Previous results of circulation in the upper Gulf of Thailand using a 2-dimensional model suggested counter-clockwise circulation and a clockwise gyre during the northeast and the southwest monsoons, respectively. A review of previous reports relevant to circulation, and the results of surface chlorophyll-a distribution estimated by OCTS (Ocean Color and Temperature Scanner) data support the results during the northeast monsoon. Anyway, residual circulation analyzed from oceanographic buoys deployed in the upper gulf illustrated both clockwise and counter-clockwise circulations during the southwest monsoon. Therefore, the objective of this study is trying to investigate the current patterns and their controlling factors especially during the southwest monsoon. The 2-dimenional circulation model is applied again but this time uses meteorological wind data from local stations around the study area as inputs. The experiments also include investigation of current patterns due to the difference of wind gradients in east-west and north-south direction. The results show that counter-clockwise circulation can be developed during the southwest monsoon when wind magnitude in the east or the south is significantly stronger than that in the west or the north, respectively, which possibly happens in the real situation. It can be concluded that the circulation patterns in the area are controlled by not only wind direction but also its gradient. We also found that external flows through the open boundary can change the patterns of circulation just in the southernmost area
