The Bifurcation of the North Equatorial Current in the Pacific*

JANUARY 2003 QU AND LUKAS 5

The Bifurcation of the North Equatorial Current in the Paci®c*

TANGDONG QU International Paci®c Research Center, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii

ROGER LUKAS Department of Oceanography, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii

(Manuscript received 28 December 2001, in ®nal form 25 May 2002)

ABSTRACT A new climatology using historical temperature and salinity data in the western Paci®c is constructed to examine the bifurcation of the North Equatorial Current (NEC). Integrating dynamically calculated circulation from the sea surface to 1000 m and combining it with surface Ekman transport, it is shown that the bifurcation of the NEC occurs at the southernmost position (14.8ЊN) in July and the northernmost position (about 17.2ЊN) in December. This annual signal lags behind the seasonal meridional migration of the zero zonally integrated wind stress curl line by 4±5 months but corresponds pretty well with the local Ekman pumping associated with the Asian monsoon winds. The bifurcation latitude of the NEC is depth dependent. On the annual average, it shifts from about 13.3ЊN near the surface to north of 20ЊN at depths around 1000 m. There is a time lag of 1± 2 months from the sea surface to the subsurface (300±700 m) for the annual cycle. Below 700 m, the bifurcation of the NEC approaches as far north as 22ЊN during the northeast monsoon (November±January), and as a result an anomalous transport of subtropical water is shown to ¯ow equatorward along the western boundary. The bifurcation of the NEC below 700 m becomes unrecognizable when the prevailing wind is from the southwest (June±August).

1. Introduction Sverdrup theory that the bifurcation of the NEC should occur at the zero zonally integrated wind stress curl line The western equatorial Paci®c is a crossroads for wa- at 14.6 N (Fig. 1), though this latitude varies slightly ter masses formed at middle and high latitudes (Fine et Њ al. 1994), in which low latitude western boundary cur- among different wind products (e.g., Kessler and Taft rents play a pivotal role (McCreary and Lu 1994; Lu 1987; Wajsowicz 1999). The steady Sverdrup theory, and McCreary 1995; Lukas et al. 1996). In the Northern however, masks the variable interactions and exchanges Hemisphere, the westward ¯ow of the North Equatorial of water masses between oceanic gyres, and therefore Current (NEC) splits as it encounters the Philippine is not suf®cient to describe the actual bifurcation latitude coast, feeding the poleward-¯owing Kuroshio and the of the NEC. Surface wind forcing changes both in time equatorward-¯owing Mindanao Current (MC). An in- and space, and these changes exert a particularly large dicator of the partition of the NEC mass, heat, and salt impact on the bifurcation latitude of the NEC both by transport between the Kuroshio and the MC is the bi- local (Asian monsoon winds) and remote effects (Ross- furcation latitude of the NEC, which is involved in de- by waves generated by basinwide winds). termining how much subtropical water bends equator- Another important issue that the Sverdrup theory does ward to ¯ow into the equatorial region via the MC and not accommodate is the depth dependence of the sub- how much turns poleward to return to the subtropics via tropical gyre circulation. The bifurcation latitude of the the Kuroshio. NEC is observed at a mean latitude of about 13ЊN near For the long-term mean it is easy to show from the the surface (Nitani 1972; Toole et al. 1988, 1990), but it shifts northward with depth, approaching as far north as 20ЊN at depths around 800 m (Qu et al. 1998, 1999). * School of Ocean and Earth Science and Technology Contribution Number 5996, and International Paci®c Research Center Contribution A manifestation of this northward shift is the appearance Number IPRC-158. of the Luzon Undercurrent (LUC) ¯owing southward beneath the Kuroshio (Qu et al. 1997), by which a signi®cant amount of the low-salinity North Paci®c Inter- Corresponding author address: Dr. Tangdong Qu, IPRC/SOEST, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822. mediate Water is advected into the equatorial ocean (Tal- E-mail: [email protected] ley 1993; Bingham and Lukas 1994; Qu et al. 1997).

᭧ 2003 American Meteorological Society

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boundary currents. The new climatology, as will be shown below, resolves most of the detailed nontransient phenomena identi®ed from the individual cruise data and has a reasonable representation of the NEC bifurcation in the western Paci®c. The results of this study are presented in the following sections. In section 2, we describe the data and methods of analysis. In section 3, we show the vertically integrated circulation. In section 4, we examine the vertical FIG. 1. Sverdrup transport (Sv) calculated from Hellerman and structure of the western boundary currents and the depth Rosenstein's (1983) annual mean wind stress (1Њϫ1Њ) in the North dependence of the NEC bifurcation latitude. In section Paci®c. 5, we discuss the mechanisms that determine the seasonal excursion of the NEC bifurcation latitude. Results are summarized in section 6. The appearance of the LUC has been linked with the basinwide circulation of the North Paci®c subtropical gyre that contracts to the north with increasing depth 2. Data and method of analysis (Reid and Arthur 1975; Reid 1997; Qu 2001). Without temporally continuous and spatially exten- The data used for this study consist of all temperature, sive measurements, a comprehensive study of the NEC salinity, and dissolved oxygen concentration pro®les at bifurcation latitude has been impossible, given that the observed levels recorded on the CD-ROMs of the World ¯ow ®eld in the tropical western Paci®c is highly var- Ocean Database 1998 of NOAA/NESDIS/NODC from iable and populated with various eddies and waves. Over the region 0Њ±30ЊN, 120Њ±165ЊE. We ®rst select the data the past decade, numerical models have become useful by dropping those pro®les that were ¯agged as ``bad'' tools in synthesizing observations and analyzing vari- or as not passing the monthly, seasonal, and annual ability of the ocean. Using results from a high-resolu- standard deviation checks (Levitus 1982, 1994). Then, tion, reduced-gravity model, Qiu and Lukas (1996, here- we remove those pro®les with obviously erroneous re- after referred to as QL96) provided the ®rst picture on cords (e.g., temperature higher than 10ЊC or salinity the seasonal excursion of the NEC bifurcation latitude. lower than 33.5 psu below 800 m) and those pro®les According to their model results, the seasonal bifurca- that extend shallower than 100 m, simultaneously elim- tion of the NEC occurs at the northernmost position in inating coastal stations. Some of the early observations October and the southernmost position in February. But, are of poor quality, and extreme outliers are not un- to the best of our knowledge, no observational con®r- common in some areas, requiring extensive hand editing mation has been published so far. Due to the limited to remove. vertical structure and the absence of the Indonesian The statistics of the data used for the present study Through¯ow of their model, the situation in the inter- is shown in Table 1. Most of the 198 597 temperature mediate ocean remains unknown. pro®les (Fig. 2a) are from expendable bathythermo- This study is intended to construct a new climatology graph (XBT) measurements and con®ned within 500 m of temperature and salinity similar to that of Levitus of the sea surface, with only 45 616 extending deeper (1982, 1994), using all available historical data in the than 500 m, 23 860 deeper than 800 m, and 13 793 western North Paci®c, but focusing on the structure of deeper than 1200 m. Though smaller by a factor 6.7 the low-latitude western boundary currents. Dictated by near the surface (Fig. 2b), the number of salinity ob- the need to have a single mapping scale for the entire servations is comparable with that of temperature ob- global ocean, the objective analysis used in preparing servations in the upper intermediate layers. The number the Levitus climatology involves smoothing over 700 of oxygen pro®les is somewhat smaller (18 010), about km, and this results in oversmoothing of important mean 2/3 of which extend deeper than 800 m (Table 1). The structures near the western boundary where zonal scales spatial distributions of the temperature and salinity pro- are short (Qu et al. 1999). Without this constraint, we ®les used for this study in two opposite seasons (De- are able to grid the data at 0.5Њ with a smoothing e- cember±February and June±August) are shown in Fig. folding scale of about 150 km (see section 2), giving 3, and the situation is essentially the same for the other signi®cantly better resolution of the narrow western two seasons, except for a bias in the density of sampling

TABLE 1. Statistics of the data used for the present study. Total (Ն100 m) Ն500 m Ն800 m Ն1200 m Temperature 198 597 45 616 23 860 13 793 Salinity 29 525 22 301 18 702 12 663 Oxygen 18 010 13 552 11 858 8418

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FIG. 2. Spatial distributions of (a) temperature and (b) salinity pro®les (asterisk) used for this study. toward summer. At 800 m, for example, the numbers running average is applied, that is, monthly mean tem- of observations for the four seasons are 5747, 5118, perature is derived from observations in three overlap- 8500, and 4495 for temperature; 4524, 3728, 7074, and ping months (January±February±March; February± 3376 for salinity; and 3294, 2465, 3928, and 2171 for March±April, etc.). For salinity and oxygen, the 3- oxygen. month running average is applied to the entire water The temperature, salinity, and oxygen data from in- column from the surface down to 1200 m. dividual pro®les are ®rst interpolated onto a 10-dbar The upper ocean is best sampled in the northwestern uniform pressure series using cubic spline. Then, con- part of the region studied, where the number of samples sidering that there are at least 6.7 times as many tem- is usually larger than ®ve at each grid cell and can be perature pro®les as salinity and oxygen pro®les, we grid as large as several tens. In regions of sparse sampling, the temperature, salinity, and oxygen data in slightly we choose a variable horizontal radius to include at least different ways. For temperature, the data in the upper ®ve samples at each grid cell. The use of variable radius 500 m are averaged in a 0.5Њϫ0.5Њ grid for each month gives better resolution of the narrow western boundary regardless of the year of observations. Below 500 m, currents, where its typical values for temperature, for data coverage is relatively sparse, and a three-month- example, are 0.3Њ at 100 m, 0.6Њ at 800 m, and 0.9Њ at

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FIG. 3. Same as Fig. 2 but for those (left) temperature and (right) salinity pro®les that extend below 800 m in (top) Dec±Feb and (bottom) Jun±Aug.

1200 m. This radius is somewhat larger in the interior mocline, and on the regional average, falls below 0.5ЊC ocean, namely east of 140ЊE, often exceeding 1.0Њ at at 500 m and 0.2ЊC at 800 m. Salinity standard deviation 800 m and 1.5Њ at 1200 m. The gridded monthly data at the surface is higher (ϳ0.15 psu) near the equator, are then smoothed using a two-dimensional Gaussian but it decreases toward the north, falling below 0.1 psu ®lter, with e-folding scales of 1 month and 1.5Њ longitude at about 15ЊN. This trend is reversed below the ther- and latitude. mocline. At 500 m, for example, the typical salinity The smoothed temperature and salinity ®elds are ®- standard deviation ranges from 0.02 psu south of 10ЊN nally converted to dynamic heights, from which geo- to 0.07 psu north of it. Oxygen standard deviation at strophic velocities are determined. While deeper obser- 800 m is of the order 0.1 ml LϪ1 in most part of the vations are available at some locations and seasons, the Tropics, but can be as large as 0.2 ml LϪ1 in the central number of observations decreases rather rapidly below subtropical gyre. 1200 dbar (Qu et al. 1999). To optimize uniformity of The uncertainty of dynamic height cannot be eval- the database, we select the reference level at 1200 dbar. uated from individual pro®les directly because many For the inshore grids where water depth is shallower, are shallower than 1200 dbar. Instead, it is evaluated the properties at deeper levels are extrapolated linearly from temperature and salinity standard deviations. As- from their nearby offshore grids. suming that a positive temperature deviation always Standard deviations are also estimated and used to corresponds to a negative salinity deviation and then edit the mean ®elds. If a property from an individual integrating vertically their contributions from each in- pro®le deviates from the grid mean by a value three dividual depth to 1200 dbar, we obtain a typical dy- times larger than the standard deviation, it is excluded, namic height standard deviation ranging from 15 dyn and the mean and standard deviation are recalculated. cm at 100 m to 1.5 dyn cm at 800 m. Thus the standard The spatial distributions of temperature and salinity errors of the mean dynamic height ®eld, de®ned as the standard deviations, though not shown, are chie¯y char- standard deviations divided by the square root of the acterized by east±west oriented contours. Typical sur- number of measurements, are of order 5 dyn cm at 100 face temperature standard deviation is 0.5ЊC, ranging m and 0.5 dyn cm at 800 m based on 5±10 independent from 0.3ЊC near the equator to 0.8ЊC north of 20ЊN. samples at each grid cell. These values might be over- Temperature standard deviation is largest at depths of estimated because temperature and salinity deviations the thermocline around 100 m, where its typical value are not always negatively correlated. In an extreme exceeds 1ЊC. It decreases with depth below the ther- case, where they are positively correlated, as is the

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are representative. Although there is no easy way to quantify the uncertainty in the bifurcation latitude, its location and seasonal excursion as shown below are consistent with the large-scale circulation, and this in- creases our con®dence in the signi®cance of our estimates.

b. Seasonal variation Monthly mean depth-integrated dynamic heights are presented in a smaller region near the Philippine coast (5Њ±25ЊN, 120Њ±140ЊE) to better illustrate the structure of the western boundary currents (Fig. 5). In the south, the ¯ow ®eld is dominated by a cyclonic circulation centered at 6Њ±8ЊN off Mindanao. This cyclonic circu- FIG. 4. Annual mean depth-integrated (0±1000 m) dynamic height (m2) relative to 1200 dbar. The asterisk indicates the location of the lation, often referred to as the Mindanao Dome (MD; NEC bifurcation. Masumoto and Yamagata 1991), is fully developed in December/January, when the northeast monsoon prevails (Fig. 6). It decays during the rest of the year with case often observed in the main thermocline, the stan- the diminishing of local Ekman pumping combined with dard errors of the mean dynamic height ®eld are re- the westward propagation of downwelling Rossby duced by at least a factor of 2. waves excited by the northeast trade wind farther eastward near 160ЊE (Masumoto and Yamagata 1991; To- 3. Vertically integrated circulation zuka et al. 2002). In the north, water tends to recirculate as part of the subtropical gyre; its southern boundary a. Mean state is de®ned by the location of the NEC bifurcation at the Depth-integrated (0±1000 m) dynamic height, P ϭ Philippine coast. 0 1/g#1000m D dz, provides an overview of the geostrophic Given that the western boundary currents presented circulation in the upper ocean (Fig. 4), where g is the in this study (Fig. 5) are considerably weaker and acceleration due to gravity and D represents the dynamic wider than what we have known from synoptic mea- height relative to 1200 dbar. Given depth-integrated dy- surements presumably as a result of smoothing, we namic heights P at any two points A and B, the geo- de®ne the NEC bifurcation latitude to be where the strophic volume transport between these two points, meridional velocity averaged within a 5Њ- instead of

QAB, can be determined by a2Њ-longitude band as previously used by QL96 off the continental slope is zero. At the Luzon strait Q ϭ (P Ϫ P )g/ f, AB A B (18.5Њ±22ЊN), where western boundary is discon- where f is the Coriolis parameter. nected, the average is made from 120Њ to 125ЊN. Our The westward NEC splits at the western boundary (Fig. calculation shows that the annual signal of the NEC 4); its two branches (i.e., the Kuroshio and MC) close bifurcation latitude is insensitive to the selection of the interior Sverdrup circulations of the subtropical gyre longitude bands in the range between 2Њ and 5Њ, but in the north and the tropical gyre in the south. The di- its amplitude (maximum±minimum) tends to increase vision between the two gyres (i.e., the NEC bifurcation) by up to 30% with a selection near the larger end of occurs around 15.4ЊN near the western boundary. If sur- this range, re¯ecting the wide spread signal of the face Ekman transport is considered, this latitude moves western boundary currents in this climatology. to 15.2ЊN, suggesting that, despite the presence of bar- The annual march of the NEC bifurcation is evident oclinic processes, inertial effects, and locally wind-driven in Fig. 5. It occurs at the southernmost position (14.6ЊN) ¯uctuations, Sverdrup theory provides an useful estimate in July and the northernmost position (17.8ЊN) in No- (14.6ЊN) for the mean bifurcation latitude of the NEC in vember (Fig. 7, top). This annual signal will be modi®ed the western Paci®c (Fig. 1). if the contribution of surface Ekman transport, whose Based on 5±10 independent samples in each grid cell typical values within the 5Њ-longitude band are 1.5 Sv and assuming dynamic height errors at different depths (1 Sv ϵ 106 m3 sϪ1) (northward) in winter and Ϫ0.5 are all positively correlated, an uncertainty of order 20 Sv (southward) in summer, is considered. Here, we see m2 is obtained in the mean ®eld of depth-integrated that the Ekman transport associated with the northeast dynamic height. However, if we assume that dynamic monsoon (October±December) pushes the NEC bifur- height errors at different depths are uncorrelated, this cation southward by up to 1Њ, while its northward move- uncertainty would be reduced by a factor 10. With such ment forced by the southwest monsoon (June±August) an uncertainty ranging between 2 and 20 m2, most of is relatively small. Adding the contribution from the the large-scale phenomena presented here (e.g., Fig. 4) Ekman transport reduces the peak-to-peak seasonal var-

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FIG. 5. Monthly depth-integrated (0±1000 m) dynamic height (m2) relative to 1200 dbar. The asterisks indicate the location of the NEC bifurcation.

FIG. 6. Same as Fig. 5 but for wind stress curl (10Ϫ8 NmϪ3). The heavy dashed lines represent zero zonally integrated wind stress curl.

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NmϪ3) in November and a minimum (Ͻ1 ϫ 10Ϫ8 N mϪ3) in May/June (Fig. 7, bottom), which is almost in phase with the seasonal excursion of the NEC bifurcation latitude.

4. Depth dependence of the circulation a. Mean state The mean dynamic height and geostrophic ¯ow (Fig. 8) clearly demonstrate the northward shift of the NEC at increasing depth. Near the sea surface (0±100 m), the NEC is well con®ned within the latitude band between 8Њ and 16ЊN, and bifurcates as it encounters the western boundary at 14.2ЊN. This bifurcation latitude is signif- icantly affected by surface Ekman current. If we assume, for example, that Ekman current is uniformly distributed in the upper 100 m, the bifurcation near the sea surface (0±100 m) will move to 13.3ЊN, about 0.9Њ farther southward than that obtained from the geostrophic circulation alone. At 200 m, the NEC becomes broader, with the northern boundary reaching at least 22ЊN, and as a consequence, its bifurcation occurs about 2Њ farther northward than at the sea surface. At 400 m, the NEC ¯ows entirely north of 12ЊN, leaving a broad area of almost no motion to the south. At 600 m, there is a dynamic height minimum (Ͻ51 dyn cm) centered at about 12ЊN. North of FIG. 7. (top) Seasonal variation of the NEC bifurcation latitude it, the NEC ¯ows westward with a maximum speed determined from Sverdrup transport (solid line with open circle), Ϫ1 geostrophic transport (the dashed line), and geostrophicϩEkman exceeding 3 cm s at 18Њ±20ЊN; south of it, a narrow transport (solid line with closed circle). (bottom) Wind stress curl eastward ¯ow is present along 9Њ±11ЊN. A similar pat- (10Ϫ8 NmϪ3) averaged in the tropical western Paci®c (10Њ±20ЊN, tern is shown at 800 m, except that the MD identi®ed 120Њ±140ЊE). in the shallower waters has completely disappeared at this depth. Instead, an anticyclonic circulation charac- terized by a local dynamic height maximum (Ͼ32 dyn iability of the NEC bifurcation latitude by up to 20%, cm) appears southeast of Mindanao, through which wa- which yields a southernmost bifurcation at about 14.8ЊN ter of South Paci®c origin intrudes northward along the in July and a northernmost bifurcation at about 17.2ЊN western boundary (Qu et al. 1999). in December. The velocity section along 130ЊE shows the vertical With a time lag of 4±5 months, this annual signal of structure of the NEC as it enters the Philippine Sea (Fig. the NEC bifurcation latitude is weaker by a factor at 9). At about 10ЊN, the near-surface westward ¯ow as- least 2 compared with the seasonal migration of the zero sociated with the NEC exceeds 20 cm sϪ1. The core of zonally integrated wind stress curl line, whose peak-to- maximum velocity moves toward the north with in- peak amplitude exceeds 9Њ. This large discrepancy in creasing depth, approaching 18ЊN at depths below 300 amplitude was interpreted by QL96 as a result of phase m. The North Equatorial Countercurrent ¯ows eastward cancellation between local forcing and remotely forced between 3Њ and 8ЊN, forming the southern ¯ank of the Rossby waves, but no phase difference was apparent in MD. their simple reduced-gravity model. Upon reaching the western boundary, the NEC bi- Despite the phase difference noted above, the tem- furcates into the northward-¯owing Kuroshio and the poral correspondence of the seasonal bifurcation of the southward-¯owing MC and LUC. Although the narrow NEC with local Ekman pumping is striking. In Fig. 6, western boundary currents are signi®cantly weaker in we see that the zero zonally integrated wind stress curl this study compared with synoptic measurements (e.g., line migrates from about 11ЊN in January/February to Lukas et al. l991; Qu et al. 1998), as a result of averaging about 20ЊN in August/September (also see Fig. 7). Su- and smoothing, their basic structure is well resolved in perimposed on this basinwide wind system is a dramatic the present climatology. At 9ЊN, a southward ¯ow as- change in local wind associated with the Asian mon- sociated with the MC is dominant in the upper 700 m, soon. Averaged in the region 10Њ±20ЊN, 120Њ±140ЊE, with a velocity core of about 14 cm sϪ1 located at about the local wind stress curl has a maximum (Ͼ10 ϫ 10Ϫ8 50 m (Fig. 10). The Mindanao Undercurrent (MUC; Hu

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FIG. 8. Annual mean dynamic height (dyn cm) and geostrophic ¯ow relative to 1200 dbar at 0, 100, 200, 400, 600, and 800 m. et al. 1991; Lukas et al. 1991) stands out as two velocity 15ЊN, overlying the southward-¯owing LUC that ex- cores. The deeper one, lying at 800±900 m, is weaker tends from about 200 m to at least 1000 m. At 19ЊN, and closer to the coast, and the shallower one is some- the maximum speed of the Kuroshio exceeds 10 cm sϪ1 what offshore. This velocity structure shows a remark- at about 50 m, while the LUC appears as a weak south- able agreement with that from repeat hydrographic ob- ward ¯ow only at depths below 700 m. servations reported by Qu et al. (1998). Similar structure The bifurcation latitude of the NEC, by de®nition, is is seen at 11ЊN, except that the velocity core of the MC the place where the western boundary currents reverse. tends to be deeper along the northern section. At 13ЊN, Meridional velocity averaged within a 5Њ-longitude band surface velocity drops to near zero, and the core of off the Philippine coast shows that the bifurcation of southward ¯ow occurs at about 300 m. The Kuroshio the NEC occurs at about 14ЊN near the surface (Fig. starts to appear as a northward surface ¯ow around 11). This latitude shifts to the north with increasing depth, extending north of 20ЊN at 800±1000 m. Below the MC, the northward ¯ow is con®ned basically to the south of 15ЊN.

b. Seasonal variation The circulation near the surface for each individual month contains essentially the same pattern as that shown in Fig. 8. Further inspection of the anomalous ®eld, however, shows considerable discrepancies among different seasons, and these discrepancies seem to cor- respond with local Ekman pumping (Fig. 6). In winter, when the northeast monsoon prevails, strong positive Ekman pumping produces an anomalous cyclonic circulation over a large part of the Philippine Sea (Fig. FIG. 9. Annual mean geostrophic velocity (cm sϪ1) section at 12). This situation is reversed in summer. As the south- 130ЊE. Positive value indicates eastward ¯ow. west monsoon develops, local Ekman pumping reaches

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FIG. 10. Annual mean geostrophic velocity (cm sϪ1) sections across the western boundary at 9Њ,11Њ,13Њ,15Њ,17Њ, and 19ЊN. Positive values indicate northward ¯ow. its seasonal minimum (Fig. 6), and thus generates an through which water from the South Paci®c may be anomalous anticyclonic circulation in much of the re- advected northward along the coast of Mindanao. Part gion studied. of this water mass appears to be carried offshore before The circulation in the intermediate layers is generally heading eastward along 9Њ±11ЊN. Another contribution weak (Ͻ3cmsϪ1). Its seasonality is apparent even in is from the NEC through the deep extension of the LUC the monthly mean velocity ®elds (Fig. 13). At 800 m, (Fig. 13). From November to January, when the north- the NEC has moved to the latitudes north of 18ЊN. South east monsoon develops, the LUC reaches its maximum of this latitude, a narrow eastward ¯ow is present at strength, and part of the NEC water is transported south- about 10ЊN during most seasons of the year. Two path- ward along the western boundary to feed into the east- ways can be identi®ed that contribute to this eastward ward ¯ow at about 10ЊN. This does not seem to be the ¯ow. One is the anticyclonic circulation off Mindanao, case during the southwest monsoon season. From June to August, the deep part of the LUC becomes unrecognizable, with no apparent southward ¯ow along the Philippine coast below 700 m (Fig. 14). Water property distributions provide independent ev- idence for the monsoon-related circulations. In the months of November±January, when the northeast monsoon prevails, water of eastern subtropical origin with Ϫ1 O2 Ͻ 1.8 ml L and 34.3 Ͻ S Ͻ 34.5 psu (Reid 1965; Bingham and Lukas 1994) is seen extending all the way toward the western boundary at depths around 800 M (Fig. 15). Upon approaching the Philippine coast, part of this water mass turns southward via the LUC and can be traced continuously to the southern tip of Mindanao (Bingham and Lukas 1995), despite considerable mod- 1 FIG. 11. Annual mean geostrophic velocity (cm sϪ ) averaged with- i®cation of properties as a result of mixing. In the ina5Њ-lon band off the Philippine coast. Positive value indicates northward ¯ow, and the contour of zero velocity represents the bi- months of June±August, when the prevailing wind is furcation of the NEC. from the southwest, this water mass is basically east±

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FIG. 12. Anomalous geostrophic ¯ow (cm sϪ1) at 100 m in Jul and Dec relative to the annual mean ®eld. west oriented, with its extreme properties centered at sibly providing a direct pathway for the Antarctic In- 18Њ±20ЊN (Fig. 15). There is little indication that this termediate Water (AAIW) to leak into the subtropical water mass extends farther southward than 14ЊN during North Paci®c. this period of the year. The seasonal cycle of the NEC bifurcation is depth dependent (Fig. 16). Near the sea surface (Ͻ100 m), 5. Some thoughts on the forcing mechanisms the dynamically calculated circulation suggests a southernmost bifurcation (13.4ЊN) in June and a northernmost Before proceeding to the conclusions, we discuss in bifurcation (14.8ЊN) in November. As we progress to this section the mechanisms that determine the seasonal the deeper levels, this bifurcation trends northward, ap- bifurcation of the NEC. The theoretical background of proaching 17.3ЊN in August and 20.9ЊN in January at the bifurcation has been presented by QL96, and only depths around 700 m. a brief summary is presented here. In the sense of linear Below 700 m, the bifurcation of the NEC reaches at dynamics, the variability of large-scale, low-frequency least 22ЊN from November to January, but becomes circulation in the ocean is essentially governed by a unrecognizable from June to August (Fig. 14). In fact, combination of local Ekman pumping and remotely during the latter period of time, the weak MUC appears forced Rossby waves (Meyers 1979). This simple mod- to merge with the deep extension of the Kuroshio, pos- el, for the long-term average, yields the Sverdrup re-

FIG. 13. Geostrophic ¯ow (cm sϪ1) at 800 m in Jul and Dec.

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FIG. 14. Same as Fig. 11 but for monthly mean velocity in Jul and Dec.

FIG. 15. Salinity (psu) and oxygen concentration (ml LϪ1) at 800 m in Jul and Dec. Areas with oxygen concentration Ͻ1.8 ml LϪ1 are shaded.

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boundary results in a shift of the NEC bifurcation to its southernmost position. Another process that might be important in modu- lating the bifurcation latitude of the NEC is the equatorward-propagating coastal Kelvin waves (Lukas 1996). To the extent that linear dynamics dominates, annual, westward-propagating Rossby waves generated at midlatitudes should re¯ect at the western boundary into equatorward-propagating coastal Kelvin waves and eastward-propagating short Rossby waves. These equatorward-propagating coastal Kelvin waves may alter the circulation pattern of the western boundary at lower latitudes and consequently the bifurcation latitude of the NEC. Lukas (1996) also pointed out the potential im- FIG. 16. Time±depth section of the NEC bifurcation latitude in the portance of the eastward-propagating short Rossby upper 700 m. waves, but Qiu and Lukas's (1996) reduced gravity model does not support this idea. This has to be inves- tigated further by research. lationship that is well satis®ed in most parts of the global In addition to the above, we note that the ocean process within the marginal seas may also play a role in ocean. determining the seasonal bifurcation of the NEC. In the However, because the time taken for annual Rossby southern hemisphere, it is often believed that the pres- waves to cross the Paci®c in the latitudes of the NEC ence of the Indonesian through¯ow, a component of the bifurcation is more than a year, Sverdrup theory is not circulation around Australia±Papua New Guinea as sug- applicable to the seasonal cycle (Fig. 7). In a linear, gested by Godfrey's (1989) ``Island Rule,'' augments reduced-gravity model, QL96 noticed that waves gen- the region of equatorward western boundary current, erated at different longitudes arrive at the western thus producing a bifurcation farther to the south (Qu boundary at different phases of the seasonal cycle, and and Lindstrom 2002). The situation seems to be reversed as a result of phase cancellation, their integral effect in the Northern Hemisphere, where the circulation upon the NEC bifurcation is small compared with the around the Philippines tends to push the bifurcation to- annual migration of the zonally integrated wind stress ward the equator (Metzger and Hurlburt 1996; Qu et al. curl line. The present results show that the amplitude 2000). In a 1.5-layer reduced-gravity model forced by of the seasonal excursion of the NEC bifurcation latitude climatological wind, Metzger and Hurlburt (1996) found is only about half of that of the annual migration of the that this equatorward movement can be up to several zonally integrated wind stress curl line, giving support degrees, but its coherence with the ¯uctuation of the to the QL96 conclusion. South China Sea circulation is not understood. We will We note, however, that in addition to the difference leave this problem for a future study using results from in amplitude, there is a phase lag of 4±5 months between high-resolution general circulation models. the seasonal bifurcation of the NEC and the annual migration of zero zonally integrated wind stress curl line. This result differs from QL96. One possible explanation 6. Summary and discussion for this difference is that the QL96 result includes ageo- Based on a new climatology derived from historical strophic ¯ow and applies only to the surface ocean. temperature and salinity data, this study provides a de- Here, we emphasize the importance of local Ekman tailed description of the seasonal variation and depth pumping associated with the monsoonal winds in de- distribution of the NEC bifurcation in the western Pa- termining the seasonal bifurcation of the NEC. In the ci®c. The results are summarized as follows. months of November±December, for example, the local Near the sea surface (0±100 m), the mean bifurcation wind stress curl in the tropical western Paci®c is usually of the NEC occurs at about 14.2ЊN. Adding surface larger than 10 ϫ 10Ϫ8 NmϪ3, and can be as large as Ekman transport moves this latitude southward to about 30 ϫ 10Ϫ8 NmϪ3 near the coast of Luzon (Fig. 6). This 13.3ЊN. Below the sea surface, the NEC bifurcation lat- large positive wind stress curl produces an anomalous itude shifts with depth, reaching as far north as 20ЊNat cyclonic circulation in the Philippine Sea (Figs. 12 and depths around 800 m. A manifestation of this northward 13), whose southward component near the western shift is the appearance of the LUC ¯owing southward boundary causes the NEC bifurcation to occur at a high- beneath the Kuroshio. A subsurface northward coun- er latitude. During the southwest monsoon, on the con- tercurrent also exists along the coast of Mindanao, rep- trary, the weak positive/negative wind stress curl pro- resenting the in¯uence of South Paci®c sources, but its duces an anomalous anticyclonic circulation (Figs. 12 northward extension is con®ned primarily to the south and 13), and its northward component near the western of 15ЊN.

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A combination of the vertically integrated (0±1000 many valuable discussions. Thanks are also extended to m) geostrophic circulation with surface Ekman transport G. Meyers, T. Yamagata, and E. Lindstrom for useful indicates that the seasonal bifurcation of the NEC occurs communication on the present topic and to L. D. Talley at the southernmost position (about 14.8ЊN) in July and and two anonymous reviewers for their thoughtful com- the northernmost position (about 17.2ЊN) in December. ments on the earlier manuscript. This annual signal is smaller by a factor of at least 2 in amplitude than that of the annual migration of the zero zonally integrated wind stress curl line, supporting REFERENCES the QL96 conclusion that the integral effect of Rossby waves on the NEC bifurcation latitude is reduced by Bingham, F. M., and R. Lukas, 1994: The southward intrusion of the North Paci®c Intermediate Water along the Mindanao coast. J. phase cancellation. Phys. Oceanogr., 24, 141±154. 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