A numerical study of effects of submesoscale anticyclonic eddies
induced by Kuroshio around Ryukyu Islands
Yuki Kamidaira 1, Yusuke Uchiyama 1, Satoshi Mitarai 2and Taichiro Sakagami 212

Department of Civil Engineering, Kobe University, Japan
Marine Biophysics Unit, Okinawa Institute of Science and Technology, JapanAbstract

In order to maintain biological diversity in the coral coasts around Ryukyu Islands, Japan, a role played by the adjacentKuroshio warm current is anticipated to be essential for larval and nutrient transport. For better understanding dynamicsand mixing between the Kuroshio and the islands, we develop a detailed oceanic downscaling model in a doubly nested
configuration using ROMS at horizontal resolutions down to 1km, forced by the assimilative JCOPE2 and JMAGSM/MSM. A model-data comparison is conducted with the field observation and satellite altimetry data to demonstrate aquite close agreement. According to a diagnostic eddy heat flux analysis, the dominance of negative vorticity is substantial
in emergence of submesoscale anticyclonic eddies and the associated asymmetric heat transport from the Kuroshio.
Keywords - Kuroshio, Ryukyu Islands, ROMS
1Introduction

For preserving biodiversity and marine ecosystem in the coral
coasts around Ryukyu Islands, Japan, a role played by theadjacent Kuroshio warm current is considered to be substantial
for larval and nutrient transport. Based on a triply nested ocean
modeling using POM, Guo et al., (2003) reported that the pathand vertical structure of Kuroshio in the East China Sea,
encompassing Ryukyu Islands, become more realistic as themodel horizontal resolution increases. A numerical study usinga high-resolution ocean model in this area indicates that thesouthwestward counter-Kuroshio Current and mesoscale eddies
have non-trivial influence on volume and heat transport between
Kuroshio and the Islands (Nadaoka et al., 2006).
Recently, effects of submesoscale dynamics, O (10) km or less,on the mean structure, eddies, frontal processes, stratification,etc. have widely been recognized crucial in the upper oceandynamics (Uchiyama et al., 2012). However, the submesoscale
influences on the oceanic structure in the study area have notbeen fully investigated yet. Furthermore Kuroshio flowing up
on the continental shelf off the Okinawa Trough is considered tobe largely skewed by the shallow island topography on its eastside that may result in unique turbulence. Therefore, in order tobetter understand dynamics and mixing between Kuroshio andthe islands, we develop a detailed ocean downscaling model in a
doubly nested configuration using ROMS at horizontal
resolution down to 1km around Ryukyu Islands, forced byrealistic assimilative meteorological and oceanic products.
2

dimensional TS-nudging (nudging strength = 1/20 day−1)
towards the 10 day-averaged JCOPE2 temperature and salinity
for consistency of the Kuroshio path reproduced by the ROML1 with that of JCOPE2.
3
3.1

ResultSurface normalized relative vorticity

An extensive model-data comparison is performed against thefield observation and satellite altimetry data to demonstrate aquite close agreement, particularly in stratification in the East
China Sea over the Okinawa Trough, and in mean and eddy
kinetic energy at surface. Horizontal shear of Kuroshio
generates axisymmetric pairs of positive and negative relative
vorticity on the west and east sides of the Kuroshio path around
Ryukyu Islands. However, the inner-most model at the highest
resolution captures the negative vorticity retained significantly
on the eastern side of Kuroshio, while the centrifugally-stable
positive vorticity is attenuated rather quickly. In order toquantitatively evaluate this negative bias of relative vorticity
with increased model resolutions, we define two sub-regions(sub 1 and sub2 as shown in Fig. 2 Left) to examine the effects

Numerical model

We rely on a one-way offline nesting approach to successively
reduce the horizontal grid size from about 10 km (JCOPE2) à 3km (ROMS-L1) à 1 km (ROMS-L2) (Fig. 1). The outer-mostboundary and initial conditions are provided with theinterpolated fields of the assimilative JCOPE2 daily-averaged
data. The model topography is taken from SRTM 30, which
covers the global ocean at 30 geographic arc seconds (about 1km). QuikSCAT-ECMWF blended wind and JMA GPVMSM/ GSM data are utilized for the surface momentum forcing.
Surface heat and freshwater fluxes are given by the COADS
monthly climatology. A 20 day-averaged JCOPE2 data isapplied to the sea surface temperature and salinity. The monthly
climatology of major river discharges in Dai et al. (2009) is
considered for Yangtze River in L1. We perform a four-Fig. 1.	 Bathymetry and domains of doubly nested models:
ROMS-L1 (outer box) and ROMS-L2 (inner box).

Fig. 2. Fx (eddy heat flux in ℃m/s across the Kuroshio axis) from the 3 models. The boxes in the left panel indicate sub 1and sub 2.

Fig. 3.	 PDFs of normalized relative vorticity in sub 1 from the 3 models and that in sub 2 of ROMS-L2 (right panel) as function of thedistance from the Okinawa Island (in km). Red vertical lines are the approximate location of the Kuroshio axis.	 
of islands and resolutions. PDFs (probability density functions)of nomalized relative vorticity (ζ / f) at surface for the two-year
period is diagnosed as a function of the distance from theOkinawa Island normal to the Kuroshio axis (Fig. 3). The PDF
is nearly symmetric to the Kuroshio axis although peaks at ζ / f< 0 on the eastern side of the Kuroshio axis while converges tozero as away from the axis to the west. This negative bias on
the east is most evident at the highest resolution (L2-sub1, thesecond right panel). Nevertheless, the asymptote of the PDF
peak on the east in L2-sub2 (without islands) is ζ / f → ± 0, andthus the negative bias is less visible. These findings suggest that
submesoscale eddies tend to be more anti-cyclones (ζ / f <0)that are prominently enhanced by the island topography.
3.2

Heat flux analysis

4References[1]

The submesoscale anti-cyclonic eddies induced by Kuroshio
are anticipated to promote eastward material transport to thewest coast of the Okinawa Island through lateral eddy mixing.
To quantify this effect, we assess the lateral turbulent mixing ofa tracer (heat) in the upper ocean. The time-averaged, verticallyintegrated heat (potential temperature) transport equation isrepresented as (e.g., Marchesiello et al., 2003)
η " ∂uT ∂vT %η " ∂u*T * ∂v*T * %∫−h $++'dz + ∫−h $'dz∂y &# ∂x ∂x &# ∂x
η+ ∫−h +,Q T + D T . dz = 0() ()Conclusion

A detailed downscaling model is developed to investigate
submesoscale dynamics and mixing around Ryukyu Islands. The
submesoscale eddy-resolving model (i.e., L2) successfully
reproduces negative vorticity significantly retained on thewestern side of the islands presumably because of topographic
eddy shedding. A diagnostic eddy heat flux analysis suggeststhat the dominance of the negative vorticity is substantial inemergence of more energetic submesoscale anti-cyclonic eddies
that lead to promoting lateral material exchange between
Kuroshio and the islands.

[2]

[3]

[4]

where T: potential temperature, (u, v): horizontal velocity, Q:[5]sea surface heat flux, D: parameterized, vertical and horizontal
subgrid-scale mixing of heat. The variables assigned with theprime are eddy components obtained by removing the seasonal[6]variations. We focus on the transport by eddy flow, that is adivergence of lateral eddy fluxes F =（Fx, Fy）=（𝑢 ! 𝑇′，  𝑣′𝑇′）
where Fx and Fy are the across and along Kuroshio axis eddy
heat flux. Figure 2 demonstrates that Fx becomes larger as thelateral model resolution gets finer, particularly near the islands.

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