Effects of Monsoon Activity on Monthly Phytoplankton Blooms in the Gulf of Thai Land in El Nino Year 2002

VNU Journal of Science: Earth and Environmental Sciences, Vol. 34, No. 2 (2018) 22-27 
 22 
Effects of Monsoon Activity on Monthly Phytoplankton 
Blooms in the Gulf of Thai Land in El Nino Year 2002 
Le Van Thien * 
Hanoi University of Natural Resources and Environment, Cau Dien, Nam Tu Liem, Hanoi, Vietnam 
Received 19 March 2018 
Revised 14 April 2018; Accepted 18 April 2018 
Abstract: The Gulf of Thailand is a semi-closed Gulf on the west and southwest side of the 
Indochina Penisula and experiences reversal monsoon. The object of the present study is to 
investigate monthly and spatial distributions of the phytoplankton in the Gulf of Thailand during 
whole El Nino year 2002 by using remote-sensing measurements of chlorophyll-a (Chl-a) and 
surface wind vectors. Results show that monthly and spatial variations of the phytoplankton 
blooms are primarily associated with the monsoonal winds. In general, the average monthly Chl-a 
concentrations were quite low (<0.5 mg m
-3
) most area of the Gulf, with a belt of higher Chl-a 
concentrations along the coast during throughout year. Phytoplankton blooms extensively offshore 
in the near-coastal area of the Gulf in January and February, which is consistent with the winter 
northeast monsoon. In particular, one peak of Chl-a concentrations was observed in December. 
Areas with higher Chl-a concentrations along the coast were observed in both winter and summer 
monsoon months. 
Keywords: Phytoplankton blooms, Monsoon, Gulf of Thailand, El Nino. 
1. Introduction
The Gulf of Thailand is a semi closed sea 
and on the west and southwest side of the 
Indochina Penisula (Fig 1). The population in 
the coastal area of Gulf of Thailand is large, and 
the Gulf of Thailand is a rapidly developing 
area both in economics and society, particularly 
in aquaculture sectors. Physical, chemical and 
biological processes in the ocean are in an 
_______ 
 Tel.: 84-1676171337. 
 Email: thienmet@gmail.com 
 https://doi.org/10.25073/2588-1140/vnunst.4232 
intimated relationship [1, 2]. The physical 
properties such as the horizontal distribution of 
bottom cold, saline, and heavy water masses in 
the Gulf of Thailand could be related to the 
incidence and direction of monsoon winds in 
that gulf [3]. The monthly variation of the heat 
flux could be correlated with the sea surface 
wind in the Gulf of Thailand [4]. Chlorophyll-a 
is an index of phytoplankton biomass. 
However, characteristics of chlorophyll-a and 
its distribution associated with monsoon 
activity have remained unknown or poorly 
known for most of the gulf. In the present 
study, we investigated monthly and spatial 
L.V. Thien / VNU Journal of Science: Earth and Environmental Sciences, Vol. 34, No. 2 (2018) 22-27 
23 
variations of Chlorophyll-a (Chl-a) and sea 
surface wind conditions in the Gulf of Thailand 
during the whole El Nino year 2002 by 
examining satellite measurements. 
2. Study area and satellite data, and methods 
2.1. Study area 
The study region is the Gulf of Thailand 
(area in Fig. 1, 100
0
E – 1040E, 60N – 120N). 
The average depth of Gulf of Thailand is about 
40m. This region experiences reversal 
monsoons with the southwest monsoon in the 
summer and northeast monsoon in the winter. 
2.2. Satellite-derived chlorophyll-a 
Sea viewing Wide Field-of View Scanner 
(SeaWiFS) derived Chlorophyll-a was 
processed using the Ocean Color 4-band 
algorithm (OC4) [5, 6]. Monthly averaged Chl-
a concentrations with 3x3km spatial resolution 
were obtained and processed for the study 
region. Ocean Color and Temperature Scanner 
(OCTS) aboard Advanced Earth Observing 
Satellite observed the Chl-a concentration in the 
surface layer from October 1996 to June 1997 
with quality similar to that of SeaWiFS [7]. 
SeaWiFS-derived Chl-a concentrations are 
consistent with survey measurement in most 
area in the western South China Sea, including 
coastal waters [2]. 
2.3. Satellite-derived surface vector winds 
Sea surface vector winds have been 
measured from the microwave scatterometers 
[8]. We used 0.5-degree monthly mean wind 
fields obtained from the QuickBird satellite 
which was launched in June 1999. QuikScat is a 
radar device that transmits radar pulses down to 
the Earth’s surface and then measures the 
power that is scattered back to the instrument. 
Wind speed and direction over the ocean 
surface are obtained from measurements of the 
QuikScat backscattered power [8]. 
Figure 1. Bathymetry of the study area. 
Figure 2. Monthly mean SeaWiFS Chl-a 
for January 2002. 
3. Conditions of surface winds and Chl-a 
distributions and phytoplankton blooms 
The monthly variations and spatial 
distributions of Chl-a concentrations and 
surface winds from January to December 2002 
were analyzed and shown by some 
representative figures. During January, the 
Chl-a in the center of the Gulf is very low (<0.5 
mg m
-3
). However, a belt of high Chl-a 
L.V. Thien / VNU Journal of Science: Earth and Environmental Sciences, Vol. 34, No. 2 (2018) 22-27 
24 
concentrations along the coast of the Gulf (Fig. 
2) and strong northeast monsoon winds (> 
7m/s) were observed on the south side of the 
gulf below latitude 9N (Fig 3). Particularly, the 
strong phytoplankton blooms with high Chl-a 
concentrations (> 1.5 mg m
-3
) appeared in the 
offshore region with a tongue shape in this 
month (Fig. 3). 
These characteristics were found to be 
similar in February although the extended area 
of high Chl-a and the magnitudes of winds were 
smaller than in January (not shown). The 
distribution of Chl-a concentration has similar 
patterns with the coastal phytoplankton blooms 
and values during March and April (not 
shown). The weaker south and southeast 
monsoon winds dominated almost entire the 
gulf and ranged from 4-5.5 m/s during these 
two months (not shown). The bloom 
strengthens in May along the eastern coast area 
and the southwest monsoon onset was obvious 
as the monsoon winds started changing in the 
direction to south and southwest all over the 
Gulf (not shown). The bloom developed in the 
eastern gulf and weakened in the western gulf 
along the coastal lines from June to September 
(Fig. 4). 
Figure 3. Monthly mean QuikScat surface vector 
winds for January 2002. 
Figure 4. Monthly mean SeaWiFS Chl-a 
for July 2002. 
The prevailing winds in the gulf were very 
strong southwesterly winds with surface wind 
speed reached from 5-10m/s during these 
months (Fig. 5). 
The bloom seems a little weakened in 
October (not shown). The monthly mean winds 
lessened during this month (not shown). A 
longer intense bloom was found in November 
and December near the coast (Fig. 6). 
Figure 5. Monthly mean QuikScat surface vector 
winds for July 2002. 
L.V. Thien / VNU Journal of Science: Earth and Environmental Sciences, Vol. 34, No. 2 (2018) 22-27 
25 
The strong extended bloom father offshore 
has a similar patch of high Chl-a in both 
December and January. This behavior of 
phytoplankton is the same as shown in 
November and February. It is worth to note that 
the prevailing winds were the strongest 
northeast winds through the year in these two 
months (Fig. 7). 
Figure 6. Monthly mean SeaWiFS Chl-a for 
December 2002. 
Figure 7. Monthly mean QuikScat surface vector 
winds for December 2002. 
4. Discussion 
In general, Chl-a concentration in the 
coastal area of the Gulf of Thailand was higher 
than that in the offshore area. The 
phytoplankton blooms with high Chl-a 
concentration (>1.5 mg m
-3
) appeared in the 
extended offshore regions in January, February, 
November, and December and decreased during 
transition month in April. In the center area of 
the Gulf, Chl-a concentrations were usually 
relatively low (<0.5 mg m
-3
) throughout the 
year. In the coastal zones, Chl-a concentrations 
were generally high throughout the year and 
further enhanced during the strong northeast 
winter monsoon winds of November, December 
and January and strong southwest summer 
monsoons winds of June-September. In 
particular, Chl-a concentration was peak during 
the strong northeast monsoon winds in January, 
November, and December. 
The Gulf of Thailand dominates by the 
Asian monsoon, which obviously illustrates the 
reversed wind direction in a year with northeast 
winds in the winter and southwest winds in the 
summer. 
So hat factors that may cause the bloom in 
this gulf? In this section, we discuss some 
physical processes that may contribute to the 
bloom. First, the coastal upwelling which is the 
consequence of the offshore transport of wind 
drove surface current due to the Coriolis effects. 
The upward movement of waters causes the 
intense phytoplankton blooms. However, the 
northeasterly and southwesterly monsoon winds 
during months in this year do not favor coastal 
upwelling along the coast. Another process that 
contributes to the bloom is from Ekman 
pumping. This Ekman pumping was thought to 
cause strong upward motion. Upwelling by 
Ekman pumping during the monsoon activity is 
able to enhance Chl-a concentration and induce 
the bloom. A comprehensive study is required 
for this elucidation when mean Ekman pumping 
calculated from winds. We will leave it for a 
future study. 
L.V. Thien / VNU Journal of Science: Earth and Environmental Sciences, Vol. 34, No. 2 (2018) 22-27 
26 
In addition, many previous papers have 
demonstrated that vertical mixing is associated 
with abundant plant and animal biomass [9-11]. 
Entrainment of nutrient-rich water by wind 
mixing may act to enhance phytoplankton 
blooms during monsoon in this gulf. The strong 
winds during northeast monsoon in the winter 
mix water to deeper depths and thus bring 
nutrients to the mixed layer induced high Chl-a. 
During the mature phase of El Nino, [12] 
showed that a decrease in cloudiness over the 
Gulf induces an increase in the shortwave 
radiation in November. Thus the strong winds 
during the northeast winter monsoon may mix 
water to deeper depths and consequently induct 
nutrients to the mixing layer resulting in high 
Chl-a in the clear sky period of El Nino year. 
Thus, the importance of monsoonal winds in 
the Gulf as a physical process which may 
enhance Chl-a appears to be a major forcing 
factor during the northeast monsoon in this El 
Nino year over the Gulf of Thailand. 
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Hooker, and E. R. Firestone, NASA Tech. 
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[6] O'Reilly J.E., Maritorena S., Mitchell B. G., 
Siegel D.A., Carder K.L., Garver S.A., Kahru 
M., and McClain C., 1998. Ocean color 
chlorophyll algorithms for SeaWiFS, J. Geophys. 
Res, 103(24), p.937–24,953. 
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mission overview, J. Oceanogr, 54, p.383–399. 
[8] Wentz F.J., Smith D.K., Mears C.A., and 
Gentemann C.L, 2001. Advanced algorithms for 
QuikScat and SeaWinds/AMSR, paper presented 
at IGARSS '01, NASA, Washington, D. C. 
[9] McGowan J.A., and Hayward T.L., 1978. Mixing 
and oceanic productivity, Deep Sea Res, 25, 
p.771-793. 
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productivity, in Primary Productivity in the Sea, 
edited by P. G. Falkowski, p. 121-137, Plenum, 
New York. 
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observed winter blooms of phytoplankton in the 
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L.V. Thien / VNU Journal of Science: Earth and Environmental Sciences, Vol. 34, No. 2 (2018) 22-27 
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Ảnh hưởng của hoạt động gió mùa đến sự bùng nổ thực vật 
phù du trong các tháng của năm El Nino 2002 
ở Vịnh Thái Lan 
Lê Văn Thiện 
Trường Đại học Tài nguyên và Môi trường Hà Nội, Cầu Diễn, Nam Từ Liêm, Hà Nội, Việt Nam 
Tóm tắt: Vịnh Thái Lan là một vịnh gần như khép kín ở phía Tây Nam và Tây của bán đảo Đông 
Dương và là vịnh có sự dịch chuyển ngược chiều của hoạt động gió mùa. Mục tiêu của nghiên cứu này 
là nghiên cứu sự phân bố theo không gian và theo các tháng của thực vật phù du ở Vịnh Thái Lan 
trong toàn bộ một năm El Nino 2002 bằng việc sử dụng số liệu quan trắc từ vệ tinh của nồng độ 
chlorophyll-a (Chl-a) và véc tơ gió bề mặt. Các kết quả nghiên cứu chỉ ra rằng sự biến đổi theo không 
gian và theo các tháng của việc bùng nổ thực vật phù du là chủ yếu liên quan đến sự hoạt động của gió 
mùa. Nhìn chung, nồng độ Chl-a trung bình hằng tháng là khá thấp (<0,5 mg m-3) ở hầu hết các khu 
vực trong Vịnh, tuy nhiên có một dải có nồng độ Chl-a cao hơn dọc theo ven biển Vịnh trong suốt cả 
năm. Sự bùng nổ thực vật phù du mở rộng ra ngoài khơi trong các khu vực gần ven biển của Vịnh 
trong tháng 1 và tháng 2, đây cũng là tháng phù hợp với hoạt động của gió mùa đông bắc. Đặc biệt, 
nồng độ Chl-a lớn nhất được quan trắc thấy vào tháng 12. Các khu vực có nồng độ Chl-a cao hơn dọc 
theo ven biển là được quan trắc thấy trong cả các tháng mùa đông và các tháng mùa hè. 
Từ khóa: Bùng nổ thực vật phù du, gió mùa, Vịnh Thái Lan, El Nino.