Residual circulations and associated watermass transport in the South China sea
analyzed with a coupled HYCOM-ROMS
downscaling ocean model
Cite as: AIP Conference Proceedings 2157, 020029 (2019); https://doi.org/10.1063/1.5126564
Published Online: 18 September 2019
Yusuke Uchiyama, Naru Takaura, Nobue Okada, and Akihiko Nakayama

ARTICLES YOU MAY BE INTERESTED INDevelopment of handy measurement system for river flow environment based on imagebased technique
AIP Conference Proceedings 2157, 020028 (2019); https://doi.org/10.1063/1.5126563
Numerical study of transport and accumulation of floating objects in Perak River during floods
AIP Conference Proceedings 2157, 020003 (2019); https://doi.org/10.1063/1.5126538
Study on stability evaluation of steel slag as road embankment material focused on theimprovement effect of low quality soil
AIP Conference Proceedings 2157, 020033 (2019); https://doi.org/10.1063/1.5126568

AIP Conference Proceedings 2157, 020029 (2019); https://doi.org/10.1063/1.5126564© 2019 Author(s).

2157, 020029

Residual Circulations and Associated Water Mass
Transport in the South China Sea Analyzed with a Coupled
HYCOM-ROMS Downscaling Ocean Model
Yusuke Uchiyama1, *, Naru Takaura1, Nobue Okada1 and Akihiko Nakayama212

Department of Civil Engineering, Kobe University, Japan
Department of Environmental Engineering, Universiti Tunku Abdul Rahman, Malaysia*

Corresponding author: uchiyama@harbor.kobe-u.ac.jp

Abstract. 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 andforced 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 heat budget at surface, freshwater
influences from the atmosphere and major rivers, and tidal variability. Prospective applications of the system include
assessments of oceanic dispersal of wastewater, quantification of marine ecosystem network, analyses of micro plastictransport and its coastal accumulation, etc. In prior to applied studies, we extensively investigated the validity andperformance of the developed system. We found that it 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 the northern
SCS through the Luzon Strait. The North Equatorial Current (NEC) drifting near Bismarck Archipelago, New Guinea,
sporadically intrudes into the Indonesian water and adjacent 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
sub-basins has considerably been demanded for many purposes including maintenance of platforms and rigs formining, maritime navigations, and oceanic material dispersal responsible for aquatic ecosystems. It has been wellknown that the dynamics of the northern SCS is considerably affected by energetic internal waves primarily excitedin the Luzon Strait (Fig. 1). Nevertheless, dynamics in the southern SCS is much poorly understood because of thetopographic complicatedness associated with thousands of islands around Malaysia, Indonesia, and their neighbors.
Challenges toward rigorous ocean circulation modeling for the SCS involve energetic internal tides and influencesfrom oceanic western boundary currents, in particular the Kuroshio and Mindanao Current flowing on the east off
the SCS. Complex coastline topography and thousands of islands introduce additional difficulty 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 child domain through open boundaries (e.g., Mason et al [1]).
In the present study, we primarily aimed to develop a high-resolution, high-precision downscaling modelfor the SCS based on the Regional Oceanic Modeling System (ROMS, Shchepetkin and McWilliams, 2005 [2]),initialized and forced by the global HYbrid Coordinate Ocean Model (HYCOM) reanalysis conducted by theHYCOM consortium. A prototype of the proposing modeling system is the JCOPE2-ROMS system (e.g., Uchiyama
et al., 2018 [3], 2017 [4], 2018 [5]; Tada et al. 2018 [6]), where the child model relies on large-scale oceanic signals

International Symposium on Green and Sustainable Technology (ISGST2019)
AIP Conf. Proc. 2157, 020029-1–020029-6; https://doi.org/10.1063/1.5126564
Published by AIP Publishing. 978-0-7354-1902-5/$30.00

020029-1

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 the bathymetry in meter, supplemented bywhite thin contours for the selected shallow isobaths of 50 and 100 m in (b). Texts are acronyms of major sub-basins of theSCS: 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 used in the volume flux analysis.

FIGURE 2. Time series of volume-averaged kinetic energy in the upper ocean from the surface down to the depth of 400 m.
Red curve: ROMS, black: HYCOM.

evaluated by the assimilative JCOPE2 reanalysis (Miyazawa et al., 2009 [7]). The developed HYCOM-ROMS
downscaling system has a at lateral grid spacing of 5 km, which is sufficiently fine to resolve complex topography
and bathymetry of the SCS, with covering not only the northern SCS but also the southern SCS including the Gulfof Thailand among others. A multi-year reanalysis was conducted using the HYCOM-ROMS system that properly
accounts for wind stress and heat/freshwater budget at surface, buoyancy inputs from major rivers (e.g., Yangtze,Pearl, Mekong rivers), and tidal variability, etc., as accurately as possible. Then a careful model validation wascarried out though extensive comparison of the model results with in situ measurement and satellite data todemonstrate a good reproducibility of the downscaling system. Inter- and intra-annual variability of the surface
currents in the SCS is discussed in conjunction with seasonally varying surface wind stresses characterized by the020029-2

FIGURE 3. Comparisons of five year-averaged surface currents (upper panels) and sea surface temperature (SST, lowerpanels). From left to right, the results from the developed HYCOM-ROMS model, the assimilative HYCOM model, andsatellite observations are shown. 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 is water temperature of the top-most model layer,
whereas the MGDSST composite data of oceanic skin temperature measured by multiple IR satellites (oC) is plotted as theobservation.

winter monsoons. A fundamental, time-varying 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 by Center for Ocean-AtmosphericPrediction 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 model at lateralgrid 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, anassimilative 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 atthe 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 et al. (2009) [10]. The temperature-salinity (T-S)
nudging (also know as robust diagnostic; e.g., Uchiyama et al., 2018 [3]) was employed to enhance the synoptic and020029-3

FIGURE 4. Time series plots of volume fluxes F in Sv (n.b., 1 Sv = 106 m3/s) at several transects defined in Fig. 1b. The
sign is defined in such a way that positive F corresponds to incoming flux toward the SCS. A Kuroshio-originated watermass enters the SCS through the 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 theCelebes Strait (C), while SCS water enters from the Sulu Strait (S). As a consequence, both waters have to exit the CS at theMakassar Strait (M), leading to formation of through flow toward the Java Sea (JS).

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 2 depicts time series ofspatially averaged kinetic energy (= [u2 + v2]/2) in the upper ocean from the surface down to the depth of 400 m. The
surface kinetic energy is viewed as a proxy of transient behavior of major oceanic currents such as the Kuroshio andassociated mesoscale eddies. The assimilative HYCOM and forward ROMS models coincide each other, illustratingthat the downscaled ROMS model properly reproduces the surface currents and eddies. Model-reproduced timeaveraged surface currents and surface temperature (SST) distributions are also quite consistent with the satellite
observations, especially in terms of patterns of the major 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 the validations successfully
demonstrate that the present HYCOM-ROMS model has a good agreement with the observed structures and theirtransient natures in and around the SCS.

DYNAMICS IN THE SOUTH CHINA SEA
Spatial distributions of surface currents (Fig. 3a) and vertically integrated volume transports (not shown)represent that the SCS is notably influenced by the offshore currents particularly two western boundary currents, theKuroshio (KC) and Mindanao Current (MC) either in direct and indirect manners as discussed below. Both currents
are originated from the North 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 and southward 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 examine mass budget of the SCS (Fig. 1b). At an arbitrary transect with thecross-sectional area of A (m2), a volume flux F (m3/s) at a given time t is computed byF (t ) = ∫ u n dA ,(1)A

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 the westward component that leads to a remarkableinflow into the SCS from the east. On the contrary, 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 the SCS

020029-4

through the Sulu Sea (SS) that is fringed and surrounded by many islands. The water 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 Indonesian Through 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 the SCS water because thethrough flux at the Celebes Strait correlates quite well with the exchanging flux between the Sulu Sea and the SCS.
Moreover, the KC and MC have pronounced 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 and Celebes
straits, consequently resulting in altering the mass balance and associated dynamics of the SCS significantly.

CONCLUSIONS
We developed a high-resolution HYCOM-ROMS downscaling oceanic circulation model suitable toscientific and engineering applications for the South China Sea (SCS) and its surrounding waters. The model wasfound 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 theKuroshio (KC) and the Mindanao Current (MC) derived from the North Equatorial Current (NEC). Because theglobal HYCOM does not sufficiently detect such detailed physics for its coarser grid resolution, the present highresolution downscaling model improves representation of the regional 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]).

ACKNOWLEDGMENTS
The present research was financially supported by Japan Society for the Promotion of Science (JSPS) Grants-in-Aidfor Scientific Research 15KK0207, 15H04049, and 18H03798 (PI: Y. Uchiyama) at Kobe University, andFundamental Research Grant Scheme of Malaysia 230298-243290 (PI: Z. Nizamani) at Universiti Tunku AbdulRahman (UTAR). Thanks are also due to N. Sengo at Kobe University for his help in earlier stage of the numericalmodel development.

REFERENCES
1.
2.
3.

4.
5.

6.
7.

Mason, E., Molemaker, M. J., Shchepetkin, A.F., Colas, F., McWilliams, J.C., Sangrà, P. Procedures foroffline grid nesting in regional ocean models. Ocean Model. 35, 1–15, 2010.
Shchepetkin, A.F., McWilliams, J.C. The regional ocean modeling system (ROMS): a split-explicit, freesurface, topography-following-coordinate oceanic model. Ocean Model. 9, 347–404, 2005.
Uchiyama, Y., Kanki, R., Takano, A., Yamazaki, H. and Miyazawa, Y. Mesoscale reproducibility in regional
ocean modeling with a 3-D stratification estimate based on Aviso-Argo data, Atmosphere-Ocean, 56, 212–229,
2018, doi: 10.1080/07055900.2017.1399858
Uchiyama, Y., Suzue, Y. and Yamazaki, H. Eddy-driven nutrient transport and associated upper-ocean primary
production along the Kuroshio, J. Geophys. Res. Oceans, 122, 5,046–5,062, 2017, doi:10.1002/2017JC012847
Uchiyama, Y., Zhang, X., Suzue, Y., Kosako, T., Miyazawa, Y. and Nakayama, A. Residual effects of treatedeffluent diversion on a seaweed farm in a tidal strait using a multi-nested high-resolution 3-D circulationdispersal model, Mar. Pollut. Bull., 130, 40–54, 2018, doi: 10.1016/j.marpolbul.2018.03.007
Tada, H., Uchiyama, Y. and Masunaga, E. Impacts of two super typhoons on the Kuroshio and marginal season the Pacific coast of Japan, Deep-Sea Res. Part I, 132, 80–93, 2018, doi: 10.1016/j.dsr.2017.12.007
Miyazawa, Y., Zhang, R., Guo, X., Tamura, H., Ambe, D., Lee, J., Okuno, A., Yoshinari, H., Setou, T.,
Komatsu, K. Water mass variability in the western North Pacific detected in 15-year eddy resolving ocean
reanalysis. J. Oceanogr. 65, 737–756, 2009.

020029-5

8.

Dauhajre, D.P., McWilliams, J.C. and Uchiyama, Y. Submesoscale coherent structures on the continental shelf,J. Phys. Oceanogr., 47, 2,949 - 2,976, 2017, doi:10.1175/JPO-D-16-0270.1
9. Masunaga, E., Uchiyama, Y., Suzue, Y. and Yamazaki, H. Dynamics of internal tides over a shallow ridge
investigated with a high-resolution downscaling regional ocean model, Geophys. Res. Lett., 45:8, 3,550-3,558,
2018, doi: 10.1002/2017GL076916.6
10. Dai, A., Qian, T., Trenberth, K. E., and Milliman, J. D. Changes in continental freshwater discharge from 1948
to 2004. J. Climate, 22 (10), 2,773–2,792, 2009.
11. Kamidaira, Y., Uchiyama, Y., Kawamura, H., Kobayashi, T. and Furuno, A. Submesoscale mixing on initial
dilution of the radionuclides released from the Fukushima Dai-ichi Nuclear Power Plant. J. Geophys. Res.
Oceans, 123, 2,808–2,828, 2018, doi: 10.1002/2017JC013359

020029-6