The phenomenon or process involving the ascending motion of water in the ocean. Vertical motions are an integral part of ocean circulation, but they are a thousand to a million times smaller than the horizontal currents. Vertical motions are inhibited by the density stratification of the ocean because with increasing depth, as the temperature decreases, the density increases, and energy must be expended to displace water vertically. The ocean is also stratified in other properties; for example, nutrient concentration generally increases with depth. Thus even weak vertical flow may cause a significant effect by advecting nutrients to a new level. Geophysical fluid dynamics

There are two important upwelling processes. One is the slow upwelling of cold abyssal water, occurring over large areas of the world ocean, to compensate for the formation and sinking of this deep water in limited polar regions. The other is the upwelling of subsurface water into the euphotic (sunlit) zone to compensate for a horizontal divergence of the flow in the surface layer, usually caused by winds.Ocean circulation