1. El niño y La niña 1
Jim Stevens
NSCI 170 Section 6380
Professor Berman
February 14, 2015
ATMOSPHERIC CIRCULATIONS
EL NIÑO Y LA NIÑA

2. El niño y La niña 2
Atmospheric Circulations
El Niño y La Niña
Figure 1: Current Conditions at the Equator
http://www.ssec.wisc.edu/data/sst/latest_sst.gif
The map above (Figure 1) represents the sea surface temperatures in the equatorial
Pacific Ocean on February 13, 2015. According to the National Oceanographic and
Atmospheric Administration (2015), these measurements are taken by an array of buoys, shown
in Figure 2 below; their most important function is to monitor potential El Niño and La Niña
conditions (United States Department of Commerce, 2015). While these two conditions occur in
the equatorial Pacific Ocean, their effects can be felt around the globe. Although neither of these
conditions are presently occurring, NOAA is currently predicting "a 50-60% chance that an El
Niño event will occur in the next two months" (United States Department of Commerce, 2015).
Figure 2: TAO/Triton Array
http://www.pmel.noaa.gov/tao/images/tao-array-huge.gif

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So what are El Niño and La Niña, and what do they mean for the weather patterns around
the world? Before we answer this, we should understand what the normal conditions are in this
important area of the Pacific. As one might expect, temperatures at the sea surface in the Pacific
Ocean along the equator are much warmer than they are in the middle and higher latitudes of
both hemispheres. Under normal conditions, these temperatures are roughly 8 °C higher on the
Indonesian side of the Pacific than those on the South American shores. In addition, the cooler
water below the surface of the latter, the thermocline, lies at approximately 50 m deep, much
more shallow than the shores of the former. According to NOAA, "This cold water is nutrient-
rich, supporting high levels of primary productivity, diverse marine ecosystems, and major
fisheries (United States Department of Commerce, 2015). As Figure 3 below illustrates, sea
surface level ocean and wind currents (tradewinds) flow in a westard direction, precipitation
forms off the coast of Indonesia, and the upper air convective air currents flow eastward in a
consistent cycle.
Figure 3 : Normal Equatorial Pacific Oceanic and Atmospheric Conditions
http://www.pmel.noaa.gov/tao/elnino/images/normal-only.gif
During an El Niño event, sea surface temperatures are warmer than usual by up to 6 °C.
Warmer sea surface temperatures begin to spread much farther east and the trade winds weaken.
In addition, the thermocline deepens in the east while rising closer to the surface in the west.
The result, as Figure 4 shows, is an eastward ocean current, precipitation that forms much farther
east, and central pacific vertical convection that circulates outward toward both coasts.
The effects of an El Niño event can be serious on both sides of the Pacific and can affect
weather systems in other parts of the world as well. As the trade winds weaken, even
occasionally becoming eastward winds, warm water is also pulled in an eastward direction
lowering sea levels in the west and increasing sea levels in the east. This, coupled with an
increase in water vapor in the atmosphere in the mid- to eastern Pacific may cause an increase in
storminess and precipitation resulting in coastal flooding in South America. Conversely, this

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may lead to drought in other parts of the world as it receives lessened rainfall including a calmer
than usual hurricane season in the tropical Atlantic (Ahrens, 2015, p. 215). In addition, the
sinking thermocline in the east can diminish the supply of nutrient-rich cool water to the region
and have devastating effects on marine life and the fishing industry.
Figure 4: El Niño
http://www.pmel.noaa.gov/tao/elnino/images/nino-only.gif
In contrast to La Niña conditions are typically periods of cooler than normal temperatures
in the equatorial Pacific Ocean. The localized effects are typically the reverse of El Niño
periods. For example, the thermocline rises in the east, sinking in the west; trade winds are
strengthened and are once again easterly; precipitation moves toward the Indonesian coast; and
cooler water extends much farther west than normal.
Just as El Niño can affect weather patterns over the entire globe, so too can La Niña.
According to NOAA, “In the U.S., winter temperatures are warmer than normal in the Southeast,
and cooler than normal in the Northwest" (United States Department of Commerce, 2015). In
addition, severe weather events such as thunderstorms and tornadoes can increase in the central
and eastern portions of the U.S. (Ahrens, 2015, p. 215) Figure 5 illustrates the La Niña
characteristics.
According to Ahrens (2015), "This see-saw pattern of reversing surface air pressure at
opposite ends of the Pacific Ocean is called the Southern Oscillation (p. 214). Moreover, while a
La Niña cycle often follows an El Niño cycle, it does not always. Likewise, although these
atmospheric cycles share similar characteristics each time they occur, they can vary somewhat in
behavior, duration, and intensity.

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Figure 5: La Niña
http://www.pmel.noaa.gov/tao/elnino/images/nina-only.gif
Figure 6: Normal Equatorial Pacific Ocean Surface Wind Conditions
http://www.pmel.noaa.gov/tao/elnino/images/lat_lon_9312_sst_lf.gif

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Figure 6 above illustrates the normal surface wind conditions (top) of the equatorial
Pacific Ocean. We can see that the wind tends to be easterly creating the trade winds. There are
limited anomalies as shown (bottom), where wind direction is in most cases, northerly.
Figure 7 below shows the conditions that occur during an El Niño event. Examining the
mean winds (top), it becomes immediately apparent that the wind directions have changed.
Winds in the eastern Pacific tend to be southeasterly, while the central Pacific have northeasterly
winds. Moreover, we can see a shift in the west toward westerly winds, a reverse of the norm.
We can also notice a major increase in wind anomalies (bottom) toward westerly winds in the
western Pacific and northerly winds in the east.
Figure 7: El Niño Surface Wind Conditions
http://www.pmel.noaa.gov/tao/elnino/images/lat_lon_9712_sst_lf.gif

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Figure 8: El Niño Sea Surface Height Anomalies
https://sealevel.jpl.nasa.gov/images/ostm/files/images/browse/120202.jpg
Figure 8, above, is a satellite photo illustrating the sea level anomalies of the December
2002 El Niño event. The color scale below the photo corresponds to sea level increases or
decreases around the world in millimeters. Anomaly refers to the change from the normal sea
levels as a result of this event, just as temperature and wind anomalies refer to changes from the
norms of their respective states.

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Works Cited
Ahrens, C. D. (2015). Essentials of Meteorology: An Invitation to the Atmosphere (Seventh ed.).
(S. Kenter, Ed.) Stamford, CT, United States of America: Cengage Learning. Retrieved
February 14, 2015
Historical El Niño/La Niña Watch. (n.d.). Retrieved February 14, 2015, from Nasa Ocean
Surface Topography From Space:
https://sealevel.jpl.nasa.gov/images/ostm/files/images/thumbnail/120202-th.gif
United States Department of Commerce. (2015, February 14). NOAA's El Niño Portal. Retrieved
from National Oceanic and Atmospheric Administration Web Site:
http://www.elnino.noaa.gov/