Environmental Problems, Their Causes, and Sustainability 1

Winds can also affect regionalclimates and distributionof some forms of aquaticlife. For example, wind blowingalong steep western coastsof some continents pushessurface water away from theland. This outgoing surfacewater is replaced by an upwellingof cold, nutrient-richbottom water, as shown in Figure6-11.Upwellings, whether farfrom shore or near shore,bring plant nutrients from thedeeper parts of the ocean tothe surface. In turn, these nutrientssupport large populationsof phytoplankton, zooplankton,fish, and fish-eatingseabirds.UpwellingMovement ofsurface waterFishZooplanktonPhytoplanktonNutrientsWindDiving birdsFigure 6-11 A shore upwelling (shown here) occurs when deep, cool, nutrient-rich waters aredrawn up to replace surface water moved away from a steep coast by wind flowing along the coasttoward the equator. Such areas support large populations of phytoplankton, zooplankton, fish, andfish-eating birds. Equatorial upwellings occur in the open sea near the equator (Figure 6-6) whennorthward and southward currents interact to push deep waters and their nutrients to the surface,thus greatly increasing primary productivity in such areas.What Are El Niño and La Niña?Changing Winds, Altered Upwellings, andFreaky WeatherEl Niño occurs when a change in the direction of tropicalwinds warms coastal surface water, suppressesupwellings, and alters much of the earth’s weather. LaNiña is the reverse of this effect.Every few years in the Pacific Ocean, normal shoreupwellings (Figure 6-12, left) are affected bychanges in climate patterns called the El Niño–Southern Oscillation, or ENSO (Figure 6-12, right).Trace the flows and components of the diagram inFigure 6-12.Surface windsblow westwardDrought inAustralia andSoutheast AsiaWinds weaken,causing updraftsand stormsEQUATOREQUATORAUSTRALIAWarm waterWarm waterspushed westwardThermoclineSOUTHAMERICACold waterAUSTRALIAWarm waterWarm waterflow stoppedor reversedThermoclineSOUTHAMERICAWarm water deepens offSouth AmericaCold waterNormal ConditionsEl Niño ConditionsFigure 6-12 Normal trade winds blowing westward cause shore upwellings of cold, nutrient-rich bottomwater in the tropical Pacific Ocean near the coast of Peru (left). A zone of gradual temperature changecalled the thermocline separates the warm and cold water. Every few years a climate shift known as theEl Niño–Southern Oscillation (ENSO) disrupts this pattern. Westward-blowing trade winds weaken or reversedirection, which depresses the coastal upwellings and warms the surface waters off South America (right).When an ENSO lasts 12 months or longer, it severely disrupts populations of plankton, fish, and seabirds inupwelling areas and can trigger extreme weather changes over much of the globe (see Figure 6-13).108 CHAPTER 6 Climate and Terrestrial Biodiversity