Meridional overturning circulation in the South ... - Wiley Online Library

PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE 10.1002/2013JC009583

Special Section: Pacific-Asian Marginal Seas Key Points: Depict the structure of SCS MOC How inflows from different depths/ straits contribute to the SCS MOC Horizontal distribution of upwelling and sinking areas and possible mechanism

Meridional overturning circulation in the South China Sea envisioned from the high-resolution global reanalysis data GLBa0.08 Yeqiang Shu1,2, Huijie Xue1,2, Dongxiao Wang1, Fei Chai2, Qiang Xie1,3, Jinglong Yao1, and Jingen Xiao1 1

State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, People’s Republic of China, 2School of Marine Sciences, University of Maine, Orono, Maine, USA, 3Sanya Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences, Sanya, People’s Republic of China

Abstract The pattern of meridional overturning circulation (MOC) in the South China Sea (SCS) is studied Correspondence to: H. Xue, [email protected] Citation: Shu, Y., H. Xue, D. Wang, F. Chai, Q. Xie, J. Yao, and J. Xiao (2014), Meridional overturning circulation in the South China Sea envisioned from the high-resolution global reanalysis data GLBa0.08, J. Geophys. Res. Oceans, 119, 3012–3028, doi:10.1002/ 2013JC009583. Received 6 NOV 2013 Accepted 29 APR 2014 Accepted article online 3 MAY 2014 Published online 22 MAY 2014

using a numerical Lagrangian tracing method with the HYCOM1NCODA Global 1/12 Analysis (GLBa0.08) data. The SCS MOC has a ‘‘sandwich’’ structure, which consists of a layer of stronger clockwise circulation above 500 m depth, a counterclockwise layer in the mid layer between 500 and 1000 m depth, and a weaker clockwise layer below 1000 m. The deep (below 1000 m depth) clockwise layer is divided into three cells, namely, the deep southern MOC cell, DSMOC; the deep middle MOC cell, DMMOC; and the unclosed deep northern MOC cell, DNMOC. The inflow through the Luzon Strait is the main source for the SCS MOCs. The upper layer Luzon Strait inflow dominates the upper SCS MOC structure but has relatively less contribution to the DNMOC, whereas the deep layer Luzon Strait inflow mainly influences the DNMOC and it mostly rises near 18 N. The inflow through the Taiwan Strait mainly contributes to the upper layer MOC. Moreover, inflows from the Mindoro and Karimata straits contribute negatively to the upper MOC but play a significant role on the DSMOC. The backward integration of Lagrangian trajectories further validates that the SCS deep water comes not only from the deep inflow but also from the entrainment of the middle and upper layer inflow through the Luzon Strait. In the SCS basin, there are three northwest-southeast tilted zones where tracers upwell, which correspond to the three deep MOC cells. One possible mechanism for these upwelling zones is the interaction between the continental slope-trapped waves and the westward planetary Rossby waves.

1. Introduction The South China Sea (SCS) is the largest tropical marginal sea that covers a region from the equator to 23 N and from 99 N to 121 E with an average depth over 2000 m (Figure 1). It connects to the East China Sea through the Taiwan Strait, to the Pacific Ocean through the Luzon Strait, to the Sulu Sea through the Mindoro Strait, and to the Java Sea through the Karimata Strait. The Luzon Strait is the only deep channel connecting the SCS and its surrounding oceans with the sill depth of about 2400 m. Hence, the SCS is a completely isolated basin below the 2400 m depth. In the interior regions, there are four groups of islands, namely, the Dongsha Islands, Xisha Islands, Zhongsha Islands, and Nansha Islands, which result in the complex topography in the SCS. The upper layer circulation in the SCS has been widely studied since Wyrtki’s first report [Wyrtki, 1961], and a widely accepted notion is that, driven by the seasonally reversed monsoon, the upper circulation exhibits a distinct seasonal variability with a cyclonic circulation over the whole SCS basin in winter and an anticyclonic circulation in the southern part of the SCS and an eastward jet off the Vietnam coast in summer [Wrytki, 1961; Chu et al., 1999; Shaw and Fu, 1999; Chu and Li, 2000; Qu, 2000; Hu et al., 2000; Liu et al., 2001; Su, 2004; Fang et al., 2009]. In contrast, the middle and deep layer SCS circulations, likely induced by the Luzon Strait transport, present anticyclonic and cyclonic pattern, respectively [Yuan, 2002; Li and Qu, 2006; Qu et al., 2006; Wang et al., 2011; Lan et al., 2013]. Thought to be driven by the Luzon Strait overflow the deep SCS water flows southward, and arises in the southern basin, forming the general SCS deep meridional overturning circulation (MOC) [Liu et al., 2008;

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Figure 1. (a) Three-dimensional (b) and two-dimensional view of the SCS topography extracted from the HYCOM data set. Only the area with water depth greater than 1000 m is shown in Figure 1b. The white dashed curve and the black solid curve are the 3500 m isobaths in the northern and western-southern-eastern side of the SCS Basin.

Tian and Qu, 2012; Xie et al., 2013]. MOC plays an important role in the world ocean in controlling the stratification and distribution of water masses, the exchange of heat and the storage of chemical species such as carbon dioxide in the deep ocean [e.g., Kuhlbrodt et al., 2007]. The MOC is also considered as a very important factor that modulates the energy balance in the world ocean [e.g., Rahmstorf, 2002]. There are two driving mechanisms for MOC in the world oceans: diapycnal mixing and convergence/divergence of the winddriven Ekman transport [Toggweiler and Samuels, 1993; Munk and Wunsch, 1998]. The latter can be found in the southern ocean [Rahmstorf, 2002], while the most commonly referred example of the former is the deep-water formation in Labrador and Nordic seas in the north Atlantic as well as in Weddell and Ross seas in the Southern Ocean [Kuhlbrodt et al., 2007]. However, the SCS is located at low latitudes, and it is unlikely to form deep water by itself. By analyzing the potential density, potential vorticity, dissolved oxygen, and sediment distribution in the deep SCS, previous studies found that the deep water in the SCS has the similar characteristics as the Pacific water at 2000 m depth, which points to the existence of a deepwater overflow from the Pacific passing through the Luzon Strait into the deep SCS [Li and Qu, 2006; Qu et al., 2006; Wang et al., 2011]. It is thus concluded that the SCS deep water results from the transformation of the northwestern Pacific deep water that flows into the SCS via the Luzon Strait overflow. The SCS is not a widely open ocean and the wind-driven upwelling mechanism in the deep sea is not applicable for the SCS MOC. The vertical motion induced by mixing is considered as the primary driving mechanism in the deep SCS [Qu et al., 2006; Xie et al., 2013; Lan et al., 2013]. Tian et al. [2009] estimated the enhanced diapycnal diffusivity in the SCS is about O(1023 m2 s21) below 1000 m and reaches O(1022 m2 s21) in the Luzon Strait below 500 m, which is 2 orders larger than that in the North Pacific. This suggests the intensified diapycnal mixing might be responsible for the transformation of deep water and overturning circulation in the SCS. Nevertheless where the deep SCS water upwelled is still unknown. Moreover, the strong diffusivity in the SCS also enhances the SCS water transformation by itself in the vertical direction. Whether the middle and upper intruded water through the Luzon Strait can sink to the deep layer by the strong diapycnal mixing and contribute to the SCS deep water is unclear. The goal of this paper is to depict the SCS MOC and understand how inflows from different depths/straits contribute to the SCS MOC by using Lagrangian trajectories based on a global high-resolution reanalysis data set. Moreover, we attempt to illustrate in the horizontal planes the spatial distribution of upwelling and sinking areas associated with the SCS MOC and discuss the possible reasons that lead to this distribution. The rest of the paper is organized as follows. The reanalysis data and methods are introduced in section 2. The derived MOC is presented in section 3. Sections 4 and 5 give discussion and conclusion, respectively.

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2. Data and Methods 2.1. Data The 7 year product (from 2004 to 2010) of Hybrid Coordinate Ocean Model 1 Navy Coupled Ocean Data Assimilation (HYCOM 1 NCODA) global 1/12 Analysis (GLBa0.08, http://hycom.org/dataserver/glb-analysis) run by the Naval Research Laboratory of U.S. Navy is used in this study. The HYCOM is designed as a generalized (hybrid of isopycnal, r and z) coordinate ocean model [Bleck, 2002]. The K-Profile Parameterization (KPP) mixing scheme is employed for the vertical diffusion of momentum, heat, and salt [Thoppil et al., 2011]. It was initialized using temperature and salinity from the 1/4 Generalized Digital Environmental Model (GDEM3) climatology and forced with the data from Navy Operational Global Atmospheric Prediction System (NOGAPS). The NCODA is a multivariate optimal interpolation scheme that assimilates available operational ocean observations such as surface satellite observations, sea ice concentration, as well as profile data [Cummings, 2006]. The HYCOM1NCODA system runs daily and produces a 5 day hindcast (analysis) and a 5 day forecast. The HYCOM system was restarted in September 2008 and May 2009 when the assimilation scheme for the Sea Surface Height Anomaly (SSHA) was switched from Cooper-Haines downward projection to using the MODAS synthetic profiles. The daily analysis data used in this study include sea surface height (SSH) and three-dimensional current. The latter has 33 levels in the vertical direction at 0, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1750, 2000, 2500, 3000, 3500, 4000, 4500, 5000, and 5500 m. The GLBa0.08 data have been used to describe the general circulation and overflows in the world oceans [e.g., Kelly et al., 2007; Chang et al., 2009; Joseph et al., 2012]. In the SCS, the GLBa0.08 data exhibit a flow pattern similar to the observations in the Luzon Strait [Zhang et al., 2010], and it is superior in simulating the SCS deep and bottom circulation among eight global analysis data [Xie et al., 2013]. Therefore, this data set is selected in this study to diagnose the SCS MOC. 2.2. Methods €o €s [1995] is used in this study to calculate Lagrangian trajectories The TRACMASS code developed by Do from Eulerian velocity fields. The code computes numerically the trajectory through each grid cell by solving €o €s, 1995; Do €o €s et al., 2008]. a differential equation that depends on the velocities on the grid box walls [Do The chosen water mass (trajectories) from any location can be followed along the path both forward and backward in time, and each conservative trajectory is associated with a certain amount of net transport in and out of the grid cell. The transports are then integrated zonally to construct the meridional overturning stream function. When enough trajectories are released from vertical sections, the results should converge €o €s, 1995; Do €o €s et al., 2008]. to the Eulerian stream function [Do €o €s et al. [2008], the Lagrangian meridional overturning stream function can be written as As noted by Do y wj;k 2wj;k21 52Tj;k 52

XX i

y Ti;j;k;n

or

wj;k 2wj21;k 5

n

XX i

z Ti;j;k;n

(1)

n

y where wj,k is the Lagrangian stream function, Tj;k is zonally integrated Lagrangian meridional volume transy z port, and Ti;j;k;n and Ti;j;k;n are trajectory-derived volume transports in the meridional and vertical direction, respectively. (i, j, k) represent grid indices in the x (zonal), y (meridional), and z (vertical) direction, respectively, while n represents the time step. Correspondingly, the meridional overturning stream function in Eulerian space is defined by

xðe

ðz

wðy; zÞ5 dx xw

vdz

(2)

2H

where w(y,z) is the Eulerian meridional stream function. In this study, 18 experiments are conducted, and details are summarized in Table 1. Exps. 1.1 and 1.2 are used to obtain the SCS MOC pattern. In Exps. 1.1 and 1.2, the trajectories are initiated from the four straits in the SCS, namely, the Luzon Strait, Taiwan Strait, Karimata Strait, and Mindoro Strait. The trajectories are

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Table 1. List of 18 Lagrangian Trajectory Experiments and the Summary of Their Designs Experiment Exp. 1.1 Exp. 1.2 Exp. 2.1 Exp. 2.2 Exp. 2.3 Exp. 2.4 Exp. 3.1 Exp. 3.2 Exp. 3.3 Exp. 4.1 Exp. 4.2 Exp. 4.3 Exp. 4.4 Exp. 5.1 Exp. 5.2 Exp. 5.3 Exp. 5.4 Exp. 5.5

Data

Trajectories Horizontal Position

Vertical Position

Daily in 7 year loops Climatological annual cycle Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops Daily in 7 year loops

Perpendicular to the four straits Perpendicular to the four straits Perpendicular to the Luzon Strait Perpendicular to the Taiwan Strait Perpendicular to the Karimata Strait Perpendicular to the Mindoro Strait Perpendicular to the Luzon Strait Perpendicular to the Luzon Strait Perpendicular to the Luzon Strait All SCS domain All SCS domain All SCS domain All SCS domain All SCS domain All SCS domain All SCS domain All SCS domain All SCS domain

All depths All depths All depths All depths All depths All depths 0–500 m 500–1500 m Below 1500 m 50 m 1000 m 2000 m 3000 m 50 m 500 m 1000 m 2000 m 3000 m

Integration in Time Forward Forward Forward Forward Forward Forward Forward Forward Forward Forward Forward Forward Forward Backward Backward Backward Backward Backward

released from a y-z plane at the Luzon Strait and Mindoro Strait and from a x-z plane at the Taiwan Strait and Karimata Strait. Since the trajectories conserve their volume transport from the initial position to the final position, there are no sources or sinks in the internal regions of the SCS. Hence, the relative contribution of the inflow from each strait to the SCS MOC can be determined from the computed trajectories in terms of Exps. 2.1–2.4. Similarly, Exps. 3.1–3.3 illustrate the different roles of different layer inflows from the Luzon Strait to the SCS MOC. Exps. 4.1–4.4 are used to study the departing time of SCS waters, and lastly the origins of different layers of SCS waters are revealed by backward tracking in Exps. 5.1–5.5. The tracers are released every 10 days in the first 7 years for all experiments. All the trajectories are tracked from when they enter the SCS through the four main straits to when they leave the SCS domain, which may take more than 100 years. Hence, the GLBa0.08 velocity fields are looped in time, so that after the end of 2010 the velocity field is returned to the beginning of 2004. This introduces a jump for the velocity field, which may result in an error in the Lagrangian stream function. This error can be estimated by taking the difference €o €s et al., 2008]. Moreover, this between the Eulerian stream function and the Lagrangian stream function [Do loop approach tends to represent better the magnitude of seasonal cycle as well as its interannual variation compared to the climatology.

3. Results 3.1. The Mean Circulation in the SCS and the Transport Through the Four Straits 3.1.1. Circulation The 50, 1000, and 3000 m depths are selected to represent the upper, middle, and deep horizontal circulation in the SCS revealed by the GLBa0.08 data (Figure 2). The upper layer horizontal circulation of SCS is driven primarily by the East Asia Monsoon that is stronger and northeasterly from December to April and relatively weaker and southwesterly during the rest of the year. As a result, the time-averaged upper horizontal circulation in the SCS is cyclonic with a strong western boundary current (Figure 2a). The intrusion of the Kuroshio water is significant in the upper layer. Driven by the outflow through the middle layer of the Luzon Strait, the horizontal circulation in the middle depth is anticyclonic (Figure 2b). There are three prominent anticyclonic eddies along the western and northern boundary of the basin. The deep circulation shown in Figure 2c is cyclonic, which is driven by the inflow of saline and cold Pacific deep water through the Luzon Strait [Qu et al., 2006; Wang et al., 2011; Xie et al., 2013]. There is an accelerated southwestward current to the west of the Luzon Strait where the horizontal source from the east does not exist at the 3000 m depth because the maximum depth of the Luzon Strait is about 2400 m. Therefore, a strong overflow is expected above 3000 m in the Luzon Strait. 3.1.2. Transport The volume transports through the four main straits are the sources and sinks of SCS waters, which influence the structure and intensity of the SCS MOC. Figure 3 shows the volume transport and inflow through

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Figure 2. The GLBa0.08 7 year (2004–2010) climatological horizontal circulation in the SCS at (a) 50 m, (b) 1000 m, and (c) 3000 m depths, respectively.

Figure 3. Time series of (a) net transport and (b) inflow through the four main straits connected to the SCS (units: Sv). The time-averaged net transport and inflow are also presented at the upper left corner. The Luzon Strait transport (black curve) uses the scale on the left, and the transport through other three straits uses the scale on the right.

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the Luzon Strait, Taiwan Strait, Karimata Strait, and Mindoro Strait derived from the GLBa0.08 data. The annual mean Luzon Strait transport (LST) is about 5.23 Sv westward (1 Sv 5 106 m3 s21), which is close to the recent model estimate of 4 Sv by Hsin et al. [2012] and the observation estimates of Chu and Li [2000] at 6.5 Sv, Qu [2000] at 4 Sv, and Su [2004] at 4.2–5.0 Sv. The annual mean Taiwan Strait transport (TST) is 1.70 Sv northward, which is consistent with observations of Fang et al. [1991] at 2 Sv, Wang et al. [2003b] at 1.8 Sv, Guo et al. [2005] at 1.27 Sv, and the model estimate by Wu and Hsin [2005] at 1.09 Sv. The annual mean Mindoro Strait transport (MST) is 2.53 Sv eastward from the GLBa008, which is also similar to the results of 2.3 Sv by Bao et al. [2002] and 1.77 Sv by Fang et al. [2005]. However, the annual mean Karimata Strait transport (KST) is only 0.64 Sv southward, which is significantly smaller than the previous estimates of 1.3 Sv by Bao et al. [2002] and Fang et al. [2005]. The sum of transports through the four main

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straits is 0.36 Sv out of the SCS. The transport through the Balabac Strait is about 0.10 Sv out of the SCS. The transport through all straits is not balanced with a net of about 0.26 Sv. The reason may be that the GLBa0.08 data are interpolated from the HYCOM native hybrid coordinates to the z-level coordinate.

Figure 4. Same as Figure 3 except for different layers of the Luzon Strait. The upper layer: 0–500 m; the middle layer: 500–1500 m; and the deep layer: 1500 m–bottom. The upper layer transport (black curve) uses the scale on the left, and the middle layer transport (red curve) and the deep layer transport (blue curve) use the scale on the right.

The volume transports through the four straits exhibit significant seasonal variations, which is a response to the seasonal reverse of the East Asia monsoon [Hsin et al., 2012]. The net LST is larger in winter, but it is much weaker in summer (Figure 3a). There is a similar seasonal variation for MST but a reversed variation for TST, both of which are, however, outflows. The KST is a strong outflow in winter and a weaker inflow in summer, but it appears to be rather difficult to reproduce the precise KST so that there is much difference between different models [Wang et al., 2009].

The inflow from each strait is more important to Lagrangian stream function than the net transport because only the inflow trajectories are used to track their paths. The annual mean inflows from the Luzon Strait, Taiwan Strait, Karimata Strait, and Mindoro Strait shown in Figure 3b are 16.19, 0.3, 0.86, and 0.88 Sv, respectively. The inflow via the Taiwan Strait mainly happens in winter, whereas the inflow through the Karimata Strait mainly appears in summer. Inflow prevails in the Luzon Strait and Mindoro Strait all year round, and the stronger inflow occurs in winter for the former but in summer for the latter. Inflows to the SCS are mostly from the Luzon Strait (Figure 3b). In order to conveniently distinguish the contribution to the SCS MOC from inflows in different layers of the Luzon Strait, three layers of 0–500 m (the upper layer), 500–1500 m (the middle layer), and below 1500 m (the deep layer) are divided based on the vertical structure of LST (not shown). The annual mean transport for the upper, middle, and deep layers is 5.28 Sv westward, 1.21 Sv eastward, and 1.16 Sv westward, respectively (Figure 4). The transport in different layers also has significant seasonal variations. The upper layer westward transport is stronger in winter and weaker in summer. The transport in middle layer is out of phase with the upper layer transport, and the deep layer transport lags the upper layer transport by about 5 months. The annual mean inflow in the upper, middle, and deep layer is 12.09, 2.55, and 1.54 Sv, respectively. The principal inflow by far occurs in the upper layer. Though the net transport in middle layer is eastward, the westward inflow exists all year round and is larger than that in the deep layer. The seasonal variation of inflow is similar to the total transport in the upper and deep layers, whereas it is less regular in the middle layer. 3.2. The SCS MOC and Diagnoses The Lagrangian stream function is obtained by summing up the volume transport of all trajectories that have been followed in the SCS domain and then dividing by the number of released trajectories. The meridional overturning stream functions calculated by using Eulerian and Lagrangian methods, termed as Eulerian meridional stream function (EMSF) and Lagrangian meridional stream function (LMSF), are

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Figure 5. (a) Eulerian and (b and c) Lagrangian meridional overturning stream function in the SCS based on GLBa0.08 data (units: Sv). Figure 5c is derived from the climatological mean velocity. Negative streamlines correspond to clockwise circulation. (d) The Lagrangian meridional transport for total inflow through Luzon Strait (units: Sv).

shown in Figures 5a and 5b. Albeit the quantitative differences, the LMSF has the same pattern as the EMSF. The differences could be induced because of the cycling application of the velocity field from 2004 to 2010 and by the insufficient number of trajectories. In order to assess any potential error induced by the data jump, the time-averaged (climatological) data is used in Exp. 1.2 where the jump should not exist. The result shows the LMSF derived from the climatological data in Exp. 1.2 (Figure 5c) is similar to EMSF and LMSF derived from Exp. 1.1. This indicates the data jump should not have any significant impact on this study. In Exp. 1.2, there are still differences between the LMSF and the EMSF; however, the differences should disappear if the number of trajectories is large enough. The overall pattern of the SCS MOC is that the upper inflow water through the Luzon Strait sinks in the northern SCS and upwells mostly in the southern SCS. Another obvious character of the meridional overturning stream function is the ‘‘sandwich’’ pattern, which is consistent with the basin-scale horizontal SCS circulation of the three different layers (see Figure 2) as well as the LST (see Figure 4). A strong clockwise MOC appears above 500 m depth with the strongest downdraft found at 21 N, which then gradually rises from north to south. The weaker counterclockwise structure is found in the middle layer between 500 and 1000 m depths. The counterclockwise overturning is stronger in the south than in the north. The third layer of the ‘‘sandwich’’ is clockwise located below 1000 m. This deep (below 1000 m depth) clockwise MOC is divided into three cells of deep southern MOC cell (DSMOC), deep middle MOC cell (DMMOC), and deep northern MOC cell (DNMOC). It should be noted that the DNMOC is unclosed because of the presence of both sink and source in the Luzon Strait. The maximum depth of these three deep MOC cells reaches about 3500 m. Having three deep cells means the sunken deep water in the northern SCS upwells not only in the southern domain but also on the south side of each deep cell. It also indicates the source water of deep MOC is not only from the northern sunken water but also the sunken water from the north side of each deep cell. It is important for us to understand this panorama of deep SCS MOC because the previous studies suggest the SCS deep water is formed by sinking of the deep Pacific water west of the Luzon Strait after

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Figure 6. Contributions of inflow from different straits to the Lagrangian meridional overturning stream function (units: Sv): (a) Luzon Strait, (b) Taiwan Strait, (c) Karimata Strait, and (d) Mindoro Strait. Negative streamlines correspond to clockwise circulation.

intruding the strait [Qu et al., 2006; Wang et al., 2011]. The zonally integrated Lagrangian meridional volume transport derived from equation (1) for intruded water through the Luzon Strait also shows the ‘‘sandwich’’ pattern directly, which is southward transports in the upper and deep layers but a northward transport in the middle layer (Figure 5d). €o €s et al. [2008], the Lagrangian stream function can be decomposed into partial stream As introduced by Do functions that describe the contributions from different trajectories. The trajectories released from each straits in Exps. 2.1–2.4 are used to illustrate the relative roles of the inflows through different straits to the SCS MOC (Figure 6). The meridional overturning stream function of Exp. 2.1 has a similar pattern and magnitude to that of Exp. 1, which indicates that the inflow from the Luzon Strait dominates the SCS MOC structure (Figure 6a). This is intelligible because the inflow through the Luzon Strait at 16.19 Sv is much larger than the sum of inflows from the other three straits (2.04 Sv). Influences of the Taiwan, Karimata, and Mindoro Strait inflows are very limited with the maximum amplitude of 0.2 Sv located near the surface. The Taiwan Strait inflow mainly contributes to the upper layer MOC and hardly influences the deep layers, implying the water from the Taiwan Strait hardly sinks in the SCS (Figure 6b). The inflow through the Karimata Strait plays a negative role in the upper MOC but a small positive role in the DSMOC (Figure 6c). The Mindoro Strait inflow contributes to the upper MOC negatively, especially in the north, while it contributes

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Figure 7. Contributions of different layer inflows through the Luzon Strait to Lagrangian meridional overturning stream function (units: Sv): (a) the total inflow, (b) the upper layer inflow, (c) the middle layer inflow, and (d) the deep layer inflow.

positively to the DSMOC and DMMOC (Figure 6d). These results suggest that part of the inflow from the Karimata and Mindoro straits can also sink to the deep layer and has minor contributions to the deep MOC in the southern and middle SCS though their roles are minor compared to that of the inflow from the Luzon Strait. Since the Luzon Strait inflow is the main source driving the SCS MOC and the LST has a three-layer structure with net inward transports in the upper and deep layers but a net outward transport in the middle layer (see Figure 4a), different contributions from these three layers to the SCS MOC are demonstrated in Exps. 3.1, 3.2, and 3.3 (Table 1). The upper layer inflow through the Luzon Strait dominates the upper and middle layers of the ‘‘sandwich’’ structure of the SCS MOC as suggested by the similar pattern and magnitude above the 1000 m depth to that induced by the total inflow through the Luzon Strait (Figures 7a and 7b). Surprisingly, it contributes considerably to the deep layer MOC as well, more than 0.5 Sv for all three deep cells (Figure 7b). The upper layer inflow is thus proportionally more important to the DSMOC and DMMOC than that to the DNMOC. The Luzon Strait middle layer inflow contributes to the upper MOC by about 1 Sv and to the three deep MOC cells by about 0.2 Sv (Figure 7c). This indicates that the Luzon Strait middle layer inflow also has a considerable role in the SCS MOC although the net transport is outflow. The intruded deep layer water through the Luzon Strait affects mostly the DNMOC, which accounts for about 40% of the DNMOC intensity, whereas it has limited contributions to the DSMOC and DMMOC (Figure 7d).

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Figure 8. (a) Normalized number of trajectories remaining in the SCS for Exps. 2.1 (red solid line), 3.1 (red dash-dotted line), 3.2 (red dashed line), and 3.3 (red dotted line). (b) Same as Figure 8a except that the trajectories are released from horizontal planes of different depths in the SCS from Exps. 4.1 (blue dotted line), 4.2 (blue dash-dotted line), 4.3 (blue solid line), and 4.4 (red solid line). The black line in Figures 8a and 8b represent a theoretical decay curve with the time scale of 12 and 21 years, respectively.

To summarize, except the northern most SCS where the MOC is clockwise throughout the water column, the SCS MOC is counterclockwise from 500 to 1000 m, which is sandwiched between the clockwise MOCs above and below. Such a MOC pattern results mostly from the inflows through the Luzon Strait. About 60%/30%/10% of the upper clockwise cell can be attributed to the upper/middle/deep Luzon Strait inflow, respectively. Inflows from the other three straits contribute a few percentages to the upper cell, but the contribution is positive for the Taiwan Strait and negative for Karimata and Mindoro straits with the sum from the three being very close to zero. Interestingly, the inflows from the Luzon Strait are the main drivers for the southern portion of the middle counterclockwise cell with more than half the magnitude from the upper inflow and another 30% from the middle and deep inflow combined. The rest 10% of the southern counterclockwise cell is induced by the inflow through the Karimata Strait. In contrast, the northern portion of the middle counterclockwise cell is induced largely by the inflow through the Mindoro Strait supplemented by the upper inflow from the Luzon Strait. Below 1000 m, there are actually three clockwise cells. The DNMOC results almost entirely from the inflows through the Luzon Strait with about 40%/20%/40% from the upper/middle/deep inflow, while the upper and middle inflow from the Luzon Strait contributes, respectively, about 60% and 30% to both the DMMOC and DSMOC with the Karimata and Mindoro inflows, accounting for 10% or so. Most of the results above are intuitively clear, but how the upper and middle layer inflows from the Luzon Strait induce the DMMOC and DSMOC is not as straightforward. This is to say that most of the SCS deep water comes from the Luzon Strait upper layer intrusion. Moreover, the water from the upper layer Luzon Strait inflow sinks not only in the northern SCS but also in the middle of the basin, i.e., the north side of DSMOC and DNMOC. The trajectories from the first year of Exps. 3.1–3.4 are also used to investigate the residence time of the Luzon Strait inflow water in the SCS. Figure 8a shows the normalized temporal evolution of the number of trajectories remaining in the SCS, which represents a quantity intimately related to the residence time of the Luzon Strait inflow water in the SCS. As a comparison, the exponential decay with the formula VðtÞ=V0 5e2t=s is also shown in Figure 8a, where VðtÞ=V0 is the ratio between the remaining trajectories in the SCS at time t and the total number of trajectories initially released, and s 5 12 years is the e-fold time scale. This €o €s et al., criterion is widely used in the atmospheric and oceanic studies [e.g., Bolin and Rodhe, 1973; Do 2004]. Because a portion of the Luzon Strait inflow water flows out of the SCS quickly through the same strait (i.e., via the Luzon loop current) and it does not mix fully with the interior SCS water, this portion of water should not be considered when calculating the residence time. Furthermore, it takes about 3 months for the upper SCS water to make one round along the 1 km isobath. The trajectories with less than 3 months of lifetime in the SCS are thus excluded from the calculation. Figure 8a indicates the e-fold time scale of the Luzon Strait inflow water is slightly less than 12 years. What appears to be intriguing is that the residence time of the deep inflow water is shorter than that of the middle and upper layers’ and the upper layer’s residence time is the longest. The reason is likely that most of the Luzon deep inflow water upwells

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Figure 9. (a) Normalized contribution of the inflow from the Taiwan Strait (TW Str.), Luzon Strait (LZ Str.), Karimata Strait (KM Str.), and Mindoro Strait (MD Str.) to SCS waters at 50, 500, 1000, 2000, and 3000 m derived from Exps. 5.1–5.5, respectively. (b) Normalized contribution of different layer inflows (0–500, 500–1500, and below 1500 m) from the Luzon Strait to SCS waters at 50, 1000, 2000, and 3000 m derived from Exps. 5.1–5.5, respectively.

in the northern domain after it sinks west of the Luzon Strait and arrives at the southern deep SCS, whereas a significant portion of the upper and middle inflow waters through the Luzon Strait travel to the deep basins in the south (Figure 7). Qu et al. [2006] estimated the SCS deep-water residence time of about 24 years using the water volume below 1500 m divided by the estimated deep inflow of 2.5 Sv. Such a calculation implies that all the SCS deep water is from the deep inflow through the Luzon Strait. However, as shown in Figure 7, the SCS deep water originates from Luzon intrusions of all depths and, moreover, the deep Luzon Strait inflow contributes mostly the cycling of the deep water in the northern SCS. It is also worth noting that the e-folding time scale of 12 years derived here should not equal the SCS deep water residence time as it represents, in a more strict term, the mean residence time of the intruded Northern Pacific water in the SCS. On the other hand, normalized number of tracers remaining in the SCS for those initiated in the SCS basin derived from Exps. 4.1–4.4 (Table 1) is shown in Figure 8b. The time needed for those trajectories initiated at 3000 m depth to depart from the SCS is about 21 years, which is larger than those at upper depths and is comparable to the estimate of Qu et al. [2006]. 3.3. Diagnosing the Sources of SCS Waters In order to quantify the contribution of inflows from different straits and from different layers of the Luzon Strait to the different layers of SCS waters, the trajectories are released from the horizontal planes at 50, 500, 1000, 2000, and 3000 m depths (Exps. 5.1–5.5), respectively. All trajectories have the uniform distribution initially with one release per model grid, and they are followed backward in time. Figure 9a shows the normalized contributions of the inflow from the four straits to SCS waters at different depths. The Luzon Strait inflow is again the main source of SCS waters, which contributes about 77% at the 50 m depth, about 80% at 500 and 1000 m depths, and 82% at 2000 and 3000 m depths. The Karimata Strait inflow is the secondary source for the SCS waters and its contribution reaches about 13% at the 50 m depth and about 10% at the lower depths. The Mindoro Strait inflow has a similar contribution to SCS waters at all depths with 6%. Contributions from the Taiwan Strait inflow are relatively less, from 4% at the 50 m depth to