Currents, Circulation, and El Nino

The surface waters of the ocean are set in motion primarily by wind, and their movement is further shaped by the Coriolis effect, the apparent deflection caused by Earth's rotation, which curves moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The combined action of persistent wind belts - the trade winds near the equator and the westerlies at mid-latitudes - together with this deflection organizes surface currents into five major subtropical gyres, enormous circular current systems centered roughly on 30 degrees north or south latitude: the North Pacific, South Pacific, North Atlantic, South Atlantic, and Indian Ocean gyres. Within each gyre, warm currents flow poleward along the western side of the ocean basin, such as the Gulf Stream in the North Atlantic and the Kuroshio Current in the North Pacific, while cooler currents flow back toward the equator along the eastern side. At a finer scale, wind-driven surface water is deflected further with depth in a pattern called the Ekman spiral, producing a net transport of water at roughly 90 degrees to the wind direction, known as Ekman transport, a mechanism that plays a central role in generating coastal upwelling.