Nghiên cứu động học và cấu trúc khối nước tại vùng biển thềm lục địa nam việt nam vào mùa hè năm 2010

An interesting result was an observation of very high turbidity bottom layer of 20-30 m thick located at the outer shelf area which probably corresponds to advection of coastal water w

WATER MASS STRUCTURE AND DYNAMICS OVER THE SOUTHERN

VIETNAM SHELF IN SUMMER 2010

Vvacheslav Lobanov' Nguyen Ba Xuan', Aleksandr Sergeev", Nguyen Kim

Vinh^ Nguyen Van Tuan', Igor Gorin', Pham Xuan Duong', Pham Sy Hoan',

Pavel Sbcherbinin', Aleksandr Voronin'

'V.L II'ichev Pacific Oceanological Institute, Far Eastern Branch, Russian

Academy of Sciences, 43 Baltiyskaya Street, Vladivostok, 690041, Russia;

'Inslitute of Oceanography, Viemam Academy of Science and Technology, 01

Cauda, Nha Trang, Vietnam, (lobanov@poi.dvo.ru)

Abstract:

The paper presents results of first survey implemented in summer of 2010

in accordance with the Sub-project 2 of Vietnam-Russia collaboration on

marine meteorological hydrological and dynamical investigation the

hydrographic observations The survey included observations of physical

and ecological parameters with a goal to characterize summer conditions in

the continental shelf waters of Khanh Hoa province, southem Vietnam

Based on these observations a verticgl structure and spatial distribution of

main oceanographic parameters and structure of currents and frontal zones

off central Vietnam were analyzed In Summer of 2010 an anomalous

anticyclonic circulation and warm water pool was observed off southem

coast of Vietnam which is different from the situation of previous years This

dynamic system caused convergence of warm surface water at the coastal

area instead of cold water upwelling zone usually occurred there in previous

summer seasons Water temperature was higher and exceeded 30^C at the

surface Vertical profiles of other oceanographic parameters demonstrated

baroclinic structure over the shelf with two or three layers of interleaving

water masses An interesting result was an observation of very high turbidity

bottom layer of 20-30 m thick located at the outer shelf area which probably

corresponds to advection of coastal water with high content of suspended

matter or high productivity area over the shelf which leads to formation of

nepheloid layer The measurements of currents had confirmed a strong

diurnal tide impact on shelf water dynamics off central Vietnam Maximum

currents were observed up to 50 cm/s in surface layer and more then 30 cm/s

at the bottom Vertical structure of tidal flow had opposite directions in

surface and bottom layer during the tidal phase shift A soliton like feature

was observed propagating in Nha Trang Bay and increasing currents speed

by two times and thus enhancing vertical mixing which may have important

ecological consequences

Introduction

Water circulation off Vietnam is characterized by strong monsoon impacts with sharp changes between winter and summer seasons [1,2] Northeastward current along the coast is typical for summer season (Fig.l) At the same time an anticyclonic eddy or gyre has been often observed located to the southeast [3, 4,5] and thus influencing on intensiflcation of the current and formation of upwelling along the Vietnam coast Existence of mesoscale eddies and their motion makes water dynamics on the shelf to be complicated [6] In summer 2010 in accordance with the Sub-project-2 of Vietnam-Russia collaboration on marine meteorological, hydrological and dynamical investigation the hydrographic observations were implemented over the shelf area of southem Vietnam to study water mass structure and dynamics in the peak of summer season

1097 -ma less

Fig 1 Scheme of the survey implemented in July-August 2010 and location of stations Left figure shows typical currents off southern Vietnam in summer CTD stations are indicated by numbers Insertion on right figure shows detailed survey of Nha Trang Bay Location of stations with repeated CTD casts (Ml and M2) and mooring current meter

stations (Cl, C2 and C3) are indicated by stars

Observations

The survey was implemented during July 24 - August 4, 2010 using research vessel NCB-95 belonged to 10 VAST (Fig.l) The observations basically included multi-parameter CTD survey along flve sections with the SBE19 plus instruments equipped with temperafaire, conductivity and pressure, dissolved oxygen (DO), chlorophyll-a (Chl-a), turbidity, pH and photosynthetic active radiation (PAR) sensors and measurements of currents at stations with the Compact-AEM current meter At two locations (MI and M2) a repeated CTD casts were carried out during 21-28 hours with 1-2 hour interval between the observations Additionally,

measurements of currents were carried out along the ship track with acoustic

Doppler current profiler ADP250 and also at three long-term mooring sites (CI,

C2, and C3) with acoustic profiler RDCP600 equipped with additional temperature,

dissolved oxygen and turbidity sensors A main survey consisted of 4 sections of

10 stations each directed off the coast for around 50 kin However because of gale

weather conditions observations at distant parts of most of the sections were not

available In addition a more detailed survey of Nha Trang Bay was carried out as

shown on insertion of right part of Fig 1 The main survey area covered latitude

1 r45'-12°30' N and longitude 109°13'-109°42' E and depth lange from 15 to 147

m As a whole the 47 stations were taken during the survey including 2 stations

with repeated CTD casts, in total 111 CTD casts were obtained

Distribution of water properties

Vertical profiles A vertical distribution of measured parameters of Vietnam

shelf waters are presented at Fig 2 A water temperature varied from 28-31 °C at

the surface down to 16-18 °C below 100 m Surface salinity were from 31,5 at the

coastal zone to 33,7 at the sea and increasing with the depth to 34,5 Dissolved

oxygen content was in general high at surface layer (200-220 umol/kg) with some

decreasing at coastal area down to 140 umol/kg, and it was low as 110-120

umol/kg below 50 m depth layer A deep maximum of dissolved oxygen occurred

around 70-90 m depth Maximum of Chl-a content was observed around 25-40 m

over the whole survey area indicating phytoplankton activity at the surface

thermocline layer Maximum values of Chl-a were up to 1,0-1,5 ug/1 which is quite

normal Vertical distribution PAR shows a lower boundary of euphotic zone to be

around 30-40 m A distribution of turbidity presented a surprising feature of very

high turbidity layers located at the bottom This turbid bottom layer was observed

off the coast starting from depth 40 m

Vertical sections An example of water properties distribution along the sections

is presented at Fig 3 for section 2 located in Nha Trang Bay All main features of

the properties distributions described above can be seen here Surface waters are

characterized by warmer and less saline water at coastal area and slightly colder

and much saline waters far from the coast Deep sea waters with lowest

temperature, highest salinity and low oxygen content can be seen at outer shelf at

depth below 100 m Along bottom high turbidity layer is visible along whole

section 2, while it is presented at only some stations of other sections (not shown

here) The thickness of this layer is 15-30 m Deep dissolved oxygen maximum is

located at 70-90 m at far end of the section and not close to the cost Low values of

PAR at western part of the sections are because of completion of the sections after

the sunset

Spatial distribution Distribution of water properties at surface and bottom

layers of the survey area is presented at Fig 4 At surface layer (upper panel), a

low salinity water is observed in Nha Trang Bay coastal zone (as a result by rivers

discharge) while it is not seen in northern and southem areas This water is

associated with higher turbidity which confirms its origin Important feature of

bottom layer distribution (lower panel) is an area of high turbidity located as, meridional zone from north to south wifli maximum values in depth interval 40.61

m (Fig 4, f) High ttirbidity bottom layer is not presented at southern part cf % survey area

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Fig 2 Vertical profiles of water temperature (a), salinity (b), dissolved oxygen (c), chlorophyll-a (d), PAR (e), and turbidity (f) for all stations of the survey Temporal variability Variations of vertical profiles of water properties from surface down to bottom (40 m) during 28 hours based on every hour repeated CTD casts at station M2 are shown at Fig 5 The station M2 was located close to the station 13 of section 2 (Fig.l) just at the westem (coastal ward) edge of bottom high turbidity area (Fig 4) A most notable feature is changes of water properties al bottom layer with decreasing of temperature and oxygen and increasing of salinity and turbidity at the phase of high tide

This means that waters with high turbidity moves toward the coast with tidal flow During ebb tide less turbid waters came to the M2 site from the coast The thickness of this layer is around 15 m This water advection is also associated with vertical motion of thermo- and halo-cline as well as lower boundary of high oxygen content upper layer and lower boundary of euphotic layer (Chl-a maximum layer) On the background of drastic changes associated with day and night periods, variations of PAR do also reflect short-term vertical displacements of euphotic layer probably associated with internal waves

186

Fig 3 Vertical distribution of water temperature (a), salinity (b), turbidity (c), dissolved oxygen (d), Chl-a (e) and PAR (f) along section 2, Nha Trang Bay

r m p a n a m f ^ S Otpth /rn/-J SalMHy ff»Mj A OWA fntf^ TuttiOity IFTU} e O^Vi fm/*J

Fig 4 Spatial distribution of water temperature (a, d), salinity (b, e) and turbidity (c, f) at

surface (a-c) and bottom (d-f) layers of the study area

Workshop: "International Cooperation op

Fig 5 Temporal variation of temperature (a), salinity (b), turbidity (c), dissolved oxygen (d), Chl-a (e) and PAR (i) at every hour repeated CTD cast station M2 ftom 10:00 ot

August 1 through 14;00 of August 2, 2010

Water masses over the shelf

Based on water mass properties discussed above at T,S-diagrams (Fig.6) one can suggested 3 water masses of the survey area: a surface water dov™ to 20 m depth characterized by low salinity (below 33,5), intermediate waters between 20 and 80 m and the coldest and saline deep shelf waters (below 80 m) It should be noted that high turbidity layer is associated with intermediate waters of 23 sigma isopicnal surface, while a high Chl-a layer is located higher at 22 sigma This may suggest a relation between high productivity of phytoplankton (Chl-a max) and located below high turbidity layer as a result of plankton decay and sinking of large amount of organic matter

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Despshdf

(a) water i)

Fig 6 T, S diagrams for the stady area Color scale indicates depth (a), turbidity (b) and

Chl-a (c)

Water dynamics

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Fig 7 Variations of currents at mooring station C2 in central part of Nha Trang Bay from 09:59 of August 1 through 12:11 of August 2,2010 based on acoustic current profiler

measurements

Fig 8 Variability of current speed (a) and turbidity in bottom layer (b) at mooring station

Cl in southem part of Nha Trang Bay firom 11:51 tiirough 17:07 on31 July 2010 based on acoustic current profiler measurements Occurrence of soliton-like feature that increased

currents by two times and increased turbidity is shown by ellipse

Direct measurements of sea currents were implemented by acoustic Doppler current profiler (ADCP) in 3 locations of the study shown at Fig 1 As an example results of observations at station C2 located close to repeated CTD station M2 are shown at Fig 7 Vertical profile of currents was measured by RDCP600 located at the bottom (depth around 40 m) with 1 m step starting 1.5 m from the instrament sensor Adtlitional sensors located at the RDCP measured water temperature, dissolved oxygen content and turbidity at bottom layer Fig 7 shows strong diumal

tidal currents in the area with strong baroclinic component Two maxima of current velocity are located at surface and bottom layers and are out of phase, e.g currents toward the coast (high tide) at surface layer may be associated with currents toward the sea (ebb tide) at bottom layer and vice versa

A statistics of all 3 experiments of current measurements (Fig 1) are presented

at Table 1 Maximum currents were observed up to 50 cm/s in surface layer and more then 30 cm/s at the bottom Strong tidal currents at bottom layer may be responsible for high tiirbidity layer formation A soliton like feature was observed propagating in Nha Trang Bay and increasing currents speed by two tirnes and thus enhancing vertical mixing and turbidity in a bottom layer (Fig 8) Solitary waves generated by internal tides are quite common in the area, e.g [7, 8] Their propagation in the coastal area may thus have important ecological consequences

Tab 1 Statistics of currents measurements at 3 locations Cl, C2 and C3

Station

Cl

C2

C3

Date

0731

0801

0804

Depth,

m 2,6 13,6 2,4 15,4 34,4 3,1 15,1

Max

current speed, cm/s 28,8 33,6 50,5 35,6 30,3 32,4 22,0

Mean current speed, cm/s 11,9 15,0 25,9 7,7 13,8 17,9 11,0

Residual currents Speed,

cm/s 10,6 14,4 12,9 0,8 3,3 17,5 9,4

Direction, deg 314,8 321,2 28,4 24,0 86,8 222,8 194,8 Conclusion

The first joint Russian-Vietnam survey implemented in summer of 2010 allowed to obtain information on water dynamics and properties distribution to characterize summer conditions in the continental shelf waters of Khanh Hoa province, southem Vietnam Based on these observations a vertical structure and spatial distribution of main oceanographic parameters and structijre of currents and frontal zones off central Vietnam were analyzed In Summer of 2010 an anomalous anticyclonic circulation and warm water pool was observed off southem coast of Vietnam which is different from the situation of previous years [9] This dynamic system caused convergence of warm surface water at the coastal area instead of cold water upwelling zone usually occurred there in previous summer seasons Water temperature was higher and exceeded 30°C at the surface Vertical profiles

of other oceanographic parameters demonstrated baroclinic sfructure over the shelf with three layers of interleaving water masses An interesting result was an observation of very high turbidity bottom layer of 20-30 m thick located at the outer shelf area which probably con-esponds to advection of coastal water with, high content of suspended matter or high productivity area over the shelf which

190

leads to formation of nepheloid layer The measurements of currents had confirmed

a strong diumal tide impact on shelf water dynamics off central Vietnam Maximum currents were observed up to 50 cm/s in surface layer and more then 30 cm/s at the bottom Vertical structure of tidal flow had opposite directions in surface and bottom layer during the tidal phase shift A soliton like feature was observed propagating in Nha Trang Bay and increasing currents speed by two times and thus enhancing vertical mixing which may have important ecological consequences

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195 pp [Available from Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093.]

2 Hu J., Kawamura H., Hong H and Qi Y (2000) A Review on the Currents in the South China Sea: Seasonal Circulation, South China Sea Warm Current and Kuroshio Intrusion J Oceanogr Vol 56, N6, pp 607-624

3 Pohlman T., B.H.Long, H.Hein, N.K.Vinh and J.Dippner (2008) Investigation of the upwelling in South Vietnam in the frame of the joint German-Vietnamese cooperation project Proc Nat Sci Conf "Bien Dong- 2007", Nha Trang, 12-14.09.2007, Publishing House for Sci and Tech., Hanoi, Vietnam, pp 15-42

4 Vinh N.K (2008) On the characteristics of circulation in the upwelling region of the south of Vietnam Proc Nat Sci Conf "Bien Dong- 2007", Nha Trang, 12-14.09.2007, Publishing House for Sci and Tech., Hanoi, Vietnam, pp 601-614

5 Dippner J., B.H.Long, N.K.Vinh and T.Pohlmann (2008) The Vietnamese upwelling in the light of climate variability Proc Nat Sci Conf "Bien Dong- 2007", Nha Trang, 12-14.09.2007, Publishing House for Sci and Tech., Hanoi, Vietnam, pp 573-590

6 Zhuang W., S.P Xie, D: Wang, B Taguchi, H Aki, and H Sasaki (2010) Intraseasonal variability in the sea surface height over the South China Sea, J Geophys Res., 115 {C04010), doi: I0.1029/2009JC005647

7 Wang Y.-H., C.-F Dai and Y.-Y Chen (2007), Physical and ecological processes of internal waves on an isolated reef ecosystem in the South China Sea, Geophys Res Lett., 34, LI 8609

8 Xuan N.B and T.V Chung (2008) Some results of experimental computation and study

of the amplitude and influence of internal wave to the hydroacoustic velocity field in the central region of Vietnamese waters Proc Nat Sci Conf "Bien Dong- 2007", Nha Trang, 12-14.09.2007, Publishing House for Sci and Tech., Hanoi, Vietnam, pp

627-644

9 Polyakova A.M (2011) Classification of atmospheric processes over the South China Sea Meteorology and Hydrology N 5 P 17-24 (in Russian)