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Interannual Variability of the Mindanao Current/Undercurrent in Direct Observations and Numerical Simulations

SHIJIAN HU AND DUNXIN HU Institute of Oceanology, and Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, and Laboratory for Ocean and Climate Dynamics, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China

CONG GUAN Institute of Oceanology, and Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, Qingdao, and University of Chinese Academy of Sciences, Beijing, China

FAN WANG,LINLIN ZHANG,FUJUN WANG, AND QINGYE WANG

Institute of Oceanology, and Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, and Laboratory for Ocean and Climate Dynamics, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China

(Manuscript received 18 May 2015, in final form 30 November 2015)

ABSTRACT

The interannual variability of the boundary currents east of the Mindanao Island, including the Mindanao Current/Undercurrent (MC/MUC), is investigated using moored acoustic Doppler current profiler (ADCP) measurements combined with a series of numerical experiments. The ADCP mooring system was deployed east of the Mindanao Island at 78590N, 127830E during December 2010–August 2014. Depth-dependent in- terannual variability is detected in the two western boundary currents: strong and lower-frequency variability dominates the upper-layer MC, while weaker and higher-frequency fluctuation controls the subsurface MUC. Throughout the duration of mooring measurements, the weakest MC was observed in June 2012, in contrast to the maximum peaks in December 2010 and June 2014, while in the deeper layer the MUC shows speed peaks circa December 2010, January 2011, April 2013, and July 2014 and valleys circa June 2011, August 2012, and November 2013. Diagnostic analysis and numerical sensitivity experiments using a 2.5-layer reduced- gravity model indicate that wind forcing in the western Pacific Ocean is a driving agent in conditioning the interannual variability of MC and MUC. Results suggest that westward-propagating Rossby waves that generate in the western Pacific Ocean (roughly 1508–1808E) are of much significance in the interannual variability of the two boundary currents. Fluctuation of Ekman pumping due to local wind stress curl anomaly in the far western Pacific Ocean (roughly 1208–1508E) also plays a role in the interannual variability of the MC. The relationship between the MC/MUC and El Niño is discussed.

1. Introduction the El Niño–Southern Oscillation (ENSO) cycle and in regulating the western Pacific warm pool that is a key Low-latitude western boundary currents (LLWBCs) in factor in the tropical Pacific Ocean (Hu and Cui 1991; the Pacific Ocean are of much importance in the tropical Lukas et al. 1996; Jin 1997; Hu and Hu 2012; Hu et al. climate system for their role in the recharge/discharge of 2015). The Mindanao Current (MC) and the underlying Mindanao Undercurrent (MUC) are remarkable com- ponents of the LLWBCs in the Pacific Ocean (Hu and Corresponding author address: Shijian Hu, Institute of Ocean- ology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao Cui 1989). They are suggested to be important in the 266071, China. global for their significant con- E-mail: [email protected] tribution to the Indonesian Throughflow (ITF) that

DOI: 10.1175/JPO-D-15-0092.1

Ó 2016 American Meteorological Society Unauthenticated | Downloaded 10/04/21 07:08 AM UTC 484 JOURNAL OF VOLUME 46 connects different ocean basins (e.g., Gordon and Fine suggested that ‘‘the stationary northward undercurrent, 1996; Sprintall et al. 2014). The MC is also a crucial the Mindanao Undercurrent, was also not found at 78N pathway between the midlatitudes and equatorial region east of Mindanao’’ on the basis of observations shallower and influences the interdecadal climate variability (e.g., than 600 m (thus shallower than the MUC layer). Gu and Philander 1997). The significance of LLWBCs Kashino et al. (2015,p.1)proposedthatthe‘‘Mindanao in the Pacific Ocean has led to international research ef- Undercurrent was not confirmed at the 78N line,’’ but the forts such as the Northwestern Pacific Ocean Circulation measurements in that paper are confined to the upper and Climate Experiment (NPOCE; Hu et al. 2011)and 350-m layer and depths of 560, 960, and 1460 m, which Southwest Pacific Ocean Circulation and Climate Exper- expectedly might miss the MUC core. Lack of enough iment (SPICE; Ganachaud et al. 2014). observations gives rise to some studies using model out- The MC and MUC are of particular interest to the puts (e.g., Kashino et al. 2015), but unfortunately few of oceanography community for their spatial complexity these simulations can be validated by observations. Re- (e.g., Hu and Cui 1989, 1991; Wijffels et al. 1995; Wang cently, Qiu et al. (2015) pointed out that a time-mean et al. 2015) and multi-time-scale variabilities (e.g., Lukas MUC is observed from 68 to 138N and extends from the 1988; Qiu and Lukas 1996; Tozuka et al. 2002; Kashino about 400- to 1200-m layer at 68N to the about 800- to et al. 2005, 2009, 2011; Zhang et al. 2014). On an in- 1200-m layer at about 128N, on the basis of 14-yr Argo terannual time scale, Lukas (1988) found that fluctua- float profiling data (obviously with depth up to 2000 m) tions of the MC transport are at least 50% of the mean from 2001 to 2014. The study by Qiuetal.(2015)together transport, with a period of 2 yr, but have no apparent with other previous observations seems to confirm that relationship to the strength of ENSO, on the basis of sea the MUC is a mean flow along the east coast of Mindanao level observations at the islands of Mindanao in the Island (e.g., Zhang et al. 2014; Hu et al. 2015). and Malakal in Palau. Using hydrographic Previous studies have tremendously facilitated our observations from eight cruises, Wijffels et al. (1995) understanding of the structure and variability of these suggested that the MC is a stable coastal jet with a currents. However, in a situation of a lack of sustained 2 maximum speed of 1 m s 1 and speculated that the measurements of these boundary currents, the feature variation of MC is related to the eddy or meanderlike and mechanism of the interannual variability of MC are anomalous circulation. Qiu and Lukas (1996) pointed still controversial. For example, we do not know much out that the interannual variability of the MC is influ- about the vertical feature of the interannual variability, enced by both the ENSO wind on a time scale of 3–7 yr and it is unclear yet what the relative contributions are and the quasi-biennial wind that is confined to the from remote forcing and local wind forcing. Geostrophic tropical gyre of the North Pacific. Kashino et al. (2005) calculation in the MC/MUC region is possibly of non- examined the variability of the MC using mooring ob- ignorable error, and thus direct observations of the un- servations conducted east of the Mindanao Island and dercurrents in this region are extremely needed. The suggested that the MC was enhanced during the onset of interannual variability of MUC in the direct observa- 2002 El Niño. tions is even nearly unknown yet. With regard to the MUC, direct observations suggest Here, we present the direct observations of the MC that the MUC around 88N is probably below 600 m to and MUC from December 2010 to August 2014 with the depth deeper than 1000 m and features significant emphasis on an interannual time scale. Spatial and intraseasonal variability (Wang et al. 2014; Zhang et al. temporal features of the mean current and interannual 2014; Qiu et al. 2015; Wang et al. 2015). It seems that the variability will be described in sections 2 and 3. Dynamic MUC is a relatively weak northward continuous mean mechanisms about the MC/MUC interannual variability flow with a mean speed of several centimeters per sec- will be explored in sections 4 and 5 by combining dy- ond, possesses a double-core structure [a major inshore namic diagnostic analysis and sensitivity numerical ex- core and a secondary offshore core (e.g., Qiu et al. periments using a 2.5-layer reduced-gravity model. 2015)], and probably is related to the subthermocline Section 6 will discuss the relationships between the eddies. But debate persists as to if it is a transient currents and ENSO and between currents and in- or permanent current (Hu and Cui 1989, 1991; terannual modulation of mesoscale subthermocline Lukas et al. 1991; Wijffels et al. 1995; Wang and Hu eddies. Results will be summarized in section 7. 1998; Qu et al. 1998; Firing et al. 2005; Kashino et al. 2005; Qu et al. 2012; Kashino et al. 2015). This debate 2. Observations and mean structure might be due to the absence of enough observations of the MUC, especially the shortage in the depths of mea- To monitor the boundary currents east of Mindanao surements. For example, Kashino et al. (2013, p. 1) Island, a subsurface mooring has been sequentially

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FIG. 1. Topography (color) around the subsurface mooring (black dot) and mean current during 2011–13 in the upper 410-m layer from the ECCO2 dataset. Major surface currents, including the NEC, NECC, ITF, and the MC, are labeled. deployed and recovered at 78590N, 127830E since De- documented methods (Hu et al. 2013). All the time cember 2010 (Fig. 1), as a part of the observation project series are low-pass filtered with a cutoff period of 3 days of the NPOCE. The mean horizontal structure of upper- to remove the tidal effect. layer (upper 410 m) currents, including the North Figure 2a presents the daily mean y observed by the Equatorial Current (NEC), North Equatorial Counter- moored ADCPs. Northward current in the sub- current (NECC), ITF, and MC, is presented in Fig. 1 thermocline layer was observed during most of the time. using the Estimating the Circulation and Climate of the The maximum instantaneous meridional speeds of the Ocean, Phase II (ECCO2), dataset over 2011–2013. northward and southward currents exceed 48 and 2 According to Qiu et al. (2015), the inshore core of the 140 cm s 1, respectively. To obtain approximate means mean MUC is much stronger than the offshore core, and of the observed currents, we apply the bootstrap method the MC core and the inshore core of the MUC at 88N are to the daily mean meridional velocities with a trial approximately located at the same longitude, that is, number of 300 at depths where the numbers of samples between about 1278 and 127.58E. As shown in Fig. 1, the are greater than 365 (Efron 1979). Figure 2b presents the mooring was located at the principal axis of the major bootstrapped means of daily y and corresponding errors. cores of the MC and MUC. It shows that maximum mean northward and southward 2 The first mooring was deployed in December 2010 speeds are about 4 and 78 cm s 1 and statistically sig- and recovered in July 2011, the second one was nificant relative to the errors, though the observed mean deployed in July 2011 and recovered in December undercurrent is weak as expected. Note that the mean 2012, and the third mooring was deployed in December undercurrent is enhanced with the increase of depth 2012 and recovered in August 2014. Each mooring was below 600 m; it is very likely stronger at deeper layers equipped with an upward-looking and a downward- (e.g., Qiu et al. 2015). The observed mean surface layer looking 75-kHz Teledyne RD Instruments (TRDI) current should be the MC, while the mean subsurface acoustic Doppler current profilers (ADCPs) at about current should be a part of the MUC, as mentioned in 400-m depth. The velocity accuracy is 1% of the current previous studies of the boundary currents in this region 2 magnitude 65mms 1. The raw data are quality con- (Hu and Cui 1989, 1991; Lukas et al. 1991; Kim et al. trolled using percent good quality control, internal 2004; Qu et al. 2012; Wang et al. 2014; Zhang et al. 2014; ADCP quality control, and correlation quality control Wang et al. 2015; Qiu et al. 2015). (Book et al. 2007). Vertical movement of the in- To examine the velocity of the subsurface MUC, we struments wADCP is estimated using the records of the plot in Fig. 2c the core-mean (i.e., vertical average of the pressure sensors mounted in the ADCPs. Result shows MUC speed) northward velocity yMUC, which is 24 21 that wADCP is O(10 )ms , which is much less than smoothed to remove high-frequency variability (mainly the vertical or horizontal velocities of the seawater. including intraseasonal and seasonal signals). Fluctua-

Thus, the errors in the horizontal velocities induced by tion of yMUC in Fig. 2c is induced by both the natural the up–down motion of the buoy are not taken into variability of the MUC and the variation of depths of the consideration in the present paper. Then daily meridi- ADCPs, and the maximum velocity of the MUC is onal and zonal components of current velocity probably larger than the velocity in Fig. 2c. The yMUC y and u are obtained from the quality-controlled hourly has been positive in the entire duration of the mooring records and further processed according to our observation, suggesting that a steady mean current does

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21 FIG. 2. (a) The 3-day low-passed daily mean meridional velocity y (cm s ) measured by the moored ADCPs. (b) Bootstrapped means of y and corresponding errors at depths where the number of samples are greater than 365. Errors are defined as the standard deviations of the bootstrapped means. (c) Time series of smoothed core mean meridional velocity yMUC and (d) distribution of density of bootstrapped means of yMUC. exist below the MC at this place. The bootstrap method MC and MUC) were observed by the moored ADCPs in is applied to the yMUC to calculate its mean with trial most (about 89%) of the duration. The MUC is roughly number of 1000. Figure 2d presents the density of below 600 m with several centimeters per second, as we bootstrapped means of yMUC (i.e., yMUC) and indicates mentioned in Fig. 2, but apparently the mooring has 21 that the yMUC is about 6 cm s . missed the deeper part of the undercurrent. This result In addition, variation sy of the observed currents, implies that measurements at 88N shallower than 600 m defined as the standard deviation of daily y, is further naturally cannot capture the MUC. examined by applying the bootstrap method. The sy shown in Fig. 3a covers time scales from daily to in- 3. Observed interannual variability terannual scales, say synoptic, intraseasonal, seasonal, and interannual variabilities. For the subsurface layer, The interannual variations of boundary currents are 2 the variation is about 12 cm s 1, which is about twice the significant during 2010–2014, though the duration of the yMUC. Of the total variations of y for the MC and MUC ADCP measurements is very limited. The MC is stronger in Fig. 3, about 33% and 14%, respectively, are induced during December 2010–August 2011 and August 2013– by interannual variability (estimated by comparing with August 2014 than October 2011–January 2013 (Fig. 2a). the sy of 1-yr low-pass filtered y). Figure 3b presents the To illustrate the interannual features, we extract the in- low-pass filtered y using a Fourier filter (Walters and terannual series of daily meridional velocity anomalies Heston 1982) with a cutoff period of 365 days to remove relative to the mean velocities over the observation du- the mesoscale to small-scale effects. In spite of the ration by low-pass filtering with a cutoff period of 365 strong variability as in Fig. 3a, the mean flows (i.e., the days. As shown in Fig. 4a, there are three interannual

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21 FIG. 3. (a) Variation sy (cm s ) of the observed currents at depths defined as the boot- strapped standard deviation of daily y. (b) Low-passed ADCP meridional velocity y using a Fourier filter with a cutoff period of 365 days. phases of y0 in the upper 550-m layer (i.e., the MC layer). 2013–2014) negative y0 was dominant. The amplitude of 2 Positive y0 was significant during the second half of 2011 interannual MC exceeds 15 cm s 1 with an approximate 3-yr and 2012 (phase B for simplification), but in other periods period, though it is difficult to assess the statistical signifi- (phase A during the first half-year of 2011 and phase C in cance. Higher-frequency but weaker interannual variability

0 21 0 FIG. 4. (a) Low-passed anomalous meridional velocity y (cm s ) and (b) vertical mean of y in 550–750- and 80–550-m layers. Error bars in the right panel denote the vertical variation of y0 (standard deviations over depths).

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FIG. 5. (a) Hovmöller diagrams of low-passed (a) observed sea level anomaly (m) and 2 (b) wind stress curl anomaly (Pa m 1) averaged over 48–128N relative to the mean fields over 1997–2013. Wind data are extracted from ERDDAP, version 1.62 Fleet Numerical Meteo- rology and Oceanography Center 10-m surface wind. Bottom panels are the same as upper panels [(c) SLA and (d) wind stress curl anomaly] but for the years after 2009. Black rectangle in (a) and (c) highlights the region of relative maximum signals near the coast. exists in the ocean below 550 m (i.e., the MUC layer) in 4. Related dynamics contrast to the interannual variability of MC (Figs. 4a,b). The amplitude of interannual variability of MUC is no Previous studies suggest that the MC and MUC are 2 more than 5 cm s 1 with an approximate period of 1.2 yr. approximately geostrophic flows (e.g., Qu et al. 2012; We averaged y0 vertically over the 80–550-m layer and Wang and Hu 1999). An eastward-directed cross-shelf the 550–750-m layer to present the respective interan- pressure gradient is set up in the upper layer and bal- nual features in the upper- and deeper-layer currents, ances the westward associated with the respectively (Figs. 4b). Over the observation duration, southward-flowing MC, while a westward-directed the weakest MC was observed in June 2012, in contrast cross-shelf pressure gradient is built up in the sub- to the maximum peaks in December 2010 and June thermocline layer and balances the eastward Coriolis 2014. In the deeper layer, the MUC shows speed peaks force associated with the northward-flowing MUC. around December 2010, January 2011, April 2013, and Thus, local wind forcing and Rossby waves forced by July 2014 and valleys in June 2011, August 2012, and remote winds, which are major dynamical processes that November 2013. affect the pressure structure, might play an important

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FIG. 6. Hovmöller diagrams of Argo temperature anomaly (8C) at depth: (a) at 200 dbar, (b) vertical mean in the 2.5–200-dbar layer, and (c) vertical mean in the 550–750-dbar layer. Bottom panels are as in the upper panels but relative to the mean during November 2010– February 2014. role in controlling the interannual variability of both the variability of SLA in the eastern to central Pacific Ocean. MC and MUC. But in the western Pacific Ocean, in particular the far Figure 5a presents the Hovmöller diagram of sea level western Pacific Ocean centered at 1408E, the wave signals anomaly (SLA) averaged over 48–128N. Here, the SLA generally originate in the central to western Pacific Ocean data are provided by the Data Unification and Altim- most of the time. Interannual variability of SLA in the eter Combination System (DUACS) and distributed western Pacific Ocean seems to be composed of local, by Archiving, Validation, and Interpretation of Satel- low-frequency variation and relatively higher-frequency lite Oceanographic Data (AVISO)/Centre National propagating variation from the western-central Pacific d’Etudes Spatiales (Dibarboure et al. 2009). High- Ocean and is much different from that in the eastern frequency variability (periods less than 1 yr) are ex- Pacific Ocean. Power spectra of SLA at 88N shows that cluded by applying a 13-month running mean to the the far western Pacific Ocean (1208–1508E) possesses an monthly SLA. Correspondingly, Fig. 5b presents the interannual variability with a period of about 3 yr and is Hovmöller diagram of wind stress curl anomaly to consistent with the MC feature. The eastern part of the compare with SLA, where the wind stress curl data are western Pacific Ocean (1508E–1808)isanoriginofthe provided by the Environmental Research Division Data Rossby waves (Fig. 5a) and is characterized by interan- Access Program (ERDDAP) data server at the National nual variability with a peak of period between 1 and 2 yr Oceanic and Atmospheric Administration (NOAA) that is in agreement with the observed MUC feature (Smith 1988). The difference of SLA is clear between (figure not shown). Comparison indicates that the SLA the western and central-eastern Pacific Ocean. As variation is in agreement with the fluctuation of wind shown in Fig. 5a, interannual anomalies generated in the stress curl phase to phase; SLAs were positive during July eastern to central Pacific Ocean propagate westward to 1998–July 2001 and July 2007–July 2013, corresponding the central Pacific Ocean and dominate the interannual to negative wind stress curl anomalies, and negative

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FIG. 7. (a) Composite AVISO SLAs (cm) and (b) ERDDAP wind stress curl anomalies 2 (Pa m 1) over the three interannual phases (phase A: March–July of 2011; phase B: July 2011– January 2013; and phase C: January 2013–June 2014) of the observed currents. during July 2001–July 2007 when wind stress curl anoma- frequency than the MUC. This is reasonable because the lies were positive. The strongest wind stress curl anomalies local wind forcing in the far western Pacific Ocean gives are presented in the western Pacific Ocean. The above rise to great upper-layer influence on the signals propa- features imply that the western Pacific Ocean is possibly gating from the eastern part of the western Pacific Ocean very important in the interannual variability of the western (1508E–1808). Disparity of the temperature anomaly in boundary currents like the MC. Fig. 6 and MUC velocity in Fig. 4 also exists, suggesting Rossby waves forced by wind forcing in the western that the interannual variability of the MUC is also mod- Pacific Ocean propagate westward until the Philippine ulated by other processes besides the Rossby waves. Sea, but at different depths the propagation is different. Rossby waves triggered by remote wind forcing get To examine the vertical feature of the wave propaga- enhanced toward the west as shown in Fig. 5a. It should tion, we present in Fig. 6 a Hovmöller diagram of tem- be noted that almost all the signals reach a perature anomalies at depths along 88N using gridded zonal maximum (minimum for negative phases) around Argo observations since 2004 (Roemmich and Gilson 1308E, but in contrast the signal is very small in the re- 2009). For the upper layer (at 200 dbar and 2.5–200-dbar gion between the coast and the maximum (minimum for mean), the fluctuation is strong and the propagation is negative phases). Because the MC and MUC are ap- very clear. But the anomalies in the deeper layer (at proximately geostrophic flows, this feature implies that a 650 dbar) are relatively small, as expected. A notable downwelling Rossby wave (positive SLA) will cancel the characteristic is that whether the upper layer or the eastward pressure gradient, reduce the southward- deeper layer, the Rossby waves are generally generated flowing MC, but enhance the northward-flowing MUC at the region west of the date line except for a few special and vice versa for the response of the MC and MUC to years like 2008 and 2010. The temperature phases over upwelling Rossby waves. Recently, Qiu et al. (2015) the mooring duration are consistent with the interannual suggested that the MUC are related to baroclinic in- variation of the MC: cooling and upwelling during stability of the overlying wind-driven western boundary phases A and C but relative warming during phase B. It currents. Thus, beside the direct influence of Rossby should be noted that the deeper-layer temperature waves, the interannual variability of the MUC might variability is different from the upper layer in the far also be influenced by the Rossby waves through modu- western Pacific Ocean, that is, west of about 1508E: it is lating the baroclinic instability in the MC. of higher frequency but weaker amplitude. This difference To examine the role of local wind forcing, we present might explain why the MC shows stronger and lower the composite maps of AVISO SLAs and ERDDAP

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negative wind stress curl anomaly gives rise to anomalous downwelling in the Mindanao Dome (Masumoto and Yamagata 1991) and anticyclonic current anomaly and thus causes southward current anomaly near the western boundary and interannual intensification of MC (Figs. 5, 6).

5. Sensitivity experiments

To further understand the variability of MC and MUC, we adopt in this section a 2.5-layer (the third layer is assumed to be inert) reduced-gravity model (RGM) built by Qiu et al. (1997). On the basis of this RGM, several sensitivity numerical experiments are conduct- ed. A 2.5-layer RGM is found to be suitable to study the wind-driven circulations and oceanic adjustment by the Rossby waves (e.g., McCreary and Lu 1994; Qiu et al. 1997). Equations that govern the upper two layers in the RGM and basic model sets are the same as those de- scribed by Qiu et al. (1997), except for the following changes. First, the model domain covers a closed region from the tropical South Pacific Ocean to subtropical North Pacific Ocean (208S–408N, 1208E–708W) with a horizontal resolution of 0.25830.258. Second, the mean thicknesses for the upper two layers are chosen ac- cording to the vertical structure of the MC/MUC system, and we require that the RGM can reproduce the upper MC and the subsurface MUC and their interannual FIG. 8. Surface horizontal current anomalies during the three variability. As shown in Figs. 2 and 4, because the MC interannual phases from the monthly OSCAR. Cyclonic and anti- core is mainly shallower than 400-m and interannual cyclonic current anomalies are denoted in red and blue ellipses. variability in the upper 400-m layer is distinct from the deeper layer, the initial thickness of the first layer is wind stress curl anomaly during phases A, B, and C. As chosen to be 400 m. Figure 2 also indicates that the shown in Fig. 7a, SLAs were negative during phases A subsurface current underlying the MC is possibly as and C but positive in phase B, suggesting that anomalous deep as 1000 m; thus, the initial thickness of the second upwelling was significant east of Mindanao during pha- layer is set to be 600 m. Third, the horizontal eddy vis- 2 ses A and C, while anomalous downwelling governed cosity coefficient is 700 m2 s 1 in the whole model do- 2 the current anomaly. To examine the horizontal current main but increases linearly to 1500 m2 s 1 near the pattern during these phases, we extract the surface model boundaries to suppress the exaggerated in- current anomalies from monthly Ocean Surface Cur- stabilities. Finally, the RGM in the present paper is spun rents Analyses–Real Time (OSCAR) datasets. As a up from rest by the climatological wind stress from the result, surface currents show cyclonic current anomalies European Centre for Medium-Range Weather Fore- and southward current anomalies near the western casts (ECMWF) Ocean Reanalysis, system 3 (ORA-S3; boundary during negative phases of the MC, and on the Balmaseda et al. 2008), wind stress data (1992–2011) and contrary, anticyclonic current anomaly and northward ERDDAP wind stress data (2012–2014) for 30 yr to anomaly are distinct during positive phases of MC reach a quasi-steady state. We extract the monthly wind (Fig. 8). The composite wind stress curl anomalies in stress data (1992–2011) from the ECMWF ORA-S3 and three MC phases are consistent with the corresponding ERDDAP (2012–14) and remove the seasonal variation SLAs in the south . Positive wind stress in the wind field to focus on the interannual variability. curl anomalies in phases A and C lead to local Ekman The RGM ocean circulation is then driven by these pumping, cyclonic current anomalies surrounding the monthly wind stress data, namely, FP run. wind stress curl anomalies, and thus northward current At first, model results are validated by observations anomaly close to the western boundary. Meanwhile, from satellites and mooring ADCPs (Figs. 9–11). The

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FIG. 9. Comparison between the first-layer thickness in the RGM (blue) and the AVISO SLA (red) at (a) the mooring point of 88N, 1278E in the MC region and (b) 138N, 1378E in the NEC region.

0 8 8 anomalous thickness of the first-layer H1 is compared at 13 N, 137 E, indicating that the RGM outputs are with the observational SLAs from AVISO at two points: consistent with the satellite observations. 88N, 1278E in the MC/MUC region and 138N, 1378Ein The RGM current is also compared with the observa- the NEC region (Fig. 9). Correlation coefficients be- tions from the ADCPs of the moorings at the 88N, 1278E. 0 8 8 tween SLAs and H1 in Fig. 9 are 0.9 at 8 N, 127 E and 0.9 Figure 10 presents the anomalous meridional current

0 FIG. 10. Comparison of the anomalous meridional current velocities y at the 88N, 1278E between RGM and observations from the ADCPs of the moorings at (a) RGM layer 1 (80– 550 m for ADCP observations) and (b) layer 2 (550–750 m for ADCP observations). (c) The power spectra of the RGM transport anomalies of (left) the MC and (right) the MUC during 1992–2014. Dashed black lines in (c) indicate the 95% confidence level.

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FIG. 11. Comparison of Hovmöller diagrams between (a) AVISO SLA and (b) the first-layer 0 8 8 thickness in the RGM, that is, H1, averaged over 4 and 12 N. Black boxes roughly denote the longitudes of maximum anomalies around the offshore part of the MC/MUC. velocities in the RGM layers 1 and 2 and ADCP obser- presents significant westward propagation and is in good vations averaged over 80–550 and 550–750 m. Correlation agreement with the observed SLA phase by phase. coefficients between RGM and ADCP observations are Based on Radon transform calculation (Chelton and 0 0.8 in the upper layer and 0.5 in the second layer and are Schlax 1996), the phase speeds of H1 and SLA in Fig. 11 2 both statistically significant at the 98% confidence level. are about 240 cm s 1, which is close to the baroclinic Transports of the MC and MUC are calculated along Rossby waves at the latitude of about 88N and in good 88N as below: agreement with the results by Chelton and Schlax (1996). ð Both the SLA and H0 are increased with the westward 1308E 1 52 y propagation of the Rossby waves and reach a peak in the TMC ( 1H1) dx, and (1) 1268E eastern part of the MC/MUC (roughly 1278–1308E), but ð 1308E they are very small near the Mindanao coast. As we dis- 52 y TMUC ( 2H2) dx, (2) cussed above, this characteristic gives rise to significant 1268E influence of Rossby waves on the MC/MUC by modu- where y1 and y2 are meridional velocities in the first and lating the zonal pressure gradient in the two currents. second RGM layers, and H1 and H2 are the thicknesses Figure 12 presents the temporal mean field of the of the first and second RGM layers. RGM currents in the first and second layers and com- Power spectra of the MC and MUC transport anom- pares with satellite observed mean surface geostrophic alies are calculated using the monthly RGM outputs current from the AVISO dataset averaged over 1993– during 1992–2014. As shown in Fig. 10c, both the MC 2013. The RGM layer 1 shows a similar horizontal cur- and MUC have significant seasonal variability. On the rents pattern to the AVISO surface geostrophic current. interannual time scales, the spectral peaks of the MC Disparity between the RGM layer 1 and AVISO surface transport are located at 1–2 and 2–3 yr, and the latter currents might be due to the difference of depths of the near the 3-yr period is much stronger. In contrast, the two datasets and the ideal topography of the RGM. significant interannual period of the MUC transport is Both the MC and MUC are well reproduced by the about 1–2 yr and less than the MC periods. The spectral RGM in terms of spatial structure, path, and amplitude. feature in Fig. 10c is consistent with the moored ADCP In the second layer, although horizontal currents possess observations as described in section 3. strong mesoscale eddy activities, undercurrents, the The RGM outputs are further validated in terms of MUC for instance, are also very distinct from eddies. wave propagation by comparing the Hovmöller diagram The RGM MUC originates at about 5.58N and flows 0 8 of the first-layer thickness anomaly of the RGM H1 with northward until 14 N. The RGM currents are consistent that of AVISO SLA along the zonal band between 48 with the geostrophic currents derived from 14-yr Argo 8 0 and 12 N(Fig. 11). Results show that the simulated H1 float profiling data presented by Qiu et al. (2015).In

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FIG. 12. (a) Satellite-observed mean surface geostrophic current from the AVISO dataset averaged over 1993–2013 and mean hori- zontal currents in (b) layer 1 and (c) layer 2 of the RGM. Vectors of currents are superposed on the shaded color that indicates the 2 meridional components of mean velocities (cm s 1). Red and blue squares denote the mooring site. particular, Qiu et al. (2015) pointed out that the MUC relative to EP and CP for both the MC and MUC, and in was observed from 68 to 138N. the western Pacific, EWP is more important than the Sensitivity experiments include five parts: the east- FWP region. Table 1 shows some statistics in terms of ern Pacific (EP) run, the central Pacific (CP) run, the the MC and MUC transports in Fig. 13. Correlation western Pacific (WP) run, the eastern part of the coefficients between the FP and WP are 0.8 for the MC western Pacific (EWP) run, and the far western Pacific and 0.7 for the MUC. But for the MC, the FP–FWP (FWP) run. All the experiments utilize a unified model correlation coefficient (0.6) is also significant. To quan- set as introduced above but forced by different wind tify the contributions to the total MC/MUC transport

fields. In the EP run, interannual variability of wind variability, we define the variations Si in the i run as stress is removed except the eastern Pacific Ocean 5 3 (1308–708W).Similarly,fortheCP,WP,EWP,and Si CORRi STDi, (3) FWP runs, interannual variability is permitted only in 5 the central Pacific (1808–1308W), the western Pacific where i FWP, EWP, CP, and EP; CORRi is correla- (1208E–1808), the eastern part of western Pacific tion coefficient between the FP run and the i run; and (1508E–1808), and the far western Pacific (1208–1508E) STDi is the standard deviation of the i run. The relative Ocean, respectively. contribution is then calculated as below: MC and MUC transports in the control run FP are S [ 6 3 21 P 5 i 3 100%. (4) about 27 Sverdrups (Sv; 1 Sv 10 m s ) and 5 Sv and i å Si are consistent with previous studies (e.g., Hu and Cui i5FWP,EWP,CP,EP 1991; Lukas et al. 1991; Wang and Hu 1998; Qu et al. 2012). For example, Qu et al. (2012) reported that the MC and MUC are 23.9 and 3.8 Sv. Transport anomalies As shown in Fig. 14, the WP wind (FWP plus EWP) 0 0 6 of the MC (TMC) and MUC (TMUC) in various sensitivity accounts for (72% 3%) of the MUC variation and experiments are presented in the Fig. 13. Among the (54% 6 2%) of the MC variation, suggesting that the three parts of Pacific regions, WP contributes the most interannual variability of both the MC and MUC are

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FIG. 13. Anomalous volume transports (Sv) of the (a) MC and (b) MUC in various numerical experiments including FP, CP, EP, WP, FWP, and EWP runs. largely controlled by the WP wind forcing. Meanwhile, well as their lead–lag correlation. Result indicates that the EWP wind forcing contributes (29% 6 1%) to the the SLA varies out of phase with the Niño-3.4 index, MC variability and (47% 6 2%) to the MUC variability, and the latter leads the former by about 1 month, supporting the results discussed above: Rossby waves suggesting a fast response of the SLA in the MC region generate at the EWP, propagate westward, and contrib- to the Niño-3.4 index. ute to the interannual variability of the MC and MUC in During the mooring observations, two La Niña events the FWP. Considering that the MUC are related to the occurred during December 2010–April 2011 and August baroclinic instability of the overlying wind-driven west- 2011–March 2012, respectively. Here, the observed ve- ern boundary currents (Qiu et al. 2015), it is possible that locity anomalies of the MC and MUC in Fig. 4b are the baroclinic Rossby waves induced by the EWP wind normalized and compared with the Niño-3.4 index forcing also influence the MUC through modulating the (Fig. 16). During the two La Niña events, the MUC and baroclinic instability in the MC. But this hypothesis needs MC showed no abnormal variations. The MUC velocity to be confirmed by more subthermocline observations. and Niño-3.4 index shows a correlation with a correla- For the upper-layer MC, the FWP wind is also of signif- tion coefficient of 20.33, but no significant relation is icance, pointing to the importance of local Ekman found between the MC velocity and Niño-3.4 index. This pumping in the FWP as discussed in section 4. is consistent with the result by Lukas (1988) and also

6. Discussion TABLE 1. Statistics of the MC and MUC transports (Sv) in the Interannual variability of ocean circulations in the sensitivity experiments. CORR is the correlation coefficients western Pacific Ocean are commonly related to the between sensitivity experiments and control run FP. Mean and std ENSO cycle (e.g., Qiu and Lukas 1996; Kashino et al. dev are mean transports and corresponding standard deviations. 2009; Kashino et al. 2011; Hu and Hu 2014; Hu et al. FP WP FWP EWP CP EP 2015). During El Niño development, the MC is sug- MC CORR 1 0.8 0.6 0.8 0.5 0.6 gested to be enhanced, transport more cool water Mean 27 26 25 25 27 27 equatorward, and contribute to the discharge of the Std dev 3.5 2.3 1.6 1.4 2.0 1.2 ENSO cycle (Kashino et al. 2009; Hu et al. 2015). MUC CORR 1 0.7 0.4 0.7 0.1 0.3 Figure 15 depicts the Niño-3.4 index and the SLA av- Mean 5 3 2 3 3 4 eraged over 1268–1308E along 88N in the MC region as Std dev 1.8 1.2 1.1 0.9 1.0 0.8

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FIG. 14. Percentages of contributions Pi of wind forcings in different regions (FWP, EWP, CP, and EP) to the interannual variability of the (top) MC and (bottom) MUC. probably because the ENSO signal during the present energy (EKE) averaged over 58–108N and 1268–1308Eat mooring observation is not strong enough. 605 m. EKE is defined as Previous studies also suggest that subthermocline eddies exist in the western Pacific Ocean below the up- EKE 5 u02 1 y02 , (5) per layer and influence the subsurface currents (e.g., Qu et al. 2012; Chiang and Qu 2013). Figure 17 presents where u0 and y0 are 3-day zonal and meridional velocity y0 meridional velocity anomaly 88N and eddy kinetic anomalies relative to the temporal mean over 1980–2011

FIG. 15. (a) Monthly Niño-3.4 index and SLA averaged over 1268–1308E along 88N in the MC region, and (b) their lead–lag correlation. Both the Niño-3.4 index and SLA are 13-month running means.

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FIG. 16. Comparison between the Niño-3.4 index and the velocity anomalies averaged over 80–550 (MC) and 550–750 m (MUC) as in Fig. 4b. The velocity anomalies are normalized by their standard deviations. from the Ocean General Circulation Model for the such a time scale probably also exists but remains to be Earth Simulator (OFES) output, which is eddy re- addressed. solving with a horizontal resolution of 0.1830.18 In addition, two serious facts should be noted: First, (Sasaki et al. 2008). It seems that the subthermocline the depth of MUC is roughly about 600 m to deeper EKE possesses significant interannual fluctuations than 800 m (e.g., Qiu et al. 2015). This implies that y0 and a close relationship with 88N (Fig. 17), and sta- observations shallower than 600 m naturally miss the tistically the latter leads the EKE by about 3 months. MUC core. Deductions of the MUC based on this kind This indicates that the interannual variability of the of observations are probably misleading to readers background mean currents probably play an impor- and to a certain extent lead to the dispute on the ex- tant role in the interannual variation of the sub- istence of the MUC. Second, the MUC is referred to thermocline eddies. But the interaction between the be a mean flow and an undercurrent. A mean flow like mean circulation and subthermocline eddies needs MUC typically has a very different time scale from further studies. As suggested by Qiu et al. (2015),the mesoscale to small-scale processes, as suggested by broad-scale subthermocline boundary flows (including Qiu et al. (2015). the MUC) are related to the baroclinic instability of the overlying wind-driven western boundary currents. 7. Conclusions Kashino et al. (2011) pointed out that the interannual variability of temperature and heat content at 88N (1308 On the basis of nearly 4-yr ADCP measurements from and 1378E) are different from that at 58N, 1378E. For the subsurface moorings in the south Philippine Sea, we MC and MUC, the latitude dependence of variability on investigate the interannual variability of the observed

FIG. 17. (a) OFES EKE and meridional velocity anomalies averaged over 58–108N and 1268– 1308E, and the (b) lead–lag correlation between them. Both the time series are 1-yr running averaged. Positive time lag indicates EKE lags the meridional velocity anomalies.

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MC and MUC during December 2010–August 2014. html). The OFES outputs are provided by the JAMSTEC Both the MC and MUC are characterized by significant (http://www.jamstec.go.jp/esc/research/AtmOcn/product/ interannual variability but different frequencies and ofes.html). The mooring data are available at the NPOCE amplitudes. By combining diagnostic analysis and nu- website (http://npoce.qdio.ac.cn/moored).This work was merical sensitivity experiments, we conclude that the supported by the National Natural Science Founda- wind forcing over the western Pacific Ocean acts as a tion of China (Grants 41406016 and 41330963), the driving agent in conditioning the interannual variability NSFC-Shandong Joint Fund for Marine Science Re- of MC and MUC. Westward-propagating Rossby waves search Centers (Grant U1406401), the CAS Strategic Pri- generated by the EWP wind forcing might play an es- ority Research Program (Grant XDA11010101), and the sential role in the interannual variability of the MC/ National Key Basic Research Program of China (Program MUC by modulating the zonal pressure gradient in the 973, Grant 2013CB956202). two boundary currents and changing the baroclinic in- stability in the MC. For the upper-layer MC, the fluc- REFERENCES tuation of local wind stress curl over the FWP in the Balmaseda, M. A., A. Vidard, and D. L. T. Anderson, 2008: The south Philippine Sea is also of much importance. Re- ECMWF ocean analysis system: ORA-S3. Mon. Wea. Rev., lationships between the MC/MUC and ENSO cycle and 136, 3018–3034, doi:10.1175/2008MWR2433.1. between the MC/MUC and subthermocline eddy activ- Book, J. W., H. Perkins, R. P. Signell, and M. Wimbush, 2007: The ities are also discussed. 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