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The Weather Classroom
Your Alaskan guide through the murky waters of El Nino and La Nina -- Section 1
John Papineau -- NWS Anchorage
Weather in Alaska varies on a variety of timescales- from weeks, months to years.
A considerable part of that variation but certainly not all of it is due to semi-periodic
events like El Nino and La Nina. For example, the El Nino which occurred during the
winter of 2002-2003 produced very warm temperatures across most of Alaska, while the
El Nino of 1972-1973 produced cooler than normal conditions. Over the past several
decades the national media has spent considerable time highlighting the impacts that
these events have on North American weather. It is important to keep in mind that
impacts differ greatly from one region of the continent to another.
This paper will cover basic El Nino and La Nina concepts as well as explore the
impacts that these events have on weather across ALASKA.
Some of the questions that this discussion will attempt to answer are: What are El Nino's,
La Nina's and how do they differ? What produces these events and why do they occur at irregular
intervals? How do weather patterns in the tropics influence weather that occurs
thousands of miles away in Alaska? And finally, how do these events specifically
impact weather in Alaska?
Basic Concepts
An El Nino (EN) occurs when there is a significant increase in water temperature
in the eastern equatorial Pacific Ocean, which is the area between the dateline and
Ecuador. This warming is often, but not always first noticeable during spring.
Most EN have a duration of 10 to 16 months, but in some cases it may extend for
several years as it did in 1976-1978 and again in 1986-1988. Sea water temperatures
increase different amounts but typically range from 1° to 4°F above normal over tens
of thousands of square miles of ocean. This rise in temperature not only occurs at
the surface but at depths of several hundred feet as well. An increase of few degrees
does not sound like a very impressive warming; nevertheless it represents a very large
amount of energy. While water temperatures rise in the eastern Pacific they simultaneously
cool in the western equatorial Pacific. This sea-saw pattern of cooling and warming
of large areas of the tropical ocean has a very significant impact on the generation of
clouds, rainfall, and winds all across the tropics and beyond.
A La Nina (LN) on the other hand generates, among other signals, cooler than normal
water in the eastern equatorial Pacific Ocean. The longevity of LN is about the same
as for EN. On occasions the end of a LN is in a few months, followed by an EN and vise
versa, as occurred in 1987-1989 when an EN turned into a strong LN and again in 1998-1999.
A very commonly used acronym in the climate business is: ENSO, which stands for
El Nino and Southern Oscillation. In the broadest sense, this term refers to the
oscillation between the three different modes: El Nino, La Nina, and neutral.
A neutral mode occurs when there is no EN or LN. The southern oscillation part of ENSO we
will consider later on, but is a reference to different sea-level pressure patterns that
form in the sub-tropics of the Southern Hemisphere during EN and LN.
There are additional affects produced by EN and LN besides the warming and cooling of the
ocean in the tropics (strictly speaking within about 500 miles of either side of the equator).
During EN's wind direction changes from east (normal direction in the tropics) to the west.
In addition, the central and eastern tropical Pacific receives considerably more rainfall
than in other years. This often results in flooding in coastal Ecuador and Peru. The western
Pacific during an EN is most often drier than normal, leading to drought and an increase in
wildfires in Indonesia and Australia. Although not as easy to observe, during EN there seems
to be some type of modification to the ocean currents that transport warm water out of the
tropical Pacific and cool water back into the tropics. This type of disruption in the 'normal'
weather pattern (called 'anomalies') is important for short and long-term variation in the earths'
climate. During a LN winds remain out of the east while rainfall over the western Pacific may be
higher than normal. During neutral modes the weather in the tropics can still have significant anomalies.
These anomalies are a result of other physical processes that operate independently of EN and LN.
One interesting aspect of the ENSO phenomena is the fact that LN's and EN's occur at irregular intervals.
No one really knows at the present time how these events are initiated. The most commonly accepted theory
for the creation of an El Nino for example, is that abnormally warm water in the Indonesia area is
'released' into the central and eastern Pacific. The body of warm water moves to the eastern Pacific
over a period of two to four months. As noted earlier, this warm pool of water is typically several
hundred feet deep. LN's occur as cool water moves into the central and eastern Pacific.
Once in place, these warm or cool ocean water anomalies alter the winds, clouds and rainfall
in and outside of the tropics.
Continue to the Mid-latitude Impacts
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