The water circulation in area IV was calculated by 2 methods. The circulation for the area where water depth exceeded 600m was calculated by the geostrophic balance method. In shallow water area effect of wind absolutely surpasses geostrophic balance, so wind induced drift current is greater than geostrophic current many times. So, for the whole area (shallow deep) the drift current was calculated by two-dimensional nonlinear shallow water equation based on typical monsoon fields. The results of 2 methods showed common picture of the circulation with divergence and convergence changing by season

Lien, Nguyen Thi Viet

Quynh, Do Ngoc

Van Lanh, Vo

Van Ninh, Pham

Aquatic Commons

365
Proceedings of the SEAFDEC Seminar on Fishery Resources
               in the South China Sea, Area IV : Vietnamese Waters
Geostrophic and Drift Current in the South China Sea,
Area IV: Vietnamese Waters
1
 Prof. Dr. Pham Van Ninh, 1Assoc. Prof. Dr. Do Ngoc Quynh
2Assoc. Prof. Dr. Vo Van Lanh and 1Dr. Nguyen Thi Viet Lien
1
 Center for Marine Environment Survey, Research and Consultation,
 264 Doi Can Str., Ba Dinh Dist., Hanoi, Vietnam
2 Institute of Oceanography, Nha Trang, Vietnam
1 Cau Da  Str., Nha Trang City, Vietnam
 ABSTRACT
The water circulation in area IV was calculated by 2 methods. The circulation for the area
where water depth exceeded 600m was calculated by the geostrophic balance method. In a shallow
water area effect of wind absolutely surpasses geostrophic balance, so wind induced drift current is
greater than geostrophic current many times. So, for the whole area (shallow deep) the drift current
was calculated by two-dimensional nonlinear shallow water equation based on typical monsoon fields.
The results of 2 methods showed common picture of the circulation with divergence and convergence
changing by season.
 Introduction
The water circulation is one of the most essential characters in the marine hydrodynamical
regime. They are always very important factors for the management of fisheries and living resources in
the Ocean. There are some methods to determine currents in the sea. The net current can be determined
directly by the observation data measuring the direction and speed of the current at each station. This
method requires observation duration of at least 25 hours at each station in a study area in order to
obtain separately tidal components and remainder current (non-periodic or stable components). Clearly
that, the observation data of the Survey Cruise (30 April - 29 May 1999) of SEAFDEC in Vietnamese
sea (Area IV) don't satisfy the requirements of a direct method for current calculation.
The indirect methods to determine the net movement of sea water are enough abundant now.
The classical geotrophic balance is one of widely used indirect methods in Oceanography. It is somewhat
appropriate under the conditions of this survey. It requires accurate temperature, salinity and pressure
data which could be measured by CTD at each station. The current obtained from this geostrophic
balance method will be relative current between 2 layers. It is necessary to choose a deep water layer
which can be assumed to be the level of no motion. The absolute current at any levels above the level
of no motion can be then calculated. This procedure can be realized only in deep ocean, where the
current speed at a sufficiently deep water layer is usually very slow relatively to the surface current (less
than 1 cm/s). So the method of classical geotrophic balance is calculated suitably only for deep ocean.
In a shallow water the level of no motion couldn't be chosen, the results calculated by this method will
be incorrect. On the other hand, in a shallow water the effect of wind always surpasses absolutely
geostrophic balance and wind induced drift current always is dominant in the remainder current.
The drift current will be calculated by numerical method from two dimensional nonlinear shallow
water equation system with the typical wind fields taken into account.
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   Southeast Asian Fisheries Development Center
Thus, combining two methods (calculation from geostrophic balance and numerical modelling),
it is possible to obtain the common picture of water circulation in the study area: in the deep water the
current abides by the laws of geostropic balance, in the shallow water the current has an essence of drift
current.
Materials and Methods
 The Study Area
The study area is the surrounding Vietnam sea included the Gulf of Tonkin and the continental
shelf of Central and Southern Part of Vietnam. The study area is spreading from 6o to 22oN latitude and
from 105o to 112oE longitude. The shallow water zone occupied most of area. Among the total of 58
survey stations of Cruise from 30 April to 29 May 1999, only 16 stations had bottom depth exceeded
600 meter and at these station there level of no motion can be assumed.
The location of these stations and the bottom topography of the study area are shown in Fig.1.
 Geostrophic Current
The geostrophic balance method has been used widely for deep ocean and it can be said that
probably most of the subsurface circulation of the World Ocean up to now was obtained by this
approach. The basic assumption of this method is that in the case of an isobaric surface, for example,
sea  surface,  to maintain an unequal level then the horizontal pressure gradient force due to gravity must
be balanced by Coriolis force due the movement of water.
The general expression of the classical dynamic method is following:
L
DYNHDYNH
fu 21
−
=
           (3.1)
Fig. 1. Depth contours (m) of the study area
            and the location of stations (+).
102
?
103
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104
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105
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106
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115
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102
?
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115
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6
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6
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0
50
100
500
1000
2000
3000
4000
367
Proceedings of the SEAFDEC Seminar on Fishery Resources
               in the South China Sea, Area IV : Vietnamese Waters
where    f  - Coriolis factor,
u - Current speed in the direction perpendicular to the pressure gradient,
L - Distance (m) between the 2 stations,
DYNH1,2 - Dynamic height (in dyn.m). Subscript refers to station.
For each station, dynamic height was calculated following.
                                                             DYNH = gz                                                            (3.2)
where   g -  gravitational acceleration,
z - vertical distance between the interested surface and the reference surface (in m).
A lot of observation data on temperature, salinity and pressure for many years in the study area
has been collected. These are data in the summer period and the winter period. The summer consists of
May, June, July and August. The winter consists of December, January, February and March. The data
were collected for each square with one degree size. Based on the data collected during the Cruise 30
April - 29 May 1999 with these available data the picture of geostrophic current in the deep sea has
been calculated for each season (summer and winter).
 Wind Induced Drift Current
In order to simulate circulation and sea water level oscillation caused by wind stress, the TIDE-
2D software developed in Center for Marine Environment Survey, Research, Consultation (CMESRC)
was used. The calibration and verification of this model were implemented rather carefully by observation
data for several coastal and offshore zones of Vietnam sea, for example Ha Long Bay, Hai Hau, Thanh
Hoa, the mouth of the Gulf of Tonkin, Dinh An, Ca Mau...  This software has been used to calculate the
current regime in many coastal areas of Vietnam sea. This model is based on the two - dimensional
nonlinear shallow water equations:
Conservation of mass:
                       (4.1)
 Conservation of momentum:
)(
)(
2
2
2
22222
2
2
2
22222
y
v
x
v
D
d
u
d
vuv
f
y
z
g
y
v
v
x
v
u
t
v
y
u
x
u
D
d
v
d
vuu
f
x
z
g
y
u
v
x
u
u
t
u
yxy
yxx
∂
∂
+
∂
∂
+
+
+Ω−+−
∂
∂
−=
∂
∂
+
∂
∂
+
∂
∂
∂
∂
+
∂
∂
+
+
+Ω++−
∂
∂
−=
∂
∂
+
∂
∂
+
∂
∂
τττ
γ
τττ
γ
           (4.2)
where: u, v - Depth averaged components of velocity in the x - direction and y - direction (m/s),
z - Water surface elevation above reference level (m),
h - Water depth below reference level (m),
d - Total water depth (d = h + z),
W - Coriolis parameter (S-1),
g - Acceleration of gravity (m/s2),
t
x
, ty - The components of wind stress in x - and y - direction with the wind velocity taken at
           10m above the Mean Sea Level,
f - The friction coefficient,
g - The wind drag coefficient,
D - The horizontal turbulent coefficient.
The above equations are solved using implicit different scheme and alternating direction method
0)()( =
∂
∂
+
∂
∂
+
∂
∂
vd
y
ud
xt
z
368
   Southeast Asian Fisheries Development Center
in space (not in time).
Because the study area is influenced strongly by the Monsoons: the North-Eastern wind in
winter and the South-Western Wind in summer, the circulation regime also has the characters changing
in accordance with seasons. The drift current in the study area has been calculated by model TIDE-2D
for two season - winter and summer - in correspondence with two wind fields: NE and SW. The
chosen wind fields are the average wind in the typical month in winter and in summer (January and
June).
 Results and Discussion
 Geostrophic Current in the Deep Sea
The data of temperature, salinity and pressure in the study area were collected for the winter
(from December to March) and the summer (from May to August) at each square with one degree size
from oceanographic database. In addition the summer data also were supplemented by the observation
data of the survey cruise (May 1999).
Based on the collected data the dynamic height was calculated and corrected to 600 dbar
level, the assumed level of no motion. From the obtained dynamic height the dynamic topography map
was established for each isobaric level: sea surface, 50, 150 and 500 dbar. After that these results were
used to calculate velocity components u and v for each grid cell at each isobaric level.
Fig.2-Fig.5 show dynamic height and current vectors at different isobaric level in summer.
Fig.6-Fig.9 present dynamic height and current vectors and different isobaric level in winter.
Some characteristics of circulation calculated from geostrophic balance at the deep water are
following:
In  Summer  (the period of Survey Cruise)
On the maps for dynamic topography and geostropic current at isobaric level 0, 50, 150 and
500 dbar it can be noted that in general the isolines of dynamic height were parallel to the coastal line.
The current has Northern and North-Eastern direction (N and NE). There are two rather large and
strong eddies at the Southern zone: anticyclonic eddy near Vietnam shelf and cyclonic eddy near
Kalimantan  Island. The current in anticyclonic eddy was strengthened  at the South Central Part of
Vietnam zone with maximum speed over 30cm/s.
Near the Central Part of Vietnam there exist the cyclonic eddy at the area zone from 13oN to
16oN latitude with maximum speed about 25-30cm/s.
Generally spreading the system of eddies, general direction and tendency of the circulation are
similar from sea surface to deep levels, only the value of speed is quickly decreased with depth. Example,
of maximum speed at the surface was over 30cm/s then at 500 dbar level that is less than 5cm/s.
In correspondence with the cyclonic and anticyclonic eddies there exist the divergence zone
and the convergence zone. In the divergence zone the upwelling process occurs usually and on the
contrary in the convergence zone there is a downwelling process. These vertical processes could be
very important for fishery because it relates to fishing grounds in the sea.
Thus in the summer there exists the upwelling at offshore zone of the Central Part of Vietnam
about 13oN - 16oN latitude and the downwelling  at South - Eastern zone about 300 km far from the
coast.
In Winter
From Fig.6-Fig.9 presenting dynamic height and geostrophic current at  different levels (0, 50,
150 and 500 dbar) it would be possible to recognize that the system of dynamic height isolines on the
369
Proceedings of the SEAFDEC Seminar on Fishery Resources
               in the South China Sea, Area IV : Vietnamese Waters
           : V <= 5
          :  5  < V <= 10
          : 10 < V <= 15
          : 15 < V <= 20
          : 20 < V <=25
          :  >= 25
105                                   110                                    115
05
10
15
20
whole is oriented North-East (NE) to South-West (SE) direction similar to that in summer, although
these isolines in winter are more complicated.
There are two large and strong eddies: the cyclonic eddy in the Central Part of Vietnam zone
from 9oN to 16oN latitude and anticyclonic eddy in the South Part from 5oN to 8oN latitude. In the
cyclonic eddy zone the current is strengthened at the zone near to the South-Eastern coast of Hai Nam
Island and along the shore of Central Part of Vietnam from Da Nang to Phan Thiet. The speed can
reach 30 cm/s at the surface. In the anticyclonic eddy zone the current is strengthened at the zone along
the shelf of South Vietnam. The speed is over 30 cm/s at the surface.
Thus along the Vietnam shoreline over the area of 200 km width from Central Part to South
Part there exist the cold water current flowing from North to South in winter.
As in summer the eddies and tendency of circulation are somewhat similar from sea surface to
deep levels in winter, only the speed value quickly decrease with depth.
In correspondence with the cyclonic and anticyclonic eddies there exist divergence and
convergence zones. Specially, the divergence zone in winter is very large.
Fig. 3.  Dynamic height (dyn.mm) and
geostrophic current at 50  dbar
           in summer.
Fig. 2.  Dynamic height (dyn.mm) and
geostrophic current at sea
surface (0 dbar) in summer.
isolines of dynamic height
o o o divergence zone
+ + + + convergence zone
~
           : V <= 5
(cm/s )          :  5  < V <=
10 (cm/s)          : 10 < V <=
15 (cm/s)          : 15 < V <=
20 (cm/s)          : 20 < V
<=25 (cm/s)          :  >=
25 (cm/s)
105                     110                                115
05
10
15
20
370
   Southeast Asian Fisheries Development Center
           : V  < =  5   (cm /s)
          :  5  <  V  < =  10 (cm /s)
          : 10 <  V  < =  15 (cm /s)
          : 15 <  V  < =  20 (cm /s)
          : 20 <  V  < = 25 (cm /s)
          :  > =  25 (cm /s)
105                                   110                                    115                                   120
05
10
15
20
           : V  < =  5   (cm /s)
          :  5  <  V  < =  10 (cm /s)
          : 10 <  V  < =  15 (cm /s)
          : 15 <  V  < =  20 (cm /s)
          : 20 <  V  < = 25 (cm /s)
          :  > =  25 (cm /s)
105                                   110                                    115                                   120
05
10
15
20
           : V  < =  5   (cm /s)
          :  5  <  V  < =  10 (cm /s)
          : 10 <  V  < =  15 (cm /s)
          : 15 <  V  < =  20 (cm /s)
          : 20 <  V  < = 25 (cm /s)
          :  > =  25 (cm /s)
105                                   110                                    115                                   120
05
10
15
20
           : V <= 5
(cm/s )          :  5  < V <=
10 (cm/s)          : 10 < V <=
15 (cm/s)          : 15 < V <=
20 (cm/s)          : 20 < V
<=25 (cm/s)          :  >= 25
(cm/s )
105                               110                              115
05
10
15
20
Fig. 4. Dynamic height (dyn.mm)
and geostrophic current at
150 dbar in summer.
Fig. 5. Dynamic height (dyn.mm) and
geostrophic current at 500 dbar
in summer.
Fig. 6. Dynamic height (dyn.mm) and
       geostrophic current at sea
           surface 0 dbar in winter.
Fig. 7. Dynamic height (dyn.mm) and
geostrophic current at 50 dbar
in winter.
371
Proceedings of the SEAFDEC Seminar on Fishery Resources
               in the South China Sea, Area IV : Vietnamese Waters
           : V  < =  5   (cm /s)
          :  5  <  V  < =  10 (cm /s)
          : 10 <  V  < =  15 (cm /s)
          : 15 <  V  < =  20 (cm /s)
          : 20 <  V  < = 25 (cm /s)
          :  > =  25 (cm /s)
105                                   110                                    115                                   120
05
10
15
20
           : V <= 5
(cm/s)          :  5  < V <=
10 (cm/s)          : 10 < V <=
15 (cm/s)          : 15 < V <=
20 (cm/s)          : 20 < V
<=25 (cm/s)          :  >= 25
(cm/s)
105                                110                                115
05
10
15
20
The Drift Current in the Shallow Water
The model TIDE-2D was used to calculate the depth averaged current induced by wind stress.
As above said that in the shallow sea the effect of wind is more important in comparison with geostrophic
balance. So it is necessary to calculate the drift current in a shallow water area. Together with the
geostrophic current in the deep water the drift current in the shallow water establishes the picture of
circulation in whole study area.
In correspondence with averaged monthly wind field (see Fig.10-Fig.12) the averaged drift
current fields are calculated as follows:
- The drift current field averaged for May (coincidence with the period of the Survey Cruise)
- The drift current field averaged for June (typical for Summer)
- The drift current field averaged for January (typical for Winter)
The Fig.13-Fig.15 shows the averaged current fields for May, Jun and January respectively.
The picture of drift current changes by season has some characteristics as follows:
In Summer
At first it would be possible to realize that the averaged drift current manifests itself clearly only
in shallow water zone such as the Gulf of Tonkin or coastal zone along shore from North to South. In
the Gulf of Tonkin there exist the large and strong cyclonic eddy, the center of which is in the middle of
the Gulf. Thus along Vietnam shoreline from 21oN to 15oN latitude the current always flows to the
South. The depth averaged speed can reach 20 cm/s over there. In the North part of Gulf the current
is rather weak.
In the Coastal zone of the Central and Southern  Parts of Vietnam from 8oN to 15oN latitude
there exist the current flowing along shoreline to the North, the value of the depth averaged speed is
Fig. 8. Dynamic height (dyn.mm)
and geostrophic current at
150 dbar in winter.
Fig. 9. Dynamic height (dyn.mm)
          and geostrophic current at
          500 dbar in winter.
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   Southeast Asian Fisheries Development Center
Fig. 12. The wind field averaged for May. Fig. 13. The depth averaged drift current
for May.
Fig. 10. The wind field averaged for January. Fig. 11. The wind field averaged for June.
over 20 cm/s in the Southern coastal zone.
In Winter
It would be possible to say that in winter the drift current is stronger than in summer in the whole
coastal zone. Almost everywhere in the coastal zone from North to South the value of the depth
averaged speed is about 20 cm/s.
The drift current flowed along the shoreline to the South everywhere. Whole the Gulf of Tonkin
is a half of big eddy. The Cold water from the South China Coastal zone flows into the Gulf of Tonkin
by strait Quynh Chau and around Hai Nam Island, after that the current continued to flow along the
Vietnam shoreline to the South.
373
Proceedings of the SEAFDEC Seminar on Fishery Resources
               in the South China Sea, Area IV : Vietnamese Waters
Fig. 14. The depth averaged drift
              currentfor June.
Fig. 15. The depth averaged drift
              current for January.
Conclusion
The study area consists of the shallow water and the deep sea. The system of the circulation
there includes correspondingly the wind induced drift current and geostrophic current.
The circulation in study area changes clearly by seasons. The pictures of the circulation in
summer and winter have contrary characters.
There existed the divergence and convergence zones in correspondence with the upwelling and
downwelling processes, which could be important for fisheries purposes.
References
Fomin L.M. 1964.  The Dynamic Method in Oceanography. Elsevier, Amsterdam,  212pp.
NODC.  1994.  World Ocean Atlas. CD-ROM Data Sets.
VODC. 1999.   Oceanographic Data Sets. CD-ROM and Hard Disk.


Geotrophic and drift current in the South China Sea, Area IV: Vietnamese waters

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Geotrophic and drift current in the South China Sea, Area IV: Vietnamese waters

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