1712 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31

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

E. JOSEPH METZGER AND HARLEY E. HURLBURT Naval Research Laboratory, Stennis Space Center, Mississippi

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

ABSTRACT A ⅛Њ, 6-layer Paci®c version of the Naval Research Laboratory Layered Ocean Model is used to investigate the nondeterministic nature of Kuroshio intrusion and eddy shedding into the South China Sea (SCS) on annual and interannual timescales. Four simulations, which only differ in the initial state, are forced with 1979±93 European 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 Kuroshio penetration across all four simulations for the yearly means, indicating a large degree of nondeterminism at this timescale. This nondeterminism is quanti®ed by a technique that separates the variability of a model variable into deterministic (caused by direct atmospheric forcing) and nondeterministic (caused by mesoscale ¯ow 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 is presented that allows interexperiment comparisons and investigation of interannual variability, and attempts are made to positively correlate it with the oceanic and atmospheric environment. Yearly mean Kuroshio intrusion is 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 found that linked interannual variability of yearly mean Kuroshio intrusion or monsoon season mean Luzon Strait transport with the corresponding zonal or meridional wind components, wind stress magnitude, or wind stress curl. However, there was a close relationship between the mean seasonal cycles of the Luzon Strait transport and the northeast±southwest monsoon. Eddy shedding and deep Kuroshio intrusion are rare events during the period 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 in the 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±south gradient 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 simulated Kuroshio intrusion, the marked differences in curl between the two atmospheric products did have a substantial impact.

1. Introduction Sea). To the north, Taiwan Strait connects the SCS with The South China Sea (SCS) is the largest semien- the East China Sea but it is generally believed that the closed marginal sea in the western tropical Paci®c Ocean Taiwan Warm Current ¯ows northward throughout the (Fig. 1). Its main connection with the Paci®c Ocean is year (Chuang 1986; Zhao and Fang 1991). Thus, the via Luzon Strait, which is very wide and has a sill depth main interaction of the SCS with the surrounding waters of approximately 2400 m. The connections to the south is through Luzon Strait. are either very narrow (Sibutu Passage in the southern Metzger and Hurlburt (1996, hereafter MH) and the Sulu Sea) or shallow (a 29-m sill depth at Karimata present study have considered three major components Strait, which connects the Sunda Shelf and the Java of upper ocean ¯ow through Luzon Strait. The ®rst one enters Luzon Strait and exits through another strait. It appears as a branch off the Kuroshio into the SCS just * Naval Research Laboratory Contribution Number JA/7323-99- 0038. north of the Philippine Islands and is the only one of these that contributes to the long-term mean Luzon Strait transport. The second component is due to vari- Corresponding author address: Mr. E. Joseph Metzger, Naval Re- ations in the volume of water above the thermocline search Laboratory, Code 7323, Stennis Space Center, MS 39529- 5004. within the SCS. This one has a strong seasonal cycle E-mail: [email protected] associated with the northeast±southwest monsoon. De-

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1713

FIG. 1. Geometry of the South China Sea region plotted on the ⅛Њ NRL Layered Ocean Model grid. The model land±sea boundary is the 200-m isobath, except for Taiwan Strait and the two interior Philippines passages. pending upon the direction of the ¯ow, it can appear as winter and from the southwest during boreal summer. a branch off the Kuroshio into the SCS on the southern Shaw et al. (1999) suggest that the wind stress curl is side of Luzon Strait or as a current feeding into the the main driving force of the circulation in the deep Kuroshio on the northern side of the strait. The third basin of the SCS except near Luzon Strait. Huang et al. component appears as a Kuroshio loop or intrusion into (1994) and Fang et al. (1998) indicate a central basin the SCS that enters through the southern part of Luzon cyclonic (anticyclonic) circulation during the winter Strait and exits through the northern part. In relation to (summer) monsoon. In the northern SCS along the shelf the ¯ow through Luzon Strait, MH concentrated on the break at a depth of 200±400 m, and most likely orig- ®rst two components, while the present study focuses inating near Hainan Island, the South China Sea Warm primarily on the third. However, it is not always clear Current (SCSWC) ¯ows northeastward, even during the how to sort out observations in relation to these three periods of the northeast monsoon (Guan 1978, 1985). components as well as locally driven ¯ows. Thus, at However, the existence of the SCSWC is somewhat con- times this will not be clear-cut in the subsequent dis- troversial. Repeated measurements have yet to con®rm cussions of observational evidence. that this warm current is a permanent feature. Imme- The upper ocean circulation within the SCS is dom- diately offshore of the SCSWC, Guo et al. (1985) have inated by the seasonally reversing monsoon winds, found an equally strong southwestward ¯owing current which typically blow from the northeast during boreal during the winter. The coastal current along Vietnam

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1714 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 also responds to the seasonally changing monsoon erature. These again indicate the variability of the Ku- winds; it ¯ows to the north during the southwest mon- roshio's interaction with the SCS. soon and to the south during the northeast monsoon Evidence of warm core (anticyclonic) eddies in the (Wyrtki 1961). northern SCS have been found by Wang and Chern (1987a) and Li et al. (1998). The former study reports an eddy diameter of approximately 200 km, while the a. Hydrographic evidence for Kuroshio intrusion into latter found an eddy with a diameter of approximately the SCS 150 km and a vertical expression as deep as 1000 m. The Kuroshio (Black Stream) originates where the Li et al. (1998) have also performed a hydrographic North Equatorial Current (NEC) bifurcates along the analysis that suggests the subsurface water of the eddy eastern coast of the Philippines at approximately is of Kuroshio origin, while the surface waters associ- 13.5ЊN; the southward ¯owing Mindanao Current has a ated with the ring are believed to consist of entrained volume transport of approximately 27 Sv (Sv ϵ 106 m3 SCS waters. Although some eddies have been observed, sϪ1)at8ЊN, while the northward-¯owing Kuroshio the hydrographic observations are insuf®cient to deter- transport is approximately 14 Sv at 18ЊN (Qu et al. mine how frequently eddies are shed from the main core 1998). As the Kuroshio ¯ows northward and enters the of the Kuroshio. Clearly, satellite data will be required region of Luzon Strait, it has been observed to bypass to answer this question. the strait (Wang and Chern 1996) as well as penetrate deeply into the SCS and shed eddies. When penetrating, b. Dynamical mechanisms for the Kuroshio intrusion the in¯ow is typically northwestward in the southern Luzon Strait (Balintang Channel). It turns anticycloni- The dynamics that govern the intrusion of the Ku- cally and exits as a northeastward-¯owing current in the roshio into the SCS are also not clearly understood. A northern Luzon Strait (Bashi Channel) (Nitani 1972; hypothesis promoted by Wang and Chern (1987b) pro- Huang et al. 1994). poses that the Kuroshio intrusion only occurs during the Hydrographic data exist for the northern SCS/Luzon northeast monsoon season as the winds de¯ect the Ku- Strait region, but repeated cruises during all seasons are roshio transport westward into Luzon Strait. Farris and lacking and, thus, the seasonal climatology is not fully Wimbush (1996) concur with this theory, as they have known. A few studies are described here, but it should examined satellite derived sea surface temperature im- be noted that cruises taken in the same season do not ages and linked the intrusion current development pro- indicate the same degree of Kuroshio intrusion. Thus, cess with an integrated supercritical wind stress param- they provide evidence that the interaction between this eter. However, Shaw (1991) suggests that the general Paci®c western boundary current and the SCS is gov- circulation in the northern SCS is probably driven by erned by deterministic interannual variability, nonde- higher sea level in Luzon Strait. A weakening of this terministic variability, or a combination of both. Spring pressure gradient in summer may prevent a strong in- (March) cruise data were analyzed by Xu et al. (1995), trusion current from developing. He also suggests that who found the Kuroshio penetrating to approximately the intrusion process may be related to the transport of 119ЊE and mostly in the northern half of the strait, while the Kuroshio or the reversal of the wind direction from the autumn (September) cruise data of Guo and Fang the northeast in winter to the southwest in summer. (1988) indicated intrusion to about 120ЊE. As opposed to these one-time cruises, Shaw (1991) looked at his- c. Numerical modeling studies of the SCS/Luzon torical hydrographic data across all seasons and over a Strait region wide region of the northern SCS and found evidence of an intrusion current over the entire continental margin Kuroshio intrusion into the SCS has also been studied of the northern SCS in late autumn and winter. That by employing numerical ocean models of different study, along with evidence gathered by Shaw (1989), types, covering different domains, and applying differ- describes the seasonal cycle as follows: (i) from May ent forcing. The idealized geometry in the model of W. to September the Kuroshio front meanders into Luzon Li et al. (1996) can be applied to Luzon Strait, and it Strait (an intrusion current may not exist), (ii) from indicates an intrusion current that penetrates deeper October to January an intrusion current may begin to when the gap in the western boundary becomes wider form and penetrate along the continental margins, and or the latitude decreases. Other models that use realistic (iii) from February to May the intrusion current decays. coastlines have applied winds only for a speci®c season Shaw (1991) also suggests that it is unlikely that an in an attempt to examine the response of the Kuroshio. inertial current ¯ows hundreds of kilometers into the For example, Chern and Wang (1998) applied sum- SCS, but the intrusion current is part of the general mertime forcing to a very limited area model (10-km circulation of the northern SCS. A more recent study resolution) of Luzon Strait and suggest that the retreat published by Fang et al. (1998) provides a concise de- of the intrusion current may be related to the northward scription of observational studies related to the Kuroshio movement of the strongly strati®ed SCS waters driven intrusion, many of which come from the Chinese lit- by southwesterly winds, while Li et al. (1993) used

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1715 wintertime forcing in their ½Њ barotropic full basin Pa- an accurate representation of the islands and coastlines ci®c model and found a branch of the Kuroshio that appears to be one of the critical elements in modeling penetrated as far east as 113ЊE. Monthly varying forcing this region. was used by R.-F. Li et al. (1996), who noted Kuroshio Section 2 of this paper provides a description of the penetration to at least 116ЊE in all months. They suggest Naval Research Laboratory Layered Ocean Model that variations in the Kuroshio and the whole current (NLOM) and the associated experiments used in this system of the Paci®c have an important in¯uence on the study. Annual and seasonal means are presented along current entering the SCS via Luzon Strait. Shaw and with a description of the Luzon Strait transport. Next Chao (1994) used a limited-area model of the SCS re- comes a discussion of a quantitative measure of the gion, also with monthly wind forcing, that only pro- Kuroshio penetration. This is followed by an investi- duced the Kuroshio intrusion during the winter months; gation of the nondeterministic nature of Kuroshio in- they suggest that summertime intrusion is blocked by trusion with emphasis on interannual timescales. The the northeastward ¯ow from the Vietnamese coast to- model sensitivity to wind forcing is discussed in section ward Luzon Strait. They also indicate that most water 5 and a summary completes this work. exchange between the SCS and Paci®c is concentrated in the upper 300 m. 2. The numerical model A ½Њ global, 1.5-layer reduced gravity global model was used by MH to study the coupled dynamics of the a. NLOM description SCS and Paci®c Ocean region. That paper focused main- The numerical ocean model used is a primitive equa- ly on the controlling factors of the mean Luzon Strait tion layered formulation where the equations have been transport and its variability. The Kuroshio intrusion was vertically integrated through each layer. The NLOM is brie¯y addressed in their Plate 1, which compared the a descendent of the model by Hurlburt and Thompson annual mean sea surface height (SSH) from a ½Њ global (1980) with signi®cant enhancements by Wallcraft 1 model with higher resolution (⅛Њ and ⁄16Њ) global and (1991), Wallcraft and Moore (1997), and Moore and Paci®c counterparts. They noted that the Kuroshio pen- Wallcraft (1998). It is available in both reduced-gravity etration was pronounced in the ½Њ model, but much less versions, where the lowest layer is in®nitely deep and so at higher resolutions. This is probably due to a much at rest, and ®nite depth versions, which allow realistic better model representation of the north±south Babuyan bottom topography. Additionally, NLOM can be run in and Batan Island chains; namely, the higher horizontal a hydrodynamic mode (spatially and temporally con- resolution allows more islands to be resolved and in- stant density within each layer) or in a thermodynamic cluded as part of the coastline geometry. This is sup- mode (spatially and temporally density within each lay- ported by a ®nding in Hurlburt and Thompson (1982) er); that is, density is a prognostic variable. A 6-layer, 1/2 that gives a length scale (␷ c/␤) for westward bending thermodynamic, ®nite depth model of the Paci®c Ocean of the Loop Current in the Gulf of Mexico. This means north of 20ЊS is used in this study. that higher velocity at the core of the current (␷ c)in The vertically integrated equations of motion used in higher resolution models tends to increase the length the volume conserving n-layer ®nite-depth thermody- scale for westward bending and to reduce the westward namic model are given below, for layers k ϭ 1,...,n, 1/2 bending on a given length scale Ͻ(␷ c/␤) , where ␤ is with k ϭ 1 for the top layer. When k is used to index the planetary vorticity. The westward bending is further the model interfaces, k ϭ 0 at the surface and k ϭ n at reduced by the narrower strait at higher resolution. Thus, the bottom:

(Vu cos␪)ץ (Uu)ץ U 1ץ kkkkkϩϩϪV (u sin␪ ϩ a⍀ sin2␪) ␪ kkץ ␾ץ[]tacos␪ץ

ϭ max(0, Ϫ␻kϪ1)ukϪ1 ϩ max(0, ␻kk)u ϩ1 Ϫ (max(0, Ϫ␻kk) ϩ max(0, ␻ Ϫ1))ukMkϩ max(0, ϪC ␻ Ϫ1)(ukϪ1 Ϫ uk) ␳ץGgϪ1 hץ ( h ϪH)ץh nk ϩ max(0, C ␻ )(u Ϫ u ) Ϫ kkjkkG jjϩ h Ϫ h ϩϩ(␶ Ϫ ␶ )/␳ Mk kϩ1 kkjjj͸͸␾␾kϪ1 k o ␾ץ ␾␳jϭ1 2ץ ␾ץ a cos␪ jϭ1 []΂΃΂΃o (he cos2␪)ץ (he cos␪)ץ A ϩϩH k ␾␾kkk ␾␪ ␪ץ ␾ץ[]a22cos ␪ (V ␷ cos␪)ץ ( U ␷)ץ V 1ץ kkkkkϩϩϩU (u sin␪ ϩ a⍀ sin2␪) ␪ kkץ ␾ץ[]tacos␪ץ

ϭ max(0, Ϫ␻kϪ1)␷kϪ1 ϩ max(0, ␻kk)␷ ϩ1 Ϫ (max(0, Ϫ␻kk) ϩ max(0, ␻ Ϫ1))␷kMkϩ max(0, ϪC ␻ Ϫ1)(␷kϪ1 Ϫ ␷k)

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1716 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31

␳ץGgϪ1 hץ ( h ϪH)ץh nk ϩ max(0, C ␻ )(␷ Ϫ ␷ ) Ϫ kkjkkG jjϩ h Ϫ h ϩϩ(␶ Ϫ ␶ )/␳ Mk kϩ1 kkjj͸͸ j ␪␪kϪ1 k o ␪ץ ␾␳jϭ1 2ץ ␾ץ a jϭ1 []΂΃΂΃o (he cos2␪)ץ (he cos␪)ץ A ϩϩH k ␾␪kkk ␪␪ ␪ץ ␾ץ[]a22cos ␪ ␳ץ ␳ hץhhץ k à 22 kk kk Vkkkϭ ␻ Ϫ ␻ Ϫ1 Ϫ KHkk4ٌ [ٌ (h Ϫ H )] ϪϪ ´ ١ ϩ tץ t ␳ץt ΂΃␳kkץ dd (␳ max(0, ␻ ) max(0, Ϫ␻ kϪ1ץ␳kkkץ (١␳ ϭϩ max(0, ␳ kϩ1 Ϫ ␳kkϪ⌬␳ ) Ϫ max(0, ␳ kkϪ ␳ Ϫ1 Ϫ⌬␳kϪ1 ´ϩ v kk thkk hץ tץ d ␳ 1 Kץ␳ HQץ , ␳((١ h)´ ١) K ␳ Ϫ H4 ((١ h)´ ١) koϭ ␴ (␳à Ϫ␳ ) Ϫ ␦ 1 ϩ Th hץ th␳ kk 1kkHkkk c␳ hץ kpo1 kk[] d where g acceleration due to gravity

hk kth layer thickness ϩ hk kth layer thickness at which entrainment AH coef®cient of horizontal eddy viscosity C coef®cient of bottom friction starts b Ϫ C coef®cient of interfacial friction hk kth layer thickness at which detrainment k starts CM coef®cient of additional interfacial friction associated with entrainment max(A, B) use the maximum value of the arguments D(␾, ␪) total depth of the ocean at rest A and B t time F cos␪ץ F␾␪1ץ 1 (F ϭϩ vk kth layer velocity (ϭϩe␾uk e␪␷ k ´ ١ ␪ץ ␾ a cos␪ץ a cos␪ ⌽ץ ⌽ 1ץ 1 ١⌽ϭ e␾␪ϩ e g, for j Ն k ␪ץ ␾ aץ G ϭ a cos␪ kj g Ϫ g(␳ Ϫ ␳ )/␳ , for j Ͻ k ⌽ץץ ⌽2 1ץ Ά kjo 1 ⌽2 ϭ ϩ cos␪ٌ 22 2 2 ␪ץ␪΂΃ץ ␾ a cos␪ץ Hk kth layer thickness at rest a cos ␪ nϪ1 H ϭ D(␾, ␪) Ϫ H ⍀ earth's angular rotation rate (7.292205 ϫ nj͸ jϭ1 10Ϫ5 sϪ1 ) longitude Ho constant reference layer thickness ␾ latitude KH coef®cient of horizontal density diffusivity ␪ ␳k kth-layer density KH4 coef®cient of biharmonic horizontal den- sity diffusivity ␳à k(␾, ␪) kth-layer density climatology à KH4 coef®cient of biharmonic horizontal layer ⌬␳k kth-interface minimum density shear thickness diffusivity ␳o constant reference density Q total surface heat ¯ux ␳1 Ãץ à layer 1 temperature climatology ϭ 0.0757 ϩ 0.00946T1 Tץ T1 Vkkkϭ h v ϭ e␾Uk ϩ e␪Vk ␴␳ reference coef®cient of density climatol- Wk(␾, ␪) kth interface weighting factor for global vertical mixing designed to conserve mass ogy relaxation within a layer in compensation for explicit  , for k 0 ϩ ␶ w ϭ vertical mixing due to hhkkϽ  X(␾, ␪) domainwide area average of X Cko␳ |v kϪ v kϩ1 |(vkkϪ v ϩ1), ␶ ϭ  a radius of the earth (6371 km) k  for k ϭ 1,´´´, n Ϫ 1 cp coef®cient of speci®c heat for seawater Cbo␳ |v n|v n, for k ϭ n ek angular deformation tensor

␷ ␶ w wind stress ץukk ץ e␾␾ϭϪcos␪ ϭϪe ␪␪ ␪ cos␪ k ץ ␾ cos␪ץ k ΂΃ ΂΃ 0, for k ϭ 0, n , u ␻ ϭ ␻ϩϪϪ ␻ Ϫ W ␻Ã ץ ␷ ץ e ϭϩkkcos␪ ϭ e kkkkk ␪ ΂΃cos␪ ␪␾k  for k ϭ 1,´´´, n Ϫ 1ץ ␾΂΃cos␪ץ ␾␪k ϩϩϩ2 ␻kkϭ ␻Ä (max(0, h kkkϪ h )/h )

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1717

TABLE 1. Unvaried model parameters. layer can be modi®ed by advection, diffusion, entrain- Parameter Value De®nition ment, heating (not used), and relaxation to an annual mean density climatology, here based on Levitus and 2 Ϫ1 AH 100 m s Coef®cient of horizontal eddy viscosity Boyer (1994) and Levitus et al. (1994). Ϫ3 The 1/12 ETOP05 bottom topography (NOAA 1986) Cb 2 ϫ 10 Bottom drag coef®cient Њ

Ck 0 kth interfacial stress coef®- was used in the model with some modi®cations. It was cient interpolated to the model grid, then smoothed twice with C 1 Coef®cient of additional in- M a nine-point real smoother to reduce energy generation terfacial friction associat- ed with entrainment at small scales that are poorly resolved by the model. g 9.8msϪ2 Acceleration due to gravity The 200-m isobath generally de®nes the model land±

Hk 80/140/160/245 kth layer thickness at rest sea boundary but there are some exceptions, namely the 375/bottom (in meters) Taiwan Strait, which has been opened at the 25-m iso- ϩ hk 50m(k ϭ 1) Thickness of layer k at 40m(k ϭ 2±6) which entrainment starts bath, and two interior Philippines passages, Leyte Gulf Ϫ (10.5 N, 125.5 E) and San Bernadino Strait (8.0 N, hk Thickness of layer k at Њ Њ Њ which noncompensating 124.5ЊE), are opened. The model does not allow the detrainment starts; not interfaces to intersect the bottom topography, so the used by setting to a large topography must be con®ned to the lowest layer. Thus, value 2 Ϫ1 in order to prevent unrealistically high transport through KH 300 m s Coef®cient of horizontal density diffusivity Taiwan Strait, depth-weighted friction is applied (as dis-

KH4 0 Coef®cient of biharmonic cussed in the appendix of MH). In general, ¯ow is con- horizontal density diffu- strained to near zero in layers completely below the sill sivity à depth. The model produces a net northward Taiwan KH4 0 Coef®cient of biharmonic horizontal layer of thick- Strait transport of 0.7 Ϯ 0.3 Sv in comparison to the ness diffusion 2.1 Sv estimate of Zhao and Fang (1991). The model ␳kϩ1 Ϫ ␳k 1.86/1.38/0.56/0.38/0.43 Strati®cation at rest be- boundaries are closed along 20ЊS, at the southern In- tween layer k and k ϩ 1 donesian archipelago, and at the Sunda Shelf in the in kg mϪ3 Q 0 Total surface heat ¯ux southwest SCS. The Sunda Shelf region covers a large Ϫ1 ␻Ä k 0.04 m s kth interface reference ver- geographical area, but it is very shallow. The Karimata tical mixing velocity Strait that joins the Sunda Shelf with the Java Sea is approximately ¼Њ wide at the 25-m isobath and has a sill depth of 29 m. Metzger and Hurlburt has shown ␻ϪϪϩϭ ␻Ä (max(0, h Ϫ h )/h )2 that closing the Paci®c to Indian Ocean through¯ow and kk kkk the Sunda Shelf does not signi®cantly impact the ¯ow ϩϪ ␻à kkkkϭ (␻ Ϫ ␻ )/W in the SCS in NLOM. In addition, a ⅛Њ global version

␻Ä k kth-interface reference vertical mixing ve- of NLOM that includes the Sunda Shelf/Java Sea and locity can accurately resolve the narrow Karimata Strait has shown the net southward transport to be small (0.25 Ϯ The model boundary conditions are kinematic and no 0.25 Sv). The mean transport across Sunda Shelf in slip. No heat ¯ux forcing is applied in the experiments global NLOM is not much different from the mean 0.6 used here. Table 1 de®nes the nonvarying model pa- Sv estimate by Wyrtki (1961), but the model variability rameters. is more than an order of magnitude smaller than The model domain covers the Paci®c Ocean from Wyrtki's seasonal estimates (summer: 3.0 Sv, winter: 20ЊSto62ЊN in latitude and 109.125ЊE to 77.21ЊWin 4.5 Sv). Estimating the cross-sectional area of the choke longitude, but here the focus is on the subregion of the point as 28 km wide by 25 m deep yields cross-sectional SCS. The horizontal resolution of each prognostic mod- average velocities of 0.85, 4.3, and 6.4 m sϪ1 respec- el variable is ⅛Њ in latitude by 45/256Њ in longitude. The tively for the mean, summer, and winter transport es- model contains horizontal mixing in the form of La- timates of Wyrtki (1961). These seasonal values are placian friction and allows vertical exchange of mo- unrealistically high. Global NLOM values are more re- mentum, mass, and heat across the interfaces. The model alistic and yield average velocities of 0.36 Ϯ 0.36 m permits ventilation of the interfaces between the layers sϪ1 using the same cross-sectional area de®ned above. (outcropping). Vertical mixing (entrainment) is applied The Hellerman and Rosenstein (1983, hereafter HR) when a layer thins beyond a prescribed minimum layer monthly mean wind stress climatology is used to spin ϩ thickness (hk ) and water from the layer below is en- up the model to statistical equilibrium. From this spinup trained at a velocity (␻Ä k) needed to prevent nonpositive model, four different initial states (each one year apart) layer thickness. Volume is conserved within a layer by are used as the restart conditions for the interannually domainwide vertical mixing using a weighting scheme wind-forced experiments. Since these experiments differ based on oxygen saturation as discussed in Shriver and only in the initial state, any differences between them Hurlburt (1997). Horizontal density gradients within a can be attributed to nondeterministic differences in both

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1718 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31

FIG. 2. (a) Mean SSH and upper-layer currents from the ⅛Њ, 6-layer ®nite depth thermodynamic model of the Paci®c Ocean north of 20ЊS with realistic bottom topography. Only the SCS subregion is shown. The mean covers the 1979±97 time frame and is an average of four model realizations, which only differ in their initial state. The simulations were forced with the ECMWF/HR hybrid winds (see text for details). The SSH contour interval is 5 cm and the reference vector at the top is 1 m s Ϫ1. (b) Rms SSH over 1993±97 from the same four simulations with a contour interval of 2 cm. The x's represent the mesoscale SSH variability maxima along TOPEX/Poseidon altimeter tracks over the same time period as determined by Wang et al. (2000). the initial conditions and the evolution of the simula- spheric forecast model and, thus, are unaffected by prod- tions. Thus, we can use these simulations to differentiate uct upgrades. The switch in ECMWF products across deterministic variability in response to atmospheric the 1993/1994 time boundary was investigated to make forcing versus nondeterministic variability due to ¯ow sure no large-scale response might be introduced into instabilities. The spinup experiment exhibits a large de- the ocean model. Paci®c basin energy levels of the two gree of variation in the Kuroshio intrusion, even though wind products showed no signi®cant change between it is forced with monthly climatological winds. (Section the reanalysis/operational (Re/Op) time frames. This 3 describes a quantitative measure of Kuroshio pene- was not the case when the ECMWF 10-m winds were tration and, for reference, the four initial states have examined, hence the decision to use the 1000-hPa prod- penetration values of Ϫ1.1, 6.5, 8.7, and 15.8 cm.) This uct. Smaller-scale differences between the two products provides one of the ®rst insights into the nondetermin- are discussed in section 5. istic nature of the system. Interannual forcing is by European Centre for Me- dium-Range Weather Forecasts [ECMWF (1995)] 1000- b. NLOM results hectopascal (hPa) winds, and in all cases the temporal mean is replaced by the annual mean from HR as dis- 1) ANNUAL SSH MEAN AND VARIABILITY cussed in Metzger et al. (1992). This interannual forcing is referred to as an ECMWF/HR hybrid. The magnitude Long-term means were calculated for each of the four (but not direction) of the ECMWF winds is modi®ed to interannually forced experiments. At this timescale, dif- account for the 1000-hPa level not always being at the ferences were minor between the realizations. Figure 2a surface (Hundermark et al. 1999). The winds are con- shows the combined four-experiment mean SSH and verted to a stress using the density of air (␳air) ϭ 1.2 upper-layer currents and can be compared to Plate 1 of Ϫ3 kg m and a constant drag coef®cient (cd) ϭ 1.5 ϫ MH (which shows four simulations forced by HR 10Ϫ3. The model is forced using 1979±93 ECMWF re- monthly winds). The results from the combined exper- analysis 6-hourly data (Gibson et al. 1997) and then iment mean are very similar to the MH results, so this continued using 1994±97 ECMWF archived operational leads to con®dence in using the ECMWF/HR hybrid 12-hourly winds. The reanalysis winds have the advan- wind approach. Section 2.2 of MH also went into detail tage of using a constant data assimilation and atmo- comparing NLOM transport values with observational

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1719 data. While similar comparisons are not reported here, Kuroshio within the SCS, but the eddy usually fails to the ⅛Њ Paci®c model compares favorably. separate from the main core and eventually coalesces The NEC bifurcates at the Philippines coast at ap- back into the current. During the 15 year reanalysis time proximately 14ЊN and, as the northward ¯owing Ku- frame, each realization typically had only two or three roshio reaches Luzon Strait, one branch bypasses the eddy-shedding events (which usually did not occur in strait with ¯ow east of the Babuyan/Batan island chain. contemporaneous years). This contrasts markedly with A northwestward branch enters Luzon Strait through the the 4 year time frame forced by ECMWF-operational/ Balingtang Channel, penetrates as far east as 118ЊE, HR winds (1994±97) in which all realizations exhibited turns anticyclonically, and exits as a northeastward ¯ow- very deep Kuroshio penetration and eddy shedding in ing current through the Bashi Channel. These alternating almost all years. This difference will be discussed in branches of in¯ow/out¯ow agree with observational more detail in section 5. The maximum model eddy data (Nitani 1972; Huang et al. 1994), and the separation diameter reaches approximately 200 km, which agrees of the Kuroshio into two velocity cores as it ¯ows past with the observational evidence of Wang and Chern Luzon Strait is consistent with the modeling work of (1987a), and they have a vertical extent to the lowest Chern and Wang (1998). Within the SCS basin, a well- model interface (1000 m), which agrees with Li et al. de®ned cyclonic gyre exists north of 13ЊN except along (1998). The animations also show other areas of favored the eastern boundary. The gyre includes a through¯ow eddy generation and propagation. In the lee of the Ba- component with in¯ow through Luzon Strait and out- buyan Island chain, anticyclonic eddies develop in Au- ¯ow into the Sulu Sea. A more diffuse cyclonic cir- gust/September in about half of the model years and culation can be seen in the southern half of the basin. propagate westward. Their signature can be seen in Fig. In the annual mean (and the seasonal means to be shown 2b as a tongue of slightly higher variability west of the later), the model does not produce a SCSWC as found Babuyan Islands along 19ЊN. The model maintains a by Guan (1978, 1985). The relatively strong south- one-gridpoint strait between Luzon Island and the shoals westward ¯owing current seen along the boundary in at 19ЊN, 121.5ЊE, and the eddies often develop when a the northwest SCS corresponds to the current found by jet of water passes through the channel. Many eddies Guo et al. (1985). reach the western boundary in winter and travel south- The 1993±97 SSH variability is shown in Fig. 2b. An westward where they often amplify. Some propagate area of high variability exists west of Luzon Strait at into the central part of the basin and merge/interact with 21ЊN, 119ЊE that is associated with Kuroshio penetra- the eddies in the interior, while others simply dissipate tion and eddy shedding. The corridor of high variability as they travel westward. In winter there is a direct cur- extends to the southwest as the eddies propagate in that rent from the Paci®c entering the SCS. As it becomes direction and the boundary current is unstable. A sec- a western boundary current, eddies occasionally develop ondary maximum is seen off the coast of Vietnam at along the continental coastline and amplify as they prop- 12±15ЊN, 111ЊE with a weaker high variability corridor agate southwestward. However, these coastal eddies are oriented southwest to northeast across the central SCS. more commonly generated from eddies that have either These corridors of high SSH variability agree with con- pinched off from the main core of the Kuroshio or have temporaneous TOPEX/Poseidon (T/P) altimeter data. formed in the lee of the Babuyan Islands. In this case The x's plotted on Fig. 2b are from Wang et al. (2000) the eddies become ®nite amplitude perturbations for and represent the 1993±97 mesoscale SSH variability ¯ow instabilities and thus amplify as they propagate maxima along the T/P altimeter tracks used in their west-southwestward. study. They note decreasing variability from northeast to southwest in the northern corridor and decreasing 3) SEASONAL MEANS variability from southwest to northeast in the southern corridor, as is seen in NLOM. Three-month running means were combined for the four interannually forced realizations and are shown in Fig. 3. The seasonal cycle of Kuroshio penetration 2) SSH ANIMATIONS agrees qualitatively with the historical surface drift data Sea surface height animations from the four reali- of Dale (1956) and Wyrtki (1961) and the hydrographic zations give an indication of the nondeterministic nature analysis of Shaw (1991), namely, increased intrusion in of Kuroshio penetration into the SCS. Side-by-side the winter months and a main core of the Kuroshio that model comparisons rarely show the Kuroshio to be in penetrates only shallowly before retro¯ecting in the the same con®guration, suggesting that nonlinear me- summer months. From November to April, a direct soscale ¯ow instabilities are dominant in this region. branch of the Kuroshio enters the SCS through Baling- Eddy shedding from the main core of the Kuroshio is tang Channel. Guo et al. (1985) have named this win- a relatively rare event during the time frame forced by tertime current the SCS Branch of the Kuroshio ECMWF-reanalysis/HR winds (1979±93). The anima- (SCSBK), and Shaw and Chao (1994), R.-F. Li et al. tions reveal instances of relatively deep intrusion and (1996), and MH are numerical models that also depict an anticyclonic eddy forming on the Paci®c side of the it. From May to October, the Kuroshio also enters

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1720 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31

FIG. 3. The combined experiment 3-month running mean SSH from the four NLOM realizations centered over (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- area model of the SCS forced by Hellerman and Ro- tro¯ects on average at 120ЊN and exits through the Bashi senstein (1983) monthly winds and bimonthly transport Channel. The lack of the SCSBK in summer is consis- estimates at the southern and northern boundaries from tent with observational studies of Xu et al. (1995) and Wyrtki (1961). Sea level in August and October is qual- Wang and Chern (1996). Beginning in July, a sea level itatively similar between NLOM and Shaw and Chao dipole forms off the coast of Vietnam, peaks in strength (1994), but different in December and February. A like- in August, and can still be seen in September. Shaw et ly cause for some of the seasonal sea level differences al. (1999) and Wu et al. (1999) present altimeter data is different boundary condition forcing between the that support such a pattern. The standard deviation of models. NLOM (Fig. 3a) produces a more complete the sea surface (not shown) indicates the highest vari- wintertime cyclonic gyre, whereas Shaw and Chao ability in the western Luzon Strait (21ЊN, 119ЊE) during (1994) indicate that the sea level contours are open on the winter months associated with the deepest Kuroshio the eastern side of the SCS. Satellite data published by intrusion and eddy shedding. Shaw et al. (1999) and Wu et al. (1999) provide an Shaw and Chao (1994) present summer (August) to independent data source. Shaw et al. show T/P altimeter winter (February) bimonthly sea level elevation ®gures data for three days in December 1992, 1993, and 1994. from their 0.4Њ horizontal resolution, 21-level limited- Focusing on 1993, which is from a non-ENSO time

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1721

FIG. 4. As in Fig. 3 except for upper-layer current vectors. The reference vector (at the top) is 1 m s Ϫ1. frame, the satellite data indicate a basinwide cyclonic formative in depicting the surface (non-Ekman) circu- gyre system similar to the NLOM results (Fig. 3a). Ad- lation of the SCS. It can be compared against Figure 10 ditionally, the T/P data suggest that it is a two-gyre in Qu et al. (2000), which shows the seasonal variation system with one center near 17ЊN, 117ЊE and one in the of geostrophic velocity at 100 m. While differences exist southern basin near 11ЊN, 112ЊE. The 1992 and 1994 between the modeled and observed circulation patterns, data indicate weaker basinwide cyclonic gyres than in NLOM qualitatively agrees with these seasonal data. In 1993, and Shaw et al. (1999) attribute this to weaker all seasons, a southwestward current exists in NLOM SCS circulation during ENSO events. Wu et al. (1999) along the continental coastline in the northwest SCS. use the same numerical model as Shaw and Chao (1994) This current is much broader in the winter months and but force it with NCEP/NCAR daily winds beginning is only a narrow (Ͻ1Њ) boundary current in July (Fig. in 1992. Their control run for December 1993 produces 4c). Qu et al. (2000) point out that, despite considerable a very complete cyclonic gyre (their Fig. 3f) that is even variability, a westward ¯ow is seen during all seasons more pronounced when they assimilate the T/P altimeter of the year. From late spring to early fall, their data data (their Fig. 4f). suggest that the ¯ow is narrowly con®ned in the con- Figure 4 is identical to Fig. 3 except that it shows tinental slope south of China but does not always follow seasonal mean upper-layer currents and it is more in- the western boundary. In NLOM, a basinwide cyclonic

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1722 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 gyre exists in the SCS from November to February, with across the ECMWF Re/Op time boundary does not a stronger closed cyclonic gyre in January (Fig. 4a) change signi®cantly either. Metzger and Hurlburt sug- centered at 8.5ЊN, 111ЊE. Fang et al. (1998) refer to this gests that, from a mass balance point of view, the mean as the SCS Southern Cyclonic Gyre and include ®gures Luzon Strait transport is actually an extension of the from Xu et al. (1982) and Fang et al. (1997) that ob- northern tropical gyre. Thus, the wind forcing that con- servationally locate this wintertime gyre at 6.5ЊN, trols the strength of this gyre appears relatively consis- 109.5ЊE. NLOM's displacement of this gyre to the north- tent for both atmospheric products that forced NLOM. east may be due to the closed boundary at the Sunda Despite the large interannual variability, the net trans- Shelf or to the spatial scales resolved by the wind forc- port is very deterministic between the realizations at the ing. Shaw et al. (1999) indicate the importance of the yearly mean scale. Correlations are Ն0.97. Layer by wind stress curl in setting up the sea level in the southern layer, this determinism generally holds, as correlations basin. Adequate resolution of the wind forcing is there- are typically Ͼ0.9 for the upper four layers and Ͼ0.8 fore important and will be discussed later. NLOM also for the abyssal layer; layer 5 shows the most nondeter- reproduces the wintertime north-northeastward Natuna minism. Finally, the abyssal layer is generally in neg- Off-Shelf Current near 6ЊN, 112ЊE (Fang et al. 1997). ative phase with the upper ®ve layers. The circulation in the southern SCS turns anticyclonic Within the model, the in¯ow of Kuroshio waters into in summer (Fig. 4c), which again agrees with Xu et al. the SCS is con®ned between the straits north of Luzon (1982) and Fang et al. (1997). Likewise, the Southeast and the northernmost Batan Islands. On average, the Vietnam Off-Shore Current seen in these observational in¯ow is approximately 16 Sv summed over all six lay- studies at 11ЊN shows up well in the modeled output. ers, but it is concentrated (Ͼ80%) within the top three layers, which have a mean depth of 440±490 m. In the annual mean the majority of the transport within the 4) LUZON STRAIT TRANSPORT southern in¯ow branch simply retro¯ects (approximate- Over 1979±97, the model produces a net Luzon Strait ly 85%) while the remaining portion enters the SCS via transport (sum across all six layers) of 1.8 Ϯ 1.0 Sv Luzon Strait (Fig. 5a). The SCS out¯ow between the from the Paci®c Ocean to the SCS. Qu (2000) used northernmost Batan Islands and Taiwan is simply the historical temperature pro®les combined with climato- in¯ow minus the net Luzon Strait transport and, as logical T/S relationships to obtain a Luzon Strait trans- would be expected, it is highly correlated with the in¯ow port of 3.0 Sv, and Qu et al. (2000) employed the mod- (r ϭϪ0.83). The majority of the out¯ow transport is i®ed ``island rule'' to estimate the transport at 4.2 Sv. retro¯ected in¯ow, but a portion of the volume transport The latter study suggests that the Luzon Strait transport is due to an emptying of the SCS basin from seasonally is a result of the counterbalance between the large-scale stored water. forcing in the Paci®c Ocean and the frictional effect At the seasonal timescale, the southern Luzon Strait along the western coast of the Philippines. Because of in¯ow transport is essentially constant in the upper three this, they indicate that, in order to produce a reasonable model layers. This contrasts with the upper-layer net estimate of Kuroshio intrusion into the SCS, the model (Paci®c to SCS) Luzon Strait and northern Luzon Strait domain must extend southward to the southern tip of out¯ow transports. In winter, the net (northern branch) the Philippines and eastward to the western coast of Luzon Strait transport is at maximum in¯ow (minimum America. The spatial resolution must also be adequate out¯ow), but reverses to weak out¯ow (maximum out- enough to resolve most of the narrow passages that con- ¯ow) in the summer. Because of the nearly constant nect the SCS with the Paci®c and Indian Oceans. These southern in¯ow, the net Luzon Strait and northern out- criteria are met with the version of NLOM used here. ¯ow branches have a very high correlation, r ϭ 0.99. Figure 5a depicts the annual average Luzon Strait The upper-layer northern out¯ow branch (and, hence, transport for the four realizations plus the combined the net Luzon Strait transport) clearly responds to the experiment average. Relatively large interannual vari- seasonal northeast±southwest monsoon as depicted in ations are seen from 1984 to 1990 with a trend toward Fig. 5b. Signi®cant at the 99% con®dence interval, the higher SCS in¯ow transport. The net ¯ow through Lu- correlation of the out¯ow transport with the ECMWF zon Strait is always directed into the SCS with the ex- zonal and meridional wind stress and the wind stress ception of two realizations in 1984. Layer by layer, the magnitude is 0.95, 0.95, and Ϫ0.88, respectively. annual mean indicates Paci®c to SCS in¯ow in all layers, The relationship between interannual variability in the but it is very weak (Ͻ0.1 Sv) in the lowest three. In- northern branch transport and interannual variability in terannually, the upper layer exhibits in¯ow in all years, the northeast±southwest monsoon was investigated by as do layers 2 and 3 with the exception of 1982±84. computing a June±July±August (JJA) and a November± This contrasts with the bottom three layers that switch December±January (NDJ) average of the ECMWF between in¯ow and out¯ow with an approximate 3±4 winds and Luzon Strait transports for each year 1979± yr cycle. Using hydrographic data, Chen and Wang 96. These 3-month bins represent the height of the (1998) found intermediate water out¯ow in all seasons southwest and northeast monsoons, respectively. For the between 350 and 1350 m. Note that the total transport summer season (JJA), the southern in¯ow, northern out-

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1723

FIG. 5. (a) Model transport (in Sv) vs time through Luzon Strait (between Taiwan and Luzon along 120.7ЊE) for the four ⅛Њ NLOM realizations plus their average. Data points are means for each model year and the sum of all six model layers. (b) Monthly upper-layer model transport (blue) for the northern Luzon Strait out¯ow (i.e., between Taiwan and the northernmost Batan Islands), zonal (red solid) and meridional (red dashed) wind stress, and wind stress magnitude (green). The transport (in Sv) is an average of the four ⅛Њ NLOM realizations, and the ECMWF winds are averaged over a 4Њϫ4Њ box centered at 20ЊN, 120ЊE as de®ned by Farris and Wimbush (1996) with units of N mϪ2 ϫ 100.

¯ow, and net Luzon Strait transport showed similarly 3. A quantitative measure of Kuroshio penetration moderate correlations with the zonal wind stress com- ponent (0.71, Ϫ0.67, and 0.59, respectively), but the In order to better link causal effects to Kuroshio pen- correlations were much lower for the meridional com- etration, a quantitative measure of the Kuroshio intru- ponent and the wind stress magnitude. For the winter sion was developed for the yearly mean timescale. Fig- season (NDJ), no moderate correlation was found be- ure 6 shows the combined experiment long-term mean tween the winds and the Luzon Strait transport. It is SSH for the Luzon Strait region. The difference between unclear why moderate correlations were found only for the areal average SSH of the diagonal box centered the summer zonal wind stress. along the penetrating ridge and the average for the north-

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1724 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31

FIG. 7. Kuroshio penetration vs time for the four NLOM realiza- tions plus their average. Penetration is de®ned as the SSH difference (in cm) of the areal average of the two boxes de®ned in Fig. 6. Larger SSH differences indicate deeper Kuroshio intrusion. The plus signs at 1992 and 1993 measure Kuroshio intrusion from two additional experiments forced by the ECMWF operational winds that overlap the reanalysis winds in time; the blue (red) plus signs are for the twin to experiment 1 (2).

FIG. 6. The combined experiment 1979±97 mean SSH for the Luzon Strait region. The contour interval is 2.5 cm. The two boxes de®ne the locations over which the areal average SSH is calculated and then The quantitative measure of Kuroshio penetration ap- differenced to produce a quantitative measure of Kuroshio penetration pears to work well for the annual timescale, but breaks into the SCS. down when looking at the seasonal cycle. The technique indicates maximum Kuroshio intrusion during the sum- mer months with weakest intrusion in the winter (not ern box in the southern Taiwan Strait comprises this shown), while the monthly mean SSH indicates just the quantitative measure. SSH in the northern box shows opposite (note the 30-cm isoline in Fig. 3). This occurs relatively low variability, while the SSH over the di- because wintertime storage of upper ocean water within agonal box provides a good measure of how deep the the SCS affects the measure. That upper-layer water ridge penetrates into the basin. The difference then, al- stored in the SCS in winter exits through Luzon Strait lows for a quantitative comparison between the simu- in the summer, as seen in Fig. 3c, consistent with the lations. observational study by Chu (1972). Note how the SSH Figure 7 depicts the Kuroshio intrusion (based on 1- contours emanate from the central SCS basin and exit yr annual mean SSH) as a function of time for the four in the northern half of the strait. In doing so they add interannually forced realizations plus the combined ex- to the areal average of diagonal box and bias the results. periment average. The simulations show general agree- ment in the intradecadal variability. Moderate penetra- tion (subjectively de®ned as SSH differences between 4. Nondeterminism in the SCS 3 and 8 cm) exists in 1979. This gradually weakens to a. Kuroshio intrusion into the SCS shallow penetration (SSH differences Ͻ3 cm) between approximately 1983 and 1988 and then builds to deep Metzger et al. (1994) developed a technique to sep- penetration (SSH differences Ͼ8 cm) after about 1993. arate the variability of a given model variable into two Despite the general agreement, there is a lot of inter- components, one caused by direct atmospheric forcing experiment variability, which provides some insight into (deterministic) and the other caused by mesoscale ¯ow the nondeterministic nature of Kuroshio intrusion. Cor- instabilities (nondeterministic). This can be applied here relations between the curves ranged from 0.42 to 0.77 and is possible because all the simulations are identical, over the 1979±97 time period, but the deep penetration except for their initial state. Spatial maps can then be during the ECMWF operational wind period appears created to determine the relative importance of the two related to differences in the annual mean wind curl com- anomaly generation mechanisms. pared to the reanalysis period. This will be discussed Using SSH as an example, the technique is as follows. in more detail in section 5. The correlations computed First, the long-term mean SSH is computed for each just over the reanalysis time period are very low, Ͻ0.42, experiment and averaged over the four realizations (Fig. with the exception of one experiment pair. 2a). This is then subtracted from each record (3.05 days

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1725

FIG. 8. The fraction of the modeled SSH variability for (a) 1979±93 and (b) 1994±97 from the four ⅛Њ NLOM simulations caused by direct wind forcing (predominantly deterministic: blue areas) and by ¯ow instabilities (predom- inantly nondeterministic: orange/red areas). The ®eld has been normalized by the total SSH variability at each grid point. The velocity ®eld (not shown) is more nondeterministic than the SSH ®eld. The combined experiment temporal means, used to calculate anomalies, were calculated separately for the time periods 1979±93 and 1994±97. That way the temporal means of the deterministic anomalies were zero over the time frame of each analysis, an essential step to avoid misleading results concerning the relative importance of ¯ow instabilities before and after the wind product change. apart) of the individual simulations to form four time such as the Kuroshio Extension region. Consider the series of SSH anomaly ®elds (␩). At each grid point, case where the true deterministic component is zero. the temporally corresponding records of the four real- Generally the average over the realizations will not be izations are averaged (␩ ) and deviations (␩Јϭ␩ Ϫ zero, but closer to zero as the number of realizations ␩) computed about the combined experiment mean. increases. The fraction of variability was computed for These are then combined and averaged as follows: two, three, and four realizations and with the addition of each successive model run, the fraction due to ¯ow ␩22ϭ ␩ 2 ϩ ␩Ј , instabilities grew. While convergence has probably not where the bar is the average of the four realizations. been achieved for this technique, the ®elds nonetheless Except for the removal of the combined temporal mean, indicate regions where deterministic and nondetermin- no averaging through time is required to obtain these istic variability are relatively important. results. However, the most meaningful results are ob- Figure 8 shows the fractional SSH variability for the tained by temporally averaging the above results over SCS region and has been broken down into two periods, the same period as the temporal mean removed. That 1979±93 and 1994±97, because of the different char- way the temporal average of the deterministic anomalies acter of Kuroshio intrusion when NLOM is forced with is zero. The left-hand side represents the total SSH var- reanalysis and operational winds, respectively. Dark iability and the right-hand side represents the SSH var- blue (orange) areas are regions where SSH variations iability due to atmospheric forcing plus the variability are predominantly caused by wind forcing (mesoscale due to ¯ow instabilities, assuming␩ is accurately de- ¯ow instabilities). Most areas represent a combination ®ned by a large number of realizations. After dividing of the two variability mechanisms, but relative orange the equation by the total SSH variability (␩ 2 ), maps can maxima for both periods are seen in the vicinity of be produced that show the fraction of the SSH variability Luzon Strait, indicating that the intrusion of the Ku- due to either atmospheric forcing or ¯ow instabilities, roshio into the SCS has a substantial nondeterministic respectively. component in its variability. The SSH variability as- With a small number of model realizations the ¯ow sociated with in¯ow in the southern half of the strait instabilities component is underestimated and the wind appears more deterministic than that associated with forcing component is overestimated. This is especially out¯ow in the northern half of the strait. Note also the true for areas dominated by mesoscale ¯ow instabilities corridor of relatively nondeterministic SSH variability

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1726 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 leading southwestward away from the strait. This is as- increases (decreases). Corroborating the analysis above, sociated with eddy development and propagation in the no meaningful relationship between Kuroshio penetra- model. Substantial differences are also seen between the tion and the NEC bifurcation latitude was found; cor- two time periods with the operational wind forcing re- relations ranged from 0.1 to 0.5 with those correlations sulting in a larger degree of nondeterminism in the vi- greater than 0.4 being statistically signi®cant at the 90% cinity of Luzon Strait and in the interior of the SCS con®dence interval. basin. While the operational winds produce a more con- In addition, a causal relationship between the Kuro- sistent, deep Kuroshio intrusion than the reanalysis shio intrusion and the atmosphere is explored. Looking winds, it is clear that the magnitude and spatial extent at just the reanalysis time frame, the wind stress is av- varies greatly between the realizations. The higher de- eraged over a 4Њϫ4Њ box centered at 20ЊN, 120ЊE, as gree of nondeterminism in the interior of the basin also de®ned by Farris and Wimbush (1996). Neither the zon- suggests that the higher resolution operational winds al or meridional components nor the wind stress mag- (more details regarding this in section 5) may be re- nitude correlate well with the measure of yearly mean solving smaller-scale wind features, which might in turn Kuroshio intrusion. trigger more oceanic ¯ow instabilities. The fractional velocity variability is much more non- b. The SCS and the western equatorial Paci®c deterministic in the Luzon Strait region (not shown). The nondeterministic component accounts for Ͼ65% of Using the quantitative measure of Kuroshio penetra- the velocity variability for both the zonal and meridional tion, previous analysis has indicated that the four sim- components. Unlike the SSH variability, the southern ulations show a lot of interexperiment variability. But, half of the strait is equally as nondeterministic as the is this true for regions away from Luzon Strait or for a northern half. larger subregion within the strait? This is examined by From the preceding analysis, it appears then that Ku- correlating yearly mean SSH anomalies over wider spa- roshio penetration on the interannual timescale is not tial areas. One-year means are calculated for each re- solely related to direct atmospheric forcing. Given that alization and the combined experiment long-term mean all four simulations were forced with identical winds, (with respect to 1979±97 and 1979±93) is subtracted to perturbations caused by the different initial states of create four time series of yearly mean anomalies. The each realization are introducing a nondeterministic com- calculations are performed relative to the two long-term ponent as a result of mesoscale ¯ow instabilities. Mod- means because of the signi®cant change in behavior of eling this region at an even higher horizontal resolution the Kuroshio intrusion during the ECMWF operational would likely increase the nondeterministic component. wind period (1994±97). Three subregions are examined: Despite the large element of nondeterminism in the 1) the Luzon Strait region where the ridge of SSH pen- system, attempts were made to relate the Kuroshio in- etrates into the SCS (20Њ±22.5ЊN, 118Њ±121ЊE), 2) the trusion to the oceanic environment. For example, is the entire SCS (5Њ±22.5ЊN, 109Њ±121ЊE) region, and 3) the penetration related to current transports in the vicinity extreme western equatorial Paci®c (0Њ±22.5ЊN, 121Њ± of Luzon Strait? Annual mean modeled transport was 135ЊE). Within each subregion, experiment by experi- determined for four sections (which comprised a box): ment correlation analysis is performed over all model across Luzon Strait along 120.7ЊE (Fig. 5a), east of years to see if a degree of determinism can be found. Taiwan along 22ЊN, a north±south section east of Luzon As expected over the Luzon Strait subregion, yearly Strait along 124.6ЊE, and east of Luzon Island along SSH anomaly correlations between the simulations var- 18ЊN. Correlations between the quantitative measure of ied widely for a given year and showed large interannual Kuroshio penetration (Fig. 7) and the time series of variations. The median correlation over all the experi- annual mean transport for the four sections were low; ments and all years was 0.59 with respect to the 1979± this applied to layer by layer analysis, as well as the 97 mean, but this fell to 0.3 with respect to the 1979± sum of all six model layers. For the three sections away 93 mean. This large difference is due to the consistent, from Luzon Strait, correlations ranged from Ϫ0.1 to deep penetration of the Kuroshio during 1994±97, Ϫ0.3. The range in correlation between the Kuroshio which tends to make the intrusion process appear more intrusion and Luzon Strait transport was stronger, Ϫ0.4 deterministic over this subregion. to Ϫ0.55, correlations signi®cant at the 95% con®dence Over the SCS subregion, there was not as much over- level. However, Kuroshio transport did not affect Ku- all interexperiment variation for a given year compared roshio penetration. to the Luzon Strait subregion, but there was still a lot The NEC bifurcates at 14ЊN along the Philippines of interannual variability in these correlations. Corre- coast on average in NLOM, but interannually (season- lating the anomalies relative to the two long-term means ally) this latitude can vary by 3Њ (2Њ). (The bifurcation did not produce as large a difference in the overall me- latitude across the four realizations is relatively deter- dian correlation, which was 0.57 (0.49) relative to the ministic and correlations between the simulations are 1979±97 (1979±93) mean. Thus, the SCS also exhibits typically Ͼ0.8.) As the bifurcation latitude shifts south- a large degree of nondeterminism in the yearly mean ward (northward), the Kuroshio transport east of Luzon SSH signal due to ¯ow instabilities.

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1727

In contrast to the two subregions above, the western creases, the Luzon Strait transport should increase, equatorial Paci®c subregion is much more deterministic while the Kuroshio transport just north of Luzon Strait at both annual and interannual timescales. For a given would remain the same. Conversely, if the strength of year there is very little interexperiment variation of the the subtropical gyre increases, the Kuroshio transport yearly SSH anomaly correlations, and the median cor- just north of the Luzon Strait should increase, while the relation over all experiments and years is 0.91 (relative Luzon Strait transport would remain the same. No sig- to both long-term means). Thus, the yearly mean SSH ni®cant change in either the Luzon Strait transport (Fig. variations in the extreme western equatorial Paci®c, 5a) or Kuroshio transport was detected across the which are predominantly larger in scale, appear to be ECMWF Re/Op time boundary, so large-scale atmo- largely deterministic, wind-forced phenomena. spheric changes do not appear to be the cause. This is further corroborated by analyzing the atmospheric en- ergy levels over the northern tropical and subtropical c. Intradecadal variability gyres (not shown). No change is detected across the After looking at the Kuroshio intrusion as a function ECMWF Re/Op time boundary. of time (Fig. 7), there appear to be periods when the The local wind stress in the SCS/Philippines region penetration is similar for several years in a row. Overall, is examined next to see if it might provide insight con- 1983±90 are years of shallow Kuroshio penetration cerning the different character of Kuroshio penetration. while 1994±97 are years of deep penetration; 1979±82 Atmospheric energy levels (not shown) are computed and 1991±93 are transition years between the two ex- for three regions: 1) the entire Philippines/SCS (0Њ± tremes with moderate Kuroshio intrusion. Means are 30ЊN, 110Њ±135ЊE), 2) the Luzon Strait region and the created over these four time groups and anomalies of area of strong monsoonal winds to the northeast (18Њ± these relative to both the 1979±97 and 1979±93 com- 30ЊN, 110Њ±139ЊE) similar to the area chosen for wind bined experiment long-term means are again computed analysis in MH, and 3) a more locally con®ned Luzon for each experiment. Correlation analysis is performed Strait region (16Њ±26ЊN, 113Њ±125ЊE). From the mid- over the same three subregions de®ned in the previous 1980s to 1990 the two products show very consistent section. energy levels over all the subregions, but in 1991 a small Unlike the yearly anomaly correlations, the 1983±90 divergence between the curves begins with the opera- and 1994±97 time group interexperiment anomaly cor- tional winds about 10% stronger. This is probably due relations for Luzon Strait are quite high, Ͼ0.9. However, to a change in the atmospheric model resolution, which the 1979±82 and 1991±93 time groups had very weak is described later. However, this approximate 10% up- correlations between the experiments with median cor- ward shift is still within the range of the interannual relations near zero. This suggests a certain amount of variability of the atmospheric energy levels, and thus intradecadal determinism when the Kuroshio intrusion we suggest that wind stress changes across the ECMWF is in one of two extreme modes, either deep penetration Re/Op time boundary are not causing the marked shift or very shallow penetration. These results apply for in Kuroshio intrusion. anomalies relative to both long-term means. Then what about the wind stress curl in Luzon Strait? For the SCS region, the same results apply, but the Yearly mean curl ®elds indicate a signi®cant difference median correlations for the extrema year time groupings over Luzon Strait and in many other regions. Figure 9 (0.8) are not substantially higher than the median cor- shows yearly mean curl for 1993, the last year of overlap relation for the transitional year time groups (0.6). In between the two products. The curl pattern for this year the western equatorial Paci®c region, correlations are is representative of most other years in each dataset. greater than 0.9 for all time groups. Both products indicate negative curl in the northern half of the strait and positive curl in the southern half of the strait; yet the magnitudes differ substantially. Wind curl 5. ECMWF reanalysis versus operational wind extrema from the operational winds are more than twice forcing as large, and the gradient in curl is much sharper. In As mentioned previously, the character of the Ku- addition, the pattern of curl over the interior of the SCS roshio intrusion into the SCS is greatly different de- is also quite different. So, while wind stress magnitude pending upon the time frame of the ECMWF forcing. is fairly consistent between the two products, the dis- During the reanalysis wind period (1979±93), deep Ku- tribution of curl has some substantial differences. roshio penetration is a relatively rare event, but during The ECMWF operational winds are obtained from the operational wind period (1994±97) there is a per- NCAR and are named the ECMWF Tropical Ocean sistent deep intrusion of the Kuroshio. An initial hy- Global Atmosphere (TOGA) global basic surface and pothesis for the different behavior is possible strength- upper-air analyses (NCAR dataset DS111.2). Since Sep- ening of the northern tropical or subtropical gyre. Metz- tember 1991, the operational atmospheric model run- ger and Hurlburt points out that the Luzon Strait trans- ning at ECMWF has had a T213 spectral resolution port is actually an extension of the northern tropical (0.85Њ along the equator). However, a spherical trun- gyre. If the strength of the northern tropical gyre in- cation to T63 (2.8Њ along the equator) is performed and

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1728 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31

FIG. 9. The 1993 mean wind stress curl from the ECMWF 1000-hPa (a) reanalysis and (b) operational atmospheric products. In both, the 1979±93 ECMWF reanalysis mean has been subtracted and the Hellerman and Rosenstein (1983) annual mean added back in to create the hybrid wind set. The contour interval is 7.5 Pa mϪ1 ϫ 10Ϫ8. then interpolation to a 2.5Њ Cartesian grid before dis- dicate the quantitative measure of Kuroshio intrusion tribution. In contrast, the reanalysis winds obtained for the two new experiments. In both cases, the exper- through NCAR are the ECMWF reanalysis basic global iments forced by ECMWF operational winds exhibit upper-air dataset (DS115.1). The model resolution is deeper Kuroshio penetration than its twin forced with spectral T106 (1.7Њ along the equator) with a spherical ECMWF reanalysis winds. This suggests that the larger harmonic truncation to T47 (3.8Њ along the equator) be- negative curl over the northern Luzon Strait creates fore ®nal interpolation to the 2.5Њ grid. In the interior more Ekman pumping, which causes the thermocline to of the SCS, the large and small scale structure of the deepen and in turn leads to greater intrusion of the Ku- wind curl differs. It is clear that the higher-resolution roshio. Given this hypothesis, one might expect wind operational winds pick up more small-scale structure, curl over Luzon Strait to be highly correlated with Ku- in addition to producing larger extreme values. These roshio penetration, but this is not the case. Over the differences appear to be related to the truncation tech- reanalysis period, the correlation coef®cient between the nique employed within each dataset, but further study Kuroshio intrusion and wind curl (averaged over the is needed to corroborate this. region of the mean penetrating Kuroshio) is small (r ϭ Because of an overlap between the ECMWF opera- Ϫ0.25). This again highlights the nondeterministic na- tional and reanalysis winds, it is possible to investigate ture of the Kuroshio intrusion into the SCS. However, model sensitivity to these two wind products. Two ad- the results imply that large changes in wind stress curl ditional experiments are integrated over 1992±93, but magnitudes and patterns over Luzon Strait do signi®- the ECMWF operational winds force the model. The cantly affect the modeled Kuroshio pathway and its in- initial states for these two simulations are the ®rst record teraction with the SCS. The operational products pro- of 1992 from two of the simulations run with ECMWF duce curl maxima that are more than twice as large as reanalysis winds. Thus, there will be two sets of ex- the reanalysis products, and this is greater than the in- periment twins that start from the same initial state but terannual variability of the wind stress curl over this have different wind forcing. Figure 7 indicates a lot of region. interexperiment variability with regard to Kuroshio pen- Note also in Fig. 10 the differences in the circulation etration in 1992±93, with experiment 1 showing mod- in the interior of the SCS brought about by wind stress erately deep intrusion and experiment 2 showing shal- curl changes. Both simulations forced with ECMWF low intrusion. These two extreme cases are chosen as reanalysis winds (Figs. 10a,c) produce a single, northern the initial states for the new experiments described basin cyclonic gyre while the ECMWF operational wind above. forced experiments (Figs. 10b,d) have two distinct Figure 10 shows the 1993 annual mean SSH for the gyres, one northeast of Vietnam near 15ЊN, 111ЊE and two twin experiments and the plus signs on Fig. 7 in- one west of Luzon near 17ЊN, 117ЊE. Thus, the ocean

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1729

FIG. 10. The 1993 mean SSH from (a) expt 1 and (c) expt 2 forced with ECMWF reanalysis winds and from (b) expt 5 and (d) expt 6 forced by ECMWF operational winds. Experiments 5 and 6 begin from the 1992 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 the winds). However, these products are more expensive and ECMWF operational wind curl. It appears then, to most have substantially increased storage requirements com- accurately model the SCS region, numerical modelers pared to the products distributed on the 2.5Њ Cartesian would be wise to choose a wind-forcing dataset with grid. the highest possible horizontal resolution. These prod- ucts are available at NCAR and are the ECMWF re- 6. Summary analysis advanced global dataset (DS115.4) or the ECMWF TOGA global advanced operational spectral The results presented here suggest a large degree of analyses (DS111.0). Both are output on a T106 spectral nondeterminism regarding the Kuroshio intrusion into grid (which is the full model resolution during the re- the SCS. Four numerical experiments were forced with analysis, but a truncated resolution of the operational 1979±97 ECMWF reanalysis and operational winds and

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1730 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 only differ in their initial state. Because the evolution relationship between the mean seasonal cycles of the of the Kuroshio's interaction with the SCS can be quite Luzon Strait transport and the northeast±southwest different between all four realizations, mesoscale ¯ow monsoon. instabilities must be contributing to the nondeterminism As mentioned, the quantitative measure of the Ku- of the system. One would expect these ¯ow instabilities roshio intrusion showed substantial interexperiment in- to dominate even more in the real ocean and at model terannual variability. Wider spatial areas in and away resolutions higher than the ⅛Њ used here. Thus, the re- from Luzon Strait were examined to see if the same was sults suggest that on the yearly mean timescale, Kuro- true. Yearly mean SSH anomalies from each experiment shio intrusion does not appear to be a wind-forced, de- were intercorrelated for a larger Luzon Strait subregion, terministic event. the SCS basin subregion and the extreme western equa- A description of the long-term mean SSH is given torial Paci®c Ocean. The interexperiment correlations along with a discussion of its time evolution. In the for all the ECMWF reanalysis forced years produced Luzon Strait region, contemporaneous interexperiment median correlations of 0.3, 0.49, and 0.91 for the three comparisons of instantaneous SSH rarely show the Ku- subregions, respectively. Thus the SCS basin also ex- roshio to be in the same con®guration. Eddy generation hibits a large degree of nondeterminism in the yearly in Luzon Strait can occur when the Kuroshio intrudes mean SSH signal due to ¯ow instabilities, but the west- deeply into the SCS basin and pinches off a vortex. ern equatorial Paci®c appears to be a region governed Deep penetration and eddy shedding are relatively rare by largely deterministic wind-forced phenomena. events, occurring only two or three times during the Much greater Kuroshio penetration was found after ECMWF reanalysis forcing (1979±93) but are persistent switching to archived operational winds at the end of features in almost all years for the ECMWF operational the ECMWF reanalysis. An initial hypothesis for the winds (1994±97). In August/September, a secondary change in behavior was possible strengthening of the area of favored eddy generation exists in the lee of the northern tropical or subtropical gyres. Since the Luzon Babuyan Island chain. Strait transport is actually an extension of the northern In order to quantify the Kuroshio intrusion into the tropical gyre, the transport should increase with an in- SCS, a measure was developed for the yearly mean crease in the strength of this tropical Paci®c gyre. Like- timescale that examined the difference between the areal wise, increases in subtropical gyre strength should pro- average SSH in the southern Taiwan Strait and along duce an increase in transport just north of Luzon Strait. the penetrating SSH ridge axis. This difference allowed No signi®cant change in these transports was detected quantitative intercomparison of the simulations and in- across the ECMWF reanalysis/operational time bound- vestigation of interannual variability. It showed general ary, so large-scale atmospheric changes do not appear interexperiment agreement for the intradecadal varia- to be the cause. tions, but a lot of interexperiment variability for any The local wind stress in the Luzon Strait/SCS region given year. Kuroshio intrusion correlations between ex- was also studied to provide insight into the different periments were low for the interannual time series. character of the Kuroshio intrusion. No signi®cant Using this intrusion measure, hypotheses relating Ku- change across the reanalysis/operational time boundary roshio penetration to the oceanographic environment or was found for the wind stress. However, yearly mean atmospheric forcing were also investigated. The results wind stress curl ®elds revealed marked differences. Both indicate that the Kuroshio intrusion is not a function of reanalysis and operational products produce negative the oceanic transport through Luzon Strait nor of the curl in the northern Luzon Strait and positive curl in surrounding Paci®c Ocean waters, including Kuroshio the southern strait, but the magnitudes differ substan- transport north or south of Luzon Strait. Yearly mean tially. Wind curl extrema from the operational winds are transports from transects across Luzon Strait, east of more than twice as large, and the gradient in curl is Luzon, east of Taiwan, and a north±south section east much sharper. The pattern of curl over the interior of of Luzon Strait along 124.6ЊE did not strongly correlate the SCS is also quite different. The net effect in the with the Kuroshio intrusion. The NEC bifurcation lat- model is larger Ekman pumping, a deepening of the itude was also examined since, as it shifts southward thermocline, and in turn greater westward Kuroshio pen- (northward), the Kuroshio transport east of Luzon in- etration into the SCS during the 1994±97 ECMWF op- creases (decreases). Corroborating the previous analy- erational forcing time frame. There are also substantial sis, no meaningful relationship was found with Kuroshio changes within the SCS basin. penetration. While the simulated Kuroshio intrusion clearly re- In addition, interannual variations of the wind in the sponds differently to the ECMWF operational and re- vicinity of Luzon Strait were studied. No tangible re- analysis winds, neither period exhibits much determin- lationship could be found that linked interannual vari- ism in the variability. Deep Kuroshio intrusion is spo- ability of yearly mean Kuroshio intrusion or monsoon radic during the reanalysis wind forcing, whereas the season mean Luzon Strait transport with corresponding operational winds produce a consistent, deep penetra- zonal or meridional wind components, wind stress mag- tion. It might appear that the operational winds produce nitude, or wind stress curl. However, there was a close a more deterministic response, but the strength of the

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC JULY 2001 METZGER AND HURLBURT 1731 deep intrusion during 1994±97 is highly variable (Figs. Guo, Z., and W. Fang, 1988: The Kuroshio in Luzon Strait and its 7 and 8b). In fact, the interexperiment root mean square transport during September 1985, (in Chinese with English ab- stract). Trop. Oceanol., 7 (2), 13±19. of the quantitative measure of Kuroshio intrusion cal- ÐÐ, T. Yang, and D. Qiu, 1985: The South China Sea Warm Current culated over the operational wind forcing period (6.4 and the southwestward current on its right side in winter (in cm) is higher than that of the reanalysis wind period Chinese with English abstract). Trop. Oceanol., 4 (1), 1±9. (5.0 cm). Thus, the determinism does not seem to be Hellerman, S., and M. Rosenstein, 1983: Normal monthly wind stress dependent on the ¯ow regime. over the World Ocean with error estimates. J. Phys. Oceanogr., 13, 1093±1104. The difference in the curl distribution is probably Huang, Q., W. Wang, Y. Li, and C. Li, 1994: Current characteristics related to the manner in which the two atmospheric of the South China Sea. Oceanology of China Seas, D. Zhou, products have been truncated on their spectral grid be- Y.-B. Liang, and C.-K. Zeng, Eds., Vol. 1, Kluwer Academic, fore distribution on the 2.5Њ grid. Investigation of this 39±47. Hundermark, B. W., H. E. Hurlburt, E. J. Metzger, and J. F. Shriver, topic requires further study and is beyond the scope of 1999: A comparison of wind stresses derived from archived op- this paper, but as ocean model resolution becomes ®ner, erational ECMWF 1000 mb winds and FSU pseudo stresses over the resolution of the atmospheric forcing driving these the tropical Paci®c Ocean, 1981±93. Naval Research Laboratory models must also increase in order to represent smaller Rep. NRL/FR/7320-99-9643, Stennis Space Center, MS, 40 pp. spatial scales. Hurlburt, H. E., and J. D. Thompson, 1980: A numerical study of Loop Current intrusions and eddy shedding. J. Phys. Oceanogr., 10, 1611±1651. Acknowledgments. This work has been performed as ÐÐ, and ÐÐ, 1982: The dynamics of the Loop Current and shed part of the 6.1 Dynamics of the Low Latitude Western eddies in a numerical model of the Gulf of Mexico. Hydrody- Boundary Currents project under program element namics of Semi-Enclosed Seas, J. C. J. Nihoul, Ed., Elsevier 601153N sponsored by the Of®ce of Naval Research. Scienti®c, 243±298. Levitus, S., and T. P. Boyer, 1994: World Ocean Atlas1994. Vol. 4: Thanks are extended to Alan J. Wallcraft for his con- Temperature. NOAA Atlas NESDIS, 117 pp. tribution in developing the ocean model. The numerical ÐÐ, R. Burgett and T. P. Boyer, 1994: World Ocean Atlas 1994, simulations were performed under the Department of Vol.3: Salinity. NOAA Atlas NESDIS, 99 pp. Defense High Performance Computing Initiative on Sil- Li, L., W. D. Nowlin, and J. Su, 1998: Anticyclonic rings from the Kuroshio in the South China Sea. Deep-Sea Res. Part I, 45, icon Graphics, Inc. Origin 2000 computers at the Aero- 1469±1482. nautical Systems Center, Dayton, Ohio, and the Naval Li, R.-F., Q.-C. Zeng, Z.-J. Gan, and W.-Z. Wang, 1993: Numerical Oceanographic Of®ce, Stennis Space Center, Mississip- simulation of the South China Sea Warm Current and the Taiwan pi. Strait ¯ow in winter. Prog. Natl. Sci., 3 (2), 123±129. ÐÐ, D.-J. Guo, and Q.-C. Zeng, 1996: Numerical simulation of interrelation between the Kuroshio and the current of the north- REFERENCES ern South China Sea. Prog. Natl. Sci., 6 (3), 325±332. Li, W., Q. Liu, and S.-P.Cheng, 1996: The effect of a break in western boundary on the Western Boundary Current. Acta Oceanogr. Chen, C.-T., and S.-L. Wang, 1998: In¯uence of intermediate water in the western Okinawa Trough by the out¯ow from the South Taiwan., 35, 141±153. China Sea. J. Geophys. Res., 103, 12 683±12 688. Metzger, E. J., and H. E. Hurlburt, 1996: Coupled dynamics of the Chern, C.-S., and J. Wang, 1998: A numerical study of the sum- South China Sea, the Sulu Sea and the Paci®c Ocean. J. Geophys. mertime ¯ow around the Luzon Strait. J. Oceanogr., 54, 53±64. Res., 101, 12 331±12 352. Chu, T.-Y., 1972: A study on the water exchange between the Paci®c ÐÐ, ÐÐ, J. C. Kindle, Z. Sirkes, and J. M. Pringle, 1992: Hind- Ocean and the South China Sea. Acta Oceanogr. Taiwan., 2, casting of wind-driven anomalies using a reduced gravity global 11±24. ocean model. J. Mar. Technol. Soc., 26 (2), 23±32. Chuang, W. S., 1986: A note on the driving mechanism of current ÐÐ, ÐÐ, G. A. Jacobs, and J. C. Kindle, 1994: Hindcasting wind- in the Taiwan Strait. J. Oceanogr. Soc. Japan, 42, 355±361. driven anomalies using reduced gravity global ocean models Dale, W. L., 1956: Wind and drift currents in the South China Sea. with ½Њ and ¼Њ resolution. Naval Research Laboratory Rep. Malays. J. Trop. Geogr., 8, 1±31. NRL/FR/7323-93-9444, Stennis Space Center, MS, 21 pp. ECMWF,1995: User guide to ECMWF products. Meteorological Bul- Moore, D. R., and A. J. Wallcraft, 1998: Formulation of the NRL letin M3.2, 71 pp. Layered Ocean Model in spherical coordinates. Naval Research Fang, G., W. Fang, Y. Fang, and K. Wang, 1998: A survey of studies Laboratory Rep. NRL/CR/7323-96-0005, Stennis Space Center, on the South China Sea upper ocean circulation. Acta Oceanogr. MS, 24 pp. Taiwan., 37, 1±16. Nitani, H., 1972: Beginning of the Kuroshio. Kuroshio: Physical Fang, W., Z. Guo, and U. Huang, 1997: Observation and study on Aspects of the Japan Current, H. Stommel and K. Yoshida, Eds., the circulation in the southern South China Sea (in Chinese). University of Washington Press, 129±163. Chin. Sci. Bull., 42, 2264±2271. NOAA, 1986: ETOP05 digital relief of the surface of the Earth. Data Farris, A., and M. Wimbush, 1996: Wind-induced Kuroshio intrusion Announcement 86-MGG-07, National Geophysical Data Center, into the South China Sea. J. Oceanogr., 52, 771±784. Boulder, CO, 1 p. Gibson, J. K., P. KaÊllberg, S. Uppala, A. Hernandez, A. Nomura, and Qu, T., 2000: Upper layer circulation in the South China Sea. J. Phys. E. Serrano, 1997: ERA description. ECMWF Re-Analysis Pro- Oceanogr., 30, 1450±1460. ject Report Series, No. 1, ECMWF, 72 pp. ÐÐ, H. Mitsudera, and T. Yamagata, 1998: On the western boundary Guan, B., 1978: New evidence of the South China Sea Warm Current currents in the Philippine Sea. J. Geophys. Res., 103, 7537±7548. (in Chinese). Ocean Sci., 100±103. ÐÐ, ÐÐ, and ÐÐ, 2000: Intrusion of the North Paci®c waters ÐÐ, 1985: Some features of the temporal and spatial distributions into the South China Sea. J. Geophys. Res., 105, 6415±6424. of the ``counter-wind'' current in the northern South China Sea Shaw, P.-T., 1989: The intrusion of water masses into the sea south- in winter (in Chinese with English abstract). Oceanol. Limnol. west of Taiwan. J. Geophys. Res., 94, 18 213±18 226. Sin., 16 (6), 429±438. ÐÐ, 1991: The seasonal variation of the intrusion of the Philippine

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC 1732 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31

sea water into the South China Sea. J. Geophys. Res., 96, 821± ÐÐ, and ÐÐ, 1996: Some aspects on the circulation in the northern 827. South China Sea. La Mer, 34, 246±257. ÐÐ, and S.-Y. Chao, 1994: Surface circulation in the South China Wang, L., C. J. Koblinsky, and S. Howden, 2000: Mesoscale vari- Sea. Deep-Sea Res. Part I, 41, 1663±1683. ability in the South China Sea from TOPEX/Poseidon altimetry ÐÐ, ÐÐ, and L.-L. Fu, 1999: Sea surface height variations in the data. Deep-Sea Res. Part I, 47, 681±708. South China Sea from satellite altimetry. Oceanol. Acta, 22 (1), Wu, C.-R., P.-T. Shaw, and S.-Y. Chao, 1999: Assimilating altimetric data into a South China Sea model. J. Geophys. Res., 104, 1±17. 29 987±30 005. Shriver, J. F., and H. E. Hurlburt, 1997: The contribution of the global Wyrtki, K., 1961: Physical oceanography of the southeast Asian wa- thermohaline circulation to the Paci®c to Indian Ocean through- ters. NAGA Rep. 2, Scripps Institute of Oceanography, La Jolla, ¯ow via Indonesia. J. Geophys. Res., 102, 5491±5511. CA, 195 pp. Wallcraft, A. J., 1991: The Navy Layered Ocean Model user's guide. Xu, J., J. Su, and D. Qiu, 1995: Hydrographic analysis on the intruding NOARL Rep. 35, Naval Research Laboratory, Stennis Space of Kuroshio water into the South China Sea. Proc. Symp. of Center, MS, 21 pp. Marine Sciences in the Taiwan Strait and its Adjacent Waters, ÐÐ, and D. R. Moore, 1997: The NRL Layered Ocean Model. Guangzhou, China, Chinese National Sciences Foundation, 30± Parallel Comput., 23, 2227±2242. 44. Wang, J., and C.-S. Chern, 1987a: The warm-core eddy in the northern Xu, X., Q. Zhang, and H. Chen, 1982: The general descriptions of the horizontal circulation in the South China Sea (in Chinese South China Sea, I. Preliminary observations on the warm-core with English abstract). Proc. Symp. of the Chinese Society of eddy. Acta Oceanogr. Taiwan., 18, 92±103. Marine Hydrology and Meteorology, Chinese Society of Ocean- ÐÐ, and ÐÐ, 1987b: The warm-core eddy in the northern South ology and Limnology, 119±127. China Sea, II. A simple mechanism for the establishment and Zhao, B., and Fang, G., 1991: Estimation of water volume transports development of the warm-core eddy. Acta Oceanogr. Taiwan., through the main straits of the East China Sea. Acta Oceanol. 18, 104±113. Sin., 10 (1), 1±13.

Unauthenticated | Downloaded 09/29/21 05:11 PM UTC