GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 12: El Niño
POWERPOINT SLIDE SHOW NOTES
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Topic 12: El Niño
El Niño is a phenomenon much talked about in the news. However, it’s a little hard to
define except by starting with these general statements.
Typically, strong trade winds push warm surface water away from South America and
across the Pacific Ocean. So much water is pushed to the west that the sea level in
Darwin, Australia, is actually a little bit higher than in Tahiti. El Niño is a disruption of the
normal atmospheric and ocean conditions.
El Niño resources
El Niño Characteristics
Normal conditions. Strong trade winds mean higher sea level and a mound of warm
water in the Western Pacific. Warm water means warm, moist air which rises and
creates rainfall. So much surface water being pushed away from South America means
deeper cold water is brought to the surface (upwelling). The thermocline separates
warm surface water from deep cold water. Upwelling is very important to the
ecosystem. It brings nutrient- and carbon dioxide-rich water to the surface where it is
available for use by single-celled plants. The plants grow in great abundance providing a
tremendous amount of food for animals, and the fishing industry flourishes. Much of the
economy of the region is based on the fishing industry.
During an El Niño, the trade winds are weaker. That means less surface water pushed to
the western Pacific, and the mound of warm water only extends part way across the
ocean, which means the low-pressure center moves to the east. Where under normal
conditions rain is plentiful in the west and conditions are drier in the central Pacific,
during El Niño it is dry in the west and wet in the central Pacific. Also, with less surface
water pushed away from South America, upwelling is weaker and fewer nutrients are
brought to the surface. This impedes the plant productivity and the fishing industry
suffers.
El Niño and Productivity
El Niño Sea Surface Temperatures. Reds are higher sea surface temperatures. Warm
surface water does not extend as far to the west during an El Niño as it does under
normal conditions.
La Niña
La Niña conditions. Strong trade winds, strong upwelling, low-pressure center farther to
the west, thermocline even steeper than normal.
Sea surface temperatures are cooler in the western Pacific during La Niña because so
much upwelling occurs in the east that cool water is pushed to the west by the trade
winds.
El Niño condition: departures from normal
El Niño mean temperature minus the neutral (normal) mean temperature. The yellows,
oranges, and reds mean warmer conditions during El Niño. The blues and purples mean
cooler conditions during El Niño.
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El Niño mean precipitation minus the neutral (normal) mean precipitation. The yellows,
golds, and oranges mean more precipitation during El Niño. The blue colors mean less
precipitation during El Niño.
La Niña Impacts in U.S.
El Niño and La Niña and the Asian monsoons.
ENSO
ENSO illustrated
Southern oscillation (the “SO” in ENSO)
Southern Oscillation Index (SOI). The SOI measures the difference in atmospheric
pressure between Tahiti and Darwin, Australia. The top graph is sea surface temperature
departure from normal. The bottom graph is the Tahiti-Darwin SOI. Note how when the
SOI is negative (red on the bottom graph) the sea surface is warmer than normal (red on
the top graph). And When the SOI is positive (blue on the bottom graph) the sea surface
is cooler than normal (blue on the top graph).
Refer back to slide 19. During normal conditions, the pressure in Darwin is lower than
Tahiti (Tahiti pressure minus Darwin pressure is a positive number—blue on the SOI
graph). During El Niño conditions, the pressure in Darwin is higher than Tahiti (Tahiti
pressure minus Darwin pressure is a negative number—red on the SOI graph).
Coupled Ocean-Atmosphere Circulation. Coupling means the ocean and atmosphere are
linked. During normal conditions (non El Niño) the trade winds (red arrows at the ocean
surface) are strong, warm ocean surface water is pushed to the west (gold arrows), and a
mound of warm water builds in the western Pacific. During El Niño events, the trade
winds (red arrows at the ocean surface) are weaker, warm ocean surface water is not
pushed as far to the west (gold arrows), and the mound of warm water sloshes eastward
to the central Pacific.
Ocean’s Response to the SO
ENSO observing system. Meteorological data from moored buoys, drifting buoys, tide
gages, and observations relayed to researchers by satellites allow monitoring and
prediction of potential El Niño and La Niña events.
El Niño and Hurricanes El Niño and Hurricanes
TAO/TRITON. See http://www.pmel.noaa.gov/tao/index.shtml.
Moored Buoy locations. Moored buoys record conditions in one location.
Moored buoys need maintenance which is carried out by NOAA, the National Oceanic
and Atmospheric Administration.
Example of a maintenance cruise in 2006. Service was performed on several buoys to
the southwest of Hawaii.
Drifters, or drifting buoys, record conditions as them move with the currents.
The vast drifter array collects valuable data from around the world.
Visit http://www.pmel.noaa.gov/tao/jsdisplay/ani.html for TAO/TRITON displays,
including animations.
CTD’s take measurements as they descend through the water column.
Deploying a CTD array.