further reading

Glantz, M.H. 1994. The Impacts of Climate on Fisheries. UNEP Environment
Library No. 13. Nairobi, Kenya: UN Environment Programme, 36 pp. 

Glantz, M.H. (Ed.). 1992. Climate Variability, Climate Change and
Fisheries. Cambridge, UK: Cambridge University Press, 450 pp. 

Glantz, M.H., R.W. Katz, and N. Nicholls (Eds.). 1991. Teleconnections
Linking Worldwide Climate Anomalies. Cambridge, UK: Cambridge University
Press, 535 pp. 

P.W. Glynn, 1990. Global ecological consequences of the 1982-83 El
Nino-Southern Oscillation.

Henry F. Diaz and Vera Markgraf (Eds.), 1992. El Niņo: Historical and
Paleoclimatic Aspects of the Southern Oscillation

S. George Philander, 1990.  El Niņo, La Niņa, and the Southern
Oscillation
         
Warren S. Wooster and David L. Fluharty, (Eds.), 1985. El Niņo North:
Niņo Effects in the Eastern Subarctic Pacific Ocean


                  ______________________________________
                            El Nino and La Nina

                          Written by Tom Murphree
                         for the Forthcoming Book,
                            "Watching Weather"
                      By Tom Murphree and Mary Miller
                          with the Exploratorioum
                 
                       Written for a Popular Audience 
                   Published by Henry Holt, New York, NY,
                          Due out in August 1998
                   ______________________________________

     In the western U.S., Arizona and New Mexico are famous for their dry
weather and deserts, while Oregon and Washington are known for their
persistent rain and dense forests.  But in each of these regions, the average
weather and the actual weather can be quite different from each other.  During
the winter of 1992-1993, the Southwest had exceptionally high rain and snow
falls, and major flooding.  At the same time, the Pacific Northwest was
suffering through a drought.  Three years later, during the winter of 1995-
1996, the stories were reversed.  Oregon and Washington had extremely heavy
rain and snow, and devastating floods, while the Southwest went through one
of its worst droughts in years.  Despite their differences, these two regions and
these two extreme winters had a very important feature in common:  Their
extreme weather could be traced back to unusual ocean temperatures in the
tropical Pacific, over five thousand miles away.

     During the 1992-1993 winter, the sea surface was cooler than normal
in the western part of the tropical Pacific and warmer in the central and eastern
part.  Since tropical storms tend to form over the warmest water, these
changes in the sea surface temperature (SST) led to fewer tropical storms than
usual in the western Pacific and more than usual in the central and eastern
Pacific.  These unusual patterns of SSTs and storms is part of what's called an
El Nino event, a closely linked set of unusual conditions in the tropical Pacific
ocean and atmosphere that can lead to dramatic weather changes all over the
world.

     The name El Nino means "the boy" and is a shortened version of the Spanish
name given by fishermen along the tropical Pacific coast of South America to an
ocean warming that they often noticed occurring around Christmas and which they
called El Nino de Navidad, or the Christ Child.  Scientists have since recognized
that this coastal warming is just one small part of a grand rearrangement of the
entire tropical Pacific ocean and atmosphere that occurs about every two to
seven years.  In this rearrangement, the normal patterns of SST and storms
change, and remain changed for about a year.  During this period of El
Ni¤o conditions, major disruptions of the weather and climate far from the
tropical Pacific are likely.  In the U.S., for instance, El Nino events generally
lead to winters that are drier in the Pacific Northwest and wetter in the
Southwest.

     These remote impacts of El Nino events occur when changes in the
tropical storms alter the winds blowing into and out of the tropics.  The wind
changes lead to disruptions in the normal path of the subtropical and
midlatitude jets and storm tracks.  So during the 1992-1993 winter, an
unusually persistent jet and storm track developed that extended from near
Hawaii into southern California, Arizona, and New Mexico and brought an
exceptional number of warm wet storms out of the subtropics.  February 1993
was especially wet in Arizona, leading to flooded homes, the loss of many
winter vegetable crops, and a big increase in produce prices.  Meanwhile, the
jet and storm track that would normally have spent much of the winter bringing
rain and snow to Oregon and Washington shifted north into northern British
Columbia, leaving the Northwest dry.

     Three years later, in 1995-1996, the tropical Pacific SST and storm
anomaly patterns were reversed, so that the western part was warmer and
stormier than normal, while the central and eastern was cooler and less stormy. 
Again, the atmosphere far away responded to these changes, only this time the
responses tended to be reversed.  Over the western U.S., the shifts in the jets
and storm tracks brought many warm moist winter storms and record amounts
of rain and snow to Oregon and Washington, and unusually few and weak
storms to the Southwest.  The reversed conditions in the tropical Pacific are
part of what's called a La Nina event.  La Nina is Spanish for the girl and is
used by scientists to describe how this event is the opposite of an El Nino
event.  Like El Ninos, La Ninas also occur about every two to seven years and
last about a year.  The changes a La Nina brings to the tropical Pacific, and
its remote impacts, can be just as large as those during an El Nino.

     One of the main reasons they are so large is that the tropical Pacific
takes up a very big chunk of Earth's surface, extending from Indonesia in the
west to South America in the east, and from about 15 south to 15 north --- an
area of about 20 million square miles, or more than five times the area of the
48 states.  This huge tropical region is a giant fluid solar collecting panel,
absorbing much of the solar energy that drives the world's weather.  The trade
winds that blow westward and toward the equator over the tropical Pacific
cause water warmed at the surface to also flow westward.  This creates an
accumulation of especially warm water near New Guinea and Indonesia,
sometimes called the western tropical Pacific warm pool, that at the surface
is about 80-85 F year-round.  

     The warm pool is actually a layer that floats on the cooler water below. 
Near New Guinea, the layer is about 700 feet thick.  This warm thick layer
makes the tropical western Pacific a tremendous reservoir of energy for the
atmosphere.  The atmosphere's response to this ready source of energy is
some exceptionally dramatic weather, including about 45% of the world's
hurricanes, known as severe tropical cyclones or typhoons or in this area.
(By comparison, the Atlantic produces only about 12% percent of the hurricanes.)
The intense storm activity, including some of the highest rainfall and latent
heating in the world, also help create the East Asian jet stream, the strongest
jet in the world.  The strength of this jet is due mainly to the large
temperature gradient between the tropical western Pacific to the south of the
jet and cold Siberia to the north (see chapter 3).  Many of the storms that
travel east with this jet hit the west coast of the U.S. and then head further
east across the country, some getting all the way to the east coast or beyond
(see chapters 3 and 4).

     As the trade winds create currents that pile up water in the warm pool,
they also remove water from the eastern Pacific, creating a sea surface that's
about 1-1/2 feet higher in the west.   The trades also produce a sea surface
temperature difference, with the eastern waters being about 65-70 F, or 15 F
cooler than in the west.  The cooler water inhibits storm formation, so the
eastern tropical Pacific is a relatively quiet region for tropical storms,
compared to further west.  This makes the jets that flow north and south of the
tropical eastern Pacific relatively weak too.

     So long as the trades blow normally, these sea level, SST, storm, and jet
patterns hold steady.  But if the trades change, then lots of rearrangements can
happen.  The trades are driven by the difference between the high pressure in
the eastern Pacific and the low pressure near Indonesia (see chapter 3).  At the
beginning of an El Nino event, the high pressure in the east, near Tahiti or Easter
Island, say, gets lower while the low pressure in the west, near Indonesia, gets
higher.  So the pressure difference gets smaller and the trades weaken.  This
causes the tropical western Pacific to cool down while the central and eastern
Pacific warms up --- for example, by an eastward flow of water out of the warm
pool.  As the sea surface temperatures change, so do the tropical storms that feed
on warm water.  The tropical western Pacific gets less stormy while the eastern
gets more stormy.  This in turn disrupts the normal midlatitude jet streams and
the storm tracks that run along side the jets (see chapter 4).

     This chain of events for an El Nino event is roughly reversed for a La Nina
event, with a La Nina event starting with an increase in the east-west pressure
difference and the trades.  The links in the chain of events that come before the
pressure and trade wind changes aren't yet known.  One possibility is a change in
Indian Ocean temperatures.  Another is a change in the snow fall over Tibet.  A
large Tibetan snow fall might cause Tibet to stay cool well into the spring and
summer, slowing down the development of the Asian summer monsoon and
causing weaker trade winds to blow across the Pacific and into Asia.  The
events by which El Ninos and La Ninas come to an end are also not understood,
although some scientists think there may be feedbacks between the tropical
Pacific ocean and atmosphere that cause an El Nino to bring itself to an end as
it sets the stage for a La Nina to follow, which then brings itself to an end and
prepares the stage for the next El Nino.

     These jet and storm changes that El Ninos and La Ninas trigger outside the
tropics are greatest during the winter, when the temperature differences between
the warm tropics and the cold higher latitudes are greatest.  That's part of why
El Ninos and La Ninas have their biggest impacts on the U.S. during November-
March.

     The changes in Earth's weather that occur during El Nino and La Nina
events can be very big and very expensive.  The cost to the U.S. of the 1982-
1983 El Nino event, one of the biggest on record, has been estimated in the
billions of dollars, due mainly to property damage and lost work time caused by
heavy rains, snows, and floods.  This has prompted a lot of research into the
causes of El Nino and La Nina events, and how to predict them and their
impacts.  One study has estimated that the benefits to the U.S. of accurate El
Ni¤o and La Nina forecasts would be worth about 1-2 Billion dollars a year.

     Good forecasts depend on thorough observations of the key regions of
the ocean, atmosphere, and land that create El Ninos and La Ninas and their
impacts.  In the tropical Pacific, an array of shore stations, balloons, planes,
buoys, ships, and satellites keep watch on changes in pressure, wind, sea level,
currents, air and sea temperatures, humidity, rain, and many other weather
factors.  But there are still large areas, such as in and over the ocean between
Hawaii and Mexico, where little data is being collected.  Even with much better
observations, El Nino and La Nina forecasts would still have a lot of weaknesses. 
What's most wanted are forecasts that give people one to several months to
prepare for the impacts of El Ninos and La Ninas.  But forecasts with that sort
of lead time have a very high predictability barrier to contend with (see chapter
5).

     Many forecasts of El Ninos and La Ninas and their impacts rely on
observations of what's gone on during past events.  For instance, during past
El Ninos, the extreme southern U.S. has generally received unusually heavy winter
rain and snow fall, so forecasts of El Nino impacts usually predict heavy
precipitation there.  But each El Nino event is different in the tropical Pacific
and, even more so, in its impacts outside the tropics.  So predictions based on
past events can be very iffy.  This is especially true for places like central
California and much of the southern Great Plains, where the impacts from one El
Nino event to the next can be quite different.  In central California, where
I live, past El Ninos and La Ninas have helped create very wet, very dry, and
very normal winters --- which makes it tough when people ask me what to expect
from future El Ninos and La Ninas.

________________________________________________________________________________

                      Sidebar for "Watching Weather"

                           Blame it on El Nino
                      (Or Maybe on Global Warming?)

     Have you had some really strange weather lately?  How about unusual
forest fires, floods, droughts, or cranky in-laws?  Maybe you could blame it
all on El Nino.  And why not?  Seems like every weird phenomenon gets pinned
on El Nino.  How about the Oakland Hills fire that wiped out hundreds of homes
in October 1991, or the Houston floods in December 1991, the marlin happily
swimming off the coast of Oregon and Washington in August 1997, and hurricane
Nora raining on southern Arizona in Septemeber 1997?  For some people, those all
look like the handiwork of El Nino.

     And maybe some really are.  But the quick and easy way in which
some people heap the blame on El Nino --- and the media leaps to report their
claims --- makes scientists who study El Nino and its impacts cringe,
or at least sigh in frustration.  The ways in which El Nino events affect the
world are complex.  They are also the ways in which many other natural
disruptions affect the world.  For example, El Ninos, volcanic eruptions in the
Philippines, weak monsoon rains in India, and heavy snow cover in Siberia can
all disrupt the jets and storm tracks that cross the North Pacific and head
into the U.S.  Jets and storm tracks are very energetic and unstable parts of
the atmosphere.  Lots of disturbances, even fairly little ones, can tweak
them and set them on strange courses, in roughly the same way that lots of
disturbances can make a pencil standing on end tip over, or a house of cards
collapse.  So figuring out what caused a tweaked storm track that led to
flooding in one area and drought in another is not an easy matter.  There are
many leads that a meteorologic detective needs to check out before blaming EN
or anything else.

     Fortunately for El Nino, it's been able to share some of the blame lately. 
It seems that if the latest natural disruption is not the work of El Nino or his
"wicked sister" (as one popular science article has labelled La Nina), then it's
surely a sign of global warming!

________________________________________________________________________________