Title
Temporal and spatial variation of the Kuroshio in Tokara Strait observed by ferryboat
ADCP
Members’ names and affiliations
Zhaojun Liu1, Xiaohua Zhu1, Xinyu Guo2, and Menghong Dong2
1

State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute ofOceanography, Ministry of Natural Resources, Hangzhou, China
2Center for Marine Environment Studies, Ehime University, Matsuyama, Japan
Aim
Transverse-vertical structure and temporal variability of the Kuroshio current
across the Tokara Strait during 2003–2012 measured by ferryboat acoustic Doppler
current profiler (ADCP) with a 2-km horizontal resolution and a 2-day interval arepresented. In the empirical orthogonal function (EOF) analyses of the cross-sectional
velocity, the EOF3 mode (19.25%) exhibits a band-like structure, with a smaller
horizontal scale than the first two EOF modes; such a band-like structure may berelated to the wakes in the lee of the Tokara Islands.
For better understanding of the character of the island wakes behind the Tokara
Islands captured by EOF3, we discussed with Prof. Xinyu Guo how to use theJCOPE-T ocean reanalysis data to examine the plain and vertical structure of theisland wakes, including: (1) validation of ocean reanalysis data, and (2) surface
pattern and vertical view of island wakes obtained from the reanalysis data.
Procedure(1) Validation of ocean reanalysis data. We first compare the JCOPE-T reanalysis
and ferryboat ADCP data for the time-averaged velocity structure of the Kuroshio
on a transection through the Tokara Strait, and then, we examine their temporalagreement by the correlation analysis.
(2) Surface pattern and vertical view of island wakes. Previous studies revealed
that area with negative Ertel’s potential vorticity appears on the left side of aseamount, where high-wavenumber internal wave shears are remarkable, and theassociated energy dissipation is strong. We examine this situation around theTokara Islands using the JCOPE-T reanalysis data.
Results
To be consistent with observations, the JCOPE-T data was linearly interpolated tothe ferry track. Next, the time frame of the JCOPE-T data was set to strictly coincidewith the observational time; in this sense, the reanalysis data in the period when theobservational data are missing were not used in the analysis. Considering that thedirection of the current across the ferry track may differ from the JCOPE-T and ADCP
data, we compared the absolute velocities on the transection but not cross-sectional
velocities. In the time-mean vertical structure of the absolute velocity on thetransection (Figure 1), both the observation and reanalysis demonstrate a multicorefeature of the Kuroshio, including the countercurrent beneath the Kuroshio, though
the current in the northern channel is excessive for the JCOPE-T data. The cause ofthe excessive northern Kuroshio core may be related to the overshooting of theKuroshio, separating from the continental slope.
The JCOPE-T time series of volume transport through the Tokara Strait during
2003–2012 is shown in Figure 1c. The JCOPE-T reanalysis agrees well withobservations but has a slightly smaller mean and variability. The JCOPE-T mean net
volume transport through the Tokara Strait (excluding the Ohsumi Strait) is 20.41 ±3.01 Sv, while the observed net volume transport by ferryboat ADCP is 23.03 ± 3.31
Sv. During the observational period, the JCOPE-T reanalysis captures the temporaltransport variability well. The correlation coefficient between the observed andreanalyzed volume transports through the Tokara Strait is 0.68 (significant at the 95%
confidence level).

Figure 1. Mean absolute velocity on the transection for (a) JCOPE-T reanalysis dataand (b) ADCP observation. The time frame spans the observational period during
January 2003–March 2012. (b) Comparison of transport time series from theJCOPE-T reanalysis (black) with ADCP observation (blue). Coefficient correlation
between two time series is given at the top right corner.

Figure 2. (a) Comparison of the sum of the absolute normalized surface relative
vorticity (|𝜁/𝑓|, red line) with the Kuroshio axis latitude at 129°E (green line) basedon JCOPE-T data (r = 0.61 for the period January 2003–March 2012). (b)–(d)Instantaneous 𝜁/𝑓, sea surface temperature anomaly, and sea surface current velocity
for June 20, 2006, when the island wakes are strongest. (e)–(g) Same as (b)–(d) but
for instantaneous distribution on October 19, 2011 when the island wakes areweakest.
Figure 2a shows the time series of the sum of absolute normalized relative
vorticity along the ferryboat track (red lines, hereafter sum|ζ/𝑓|). This time series
reveals that strong wakes with large amplitudes (> mean + std) tend to develop insummer (e.g., June–July). In contrast, weak wakes with small amplitudes (< mean –std) tend to occur in winter. Such a seasonal variation of island-induced wakeintensity is closely associated with the location in the latitude for the Kuroshio at129°E, as revealed in Figure 2a. The correlation between the sum|ζ/𝑓| (red line) andthe latitude of Kuroshio at 129°E (green line) is 0.68. In the following, we will show
the surface patterns and vertical views of the strongest wake (on June 20, 2006) andthe weakest wake (October 19, 2011), respectively, in the entire observation period.
Figure 2b shows the instantaneous ζ/𝑓 (surface relative vorticity normalized by𝑓) on June 20, 2006, and it reveals that the anticyclonic and cyclonic vorticities are on
the left and right side of the wake, respectively. As shown in Figure 2b, the area whereζ/𝑓 is less than –1 (equivalent to negative Ertel’s potential vorticity) is present in theTokara Strait where the Kuroshio current strongly interacts with the topography. Cold,thin streaks with lengths of about 50 km appear behind the islands on June 20, 2006
(Figure 2c). The water within the wakes is 0.3–0.5 °C colder than the surroundingwaters. The lee of the islands is characterized by the return current against theincoming current (Figure 2d). Figures 3a, 3b, and 3c show the vertical views ofrelative vorticity, temperature, and velocity over the section, respectively. Behind theisland, the multicore velocity structure of Kuroshio is more enhanced on June 20,
2006 than the mean state shown in Figure 1. Isotherms in the upper 100 m are domed
behind the islands, indicating that upwelling and vertical mixing occur there.
Contrary to the situation shown in Figure 2b (June 20, 2018), Figure 2e shows thatwhen the island-induced wakes become relatively weaker (October 19, 2011), theKuroshio path clearly shifts northward. In this case, the concurrent distributions ofrelative vorticity and sea surface temperature do not change as much in the lee of theislands (Figure 2e–g). Additionally, the vertical sections of the relative vorticity,temperature, and velocity (Figure 3e–f) are comparable to the mean state displayed inFigure 1.

Figure 3. Vertical sections of (a) normalized relative vorticity ζ/𝑓, (b) temperature,and (c) cross-sectional velocity on June 20, 2006 when the island wakes are strongest.
(d)–(f) Same as (a)–(c) but for vertical sections on October 19, 2011 when the islandwakes are weakest. The white curves for temperature (b and e) and velocity (c and f)
indicate isotherms from 21.6 °C to 27.6 °C with an interval of 2 °C, and zero velocity.
Publication/conference presentation
Publication: Liu, Z.-J., H. Nakamura, X.-H. Zhu, A. Nishina, Xinyu Guo, andMenghong, Dong, 2019, Tempo-spatial variations of the Kuroshio current in theTokara Strait based on long-term ferryboat ADCP data, Journal of GeophysicalResearch-Oceans, under review.
Perspectives in future
I would like to thank the Lamer project for proving this chance to discuss withProf. Guo about the usage of JCOPE-T reanalysis data. The high-resolution JCOPE-T
data improved our understanding about the island wakes obtained from ferryboat
ADCP data. In future studies, it will be important to combine the observational datawith the JCOPE-T data for understanding the impact of island wake variations
associated with the Kuroshio shape around the Tokara Strait on the surrounding
ecosystem. Looking forward to joining the Lamer next year.