The mechanism to maintain the high surface chlorophyll in the western Arabian Sea during the southwest monsoon (SWM) season is studied with an eddy-permitting, biological-physical coupled model. Using a set of artificial tracers, it is revealed that horizontal transport of nutrient subsequent to the coastal upwelling is by far more important than the open ocean upwelling due to the Ekman pumping. The horizontal transport can be classified into three processes, the Ekman transport, transport by the large-scale circulation and that by the mesoscale currents. In the model, the first has only a minor effect within the area of high chlorophyll while the latter two have comparable contributions. Considering the fact that simulated eddy activity is still less than observed by satellites, it may be possible to make the model results more realistic by assimilating sea-surface height data or running a finer resolution model.