short term climate change

el nio southern oscillation

Short-term changes in climate are common and they have many causes (Figure 1.1). The largest and most important of these is the oscillation between El Nio and La Nia conditions. This cycle is called the ENSO (El Nio Southern Oscillation). The ENSO drives changes in climate that are felt around the world about every two to seven years.

normal conditions

In a normal year, the trade winds blow across the Pacific Ocean near the Equator from east to west (toward Asia). A low pressure cell rises above the western equatorial Pacific. Warm water in the western Pacific Ocean raises sea levels by half a meter. Along the western coast of South America, the Peru Current carries cold water northward, and then westward along the Equator with the trade winds. Upwelling brings cold, nutrient-rich waters from the deep sea.

el nio

In an El Nio year, when water temperature reaches around 28o C (82o F), the trade winds weaken or reverse direction and blow east (toward South America) (Figure 1.2). Warm water is dragged back across the Pacific Ocean and piles up off the west coast of South America. With warm, low-density water at the surface, upwelling stops. Without upwelling, nutrients are scarce and plankton populations decline. Since plankton form the base of the food web, fish cannot find food, and fish numbers decrease as well. All the animals that eat fish, including birds and humans, are affected by the decline in fish. By altering atmospheric and oceanic circulation, El Nio events change global climate patterns. Some regions receive more than average rainfall, including the west coast of North and South America, the southern United States, and Western Europe. Drought occurs in other parts of South America, the western Pacific, southern and northern Africa, and southern Europe. An El Nio cycle lasts one to two years. Often, normal circulation patterns resume. Sometimes circulation patterns bounce back quickly and extremely (Figure 1.3). This is a La Nia.



la nia

In a La Nia year, as in a normal year, trade winds moves from east to west and warm water piles up in the western Pacific Ocean. Ocean temperatures along coastal South America are colder than normal (instead of warmer, as in El Nio). Cold water reaches farther into the western Pacific than normal. Other important oscillations are smaller and have a local, rather than global, effect. The North Atlantic Oscillation mostly alters climate in Europe. The Mediterranean also goes through cycles, varying between being dry at some times and warm and wet at others. Click image to the left or use the URL below. URL:


instructional diagrams

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in a normal year, the trade winds blow from _ near _.

a) east to west; the 45th parallel

b) west to eat: the 45th parallel

-->  c) east to west; the equator

d) west to east; the equator

in a normal year, the peru current carries cold water north along south america and then across the equator.

-->  a) true

b) false

in a normal year, along western south america

-->  a) upwelling brings cold, nutrient-rich water to the surface.

b) downwelling takes warm, nutrient-poor water to the bottom.

c) surface currents bring warm, nutrient rich water from the equator.

d) none of these.

the north atlantic oscillation mostly alters climate in europe.

-->  a) true

b) false

el nio events

a) alter rainfall patters so some regions receive much more rain than normal.

b) bring drought to some regions.

c) ordinarily last one to two years.

-->  d) all of the above

warm water in the western pacific ocean decreases sea levels.

a) true

-->  b) false

during el nio,

a) downwelling off of south america ends.

-->  b) the trade winds and surface currents reverse direction.

c) cold water piles up in the eastern pacific ocean.

d) all of these.

during la nia,

-->  a) surface wind and water currents flow in their normal directions.

b) cold water piles up in the western pacific ocean.

c) weather patterns mirror the patterns in el nio.

d) all of these.

what do el nio events cause to happen off of south america?

a) a rise in upwelling.

b) the pooling of cold, nutrient-rich water.

-->  c) a collapse of the food web.

d) all of these.

el nio and la nia make a cycle called the los nios oscillation.

a) true

-->  b) false

diagram questions

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