AOS04-P03

日本地球惑星科学連合2016年大会

沖縄本島周辺海域における黒潮反流に伴う海洋構造の非対称性について
Impact of mesoscale recirculation of the Kuroshio on asymmetric oceanic structure around
Okinawa Island in the East China Sea*小谷 瑳千花1、内山 雄介1、山西 琢文1、上平 雄基1、御手洗 哲史2*Sachika Odani1, Yusuke Uchiyama1, Takafumi Yamanishi1, Yuki Kamidaira1, Satoshi Mitarai2
1.神戸大学、2.沖縄科学技術大学院大学
1.Kobe University, 2.Okinawa Institute of Science and Technology Graduate University
Okinawa Island is located in the subtropical region of Japan, hosting ecologically abundant coral
reefs even though they lie at the northernmost extreme of the habitable region. The coral ecosystemin the west coast of the island is maintained by persistent intrusions of the Kuroshio warm water
through eddy-induced lateral mixing (Kamidaira et al., 2016), while the Ryukyu Under Current is amajor source of warm water from the lower latitude on the east coast. The island is situated on aridge of the Ryukyu Arc that separates the shallow East China Sea (ECS) and the deep Ryukyu Trench(RT) to the Pacific Ocean, preconditioning oceanic asymmetry between the both sides of the island.
In the present study, asymmetric oceanic responses around the island are investigated with asynoptic, triple nested downscaling ocean model based on ROMS. The model is forced by the JCOPE2
reanalysis as the outermost lateral boundary conditions and by the JMA GPV-GSM/MSM atmosphericreanalysis as the surface momentum boundary conditions. The horizontal grid spacing is decreased
from 3 km in the outermost ROMS-L1 model, to 1 km in the intermediate ROMS-L2 model, and further
down to 250 m in the innermost ROMS-L3 model. The L3 model has a 152 x 416 km domain and ten
principal tidal constituents based on the TPXO 7.0 reanalysis is newly introduced to account for
tides for more realistic reanalysis.
The harmonic analysis of the L3 model result highlights that semi-diurnal and diurnal tides
propagate differently on the both sides of the island, yielding the asymmetric distributions in
tidal amplitudes and phases. The tides are rather uniform with neither noticeable phase lags nor
amplification on the RT side, whereas bidirectional propagation occurs on the ECS side originatedfrom the northern- and southern-most tips of the island with prominent changes in amplitude near
the shore. Similarly, the baroclinic energy flux demonstrates that the diurnal internal tides are
not trapped topographically. Therefore the resultant clockwise circular propagation, which has been
observed in several islands such as Izu Oshima and Sadogashima Islands where the local inertial
period is shorter than the diurnal period, is interfered at the southernmost area off Okinawa
Island. In addition, these two tips are areas of generation of the most energetic eddy kinetic
energy (EKE). In particular, the upstream southernmost area sheds eddies that affect the nearshore
area around the island. Remarkable enhancement of EKE is found around the shallow channel lying
between Okinawa Island and Tokashiki Island (viz., Tokashiki Channel). These analyses clearlysuggest that the southernmost area of the island around Tokashiki Channel plays substantial roles
in controlling the asymmetric oceanic responses. The evaluated meridional volume flux normal to thechannel indicates seasonal variability with prominent ECS to RT transport in spring, although RT toESC transport is comparable to ESC to RT transport for the rest of the year. The transport alongthe channel is highly correlated with the volume transport of the northeastward drifting Kuroshio
centered at 150 –200 km west of Okinawa. In spring, opposing southwestward transport is generated
between the Kuroshio and the island, often referred to as the Kuroshio Counter Current (KCC).
Surface vorticity indicates that the KCC is composed of clockwise-rotating, anti-cyclonic mesoscaleeddies. Wavenumber kinetic energy spectra clearly shows seasonal transition from submesoscale-eddy
dominance in winter to mesoscale-eddy dominance in spring. Relaxation of surface cooling and mixed©2016. Japan Geoscience Union. All Right Reserved.

- AOS04-P03 -AOS04-P03

日本地球惑星科学連合2016年大会

layer deepening from winter to spring lead to this transition that is responsible for seasonal
exchange between the ECS and the RT.
キーワード：黒潮、黒潮反流、琉球海流、メソスケール渦サブメソスケール渦による乱流混合、領域海洋循環モデル
Keywords: Kuroshio, Kuroshio Counter Current, Ryukyu Under Current, meso - and submesoscale eddy
mixing, ROMS

©2016. Japan Geoscience Union. All Right Reserved.

- AOS04-P03 -