How Do We Know So Much About El Niño?

Satellites and moored instruments in the Pacific provide the measurements we need to keep track of El Niño.

Satellites

Three types of satellites are used to watch El Niño:

Weather satellites orbiting 800 km above the Earth carry instruments that map heat coming from the ocean. The maps give the temperature of the water. Warm water gives off more heat in the form of infrared radiation than cold water. If you hold your hand close to a warm oven, but be careful you don't touch the oven, you can feel the heat given off by the oven. Your hand is feeling the infrared radiation. The most important instrument for measuring infrared radiation is called AVHRR, which means Advanced Very High Resolution Radiometer. The map on the right below shows how much sea-surface temperature deviates from normal. The map was made from AVHRR measurements. It shows water in the eastern equatorial Pacific that is 2-2.5°C warmer than normal during the strong El Niño of 1997-1998.
Altimeter satellites such as Topex/Poseidon and Jason measure the height of the sea surface. The height is called sea level. Because water expands as it warms up, a layer of hot water at the sea surface is higher than a layer of cold water because of the expansion. The map on the left below shows sea level is higher than normal in the eastern tropical Pacific where water temperature is warmer than normal.
Scatterometers such as the one on QuickSCAT are special radars that measure wind speed. To watch for weak or reversing trade-winds, they map wind speed in the western equatorial Pacific. The wind measurements are helpful for predicting El Niño months in advance. The predictions give people time to prepare for difficult weather conditions.

Right: A map of the deviation of surface temperature from normal conditions. It shows warm water on the equator in the east, which is typical of El Niño. Image courtesy Center for Space Research, University of Texas.Left: A map of the deviation of surface height from normal conditions. It shows sea level higher than 28 cm above normal in the eastern tropical Pacific. The high areas are indicated by arrows. High sea level corresponds to warm water. Image courtesy Center for Space Research, University of Texas.

Moored Instruments

The National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, with the help of oceanographers in other countries, operates around 70 moorings in the tropical Pacific. A mooring is a steel or plastic rope that goes from a float on the surface to an anchor on the sea floor. The moorings holds instruments that measure surface winds, air temperature, relative humidity, sea surface temperature, and underwater temperatures. Data from the mooring are sent to Seattle by special satellite links.

Here are the conditions in the Pacific measured by the ATLAS moorings in December 1997. The maps show temperatures in the east 6°C warmer than normal. Winds in the west are weaker than normal. Arrows pointing toward the east indicate winds weaker than normal trade-winds, which have arrows that point west.

Here is what the mooring looks like. ATLAS means Autonomous Temperature Line Acquisition System. (Image courtesy NOAA)

Top: Sea surface temperature in Celsius in color, red is hotter. The arrows give wind speed and direction. Bottom: Deviations from normal conditions. Temperature deviations are in color. Wind deviations are arrows.The image is from the Tropical Atmosphere and Ocean TAO project at the Pacific Marine Environmental Laboratory of the National Oceanic and Atmospheric Administration. For the most recent data on TAO array sea surface data and wind speeds visit: http://www.pmel.noaa.gov/tao/jsdisplay/

Questions that come to mind are:

1. What is El Niño?

2. How has it come to be called El Niño (the [male] child)?

3. What causes El Niño?

4. What's the big deal? What does El Niño mean to me?

5. What do scientists know about El Niño and how do they know what they know?

6. Where and when does El Niño occur and how often?

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