Atmospheric winds generate horizontal currents that move around the ocean's surface. Wind can also generate vertical water motions in processes called upwelling and downwelling. When wind blows over water, the surface water does not move directly in front of the wind but moves about 45 degrees toward the right of the wind's motion in the Northern Hemisphere (Figure 1). This process is called Ekman transport and is a result of the Coriolis effect. In the Southern Hemisphere, surface water is deflected to the left of the wind's motion. Where winds cause the surface water to move away from a coastline or to diverge from another surface water mass, deeper water will move up to the ocean surface, creating an upwelling current. Where winds cause the surface water to move toward a coastline or to converge with another water mass, the surface water will try to move downward to create a downwelling current. For example, northerly winds are common in the summer along the California coast. Winds moving from north to south cause surface water to move toward the west, away from the coastline. Upwelling currents are created, which bring deeper, colder water to the surface. Our coastal waters are cold because of these cold upwelling currents and because of the cold California current (eastern boundary current).
Whereas surface waters are usually depleted of nutrients such as phosphates and nitrates that are critical to plant growth, deeper waters have high concentrations of these nutrients. Upwelling replenishes the surface layers with the nutritional components necessary for biological productivity. Regions of upwelling are among the richest biological areas of the world.