Surface Ocean Circulation
The circulation of the surface ocean is driven primarily by surface winds. As we have seen, winds blow from areas of high atmospheric pressure to regions of low atmospheric pressure. These winds are generally transferring heat from areas where there is excess incoming radiation (the tropics and subtropics) to temperate and higher latitude regions, where there is a net loss of heat. Typically speaking, the distribution of pressure on the Earth’s surface is zonal or meridional, with high-pressure bands covering the subtropics and polar regions and low-pressure bands, the equatorial regions, and subpolar regions.
Where winds and surface currents are moving along a coastline, they draw the surface water away from the coast. The surface waters are replaced by waters from below by the upwelling described earlier. This is shown in the figure below.
Upwelling also happens in parts of the ocean where winds cause surface currents to diverge or move away from one another. Downwelling is the opposite process to upwelling, where surface waters flow downwards and replace deep waters. This occurs in parts of the ocean where surface winds are converging. One place this happens is in the centers of gyres.