IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 29, NO. 4, OCTOBER 2004 1131 Shelfbreak Circulation and Thermohaline Structure in the Northern South China Sea—Contrasting Spring Conditions in 2000 and 2001 Glen Gawarkiewicz, Joe Wang, Michael Caruso, Steven R. Ramp, Kenneth H. Brink, and Frank Bahr Abstract—Two intensive, high-resolution hydrographic surveys during April 2000 and May 2001 are used to characterize the thermohaline and current structure at the shelfbreak in the South China Sea. In 2000, a strong anticyclonic circulation was present in the northern portion of the South China Sea with strong onshore currents east of Dongsha Island. The flow became polarized along isobaths as it encountered shallow water, with northeastward flows of over 0.9 m/s along steep topography. The flow was driven by strong density contrasts between waters of the outer shelf and upper slope. Shelf water was both cooler and more fresh than the water offshore, which had salinities close to that of Kuroshio water. In contrast, the mean flow in the northern South China Sea was predominantly cyclonic in 2001. Flow over the slope was to the southwest at up to 0.2 m/s. The water mass properties of the outer shelf and upper slope were similar, so that there were not the strong cross-shelf density gradients present as in 2000. A potential differ- ence between the water mass structure of the two years was the difference in cooling during the preceding winters. In December, 1999, unusually strong cooling may have resulted in cooler shelf waters relative to the following year. The ASIAEX study area may be a particularly sensitive region to both seasonal and interannual variability, as it is near a bifurcation point associated with the Kuroshio Intrusion into the South China Sea. Index Terms—Acoustic propagation, environmental factors, fluid flow, sea, sea measurements, underwater acoustics, wind. I. INTRODUCTION T HE CIRCULATION of the South China Sea is complex, with Kuroshio Intrusions via Luzon Strait [19], seasonal monsoon forcing, and adjacent straits such as Taiwan Strait af- fecting the flows over the continental shelf and slope. A re- cent review article [8] notes that the Kuroshio Intrusion may have three different forms, with a loop mode, an eddy-shed- ding mode, and a leaping mode entering the South China Sea only a short distance. An important issue dynamically is the role of the winter monsoon in forcing the Kuroshio Intrusion. Farris and Wimbush [5] established a criterion for initiating in- trusions based on the duration and magnitude of the wind stress from the monsoon. They found that it took a wind stress of over 0.08 for four days or longer to result in an intrusion Manuscript received February 8, 2003; revised February 26, 2004. This work was supported by the Office of Naval Research under Grant N-00014-00-1- 0210 and the National Science Council of ROC (Taiwan) under Grant NSC-90- 2611-M-002-016-OP2. G. Gawarkiewicz, M. Caruso, K. H. Brink, and F. Bahr are with the Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA. J. Wang is with the National Taiwan University, Taipei, Taiwan. S. R. Ramp is with the Naval Postgraduate School, Monterey, CA 93943 USA. Digital Object Identifier 10.1109/JOE.2004.839123 into the South China Sea. The South China Sea also has sig- nificant mesoscale variability (e.g., [23]) with different regional characteristics. Theoretical studies have examined issues of predictability and hysteresis in the current system [22]. The primary balance governing the flow across a gap was between the beta effect car- rying flow to the west versus inertia carrying flow across a gap. Nof [17] has also examined flow across gaps using two-layered models to determine the effect of rotation on inflows to mar- ginal seas. A more complicated layer model was used to study the Kuroshio Inflow [13]. They found, using a layer model which coupled the South China Sea to the North Pacific, that Kuroshio Intrusions into the South China Sea were associated with the winter monsoon, and also established criteria for the emergence of the Intrusion. In a second study, Metzger and Hurlburt [14] found that the presence of the Kuroshio Intrusion was very sensitive to mesoscale flow instabilities and was not strongly related to wind stress curl, wind stress magnitudes, and meridional and zonal wind components. A recent model study examined the seasonality of the circu- lation in the South China Sea [2]. They used a primitive equa- tion model with monthly mean wind stress forcing to generate the seasonal cycle within the South China Sea. They found that the winter monsoon generates a large cyclonic circulation in the central and southern part of the South China Sea along with an anti-cyclonic circulation in the northern portion. As the winter monsoon weakens, the anti-cyclonic gyre weakens and disap- pears, while the cyclonic gyre moves to the northwest and also weakens. A schematic of this appears in Fig. 1. The circulation during spring is complicated because it marks the transition be- tween the very different conditions of winter and summer. As part of the Asian Seas International Acoustics Experi- ment (ASIAEX) in the South China Sea, hydrographic surveys in April 2000 and May 2001 were undertaken to determine the large scale flow pattern as well as the more detailed flow near the edge of the continental shelf. The intent was to examine vari- ability within this region in order to establish the potential influ- ence of mesoscale features on acoustic propagation. As we will see, the flow in spring 2000, was typical of the winter pattern described by Chern and Wang, while the flow in spring 2001 was more typical of the summer pattern described by Chern and Wang. The results presented here are the first high-resolution sur- veys of the outer shelf and upper continental slope, and thus rep- resentthe best-resolved thermohaline fields within this region. Previous hydrographic surveys of the shelfbreak region in this 0364-9059/04$20.00 © 2004 IEEE