Dynamics of Atmospheres and Oceans

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1 Dynamics of Atmospheres and Oceans 47 (2009) 3 14 Contents lists available at ScienceDirect Dynamics of Atmospheres and Oceans journal homepage: Review An introduction to the South China Sea throughflow: Its dynamics, variability, and application for climate Tangdong Qu a,, Y. Tony Song b, Toshio Yamagata c a International Pacific Research Center, SOEST, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, HI 96822, USA b Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA c Department of Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan article info abstract Article history: Available online 17 June 2008 Keywords: South China Sea throughflow Western boundary current Indonesian throughflow Asian climate The South China Sea throughflow (SCSTF) involves the inflow through the Luzon strait and the outflow through the Karimata, Mindoro, and Taiwan straits. Recent studies have suggested that the SCSTF act as a heat and freshwater conveyor, playing a potentially important role in regulating the sea surface temperature pattern in the South China Sea and its adjoining tropical Indian and Pacific Oceans. In this introductory paper, we attempt to convey the progress that has recently been made in understanding the SCSTF. We first provide an overview of existing observations, theories, and simulations of the SCSTF. Then, we discuss its interaction with the Pacific western boundary current and Indonesian throughflow. Finally, we summarize issues and questions that remain to be addressed, with special reference to the SCSTF s dynamics, variability, and implication for climate Elsevier B.V. All rights reserved. Contents 1. Introduction Overview of the SCSTF Relation to the Pacific LLWBC Relation to the Indonesian throughflow Heat and freshwater conveyor Implication for climate Impact on the SCS surface temperature Corresponding author. Tel.: ; fax: address: tangdong@hawaii.edu (T. Qu) /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.dynatmoce

2 4 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) Influence on the Indonesian sea surface temperature Influence on the Indo-Pacific warm pool Discussion Acknowledgments References Introduction The South China Sea (SCS) is the largest marginal sea in the southeastern Asian waters. It connects in the south with the Sulu and Java Seas through the shallow Mindoro ( 200 m) and Karimata (<50 m) straits and in the north with the East China Sea through the shallow Taiwan strait (<100 m) and with the Pacific Ocean through the deep Luzon strait (>2000 m), with its maximum water depth exceeding 4000 m (Fig. 1). On the basin average, the SCS receives heat from the atmosphere at a rate ranging from 20 to 50 W m 2 (Fig. 2). Recent OAFlux data (Yu and Weller, 2007) favor a mean value around the higher end of this range, 49 W m 2, which is nearly twice as high as the estimate (23 W m 2 ) from the Comprehensive Ocean-Atmosphere Data Set (Oberhuber, 1988). Given a surface area of about m 2 in the SCS, these estimates imply a net heat gain of up to PW (1 PW = W) over the entire basin. Examination of CMAP (Climate Prediction Center Merged Analysis of Precipitation), GPCP (Global Precipitation Climatology Project), and TRMM (Tropical Rain Measuring Mission) precipitation data shows that the SCS is also a recipient of heavy rainfall, with an annual mean value of Sv (1 Sv= m 3 s 1 ) over the entire basin. An accurate estimate of evaporation from observations is not available at this time. Preliminary examination of the NCEP re-analysis data indicates that precipitation in the SCS exceeds evaporation (P E) by about 0.1 Sv (Fig. 3a), leading to the freshest sea surface in the region (Fig. 3b). Fig. 1. A schematic diagram of the South China Sea throughflow (after Qu et al., 2006a). Water entering the South China Sea through the Luzon strait is lower in temperature (blue) and higher in salinity (blue) than water leaving it through the Karimata, Mindoro, and Taiwan straits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

3 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) Fig. 2. Annual mean surface heat flux from (a) OAFlux (Yu and Weller, 2007) and (b) COADS (Oberhuber, 1988) datasets. Fig. 3. Annual mean P-E from NCEP (left) and sea surface salinity from WOA05 (right). The question which may arise immediately is where this heat and freshwater gain from the atmosphere goes. The SCS is a semi-closed basin below about 200 m. On the long-term average this heat and freshwater gain can only be balanced by horizontal advection, with inflow of cold and salty water through the Luzon strait and outflow of warm and fresh water through the Mindoro and Karimata straits. This circulation has been termed as the South China Sea throughflow (SCSTF) and extensively studied in recent years (e.g., Qu et al., 2005, 2006a; Wang et al., 2006b; Yu et al., 2007). Fig. 1 is a schematic diagram of the SCSTF (Qu et al., 2006a), based on existing observations and results from general circulation models (GCM). The diagram represents a three-dimensional circulation, including the intrusion of Pacific waters through the Luzon strait, the upwelling in the deep SCS, and the outflow through the Karimata, Mindoro, and Taiwan straits. In this introductory paper, we wish to provide an overview of the SCSTF (Fig. 1). We first summarize the results that have emerged from recent efforts and then identify areas where further efforts are clearly needed. For space limitation, we focus our attention on the large scale phenomena of the SCSTF. Additional information can be found in literature referred by the present study, as well as papers in the rest of this special issue. 2. Overview of the SCSTF The water exchange between the SCS and the Pacific Ocean through the Luzon strait has been known for decades. Based on early hydrographic data, sea level records, and ship drifts, Wyrtki (1961)

4 6 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) 3 14 first noted that water enters the SCS in winter and flows back to the Pacific in summer. This is true, however, only for the surface layer, where ocean circulation is predominantly forced by monsoonal winds. Within the depth range of the thermocline, later observations revealed that there is a Kuroshio branch toward the SCS both in winter and summer (e.g., Qiu et al., 1984; Guo, 1985), and the intrusion of the North Pacific Tropical Water seems to occur during most, if not all, seasons of the year (e.g., Shaw, 1989, 1991; Qu et al., 2000), presumably as a result of westward pressure gradient along the continental slope south of China (e.g., Qu, 2000; Hsueh and Zhong, 2004). The flow in the Luzon strait changes its sign in the intermediate layer. Both observations and numerical models have shown that water flows out of the SCS at depths between about 500 and 1500 m to compensate the inflow of the Pacific water in the surface layer (e.g., Chen and Huang, 1996; Chao et al., 1996; Qu et al., 2000; Yuan, 2002; Tian et al., 2006). In the deeper layer (>2000 m), water on the SCS side is relatively homogenous and appears to have the same characteristics as the Pacific water at about 2000 m (Nitani, 1972; Broecker et al., 1986; Chen et al., 2001). This has been interpreted as evidence for the deep ventilation of the SCS by water from the Pacific passing over the Luzon strait. Since the Pacific water is colder and of higher density, it sinks after crossing the Luzon strait (Wyrtki, 1961). To compensate this descending movement, upwelling (or intense vertical mixing) occurs elsewhere, and as a consequence, the renewal of the SCS deep water is rapid in comparison with its counterpart in the Pacific (e.g., Li and Qu, 2006). Because of the complicated coastline and topography and the highly variable nature of the currents in the region, direct measurement of the deep water overflow through the Luzon strait has proven difficult. To our best knowledge, the only direct measurement was from a single 82-day current meter time series near the bottom of Bashi Channel (Liu and Liu, 1988), showing a persistent near-bottom flow to the SCS with a mean speed of 0.14 m s 1. This provided a transport estimate of 1.2 Sv in the deep layer of the Luzon strait, consistent with hydrographic observations (e.g., Wang, 1986; Qu et al., 2006b). The total transport through the Luzon strait, often termed as the Luzon strait transport (LST), is essentially westward. Prior studies have arrived at a broad range of the LST transport estimate, varying from 0.5 to 10 Sv (e.g., Wyrtki, 1961; Metzger and Hurlburt, 1996; Lebedev and Yaremchuk, 2000; Chu and Li, 2000; Xue et al., 2004; Yaremchuk and Qu, 2004; Song, 2006; Yu et al., 2007), while recent observations favor a value near the middle of the range (Qu, 2000; Qu et al., 2000; Tian et al., 2006). On the seasonal time scale, a common feature in prior studies is that the LST is larger in winter and smaller in summer, in response to the seasonal reversing monsoon. Among others, Qu (2000) gives a maximum of 5.3 Sv in January February and a minimum of 0.2 Sv in June July, based on hydrographic observations, while Chu and Li s (2000) estimate is somewhat larger, with a maximum of 13.7 Sv in February and a minimum of 1.4 Sv in September. These discrepancies could be due largely to the uncertainties in the intermediate and deep circulations (e.g., Qu et al., 2006b; Tian et al., 2006). In a recent diagnostic analysis that combines atmospheric climatologies with drifter, satellite altimetry, and hydrographic data in the framework of a numerical model, Yaremchuk and Qu (2004) obtained a seasonal cycle of LST with a maximum of 4.8 Sv in January February and a minimum of 0.8 Sv in August, showing a better agreement with Qu s (2000) estimate. It is worthwhile to note that the LST can be contaminated by eddies of various time scales. Because of this, the intrusion of Pacific water is shown to have different pathways in the Luzon strait (e.g., Li et al., 1998; Hu et al., 2000; Wu and Chiang, 2007). The recently available satellite altimeter data have provided increasing evidence for these pathways (e.g., Yuan et al., 2006; Caruso et al., 2006). The LST also varies on interannual time scales. In the surface layer, the intrusion of Pacific waters depends on the strength of local winds and tends to be weaker during El Nino years and stronger during La Nina years (e.g., Ho et al., 2004). An accurate estimate of the LST interannual variation from observations is not available. Results from high-resolution GCMs suggest that the intrusion of Pacific waters at subsurface has different phase from that near the surface and the total LST intensifies during El Nino years and weakens during La Nina years (e.g., Qu et al., 2005; Wang et al., 2006b). These model results may be sensitive to model s topography and parameters (e.g., Metzger and Hurlburt, 2001) and need to be confirmed by further observations. As part of the Pacific tropical gyre (Metzger and Hurlburt, 1996), the LST is strongly influenced by the low-latitude western boundary current (LLWBC) in the Pacific (e.g., Qu et al., 2004), and the intrusion of Pacific waters, in turn, may have a dramatic impact on the SCS circulation, as well as the ITF

5 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) (e.g., Su, 2004; Qu et al., 2005; Wu and Chang, 2005; Guan and Fang, 2006; Yuan et al., 2007; Tozuka et al., 2007). The details are discussed below. 3. Relation to the Pacific LLWBC The North Equatorial Current (NEC) bifurcates into the northward-flowing Kuroshio and the southward-flowing Mindanao Current (MC) as it reaches the Philippine coast (e.g., Nitani, 1972; Toole et al., 1990; Qiu and Lukas, 1996; Lukas et al., 1996). The bifurcation of the NEC shifts poleward with depth, reflecting the basin-scale structure of the subtropical gyre (Reid and Arthur, 1975; Qu et al., 1997, 1998, 1999), and it experiences variations on various time scales. In concert with the seasonal reversal of monsoon, the bifurcation of the NEC moves northward in winter and southward in summer (Qu and Lukas, 2003), accomplished by a quantitative change in the partition of the NEC transport between the Kuroshio and the MC. Thus, the Kuroshio transport east of Luzon approaches its seasonal minimum in winter and seasonal maximum in summer. On interannual time scale, results from highresolution GCMs have indicated that the bifurcation of the NEC moves northward during El Nino years, resulting in a minimum transport in the Kuroshio east of Luzon (e.g., Kim et al., 2004; Qu et al., 2004). The situation seems to be reversed during La Nina years. Most of the Kuroshio water bypasses the Luzon strait and continues northward along the continental slope east of China, with only a small fraction leaking into the SCS. When the bifurcation of the NEC moves northward, the weakening Kuroshio east of Luzon provides a favorable condition for the intrusion of Pacific waters through the Luzon strait (Sheremet, 2001; Yaremchuk and Qu, 2004). As a consequence, the LST tends to be stronger during El Nino years and weaker during La Nina years (Qu et al., 2004). Because of this LST variation, the impact of ENSO can be conveyed into the SCS through the Luzon strait, thus providing an oceanic bridge between the SCS and the Pacific Ocean. Some portion of the LST exits the SCS through the Mindoro strait (Fig. 1), referred to as the Mindoro strait outflow by Qu et al. (2006a). Wyrtki (1961) noticed that the surface flow in the Mindoro strait is southward only during August October, when the southwest monsoon prevails. It turns northward during other seasons of the year, corresponding with the northeast monsoon. The local wind becomes less important at depth. Both the North Pacific Tropical and Intermediate Waters are clearly visible in the Sulu Sea (Frische and Quadfasel, 1990; Quadfasel et al., 1990), indicative of an intrusion of thermocline waters through the Mindoro strait, presumably as a result of large-scale forcing over the Pacific (Fig. 1). Though no direct measurement is available at this time, results from high-resolution GCMs have clearly demonstrated the existence of Mindoro strait outflow (e.g., Qu et al., 2006a). Water of the SCS origin exits the Sulu Sea in the south, mostly through the shallow Sibutu Passage, and this may exert direct influence on the circulation in the Celebes Sea. Results from numerical experiments have shown that transport through the Mindoro strait can shift the NEC bifurcation by as much as 0.5 (Metzger and Hurlburt, 1996). An enhanced Mindoro strait transport can result in a southward shift of the NEC bifurcation and consequently a stronger Kuroshio transport near the coast of the Philippines. The detail needs to be investigated further. 4. Relation to the Indonesian throughflow Water of the Pacific origin continues southward into the Java Sea through the Karimata strait (Wyrtki, 1961; Lebedev and Yaremchuk, 2000; Yaremchuk and Qu, 2004; Fang et al., 2005; Qu et al., 2005). As one of the primary links in the global water exchange between ocean basins, the ITF has been known to play an important role in the world s climate (Gordon, 1986). Extensive studies have been carried out in the past decades and in particular during the International Nusantara Stratification and Transport (INSTANT) program. One of the most important findings of INSTANT is that the maximum southward velocity in the Makassar strait, the primary pathway of the ITF, occurs at about 150 m and during the boreal winter, the surface flow in the strait is actually northward, leading to a considerably weaker ITF heat transport than previously thought (e.g., Vranes et al., 2002; Gordon et al., 2003).

6 8 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) 3 14 Fig. 4. A schematic diagram showing the ITF (blue) in the thermocline and the SCSTF (red) near the sea surface, both of which are primarily forced by the large-scale wind in the Pacific (after Qu et al., 2005). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.) Local Ekman current is apparently too weak and shallow to account for the subsurface velocity maximum described above. The strong vertical shear in the Makassar strait must have a remote source outside the Indonesian Seas. Based on the wind data over the past 40 years and results from highresolution GCMs, Qu et al. (2005) hypothesized that the circulation in the Makassar strait is primarily a result of interplay between the southward-flowing ITF in the thermocline and the northward-flowing SCSTF near the sea surface (Fig. 4). It is the SCSTF that inhibits the warm surface water from the Pacific from flowing southward, resulting in a subsurface velocity maximum in the Makassar strait. The SCSTF can be interpreted as a circulation around Philippine-Borneo, based on Godfrey s (1989) Island Rule. It is the large-scale wind over the Pacific that sets up the pressure head driving the SCSTF against friction (Qu et al., 2005; Wang et al., 2006b). Gordon et al. (2003) also related the northward surface flow in the Makassar strait to the intrusion of freshwater from the SCS. They argued that during the boreal winter, the southeastward monsoon wind in the Java Sea drives the buoyant, fresh SCS water into the southern Makassar strait, creating a northward pressure gradient in the surface layer of the strait. The intrusion of the SCS fresh water is part of the SCSTF. It is the SCSTF that conveys the impact of Pacific variability directly to the Indonesian Seas. Though the SCSTF volume transport is only 1 2 Sv in the Karimata strait (Wyrtki, 1961), about an order smaller than the ITF, its impact on the Pacificto-Indian Ocean heat transport is expected to be large, and this has been confirmed by results from high-resolution GCMs (Qu et al., 2006a). Tozuka et al. (2007) recently conducted a numerical experiment that provided a better insight into Qu et al. s (2005) hypothesis, based on a relatively coarse resolution GCM (0.4 in the SCS and 2 elsewhere). The numerical experiments demonstrated that the velocity structure in the Makassar strait is strongly influenced by the SCSTF. Blocking the SCSTF in the model led to an enhanced southward flow in the surface layer of the Makassar strait that eliminated most of the vertical shear reproduced in the control run (Fig. 5). Blocking the SCSTF in the model also led to a warmer ITF, with its transportweighted temperature being about 3 C higher than the control run. This result suggests that the SCSTF significantly (by as much as 47%) reduces the ITF heat transport. Its potentially important role in regulating the SST pattern in the tropical Indian and Pacific Oceans raises an important issue for climate research.

7 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) Fig. 5. Velocity structure in Makassar strait from numerical experiments (after Tozuka et al., 2007). Blocking the SCSTF leads to an enhanced southward flow in the surface layer of Makassar strait (dashed line). 5. Heat and freshwater conveyor As described in the introduction of this paper, the SCSTF primarily balances the heat and freshwater gain from the atmosphere so as to establish a stable thermocline and halocline in the SCS. Based on an analysis of surface flux data, Qu et al. (2006a) suggest that the SCSTF acts as a conveyor belt, transferring up to PW of heat and 0.1 Sv of freshwater from the SCS into the tropical Indian and Pacific Oceans. Results from numerical models seem to support this hypothesis (e.g., Fang et al., 2003; Qu et al., 2006a; Yu et al., 2008). Analysis of the high-resolution (0.1 ) OGCM for the Earth Simulator (OFES) outputs have shown that more than 65% of annually averaged heat and 90% of annually averaged freshwater gained from the atmosphere are taken away from the SCS by the SCSTF. In the model, the SCSTF is primarily responsible for the SCS upper-layer heat content variability, storing heat in the SCS during La Nina years and releasing it during El Nino years (Qu et al., 2004). The model result implies that the SCS is likely to play a more active role than previously thought in regulating the SST pattern and modulating conditions in the Maritime Continent and its adjoining Indo-Pacific warm pool. 6. Implication for climate The SCS, situated at the confluence of the tropical Indian and Pacific Oceans, is of major climate importance. As part of the Indonesian Maritime Continent, it is recognized as a site of vigorous atmospheric convection (e.g., Ramage, 1968), where small changes of SST can result in dramatic changes in weather patterns across the Indo-Pacific basin (e.g., Ashok et al., 2001; Neale and Slingo, 2003; McBride et al., 2003). As a significant heat and freshwater conveyor, the SCSTF is expected to play an important role in regulating SST patterns in the SCS and its adjoining tropical Indian and Pacific

8 10 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) 3 14 Oceans. For a detailed review of the SCS climate, the reader is referred to Wang et al. (this issue). Only a brief introduction to the SCSTF s possible impact is provided in this paper Impact on the SCS surface temperature The SCS surface temperature varies from intraseasonal to interannual time scales (e.g., Chu et al., 1997; Liu et al., 2004; Xie et al., 2007) and has a good correspondence with ENSO (e.g., Zhang et al., 1996; Ose et al., 1997; Wang et al., 2000, 2002; Xie et al., 2003; Wang and Wang, 2006). Prior to an El Nino, SST is warmer in the tropical western Pacific and cooler in the SCS, and this SST difference between the two basins further invites cold surges (westerly bursts) and provides a trigger of El Nino (e.g., Yamagata and Masumoto, 1989). As the El Nino develops, the East-Asian winter monsoon weakens (Tomita and Yasunari, 1996), leading to a reduced circulation in the SCS (Chao et al., 1996; Wu et al., 1999). Model experiments have indicated that the interannual variation of SCS circulation is dominated by two modes (Wu and Chang, 2005), modulating the winter and summer circulation patterns, respectively. In response to monsoon anomalies, SST gets warmer over much of the SCS during the mature phase of El Nino and peaks in the following summer (e.g., Wang et al., 2000; Xie et al., 2003). Recent studies further suggest that the SCS surface temperature actually has two peaks in the subsequent year of an El Nino event: one around February and the other around August (Wang et al., 2006a). During these two periods, the change in atmospheric circulation alters the near-surface air temperature, humidity, cloudiness, and monsoon wind, resulting in an enhanced surface heat flux that warms the SCS surface temperature. The warming can also be attributed to the ocean s mixed layer dynamics (e.g., Shaw et al., 1999). As the northeast winter monsoon weakens during and after the mature phase of El Nino (Tomita and Yasunari, 1996), the mixed layer depth in the SCS (MLD) shoals, and the resulting detrainment inhibits the incoming surface heat flux from being transported downward, making an additional contribution to the warming in SST (e.g., Wang et al., 2000, 2006a). The MLD variability in the SCS is closely related to the SCSTF heat transport (e.g., Chu and Chang, 1997; Qu, 2001; Liu et al., 2004; Gan et al., 2006; Qu et al., 2007). Based on a high-resolution GCM, Qu et al. (2004) noted that variability in the SCSTF explains a large fraction of the SCS upper layer heat content change. As the SCSTF and surface heat flux are not tightly coupled and they display substantially different time scales of variability (e.g., Yu and Weller, 2007), the upper layer SCS acts as a heat capacitor, storing heat in certain years and releasing it in others (Qu et al., 2006a). This result implies that the SCS is likely to play an important role in regional climate variability. The details need to be investigated further Influence on the Indonesian sea surface temperature Sea surface temperature in the Indonesian seas is influenced both locally by the monsoons and remotely by the SCSTF/ITF. Although the total SCSTF/ITF transport is insensitive to the detailed topography (Godfrey, 1989), SST within the Indonesian archipelago strongly depends on the pathways the SCSTF/ITF takes. According to a theory put forward by Wajsowicz (1993), the ITF always tends to flow through the westernmost passage, and this explains why up to 80% of the ITF transport weaves through the Makassar strait (e.g., Gordon et al., 2003; Susanto and Gorden, 2005). The SCSTF is weaker than the ITF, simply because its exit passages (i.e., the Karimata and Mindoro straits) are shallower. The sensitivity of SCSTF to bottom topography in the Mindoro and Luzon straits has been discussed by Metzger and Hurlburt (1996, 2001). In recent numerical experiments, McCreary et al. (2007, personal communication) found that almost all of the SCSTF/ITF would take the Java Sea pathway, if the Karimata strait were deeper than 700 m in the model. All these results suggest that the SCSTF/ITF variability and its partition between different passages are important for a better understanding of SST variability in the Indonesian Seas Influence on the Indo-Pacific warm pool Cold Pacific water enters the SCS through the Luzon strait (in terms of transport-weighted temperature). Most of this water upwells and exits the SCS through the Karimata and Mindoro straits in the

9 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) surface layer, taking up to PW of heat and 0.1 Sv of freshwater from the SCS into the tropical Indian and Pacific Oceans. Variation in this heat and freshwater transport may have a notable impact on SST distributions in the tropical Indian and Pacific Oceans, particularly in the region off Java and Sumatra, where heat advection by the ITF contributes directly to the coupled climate system (Saji et al., 1999; Annamalai et al., 2003; Du et al., 2005). Results from a numerical experiment suggest that blocking the SCSTF would result in a slight cooling in the tropical western Pacific and a slight warming in the tropical eastern Indian Ocean (Tozuka et al., 2007), but the details remain unknown at this time. 7. Discussion Analyses of existing observations and results from ocean GCMs have shown that the SCSTF is a heat and freshwater conveyor, which may have an important influence on the SCS heat content, the pathway and vertical structure of the ITF, and the heat and freshwater transport from the Pacific into the Indian Ocean. Preliminary model experiments suggest that it is the SCSTF that forces the ITF into a deeper level (near the thermocline) and reduces its heat transport in the Makassar strait. The implication of this result is that the SCSTF is likely to play an important role in regulating SST patterns in the SCS and its adjoining tropical Indian and Pacific Oceans. Given the importance of the region s SST in world s climate, we believe that a comprehensive study of the SCSTF is necessary. It will enhance our understanding of the ocean s role in governing the East Asian monsoon, the Pacific ENSO, and probably the Indian Ocean Dipole, an important step toward climate predictions. Despite considerable progress that has been made in the past years, our understanding of the SCSTF is far from complete. Numerous issues and questions still remain. Most fundamental is the lack of accurate estimates of the SCSTF. Yet, direct measurements of the SCSTF have proven difficult, due to the complicated coastline and topography of the SCS and to the highly variable nature of the current. The existing model estimates are not sufficiently constrained by available observations and differ in some significant ways. How the SCSTF varies and what processes are responsible for its variability are questions that need immediate attention in future study. Developing coherent measurements of transports through the Luzon, Taiwan, Mindoro, and Karimata straits is a key to this fundamental issue, and requires collaborative efforts from international communities. Concerted planning is now underway, involving scientists from China, Japan, Indonesia, U.S. and other countries in the Asia-Pacific region. With more coherent measurements combined with results from high-resolution GCMs, we anticipate an improved understanding of the SCSTF and its variability. Given that a significant portion of the SCSTF is driven by a deepwater overflow through the Luzon strait (Qu et al., 2006b), vertical mixing is also an important issue to be addressed in future study. Prior studies have shown that to achieve a mass balance, vertical mixing must be as strong as 10 3 m 2 s 1 in the mid- and deep-layers of the SCS (Wang, 1986). It is the strong vertical mixing associated with baroclinic semidiurnal tides and high-frequency internal waves that enhances the entrainment of thermocline waters into the surface mixed layer and maintains the SCSTF. Only limited observations are available at this time (e.g., Tian et al., 2006), and more work needs to be done in order to get better parameterizations of mixing for ocean models in the SCS. Another important issue left to be addressed is the SCSTF influence on the regional sea surface temperature. For example, how are the SCS mixed layer depth and surface temperature related to the SCSTF variability? Can the SCSTF play a role in modulating conditions of the Indo-Pacific warm pool? Is the SCSTF thermodynamically active in the coupled ocean-atmosphere system of the region? If so, what s its role? The existing studies summarized in this paper have addressed some aspects of these questions, but most of them need to be investigated in future studies. A major thrust of the future study is to identify the role of the SCSTF in modulating the Southeast Asian monsoon, the Pacific ENSO, and the Indian Ocean Dipole, and provide useful guidance for prediction of climate in the region. Acknowledgments This research was supported by the U.S. National Science Foundation through grant OCE and by the Japan Society for Promotion of Science through Grant-in-Aid for Scientific Research (A)

10 12 T. Qu et al. / Dynamics of Atmospheres and Oceans 47 (2009) Support for TQ was also from Japan Agency for Marine-Earth Science and Technology, from the National Ocean and Atmosphere Administration, and from the National Aeronautics and Space Administration through their sponsorship of the International Pacific Research Center (IPRC). The authors are grateful to A.M. Moore for his strong support for the special issue. School of Ocean and Earth Science and Technology contribution number 7471, and IPRC contribution number IPRC-525. References Annamalai, H., Murtugudde, R., Potemra, J., Xie, S.P., Liu, P., Wang, B., Coupled dynamics over the Indian Ocean: spring initiation of the Zonal Mode. Deep-Sea Res. II 50, Ashok, K., Guan, Z., Yamagata, T., Impact of the Indian Ocean dipole on the relationship between Indian Ocean monsoon rainfall and ENSO. Geophys. Res. 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