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The El Niño Southern Oscillation (ENSO): Formation and Effects on Weather


A version of this blog was originally posted on November 14, 2012

If you have been listening to the news or following along with weather and climate web pages, you have probably heard the term "ENSO."  ENSO, or El Niño-Southern Oscillation, is a quasi-periodic climate pattern that occurs in the tropical Pacific Ocean.  When the conditions change, the atmosphere responds in many different ways.  In certain locations, it is cloudier and it rains more, while in others it’s clear and dry.  Scientists are forecasting El Niño conditions to start later in 2014.

Formation

El Niño is a temporary change in sea surface temperatures of the Pacific Ocean near the Equator.  Generally, winds in this region blow strongly from east to west (in the mid-latitudes, like where the GLOBE Program Office is located, winds blow from west to east).  Since the winds blow this way over extended periods of time, water in the western Pacific “piles up”.  The water that piles up in this region is warm (approximately 30°C), since the wind pushes the sun-warmed shallow layer of the ocean to the west.  With warmer waters, you tend to see an increase in thunderstorm activity.  So this region that has warmer water, like the northern coast of Australia, typically sees thunderstorm activity.

The water further east is colder (approximately 22°C) because the deeper water is pulled up, or upwells, to replace the water that has been pushed away. So areas along the western coast of Equatorial South America will see cold sea surface temperatures. 

In a positive phase, also known as El Niño, the winds that push the water to the west weaken.  Since the winds are weaker or even reverse, not as much water piles up in the western Pacific, so the water slides back toward the east.  With the warmer water sliding back toward the east, not as much cold water rises along the coast, which results in warmer waters off the coast of equatorial South America.  Once this gets going, the situation continues and strengthens: the warmer waters cause the winds to weaken even further, which results in the ocean warming further, which causes the winds to weaken, which results in the ocean warming, etc..  This is known as positive feedback and allows El Niño to grow. 

What happens if the east to west winds actually strengthen? This results in even more warm water piling up in the western Pacific and even more cold water upwelling along the western coast of Equatorial South America.  This scenario is known as La Niña, or the negative phase of ENSO, and it brings with it different weather patterns.   As with El Niño, there is a positive feedback that happens with the winds and allows the event to strengthen:  the colder waters cause the winds to strengthen even further, which results in the ocean cooling further, which causes the winds to strengthen, which results in the ocean cooling, etc. 

ENSO SST
Figure 1. Sea Surface Temperatures during La Niña, normal, and El Niño conditions
Image source: http://www.pmel.noaa.gov/tao/elnino/la-nina-pacific.html

The Effects of ENSO on Weather

So, how does ENSO affect our weather? Even though the sea surface temperature anomalies are happening along the equator in the Pacific Ocean, weather all over the world is influenced by ENSO. The exact effects depends on the time of year and the phase of ENSO. Figure 2 shows the phase of ENSO (El Niño on the top, La Niña on the bottom) and the time of year (December through February and June through August). For example, if El Niño starts this June, India may experience dryer than normal conditions through August and then warmer conditions December through February (depending on how long El Niño lasts). Will ENSO potentially affect your weather? If so, how?

ENSO weather
Figure 2: The effects of ENSO on weather
Image Source: http://www.srh.noaa.gov/jetstream/tropics/enso_impacts.htm

GLOBE schools have been affected by both El Niño and La Niña.  In 1997, a historically strong El Niño event took place (note: an event is classified as weak, moderate or strong depending upon how far the sea surface temperature departs from normal over at least five consecutive months).  Students in regions affected by this El Niño took measurements to examine what the effects were locally. You can read more about GLOBE and the El Niño experiment here: http://www.globe.gov/explore-science/field-campaigns/field-campaign-archive/el-nino-experiment.

Suggested activity: No matter which region of the world you’re located, you can examine the relationship between your local weather and ENSO.  Taking air temperature and precipitation measurements are great ways to start.  You can then connect those measurements to the Oceanic Niño Index by examining the correlation.  While many studies have been performed using the combination of these observations, it is worthwhile for students to also examine these studies, as this helps makes the connection from local to global.

 
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