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Detecting El Niño

During a typical year, the trade winds that blow across the Pacific Ocean push warm surface water to the west. On a typical SST map, the surface water in the western Pacific is warmer than in the eastern Pacific. But periodically, this pattern changes. The trade winds die down or even reverse, and water temperatures in the east become warmer than usual. This change is the beginning of an El Niño.

This change can be seen by looking at a plot of sea surface temperature from west to east. During a typical year, the temperature difference between warm water in the west and cooler water in the east is evident in the slope of the line on the following temperature plot.

During an El Niño year, the area of high temperature can be seen extending farther to the east than in a typical year. The temperature difference from west to east may also be smaller.

The shift in temperature has an important effect on ocean circulation. Usually, cold water from the bottom of the ocean rises to the top in the eastern Pacific to replace the surface water that the trade winds have blown to the west. This rising of cold water to the surface is called upwelling, and is important because it brings nutrients from deeper water up to the surface. Cold deep water is separated from warm surface water by a layer called the thermocline. During El Niño, this thermocline shifts, disrupting the normal transport of cold, nutrient-rich water.

Consider the images below and check your understanding. You can examine the differences between El Niño and non-El Niño years by making maps and graphs of real data.

Check your understanding

Select three images below that might indicate an El Niño event.

Sea surface temperature graph
Sea surface temperature graph
Sea surface temperature map
Pacific circulation diagram
Pacific circulation diagram
Sea surface temperature map
 

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