DEVELOPMENT OF A COUPLED HYCOM-ROMS DOWNSCALING OCEAN
MODELING SYSTEM AND ITS APPLICATION TO THE SOUTH CHINA SEA
Yusuke Uchiyama1*, Naoto Sengo1, Kenta Kurosawa2, and Akihiko Nakayama3
1

Department of Civil Engineering, Kobe University
1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japanuchiyama@harbor.kobe-u.ac.jp
2

Data Assimilation Research Team, RIKEN Center for Computational Sciences
7-1-26 Minatojima-minami-machi, Chuo-ku, Kobe 650-0047, Japankenta.kurosawa@riken.jp
3

Department of Environmental Engineering, Universiti Tunku Abdul Rahman
Jalan Universiti Bandar Barat 31900, Kampar, Perak, Malaysia
akihiko@utar.edu.myABSTRACT: A high-resolution, high-precision downscaling oceanic circulation model for
the Southern China Sea (SCS) was developed based on the Regional Oceanic Modeling
System (ROMS) at a lateral resolution of 5 km, initialized and forced by the HYbrid
Coordinate Ocean Model (HYCOM) global reanalysis product. A multi-year reanalysis wasconducted with the HYCOM-ROMS system that properly accounts for wind stress and heatbudget at surface, freshwater influences from the atmosphere and major rivers, and tidalvariability. We successfully reproduced primary dynamics of the SCS in many aspects such as
the transient Kuroshio path in the south of Taiwan prominently affecting the circulation of thenorthern SCS through the Luzon Strait. The North Equatorial Current (NEC) drifting near
Bismarck Archipelago, New Guinea, sporadically intrudes into the Indonesian water andadjacent seas to invoke bifurcating northward and westward currents. This intrusion promotes
the northwestward mass transport into the Celebes Sea and further into the Sulu Sea, and thenenters the SCS through the Palawan Strait.
INTRODUCTION
A precise operational oceanic reanalysis-forecast system for the South China Sea(SCS) and its surrounding subbasins has considerably been demanded for many purposes
including maintenance of platforms and rigs for mining, maritime navigations, and oceanic
material dispersal responsible for aquatic ecosystems. It has been well known that thedynamics of the northern SCS is considerably affected by energetic internal waves primarily
excited in the Luzon Strait (Fig. 1). Nevertheless, dynamics in the southern SCS is much
poorly understood because of the topographic complicatedness associated with thousands ofislands around Malaysia, Indonesia, and their neighbors. Challenges toward rigorous ocean
circulation modeling for the SCS involve energetic internal tides and influences from oceanic
western boundary currents, in particular the Kuroshio and Mindanao Current flowing on theeast off the SCS. Complex coastline topography and thousands of islands introduce additionaldifficulty in the modeling. A solid solution to such oceanic conditions is evidently a nesting
approach, where large-scale information is conveyed into the embedded high-resolution childdomain through open boundaries (e.g., Mason et al., 2011[1]).
In the present study, we primarily aimed to develop a high-resolution, high-precision
downscaling model for the SCS based on the Regional Oceanic Modeling System (ROMS,Figure 1 – (a) Left: the study area around the South China Sea (SCS), and (b) right: its
enlargement. The black rectangle in (a) is the ROMS model domain. Arrows in (a)represent three major currents, KC, MC, and NEC, which stand for the Kuroshio, theMindanao Current, and the North Equatorial Current, respectively. Colors show thebathymetry in meter, supplemented by white thin contours for the selected shallow
isobaths of 50 and 100 m in (b). Texts are acronyms of major sub-basins of the SCS:
Celebes Sea (CS), Sulu Sea (SS), Java Sea (JS), and Gulf of Thailand (GoT). The red lines
indicate transects surrounding the SCS, including Luzon Strait (L), Celebes Strait (C),Makassar Strait (M), Palawan Strait (P), and Bangka Island Strait (B), all of which are usedin the volume flux analysis.
Shchepetkin and McWilliams, 2005 [2]), initialized and forced by the global HYbridCoordinate Ocean Model (HYCOM) reanalysis conducted by the HYCOM consortium. A
prototype of the proposing modeling system is the JCOPE2-ROMS system (e.g., Uchiyama etal., 2017a [3], 2017b [4], 2018 [5]; Tada et al. 2018 [6]), where the child model relies onlarge-scale oceanic signals evaluated by the assimilative JCOPE2 reanalysis (Miyazawa et al.,2009 [7]). A multi-year reanalysis was conducted using the HYCOM-ROMS system thatproperly accounts for wind stress and heat/freshwater budget at surface, buoyancy inputs frommajor rivers, and tidal variability, etc., as accurately as possible. Then a careful modelvalidation was carried out though extensive comparison of the model results with in situ
measurement and satellite data, demonstrating a good reproducibility. A fundamental, timevarying mass balance of the SCS was subsequently investigated to quantify direct and indirect
influences of the Kuroshio and Mindanao Current on the SCS dynamics.
THE HYCOM-ROMS DOWNSCALING MODEL
The parent large-scale oceanic information is attained by the HYCOM + NCODA
Global 1/12° assimilative product (expt 90.6–91.2) of the HYCOM consortium distributed byCenter for Ocean-Atmospheric Prediction Studies (COAPS), Florida State University(https://hycom.org/). The native Mercator-curvilinear HYCOM horizontal grid corresponds to~10 km lateral resolution in mid latitudes. The daily-averaged 3-D velocity, salinity,temperature, and surface elevation were projected on to the perimeter of the child ROMS

Figure 2 – Time series of volume-averaged kinetic energy in the upper ocean from thesurface down to the depth of 400 m. Red curve: ROMS, black: HYCOM.
model at lateral grid resolution of ~5 km with spatial trilinear and temporal linear
interpolations for the initial and open boundary conditions in an offline-nesting configuration.
The embedded ROMS model was forced by the GPV-GSM, an assimilative operational global
atmospheric product of the Japan Meteorological Agency (JMA), for the surface wind stresses,
the global monthly climatology of the NOAA-COADS dataset for heat, freshwater, radiation
fluxes at the surface, the TPXO 7.0 for global tidal constituents (e.g., Dauhajre et al., 2017[8]; Masunaga et al., 2018 [9]), and the global climatological river discharge dataset of Dai etal. (2009) [10]. The temperature-salinity (T-S) nudging (also know as robust diagnostic; e.g.,Uchiyama et al., 2017a [3]) was employed to enhance the synoptic and mesoscale
reproducibility of transient large-scale currents such as the Kuroshio. The HYCOM-ROMS
reanalysis was performed for a five-year period from 2011 until 2015 excluding the first
several months for the model spin-up.
MODEL VALIDATIONS
We successfully reproduced primary dynamics of the SCS in many aspects. Figure 2depicts time series of spatially averaged kinetic energy (= [u2 + v2]/2) in the upper ocean fromthe surface down to the depth of 400 m. The surface kinetic energy is viewed as a proxy oftransient behavior of major oceanic currents such as the Kuroshio and associated mesoscaleeddies. The assimilative HYCOM and forward ROMS models coincide each other, illustratingthat the downscaled ROMS model properly reproduces the surface currents and eddies.
Model-reproduced time-averaged surface currents and surface temperature (SST) distributions
are also quite consistent with the satellite observations, especially in terms of patterns of themajor currents and frontal structures in and around the SCS (Fig. 3). We have further
conducted comprehensive model-data and model-model comparisons extensively, for instanceon the vertical stratifications and subsurface dynamical structures (not shown). All thevalidations successfully demonstrate that the present HYCOM-ROMS model has a good
agreement with the observed structures and their transient natures in and around the SCS.
DYNAMICS IN THE SOUTH CHINA SEA
Spatial distributions of surface currents (Fig. 3a) and vertically integrated volumetransports (not shown) represent that the SCS is notably influenced by the offshore currents
particularly two western boundary currents, the Kuroshio (KC) and Mindanao Current (MC)Figure 3 – Comparisons of five year-averaged surface currents (upper panels) and sea
surface temperature (SST, lower panels). From left to right, the results from the developed
HYCOM-ROMS model, the assimilative HYCOM model, and satellite observations areshown. Upper: subsampled horizontal velocity vectors and their magnitudes (color, m/s).
The satellite data is from the AVISO composite altimetry data. Lower: the modeled SST iswater temperature of the top-most model layer, whereas the MGDSST composite data ofoceanic skin temperature measured by multiple IR satellites (oC) is plotted as theobservation.
either in direct and indirect manners as discussed below. Both currents are originated from theNorth Equatorial Current (NEC), which is a part of the North Pacific Subtropical Gyre, andare bifurcated off the east coast of the Philippines to form the northward drifting KC andsouthward drifting MC. To quantify effects of the KC and MC on the SCS dynamics, we next
conducted the volume flux analysis at several selected transect around the SCS to examinemass budget of the SCS (Fig. 1b). At an arbitrary transect with the cross-sectional area of A(m2), a volume flux F (m3/s) at a given time t is computed by

F (t ) = ∫ un dA ,A(1)where un is the horizontal velocity (m/s) normal to the transect.
Figure 4 shows time series of F in Sv (n.b., 1 Sv = 106 m3/s) at several transects
defined in Fig. 1b. The Kuroshio (KC) partially separates at the Luzon Strait (L) to form thewestward component that leads to a remarkable inflow into the SCS from the east. On thecontrary, the Mindanao Current (MC) has a pronounced remote effect on the outgoing flux
from the Celebes Sea (CS) at the Celebes Strait (C). The Celebes Sea is connected with theSCS through the Sulu Sea (SS) that is fringed and surrounded by many islands. The waterFigure 4 – Time series plots of volume fluxes F in Sv (n.b., 1 Sv = 106 m3/s) at severaltransects defined in Fig. 1b. The sign is defined in such a way that positive F corresponds toincoming flux toward the SCS. A Kuroshio-originated water mass enters the SCS throughthe Luzon Strait (L), which is approximately in balance with the outgoing flux through thePalawan Strait (P) toward the Celebes Sea (CS) to form the SCS water. In the Celebes Sea,
Mindanao water enters from the Celebes Strait (C), while SCS water enters from the Sulu
Strait (S). As a consequence, both waters have to exit the CS at the Makassar Strait (M),leading to formation of through flow toward the Java Sea (JS).
mass entering the Celebes Sea mainly reverts its prevailing direction toward the Java Sea (JS)via the narrow Makassar Strait (M), a part of which eventually merges with the IndonesianThrough Flow to discharge out to the Indian Ocean. Therefore, the outflow from the SCS via
the Celebes Sea has a relatively minor contribution to the formation of the SCS water.
Nevertheless, even in an indirect way, the Mindanao Current (MC) substantially affects theSCS water because the through flux at the Celebes Strait correlates quite well with theexchanging flux between the Sulu Sea and the SCS. Moreover, the KC and MC havepronounced inter-annual and inter-seasonal variability in their intensity. Thus, enhancement
and retardation of the KC and MC initiate the changes in the through fluxes at the Luzon andCelebes straits, consequently resulting in altering the mass balance and associated dynamicsof the SCS significantly.
SUMMARY AND CONCLUDING REMARKSWe developed a high-resolution HYCOM-ROMS downscaling oceanic circulation
model suitable to scientific and engineering applications for the South China Sea (SCS) andits surrounding waters. The model was found to be capable of reproducing the circulations
and temperature near the surface as well as the stratification in the SCS realistically. The mass
budget analysis clarified that the SCS water is markedly affected both by the Kuroshio (KC)and the Mindanao Current (MC) derived from the North Equatorial Current (NEC). Because
the global HYCOM does not sufficiently detect such detailed physics for its coarser grid
resolution, the present high-resolution downscaling model improves representation of theregional and small-scale dynamics, which help enhance our understanding of the SCS
circulations and dynamics substantially. Our next step in future work is definitely to conduct
one more nesting at horizontal resolution of ~1 km, which is generally sufficient to represent
submesoscale dynamics responsible for upper-ocean mixing (e.g., Kamidaira et al., 2018[11]).

ACKNOWLEDGEMENTS
The present research was financially supported by Japan Society for the Promotion of Science(JSPS) Grants-in-Aid for Scientific Research 15KK0207, 15H04049, and 18H03798 (PI: Y.
Uchiyama) at Kobe University.
REFERENCES[1] Mason, E., Molemaker, M. J., Shchepetkin, A.F., Colas, F., McWilliams, J.C., Sangrà, P.,

2010. Procedures for offline grid nesting in regional ocean models. Ocean Model. 35, 1–15.
[2] Shchepetkin, A.F., McWilliams, J.C., 2005. The regional ocean modeling system(ROMS): a split-explicit, free-surface, topography-following-coordinate oceanic model.
Ocean Model. 9, 347–404.
[3] Uchiyama, Y., Kanki, R., Takano, A., Yamazaki, H. and Miyazawa, Y., 2017. Mesoscale

reproducibility in regional ocean modeling with a 3-D stratification estimate based onAviso-Argo data, Atmosphere-Ocean, 55, 1–18.
[4] Uchiyama, Y., Suzue, Y. and Yamazaki, H., 2017. Eddy-driven nutrient transport andassociated upper-ocean primary production along the Kuroshio, J. Geophys. Res. Oceans,122, 5,046–5,062.
[5] Uchiyama, Y., Zhang, X., Suzue, Y., Kosako, T., Miyazawa, Y. and Nakayama, A., 2018.

Residual effects of treated effluent diversion on a seaweed farm in a tidal strait using a
multi-nested high-resolution 3-D circulation-dispersal model, Mar. Pollut. Bull., 130, 40–54.
[6] Tada, H., Uchiyama, Y. and Masunaga, E., 2018. Impacts of two super typhoons on theKuroshio and marginal seas on the Pacific coast of Japan, Deep-Sea Res. Part I, 132, 80–93.
[7] Miyazawa, Y., Zhang, R., Guo, X., Tamura, H., Ambe, D., Lee, J., Okuno, A., Yoshinari,

H., Setou, T., Komatsu, K., 2009. Water mass variability in the western North Pacificdetected in 15-year eddy resolving ocean reanalysis. J. Oceanogr. 65, 737–756.
[8] Dauhajre, D.P., McWilliams, J.C. and Uchiyama, Y., 2017. Submesoscale coherent

structures on the continental shelf, J. Phys. Oceanogr., 47, 2,949 - 2,976.
[9] Masunaga, E., Uchiyama, Y., Suzue, Y. and Yamazaki, H., 2018. Dynamics of internal

tides over a shallow ridge investigated with a high-resolution downscaling regional oceanmodel, Geophys. Res. Lett., doi: 10.1002/2017GL076916.6[10] Dai, A., Qian, T., Trenberth, K. E., and Milliman, J. D., 2009. Changes in continental

freshwater discharge from 1948 to 2004. J. Climate, 22 (10), 2,773–2,792.
[11] Kamidaira, Y., Uchiyama, Y., Kawamura, H., Kobayashi, T. and Furuno, A., 2018.

Submesoscale mixing on initial dilution of the radionuclides released from theFukushima Dai-ichi Nuclear Power Plant. J. Geophys. Res. Oceans, 123, doi:
10.1002/2017JC013359