South China Sea throughflow as evidenced by satellite images and numerical experiments

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 34,, doi: /2006gl028103, 2007 South China Sea throughflow as evidenced by satellite images and numerical experiments Z. Yu, 1 S. Shen, 2 J. P. McCreary, 1 M. Yaremchuk, 1 and R. Furue 1 Received 6 September 2006; revised 24 November 2006; accepted 1 December 2006; published 4 January [1] The South China Sea throughflow begins at the Luzon Strait, as an intrusion of the Kuroshio. At the present time, there are insufficient in situ measurements either to estimate accurately the transport loss or to provide a clear picture of the Kuroshio pathway at the Luzon Strait. In this study, we use newly available, multi-year, high-resolution satellite images and a numerical model to track the warm, relatively low-biomass, Pacific water carried by the Kuroshio. A suite of numerical experiments are carried out to identify key factors that influence Kuroshio paths at the Luzon Strait. The model can reproduce the satellite-inferred Kuroshio paths across the Luzon Strait only when a significant amount of the Kuroshio water is allowed to enter the Luzon Strait during December February, therefore providing strong evidence for the existence of the South China Sea throughflow. Citation: Yu, Z., S. Shen, J. P. McCreary, M. Yaremchuk, and R. Furue (2007), South China Sea throughflow as evidenced by satellite images and numerical experiments, Geophys. Res. Lett., 34,, doi: /2006gl Introduction [2] As a major western boundary current, the Kuroshio carries a significant amount of heat from low to high latitudes, a process of critical importance for North Pacific climate. Part of the Kuroshio transport enters the South China Sea (SCS) at the Luzon Strait. In recent years, estimated values of the Luzon Strait transport (LST) have converged towards an annual average of 3 4 Sv (1 Sv = 10 6 m 3 /s), with a maximum in January and a minimum in July [e.g., Metzger and Hurlburt, 1996; Qu, 2000; Lebedev and Yaremchuk, 2000; Fang et al., 2005; Cai et al., 2005]. This potential loss of more than 10% of the Kuroshio transport to the SCS represents a significant reduction in the poleward heat transport, because most of the LST goes west through the northern SCS to exit the basin in the south via the Karimata and Mindoro Straits. As such, its interannual variability could impact Indian Ocean [Qu et al., 2005], as well as North Pacific, climate. [3] Previous work suggests that during summer the Kuroshio passes by the Luzon Strait without entering the SCS [Centurioni et al., 2004; Fang et al., 2005]. What is not yet understood is the Kuroshio pathway during winter. There are at least two different views: one that a branch of 1 International Pacific Research Center, University of Hawaii, Honolulu, Hawaii, USA. 2 George Mason University and Goddard Earth Sciences Data and Information Services Center, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. Copyright 2007 by the American Geophysical Union /07/2006GL the Kuroshio enters the SCS [Wyrtki, 1961; Nitani, 1972; Centurioni et al., 2004; Fang et al., 2005], and the other that the Kuroshio loops into the northern SCS, as far west as 118 E [Li and Wu, 1989]. The difficulty in identifying Kuroshio paths at the Luzon Strait is due in part to the lack of in situ measurements and in part to the existence of intense mesoscale eddies across the Luzon Strait [e.g., Gilson and Roemmich, 2002]. There are numerous studies on this topic, including many published in Chinese journals (which can be found in a review paper by Hu et al. [2000]) and two recent ones that focus on transient features in the area using weekly satellite data sets [Caruso et al., 2006; Yuan et al., 2006]. [4] In this study, we use newly available, multi-year, high-resolution satellite images to track Kuroshio paths across the Luzon Strait, and then reproduce these paths in an eddy-resolving ocean model. Our solutions indicate that the Kuroshio path across the Luzon Strait is determined by the transports at three secondary straits, namely, the Taiwan, Karimata, and Mindoro Straits, as well as by the strength of the Kuroshio south of the Luzon Strait. In particular, only when the wintertime transports through these secondary straits are large enough does the Kuroshio pathway mimic the satellite observations, strong evidence for the existence of the SCS throughflow. 2. Ocean Model [5] The model is nearly identical to the one described by Yu and Potemra [2006]. It consists of four active layers with variables of thicknesses h i, velocities v i, salinities S i, and temperatures T i (layer index i = 1, 2, 3, or 4), overlying a deep, inert ocean where the pressure gradient vanishes (a layer system). Each of the layers represents water generated primarily by a specific process, and hence corresponds mostly to a single water-mass type: Layer 1 is the surface mixed layer, determined by Kraus and Turner [1967] physics; layer 2 is the seasonal thermocline; and layers 3 and 4 represent lower-thermocline and upperintermediate waters, respectively. To simulate the processes of upwelling, subduction and diapycnal mixing, fluid is allowed to transfer across the interfaces between adjacent layers. When this occurs, mass, heat and salt remain conserved. To prevent the mixed layer from becoming too deep in winter, we set the wind-stirring and cooling-efficiency ð parameters in the Kraus-Turner model to m = e h 1=h m Þ, h m = 300 m, and n = 0, respectively Model Basin, Initial Conditions, and Spin Up [6] The model setup is very similar to that by Shaw and Chao [1994]. The basin includes the SCS, and a small part of the Pacific Ocean east of the Luzon Strait to allow for the 1of6

2 YU ET AL.: SOUTH CHINA SEA THROUGHFLOW McCreary et al. [1993]. Forcings for Q sw, longwave radiation, and precipitation are taken from the Comprehensive Ocean-Atmosphere Data Set prepared at the University of Wisconsin-Milwaukee, constrained as recommended by the authors based on a global budget obtained from an oceanic general circulation model [da Silva et al., 1994]; specifically, we use 93% of the Q sw and 112% of the precipitation. Monthly-mean climatologies of wind stress, wind speed, and cubic wind speed are determined from surface winds from the European Centre for Medium-Range Weather Forecasts for (The 2005 data is not yet available to us.) In addition, the minimum wind speed used for the latent heat calculation is set to 8 m/s; without this minimum the model s T 1 is 1 2 C warmer in summer (not shown) Inflow and Outflow [9] Closed, no-slip conditions, u i = v i = 0, are imposed on all basin boundaries, except at the five inflow/outflow ports indicated in Figure 1. The transport for each layer i across Karimata Strait (KS), Mindoro Strait (MS), and Taiwan Strait (TS), and for Kuroshio across 17 N and 25 N are set according to Figure 1. South China Sea (SCS) throughflow. The surface flows shown are December February means from our base experiment. A branch of the Kuroshio enters the SCS after it impinges upon Taiwan, and joins the interior SCS wind-driven circulation to travel anticlockwise around the basin before it exits primarily via the Karimata Strait. The lighter shades mark water depths shallower than 100 m and 200 m, respectively. The inflow/outflow ports are marked by hatched lines. Kuroshio (Figure 1). Essentially, then, the influence of the Pacific Ocean on the SCS is reduced to the transport carried by the Kuroshio. To determine basin boundaries, model grid points are taken to be land wherever the ocean depth is shallower than 200 m in the ETOPO5 data, except for the model Karimata Strait, which is shaped more like the 100-m isobath. We specify the transports through the three secondary straits, as well as the Kuroshio transports at 17 N and 25 N (see section 2.3). [7] The resolution of the model grid is 0.1, and the integration time step is 10 minutes. Initial temperature and salinity fields are based on mean climatological fields taken from the World Ocean Atlas [Conkright et al., 1998] at depths of 20, 50, 200, and 600 m, respectively. The model is spun up from a state of rest for 3 years, by which time the two upper layers of the model have very nearly adjusted to their equilibrium states. The fields used for analyses are based on 5-day sampling of the output from model years Interior Forcing [8] Because the model is thermodynamically active, surface boundary conditions include forcing by heat, freshwater and momentum fluxes. Climatological monthly-mean shortwave radiation (Q sw ), longwave radiation, air temperature, and specific humidity, together with the model s T 1 field, are used in the calculation of sensible and latent heat fluxes, as specified by McCreary and Kundu [1989] and M iðstrþ ¼ f iðstrþ M str ; ð1þ where M str is the transport through a particular port (str), with positive values for transports out of the SCS via the three secondary straits or for the northward Kuroshio. The nondimensional factor f i(str) specifies the vertical partition of transport in each layer. The Taiwan and Karimata Straits are both shallower than 100 m, so we set f i(ts) = f i(ks) =0 for i = 3 and 4, to allow water exchange only through layers 1 and 2; the Mindoro Strait is about 400 m deep, so we set f 3(MS) =.5 and f 4(MS) = 0. The partition, f 1 /f 2 = h 1 /h 2,is applied at each port, because the upper two layers are tightly connected by mixed layer dynamics. The sum of the transports through the three secondary straits is the strength of the SCS throughflow, that is, M TF = M KS + M MS + M TS. Clearly, M TF = LST, the net loss of the Kuroshio water to the SCS. This relationship nicely expresses the importance of the LST, summarizing its role in connecting the Pacific Ocean to the Indonesian Sea, as well as its impact on the dynamic balance of the entire SCS. [10] For the Kuroshio, we distribute the transports M 17N and M 25N = M 17N M TF uniformly among layers (1 + 2), 3, and 4, setting f 1(lat) + f 2(lat) = f 3(lat) = f 4(lat) = 1 3, where lat = 17 N and 25 N, respectively. Since the thicknesses of layers (1 + 2), layer 3, and layer 4 are of the order of 100 m, 200 m, and 400 m, respectively, the vertical partition thus gives a much swifter Kuroshio in the upper 300-m depth, as in many observational and modeling results [e.g., Gilson and Roemmich, 2002; Shaw and Chao, 1994]. Note that M 25N = M 17N M TF ensures the mass conservation in the regional model. [11] Figure 2 plots the seasonally varying transports that are imposed at basin ports. The solid curves are all dynamically constrained estimates obtained through assimilations of available observations into ocean models. Specifically, M 17N in Figure 2a is based on 3-dimensional variational (3dVAR) data assimilation [Yaremchuk and Qu, 2004], which is similar in magnitude and phase to that used 2of6

3 YU ET AL.: SOUTH CHINA SEA THROUGHFLOW winter [e.g., Fang et al., 1991]. For this reason, we will use a synthetic curve for MTS, which has the same annual mean as the 4dVAR curve but with its minimum shifted to December (dashed curve in Figure 2b). The sensitivity of our solutions to the magnitude of these transport estimates is discussed in section 3. [12] Temperature and salinity of water associated with M17N are (T1, T2, T3, T4) = (T1obs, 26, 18.3, 8.4) C and (S1, S2, S3, S4) = (S1obs, 34.6, 34.78, 34.4) psu, selected according to Boyer et al. [2002] and Qu et al. [2000]. Boundary conditions at the other ports are zero normal derivatives. 3. Results Figure 2. Transport estimates. (a) Kuroshio transport at 18.5 N based on 3dVAR by Yaremchuk and Qu [2004]. (b) Transports exiting the SCS via the secondary straits based on 4dVAR (M. Yaremchuk, unpublished data, 2005) are shown in solid curves. The dashed curve is the shifted Taiwan Strait transport, to be used as MTS. by Shaw and Chao [1994]. The three solid curves in Figure 2b are based on 4dVAR data assimilation (M. Yaremchuk, unpublished data, 2005); their sum in wintertime (summertime) is similar to (much smaller than) that used by Shaw and Chao [1994], which is based on an early estimate from Wyrtki [1961]. We use the two curves for the Karimata and Mindoro Straits as MKS and MMS, respectively. Although the magnitude of the Taiwan Strait transport from the 4dVAR agrees well with the available data, its phase differs from observation-based estimates, which suggest a maximum in summer and minimum in [13] Figure 3 shows the Chlorophyll a (Chl a) concentration and nighttime sea surface temperature (NSST) near the Luzon Strait. The Chl a maps are based on the 9-km SeaWiFS climatology of September 1997 May 2005 [Campbell et al., 1995], and the NSST maps are based on the 4-km MODIS-Aqua 11-micron data processed by NASA Ocean Biology Processing Group for (B. Franz, online document, 2000, available at oceancolor.gsfc.nasa.gov/docs/modis_sst/). For appearance, 3-point smoothing is applied to each map, and shadings of log10(chl a) and NSST are selected in order to highlight the warm, relatively low-biomass Pacific water carried by the Kuroshio. The Chl a concentration is everywhere much higher in Figure 3a than in Figure 3b, probably due to the strong wintertime mixing induced by the northeasterly monsoon. The wintertime high-nutrient content in the northeastern SCS is attributed to upwelling [Shaw et al., 1996; Tang et al., 1999; Chen et al., 2006b]. [14] As implied by the low-biomass Pacific water in winter, the Kuroshio splits when it encounters the Batan Islands (near E, 20.5 N); its western part extends Figure 3. Chlorophyll a concentration in log10(mg/m3) and nighttime sea surface temperature in C during winter and summer. Light blue contours are isobaths of 100 and 200 m, respectively. 3 of 6

4 YU ET AL.: SOUTH CHINA SEA THROUGHFLOW Figure 4. (a b) Surface-layer tracer and (c d) surface-layer temperature during winter and summer from the base experiment. north-northwestward toward the southern tip of Taiwan and then seems to continue westward following the 200-m isobath. The low-biomass tongue west of Luzon results from the well-known wintertime anticlockwise circulation of the SCS, as shown by Liu et al. [2004]. Tongues of warm NSSTs in Figure 3c suggest a similar Kuroshio pathway as well as a cyclonic SCS circulation. The low-biomass tongue east of 120 E is not well defined in summer (Figure 3b), but seems to take a more northward path after passing the Batan Islands. Note that there is a region of high Chl a east of Taiwan (white-out zone) associated with coastal upwelling and offshore Ekman drift during the southwesterly monsoon. The offshore cooling east of Taiwan in Figure 3d confirms the existence of the coastal upwelling. [15] To support our interpretation of the inferred Kuroshio pathways from Figure 3, we carried out a suite of numerical experiments using the eddy-resolving regional model described in section 2. Figures 4a and 4b show surface-layer flow fields from our base experiment, namely, one forced by MKS, MMS and MTS as labeled in Figure 2b; in addition, the panels show model tracers released in the surface layer along with the imposed Kuroshio inflow for comparison with the Chl a observations. The tracer pathways indicate that during winter the Kuroshio splits into two branches after passing the Batan Islands, and that its western branch bifurcates again at the southern tip of Taiwan. The westernmost branch of the Kuroshio enters the SCS, a small part exits via the Taiwan Strait to the north, and the rest flows along the model coastline to join the cyclonic SCS circulation west of 117 E. The model SCS throughflow mainly exits the basin via the Karimata Strait to the Sunda Shelf, but some also enters the Sulu Sea via the Mindoro Strait as in the observations [Chen et al., 2006a]. The surface-layer temperature pattern west of Luzon during winter (Figure 4c) is quite similar to the NSST in Figure 3c. The model Kuroshio path and the Figure 5. Surface-layer tracer during winter from two test experiments. (a) When MTS = MKS = MMS = 0, the simulated Kuroshio no longer has a branch entering the SCS. It does not resemble a loop current, either. (b) The model Kuroshio loops into the SCS in winter, when M17N is weakened by 50%. 4 of 6

5 YU ET AL.: SOUTH CHINA SEA THROUGHFLOW circulation west of Luzon during summer (Figure 4b) are consistent with the Chl a pattern in Figure 3b: Most of the Kuroshio water bypasses the SCS, and an eastward flow in the SCS between N approaches Luzon and turns northeastward. Note that the model tracers during summer are not attached to the east coast of Taiwan (Figure 4b), and the surface-layer temperature there is cooler (Figure 4d), similar to the satellite observations (Figures 3b and 3d). [16] The Kuroshio branching at the Luzon Strait depends on the M TF. In a test case with the three outflows set to 50% of their values in Figure 2b, the Kuroshio still clearly branches into the SCS (not shown). In contrast, in a test case like the base experiment but with no throughflow (M TS = M KS = M MS = 0) there is no branching (Figure 5a). [17] Finally, none of the preceding model Kuroshio pathways appear as a loop current that penetrates deep into the northern SCS. A recent study by Sheremet [2001] shows that a western boundary current jumps across a gap (like the Luzon Strait) when it is sufficiently strong, but penetrates into the gap as a loop current when it is weak. Our test experiments show that M 17N needs to be weakened by 50% for a loop current to appear (Figure 5b), and that the Kuroshio path changes very little when M 17N is weakened by 30% (not shown). Since the mean Kuroshio transport south of Taiwan is of the order of 20 Sv [Gilson and Roemmich, 2002], it is unlikely to be weak enough to form a loop under normal conditions. A loop could form, however, under the influence of a strong eddy or when there is a significant reduction in the Kuroshio transport (say during a strong El Nino year). Since no loop current is hinted at by either Chl a or NSST monthly maps, we conclude that the monthly-averaged Kuroshio does not loop across the Luzon Strait. 4. Discussion [18] We note that Rossby waves and eddies that propagate into the SCS from the Pacific Ocean are not considered in this study. Some observations suggest that Pacific water enters the SCS in the form of warm core eddies [Li et al., 1997, 1998]. A natural extension of our study is to include such eddies and to understand their interactions with the monthly-mean Kuroshio. More in situ observations and modeling studies are needed to understand the importance of high-frequency variability from the Pacific Ocean. [19] Acknowledgments. This study was supported by NASA through grant NAG and by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) through its sponsorship of the International Pacific Research Center (IPRC). The Goddard Earth Sciences Data and Information Services Center s Web-based visualization tool, Giovanni, is greatly appreciated for the initial data exploration. 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6 YU ET AL.: SOUTH CHINA SEA THROUGHFLOW Wyrtki, K. (1961), Physical oceanography of the Southeast Asian waters: Scientific results of marine investigations of the South China Sea and the Gulf of Thailand, Naga Rep., 2, Yaremchuk, M., and T. Qu (2004), Seasonal variability of the large-scale currents near the coast of the Philippines, J. Phys. Oceanogr., 34, Yu, Z., and J. Potemra (2006), Generation mechanism for the intraseasonal variability in the Indo-Australian basin, J. Geophys. Res., 111, C01013, doi: /2005jc Yuan, D., W. Han, and D. Hu (2006), Surface Kuroshio path in the Luzon Strait area derived from satellite remote sensing data, J. Geophys. Res., 111, C11007, doi: /2005jc R. Furue, J. P. McCreary, M. Yaremchuk, and Z. Yu, International Pacific Research Center, SOEST, University of Hawaii, 1680 East-West Road, Honolulu, HI 96822, USA. (zuojun@hawaii.edu) S. Shen, George Mason University and GES Data and Information Services Center, NASA/GSFC, Code 610.2, Greenbelt, MD 20771, USA. 6of6