Oceanography

Upwelling and Down welling

Near the coast, winds blowing parallel to the shoreline cause a layers of surface waters a few tens of meters thick to move either away from or towards the coast. When surface water moves away from the coast, they expose subsurface water, which then move upward towards the surface. This is called upwelling.

The equator is major upwelling area, which is caused by the winds and by the change in the sign of the Coriolis effect at the equator. Consider the trade winds near the equator. North of the equator, Ekman transport is to the right of the winds (to the northwest) in the northeast trades, and surface waters move away from the equator. South of the equator, the Ekman transport associated with the southeast trades is to the left of the winds (toward the southwest) and again away from the equator. Thus the equator is a divergence zone, where subsurface waters are brought up into the photic zone. 

Where surface waters move toward the coast, they cause the surface layer to thicken, a process called downwelling. In this case, the resulting sea surface slopes create currents parallel to the coast.

Upwelling is especially conspicuous on the eastern side of ocean basins, where the surface layer is relatively thin. Much upwelling occurs near coasts, usually in areas few tens of kilometers across, often near capes or other irregularities in the coastline. The cells of upwelled water often have plumes of cold waters extending offshore a current gyre – a nearly circular current system. The east-west-elongated gyres are centred in the subtropical regions north and south of the equator, In addition to an equatorial current, each current gyre includes a major east-west current at higher latitudes,flowing in the opposite direction to the equatorial currents. These east-west currents are relatively slow.

Boundary currents flow parallel to the continental margins, usually north-south. Boundary currents on the western sides of oceans basins are especially strong. Eastern boundary currents are much weaker.

In the subpolar and polar regions of all ocean basins there are smaller current gyres. These high-latitude gyres circulate in the opposite direction from the subtropical gyres. Because of the positions of the continents, subpolar gyres are well developed in the Northern Hemisphere. These currents flow in a counter clockwise direction.