Page 444 - Environment: The Science Behind the Stories

California coast Figure 16.7 Upwelling is the
North movement of bottom waters
upward. This often brings nutri-
1 Wind blows from the ents up to the surface, creating
north along coastline South productive areas for marine life.
Surface water For example, north winds blow along
2 Coriolis effect the California coastline 1 , while the
causes surface Coriolis effect (p. 473) draws wind and
water to move water away from the coast 2 . Water
away from is then pulled up from the bottom 3
the coast
to replace the water that moves away
from shore.
Upwelling

Cold, deep water 3 Upwelling of cold, nutrient-
rich in nutrients rich water replaces surface
water that has moved away
from the coast

In areas where surface currents converge, or come water releases heat to the air, keeping Europe warmer than it
together, surface water sinks—a process called downwelling. otherwise would be, the water cools, becomes saltier through
Downwelling transports warm surface water rich in dis- evaporation, and thus becomes denser and sinks, creating
solved gases to deeper waters, providing an influx of oxygen a region of downwelling known as the North Atlantic Deep
for deep-water life and “burying” CO in deep ocean waters. Water (NADW).
2
Vertical currents also occur in the deep zone, where differ- Scientists hypothesize that interrupting the thermoha-
ences in density can lead to rising and falling convection cur- line circulation could trigger rapid climate change. If global
rents, similar to those in molten rock (p. 52) and in air (p. 473). warming (Chapter 18) causes much of Greenland’s ice sheet
to melt, the resulting freshwater runoff into the North Atlan-
tic would make surface waters less dense (because fresh
Ocean currents affect Earth’s climate water is less dense than salt water). This could potentially
The horizontal and vertical movements of ocean water can stop the NADW formation and shut down the northward
have far-reaching effects on climate globally and regionally. flow of warm water, causing Europe to cool rapidly. Some
The thermohaline circulation is a worldwide current system data suggest that the thermohaline circulation in this region
in which warmer, fresher water moves along the surface and is already slowing, but other researchers maintain that
colder, saltier water (which is denser) moves deep beneath Greenland will not produce enough runoff to cause a shut-
the surface (Figure 16.8). One segment of this worldwide con- down this century.
veyor-belt system includes the warm surface water in the Gulf Another interaction between ocean currents and the
Stream that flows across the Atlantic Ocean to Europe. As this atmosphere that influences climate is the el Niño–Southern
Oscillation (eNSO), a systematic shift in atmospheric pressure,
sea surface temperature, and ocean circulation in the tropi-
Greenland Sea-to-air cal Pacific Ocean. Under normal conditions, prevailing winds
heat transfer blow from east to west along the equator, from a region of
forms NADW
high pressure in the eastern Pacific to one of low pressure in
Europe the western Pacific, forming a large-scale convective loop in
Solar warming the atmosphere (Figure 16.9a). The winds push surface waters
of ocean CHAPTER 16 • M AR in E A nd Co A s TA l s ys TEM s A nd R E sou R CE s
waters westward, causing water to “pile up” in the western Pacific.
Equator As a result, water near Indonesia can be 50 cm (20 in.) higher
Warm surface current Pacific and 8°C warmer than water near South America. The west-
Ocean
ward-moving surface waters allow cold water to rise up from
Atlantic the deep in a nutrient-rich upwelling along the coast of Peru
Ocean and Ecuador.
Cold deep current el Niño conditions are triggered when air pressure

decreases in the eastern Pacific and increases in the west-
ern Pacific, weakening the equatorial winds and allowing
Figure 16.8 As part of the oceans’ thermohaline circulation, the warm water to flow eastward (Figure 16.9b). This sup-
warm surface currents carry heat from equatorial waters
northward toward Europe, where they warm the atmos- presses upwelling along the Pacific coast of the Americas,
phere. The water then cools and sinks, forming the North Atlantic shutting down the delivery of nutrients that support marine
Deep Water (NADW). Scientists debate whether rapid melting of life and fisheries. This phenomenon was called El Niño
Greenland’s ice sheet could interrupt this heat flow and cause (Spanish for “little boy” or “Christ Child”) by Peruvian
Europe to cool dramatically. fishermen because the arrival of warmer waters usually 443

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