Upwelling and Downwelling
Upwelling is the vertical movement of cold, nutrient-rich water from deep water to the surface, resulting in high productivity (plankton growth).
• Can bring cold, nutrient-rich water to the surface (photic zone) unless thermocline is strong and prevents it.
• Nutrients are not food but act like a fertilizer.
• Upwelling water rich in nutrients feeds phytoplankton, the base of the food chain.
Downwelling is the vertical movement of surface water downward in water column. Regions where downwelling is occurring typically have low biological productivity.
• Downwelling takes dissolved oxygen down where it is consumed by the decay organic matter.
Where Upwelling Occurs
Diverging surface waters occur where surface waters are moving away from an area on the ocean surface.
• Equatorial upwelling occurs where SE trade wind blow across the equator (Figure 9.19); Ekman transport forces surface water movement to the south (south of the Equator), and to the north (north of the Equator). Upwelling of deep ocean waters is most intense in equatorial regions.
• Coastal upwelling occurs where wind blowing along a coastline is influenced by Ekman current moving surface waters offshore, or winds blowing off the land pull surface waters away from the coast, pulling deeper water up to replace surface waters. See Figure 9.18 for locations where coastal upwelling most commonly occurs.
• Other locations where upwelling occurs include around underwater obstructions (guyots) or sharp bends in coastlines.
NOAA animation: Coastal Upwelling
Coastal Upwelling and Downwelling
The continental margins of the world are places where coastal upwelling and downwelling are taking place (Figure 9.18). Coastal upwelling is influenced by coastal geometry, wind directions, and the influence of the Coriolis effect (Ekman transport). Figures 9-20 and 9-21 illustrate how the direction of wind movement determines how coastal upwelling and downwelling takes place in the Northern Hemisphere (such as in California). Figure 9.22 shows regions of coastal upwelling along the California continental margin—revealed ocean-surface temperature imagery. Upwelling water along the coastline is colder than waters farther offshore.
|Figure 9.20. Coastal upwelling (example of California)||Figure 9.21. Coastal downwelling (wind reversed)|