NOAA Images of the Plumes in
the south China Sea : Some Preliminary Observations Raja Mohammad Noordin and
Ku Kassim Ku Yaacob Abstract Department of Fisheries Malaysia Marine Fishery Resources Development and Management Department ( Southeast Asian Fisheries Development Center ) Taman Perikanan Chendering, 21080 Kuala Terengganu, Malaysia. Tel : 609-6175135; Fax: 609-6175136 E-mail:seafdec@po.jaring.my To national Oceanic and Atmospheric Administration ( NOAA) images were analysed to ascertain the seasonal pattern of the major river outflows bordering the south China sea. River outflow or plumes are important in not only the transport of nutrients and riverine sediments but also the survival of fish larvae of particularly the anadromnorous species. The visible and thermal imags of the NOAA AVHRR data were used in this study and it was shown that the images have the potential of includating the flow pattern of the plumes. Abnormal flow patterns of the plumes could be used to alert the future state of the fish stock abundance. This study showed the potential use of NOAA images in the management of fisheries resources. Introduction The south china sea is still the least studied sea of the world. It is the second largest sea after the Mediterranean sea and it covers an area of about 2,505 km2 ( Couper, 1983). The climate of the land bordering the sea varies form being tropical in the south and sub-tropical in the north. It is exposed to the annual monsoons which are comprised of the Northeast winds blowing from November to February and the southwest winds from May to August. The sea is fed from the north by the Pacific waters through the Luzon Straits and the Taiwan straits . the southern part of the sea is fed by the Java sea. Thus the marine environment of the south China sea is diverse having the most variety of ecosystems which include the soft-bottom, the deep shelves, oceanic waters, mangroves swamps, lagoons, seashores, seagrasses and coral reefs. The sea has also one of the most widest continental shelves. The edges of the sea are well fed by many river mouths. Most of the major river mouths have become the sites of the highly populated regions in the world. The rivers have supported the human activities in the region and the southeast Asian region supports one of the most dense population of the world. The south China sea thus becomes the seafood bowl of the southeast Asian people. It is therefore important to maintain the supply of seafood from the sea by understanding the factors that may affect the health of the sea. The potential catches of the demersal fish and pelagic fish are estimated to be 3.3 million tones and 2.8 million tones respectively ( FAO, 1981). Changes in the terrestrial environment obviously affects the sea where the rivers flow into it. Therefore such study of he plumes in this semi-enclosed ecosystem will enable scientists to understand better the fate of the nutrients and the living resources particularity fish iarvae that are present in the plumes ( Govini et al. 1989). I the context of the south China sea, the ' terubok ' fish ( Tenualsoa toil ) of Sarawak is one anadromorous species that spawn in the river and spend their adult stage in the coastal waters. According to Yong (1994) the eggs, larvae and juveniles of 'terubok' fish are found in the Sarawak rivers all the year round. During the Northeast monsoon, the current in the South China Sea flows in the south-west direction and in the Southwest monsoon in the north-west direction. It is envisaged that the plume flow of major rivers will flow in similar directions or as near to it. Therefore in order to understand the extent of the plume flow in a wide area such as the South China Sea one approach is to use daily NOAA cloud-free images. A similar study has been conducted successfully in temperate waters of the west coast of Ireland by Haung et al., 1993). The NOAA Advanced Very High Resolution Radiometer (AVHRR) has been shown by Spitzer and Roozekrans (1990) to be useful in the synoptic assessment of the sea surface temperature and the total suspended matter in the North Sea. This paper describes the use of NOAA images to study the seasonal flow pattern of the major river of estuarine plumes that could be observed in the images. Methods and Results The data were obtained from the NOAA - 12 and NOAA - 14 satellites through the HRPT Reception System developed by Dundee Satellite Systems. HRPT (High Picture Transmission) refers to the standard format in which the data is transmitted from the satellite. The data is displayed using the STARS AVHRR Processing System provided by Sea Scan. The visible and thermal images were processed using the PCI Easi - pace software. There was no attempt to make any atmospheric or geometric corrections and only density slicing was applied to enhance the variabilities of the plume and surface temperature patterns. The surface temperature differences and suspended sediments variations were used since their relative gradients are sufficient to indicate the plume shapes and flows. The longest river in the region is the Mekong river which is 4, 189 km with its estuary located in southern Vietnam. As the Mekong River would have much influence in the South China Sea, images of it were first studied. NOAA images in the months of December-January and July-August in the years 1995-1996 were selected and cloud-free images analysed. Channels 1 and 5 of the images were used and pseudo - coloured to show the variations in the plume stages and sea surface temperature. As expected the images indicate the flow of the plume which was greatly influenced by the monsoonal winds. Discussion and conclusions Drinkwater (1986) described the profound effects of the river input which can extend to over a thousand kilometers. In the South China Sea, the effect of the Coriolis force would have some effects on the flow of the Mekong River plume. During the Southwest monsoon season, the plume would therefore be exerted by two forces, the south west widns and the Coriolis force. The plume flows in the north east direction and the extent of the plume flow is estimate d to be about 100-200 km. Conversely during the Northeast monsoon, the north east winds is stronger than the influence of the Coriolis force. Such an image is yet to be analysed and which would show the direction of the plume flowing in the southwesterly direction and perhaps extending as far down to about 300 km. In additio, the tidal flow in the area will affect the flow and consequently the timely image that would be required in this study is when the plume is flowing during the ebb tide. This is when the river runoff is high and the plume is more distinct. This kind of image is still being sorted for further analysis. The effects of the freshwater runoff of the Mekong River which effected by the Northeast winds could bring large abundance of shrimps into the coastal wat4ers of the Eastern Peninsular Malaysia. This corresponds to the seasonality of shrimps in the Terengganu waters of the area. The shrimps are abundant during the Northeast monsoon season (Ibrahim Johari and Syed Abdullah, pers. Comm..) and they seem to disappear after the monsoons. The distance between the Mekong estuary and the Terengganu coast is about 300 nautical miles (560 km). This study is a preliminary one and more images of the plumes in the area are required for analyses to further confirm this phenomenon. The extent of the plume flow is important in this area for it will show the actual the interactions of the water masses and the effects they have on the marine living resources. Acknowledgements The authors would like to thank the Director-General of Fisheries Malaysia, the Deputy Director-General of Fisheries and the Chief of MFRDMD for supporting this kind of study. References
|