CHARACTERIZING THE SEA-SURFACE CIRCULATION IN THE GULF OF TONKIN BY
USING NUMERICAL MODEL AND HF RADAR DATA
Kim-Cuong Nguyen1, Manh-Cuong Tran2, Akihiko Morimoto3
1Department of Marine Science and Technology, VNU University of Science, Vietnam
2Centre for Oceanography, Vietnam Administration of Sea and Islands.
3Center for Marine Environmental Studies (CMES), Ehime University, Japan.

I.

INTRODUCTION

The Gulf of Tonkin, which locates in the north-west of South China Sea, is enclosed byNorth Vietnamese coast in the west, the Chinese coast to the north and Leizhou peninsula andHainan island in the east. The gulf extends from 105o36’E to 109o55’E and from 17oN to 21o52’N.
Tonkin gulf is quite shallow, the water depth ranges mostly from 40 – 70m and the deep waterlocates in the south of the gulf, which also is the main entry for water interactions between Tonkin
gulf and the South China Sea.
Although having numerous studies conducted by numerical model, the circulation in Tonkin
gulf remains controversial due to the lack of observational data over the entire domain. In theprevious LaMer collaboration project “Sea-surface current features in the Gulf of Tonkin observed
from altimetry satellite and High-Frequency Radars” (June 2017), we have successfully applied
the methods and technique of combining HF Radar data and satellite altimetry data in analyzing
the surface circulation in Tonkin Gulf. The positive outcomes of the mini-project have revealed
the inter-annual and seasonal variations of the surface current in this region (Figure 1). Althoughdata collected from High-Frequency Radars and satellites are reliable with high resolution of both
spatial and temporal, the characteristics of sea-surface current in the Tonkin Gulf are investigatedand described for a relatively limited area. The reason for the limitation of the results is the lackof drifter buoys data and the spatial coverage of HF Radar in Tonkin Gulf (Figure 2). The study ofthe sea-surface current in the entire Gulf of Tonkin is necessary to not only characterize the seasurface circulation but also serve other environmental research. The purpose of this study is that
we continue to investigate the ocean hydrodynamics of the Tonkin Gulf by taking advantage ofthe state-of-the-art numerical model by ROMS and the continuous monitoring of High Frequency
radar system in Vietnam.
Surface current in Summer
Surface current in WinterFigure 1. Absolute surface current calculated from satellite data and HF radar data

II.

(b)
(a)
Figure 2. The HF Radar coverage (a) and the Drifter Buoys (b)METHODS2.1. Regional Ocean Modeling System
The ROMS (Regional Ocean Modeling System) model (Rutgers version) is a newgeneration ocean circulation calculation model built to simulate regional dynamic processes. The
ROMS model was built based on SPEM and SCRUMS hydraulic models, but was completelyrewritten by Sasha Shchepetkin, Hernan Arango and John Warner, and is a ROMS platform
community model built with contributions of many other scientists in the world. The model wasdeveloped based on a finite and numerical mesh-based finite element method of the Reynolds
average Navier-Stokes equations for 2 or 3-dimensional uncompressed fluids combined with theBoussinesq hypothesis and assumption of hydrostatic pressure. Therefore, the model includes
equations: continuous equations, momentum conservation equations, propagation equations,material diffusion, material conservation equations, state equations and they are closed by thetangled diagrams. For a three-dimensional case, the transient surface is also calculated using theapproximation to convert the sigma coordinate system. The model is written in Fortran code andruns on UNIX operating systems, so the model is capable of parallel computing with MPI orOpenMP libraries. Input files use standard NetCDF format (Hedström, 2009).
2.2. Verification Method
In order to verify the quality of the model results, we used statistical indicators as below:
a. Mean errorsnME ( Bias) = F −Oii =1i(1)nb. Mean absolute errors:nMAE =

 F −Oii =1in(2)c. Root mean square errors:nRMSE =

 (F − O )
i =1i

2i(3)

nIn which, Fi is the model results, Oi is the observed data and n is the length of records.
d. Nash – Sutcliffe indicators (1970) for assessment of model accuracy compare toobservation data: ( X − X ) −  (X − X ) (X − X )nF =2i =1n2'ii =1ini =12i2i(4)In which:F

2: Nash – Sutcliffe indicatorn: length of the record

Xi: Observation data: Model results

X i'X: Average of Observed data
Table 1: The quality of simulation results based on Nash – Sutcliffe indicator
20.9-1
0.7 - 0.9
0.5 – 0.7
0.3 - 0.5
FSimulation
Very GoodGoodAverage
Below Average
Quality
III. DATA FOR MODEL VERIFICATION
3.1. Tide Gauges
In this study, we collected the water level data from17 stations along the coast of Vietnam and compare
this data with the model results. The data arecollected during the survey in Vietnam in 2011 and2012. The position of tidal stations is indicated as
green pins in Figure 3.

Figure 3. The position of tidal stations
for mode validation

3.2. Altimetry data
The tidal gauge measurement, although provided continuous data record with sufficientaccuracy, has some limitations. The first is due to its position which is limited to the coastal
location. The data collected was strongly influent by local and shallow water dynamics (Minh etal., 2014). The second limit is the difficulty to collect the data in the open sea, which is also
important to verify the model. Therefore, in the open ocean we used the altimetry data to comparewith the model results. The satellite provides a global coverage of tidal elevation and the fourmain tidal harmonic constants (M2, S2, K1, O1) can also be estimated from this data with anaccuracy few centimeters (Minh et al., 2014). The altimetry data we in this study is collected fromthe Centre for Topographic studies of Ocean and Hydrosphere (CTOH/LEGOS, France) server.
The tidal harmonic constants were extracted and analyzed by X-TRACK toolbox which isdeveloped by CTOH.
3.3. High Frequency Radar surface current data
Three long-range HF radar observation systems have been established along the coast ofVietnam in western of Tonkin gulf since 2011. All three stations operate at 4.65 MHz withhorizontal resolution of 5.825 km and azimuthal resolution of and 5o.
The High Frequency radar measures the sea surface velocity component in the direction
toward the radar, using the Doppler frequency shift of the radio wave backscattered by ocean
surface wind waves, so that at least two radars are normally used to construct a total vector fromeach site's radial components. At the central data combining station, the radial vector maps frommultiple radar stations are merged to create a total velocity vector current map.
IV. RESULTS
4.1. Model Setup
4.1.1. Grid Generation
The domain of Tonkin gulf is defined from 105oE to 116oE and from 14oN to 22oN. We used
SEAGRID toolbox for creating the grids. SEAGRID is an old toolbox written in MATLAB forgenerating grids in oceanography numerical model. It is able to generate the rectangular or thecurved orthogonal grid and also calculate the land masking of the grid.
For the grid of Tonkin Gulf, we set up the grid asfollows:• Grid Resolution: 2.3 km• Vertical layers: 40 Sigma layers• Maximum depth is 5,000m, minimum depth is10m.
• Bathymetry data:o For the deep water: SRTM30+ with theresolutionof1km
retrievedfrom
http://topex.ucsd.edu/WWW_html/srtm30_plus.html.
o For the coastal zone: Vietnam Navy nauticalcharts from various sources: minimum resolution of50m.
Figure 4. Interpolated bathymetry ofGrid of Tonkin
4.1.2. Initial and Boundary Conditions- For the surface boundary forcing: Meteorological data is collected from European centerfor Medium-range Weather Forecasts (ECMWF) with a timestep of 3-hours: 10 m wind speed,
temperature, humidity, heat flux, solar radiation. Horizontal resolution is 0.125 degrees.
- For the fresh input: Monthly average river discharge data was collected from various
sources from 2006 to 2012, data up to the present time are repeated on the existing data series.
- For the lateral boundary forcing:o Tidal conditions are collected from the global tidal harmonic constants TPXO;
o Daily Temperature and Salinity are collected from the global HYCOM model.
4.2. Model Validation
4.2.1. Tidal Gauges comparisons
In this part we compare the water level collected from the tidal station and the results fromthe model. The results are shown in Table 2 and Figures 5-6. In most case, the model results arewell agreed with the observation data. In the northern part of Vietnam, most of the Nash-Sutcliffe
indicator shows good value (> 0.96). It is decreases in the middle of the gulf (about 0.79 to 0.85).
The absolute difference between model and observation are about 4 - 5 cm in the north andincreases to 8 – 11 cm in the middle of the gulf.
Table 2: Comparison of tidal gauges and model results
Stations

ME
(cm)

MAE
(cm)

RMSE
(cm)

Nash –
Sutcliffe

Stations

ME
(cm)

MAE
(cm)

RMSE
(cm)

Nash –
SutcliffeVân Đồn-0.07

6.30

8.50

0.95

Lạch Bạng-0.23

8.94

11.34

0.85

Hòn Dấu-0.05

4.45

6.52

0.96

Nghi Sơn-0.31

11.26

13.46

0.80

Phà Dương Áo-0.07

4.96

6.48

0.91

Cửa Hội-0.27

10.26

13.35

0.80

Diêm Điền

-0.08

5.01

6.68

0.92

Vũng Áng-0.27

11.79

14.37

0.83

Văn Lý-0.07

4.54

7.12

0.91

Lý Hòa

0.04

10.62

14.33

0.79

Ninh Cơ

-0.10

5.85

7.78

0.93

Đồng Hới

0.04

8.45

11.24

0.76

Cửa Đáy-0.05

6.75

8.13

0.96

Cửa Việt

0.05

5.94

7.53

0.84

Sông Mã-0.09

6.78

8.76

0.96

Thuận An

0.00

4.11

5.18

0.96

Cửa Đại-0.08

5.29

6.50

0.90

Surface Elevation (m)

2.00
1.00
0.00
1101

201

301

401

501

601

701

-1.00
-2.00
Observation

ROMS

Figure 5. Comparison of tidal gauges and Model results at Vân Đồn station from Oct 18,
2011 to Nov 18, 2011

Surface Elevation (m)

0.60
0.40
0.20
0.00
-0.20

1101

201

301

401

501

601

701

-0.40
-0.60
Observation

ROMSFigure 6. Comparison of tidal gauges and Model results at Cửa Đại station from Sep 14,
2012 to Oct 13, 2012
4.2.2. Altimetry Satellite Harmonic Constants comparisons
To compare mode to results with observation, we applied a harmonic analysis by MATLAB
toolbox which is called T_tide software. The T_tide toolbox is written by Rich Palowiz and utilizesthe least square fit method for decomposing tidal signal from a record of water level time series.
The model results of 1-year are used for harmonic analysis in order to well distinguish between
K2 and S2 frequency.
The difference between the model and altimetry data are represented in Figure 7 andFigure 8 along the satellite track for 4 main constituents K1, O1, M2, S2. The comparison showsthat in the deep water the model agrees well with altimeter data. The errors for the depth > 80m
are less than [2cm, 4o] for K1 and O1; less than [4cm, 6o] for M2 and S2. This means the tidal
open-boundary are properly set in the model. The errors increase when getting into the shallowerwater. In the offshore region of Tonkin Gulf, the errors are about [3-4 cm, 4o] for K1 and O1; [6 8cm, 8 – 10o] for M2 and S2. There are higher error values exists at the eastern Hainan Strait andto the north of the gulf. The increase of the error in the model is could be either the erroneous in
altimeter data in the coastal zone or the model coastal bathymetry which is not completelyaccurate. There are higher errors along the coastal of China than in Vietnam which we don’t haveany better bathymetric data. Also note that insufficient model resolution could be another causeof the error (Minh et al., 2014).

Figure 7. Tidal amplitude errors in meters

4.2.3.

Figure 8. Tidal phase errors in degrees
Ocean current comparisonsFigure 9. Comparison of sea surface current between model and HF radar from Dec 01,
2014 to Dec 31, 2014
In this part, we collected the surface current data from HF radar systems and compare withthe results of the model (Figure 9). We extracted the region of HR radar data which covers
RMSE = 9.1 cm/s
more than 90 percent of data return for this comparison. The results show that the modeland the current data are well agreed in the offshore region (Figure 10). The meandifferences between model and HF radar data are 9.1 cm/s for u component and 12.4 cm/s
for v component. Also note that there is higher error in the coastal region near DHOI site.
The erroneous of the HF current data in this region caused by the baseline of DHOI-XUAN
sites, in which the angle between radial beams are nearly parallel to each other (Kim et al.,2014).

40.00
20.00
0.00
-20.00

126
51
76
101
126
151
176
201
226
251
276
301
326
351
376
401
426
451
476
501
526
551
576
601
626
651
676
701
726

u-component (cm/s)

60.00-40.00
Radar

ROMS

20.00
0.00
-20.00

1
26
51
76
101
126
151
176
201
226
251
276
301
326
351
376
401
426
451
476
501
526
551
576
601
626
651
676
701
726

v-component (cm/s)

40.00-40.00-60.00Radar

ROMS

Figure 10. Comparison of sea surface current between Model and HF radar at position(107oE, 18oN) from Dec 01, 2014 to Dec 31, 2014
4.3. Surface circulation of Gulf of Tonkin
We ran the model from June 5, 2012 to November 15, 2014, then calculated and analyzed
the average monthly flow in the Gulf of Tonkin.
In the winter months: December, January and February, the monthly mean surface of theGulf of Tonkin has the following points: the southern flow of Hainan Island along the shore. Thewater in the western part of the island goes into the gulf then goes up to the north in combination
with a small amount going from the Quynh Chau Strait to the east of the Vietnamese coast, thiscirculation follows the topography of the North Gulf coastline then goes to the south. During thesemonths, the central area of the bay has a vortex with an average speed of about 10-15 cm/s. The
sea area south of Thanh Hoa and northern Quang Binh has a higher speed of 25-30 cm/s.
Northern part of the gulf exists a small scale with diameter of about 50 - 70 km with an average
speed of about 5-8 cm/s.In the spring months (March, April and May), the average circulation structure of the surfaceof the Tonkin Gulf has a small change compared to the winter time. At the centre of the gulf, acyclonic eddy exists with the amount of water flowing through the mouth of the bay from the westside of Hainan Island with an average speed of about 8 - 12 cm, the flow of the sea area south ofThanh Hoa and northern Quang Binh still maintain a high speed, reaching 18-25 cm/s. The
northern eddy of the gulf tends to widen and shift southward but in May the eddy is broken.
The summer months of the circulation of the Gulf of Tonkin have a big change. In June, thearea between the Gulf of Tonkin formed a reverse circulation. The velocity of this circulation is
not large, averaging about 8-12 cm/s. The north part of the gulf has a small-scale eddy. Thiscirculation exists until the end of August. In overall, the current pattern in summer seems to be
more stable than other seasons
For the autumn months (September, October and November), the cyclonic circulation inthe area between the Gulf of Tonkin is reset. The average velocity of this circulation is quite large,
about 15-20 cm, in which the southern waters of Thanh Hoa and northern Quang Binh haveaverage surface velocities of 30-40 cm/s with points up to 45 cm/s. The north of the gulf always
has two small-sized eddies, swirling at the top of the bay and swirling underneath it. The scale ofthese two vortices is about 70-90 km with an average speed of about 8-10 cm/s (Figure 11).
HF Radar
ROMS Model

Figure 11. HF Radar current and Model results
V. SUMMARY AND DISCUSSIONSThe surface seasonal circulation is shown to have similarities with previous research results.
However, according to the results of this analysis, some small circulation above the Gulf of Tonkin
operates in a seasonal cycle, which is shown quite clearly. In summary, ROMS model performed
very well to simulate the Tonkin gulf dynamic. The differences between the model and observation
are quite acceptable and the model is able to simulate the Tonkin gulf hydrodynamic features.
However, the works we did above is just limited to the hydrodynamic comparison, the temperatureand salinity comparison are not considered. In the next step, we keep continue the works tocompare both features with observation data.
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