8.8: Upwelling and Downwelling

Although wind-driven surface water motions are primarily horizontal, winds can also cause vertical water movements. Vertical movements occur because wind-driven Ekman transport drives a layer of surface water up to about 100 m deep across the underlying water layers. Winds moving surface water away from an area cause a divergence at which the surface water is replaced by water upwelling from below (Fig. 8-12a). Upwelling is important because colder water upwelled from below the pycnocline has high concentrations of nutrients, such as nitrogen and phosphorus compounds. The nutrients are needed to fertilize phytoplankton, the microscopic algae and phototrophic bacteria and archaea that grow only in near-surface waters and that are the principal source of food supporting all animal life in the oceans. Most surface waters are deficient in nutrients and cannot sustain phytoplankton growth unless nutrients are supplied by upwelling or recycling (Chap. 12).

Figure 8-12. Divergences and convergences are areas of upwelling and downwelling in the open oceans. (a) At a divergence, the surface layer is thinned and the pycnocline depth reduced. If wind mixing extends below the pycnocline depth, cold, nutrient-rich, deep water is upwelled and mixed into the surface layer. (b) At a convergence, the surface layer is thickened, and the pycnocline is driven deeper. If the pycnocline is below the maximum depth of wind influence, there is no mixing of cold, nutrient-rich subpycnocline water into surface layers, even when winds are very strong.

At a convergence, winds moving surface waters toward an area elevate the sea surface and create a high-pressure region in the subsurface water, causing downwelling (Fig. 8-12b). Surface water is transported downward at the convergence and then outward below the wind-driven layer under the influence of the horizontal pressure gradient. Downwelling thickens the layer of warmer surface water above the pycnocline and tends to isolate cold subpycnocline water, preventing it from mixing upward into the surface layers. Downwelling also occurs in areas where the surface water density is increased by cooling or evaporation sufficiently that this surface water has a higher density than the water below.

Locations of Upwelling and Downwelling 

Divergences are upwelling areas where primary productivity is high because of the supply of nutrients from below. Convergences are downwelling areas where productivity is poor. The principal open-ocean divergences include a band that parallels the equator between the North and South Equatorial Currents and a band around Antarctica between the Antarctic Circumpolar Current and the eastward coastal flow around the continent (Fig. 8-3). In the equatorial region, upwelling is inhibited by the flow of warm surface water from west to east in the Equatorial Countercurrents (Online Box 8B2). Consequently, persistent upwelling of nutrient-rich, cold subpycnocline water at the equatorial divergence is limited primarily to the east side of the Pacific Ocean off the Peruvian coast (Fig. 7-19). Upwelling in this region is inhibited by the movement of warm surface water from west to east during El Niño (Fig. 7-18b). The principal open-ocean convergences are in the center of each of the subtropical surface current gyres (Fig. 8-3).

Coastal upwelling can occur when winds blow parallel to the coast and cause Ekman transport of the surface layer offshore. Surface water transported offshore is replaced by deeper water that moves inshore and is upwelled (Fig. 8-13a). Coastal downwelling occurs when surface water is transported toward shore displaces near-shore surface water downward and forces the deeper water to move offshore (Fig. 8-13b). These processes are particularly important to local ecosystems in areas where there is a shallow pycnocline, with warm, nutrient-depleted water at the surface and colder, nutrient-rich water below. In such areas, offshore transport causes coastal upwelling that supplies large quantities of nutrients to support phytoplankton growth (Chap. 12).

Figure 8-13. Coastal upwelling and downwelling. (a) When winds produce Ekman transport of the surface water layer in an offshore direction, the pycnocline (usually a thermocline) is raised, sometimes all the way to the surface, and cold, nutrient-rich, deep water is upwelled into the surface layer. (b) When winds produce Ekman transport of the surface water layer in an onshore direction, the pycnocline (again usually a thermocline) is depressed, shelf surface water is downwelled, and cold, nutrient-rich, deep water is forced deeper and off the shelf, so it cannot enter the surface layer, even if winds are strong enough to mix coastal water to the depth of the continental shelf break.

Boundary Currents and Upwelling or Downwelling

The characteristics of ocean gyre boundary currents strongly influence the occurrence and persistence of coastal upwelling near the basin’s coasts. Western boundary currents are deep and generally compressed against the continental shelf edge. Hence, a deep layer of warm surface water generally lies over the cold, nutrient-rich subpycnocline water and prevents it from extending onto the continental shelf (Fig. 8-13b). Because coastal winds normally move aside a surface layer only a few tens of meters deep, offshore Ekman transport in western boundary current regions leads to upwelling of warm, nutrient-poor water from within the deep surface layer (Fig. 8-13b). Thus, coastal upwelling of nutrient-rich water is rare in western boundary current regions.

Eastern boundary currents are relatively shallow and wide, and they extend onto the continental shelf. Cold, nutrient-rich, deep water can migrate onto the shelf as a bottom layer below the shallow, warm surface layer (Fig. 8-13a). Only moderate coastal winds with offshore Ekman transport are needed to cause cold, nutrient-rich waters to upwell to the surface. Consequently, coastal upwelling is more frequent, widespread, and persistent on the eastern boundaries of the oceans (west coasts of continents) than on the western boundaries of the oceans (east coasts of continents).