Blog originally posted on The GLOBE Scientists' Blog: http://blog.globe.gov/sciblog/2012/11/14/esno-basics-what-is-it/
Through many posts here on the Scientists’ Blog, we, in one way or another, discuss 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. Through our “ENSO Basics” series, we’ll take a look at ENSO in many different ways, such as how ENSO forms, what type of weather it causes, and its connection to climate. Since we’re currently in a neutral scenario, it seems a great time to examine ENSO and not be partial to one phase or another. Today’s post will discuss how ENSO forms.
El Niño is a temporary change in the Pacific Ocean, in the area of 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. 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, sees thunderstorm activity.
The water further east is colder (approximately 22°C) because the deeper water is pulled up to replace the water that has been pushed away. So areas along the western coast of Equatorial South America will see cold temperatures. Figure 1, taken from the International Research Institute for Climate and Society (IRI) at Columbia University, shows a schematic of the water temperature as well as the accompanying atmospheric circulation.
Figure 1. Schematic of normal conditions. From IRI.
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. This 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. This is known as a positive feedback, and allows El Niño to grow. Figure 2, also from IRI, shows El Niño.
Figure 2. Schematic of an El Nino event. From IRI
What happens if the winds actually strengthen? This results in even more warm waters 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. Figure 3, again from IRI, shows the schematic for a La Niña event.
Figure 3. Schematic of a La Nina event. From IRI.
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 departure from normal is over at least five consecutive months). Students in regions affected by this El Niño took measurements to examine what the effects were locally. As we continue in this series, we’ll be sure to feature some of the schools making these measurements.
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. http://iri.columbia.edu/climate/ENSO/background/basics.html