Download H17-3 El Niño, La Niña, Extreme Weather, and Global Warming

H17-3 El Niño, La Niña, Extreme Weather, and Global
Warming
What is climate variability? A prominent aspect of our weather and climate is its variability. This variability ranges
over many time and space scales such as localized thunderstorms and tornadoes, to larger-scale storms, to droughts, to
multi-year, multi-decade and even multi-century time scales.
Some examples of this longer time-scale variability might include a series of abnormally mild or exceptionally
severe winters, and even a mild winter followed by a severe winter. Such year-to-year variations in the weather patterns
are often associated with changes in the wind, air pressure, storm tracks, and jet streams that encompass areas far larger
than that of your particular region. At times, the year-to-year changes in weather patterns are linked to specific weather,
temperature and rainfall patterns occurring throughout the world due to the naturally occurring phenomena known as El
Niño and La Niña.
What is El Niño? The term El Niño refers to the large-scale ocean-atmosphere climate phenomenon linked to a periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific (between approximately
the date line and 120°W). El Niño represents the warm phase of the El Niño/Southern Oscillation (ENSO) cycle, and is
sometimes referred to as a Pacific warm episode.
What is La Niña? La Niña refers to the periodic cooling of ocean surface temperatures in the central and east-central
equatorial Pacific that occurs every 3 to 5 years or so. La Niña represents the cool phase of the El Niño/Southern
Oscillation (ENSO) cycle, and is sometimes referred to as a Pacific cold episode.
What happens during El Niño or La Niña? During an El Niño or La Niña, the changes in Pacific Ocean temperatures affect the patterns of tropical rainfall from Indonesia to the west coast of South America, a distance covering
approximately one-half way around the world. These changes in tropical rainfall affect weather patterns throughout the
world.
When and why do El Niño and La Niña occur? El Niño and La Niña are naturally occurring phenomena that result
from interactions between the ocean surface and the atmosphere over the tropical Pacific. Changes in the ocean surface
temperatures affect tropical rainfall patterns and atmospheric winds over the Pacific Ocean, which in turn impact the
ocean temperatures and currents. The El Nino and La Niña related patterns of tropical rainfall cause changes in the
weather patterns around the globe. El Niño and La Niña episodes typically occur every 3–5 years. However, in the historical record this interval has varied from 2 to 7 years. El Niño typically lasts 9–12 months, and La Niña typically lasts
1–3 years. They both tend to develop during March-June, reach peak intensity during December-April, and then weaken
during May-July. However, prolonged El Niño episodes have lasted 2 years and even as long as 3–4 years.
What is ENSO (El Niño/ Southern Oscillation)? ENSO stands for El Niño/ Southern Oscillation. The ENSO cycle
refers to the coherent and sometimes very strong year-to-year variations in sea-surface temperatures, convective rainfall,
surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean. El Niño and La Niña
represent opposite extremes in the ENSO cycle.
• El Niño refers to the above-average sea-surface temperatures that periodically develop across the east-central equatorial Pacific. It represents the warm phase of the ENSO cycle, and is sometimes referred to as a Pacific warm episode.
• La Niña refers to the periodic cooling of sea-surface temperatures across the east-central equatorial Pacific. It represents the cold phase of the ENSO cycle, and is sometimes referred to as a Pacific cold episode.
What are typical El Niño characteristics? El Niño- related oceanic and atmospheric conditions are generally opposite to those of La Niña. Typical El Niño conditions include the following:
• A deep layer of very warm ocean water across the east-central equatorial Pacific, with sea-surface temperatures generally 1.5°–2.5°C above average, and subsurface ocean temperatures typically 3°–6° above average at the depth of
the oceanic thermocline.
• Enhanced convective rainfall and below average air pressure across the eastern half of the equatorial Pacific; suppressed convective rainfall and above-average air pressure across Indonesia, the western equatorial Pacific, and
northern Australia. Some areas experience much higher-than-average rainfall, increased thunderstorm activity, and
flooding; other areas experience much lower-than-average rainfall, drought, and wildfires.
Insel/Roth, Core Concepts in Health, Brief Tenth Edition © 2006 The McGraw-Hill Companies, Inc.
H17-3 (continued)
• Weaker than average easterly trade winds across the eastern half of the equatorial Pacific.
• An equatorward shift and eastward extension of the mean wintertime jet stream along the poleward flanks of these
anomalous high-pressure cells (over the eastern half of the Pacific Ocean) in both hemispheres. During El Niño, the
southern portion of the United States is more susceptible to severe weather outbreaks.
• In August–October increased upper level westerly winds lead to higher-than-average vertical wind shear and reduced
hurricane activity across the tropical North Atlantic, and to below-average vertical wind shear and increased hurricane activity over the eastern tropical North Pacific.
Example of effects in the United States: During winter El Niño episodes, there is usually an exceptionally stormy winter
and increased precipitation across California and the southern U.S, and less stormy conditions across the northern part
of the country. Also, air flow changes result in a milder than normal winter across the northern states and western
Canada.
What are typical La Nina characteristics? La Nina- related conditions are generally opposite to those of El Niño.
• A deep layer of cooler than average ocean temperatures across the east-central equatorial Pacific, with sea-surface
temperatures generally 1°–2°C below average, and sub-surface temperatures typically 2°–4°C below average at the
depth of the oceanic thermocline.
• Suppressed convective rainfall and above average air pressure across the eastern half of the equatorial Pacific;
enhanced convective rainfall and below-average air pressure across Indonesia, the western equatorial Pacific, and
northern Australia.
• A weaker mean wintertime jet stream along the poleward flanks of these anomalous low-pressure cells (over the
eastern half of the Pacific Ocean) in both hemispheres.
• In August-October, the enhanced upper-level easterly winds lead to reduced vertical wind shear and increased hurricane activity across the tropical North Atlantic, and to above-average vertical wind shear and decreased hurricane
activity over the eastern tropical North Pacific. During La Niña, the southern portion of the United States is more
susceptible to severe weather outbreaks.
Example of effects in the United States: During winter La Niña episodes, large portions of central North America experience increased storminess, increased precipitation, and an increased frequency of significant cold-air outbreaks, while
the southern states experiences less storminess and precipitation. Also, there tend to be considerable month-to-month
variations in temperature, rainfall and storminess across central North America during the winter and spring seasons, in
response to the more variable atmospheric circulation throughout the period.
Why is predicting El Niño and La Niña so important? Better predictions of the potential for extreme climate
episodes like floods and droughts could save the United States billions of dollars in damage costs. Predicting the life
cycle and strength of a Pacific warm or cold episode is critical in helping water, energy, and transportation managers,
and farmers plan for, avoid, or mitigate potential losses. Advances in improved climate predictions will also result in
significantly enhanced economic opportunities, particularly for the national agriculture, fishing, forestry, and energy
sectors, as well as social benefits.
What is the relationship between El Niño/La Niña and global warming? The jury is still out on this. Are we likely
to see more El Niño’s because of global warming? Will they be more intense? These are questions facing the science
community today. Research will help us separate the natural climate variability from any trends due to man’s activities.
If we cannot sort out what the natural variability does, then we cannot identify the “fingerprint” of global warming. We
also need to look at the link between decadal changes in natural variability and global warming. At this time we cannot
preclude the possibility of links but it is too early to say there is a definite link.
SOURCE: Excerpted from National Weather Service Climate Prediction Center. 2002. Frequently Asked Questions About El Niño
and La Niña (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.html; retrieved June 7, 2005).
Insel/Roth, Core Concepts in Health, Brief Tenth Edition © 2006 The McGraw-Hill Companies, Inc.