Chapter 2

Water circulation and material transport in the coastal areas and marginal seas of East and (Project-1)

Tetsuo Yanagi

Research Institute for Applied Mechanics, Kyushu University, Kasuga Park, Kasuga 816-8580, Japan

Introduction I introduce here some new findings on the physical coastal oceanography in the There were many biological and chemical Southeast Asia during the past ten years. coastal oceanographers but a few physical coastal oceanographers in the Southeast Scientific Accomplishment Asian countries at the time of the 1990s, about twenty years ago. The knowledge on In , the remote sensing technol- the physical conditions in the coastal sea ogy gave new information on SST (sea is indispensable for the correct biological surface temperature) and SSC (sea surface and chemical understanding of oceano- chlorophyll-a) in the coastal seas around graphic phenomena, because the biologi- Indonesia. cal and chemical oceanographers cannot The westward propagation of coastal distinguish the temporal change of biota upwelling phenomenon along the Java Is- density or chemical concentration at some land during the southeast monsoon was point in the coastal sea and the effect of clarified using NOAA AVHRR images, advection or diffusion there without the where the area with low SST propagates correct knowledge of the advection and westward (Fig. 1, Suhendar et al. 2002). diffusion around the observation point. Moreover, SSC increases due to the During the past ten years, the knowl- upwelling during the southeast monsoon edge on the coastal physical oceanography and decreases due to the downwelling dur- in the Southeast Asian region has remark- ing the northwest monsoon as shown in ably increased mainly by the JSPS (Japan Fig. 2 (Suhendar et al. 2002). Society for the Promotion of Science) The numerical ecosystem model clari- multilateral cooperative study, and the fied the biochemical characteristics of coastal oceanographers became to under- eutrophicated Bay and pristine stand that the knowledge of physical con- Bantan Bay as shown in Table 1 ditions is the base of chemical and biologi- (Nurdjaman and Yanagi 2002). Generally, cal coastal oceanography. 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. 14 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 erated production is almost one. Both nu- in dry season. Rainfall directly affects on trient load and recycling DIN (dissolved the growth of phytoplankton in Jakarta Bay inorganic nitrogen) play important roles in and the primary production in Jakarta Bay the increase of Chl.-a concentration in (416–830 mgC/m2/day) is higher than in Jakarta Bay. Bantan Bay (84–122 mgC/m2/day). Ac- The residence time of fresh water as the cording to the primary production, Jakarta indicator of the water exchange played an Bay is classified under mesotrophic and important role in the control of the water Bantan Bay is orligotrophic. In Bantan quality at Hurun Bay (Suhendar et al. Bay, the regenerated production is higher 2009). Long fresh water residence time in than the new production and plays an im- both transition periods of Wet-Dry and portant role in material cycling in the lower Dry-Wet seasons has increased the DIN trophic level ecosystem, while in Jakarta and TOM (total organic matter) accumu- Bay the ratio of new production to regen- lation in the water column, and it stimu- Water circulation and material transport 15

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). 16 T. YANAGI

Fig. 4. Satellite image of coastal habitat (Komatsu et al. 2009).

lated phytoplankton bloom at Hurun Bay (Advanced Land Observing Satellite) (Fig. 3). Such situation has caused the DO AVNIR-2 (Advanced Visible and Near In- (dissolved oxygen) concentration decrease frared Radiometer type-2) images at due to large decomposition of organic Barrang Lompo Island near Makassar, In- matter. The results recommended that in donesia (Fig. 4). both transition periods, the aquaculture In Malaysia, data from the World activity should be limited at minimum Ocean Database for the Malacca Strait level to reduce the risk of fish mass mor- were utilized to assess the seasonal varia- tality caused by the DO depletion due to tion in temperature, salinity and dissolved the phytoplankton bloom. oxygen in the Malacca Strait (Ibrahim and Komatsu et al. (2009) succeeded to Yanagi 2006). The data indicated the in- develop the efficient mapping and moni- troduction of cool, deep, saline water from toring systems of coastal habitats, such as the during the Southwest sea-grass beds and live corals, using ALOS Monsoon. During the Northeast Monsoon, Water circulation and material transport 17

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 oped in the upper Gulf of Thailand, where ingress of lower salinity water mass from the Case II water exists, based on the in- the south. This may be attributed to the tensive multi-disciplinary field observa- larger river discharge experienced during tions (Matsumura et al. 2006). At the same the Northeast Monsoon and the introduc- time, the ecosystem model coupled with tion of lower salinity water mass from the the three-dimensional hydrodynamic . The influence of the model was developed for the upper Gulf Andaman Sea and the South China Sea is of Thailand and the observed Chl.-a dis- supported by the variation in the T-S plots tribution by ocean color image was suc- for the Malacca Strait. This is especially cessfully reproduced by the coupled nu- discernible in the Northeast Monsoon and merical model (Fig. 6, Buranapratheprat et in the subsequent Inter-monsoon period al. 2008). (Fig. 5). Such results have implications for It is well known that the altimetry data the movement and exchange of material have a large tidal error in the shallow between the Andaman Sea and the South coastal area and we cannot use the China Sea via the Malacca Strait. altimetry data for the research on the sea In Thailand, local algorithm for the surface current variation in the coastal analysis of ocean color image was devel- seas, though it is possible in the open 18 T. YANAGI

Fig. 7. Tide error (M2 + S2 + K1 + O1) of altimetry data (Morimoto 2009).

Fig. 8. Seasonal variation in sea surface currents revealed by altimetry data (Sojisuporn et al. 2010). 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 Counterclockwise eddies in the inner Gulf error of AVISO (Archiving, Validation and and the western side of the Gulf entrance Interpretation of Satellite Oceanographic were associated with the upwelling in the data) attains more than 15 cm in the Yel- area. Seasonal geostrophic currents low Sea, the , near Kuril Is- showed a basin-wide counterclockwise cir- land and the northwestern parts of the culation during the southwest monsoon Okhotsk Sea in the East Asia (Fig. 7). season and a clockwise circulation during Based on this pointing, Sojisuporn et al. the northeast monsoon season. The (2010) investigated the seasonal variation upwelling was enhanced during the south- in sea surface circulation in the Gulf of west monsoon season (Fig. 8). Thailand using the correct sea surface In Vietnam, a diagnostic three-dimen- height data from the direct harmonic analy- sional numerical model has been estab- sis of altimetry data themselves. As a re- lished in order to reveal the seasonal vari- sult, the followings were revealed that the ation of residual flow, including wind- mean geostrophic current showed a strong driven current, density-driven current and southwestward flow of the South China tide-induced residual current in the South Sea water along the Gulf entrance. China Sea (Manh and Yanagi 2003). On 20 T. YANAGI

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 tide-induced residual current is relatively numerical model, it is shown that the wind- small (Fig. 9). driven current plays an important role in In June, regional phytoplankton bloom the basin-wide circulation in the South appeared as a large jet shape extending China Sea, especially in the surface layer. from the coastal waters of Vietnam east- In the lower layers, the density-driven cur- ward towards the South China Sea, about rent becomes more significant because the 200 km northeast of the mouth of the Water circulation and material transport 21

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 mary production is high, and the main in the form of a large jet or gyre from July source of DIN is the advection, due to to September, decayed in October, and dis- strong estuarine circulation development, appeared entirely in November. The gyre and the diffusion from the lower layer was about 400 km in diameter with Chl.-a where DIN is regenerated by decomposi- concentration from 0.5 to 2.0 mg m–3. Data tion. On the other hand, in dry season the on sea surface temperature, winds, and sea primary production is low, and the main surface height anomalies indicated a strong source of DIN is the decomposition in the offshore upwelling during a period of upper layer where the nitrogen cycling is strong southwesterly winds alongshore. nearly closed (Fig. 12, Hayashi et al. The upwelling coincided with the regional 2006). increase in phytoplankton biomass in terms of shape, timing, and location (Fig. 10, Conclusion Tang et al. 2004). In Philippines, the numerical model for The coastal seas in the Southeast Asia suf- tide, tidal current and residual flow in Ma- fer from many kinds of environmental nila Bay was developed in order to esti- problems such as eutrophication, oil pol- mate the mean bottom stress, which ex- lution, habitat deterioration and so on. In presses the direction of bottom sediment order to solve such environmental prob- transport. Calculation results of bottom lems, the basic knowledge on the physi- sediment transport direction are in good cal, chemical and biological coastal ocea- agreement with the observation results nography is indispensable and the close (Fig. 11, Fuji-ie et al. 2002). The calcula- scientist network in this region plays a very tion results of sedimentation reveal that the important role in the rational integrated seasonal variability of deposited clay dis- coastal area management. tribution is very large (Fuji-ie and Yanagi The human network related to coastal 2006) and it relates to the cyst accumula- marine science built by the JSPS multi-lat- tion in Manila Bay (Azanza et al. 2004). eral study “Coastal Marine Science” dur- In rainy season in Manila Bay, the pri- ing 2001–2010 has greatly contributed to 22 T. YANAGI the progress of coastal oceanography in the lated to coastal marine science for the Southeast Asia. We have to maintain and progress of coastal oceanography in the develop such useful scientist network re- Southeast Asia in the future.

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