The Northeastward current southeast of Okinawa Island observed during November 2000 to August 2001

Abstract

In order to estimate the variability of current structure and mean geostrophic volume transport in the southeast of Okinawa Island, the round‐trip acoustic travel times between sea bottom and surface at nine sites were measured by Inverted Echo Sounder with pressure gauge (PIES) from November 2000 to August 2001. Vertical sections of geostrophic velocity are calculated by using vertical profiles of specific volume anomaly derived by the Gravest Empirical Mode method from the PIES data. In the upper 500 m layer over a slope shallower than 1000 m depth, the northeastward current reached up to 60 cm s−1 during the occupation of an anticyclonic eddy, and decreased to −15 cm s−1 during the occupation of a cyclonic eddy, around the mean of 20 cm s−1. The temporal mean geostrophic volume transport relative to the 2000 dbar level was estimated to be 6.1 Sv (1 Sv ≡ 106 m3 s−1) northeastward in the west of 128.85°E.

Variability of Northeastward Current Southeast of Northern Ryukyu Islands

“…These conclusions suggest that the RCS observed to the southeast of Amami-Ohshima Island should be a continuation of the Kuroshio Branch Current east of Taiwan. However, Zhu et al (2003) The distribution of 5-year mean stream function from 1996 to 2001 estimated from the result of a data assimilating numerical model calculation by Kamachi et al (2004) is shown in Fig. 16, in which the southwestern edge of the Kuroshio recirculation gyre reaches to the south of Okinawa Island, suggesting that the main part of RCS does not originate from the Kuroshio Branch Current east of Taiwan but the westward Kuroshio recirculation flow.…”

“…The interannual signal of TVT has two maxima in April of 1999 and 2001, and one minimum in March 2000 with a variation range of 5 Sv, suggesting the existence of a quasi-biennial oscillation in TVT corresponding to the variation of SVT or SSAGC. The annual signal of TVT has a maximum in summer (July 1999, August 2000, September 2001) and a minimum in the late winter (February 1999, March 2000, March 2001, February 2002. It should be emphasized that the variation ranges of interannual and annual signals are nearly equal.…”

Mean Structure and Variability of the Kuroshio from Northeastern Taiwan to Southwestern Japan

“…At the latitudes of the East China Sea, the strongest transport variability is within the Ryukyu Current along the southeastern side of the Ryukyu Islands (Figure 1a, red dashed line), which is directly exposed to eddies arriving from the ocean interior (Zhu et al, 2003;Andres et al, 2008a). However, there is also Kuroshio transport variability within the East China Sea (at the PN Line) that is driven by these eddy arrivals outside of the island chain (Andres et al, 2008a).…”

“…Integrating over the regions of contiguous positive velocity (i.e., the red shaded areas in the sections shown in Figure 3b-d) indicates that the Kuroshio strengthens from a mean downstream transport of about 15 Sv east of Luzon to 24 ± 0.9 Sv in the East China Sea. The Kuroshio further strengthens to 65 ± 6 Sv at the ASUKA Line, fed in part by the Ryukyu Current (Zhu et al, 2003), a subsurface intensified current that flows along the southwestern side of the Ryukyu Islands and joins the Kuroshio as it exits the East China Sea through Tokara Strait (indicated with the red dashed line in Figure 1a). (Transport values are based on HPIES and ADCP data collected east of Luzon [Lien et al, 2014, and this issue], Andres et al [2008b], and Book et al [2002], with the ASUKA Line mean calculated over two years rather than from the 153-day mean sections shown in Figure 3d.…”

“…At a given latitude, eddies arriving from the interior drive strong variability in the measured flow along the boundary Lien et al, 2014). Observations suggest that the eddyinduced transport variability in the western Philippine Basin is rarely communicated downstream from Luzon to Taiwan; a clear downstream connection due to advection of transport anomalies by the Kuroshio here is the exception rather than the rule (C. Tsai et al, 2015).At the latitudes of the East China Sea, the strongest transport variability is within the Ryukyu Current along the southeastern side of the Ryukyu Islands (Figure 1a, red dashed line), which is directly exposed to eddies arriving from the ocean interior (Zhu et al, 2003;Andres et al, 2008a). However, there is also Kuroshio transport variability within the East China Sea (at the PN Line) that is driven by these eddy arrivals outside of the island chain (Andres et al, 2008a).…”