OceThe OFFiciala MaganZineog OF the Oceanographyra Spocietyhy CITATION Rudnick, D.L., S. Jan, L. Centurioni, C.M. Lee, R.-C. Lien, J. Wang, D.-K. Lee, R.-S. Tseng, Y.Y. Kim, and C.-S. Chern. 2011. Seasonal and mesoscale variability of the Kuroshio near its origin. Oceanography 24(4):52–63, http://dx.doi.org/10.5670/oceanog.2011.94. DOI http://dx.doi.org/10.5670/oceanog.2011.94 COPYRIGHT This article has been published inOceanography , Volume 24, Number 4, a quarterly journal of The Oceanography Society. Copyright 2011 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. downloaded From http://www.tos.org/oceanography SPECIAL IssUE ON THE OCEANOGRAPHY OF TAIWAN Seasonal and Mesoscale Variability of the Kuroshio Near Its Origin BY DANIEL L. RUdnICK, SEN JAN, LUCA CENTURIONI, CRAIG M. LEE, REN-CHIEH LIEN, JOE WANG, DONG-KYU LEE, RUO-SHAN TsENG, YOO YIN KIM, And CHING-SHENG CHERN Underwater photo of a glider taken off Palau just before recovery. Note the barnacle growth on the glider, fish underneath, and the twin hulls of a catamaran used for recovery in the distance. Photo credit: Robert Todd 52 Oceanography | Vol.24, No.4 AbsTRACT. The Kuroshio is the most important current in the North Pacific. Here, we present historical data and recent observations of the Kuroshio off the coasts of Taiwan and the Philippine Archipelago, with a focus on its origins. Seasonal climatologies from shipboard hydrographic and velocity measurements, and from surface drifters, demonstrate changes in the Kuroshio caused by the monsoon. In particular, seasonal monsoon forcing affects the degree of penetration of the Kuroshio through Luzon Strait. Data from surface drifters and underwater gliders describe its mesoscale variability. Velocities derived from drifters make clear the mesoscale variability associated with the Subtropical Countercurrent east of the Kuroshio. Underwater gliders document mesoscale structure prominent in salinity extrema associated with water masses. The evolution of these water masses as they progress northward near the Kuroshio indicates strong mixing in the region. InTRODUCTION Kuroshio passes between Taiwan and the Three major currents dominate the Ryukyu Islands, it is a strong, coherent general circulation in the western Pacific current with a mean transport of 21 Sv offshore of Taiwan and the Philippines (1 Sv = 106 m3 s–1) (Johns et al., 2001). (Figure 1): the North Equatorial Current The NEC supplies the western (NEC) flows westward, runs into the boundary with two distinct water Philippine coast, and bifurcates into the masses marked by salinity extrema. northward-flowing Kuroshio and the North Pacific Tropical Water (NPTW) southward-flowing Mindanao Current. is marked by high salinity near 200 m As the terminus of the NEC, and the depth, while North Pacific Intermediate origin of the Kuroshio and Mindanao Water (NPIW) is well known as a Current, this bifurcation region is glob- salinity minimum near 500 m depth. ally important. Of these three currents, On the basis of chlorofluorocarbon the Kuroshio has probably received the observations, Fine et al. (1994) conclude most study through observation, theory, that NPTW was in contact with the and modeling. Here, we review some of atmosphere less than three years before this work and present new findings, with reaching the western boundary, while a focus on variability. NPIW is less than 20 years old. As The Kuroshio starts as a relatively the water proceeds northward in the poorly defined current flowing from Kuroshio, the modification of these the bifurcation region (Figure 1). The water masses is evident. current flows northward to the northern The Kuroshio’s origin is marked by tip of Luzon Island, where it veers strong mesoscale variability, so strong, westward through Luzon Strait into the in fact, that it can overwhelm the mean South China Sea (SCS). The Kuroshio flow offshore of Luzon. The mesoscale generally does not penetrate very far variability is due to at least two sources: into the SCS, but penetrating incursions (1) the instability of the Kuroshio itself, happen occasionally. The Kuroshio and (2) westward-propagating eddies becomes well established as it flows arriving at the western boundary. northeastward out of Luzon Strait along The eastward-flowing Subtropical the east coast of Taiwan. By the time the Countercurrent (STCC) is a region of Oceanography | December 2011 53 0 observations of variability in the 24°N Taiwan Kuroshio and environs from recent 1000 22°N observational campaigns. Shipboard acoustic Doppler current profiler (ADCP) Kuroshio 20°N 2000 and surface drifter data reveal variability in currents. Autonomous underwater 18°N 3000 Luzon gliders are used to quantify variations in 16°N hydrographic properties. A special focus North Equatorial Current 4000 is the passage of the Kuroshio across the 14°N Depth (m) mouth of Luzon Strait. Mindanao Curr 5000 12°N BOTTOM TOPOGRAPHY 10°N 6000 The seafloor surrounding Taiwan has complicated topography (Figure 2). East 8°N en t Palau 7000 of Taiwan, tectonic processes related indanao M to the flipping of subduction direction 6°N 8000 lead to great seafloor roughness. The 120°E 125°E 130°E 135°E Philippine Sea Plate subducts beneath Figure 1. Schematic diagram of currents in the western Pacific off Taiwan and the Philippines. The North Equatorial Current (NEC) flows westward where the Eurasia Plate along the Ryukyu it runs into the Philippine coast. The end of the NEC is the beginning of the Trench. However, the Luzon Island Arc northward Kuroshio and the southward Mindanao Current. The Kuroshio starts that belongs to the Philippine Sea Plate as a weak, unorganized current, and strengthens as it flows northward. thrusts over the Eurasia continental margin (Liu et al., 1998). As a result, the topography east of Taiwan can be sepa- enhanced eddy activity centered at about source, the region offshore of Taiwan has rated into three parts. The northern part 22°N (Qiu and Chen, 2010), presumably the second strongest mesoscale eddies in is comprised of the northwest-southeast because of instability in the STCC. Qiu the entire North Pacific. The only region trending Okinawa Trough adjacent to and Chen (2010) also find interannual of stronger mesoscale variability in the the east-west trending Ryukyu Island changes in the strength of mesoscale North Pacific is the Kuroshio Extension Arc to the south. The middle part, variability related to vertical shear in offshore of Japan. roughly between 23°N and 24.5°N, is both STCC and NEC. Regardless of In the following, we summarize a 4,000–6,000 m deep basin called the Huatung Basin. Located south of the Daniel L. Rudnick ([email protected]) is Professor, Scripps Institution of Oceanography, east-west trending Yaeyama Ridge, the La Jolla, CA, USA. Sen Jan is Associate Professor, Institute of Oceanography, National Ryukyu Trench, with its maximal depth Taiwan University, Taipei, Taiwan. Luca Centurioni is Associate Project Scientist, Scripps over 6,200 m, becomes shallow toward Institution of Oceanography, La Jolla, CA, USA. Craig M. Lee is Principal Oceanographer, the west and loses its topographic char- Applied Physics Laboratory, University of Washington, Seattle, WA, USA. Ren-Chieh Lien is acter west of 122.5°E. In the southern Principal Oceanographer and Affiliate Professor, Applied Physics Laboratory, University of part, the north-south trending Luzon Washington, Seattle, WA, USA. Joe Wang is Professor, Institute of Oceanography, National Island Arc, which is the southern bound Taiwan University, Taipei, Taiwan. Dong-Kyu Lee is Visiting Researcher, Scripps Institution of the Huatung Basin, connects to the of Oceanography, La Jolla, CA, USA. Ruo-Shan Tseng is Professor, Department of Marine Coastal Mountain Range in south- Resources, National Sun Yat-sen University, Kaohsiung, Taiwan. Yoo Yin Kim is Senior eastern Taiwan. Green Island and Lanyu Statistician, Scripps Institution of Oceanography, La Jolla, CA, USA. Ching-Sheng Chern is Island lie on the Luzon Island Arc off Professor, Institute of Oceanography, National Taiwan University, Taipei, Taiwan. southeastern Taiwan. 54 Oceanography | Vol.24, No.4 WInds search/rescue, and general education. both hydrographic and velocity data Winds measured by satellite remote To update the climatology of the in each grid are interpolated to the sensing as well as at island weather Kuroshio east of Taiwan after Liang center of the grid and then vertically stations off eastern Taiwan suggest that et al. (2003), we analyze hydrography interpolated at 10 m depth intervals. The the East Asia monsoon dominates the and current velocity data from the corresponding climatological mean of change of wind, which is southwesterly Ocean Data Bank collected before 2009. hydrography and velocity are calculated to southerly in summer and northeast- The shipboard ADCP ocean current at each grid when the data number erly in winter. The winter northeasterly velocity is blended with wind-driven is greater than three for hydrography monsoon commences in mid-September, and tidal components fluctuating at and 30 for velocity data in each grid. prevails from October to January, periods from a half-day to seasons. The It is natural to separate data collected and weakens continuously thereafter. barotropic tidal currents computed in summer and winter because of the The summer southwesterly monsoon, by a two-dimensional numerical tidal distinct monsoonal wind forcing. prevailing from June to August, is much model described in Hu et al. (2010) are Accordingly, summer is defined as from weaker than the winter monsoon, in subtracted from the ADCP velocity data.