Tempo‐Spatial Variations of the Kuroshio Current in the Tokara Strait Based on Long‐Term Ferryboat ADCP Data

Abstract

Transverse‐vertical structure and temporal variability of the Kuroshio current across the Tokara Strait during 2003–2012 measured by a ferryboat acoustic Doppler current profiler with a 2‐km horizontal resolution and a two‐day interval are presented. The Kuroshio passing through the Tokara Strait exhibits a multicore velocity structure. Its seasonal volume transport variation is biannual for baroclinic components relative to 700 m, peaking in July and December–January. However, the barotropic transport component exhibits an annual cycle with a maximum in December. Empirical orthogonal function analysis of the cross‐sectional velocity is performed. The first two empirical orthogonal function modes reveal the north‐south shift of the Kuroshio current axis and the change in Kuroshio volume transport, respectively. Temporal variabilities of the leading two modes correspond to those of the Kuroshio Position Index and the sea level difference across the strait, respectively. The third empirical orthogonal function mode, with a relatively smaller horizontal scale, was examined in terms of turbulent mixing. The banded structure captured by this mode is likely induced by flow‐topography interaction because islands in the Kuroshio route could cause horizontal and vertical flow separation. Additional analysis based on high‐resolution reanalysis data suggested that (1) inertial instability, which is expected in the areas with negative Ertel's potential vorticity, arises to enhance vertical mixing around the islands in the Tokara Strait, and (2) when the Kuroshio directly impinges the islands, flow divergence in the lee of the islands drives upwelling and leads to uplift of isotherms.

Comprehensive Observational Features for the Kuroshio Transport Decreasing Trend During a Recent Global Warming Hiatus

“…To empirically establish a long‐term continuous Kuroshio transport time series, only daily sea‐level data that were strictly consistent with the ADCP data time frame were used. Because the Kuroshio mainstream through the Tokara Strait exhibits a nearly permanent double‐core structure located south and north of Nakanoshima (Feng et al., 2000; Liu et al., 2019), we calculated regression formulas for the two sections spanning the northern and southern Kuroshio current cores. Therefore, the observed transport was divided into northern and southern sections.…”

Controls on the Distributions of Dissolved Cd, Cu, Zn, and Cu‐Binding Organic Ligands in the East China Sea

“…Station I1 is located at the Tokara Strait, which is scattered with seamounts and small islands that induce turbulent mixing through their collision with the fast-flowing Kuroshio Current. Topography-induced upwelling has been reported at the Tokara Strait in previous studies (Cheung et al, 2017;Liu et al, 2019;Tsutsumi et al, 2017). The constant potential density anomaly in the top 100 m at station I1 indicates vertical mixing (see S1), and this mixed layer was at least 25 m deeper than that in the upstream station (B8).…”

Observation of Topographic Rossby Waves Triggered by Kuroshio Path Meander in the East China Sea

“…The model outputs used in this study ranged from 125°E–132°E, 24°N–32°N, and spanned from October 2004 to December 2005 covering the entire observation period. We focused on current variabilities below 400 m. The JCOPE outputs have been validated and used to examine circulations and tides in the western North Pacific in previous studies (Liu et al., 2019; Varlamov et al., 2015).…”

“…Moreover, Liu et al. (2019) have reported variations caused by the strong flow–topography interaction between the Kuroshio and islands in the TkS.…”

“…It then exits the ECS through the Tokara Strait (TkS) with a volume transport of 23.03 𝐴𝐴 ± 3.31 Sv according to the long-term direct current measurements in 2003-2011 (Zhu et al, 2017). Moreover, Liu et al (2019) have reported variations caused by the strong flow-topography interaction between the Kuroshio and islands in the TkS.…”