 km, while thelatter found an eddy with a diameter of approximately150 km and a vertical expression as deep as 1000 m.Li et al. (1998) have also performed a hydrographicanalysis that suggests the subsurface water of the eddyis of Kuroshio origin, while the surface waters associ-ated with the ring are believed to consist of entrainedSCS waters. Although some eddies have been observed,the hydrographic observations are insufficient to deter-mine how frequently eddies are shed from the main coreof the Kuroshio. Clearly, satellite data will be requiredto answer this question.

b. Dynamical mechanisms for the Kuroshio intrusion

The dynamics that govern the intrusion of the Ku-roshio into the SCS are also not clearly understood. Ahypothesis promoted by Wang and Chern (1987b) pro-poses that the Kuroshio intrusion only occurs during thenortheast monsoon season as the winds deflect the Ku-roshio transport westward into Luzon Strait. Farris andWimbush (1996) concur with this theory, as they haveexamined satellite derived sea surface temperature im-ages and linked the intrusion current development pro-cess with an integrated supercritical wind stress param-eter. However, Shaw (1991) suggests that the generalcirculation in the northern SCS is probably driven byhigher sea level in Luzon Strait. A weakening of thispressure gradient in summer may prevent a strong in-trusion current from developing. He also suggests thatthe intrusion process may be related to the transport ofthe Kuroshio or the reversal of the wind direction fromthe northeast in winter to the southwest in summer.

c. Numerical modeling studies of the SCS/LuzonStrait region

Kuroshio intrusion into the SCS has also been studiedby employing numerical ocean models of differenttypes, covering different domains, and applying differ-ent forcing. The idealized geometry in the model of W.Li et al. (1996) can be applied to Luzon Strait, and itindicates an intrusion current that penetrates deeperwhen the gap in the western boundary becomes wideror the latitude decreases. Other models that use realisticcoastlines have applied winds only for a specific seasonin an attempt to examine the response of the Kuroshio.For example, Chern and Wang (1998) applied sum-mertime forcing to a very limited area model (10-kmresolution) of Luzon Strait and suggest that the retreatof the intrusion current may be related to the northwardmovement of the strongly stratified SCS waters drivenby southwesterly winds, while Li et al. (1993) used


wintertime forcing in their ½8 barotropic full basin Pa-cific model and found a branch of the Kuroshio thatpenetrated as far east as 1138E. Monthly varying forcingwas used by R.-F. Li et al. (1996), who noted Kuroshiopenetration to at least 1168E in all months. They suggestthat variations in the Kuroshio and the whole currentsystem of the Pacific have an important influence on thecurrent entering the SCS via Luzon Strait. Shaw andChao (1994) used a limited-area model of the SCS re-gion, also with monthly wind forcing, that only pro-duced the Kuroshio intrusion during the winter months;they suggest that summertime intrusion is blocked bythe northeastward flow from the Vietnamese coast to-ward Luzon Strait. They also indicate that most waterexchange between the SCS and Pacific is concentratedin the upper 300 m.

A ½8 global, 1.5-layer reduced gravity global modelwas used by MH to study the coupled dynamics of theSCS and Pacific Ocean region. That paper focused main-ly on the controlling factors of the mean Luzon Straittransport and its variability. The Kuroshio intrusion wasbriefly addressed in their Plate 1, which compared theannual mean sea surface height (SSH) from a ½8 globalmodel with higher resolution (⅛8 and 1⁄168) global andPacific counterparts. They noted that the Kuroshio pen-etration was pronounced in the ½8 model, but much lessso at higher resolutions. This is probably due to a muchbetter model representation of the north–south Babuyanand Batan Island chains; namely, the higher horizontalresolution allows more islands to be resolved and in-cluded as part of the coastline geometry. This is sup-ported by a finding in Hurlburt and Thompson (1982)that gives a length scale (y c/b)1/2 for westward bendingof the Loop Current in the Gulf of Mexico. This meansthat higher velocity at the core of the current (y c) inhigher resolution models tends to increase the lengthscale for westward bending and to reduce the westwardbending on a given length scale ,(y c/b)1/2, where b isthe planetary vorticity. The westward bending is furtherreduced by the narrower strait at higher resolution. Thus,

an accurate representation of the islands and coastlinesappears to be one of the critical elements in modelingthis region.

Section 2 of this paper provides a description of theNaval Research Laboratory Layered Ocean Model(NLOM) and the associated experiments used in thisstudy. Annual and seasonal means are presented alongwith a description of the Luzon Strait transport. Nextcomes a discussion of a quantitative measure of theKuroshio penetration. This is followed by an investi-gation of the nondeterministic nature of Kuroshio in-trusion with emphasis on interannual timescales. Themodel sensitivity to wind forcing is discussed in section5 and a summary completes this work.

2. The numerical model

a. NLOM description

The numerical ocean model used is a primitive equa-tion layered formulation where the equations have beenvertically integrated through each layer. The NLOM isa descendent of the model by Hurlburt and Thompson(1980) with significant enhancements by Wallcraft(1991), Wallcraft and Moore (1997), and Moore andWallcraft (1998). It is available in both reduced-gravityversions, where the lowest layer is infinitely deep andat rest, and finite depth versions, which allow realisticbottom topography. Additionally, NLOM can be run ina hydrodynamic mode (spatially and temporally con-stant density within each layer) or in a thermodynamicmode (spatially and temporally density within each lay-er); that is, density is a prognostic variable. A 6-layer,thermodynamic, finite depth model of the Pacific Oceannorth of 208S is used in this study.

The vertically integrated equations of motion used inthe volume conserving n-layer finite-depth thermody-namic model are given below, for layers k 5 1, . . . , n,with k 5 1 for the top layer. When k is used to indexthe model interfaces, k 5 0 at the surface and k 5 n atthe bottom:

]U 1 ](U u ) ](V u cosu)k k k k k1 1 2 V (u sinu 1 aV sin2u)k k[ ]]t a cosu ]f ]u

5 max(0, 2v )u 1 max(0, v )u 2 (max(0, 2v ) 1 max(0, v ))u 1 max(0, 2C v )(u 2 u )k21 k21 k k11 k k21 k M k21 k21 k

n k21](h 2H )h ]G g h ]rj jk kj k k1 max(0, C v )(u 2 u ) 2 G 1 h 2 h 1 1 (t 2 t )/rO OM k k11 k kj j j f f ok21 k1 2 1 2[ ]a cosu ]f ]f r 2 ]fj51 j51o

2](h e cosu) ](h e cos u)A k ff k fuH k k1 12 2 [ ]a cos u ]f ]u

]V 1 ](U y ) ](V y cosu)k k k k k1 1 1 U (u sinu 1 aV sin2u)k k[ ]]t a cosu ]f ]u

5 max(0, 2v )y 1 max(0, v )y 2 (max(0, 2v ) 1 max(0, v ))y 1 max(0, 2C v )(y 2 y )k21 k21 k k11 k k21 k M k21 k21 k


n k21](h 2H )h ]G g h ]rj jk kj k k1 max(0, C v )(y 2 y ) 2 G 1 h 2 h 1 1 (t 2 t )/rO OM k k11 k kj j j u u ok21 k1 2 1 2[ ]a ]f ]f r 2 ]uj51 j51o

2](h e cosu) ](h e cos u)A k fu k uuH k k1 12 2 [ ]a cos u ]f ]u

]h h ]r h ]rk k k k k2 2ˆ1 = · V 5 v 2 v 2 K ¹ [¹ (h 2 H )] 2 2k k k21 H k k4 1 2]t r ]t r ]tk k]r ]r max(0, v ) max(0, 2v )k k k k211 v · =r 5 1 max(0, r 2 r 2 Dr ) 2 max(0, r 2 r 2 Dr )k k k11 k k k k21 k21]t ]t h hk k K]r H Q ]r 1 Hk o 1 45 s (r 2r ) 2 d 1 (= · (h =)) K r 2 (= · (h =))r ,r k k 1k k H k k k[ ]]t h c r h ]T h hk p o 1 k k

where

AH

Cb

Ck

CM

coefficient of horizontal eddy viscositycoefficient of bottom frictioncoefficient of interfacial frictioncoefficient of additional interfacial frictionassociated with entrainment

D(f, u) total depth of the ocean at rest

1 ]F 1 ]F cosuf u= · F 5 1a cosu ]f a cosu ]u

g, for j $ kG 5k j 5g 2 g(r 2 r )/r , for j , kk j o

Hk k th layer thickness at restn21

H 5 D(f, u) 2 HOn jj51

Ho

KH

KH4

constant reference layer thicknesscoefficient of horizontal density diffusivitycoefficient of biharmonic horizontal den-sity diffusivity

KH4coefficient of biharmonic horizontal layerthickness diffusivity

QT1

total surface heat fluxlayer 1 temperature climatology

V 5 h v 5 e U 1 e Vk k k f k u kW (f, u)k k th interface weighting factor for global

vertical mixing designed to conserve masswithin a layer in compensation for explicitvertical mixing due to , 1h hk k

X(f, u)a

cp

ek

domainwide area average of Xradius of the earth (6371 km)coefficient of specific heat for seawaterangular deformation tensor

] u ] yk ke 5 2 cosu 5 2eff uuk k1 2 1 2]f cosu ]u cosu

] y ] uk ke 5 1 cosu 5 efu ufk k1 2 1 2]f cosu ]u cosu

ghk1hk

acceleration due to gravityk th layer thicknessk th layer thickness at which entrainmentstarts

2hk k th layer thickness at which detrainmentstarts

max(A, B) use the maximum value of the argumentsA and B

tvk

timek th layer velocity (5 1 )e u e yf k u k

1 ]F 1 ]F=F 5 e 1 ef ua cosu ]f a ]u

21 ] F 1 ] ]F2¹ F 5 1 cosu

2 2 2 2 1 2a cos u ]f a cosu ]u ]u

V earth’s angular rotation rate (7.292205 325 2110 s )

fu

rk

longitudelatitudek th-layer density

r (f, u)k

Drk

ro

k th-layer density climatologyk th-interface minimum density shearconstant reference density

]r1 ˆ5 0.0757 1 0.00946T1]Tsr reference coefficient of density climatol-

ogy relaxation

t , for k 5 0wC r |v 2 v |(v 2 v ),k o k k11 k k11t 5k for k 5 1, · · · , n 2 1C r |v |v , for k 5 n b o n n

t w wind stress

0, for k 5 0, n1 2v 5 v 2 v 2 W v ,k k k k k

for k 5 1, · · · , n 2 11 1 1 2v 5 v (max(0, h 2 h )/h )k k k k k


TABLE 1. Unvaried model parameters.

Parameter Value Definition

AH 100 m2 s21 Coefficient of horizontaleddy viscosity

Cb 2 3 1023 Bottom drag coefficientCk 0 kth interfacial stress coeffi-

cientCM 1 Coefficient of additional in-

terfacial friction associat-ed with entrainment

g 9.8 m s22 Acceleration due to gravityHk 80/140/160/245

375/bottomkth layer thickness at rest

(in meters)h1

k 50 m (k 5 1)40 m (k 5 2–6)

Thickness of layer k atwhich entrainment starts

h2k Thickness of layer k at

which noncompensatingdetrainment starts; notused by setting to a largevalue

KH 300 m2 s21 Coefficient of horizontaldensity diffusivity

KH40 Coefficient of biharmonic

horizontal density diffu-sivity

KH40 Coefficient of biharmonic

horizontal layer of thick-ness diffusion

rk11 2 rk 1.86/1.38/0.56/0.38/0.43 Stratification at rest be-tween layer k and k 1 1in kg m23

Q 0 Total surface heat fluxvk 0.04 m s21 kth interface reference ver-

tical mixing velocity

2 2 1 2v 5 v (max(0, h 2 h )/h )k k k k k

1 2v 5 (v 2 v )/Wk k k k

vk k th-interface reference vertical mixing ve-locity

The model boundary conditions are kinematic and noslip. No heat flux forcing is applied in the experimentsused here. Table 1 defines the nonvarying model pa-rameters.

The model domain covers the Pacific Ocean from208S to 628N in latitude and 109.1258E to 77.218W inlongitude, but here the focus is on the subregion of theSCS. The horizontal resolution of each prognostic mod-el variable is ⅛8 in latitude by 45/2568 in longitude. Themodel contains horizontal mixing in the form of La-placian friction and allows vertical exchange of mo-mentum, mass, and heat across the interfaces. The modelpermits ventilation of the interfaces between the layers(outcropping). Vertical mixing (entrainment) is appliedwhen a layer thins beyond a prescribed minimum layerthickness ( ) and water from the layer below is en-1hk

trained at a velocity ( k) needed to prevent nonpositivevlayer thickness. Volume is conserved within a layer bydomainwide vertical mixing using a weighting schemebased on oxygen saturation as discussed in Shriver andHurlburt (1997). Horizontal density gradients within a

layer can be modified by advection, diffusion, entrain-ment, heating (not used), and relaxation to an annualmean density climatology, here based on Levitus andBoyer (1994) and Levitus et al. (1994).

The 1/128 ETOP05 bottom topography (NOAA 1986)was used in the model with some modifications. It wasinterpolated to the model grid, then smoothed twice witha nine-point real smoother to reduce energy generationat small scales that are poorly resolved by the model.The 200-m isobath generally defines the model land–sea boundary but there are some exceptions, namely theTaiwan Strait, which has been opened at the 25-m iso-bath, and two interior Philippines passages, Leyte Gulf(10.58N, 125.58E) and San Bernadino Strait (8.08N,124.58E), are opened. The model does not allow theinterfaces to intersect the bottom topography, so thetopography must be confined to the lowest layer. Thus,in order to prevent unrealistically high transport throughTaiwan Strait, depth-weighted friction is applied (as dis-cussed in the appendix of MH). In general, flow is con-strained to near zero in layers completely below the silldepth. The model produces a net northward TaiwanStrait transport of 0.7 6 0.3 Sv in comparison to the2.1 Sv estimate of Zhao and Fang (1991). The modelboundaries are closed along 208S, at the southern In-donesian archipelago, and at the Sunda Shelf in thesouthwest SCS. The Sunda Shelf region covers a largegeographical area, but it is very shallow. The KarimataStrait that joins the Sunda Shelf with the Java Sea isapproximately ¼8 wide at the 25-m isobath and has asill depth of 29 m. Metzger and Hurlburt has shownthat closing the Pacific to Indian Ocean throughflow andthe Sunda Shelf does not significantly impact the flowin the SCS in NLOM. In addition, a ⅛8 global versionof NLOM that includes the Sunda Shelf/Java Sea andcan accurately resolve the narrow Karimata Strait hasshown the net southward transport to be small (0.25 60.25 Sv). The mean transport across Sunda Shelf inglobal NLOM is not much different from the mean 0.6Sv estimate by Wyrtki (1961), but the model variabilityis more than an order of magnitude smaller thanWyrtki’s seasonal estimates (summer: 3.0 Sv, winter:4.5 Sv). Estimating the cross-sectional area of the chokepoint as 28 km wide by 25 m deep yields cross-sectionalaverage velocities of 0.85, 4.3, and 6.4 m s21 respec-tively for the mean, summer, and winter transport es-timates of Wyrtki (1961). These seasonal values areunrealistically high. Global NLOM values are more re-alistic and yield average velocities of 0.36 6 0.36 ms21 using the same cross-sectional area defined above.

The Hellerman and Rosenstein (1983, hereafter HR)monthly mean wind stress climatology is used to spinup the model to statistical equilibrium. From this spinupmodel, four different initial states (each one year apart)are used as the restart conditions for the interannuallywind-forced experiments. Since these experiments differonly in the initial state, any differences between themcan be attributed to nondeterministic differences in both


FIG. 2. (a) Mean SSH and upper-layer currents from the ⅛8, 6-layer finite depth thermodynamic model of the Pacific Ocean north of 208Swith realistic bottom topography. Only the SCS subregion is shown. The mean covers the 1979–97 time frame and is an average of fourmodel realizations, which only differ in their initial state. The simulations were forced with the ECMWF/HR hybrid winds (see text fordetails). The SSH contour interval is 5 cm and the reference vector at the top is 1 m s21. (b) Rms SSH over 1993–97 from the same foursimulations with a contour interval of 2 cm. The x’s represent the mesoscale SSH variability maxima along TOPEX/Poseidon altimeter tracksover the same time period as determined by Wang et al. (2000).

the initial conditions and the evolution of the simula-tions. Thus, we can use these simulations to differentiatedeterministic variability in response to atmosphericforcing versus nondeterministic variability due to flowinstabilities. The spinup experiment exhibits a large de-gree of variation in the Kuroshio intrusion, even thoughit is forced with monthly climatological winds. (Section3 describes a quantitative measure of Kuroshio pene-tration and, for reference, the four initial states havepenetration values of 21.1, 6.5, 8.7, and 15.8 cm.) Thisprovides one of the first insights into the nondetermin-istic nature of the system.

Interannual forcing is by European Centre for Me-dium-Range Weather Forecasts [ECMWF (1995)] 1000-hectopascal (hPa) winds, and in all cases the temporalmean is replaced by the annual mean from HR as dis-cussed in Metzger et al. (1992). This interannual forcingis referred to as an ECMWF/HR hybrid. The magnitude(but not direction) of the ECMWF winds is modified toaccount for the 1000-hPa level not always being at thesurface (Hundermark et al. 1999). The winds are con-verted to a stress using the density of air (rair) 5 1.2kg m23 and a constant drag coefficient (cd) 5 1.5 31023. The model is forced using 1979–93 ECMWF re-analysis 6-hourly data (Gibson et al. 1997) and thencontinued using 1994–97 ECMWF archived operational12-hourly winds. The reanalysis winds have the advan-tage of using a constant data assimilation and atmo-

spheric forecast model and, thus, are unaffected by prod-uct upgrades. The switch in ECMWF products acrossthe 1993/1994 time boundary was investigated to makesure no large-scale response might be introduced intothe ocean model. Pacific basin energy levels of the twowind products showed no significant change betweenthe reanalysis/operational (Re/Op) time frames. Thiswas not the case when the ECMWF 10-m winds wereexamined, hence the decision to use the 1000-hPa prod-uct. Smaller-scale differences between the two productsare discussed in section 5.

b. NLOM results

1) ANNUAL SSH MEAN AND VARIABILITY

Long-term means were calculated for each of the fourinterannually forced experiments. At this timescale, dif-ferences were minor between the realizations. Figure 2ashows the combined four-experiment mean SSH andupper-layer currents and can be compared to Plate 1 ofMH (which shows four simulations forced by HRmonthly winds). The results from the combined exper-iment mean are very similar to the MH results, so thisleads to confidence in using the ECMWF/HR hybridwind approach. Section 2.2 of MH also went into detailcomparing NLOM transport values with observational


data. While similar comparisons are not reported here,the ⅛8 Pacific model compares favorably.

The NEC bifurcates at the Philippines coast at ap-proximately 148N and, as the northward flowing Ku-roshio reaches Luzon Strait, one branch bypasses thestrait with flow east of the Babuyan/Batan island chain.A northwestward branch enters Luzon Strait through theBalingtang Channel, penetrates as far east as 1188E,turns anticyclonically, and exits as a northeastward flow-ing current through the Bashi Channel. These alternatingbranches of inflow/outflow agree with observationaldata (Nitani 1972; Huang et al. 1994), and the separationof the Kuroshio into two velocity cores as it flows pastLuzon Strait is consistent with the modeling work ofChern and Wang (1998). Within the SCS basin, a well-defined cyclonic gyre exists north of 138N except alongthe eastern boundary. The gyre includes a throughflowcomponent with inflow through Luzon Strait and out-flow into the Sulu Sea. A more diffuse cyclonic cir-culation can be seen in the southern half of the basin.In the annual mean (and the seasonal means to be shownlater), the model does not produce a SCSWC as foundby Guan (1978, 1985). The relatively strong south-westward flowing current seen along the boundary inthe northwest SCS corresponds to the current found byGuo et al. (1985).

The 1993–97 SSH variability is shown in Fig. 2b. Anarea of high variability exists west of Luzon Strait at218N, 1198E that is associated with Kuroshio penetra-tion and eddy shedding. The corridor of high variabilityextends to the southwest as the eddies propagate in thatdirection and the boundary current is unstable. A sec-ondary maximum is seen off the coast of Vietnam at12–158N, 1118E with a weaker high variability corridororiented southwest to northeast across the central SCS.These corridors of high SSH variability agree with con-temporaneous TOPEX/Poseidon (T/P) altimeter data.The x’s plotted on Fig. 2b are from Wang et al. (2000)and represent the 1993–97 mesoscale SSH variabilitymaxima along the T/P altimeter tracks used in theirstudy. They note decreasing variability from northeastto southwest in the northern corridor and decreasingvariability from southwest to northeast in the southerncorridor, as is seen in NLOM.

2) SSH ANIMATIONS

Sea surface height animations from the four reali-zations give an indication of the nondeterministic natureof Kuroshio penetration into the SCS. Side-by-sidemodel comparisons rarely show the Kuroshio to be inthe same configuration, suggesting that nonlinear me-soscale flow instabilities are dominant in this region.Eddy shedding from the main core of the Kuroshio isa relatively rare event during the time frame forced byECMWF-reanalysis/HR winds (1979–93). The anima-tions reveal instances of relatively deep intrusion andan anticyclonic eddy forming on the Pacific side of the

Kuroshio within the SCS, but the eddy usually fails toseparate from the main core and eventually coalescesback into the current. During the 15 year reanalysis timeframe, each realization typically had only two or threeeddy-shedding events (which usually did not occur incontemporaneous years). This contrasts markedly withthe 4 year time frame forced by ECMWF-operational/HR winds (1994–97) in which all realizations exhibitedvery deep Kuroshio penetration and eddy shedding inalmost all years. This difference will be discussed inmore detail in section 5. The maximum model eddydiameter reaches approximately 200 km, which agreeswith the observational evidence of Wang and Chern(1987a), and they have a vertical extent to the lowestmodel interface (1000 m), which agrees with Li et al.(1998). The animations also show other areas of favorededdy generation and propagation. In the lee of the Ba-buyan Island chain, anticyclonic eddies develop in Au-gust/September in about half of the model years andpropagate westward. Their signature can be seen in Fig.2b as a tongue of slightly higher variability west of theBabuyan Islands along 198N. The model maintains aone-gridpoint strait between Luzon Island and the shoalsat 198N, 121.58E, and the eddies often develop when ajet of water passes through the channel. Many eddiesreach the western boundary in winter and travel south-westward where they often amplify. Some propagateinto the central part of the basin and merge/interact withthe eddies in the interior, while others simply dissipateas they travel westward. In winter there is a direct cur-rent from the Pacific entering the SCS. As it becomesa western boundary current, eddies occasionally developalong the continental coastline and amplify as they prop-agate southwestward. However, these coastal eddies aremore commonly generated from eddies that have eitherpinched off from the main core of the Kuroshio or haveformed in the lee of the Babuyan Islands. In this casethe eddies become finite amplitude perturbations forflow instabilities and thus amplify as they propagatewest-southwestward.

3) SEASONAL MEANS

Three-month running means were combined for thefour interannually forced realizations and are shown inFig. 3. The seasonal cycle of Kuroshio penetrationagrees qualitatively with the historical surface drift dataof Dale (1956) and Wyrtki (1961) and the hydrographicanalysis of Shaw (1991), namely, increased intrusion inthe winter months and a main core of the Kuroshio thatpenetrates only shallowly before retroflecting in thesummer months. From November to April, a directbranch of the Kuroshio enters the SCS through Baling-tang Channel. Guo et al. (1985) have named this win-tertime current the SCS Branch of the Kuroshio(SCSBK), and Shaw and Chao (1994), R.-F. Li et al.(1996), and MH are numerical models that also depictit. From May to October, the Kuroshio also enters


FIG. 3. The combined experiment 3-month running mean SSH from the four NLOM realizations centeredover (a) Jan, (b) Apr, (c) Jul, and (d) Oct. The contour interval is 2.5 cm.

through the Balingtang Channel, but the main core re-troflects on average at 1208N and exits through the BashiChannel. The lack of the SCSBK in summer is consis-tent with observational studies of Xu et al. (1995) andWang and Chern (1996). Beginning in July, a sea leveldipole forms off the coast of Vietnam, peaks in strengthin August, and can still be seen in September. Shaw etal. (1999) and Wu et al. (1999) present altimeter datathat support such a pattern. The standard deviation ofthe sea surface (not shown) indicates the highest vari-ability in the western Luzon Strait (218N, 1198E) duringthe winter months associated with the deepest Kuroshiointrusion and eddy shedding.

Shaw and Chao (1994) present summer (August) towinter (February) bimonthly sea level elevation figuresfrom their 0.48 horizontal resolution, 21-level limited-

area model of the SCS forced by Hellerman and Ro-senstein (1983) monthly winds and bimonthly transportestimates at the southern and northern boundaries fromWyrtki (1961). Sea level in August and October is qual-itatively similar between NLOM and Shaw and Chao(1994), but different in December and February. A like-ly cause for some of the seasonal sea level differencesis different boundary condition forcing between themodels. NLOM (Fig. 3a) produces a more completewintertime cyclonic gyre, whereas Shaw and Chao(1994) indicate that the sea level contours are open onthe eastern side of the SCS. Satellite data published byShaw et al. (1999) and Wu et al. (1999) provide anindependent data source. Shaw et al. show T/P altimeterdata for three days in December 1992, 1993, and 1994.Focusing on 1993, which is from a non-ENSO time


FIG. 4. As in Fig. 3 except for upper-layer current vectors. The reference vector (at the top) is 1 m s 21.

frame, the satellite data indicate a basinwide cyclonicgyre system similar to the NLOM results (Fig. 3a). Ad-ditionally, the T/P data suggest that it is a two-gyresystem with one center near 178N, 1178E and one in thesouthern basin near 118N, 1128E. The 1992 and 1994data indicate weaker basinwide cyclonic gyres than in1993, and Shaw et al. (1999) attribute this to weakerSCS circulation during ENSO events. Wu et al. (1999)use the same numerical model as Shaw and Chao (1994)but force it with NCEP/NCAR daily winds beginningin 1992. Their control run for December 1993 producesa very complete cyclonic gyre (their Fig. 3f) that is evenmore pronounced when they assimilate the T/P altimeterdata (their Fig. 4f).

Figure 4 is identical to Fig. 3 except that it showsseasonal mean upper-layer currents and it is more in-

formative in depicting the surface (non-Ekman) circu-lation of the SCS. It can be compared against Figure 10in Qu et al. (2000), which shows the seasonal variationof geostrophic velocity at 100 m. While differences existbetween the modeled and observed circulation patterns,NLOM qualitatively agrees with these seasonal data. Inall seasons, a southwestward current exists in NLOMalong the continental coastline in the northwest SCS.This current is much broader in the winter months andis only a narrow (,18) boundary current in July (Fig.4c). Qu et al. (2000) point out that, despite considerablevariability, a westward flow is seen during all seasonsof the year. From late spring to early fall, their datasuggest that the flow is narrowly confined in the con-tinental slope south of China but does not always followthe western boundary. In NLOM, a basinwide cyclonic


gyre exists in the SCS from November to February, witha stronger closed cyclonic gyre in January (Fig. 4a)centered at 8.58N, 1118E. Fang et al. (1998) refer to thisas the SCS Southern Cyclonic Gyre and include figuresfrom Xu et al. (1982) and Fang et al. (1997) that ob-servationally locate this wintertime gyre at 6.58N,109.58E. NLOM’s displacement of this gyre to the north-east may be due to the closed boundary at the SundaShelf or to the spatial scales resolved by the wind forc-ing. Shaw et al. (1999) indicate the importance of thewind stress curl in setting up the sea level in the southernbasin. Adequate resolution of the wind forcing is there-fore important and will be discussed later. NLOM alsoreproduces the wintertime north-northeastward NatunaOff-Shelf Current near 68N, 1128E (Fang et al. 1997).The circulation in the southern SCS turns anticyclonicin summer (Fig. 4c), which again agrees with Xu et al.(1982) and Fang et al. (1997). Likewise, the SoutheastVietnam Off-Shore Current seen in these observationalstudies at 118N shows up well in the modeled output.

4) LUZON STRAIT TRANSPORT

Over 1979–97, the model produces a net Luzon Straittransport (sum across all six layers) of 1.8 6 1.0 Svfrom the Pacific Ocean to the SCS. Qu (2000) usedhistorical temperature profiles combined with climato-logical T/S relationships to obtain a Luzon Strait trans-port of 3.0 Sv, and Qu et al. (2000) employed the mod-ified ‘‘island rule’’ to estimate the transport at 4.2 Sv.The latter study suggests that the Luzon Strait transportis a result of the counterbalance between the large-scaleforcing in the Pacific Ocean and the frictional effectalong the western coast of the Philippines. Because ofthis, they indicate that, in order to produce a reasonableestimate of Kuroshio intrusion into the SCS, the modeldomain must extend southward to the southern tip ofthe Philippines and eastward to the western coast ofAmerica. The spatial resolution must also be adequateenough to resolve most of the narrow passages that con-nect the SCS with the Pacific and Indian Oceans. Thesecriteria are met with the version of NLOM used here.

Figure 5a depicts the annual average Luzon Straittransport for the four realizations plus the combinedexperiment average. Relatively large interannual vari-ations are seen from 1984 to 1990 with a trend towardhigher SCS inflow transport. The net flow through Lu-zon Strait is always directed into the SCS with the ex-ception of two realizations in 1984. Layer by layer, theannual mean indicates Pacific to SCS inflow in all layers,but it is very weak (,0.1 Sv) in the lowest three. In-terannually, the upper layer exhibits inflow in all years,as do layers 2 and 3 with the exception of 1982–84.This contrasts with the bottom three layers that switchbetween inflow and outflow with an approximate 3–4yr cycle. Using hydrographic data, Chen and Wang(1998) found intermediate water outflow in all seasonsbetween 350 and 1350 m. Note that the total transport

across the ECMWF Re/Op time boundary does notchange significantly either. Metzger and Hurlburt sug-gests that, from a mass balance point of view, the meanLuzon Strait transport is actually an extension of thenorthern tropical gyre. Thus, the wind forcing that con-trols the strength of this gyre appears relatively consis-tent for both atmospheric products that forced NLOM.Despite the large interannual variability, the net trans-port is very deterministic between the realizations at theyearly mean scale. Correlations are $0.97. Layer bylayer, this determinism generally holds, as correlationsare typically .0.9 for the upper four layers and .0.8for the abyssal layer; layer 5 shows the most nondeter-minism. Finally, the abyssal layer is generally in neg-ative phase with the upper five layers.

Within the model, the inflow of Kuroshio waters intothe SCS is confined between the straits north of Luzonand the northernmost Batan Islands. On average, theinflow is approximately 16 Sv summed over all six lay-ers, but it is concentrated (.80%) within the top threelayers, which have a mean depth of 440–490 m. In theannual mean the majority of the transport within thesouthern inflow branch simply retroflects (approximate-ly 85%) while the remaining portion enters the SCS viaLuzon Strait (Fig. 5a). The SCS outflow between thenorthernmost Batan Islands and Taiwan is simply theinflow minus the net Luzon Strait transport and, aswould be expected, it is highly correlated with the inflow(r 5 20.83). The majority of the outflow transport isretroflected inflow, but a portion of the volume transportis due to an emptying of the SCS basin from seasonallystored water.

At the seasonal timescale, the southern Luzon Straitinflow transport is essentially constant in the upper threemodel layers. This contrasts with the upper-layer net(Pacific to SCS) Luzon Strait and northern Luzon Straitoutflow transports. In winter, the net (northern branch)Luzon Strait transport is at maximum inflow (minimumoutflow), but reverses to weak outflow (maximum out-flow) in the summer. Because of the nearly constantsouthern inflow, the net Luzon Strait and northern out-flow branches have a very high correlation, r 5 0.99.The upper-layer northern outflow branch (and, hence,the net Luzon Strait transport) clearly responds to theseasonal northeast–southwest monsoon as depicted inFig. 5b. Significant at the 99% confidence interval, thecorrelation of the outflow transport with the ECMWFzonal and meridional wind stress and the wind stressmagnitude is 0.95, 0.95, and 20.88, respectively.

The relationship between interannual variability in thenorthern branch transport and interannual variability inthe northeast–southwest monsoon was investigated bycomputing a June–July–August (JJA) and a November–December–January (NDJ) average of the ECMWFwinds and Luzon Strait transports for each year 1979–96. These 3-month bins represent the height of thesouthwest and northeast monsoons, respectively. For thesummer season (JJA), the southern inflow, northern out-


FIG. 5. (a) Model transport (in Sv) vs time through Luzon Strait (between Taiwan and Luzonalong 120.78E) for the four ⅛8 NLOM realizations plus their average. Data points are means foreach model year and the sum of all six model layers. (b) Monthly upper-layer model transport(blue) for the northern Luzon Strait outflow (i.e., between Taiwan and the northernmost BatanIslands), zonal (red solid) and meridional (red dashed) wind stress, and wind stress magnitude(green). The transport (in Sv) is an average of the four ⅛8 NLOM realizations, and the ECMWFwinds are averaged over a 48 3 48 box centered at 208N, 1208E as defined by Farris and Wimbush(1996) with units of N m22 3 100.

flow, and net Luzon Strait transport showed similarlymoderate correlations with the zonal wind stress com-ponent (0.71, 20.67, and 0.59, respectively), but thecorrelations were much lower for the meridional com-ponent and the wind stress magnitude. For the winterseason (NDJ), no moderate correlation was found be-tween the winds and the Luzon Strait transport. It isunclear why moderate correlations were found only forthe summer zonal wind stress.

3. A quantitative measure of Kuroshio penetration

In order to better link causal effects to Kuroshio pen-etration, a quantitative measure of the Kuroshio intru-sion was developed for the yearly mean timescale. Fig-ure 6 shows the combined experiment long-term meanSSH for the Luzon Strait region. The difference betweenthe areal average SSH of the diagonal box centeredalong the penetrating ridge and the average for the north-


FIG. 6. The combined experiment 1979–97 mean SSH for the LuzonStrait region. The contour interval is 2.5 cm. The two boxes definethe locations over which the areal average SSH is calculated and thendifferenced to produce a quantitative measure of Kuroshio penetrationinto the SCS.

FIG. 7. Kuroshio penetration vs time for the four NLOM realiza-tions plus their average. Penetration is defined as the SSH difference(in cm) of the areal average of the two boxes defined in Fig. 6. LargerSSH differences indicate deeper Kuroshio intrusion. The plus signsat 1992 and 1993 measure Kuroshio intrusion from two additionalexperiments forced by the ECMWF operational winds that overlapthe reanalysis winds in time; the blue (red) plus signs are for thetwin to experiment 1 (2).

ern box in the southern Taiwan Strait comprises thisquantitative measure. SSH in the northern box showsrelatively low variability, while the SSH over the di-agonal box provides a good measure of how deep theridge penetrates into the basin. The difference then, al-lows for a quantitative comparison between the simu-lations.

Figure 7 depicts the Kuroshio intrusion (based on 1-yr annual mean SSH) as a function of time for the fourinterannually forced realizations plus the combined ex-periment average. The simulations show general agree-ment in the intradecadal variability. Moderate penetra-tion (subjectively defined as SSH differences between3 and 8 cm) exists in 1979. This gradually weakens toshallow penetration (SSH differences ,3 cm) betweenapproximately 1983 and 1988 and then builds to deeppenetration (SSH differences .8 cm) after about 1993.Despite the general agreement, there is a lot of inter-experiment variability, which provides some insight intothe nondeterministic nature of Kuroshio intrusion. Cor-relations between the curves ranged from 0.42 to 0.77over the 1979–97 time period, but the deep penetrationduring the ECMWF operational wind period appearsrelated to differences in the annual mean wind curl com-pared to the reanalysis period. This will be discussedin more detail in section 5. The correlations computedjust over the reanalysis time period are very low, ,0.42,with the exception of one experiment pair.

The quantitative measure of Kuroshio penetration ap-pears to work well for the annual timescale, but breaksdown when looking at the seasonal cycle. The techniqueindicates maximum Kuroshio intrusion during the sum-mer months with weakest intrusion in the winter (notshown), while the monthly mean SSH indicates just theopposite (note the 30-cm isoline in Fig. 3). This occursbecause wintertime storage of upper ocean water withinthe SCS affects the measure. That upper-layer waterstored in the SCS in winter exits through Luzon Straitin the summer, as seen in Fig. 3c, consistent with theobservational study by Chu (1972). Note how the SSHcontours emanate from the central SCS basin and exitin the northern half of the strait. In doing so they addto the areal average of diagonal box and bias the results.

4. Nondeterminism in the SCS

a. Kuroshio intrusion into the SCS

Metzger et al. (1994) developed a technique to sep-arate the variability of a given model variable into twocomponents, one caused by direct atmospheric forcing(deterministic) and the other caused by mesoscale flowinstabilities (nondeterministic). This can be applied hereand is possible because all the simulations are identical,except for their initial state. Spatial maps can then becreated to determine the relative importance of the twoanomaly generation mechanisms.

Using SSH as an example, the technique is as follows.First, the long-term mean SSH is computed for eachexperiment and averaged over the four realizations (Fig.2a). This is then subtracted from each record (3.05 days


FIG. 8. The fraction of the modeled SSH variability for (a) 1979–93 and (b) 1994–97 from the four ⅛8 NLOMsimulations caused by direct wind forcing (predominantly deterministic: blue areas) and by flow instabilities (predom-inantly nondeterministic: orange/red areas). The field has been normalized by the total SSH variability at each gridpoint. The velocity field (not shown) is more nondeterministic than the SSH field. The combined experiment temporalmeans, used to calculate anomalies, were calculated separately for the time periods 1979–93 and 1994–97. That waythe temporal means of the deterministic anomalies were zero over the time frame of each analysis, an essential stepto avoid misleading results concerning the relative importance of flow instabilities before and after the wind productchange.

apart) of the individual simulations to form four timeseries of SSH anomaly fields (h). At each grid point,the temporally corresponding records of the four real-izations are averaged ( ) and deviations (h9 5 h 2h

) computed about the combined experiment mean.hThese are then combined and averaged as follows:

22 2h 5 h 1 h9 ,

where the bar is the average of the four realizations.Except for the removal of the combined temporal mean,no averaging through time is required to obtain theseresults. However, the most meaningful results are ob-tained by temporally averaging the above results overthe same period as the temporal mean removed. Thatway the temporal average of the deterministic anomaliesis zero. The left-hand side represents the total SSH var-iability and the right-hand side represents the SSH var-iability due to atmospheric forcing plus the variabilitydue to flow instabilities, assuming is accurately de-hfined by a large number of realizations. After dividingthe equation by the total SSH variability ( ), maps can2hbe produced that show the fraction of the SSH variabilitydue to either atmospheric forcing or flow instabilities,respectively.

With a small number of model realizations the flowinstabilities component is underestimated and the windforcing component is overestimated. This is especiallytrue for areas dominated by mesoscale flow instabilities

such as the Kuroshio Extension region. Consider thecase where the true deterministic component is zero.Generally the average over the realizations will not bezero, but closer to zero as the number of realizationsincreases. The fraction of variability was computed fortwo, three, and four realizations and with the additionof each successive model run, the fraction due to flowinstabilities grew. While convergence has probably notbeen achieved for this technique, the fields nonethelessindicate regions where deterministic and nondetermin-istic variability are relatively important.

Figure 8 shows the fractional SSH variability for theSCS region and has been broken down into two periods,1979–93 and 1994–97, because of the different char-acter of Kuroshio intrusion when NLOM is forced withreanalysis and operational winds, respectively. Darkblue (orange) areas are regions where SSH variationsare predominantly caused by wind forcing (mesoscaleflow instabilities). Most areas represent a combinationof the two variability mechanisms, but relative orangemaxima for both periods are seen in the vicinity ofLuzon Strait, indicating that the intrusion of the Ku-roshio into the SCS has a substantial nondeterministiccomponent in its variability. The SSH variability as-sociated with inflow in the southern half of the straitappears more deterministic than that associated withoutflow in the northern half of the strait. Note also thecorridor of relatively nondeterministic SSH variability


leading southwestward away from the strait. This is as-sociated with eddy development and propagation in themodel. Substantial differences are also seen between thetwo time periods with the operational wind forcing re-sulting in a larger degree of nondeterminism in the vi-cinity of Luzon Strait and in the interior of the SCSbasin. While the operational winds produce a more con-sistent, deep Kuroshio intrusion than the reanalysiswinds, it is clear that the magnitude and spatial extentvaries greatly between the realizations. The higher de-gree of nondeterminism in the interior of the basin alsosuggests that the higher resolution operational winds(more details regarding this in section 5) may be re-solving smaller-scale wind features, which might in turntrigger more oceanic flow instabilities.

The fractional velocity variability is much more non-deterministic in the Luzon Strait region (not shown).The nondeterministic component accounts for .65% ofthe velocity variability for both the zonal and meridionalcomponents. Unlike the SSH variability, the southernhalf of the strait is equally as nondeterministic as thenorthern half.

From the preceding analysis, it appears then that Ku-roshio penetration on the interannual timescale is notsolely related to direct atmospheric forcing. Given thatall four simulations were forced with identical winds,perturbations caused by the different initial states ofeach realization are introducing a nondeterministic com-ponent as a result of mesoscale flow instabilities. Mod-eling this region at an even higher horizontal resolutionwould likely increase the nondeterministic component.

Despite the large element of nondeterminism in thesystem, attempts were made to relate the Kuroshio in-trusion to the oceanic environment. For example, is thepenetration related to current transports in the vicinityof Luzon Strait? Annual mean modeled transport wasdetermined for four sections (which comprised a box):across Luzon Strait along 120.78E (Fig. 5a), east ofTaiwan along 228N, a north–south section east of LuzonStrait along 124.68E, and east of Luzon Island along188N. Correlations between the quantitative measure ofKuroshio penetration (Fig. 7) and the time series ofannual mean transport for the four sections were low;this applied to layer by layer analysis, as well as thesum of all six model layers. For the three sections awayfrom Luzon Strait, correlations ranged from 20.1 to20.3. The range in correlation between the Kuroshiointrusion and Luzon Strait transport was stronger, 20.4to 20.55, correlations significant at the 95% confidencelevel. However, Kuroshio transport did not affect Ku-roshio penetration.

The NEC bifurcates at 148N along the Philippinescoast on average in NLOM, but interannually (season-ally) this latitude can vary by 38 (28). (The bifurcationlatitude across the four realizations is relatively deter-ministic and correlations between the simulations aretypically .0.8.) As the bifurcation latitude shifts south-ward (northward), the Kuroshio transport east of Luzon

increases (decreases). Corroborating the analysis above,no meaningful relationship between Kuroshio penetra-tion and the NEC bifurcation latitude was found; cor-relations ranged from 0.1 to 0.5 with those correlationsgreater than 0.4 being statistically significant at the 90%confidence interval.

In addition, a causal relationship between the Kuro-shio intrusion and the atmosphere is explored. Lookingat just the reanalysis time frame, the wind stress is av-eraged over a 48 3 48 box centered at 208N, 1208E, asdefined by Farris and Wimbush (1996). Neither the zon-al or meridional components nor the wind stress mag-nitude correlate well with the measure of yearly meanKuroshio intrusion.

b. The SCS and the western equatorial Pacific

Using the quantitative measure of Kuroshio penetra-tion, previous analysis has indicated that the four sim-ulations show a lot of interexperiment variability. But,is this true for regions away from Luzon Strait or for alarger subregion within the strait? This is examined bycorrelating yearly mean SSH anomalies over wider spa-tial areas. One-year means are calculated for each re-alization and the combined experiment long-term mean(with respect to 1979–97 and 1979–93) is subtracted tocreate four time series of yearly mean anomalies. Thecalculations are performed relative to the two long-termmeans because of the significant change in behavior ofthe Kuroshio intrusion during the ECMWF operationalwind period (1994–97). Three subregions are examined:1) the Luzon Strait region where the ridge of SSH pen-etrates into the SCS (208–22.58N, 1188–1218E), 2) theentire SCS (58–22.58N, 1098–1218E) region, and 3) theextreme western equatorial Pacific (08–22.58N, 1218–1358E). Within each subregion, experiment by experi-ment correlation analysis is performed over all modelyears to see if a degree of determinism can be found.

As expected over the Luzon Strait subregion, yearlySSH anomaly correlations between the simulations var-ied widely for a given year and showed large interannualvariations. The median correlation over all the experi-ments and all years was 0.59 with respect to the 1979–97 mean, but this fell to 0.3 with respect to the 1979–93 mean. This large difference is due to the consistent,deep penetration of the Kuroshio during 1994–97,which tends to make the intrusion process appear moredeterministic over this subregion.

Over the SCS subregion, there was not as much over-all interexperiment variation for a given year comparedto the Luzon Strait subregion, but there was still a lotof interannual variability in these correlations. Corre-lating the anomalies relative to the two long-term meansdid not produce as large a difference in the overall me-dian correlation, which was 0.57 (0.49) relative to the1979–97 (1979–93) mean. Thus, the SCS also exhibitsa large degree of nondeterminism in the yearly meanSSH signal due to flow instabilities.


In contrast to the two subregions above, the westernequatorial Pacific subregion is much more deterministicat both annual and interannual timescales. For a givenyear there is very little interexperiment variation of theyearly SSH anomaly correlations, and the median cor-relation over all experiments and years is 0.91 (relativeto both long-term means). Thus, the yearly mean SSHvariations in the extreme western equatorial Pacific,which are predominantly larger in scale, appear to belargely deterministic, wind-forced phenomena.

c. Intradecadal variability

After looking at the Kuroshio intrusion as a functionof time (Fig. 7), there appear to be periods when thepenetration is similar for several years in a row. Overall,1983–90 are years of shallow Kuroshio penetrationwhile 1994–97 are years of deep penetration; 1979–82and 1991–93 are transition years between the two ex-tremes with moderate Kuroshio intrusion. Means arecreated over these four time groups and anomalies ofthese relative to both the 1979–97 and 1979–93 com-bined experiment long-term means are again computedfor each experiment. Correlation analysis is performedover the same three subregions defined in the previoussection.

Unlike the yearly anomaly correlations, the 1983–90and 1994–97 time group interexperiment anomaly cor-relations for Luzon Strait are quite high, .0.9. However,the 1979–82 and 1991–93 time groups had very weakcorrelations between the experiments with median cor-relations near zero. This suggests a certain amount ofintradecadal determinism when the Kuroshio intrusionis in one of two extreme modes, either deep penetrationor very shallow penetration. These results apply foranomalies relative to both long-term means.

For the SCS region, the same results apply, but themedian correlations for the extrema year time groupings(0.8) are not substantially higher than the median cor-relation for the transitional year time groups (0.6). Inthe western equatorial Pacific region, correlations aregreater than 0.9 for all time groups.

5. ECMWF reanalysis versus operational windforcing

As mentioned previously, the character of the Ku-roshio intrusion into the SCS is greatly different de-pending upon the time frame of the ECMWF forcing.During the reanalysis wind period (1979–93), deep Ku-roshio penetration is a relatively rare event, but duringthe operational wind period (1994–97) there is a per-sistent deep intrusion of the Kuroshio. An initial hy-pothesis for the different behavior is possible strength-ening of the northern tropical or subtropical gyre. Metz-ger and Hurlburt points out that the Luzon Strait trans-port is actually an extension of the northern tropicalgyre. If the strength of the northern tropical gyre in-

creases, the Luzon Strait transport should increase,while the Kuroshio transport just north of Luzon Straitwould remain the same. Conversely, if the strength ofthe subtropical gyre increases, the Kuroshio transportjust north of the Luzon Strait should increase, while theLuzon Strait transport would remain the same. No sig-nificant change in either the Luzon Strait transport (Fig.5a) or Kuroshio transport was detected across theECMWF Re/Op time boundary, so large-scale atmo-spheric changes do not appear to be the cause. This isfurther corroborated by analyzing the atmospheric en-ergy levels over the northern tropical and subtropicalgyres (not shown). No change is detected across theECMWF Re/Op time boundary.

The local wind stress in the SCS/Philippines regionis examined next to see if it might provide insight con-cerning the different character of Kuroshio penetration.Atmospheric energy levels (not shown) are computedfor three regions: 1) the entire Philippines/SCS (08–308N, 1108–1358E), 2) the Luzon Strait region and thearea of strong monsoonal winds to the northeast (188–308N, 1108–1398E) similar to the area chosen for windanalysis in MH, and 3) a more locally confined LuzonStrait region (168–268N, 1138–1258E). From the mid-1980s to 1990 the two products show very consistentenergy levels over all the subregions, but in 1991 a smalldivergence between the curves begins with the opera-tional winds about 10% stronger. This is probably dueto a change in the atmospheric model resolution, whichis described later. However, this approximate 10% up-ward shift is still within the range of the interannualvariability of the atmospheric energy levels, and thuswe suggest that wind stress changes across the ECMWFRe/Op time boundary are not causing the marked shiftin Kuroshio intrusion.

Then what about the wind stress curl in Luzon Strait?Yearly mean curl fields indicate a significant differenceover Luzon Strait and in many other regions. Figure 9shows yearly mean curl for 1993, the last year of overlapbetween the two products. The curl pattern for this yearis representative of most other years in each dataset.Both products indicate negative curl in the northern halfof the strait and positive curl in the southern half of thestrait; yet the magnitudes differ substantially. Wind curlextrema from the operational winds are more than twiceas large, and the gradient in curl is much sharper. Inaddition, the pattern of curl over the interior of the SCSis also quite different. So, while wind stress magnitudeis fairly consistent between the two products, the dis-tribution of curl has some substantial differences.

The ECMWF operational winds are obtained fromNCAR and are named the ECMWF Tropical OceanGlobal Atmosphere (TOGA) global basic surface andupper-air analyses (NCAR dataset DS111.2). Since Sep-tember 1991, the operational atmospheric model run-ning at ECMWF has had a T213 spectral resolution(0.858 along the equator). However, a spherical trun-cation to T63 (2.88 along the equator) is performed and


FIG. 9. The 1993 mean wind stress curl from the ECMWF 1000-hPa (a) reanalysis and (b) operationalatmospheric products. In both, the 1979–93 ECMWF reanalysis mean has been subtracted and the Hellermanand Rosenstein (1983) annual mean added back in to create the hybrid wind set. The contour interval is 7.5Pa m21 3 1028.

then interpolation to a 2.58 Cartesian grid before dis-tribution. In contrast, the reanalysis winds obtainedthrough NCAR are the ECMWF reanalysis basic globalupper-air dataset (DS115.1). The model resolution isspectral T106 (1.78 along the equator) with a sphericalharmonic truncation to T47 (3.88 along the equator) be-fore final interpolation to the 2.58 grid. In the interiorof the SCS, the large and small scale structure of thewind curl differs. It is clear that the higher-resolutionoperational winds pick up more small-scale structure,in addition to producing larger extreme values. Thesedifferences appear to be related to the truncation tech-nique employed within each dataset, but further studyis needed to corroborate this.

Because of an overlap between the ECMWF opera-tional and reanalysis winds, it is possible to investigatemodel sensitivity to these two wind products. Two ad-ditional experiments are integrated over 1992–93, butthe ECMWF operational winds force the model. Theinitial states for these two simulations are the first recordof 1992 from two of the simulations run with ECMWFreanalysis winds. Thus, there will be two sets of ex-periment twins that start from the same initial state buthave different wind forcing. Figure 7 indicates a lot ofinterexperiment variability with regard to Kuroshio pen-etration in 1992–93, with experiment 1 showing mod-erately deep intrusion and experiment 2 showing shal-low intrusion. These two extreme cases are chosen asthe initial states for the new experiments describedabove.

Figure 10 shows the 1993 annual mean SSH for thetwo twin experiments and the plus signs on Fig. 7 in-

dicate the quantitative measure of Kuroshio intrusionfor the two new experiments. In both cases, the exper-iments forced by ECMWF operational winds exhibitdeeper Kuroshio penetration than its twin forced withECMWF reanalysis winds. This suggests that the largernegative curl over the northern Luzon Strait createsmore Ekman pumping, which causes the thermocline todeepen and in turn leads to greater intrusion of the Ku-roshio. Given this hypothesis, one might expect windcurl over Luzon Strait to be highly correlated with Ku-roshio penetration, but this is not the case. Over thereanalysis period, the correlation coefficient between theKuroshio intrusion and wind curl (averaged over theregion of the mean penetrating Kuroshio) is small (r 520.25). This again highlights the nondeterministic na-ture of the Kuroshio intrusion into the SCS. However,the results imply that large changes in wind stress curlmagnitudes and patterns over Luzon Strait do signifi-cantly affect the modeled Kuroshio pathway and its in-teraction with the SCS. The operational products pro-duce curl maxima that are more than twice as large asthe reanalysis products, and this is greater than the in-terannual variability of the wind stress curl over thisregion.

Note also in Fig. 10 the differences in the circulationin the interior of the SCS brought about by wind stresscurl changes. Both simulations forced with ECMWFreanalysis winds (Figs. 10a,c) produce a single, northernbasin cyclonic gyre while the ECMWF operational windforced experiments (Figs. 10b,d) have two distinctgyres, one northeast of Vietnam near 158N, 1118E andone west of Luzon near 178N, 1178E. Thus, the ocean


FIG. 10. The 1993 mean SSH from (a) expt 1 and (c) expt 2 forced with ECMWF reanalysis winds andfrom (b) expt 5 and (d) expt 6 forced by ECMWF operational winds. Experiments 5 and 6 begin from the1992 initial state derived from expts 1 and 2 respectively. For convenience, the panels from the reanalysis(operational) winds are marked by R (O) followed by the experiment number. The contour interval is 2.5 cm.

model is responding to the smaller spatial scales in theECMWF operational wind curl. It appears then, to mostaccurately model the SCS region, numerical modelerswould be wise to choose a wind-forcing dataset withthe highest possible horizontal resolution. These prod-ucts are available at NCAR and are the ECMWF re-analysis advanced global dataset (DS115.4) or theECMWF TOGA global advanced operational spectralanalyses (DS111.0). Both are output on a T106 spectralgrid (which is the full model resolution during the re-analysis, but a truncated resolution of the operational

winds). However, these products are more expensive andhave substantially increased storage requirements com-pared to the products distributed on the 2.58 Cartesiangrid.

6. Summary

The results presented here suggest a large degree ofnondeterminism regarding the Kuroshio intrusion intothe SCS. Four numerical experiments were forced with1979–97 ECMWF reanalysis and operational winds and


only differ in their initial state. Because the evolutionof the Kuroshio’s interaction with the SCS can be quitedifferent between all four realizations, mesoscale flowinstabilities must be contributing to the nondeterminismof the system. One would expect these flow instabilitiesto dominate even more in the real ocean and at modelresolutions higher than the ⅛8 used here. Thus, the re-sults suggest that on the yearly mean timescale, Kuro-shio intrusion does not appear to be a wind-forced, de-terministic event.

A description of the long-term mean SSH is givenalong with a discussion of its time evolution. In theLuzon Strait region, contemporaneous interexperimentcomparisons of instantaneous SSH rarely show the Ku-roshio to be in the same configuration. Eddy generationin Luzon Strait can occur when the Kuroshio intrudesdeeply into the SCS basin and pinches off a vortex.Deep penetration and eddy shedding are relatively rareevents, occurring only two or three times during theECMWF reanalysis forcing (1979–93) but are persistentfeatures in almost all years for the ECMWF operationalwinds (1994–97). In August/September, a secondaryarea of favored eddy generation exists in the lee of theBabuyan Island chain.

In order to quantify the Kuroshio intrusion into theSCS, a measure was developed for the yearly meantimescale that examined the difference between the arealaverage SSH in the southern Taiwan Strait and alongthe penetrating SSH ridge axis. This difference allowedquantitative intercomparison of the simulations and in-vestigation of interannual variability. It showed generalinterexperiment agreement for the intradecadal varia-tions, but a lot of interexperiment variability for anygiven year. Kuroshio intrusion correlations between ex-periments were low for the interannual time series.

Using this intrusion measure, hypotheses relating Ku-roshio penetration to the oceanographic environment oratmospheric forcing were also investigated. The resultsindicate that the Kuroshio intrusion is not a function ofthe oceanic transport through Luzon Strait nor of thesurrounding Pacific Ocean waters, including Kuroshiotransport north or south of Luzon Strait. Yearly meantransports from transects across Luzon Strait, east ofLuzon, east of Taiwan, and a north–south section eastof Luzon Strait along 124.68E did not strongly correlatewith the Kuroshio intrusion. The NEC bifurcation lat-itude was also examined since, as it shifts southward(northward), the Kuroshio transport east of Luzon in-creases (decreases). Corroborating the previous analy-sis, no meaningful relationship was found with Kuroshiopenetration.

In addition, interannual variations of the wind in thevicinity of Luzon Strait were studied. No tangible re-lationship could be found that linked interannual vari-ability of yearly mean Kuroshio intrusion or monsoonseason mean Luzon Strait transport with correspondingzonal or meridional wind components, wind stress mag-nitude, or wind stress curl. However, there was a close

relationship between the mean seasonal cycles of theLuzon Strait transport and the northeast–southwestmonsoon.

As mentioned, the quantitative measure of the Ku-roshio intrusion showed substantial interexperiment in-terannual variability. Wider spatial areas in and awayfrom Luzon Strait were examined to see if the same wastrue. Yearly mean SSH anomalies from each experimentwere intercorrelated for a larger Luzon Strait subregion,the SCS basin subregion and the extreme western equa-torial Pacific Ocean. The interexperiment correlationsfor all the ECMWF reanalysis forced years producedmedian correlations of 0.3, 0.49, and 0.91 for the threesubregions, respectively. Thus the SCS basin also ex-hibits a large degree of nondeterminism in the yearlymean SSH signal due to flow instabilities, but the west-ern equatorial Pacific appears to be a region governedby largely deterministic wind-forced phenomena.

Much greater Kuroshio penetration was found afterswitching to archived operational winds at the end ofthe ECMWF reanalysis. An initial hypothesis for thechange in behavior was possible strengthening of thenorthern tropical or subtropical gyres. Since the LuzonStrait transport is actually an extension of the northerntropical gyre, the transport should increase with an in-crease in the strength of this tropical Pacific gyre. Like-wise, increases in subtropical gyre strength should pro-duce an increase in transport just north of Luzon Strait.No significant change in these transports was detectedacross the ECMWF reanalysis/operational time bound-ary, so large-scale atmospheric changes do not appearto be the cause.

The local wind stress in the Luzon Strait/SCS regionwas also studied to provide insight into the differentcharacter of the Kuroshio intrusion. No significantchange across the reanalysis/operational time boundarywas found for the wind stress. However, yearly meanwind stress curl fields revealed marked differences. Bothreanalysis and operational products produce negativecurl in the northern Luzon Strait and positive curl inthe southern strait, but the magnitudes differ substan-tially. Wind curl extrema from the operational winds aremore than twice as large, and the gradient in curl ismuch sharper. The pattern of curl over the interior ofthe SCS is also quite different. The net effect in themodel is larger Ekman pumping, a deepening of thethermocline, and in turn greater westward Kuroshio pen-etration into the SCS during the 1994–97 ECMWF op-erational forcing time frame. There are also substantialchanges within the SCS basin.

While the simulated Kuroshio intrusion clearly re-sponds differently to the ECMWF operational and re-analysis winds, neither period exhibits much determin-ism in the variability. Deep Kuroshio intrusion is spo-radic during the reanalysis wind forcing, whereas theoperational winds produce a consistent, deep penetra-tion. It might appear that the operational winds producea more deterministic response, but the strength of the


deep intrusion during 1994–97 is highly variable (Figs.7 and 8b). In fact, the interexperiment root mean squareof the quantitative measure of Kuroshio intrusion cal-culated over the operational wind forcing period (6.4cm) is higher than that of the reanalysis wind period(5.0 cm). Thus, the determinism does not seem to bedependent on the flow regime.

The difference in the curl distribution is probablyrelated to the manner in which the two atmosphericproducts have been truncated on their spectral grid be-fore distribution on the 2.58 grid. Investigation of thistopic requires further study and is beyond the scope ofthis paper, but as ocean model resolution becomes finer,the resolution of the atmospheric forcing driving thesemodels must also increase in order to represent smallerspatial scales.

Acknowledgments. This work has been performed aspart of the 6.1 Dynamics of the Low Latitude WesternBoundary Currents project under program element601153N sponsored by the Office of Naval Research.Thanks are extended to Alan J. Wallcraft for his con-tribution in developing the ocean model. The numericalsimulations were performed under the Department ofDefense High Performance Computing Initiative on Sil-icon Graphics, Inc. Origin 2000 computers at the Aero-nautical Systems Center, Dayton, Ohio, and the NavalOceanographic Office, Stennis Space Center, Mississip-pi.

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the nondeterministic nature of kuroshio penetration and ...the ﬁrst one enters luzon strait and...

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