NTU Kuroshio Study Selected as Cover Article for Deep-Sea Research I

Dr. Ming-Huei Chang (張明輝; Figure 1), an assistant professor at the NTU Institute of Oceanography, reveals the physical processes underlying the Kuroshio meandering and transport variability east of Taiwan using an unprecedented two-year field observational dataset. The findings of Assistant Prof. Chang’s research are crucial to the application of ocean current energy, the management of fishery resources, the observation of global climate change, as well as the evolution of typhoons. This research is therefore selected by the Editors-in-Chief of Deep-Sea Research I as the cover article for the January 2018 issue (Figure 2), showing that the Kuroshio study is of both local interest and global importance.

The Kuroshio is the western boundary current in the Northwest Pacific Ocean. It transports tremendous heat, salt, and water mass from east of Luzon Island, through the Luzon Strait, along the east coast of Taiwan and the continental shelf break in the East China Sea to south of Japan. The current is important in affecting marine environments, weather, climate, as well as global change along its route. The importance of Kuroshio research therefore lies in helping better understand the dynamics of Kuroshio’s variability and, in turn, better predict its influence on marine environments and weather/climate.

The pioneering long-term Kuroshio observations were conducted at an array of moored current meters along the PCM-1 line in the East Taiwan Channel (ETC) during the World Ocean Circulation Experiment (WOCE) from September 1994 to May 1996. Observations at the PCM-1 array have helped provide insights into the variability of the Kuroshio and have been the base of Kuroshio studies over the past two decades. However, the observed Kuroshio variability, namely, the transport variations and the maximum velocity axis migrations, does not exhibit some of the typical characteristics of the Kuroshio-eddy interactions because the PCM-1 array spans the ETC. The ETC and the Ryukyu Islands chain east of the channel act somewhat as a regulator filtering out some of the variability induced by the eddy-Kuroshio interactions. The observations of Assistant Prof. Chang are part of a dataset collected through collaborative programs, including the Observations of the Kuroshio Transport Variability (OKTV) sponsored by the Ministry of Science and Technology (MOST), Taiwan and the Observations of the Kuroshio and Mindanao Currents (OKMC) supported by the US Office of Naval Research. Assistant Prof. Chang led technicians and assistants in the NTU Institute of Oceanography to maintain a cross-stream transect ~50 km south of the PCM-1 line with a 23-month (2012/11-2014/10) continuous measurement using three moored acoustic Doppler current profilers (ADCPs). The spotlights of Assistant Prof. Chang’s findings are:

1. The Kuroshio transport is 4.3 Sv (1 Sv = 1 million cubic meter per second) lower than that observed at the PCM-1 line, presumably due to the interannual variations related to the abundance of mesoscale eddies in the Subtropical Counter Current (STCC) region.
2. Our observations exhibit more energetic Kuroshio variability in its axis migration at the KTV1 than at the PCM-1 line. The relatively stable Kuroshio in PCM-1 is attributed to the presence of ETC and the Ryukyu Islands chain where the topography behaves as a regulator damping the interaction with eddies in PCM-1.
3. Transport variability east of Taiwan is mostly caused by Kuroshio-eddy interactions (Figure 3). When single anticyclonic (cyclonic) eddies encounter the Kuroshio, they enhance (reduce) poleward transport, presumably by increasing (decreasing) the sea level anomaly (SLA) along the eastern flank of the Kuroshio (correlation = 0.82). When a pair of eddies impinges on the Kuroshio, the upstream confluence and diffluence caused by the dipole eddies increase and decrease the Kuroshio transport, respectively. Furthermore, the eastward (westward) currents that result from either the single eddy or the dipole eddy produce flow divergence (convergence) adjacent to the Kuroshio's eastern edge, favoring the offshore (onshore) migration of the Kuroshio axis.

The citation of this paper is: Chang, M.-H., S. Jan, V. Mensah, M. Andres, L. Rainville, Y. J. Yang, Y.-H. Cheng (2018), Zonal migration and transport variations of the Kuroshio east of Taiwan induced by eddy impingements, Deep-Sea Research I, https://doi.org/10.1016/j.dsr.2017.11.006, and it is available at http://www.sciencedirect.com/science/article/pii/S0967063717301565.