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.
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
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.
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 is what the mooring looks like.
ATLAS means Autonomous Temperature Line Acquisition System.
(Image courtesy NOAA)
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.
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
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
6. Where and when does El Niño occur and how often?
To find out the answers to these and other
questions you may have, check out our "Helpful