PUBLICATIONS. Journal of Geophysical Research: Oceans

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1 PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE Key Points: Interannual variability of the eastward current in the western SCS is robust Wind is responsible for the migration of the eastward current The eastward current has profound effects on the summer SST and Chl a Correspondence to: G. Wang, wghocean@yahoo.com Citation: Chen, C., and G. Wang (2014), Interannual variability of the eastward current in the western South China Sea associated with the summer Asian monsoon, J. Geophys. Res. Oceans, 119, , doi: / 2014JC Received 11 JUL 2014 Accepted 19 AUG 2014 Accepted article online 23 AUG 2014 Published online 3 SEP 2014 Interannual variability of the eastward current in the western South China Sea associated with the summer Asian monsoon Changlin Chen 1 and Guihua Wang 1 1 State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, China Abstract The interannual variability of the eastward current in the western South China Sea (SCS) during the summer of is examined with satellite altimeter data and Regional Ocean Modeling System (ROMS) model output. It is found that the meridional location of the eastward current displays apparent interannual variability. The core of the eastward current shifts between 10.7 N and 17.6 N with a standard deviation of 1.6. Results from Sverdrup theory and ROMS experiments demonstrate a close dynamic linkage between the north-south migration of the eastward current and the SCS summer monsoon anomaly on the interannual time scale. When the summer monsoon has southwesterly (northeasterly) anomaly, the eastward current moves southward (northward). With the southward (northward) shift of the eastward current, the summer cold filament in the SCS moves southward (northward) as well. 1. Introduction The South China Sea (SCS) is a semienclosed marginal sea in the tropical western Pacific. The circulation in the SCS upper layer is driven mainly by the Asian monsoon. Forced by the summer southwesterly, the circulation in the SCS basin has a cyclonic gyre north of about 12 N and an anticyclonic gyre south of 12 N. Thus, there is a robust eastward current veering off central Vietnam between these two gyres [Liu et al., 2001; also see Figure 1]. The eastward jet can extend to hundreds of meters downward in the water column [Fang et al., 2002; Chen et al., 2010; Hu et al., 2011]. Since the eastward current was identified by Xu et al. [1982], it has been seen in satellite sea surface temperature (SST), sea surface height, and chlorophyll data [Kuo et al., 2000; Liu et al., 2002; Xie et al., 2003]. With the development of the summer monsoon, the eastward jet starts to appear in May or June, peaks in August or September, and disappears in October [Metzger and Hurlburt, 1996; Wang et al., 2006]. Recent studies showed that the eastward current has a strong interdecadal variability: it moved southward, northward, and again southward during the periods of , , and , respectively [Wang et al., 2010]. They concluded that the interdecadal variability of the eastward current is caused by the change of wind jet off central Vietnam and wind strength in summer. The upper layer circulation in the SCS has a strong interannual variability. The interannual variability in the Pacific Ocean, such as El Ni~no and La Ni~na events, can influence the SCS circulation through atmosphere and ocean processes [Chao et al., 1996; Xie et al., 2003; Qu et al., 2004; Fang et al., 2006]. The variability in the Indian Ocean, such as its basin mode and dipole mode, can also affect the SCS circulation through changing monsoon [Yang et al., 2010; Liu et al., 2012]. As we can see from the time series of the shift of the eastward jet [Wang et al., 2010], its interannual variability is also robust. The shift of the eastward current on the interannual time scale can also be seen in the satellite Chl a concentration data from the SeaWiFS [Tang et al., 2004]. Figure 2 also demonstrates that the eastward current is much more to the north in 1998 than in 2002 based on the Chl a concentration data. Although the monthly evolution and interdecadal variability of the eastward current were discussed [Wang et al., 2006, 2010; Gan and Qu, 2008], the interannual variability of the eastward current has not been analyzed and its dynamics is yet to be studied. This paper will investigate the interannual variability of the eastward current in the SCS using satellite altimeter data and Regional Ocean Modeling System (ROMS) model simulation and discuss the roles of East Asian monsoon through the Sverdrup theory and ROMS simulation results. CHEN AND WANG VC American Geophysical Union. All Rights Reserved. 5745

2 Figure 1. Climatological summer-mean isotherm depth (m) of 20 C (shading) from the Generalized Digital Environment Model-Version 3.0 [Carnes, 2009], and climatological summer-mean wind (vector, m/s) from cross-calibrated multiplatform data. 2. Data The sea level anomaly (SLA) data are from the French Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO) project [Ducet et al., 2000]. The product merges the measurements of TOPEX/Poseidon, European Remote Sensing Satellite (ERS-1/2), Geosat Follow-on, Jason- 1/2, and Envisat. The spatial resolution is 1/3 latitude by 1/3 longitude, and the temporal resolution is weekly. The weekly data set from 1993 to 2012 is averaged to produce monthly mean data for the present work. The data over the shelf shallower than 200 m may incorporate significant aliases from tides and internal waves and are therefore masked out. Monthly mean surface winds used are the 0.25 resolution product from the cross-calibrated multiplatform (CCMP) [Atlas et al., 2011], derived through cross-calibration and merging of ocean surface wind observations using a variational analysis method. The time period of the CCMP data used in this study is from 1993 to The monthly Advanced SCATterometer (ASCAT) wind data at 0.25 resolution [Bentamy and Fillon, 2012] in 2012 are used to extend the CCMP wind data to match the length of SLA data. Wind stress is calculated using the modified Large and Pond neutral stability drag coefficient [see the appendix of Large et al., 1994], which is most commonly Figure 2. July August composite SeaWiFS Chl a concentration (mg/m 3 ) in (a) 1998 and (b) The white area indicates missing data. The data have 9 km resolution and are provided by the Distributed Active Archive Center of NASA/Goddard Space Flight Center. CHEN AND WANG VC American Geophysical Union. All Rights Reserved. 5746

3 Figure 3. Summer-mean SLA (cm) from satellite altimeter. Black lines are for the location of the eastward current represented by zero vorticity. In the figures for 1995 and 1997, black dots indicate the hydrographic observation stations. applied to scatterometer data. The SST used in this study is the National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation SST (OISST), Version 2 [Reynolds et al., 2002]. Temperature profiles from World Ocean Database 2013 (WOD13) in August 1995 and 1997 are used. A total of nine temperature profiles on 10 August 1995 and seven temperature profiles during 3 23 August 1997 in the study area is retrieved. The locations of these stations are shown in Figure 3. Because the dipole evolution lags behind the basin-scale wind by about 40 days [Wang et al., 2006], summer mean in this study is defined as the months from June to August for wind and from July to September for SLA and SST. 3. Model 3.1. Sverdrup Model The Sverdrup theory describes the primary balance for the large-scale wind-driven ocean circulation [Sverdrup, 1947]. Many studies have applied this theory to understand the SCS circulation [Metzger and CHEN AND WANG VC American Geophysical Union. All Rights Reserved. 5747

4 Figure 4. (a) Time series of the eastward current latitude (Y E ) from satellite altimeter observation. Gray line denotes the mean location of Y E (12.3 N). (b) Same as Figure 4a, except for numerical model result, with its mean location of 11.6 N. (c) The normalized principal component (PC1) corresponding to the leading EOF mode of summer wind stress. Hurlburt, 1996; Shaw et al., 1999; Liu et al., 2001; Wang et al., 2010]. The wind-driven transport stream function for flow integrated in the entire water column can be estimated by the Sverdrup theory as wðxþ52 1 qb ð xe x curlðsþdx; where q is the density of the water, b is the northward gradient of the planetary vorticity, s is the wind stress, and x E represents the ocean eastern boundary. The boundary condition is w50 atx E ROMS The three-dimensional ROMS belongs to the family of terrain-following vertical coordinate models [Shchepetkin and McWilliams, 2005]. The model domain extends from 2 Sto28 N and from 99 E to 142 E, and the horizontal resolution is 1/12. There are 30 levels in the vertical direction. For the open boundaries, radiation and no-gradient conditions are used for velocities and surface elevation, respectively. A nonslip condition is applied to the closed boundaries. Sea surface salinity is restored to the Generalized Digital Environment Model-Version 3.0 [Carnes, 2009] monthly climatology at a time scale of 60 days. More detail about the physical model configuration can be found in Wang et al. [2013]. To reach the quasi-equilibrium state, the ROMS is first integrated for 20 years using heat fluxes from the climatological monthly Objectively Analyzed air-sea Fluxes (OAFlux) [Yu and Weller, 2007] and CCMP winds (the control run). Then, the model is forced by the CCMP monthly winds from January 1993 to December 2012 and the climatological monthly OAFlux heat fluxes. 4. Results 4.1. Observation of the Eastward Current Shift on Interannual Time Scale To present the interannual variability of the eastward current, Figure 3 shows the SLA map off central Vietnam in the summer from 1993 to Generally, the eastward current and corresponding dipole structure (cyclonic and anticyclonic eddies north and south of the eastward current, respectively) are robust. These features also vary from year to year, suggesting an obvious interannual variability. To quantify the interannual variability of the eastward current, we use the zero vorticity contour of the dipole to present its core location. The vorticity (f) is calculated @y u v 0 5 ; where g is the gravitational acceleration, f is the Coriolis parameter, and u 0 and v 0 are the geostrophic velocity anomaly for zonal and meridional components, respectively. The averaged latitude of the zero vorticity CHEN AND WANG VC American Geophysical Union. All Rights Reserved. 5748

5 Figure 5. Composite summer-mean SLA (white contours; cm) and SST (shading; C) maps in (a1) climatology, (a2) southward shift type, and (a3) northward shift type. The years of southward (northward) shift type are chosen when Y E in Figure 4a was among the five largest (smallest); year 2010 is excluded. White vectors indicate the eastward current. line between 110 E and 113 E is defined as the core location of the eastward current. The longitudinal range of 110 E 113 E is chosen because the eastward current can reach 113 E in the climatology [Xie et al., 2003]. Figure 4a displays the time series of the eastward current latitude (Y E ) for the period The 20 year mean Y E is located at 12.3 N, with a standard deviation of 1.6. The time series Y E demonstrate the robust interannual signal. It shifted to the north in some years (e.g., 1995, 1998, 2006, and 2010) and shifted to the south in other years (e.g., 1997, 2002, and 2012). The eastward current went to its northernmost location (17.6 N) in 2010 and to its southernmost location (10.7 N) in In order to distinguish the northward shift and southward shift of the eastward current, we make two composites based on Y E. For the northward shift type, the composited years include 1995, 1998, 2006, 2007, and 2008, when Y E was among the five largest [the year of 2010 is excluded because the eastward current shifted to a very north location, and an exceptionally strong anticyclonic eddy appeared between 12 N and 18 N; Chu et al., 2014]. For the southward shift type, the composite is based on the years of 1994, 1997, Figure 6. Vertical temperature distributions ( C; shading) derived from the World Ocean Database 2013, and geostrophic zonal velocity relative to 500 m (m/s; black contours, positive for eastward) in August (a) 1995 and (b) 1997 along the section marked in Figure 3. CHEN AND WANG VC American Geophysical Union. All Rights Reserved. 5749

6 Figure 7. Same as Figure 3, except for ROMS model results. 2002, 2004, and 2012, when Y E was among the five smallest. Figure 5 shows the climatology of summermean SLA and the composited SLA maps for the two different types. The locations of the eastward current and corresponding dipole structures are very different. In the climatology, the eastward current is located between 10 N and 12 N (Figure 5a). For the southward (northward) shift type, the eastward current moves about 1 southward (2 3 northward) than in the climatology (Figures 5b and 5c). Note the cyclonic eddy is weaker for the northward shift type. The hydrographic data are then used to investigate the interannual variability of the eastward current in the vertical direction. Figure 6 shows the vertical temperature distributions in the Augusts of 1995 and The geostrophic zonal velocities relative to 500 m are calculated using the thermal wind relation (Figure 6). The eastward current can be seen clearly in both years, which extended at least to 300 m and was strongest in the upper 100 m. The differences of the eastward current between 1995 and 1997 are obvious. The eastward current was more northward in 1995 than in 1997: the eastward current in 1995 was around 15 N while it was located around 10.5 N in Robust south-north shift between different years confirms the interannual variability of the eastward current. CHEN AND WANG VC American Geophysical Union. All Rights Reserved. 5750

7 4.2. Dynamics of the Shift of the Eastward Current To discuss the dynamics of the shift of the eastward current, the ROMS experiments are conducted for the SCS basin. Forcing with the monthly winds, the eastward current and dipole structure in summer are apparent from the simulated SLA maps (Figure 7). Generally, the eastward current and dipole in the model vary from year to year, suggesting prominent interannual variability. Figure 8. First EOF pattern of summer wind stress anomalies (vector) and its curl (shading) based on the CCMP data during We calculate the model Y E (Figure 4b), as we did with the satellite altimeter data. The mean location of Y E is 11.6 N in the model, a little southward than in the observation. The time series of the model output demonstrate strong interannual variability of the shift of the eastward current. The correlation between simulated and observed Y E reaches 0.80 at the 95% confidence level. The good correlation indicates that our model can generally capture the observed interannual variability of the eastward current. There are Figure 9. (top) Summer-mean wind stress (vector; N/m 2 ) and its curl (shading; N/m 2 /10 4 km) for (a1) climatology; (a2) climatology 1 southwesterly anomaly, and (a3) climatology 1 northeasterly anomaly. (bottom) Sverdrup stream functions (Sv) for the corresponding top figures. Green (black) lines are for the location of the eastward current represented by zero stream function for climatology (each component). CHEN AND WANG VC American Geophysical Union. All Rights Reserved. 5751

8 Figure 10. SLA (cm) from sensitive experiments by ROMS forced by summer-mean wind of (a) climatology, (b) climatology 1 southwesterly anomaly, (c) climatology 1 northeasterly anomaly, and (d) Gray (black) lines are for the location of the eastward current for climatology (each experiment). some slight differences between model and observation, e.g., the simulated shift of Y E in 1998 is more north than the observation. These differences may be attributed to model performance but warrant future study. Because the wind is the only forcing on the interannual time scale in our simulation, the good relationship between model and observation also indicates that the interannual variability of the eastward current is mainly wind driven. To understand the mechanism of the shift, we carry out the empirical orthogonal function (EOF) analysis of the wind stress. Figure 8 gives the leading EOF (EOF1) of summer wind stress anomalies in the SCS during This mode accounts for 73.6% of the total variance. The spatial pattern shows the summer wind stress has southwesterly (northeasterly) anomaly when its principal component (PC1) is positive (negative). Figure 4c is the normalized PC1 corresponding to the EOF1 mode. The PC1 also demonstrates robust interannual variability. The correlation between PC1 and observed Y E (between PC1 and model Y E )is20.71 (20.86), at the 95% confidence level. This indicates the meridional shift of the eastward current is strongly linked to the summer monsoon anomaly. When the summer monsoon has the southwesterly (northeasterly) anomaly as shown in Figure 8, it leads to a strengthened (weakened) wind stress curl compared to the climatology (Figures 9a1 9a3). Figures 9b1 9b3 demonstrate the summer-mean wind-driven Sverdrup ocean circulation. When the summer wind has southwesterly anomaly, the eastward current migrates about further southward (Figure 9b2) than that in climatology (Figure 9b1). When the summer wind has northeasterly anomaly, the eastward current is about 1 3 further northward (Figure 9b3) than that in climatology. This Sverdrup calculation demonstrates that the northsouth location of the eastward current is associated with large-scale circulation driven by summer monsoon. To further verify the impact of summer monsoon anomaly on the south-north shift of the eastward current, a set of sensitive experiments are conducted using the ROMS. The model initial conditions are from 31 May of the control run output, and then steadily forced with the following three sets of summer-mean wind: (a) climatology, (b) climatology plus southwesterly anomaly, and (c) climatology plus northeasterly anomaly (Figures 9a1 9a3). Note that the pattern and magnitude of the southwesterly wind anomaly is shown in Figure 8, and the northeasterly wind anomaly would have the opposite pattern. After the model reaches its quasi-equilibrium state, the SLA field in the last 3 months is analyzed. Figures 10a 10c show the SLA maps of these three experiments. The eastward current is located near 11 N under the climatology summer wind forcing (Figure 10a). The eastward current shifts slightly further southward than in the climatology when the wind has the southwesterly anomaly (Figure 10b), while the eastward current shifts 2 3 further northward than in the climatology when the wind has the northeasterly anomaly (Figure 10c). The south-north shift of the eastward current is very similar to that seen from the satellite observation composites (Figure 5) and the Sverdrup model results (Figures 9b1 9b3). In 2010, the eastward current shifted to the northernmost location (Figures 3 and 4a), because of an exceptionally strong anticyclonic eddy between 12 N and 18 N[Chu et al., 2014]. Here we conduct a sensitive experiment of the ROMS steadily forced by the summer-mean wind of Similar to the satellite altimeter observation, the eastward current shifts to the very north (around 17 N), and a strong anticyclonic eddy CHEN AND WANG VC American Geophysical Union. All Rights Reserved. 5752

9 appears north of 12 N (Figure 10d). These sensitive experiments confirm the contribution of SCS summer monsoon anomaly on the north-south location of the eastward current. Acknowledgments The SLA altimetry data are distributed by AVISO. CCMP wind data are provided by the NASA Goddard Space Flight Center. OISST data are provided by NOAA/National Climatic Data Center. Temperature profile data are from World Ocean Database 2013 provided by NOAA/National Oceanographic Data Center. This study was supported by the National Science Foundation of China (NSFC) for Distinguished Young Scholars ( ), the National Basic Research Program of China (2013CB430301), and the NSFC ( ). We wish to thank Zuojun Yu for improving the presentation of the paper. 5. Summary and Discussion The interannual variability of the eastward current in the western SCS during summer is identified with satellite altimeter data for the period of It is found that the meridional location of the eastward current displays apparent interannual variability. Sverdrup theory and ROMS experiments show that the interannual variability of the eastward current is strongly associated with the summer monsoon wind, which has apparent interannual signals. When the summer monsoon has southwesterly (northeasterly) anomaly, the eastward current moves further south (north). Figure 11. Same as Figures 4a and 4b, except for the location of summer cold filament (Y C ) in (a) OISST and (b) ROMS model output. In Figure 11a, the Y C in 2010 is missing because the cold filament was absent. As shown in Xie et al. [2003, 2007], the eastward current plays an important role in supplying cold water off central Vietnam. It is useful to see whether the eastward current can contribute to SST interannual variability in the SCS. Figure 5a shows the climatology of summer-mean SST, and Figures 5b and 5c are the composited SST maps for the southward and northward shifts, respectively. The center of the cold water displaces more northward (southward) when the eastward current moves northward (southward). To quantify the interannual variability of the displacement of the cold water, the averaged latitude of the center of cold water between 110 E and 113 E is used to define the center location of the cold water (Y C ). Both observed and simulated Y C contain strong interannual signals (Figure 11), and both have good correlations with the Y E. The correlation coefficients can reach 0.80 and 0.92 for observation and model results, respectively, and all of them are significant at the 95% confidence level. 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