Dynamically Active Areas in the South China Sea Detected from TOPEX/POSEIDON Satellite Altimeter Data Chung-Ru Ho,* Nan-Jung Kuo,* Quanan Zheng, † and Yin S. Soong ‡ TOPEX/POSEIDON (T/P) altimeter measurements ern portion of the sea (Fig. 1). This basin is surrounded from December 1992 to April 1998 were processed to by South China, the Philippines, Borneo Island, and the study the sea surface height (SSH) variation in the South Indo-China Peninsula. Its water body connects with the China Sea (SCS). Patterns of variance of the SSH devia- East China Sea, the Pacific, and the Indian Ocean tions from the four-year (1993–1996) mean show that through the Taiwan Strait, the Luzon Strait, and the there are three high-variation areas in the deep basin. Strait of Malacca, respectively. Therefore, the SCS has The first is at the northern basin west of Luzon Island been since ancient times the most important navigation (16°–18° N, 117°–119° E), the second is at the center of channel from East Asia to Southeast Asia and the Indian the SCS (14°–16° N, 113.5°–114.5° E), and the third is Ocean. The wind fields of the sea belong to the type of off the central Vietnam coast (12°–13° N, 110°–112° E). tropical monsoon. In boreal summer, warm southwest- From empirical orthogonal function (EOF) analysis, we erly winds prevail over the SCS, with an average wind found that the variations of these three active areas are speed of 6 m/s. In boreal winter, the wind direction to- dominated by annual cycles, but out of phase. The first tally reverses: cold and stronger northeasterly winds with two areas have the highest SSH in June and the lowest an average speed of 9 m/s blow over the region. SSH in December. The third one off the Vietnam coast The sea surface variation in the SCS has widely been has the highest SSH in February and the lowest SSH in investigated using sea surface temperature (SST). Dale August. The EOF analysis also reveals that the variation (1956) documented a cold SST phenomenon off the cen- of SSH in the SCS has a response to El Nin ˜ o events. tral Vietnam coast in summer. Nitani (1970) found a cold Elsevier Science Inc., 2000 pool at the northeast of the SCS off Luzon Island. Re- cently, the pool was intensively investigated using SST measurements (Chu et al., 1997a, 1997b) and airborne INTRODUCTION expendable bathythermographics (AXBT) data (Chu et al., 1998b). They found that the mesoscale pool is not The South China Sea (SCS) is the largest marginal sea always cold; in boreal spring, it is warm. Numerical mod- in southeast Asia. The bottom topography of the sea is els were also used to study sea surface thermal variations characterized by two extended continental shelves on the (Chu et al., 1998a) and velocity variations (Wu et al., northern and southern sides and a deep basin with a 1998). Their results confirmed that wind effects are the maximum depth of 5000 m situated in the central-east- major factor causing these variations. Chao et al. (1996), using a numerical model, indicated that the circulation * Department of Oceanography, National Taiwan Ocean Univer- in the SCS is modulated by El Nin ˜ o events. Satellite re- sity, Keelung, Taiwan mote sensing was also applied to observation of the me- † College of Marine Studies, University of Delaware, Newark, DE soscale variation in the SCS. Using NOAA (National ‡ Earth Science Department, Millersville University, Millers- ville, PA Oceanic and Atmospheric Administration) satellite Ad- Address correspondence to Chung-Ru Ho, National Taiwan vanced Very High Resolution Radiometer (AVHRR) im- Ocean University, Department of Oceanography, Keelung, 20243 Tai- ages, Soong et al. (1995) detected a cold mesoscale eddy wan. E-mail: chungru@sun4.oce.ntou.edu.tw Received 21 April 1999; revised 24 August 1999. and verified its existence using TOPEX/POSEIDON (T/ REMOTE SENS. ENVIRON. 71:320–328 (2000) Elsevier Science Inc., 2000 0034-4257/00/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(99)00094-2