The upwelling current brings up dense, cold and nutrient-rich water from the subsurface towards sea surface. By supplying the nutrients into the euphotic zone, primary productivity will be enhanced and the high-level productions will increase there as a result of a food chain. Therefore, in order to make rich fishing ground, studies on artificial upwelling devices such as marine structures installed on the sea bottom were made so far. Vertical stratification of water density may often occur in oceans, depending on season and latitude. The flows in these regions result in the stratified density current. In the present study, in order to investigate the effect of the stratification on the upwelling induced by the marine structures, flow visualization tests in the homogeneous and stratified flows were carried out in the small towing tank. The stratification was achieved by salt water and fresh water. Two kinds of stratification were used in the experiments, i.e. linear density profile and two-layer flows with different densities. A sea mountain range, V-shaped and X-shaped structure models were used in the present flow visualization tests. It is shown that the density stratification suppresses uplifting of water due to upwelling induced by the artificial marine structure significantly. Richardson number and densimetric Froude number are recognized to be important parameters for upwelling in the stratified density flow. An internal gravity wave in the two layer flow becomes steepest at internal Froude number of 0.8.
