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Initial Publication Date: November 12, 2013
Unit 3 Study Guide: ENSO
Use this guide to help organize your knowledge. Then test yourself by quizzing yourself on the terms with the definitions hidden and by answering the concept questions!
New Vocabulary or Terminology
- El Niño-Southern Oscillation (ENSO)
The term
El Niño was first coined over 100 years ago to describe the unusually warm waters that sometimes form along the coast of Ecuador and Peru. Today the term El Niño is used to refer to a much broader scale phenomenon associated with unusually warm water that occasionally forms across much of the tropical eastern and central Pacific. The time between successive El Niño events is irregular, but they typically tend to recur every 3 to 7 years. La Niña is the counterpart to El Niño and is characterized by cooler than normal SSTs across much of the equatorial eastern and central Pacific. These phenomena together comprise the ENSO system. (See
IRICS' Overview of the ENSO System for more details.)
A thermocline is the steep temperature gradient in a body of water marked by a layer above and below which the water is at different temperatures. In the case of ENSO, the thermocline is the boundary between hot and cold ocean waters.
Upwelling is a process in which deep, cold water rises toward the surface. Winds blowing across the ocean surface push water away. Water then rises up from beneath the surface to replace the water that was pushed away. (See
NOAA Ocean Facts: What is upwelling? for more details.)
The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics. In the case of ENSO, the strengthening or weakening of these winds drives the oscillation.
Convection is the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat. In the case of atmospheric convection, the sun heats the ground, and the ground heats the air. The air becomes less dense (lighter), so it rises. The air cools as it rises, and water vapor may condense to form a cloud. As the air cools, it becomes more dense (heavier), and it sinks to the ground again. (See
NOAA ESRL's Convection Tutorial for more details.)
A return period, or recurrence interval, is an estimate of the likelihood of an event, such as an earthquake, flood, or volcanic eruption, to occur. It is a statistical measurement typically based on historic data denoting the average recurrence interval over an extended period of time and is usually used for risk analysis. (See
Wikipedia's Return period page for more details.)
Conceptual Questions
- How do each of the following change during El Niño or La Niña?
- sea surface temperature (SST)
- thermocline
- upwelling
- trade winds
- convection/precipitation
- What are the biggest impacts of El Niño and La Niña in coastal South America and Australia?
- Why was the forecasting technique used by the Incas so successful at forecasting El Niño?
- How do El Niño and La Niña affect the jet stream in North America, and what impact does that have on North American weather?
- What similarities are there between ENSO and the NAO? What differences are there? Do you think one might impact the other?
-
SSTs during El Nino
![[reuse info]](/images/information_16.png)
Provenance: Pacific Marine Environmental Laboratory (NOAA) - Tropical Atmosphere Ocean Project
Reuse: This item is in the public domain and maybe reused freely without restriction.
SSTs during La Nina
![[creative commons]](/images/creativecommons_16.png)
Provenance: Pacific Marine Environmental Laboratory (NOAA) - Tropical Atmosphere Ocean Project
Reuse: This item is offered under a Creative Commons Attribution-NonCommercial-ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial purposes as long as you provide attribution and offer any derivative works under a similar license.
The examples on the right show the ocean surface temperature anomalies for the negative and positive phases of ENSO, along with the predominant weather effects in the tropical Pacific and Indian Oceans. As you study the images, do the following:
- draw vertical cross sections of the ocean to match the data on these maps (think about how you would draw the thermocline).
- draw vertical cross sections of the atmospheric convection loops that generated these anomalous sea surface temperatures.
You can check your work on the NOAA El Niño Theme Page and by examining the 3-D animation of ocean temperatures at:TAO Ocean Temperature Animation.
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