The Upper Gulf of Thailand (UGOT) or the so-called inner gulf is a semi-enclosed
coastal sea surrounded by land in three directions while its southern part is connected to a central gulf (Fig. 1). It is shallow with an average water depth of 15 m and is mainly influenced by seasonal monsoon and river runoffs (Saramul and Ezer,2014). The UGOT is exposed to dry northeast (November to January) and wet southwest monsoon (May to August)(Buranapratheprat et al., 2008). Four major rivers of Thailand,namely, the Mae Klong, Tha Chin, Chao Phraya, and Bang Pakong,discharge to the UGOT. Particularly, the Chao Phraya and Mae Klong Rivers, the first and second largest rivers in Thailand, dis-
charge about 38 million tons of sediment and 43 km3 water to the UGOT each year (Milliman and Farnsworth, 2011). The water and sediment discharge has distinctly seasonal variability. In wet season (May to October), for example, water discharge of the Chao Phraya River account for more than 60% of the total(Takeuchi, 1993). Generally, there are counterclockwise circulation and a clockwise gyre during the northeast and the southwest monsoons, respectively. While the counterclockwise circulation can also be developed during the southwest monsoon(Buranapratheprat et al., 2008). The current induced by tide, wind,and river runflow is weak in the Gulf of Thailand, and the mean
current is usually