NTU HIGHLIGHTS August 2017  
  Research Achievements  

Joint Taiwan-US Field Studies Reveal Reverse Current under Kuroshio

Researchers led by Prof. Sen Jan of the Institute of Oceanography have been working in coordination with counterparts in the United States in recent years to gain insight into the dynamics of the most important ocean current in the western North Pacific Ocean, the Kuroshio.

In 2012, while Prof. Jan headed the field study, the Observations of the Kuroshio Transport and Variability (OKTV), scientists at the United States Office of Naval Research conducted their own study, the Origins of the Kuroshio and Mindanao Current (OKMC). Additionally, another team from the Woods Hole Oceanographic Institution collaborated on the studies and associated data analysis.

The joint findings of thetwo companion field programs were published in the American Geophysical Union's Journal of Geophysical Research: Oceans in March in an article titled, "Downstream Evolution of the Kuroshio's Time-Varying Transport and Velocity Structure."

The article's importance was reflectedin its selection as a "Journal Highlight," which included the editor's comment: "The results reveal that the current's evolution is influenced by factors such as downstream thickening, bathymetric ridges, and eddies from the ocean interior. Given contemporary discussions about the role of Western boundary currents in global observing systems, this is a great example of how boundary observations tie together basin-wide circulation."

As the chief scientist of the OKTV, Prof. Jan worked with Prof. Yiing-Jang Yang, Prof. Ming-Huei Chang, and technicians of the Institute of Oceanography and the crew of NTU's research vessel Ocean Researcher I. The team successfully conducted a series of comprehensive ship surveys and anchored instruments observations using pressure-sensor equipped inverted echo sounders and upward-looking acoustic Doppler current profilers in 2012 and 2015.

Based on the unprecedented in-situ data acquired by the OKTV and OKMC, the researchers revealed that the time-averaged net absolute flow volume transport (i.e., integrated from the surface to the bottom) is 13.7x106m3s-1 (±3.6x106m3s-1) east of Taiwan, where the northward velocity of the Kuroshio—bounded by the 0 ms-1 isotach—reaches to the seafloor (3,000 m). The observations indicate a deep countercurrent beneath the poleward-flowing Kuroshio (-12.5±1.2 x106m3s-1).

Importantly, the studies found that the abrupt topography east of Taiwan and the effect of ocean eddies on the deep-flow volume transport modulate the deep countercurrent (deeper than 1,000 m) variations. As the shoreward (western) edge of an anticyclonic ocean eddy arrives at the offshore (eastern) side of the Kuroshio at the Yaeyama Ridge, deep flow impinges on the downstream topography (i.e. the Ryukyu Island Arc and I-Lan Ridge), and a deep recirculation (called countercurrent) is induced on the inshore side of the moored instrument array. This is the first time that the countercurrent beneath the Kuroshio east of Taiwan has been directly observed with a plausible explanation of its dynamics.