Water circulation and material transport in the coastal

Chapter 2
Water circulation and material transport in the coastal
areas and marginal seas of East and Southeast Asia
(Project-1)
Tetsuo Yanagi
Research Institute for Applied Mechanics, Kyushu University, Kasuga Park, Kasuga 816-8580, Japan
Introduction
There were many biological and chemical
coastal oceanographers but a few physical
coastal oceanographers in the Southeast
Asian countries at the time of the 1990s,
about twenty years ago. The knowledge on
the physical conditions in the coastal sea
is indispensable for the correct biological
and chemical understanding of oceanographic phenomena, because the biological and chemical oceanographers cannot
distinguish the temporal change of biota
density or chemical concentration at some
point in the coastal sea and the effect of
advection or diffusion there without the
correct knowledge of the advection and
diffusion around the observation point.
During the past ten years, the knowledge on the coastal physical oceanography
in the Southeast Asian region has remarkably increased mainly by the JSPS (Japan
Society for the Promotion of Science)
multilateral cooperative study, and the
coastal oceanographers became to understand that the knowledge of physical conditions is the base of chemical and biological coastal oceanography.
I introduce here some new findings on
the physical coastal oceanography in the
Southeast Asia during the past ten years.
Scientific Accomplishment
In Indonesia, the remote sensing technology gave new information on SST (sea
surface temperature) and SSC (sea surface
chlorophyll-a) in the coastal seas around
Indonesia.
The westward propagation of coastal
upwelling phenomenon along the Java Island during the southeast monsoon was
clarified using NOAA AVHRR images,
where the area with low SST propagates
westward (Fig. 1, Suhendar et al. 2002).
Moreover, SSC increases due to the
upwelling during the southeast monsoon
and decreases due to the downwelling during the northwest monsoon as shown in
Fig. 2 (Suhendar et al. 2002).
The numerical ecosystem model clarified the biochemical characteristics of
eutrophicated Jakarta Bay and pristine
Bantan Bay as shown in Table 1
(Nurdjaman and Yanagi 2002). Generally,
concentrations of ecosystem compartments
S. Nishida, M. D. Fortes and N. Miyazaki, eds.
Coastal Marine Science in Southeast Asia —Synthesis Report of the Core University Program of the Japan
Society for the Promotion of Science: Coastal Marine Science (2001–2010), pp. 13–22.
© by TERRAPUB 2011.
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T. YANAGI
Fig. 1. Seasonal variation in SST along the southern coast of Java Island by NOAA (Suhendar
et al. 2002).
Fig. 2.
Seasonal variation in SSC around Java Island by SeaWiFS (Suhendar et al. 2002).
in both bays are higher in wet season than
in dry season. Rainfall directly affects on
the growth of phytoplankton in Jakarta Bay
and the primary production in Jakarta Bay
(416–830 mgC/m2/day) is higher than in
Bantan Bay (84–122 mgC/m 2/day). According to the primary production, Jakarta
Bay is classified under mesotrophic and
Bantan Bay is orligotrophic. In Bantan
Bay, the regenerated production is higher
than the new production and plays an important role in material cycling in the lower
trophic level ecosystem, while in Jakarta
Bay the ratio of new production to regen-
erated production is almost one. Both nutrient load and recycling DIN (dissolved
inorganic nitrogen) play important roles in
the increase of Chl.-a concentration in
Jakarta Bay.
The residence time of fresh water as the
indicator of the water exchange played an
important role in the control of the water
quality at Hurun Bay (Suhendar et al.
2009). Long fresh water residence time in
both transition periods of Wet-Dry and
Dry-Wet seasons has increased the DIN
and TOM (total organic matter) accumulation in the water column, and it stimu-
Water circulation and material transport
Table 1. Comparison of Banten Bay and Jakarta Bay from the numerical ecosystem results
(Nurdjaman and Yanagi 2002).
Fig. 3.
Seasonal variations in freshwater residence time (a) and DIN concentration at the
surface layer (S) and bottom layer (B) (b) in Hurun Bay (Suhendar et al. 2009).
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Fig. 4.
Satellite image of coastal habitat (Komatsu et al. 2009).
lated phytoplankton bloom at Hurun Bay
(Fig. 3). Such situation has caused the DO
(dissolved oxygen) concentration decrease
due to large decomposition of organic
matter. The results recommended that in
both transition periods, the aquaculture
activity should be limited at minimum
level to reduce the risk of fish mass mortality caused by the DO depletion due to
the phytoplankton bloom.
Komatsu et al. (2009) succeeded to
develop the efficient mapping and monitoring systems of coastal habitats, such as
sea-grass beds and live corals, using ALOS
(Advanced Land Observing Satellite)
AVNIR-2 (Advanced Visible and Near Infrared Radiometer type-2) images at
Barrang Lompo Island near Makassar, Indonesia (Fig. 4).
In Malaysia, data from the World
Ocean Database for the Malacca Strait
were utilized to assess the seasonal variation in temperature, salinity and dissolved
oxygen in the Malacca Strait (Ibrahim and
Yanagi 2006). The data indicated the introduction of cool, deep, saline water from
the Andaman Sea during the Southwest
Monsoon. During the Northeast Monsoon,
Water circulation and material transport
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Fig. 5. Seasonal variation in water mass distribution at the Malacca Strait (Ibrahim and Yanagi
2006).
Fig. 6.
Sea surface Chl.-a distribution in Oct. 2003 by MERIS (left) and numerical model
(right) (Buranapratheprat et al. 2008).
the situation reversed and there was the
ingress of lower salinity water mass from
the south. This may be attributed to the
larger river discharge experienced during
the Northeast Monsoon and the introduction of lower salinity water mass from the
South China Sea. The influence of the
Andaman Sea and the South China Sea is
supported by the variation in the T-S plots
for the Malacca Strait. This is especially
discernible in the Northeast Monsoon and
in the subsequent Inter-monsoon period
(Fig. 5). Such results have implications for
the movement and exchange of material
between the Andaman Sea and the South
China Sea via the Malacca Strait.
In Thailand, local algorithm for the
analysis of ocean color image was devel-
oped in the upper Gulf of Thailand, where
the Case II water exists, based on the intensive multi-disciplinary field observations (Matsumura et al. 2006). At the same
time, the ecosystem model coupled with
the three-dimensional hydrodynamic
model was developed for the upper Gulf
of Thailand and the observed Chl.-a distribution by ocean color image was successfully reproduced by the coupled numerical model (Fig. 6, Buranapratheprat et
al. 2008).
It is well known that the altimetry data
have a large tidal error in the shallow
coastal area and we cannot use the
altimetry data for the research on the sea
surface current variation in the coastal
seas, though it is possible in the open
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Fig. 7.
Fig. 8.
2010).
Tide error (M2 + S2 + K1 + O1) of altimetry data (Morimoto 2009).
Seasonal variation in sea surface currents revealed by altimetry data (Sojisuporn et al.
Water circulation and material transport
19
Fig. 9. Seasonal variation in sea surface circulation by diagnostic model in the South China
Sea (Manh and Yanagi 2003).
ocean. Morimoto (2009) revealed that the
error of AVISO (Archiving, Validation and
Interpretation of Satellite Oceanographic
data) attains more than 15 cm in the Yellow Sea, the Celebes Sea, near Kuril Island and the northwestern parts of the
Okhotsk Sea in the East Asia (Fig. 7).
Based on this pointing, Sojisuporn et al.
(2010) investigated the seasonal variation
in sea surface circulation in the Gulf of
Thailand using the correct sea surface
height data from the direct harmonic analysis of altimetry data themselves. As a result, the followings were revealed that the
mean geostrophic current showed a strong
southwestward flow of the South China
Sea water along the Gulf entrance.
Counterclockwise eddies in the inner Gulf
and the western side of the Gulf entrance
were associated with the upwelling in the
area. Seasonal geostrophic currents
showed a basin-wide counterclockwise circulation during the southwest monsoon
season and a clockwise circulation during
the northeast monsoon season. The
upwelling was enhanced during the southwest monsoon season (Fig. 8).
In Vietnam, a diagnostic three-dimensional numerical model has been established in order to reveal the seasonal variation of residual flow, including winddriven current, density-driven current and
tide-induced residual current in the South
China Sea (Manh and Yanagi 2003). On
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Fig. 10.
Chl.-a bloom off Vietnams coast during the southwest monsoon (Tang et al. 2004).
Fig. 11.
Calculated horizontal distribution of average bottom stress vector throughout the
year (a) and the observed pattern of sediment transport path (b) (Fuji-ie and Yanagi 2002).
the basis of the calculated results by this
numerical model, it is shown that the winddriven current plays an important role in
the basin-wide circulation in the South
China Sea, especially in the surface layer.
In the lower layers, the density-driven current becomes more significant because the
tide-induced residual current is relatively
small (Fig. 9).
In June, regional phytoplankton bloom
appeared as a large jet shape extending
from the coastal waters of Vietnam eastward towards the South China Sea, about
200 km northeast of the mouth of the
Water circulation and material transport
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Fig. 12. Calculated nitrogen concentrations and fluxes in March (a) and November (b). Values
in the parenthesis represent the ratio to the photosynthesis flux (Hayashi et al. 2006).
Mekong River; this feature is intensified
in the form of a large jet or gyre from July
to September, decayed in October, and disappeared entirely in November. The gyre
was about 400 km in diameter with Chl.-a
concentration from 0.5 to 2.0 mg m–3. Data
on sea surface temperature, winds, and sea
surface height anomalies indicated a strong
offshore upwelling during a period of
strong southwesterly winds alongshore.
The upwelling coincided with the regional
increase in phytoplankton biomass in terms
of shape, timing, and location (Fig. 10,
Tang et al. 2004).
In Philippines, the numerical model for
tide, tidal current and residual flow in Manila Bay was developed in order to estimate the mean bottom stress, which expresses the direction of bottom sediment
transport. Calculation results of bottom
sediment transport direction are in good
agreement with the observation results
(Fig. 11, Fuji-ie et al. 2002). The calculation results of sedimentation reveal that the
seasonal variability of deposited clay distribution is very large (Fuji-ie and Yanagi
2006) and it relates to the cyst accumulation in Manila Bay (Azanza et al. 2004).
In rainy season in Manila Bay, the pri-
mary production is high, and the main
source of DIN is the advection, due to
strong estuarine circulation development,
and the diffusion from the lower layer
where DIN is regenerated by decomposition. On the other hand, in dry season the
primary production is low, and the main
source of DIN is the decomposition in the
upper layer where the nitrogen cycling is
nearly closed (Fig. 12, Hayashi et al.
2006).
Conclusion
The coastal seas in the Southeast Asia suffer from many kinds of environmental
problems such as eutrophication, oil pollution, habitat deterioration and so on. In
order to solve such environmental problems, the basic knowledge on the physical, chemical and biological coastal oceanography is indispensable and the close
scientist network in this region plays a very
important role in the rational integrated
coastal area management.
The human network related to coastal
marine science built by the JSPS multi-lateral study “Coastal Marine Science” during 2001–2010 has greatly contributed to
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the progress of coastal oceanography in the
Southeast Asia. We have to maintain and
develop such useful scientist network re-
lated to coastal marine science for the
progress of coastal oceanography in the
Southeast Asia in the future.
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