Download Currents: Upwelling What is an upwelling current? Why are they

Currents: Upwelling
What is an upwelling current? Why are they important?
The large ocean currents (gyres) are caused by large regional wind belts (easterlies,
westerlies) that encircle the planet. These gyres generally flow clockwise in the Northern
Hemisphere and counterclockwise in the Southern Hemisphere. There is an interesting current
that develops alongside the gyres known as upwelling, which tends to occur along coastlines
(also known as coastal upwelling).
When winds flow along coastlines, their energy is transferred into and influences the
ocean surface, causing the surface waters to move. Due to the Coriolis force’s influence on
movement, the net effect of wind on the surface creates surface water moving at a right angle to
the winds. Consider the winds off the southern Pacific coast of the United States, generally
flowing from the north to the south, as shown just below.
The wind flows south, and transfers energy to the water surface; Coriolis forces deflect
its motion to the right in the Northern Hemisphere, so the surface current slides off at a 45 degree
angle to the initial wind force. The surface water experiences friction with water at lower depths,
and transfers some energy of movement to that water at depth… and this movement is also
influenced by the Coriolis force, and deflected to the right. The lower current is now actually
flowing at a right angle to the original wind force. This is what drives the upwelling current
along coastlines. The phenomenon is known as an Ekman current or the Ekman Spiral.
Along the margins of continents where strong winds prevail, upwelling is an important
feature of oceanic circulation. It brings the cold, nutrient-rich deeper waters to the surface,
providing a supply of nutrients to oceanic plant and animal life. Zones of upwelling are often
characterized by blooms of phytoplankton that form a foundation of the oceanic food web. The
map below shows these zones along the margins of coastlines and regions in the ocean basins
where upwelling currents feed such plankton blooms. The map is derived from images captured
by NASA’s MODIS sensor on-board the Terra and Aqua satellites.