http://www.nearingzero.net/nz393.jpe
el Niño
Any comments on the video?
Sorry about the crummy quality. I looked for a place to buy a
good copy of the video; no luck.
Why are video makers so enamored of gloom and doom?
“Next to the changing of the seasons, el Niño has the greatest
influence on the Earth’s climate.”
This is something big, and we ought to understand it.
“Climate is what we expect. Weather is what we get.“—Robert Heinlein
el Niño—Lecture Outline
What is it?
What causes it?
How does it affect us?
Can we change it?
What’s happening today?
El Niño in the future?
el Niño—what is it?
I obtained the information in this lecture from a number of
web sites.
The links are listed at the end of this lecture.
I’ll do my best to give proper credit when I use somebody
else’s words or pictures. I’ll be glad to correct any omissions.
“El Niño is an intermittent
disruption of the climate
system centered in the
equatorial Pacific.”1 We
now know it has effects on
weather across much of
the world. maybe skip to slide 9.
Digression: Global Circulation Patterns
“Air rises near the equator, where it is hottest, and sinks near
the poles, where it is cooler. However, instead of a circulating
in a single loop in each hemisphere, the motion divides into
three cells in the northern and three in the southern
hemisphere.”
convection (coming soon
in a demo)
“The wind does not blow straight north or
south between the bands of rising or
falling air; the Coriolis effect bends winds
to the right in the Northern Hemisphere
and to the left in the Southern.”
surface winds
(coriolis effect)
“Trade Winds”
from http://vathena.arc.nasa.gov/curric/weather/adptcty/globcirc.html (link now dead); also
see this site
Digression: Coriolis “Force”
The Coriolis effect is the term used to describe the apparent
motion of an object relative to a rotating reference frame.
Let’s aim a big cannon at
the North Pole towards a
target on the equator.
Remember: an object in
motion moves in a straight
line unless acted on by an
external force.
Will we hit the target?
http://zebu.uoregon.edu/~js/glossary/coriolis_effect.html
(link now dead)
For you nitpickers in the audience, yes, the cannonball will follow a parabolic trajectory under the influence of gravity. We
would need to calculate the correct firing angle that would allow the cannonball to hit at the equator.
We miss every time.
There must be a force deflecting the cannonball.
The “Coriolis Force.”
Right?
Wrong. The earth rotates
“out from underneath” the
cannonball. The target
moves. The cannonball
goes in a straight line.
An observer in the rotating (accelerated) reference frame of
the earth “invents” a fictitious “Coriolis” force to explain the
apparent deflection (like “centrifugal force”).
Imagine throwing a ball to a friend on a rotating merry-goround.
throw a ball
it goes “straight”
appears to curve
Here’s a little movie. Here is the external link (for viewing on a
computer other than mine). (Looks to my eyes like friction is causing the
ball to move in a curved path as it slows down.)
Does the Coriolis effect cause your bathwater to rotate
clockwise as it drains? No. Bad coriolis.
More technical “stuff” on the Coriolis effect
(broken link, 2012)
.
back to el Niño
“El Niño is an unusual warming of the tropical Pacific Ocean
that occurs irregularly at about 3-6 year intervals in response
to large scale weakenings of the trade winds that normally
blow westward from South America toward Asia.”2
“normal”
el Niño onset
3http://www.pmel.noaa.gov/tao/elnino/nino-home.html
“El Niño…
La Niña, suggested on the slide before last, is the “opposite” of
el Niño.
el Niño—the warm western Pacific waters move eastward
towards the Americas
la Niña—the waters off the Americas
are cooler than normal
there is debate2 over whether la Niña
represents “normal” conditions, with
“in between” being the buildup
towards el Niño…
or whether the “in between” buildup towards el Niño is
“normal”
The 1997-1998 event.3
developing
“normal”
el Niño
la Niña
Reminds me of water sloshing in a bathtub.
http://www.cdc.noaa.gov/~jjb/anim.html
Click here (on my computer) or on the link below to see the
thermocline animation.
http://www.cdc.noaa.gov/~jjb/enso_anim_jjb2.html (dead link)
http://www.esrl.noaa.gov/psd/people/joseph.barsugli/enso_anim_jjb2.html (2012)
A couple of nitpicky clarifications, to satisfy my conscience…
1. I’m OK if you want to think of the trade winds as pushing
water from East to West. Mental models help our understanding. The reality is more complex.
2. Ocean waves don’t carry water molecules from one end of
the ocean to the other.
One consequence is that waves can travel much faster than
“pieces” of water.
Historical records of el Niño date back to at least 1567.
a sampling of el Niño events
“The name El Niño (referring to the Christ child) was originally given by Peruvian
fisherman to a warm current that appeared each year around Christmas. What we
now call El Niño seemed to them like a stronger event of the same type, and the
usage of the term changed to refer only to the irregular strong events. It wasn't until
the 1960s that it was widely realized that this was not just a local Peruvian
occurrence, but was associated with changes over the entire tropical Pacific and
beyond.”1
In 1904, Sir Gilbert Walker, head of the Indian Meterological Service, began attempts to predict India’s
monsoons, on which the survival of millions depends.
“He recognized some patterns of rainfall in South America and
associated them with changes in ocean temperatures. He also
found a connection between barometer readings at stations on
the eastern and western sides of the Pacific.”4
According to Walker (1923) "when pressure is high in the
Pacific Ocean it tends to be low in the Indian Ocean from
Africa to Australia."
“Changes across the tropical Pacific and beyond are not
isolated phenomena but are connected as part of a larger
oscillation.”1
“When pressure is high in the Pacific Ocean it tends to be low
in the Indian Ocean from Africa to Australia."
Darwin: Australia
Tahiti: closer to
South America
Walker was ridiculed for suggesting that there is a connection
between monsoons in India and mild winters in Canada.4
In the 1960’s, Jacob Bjerknes, at the University
of California, was the first to see a connection
between unusually warm sea-surface
temperatures and the weak easterlies and heavy
rainfall that accompany low pressure in Darwin.
Ultimately, Bjerknes' discovery led to the recognition that the
warm waters of El Niño and the pressure seesaw of Walker's
Southern Oscillation are part of the same phenomenon -sometimes referred to by the acronym ENSO.4
ENSO—“El Niño/Southern Oscillation”
“Oscillation” makes me think of something vibrating back and
forth with periodic motion.
el Niño—what causes it?
This web page says “el Niño is where the normal circulation
patterns of the atmosphere and ocean in the Southern Pacific
break down and may even reverse. La Niña on the other
hand, is an abnormal strengthening of that normal circulation.”
They are the experts, not me, but it seems that the entire el
Niño/la Niña cycle is a normal part of the Earth’s “life.”
Earlier I wrote that el Niño was a response to weakening of
the trade winds.
But why should the trade winds weaken?
“There are more things in heaven and Earth, Horatio, than are
dreamt of in your philosophy.”—Hamlet
It is clear that over time warm water
piles up in the Western Pacific.
This is an unstable situation, waiting for a “trigger” to release
the pent-up water.
The weakening of the trade winds is certainly one trigger...
…but there are other natural phenomena that affect ocean
water.
You are familiar with the jet stream, which is related to the
global circulation patterns I showed earlier.
The jet stream is an example of an atmospheric Rossby wave.
http://geography.sierra.cc.ca.us/booth/Physical/chp5_atm_pressure/rossby/Rossby.htm
There are oceanic Rossby waves in the Pacific.
Click on the picture to see a movie. For those not using my
computer, click here to view the movie from its source web site
(link dead 2012).
Rossby waves, like the trade winds, are caused by the Coriolis
effect.
The “waves” are typically 5
centimeters in height and take
months to years (dead link) to cross
the Pacific from east to west.
(Alternative link:
http://www.noc.soton.ac.uk/JRD/SAT/Rossby/Rossbyintro.html )
Rossby waves reflecting from the continents of the Western
Pacific may have something to do with triggering el Niño…
…but not in all cases (see here
).
dead link
(updated links: http://faculty.washington.edu/kessler/abstracts/jclim95-abstract.html ;
http://faculty.washington.edu/kessler/abstracts/kandmcp-1995-jclimate.pdf )
Volcanoes do not cause
el Niños…
…but there is a correlation
between major tropical
volcanic eruptions and
occurrences of el Niño.
“There was a nearly one-in-two chance of an El Niño event
occurring after a volcanic eruption in the tropical zone,
roughly double the normal probability.”
“It appears that the eruption of a tropical volcano nudges the
climate towards a more El Niño-like state.”—Michael Mann,
University of Virginia.
“Since El Niños occur against the
background existing climate,
there is little doubt that volcanic
eruptions that eject large
amounts of dust into the
stratosphere must modify the
frequency, character and
strength of El Niño events,
possibly in important ways.”
“The distinction I make is between "slowly modify the
background" and "cause" El Niño.”1
There are also oceanic Kelvin
waves that, in contrast to Rossby
waves, move from West to East.
The red band in this picture
indicates above-average sea level
water that took a month to travel
1/3 of the way around the world,
from Australia to South America.
(link dead 2012)
Kelvin waves are not el Niños (a month to travel across the
Pacific, compared to many months for the lifetime of an el
Niño), but may contribute as a “trigger.”
Some of us like to think that if we knew the position and
momentum of every molecule in the atmosphere, and all the
energy sources and sinks, and everything else that enters into
laws of physics…
…and had a powerful enough computer…
…then we could predict future weather with certainty.
prediction 1
It is possible that
weather is chaotic—
that even absolute
knowledge of current
conditions would not
allow us to predict
future weather. (dead link
2012)
Comforting, or frightening?
prediction 1
initial conditions
infinitesimally modified
“El Niños involve the full complexity of ocean-atmosphere
interaction on a global scale.”
“When you consider longer-term phenomena like El Niño… one
must consider how the ocean will evolve under the winds, and
then how the altered ocean will modify the winds, and so on,
in many tricky and sensitive feedback loops.”
In other words, we don’t really know what “causes” el Niño!
If it makes you feel any better, we
do know that sea beavers don’t
cause el Niños.
I’ve made some statements that should wake you up:
“…a tropical volcano nudges the climate towards a more El
Niño-like state…”
“…many tricky and sensitive feedback loops…”
“…initial conditions infinitesimally modified (can cause
dramatic differences in outcomes).”
Do you see why?
el Niño—how does it affect us?













floods, mudslides
drought, crop failure
fires
downpours, tornados





cyclones
coral reefs die
no fish
great fishing
 
 

la Niña—how does it affect us?
“La Niña often features drier than normal conditions in the
Southwest in late summer through the subsequent winter.
Drier than normal conditions also typically occur in the Central
Plains in the fall and in the Southeast in the winter. In
contrast, the Pacific Northwest is more likely to be wetter than
normal in the late fall and early winter with the presence of a
well-established La Niña.
Additionally, on average La Niña winters are warmer than
normal in the Southeast and colder than normal in the
Northwest.”
http://www.elnino.noaa.gov/lanina_new_faq.html
Not all the effects of el Niño are bad.
1997, US:
California suffered more than $1.1 billion in economic
losses
midwest and East Coast natural gas and heating oil
demand reduced by 10 percent—a savings of about $6.7
billion
retail sales in the US about $5.6 billion higher than normal
because of the milder winter
more home construction
fewer transportation delays and less salt used on roads
el Niño—can we change it?
Is that the “right” question to ask?
Should we instead ask “would we want to change it?”
Let’s suppose we want to cause an el Niño. NOAA scientist
Billy Kessler has some calculations for us to consider.
He calculated the energy it
would require to heat this
water by the 1.62˚C average
for el Niño 1997.
You take the surface area of the water (remember, the area
covers the length of the Pacific Ocean) times the depth of the
affected region to calculate the volume of water heated.
V=AD
The mass of water is volume times density:
m =  (A D)
The energy required is the mass times the heat capacity times
the temperature change (you had a similar homework
assignment)
E = [ (A D)] C T
Kessler estimates that 3.5x1022 joules of energy would be
needed.
That’s 3,500,000,000,000,000,000,000 joules.
That’s a ton of joules, isn’t it!
Wrong! It is the
equivalent of 389
tonnes of mass.
3,500,000,000,000,000,000,000 joules.
Kessler calculates that amount of energy would be released by
400,000 twenty-megaton hydrogen bombs.
The bomb that destroyed Hiroshima
was a 10 kiloton atomic bomb.
The energy “tied up” in an el Niño is
“worth” about 800,000,000
Hiroshima bombs.
Kessler correctly points out that el Niño actually involves a
movement of already-heated water, rather than the heating of
the water itself…
…which, give or take a few factors of 10, would require “the
same” energy as the result I just showed…
…which he also shows is equivalent to 1,500,000 huge (1000
MW) power plants operating round-the-clock for 8 months.
Alternatively, you could disrupt el Niño by dissolving a 1 km
square, 100 km high “chunk” of salt in the surface water,
which would cause the warm surface water to cool and sink.
There is a “wild card,” which I will discuss later.
The ability to forecast el Niño in advance would allow us to
take steps to save lives and money.
Actually, we can see that el Niño is coming about 6 months
before major changes take place.
That warning could be used for emergency preparations...
…except we can’t predict in advance the specific weather
events that will cause disasters. And we can’t move thousands
of people away from their homes in anticipation of a disaster
that is unlikely to happen.
el Niño—what’s happening today?
The TAO buoy array (we saw some of the buoys in the video)
covers a vast stretch of the Pacific Ocean centered on the
equator, from north of Australia almost to South America.
TAO: “Tropical Atmosphere-Ocean”
updated for 2012
These buoys provide continual sea
surface temperature (SST) and
wind data.
The figure above shows the most recent 5-day average SST
and winds, as of March 14, 2012. Updates are here.
A figure like this is not easy
to interpret visually.
After all, we know the trade winds blow to the West, and we
know it’s supposed to be hot over the western Pacific.
Also, el Niño and la Niña represent deviations from “average.”
So the TAO buoy data is also depicted in terms of deviations
from average (“anomalies”).
What can you conclude about tropical Pacific SST’s and winds,
as of March 14, 2012?
Close to normal. La Niña is weakening. Here is the latest
expert assessment:
La Niña has weakened across the tropical Pacific Ocean. Equatorial sea surface temperatures
remain at least 0.5°C below average in the central Pacific, but have warmed considerably
across the east-central and eastern Pacific Ocean over the last month. Atmospheric circulation
anomalies remain largely consistent with La Niña. La Niña is expected to transition to ENSOneutral conditions by the end of April 2012.
U. S. Seasonal Outlooks
March – May 2012
Temperature
Precipitation
The seasonal outlooks combine the effects of long-term
trends, soil moisture, and, when appropriate, ENSO.
el Niño—past and future
To predict the future, understand the past.
El Niño is a mechanism for moving excess heat from the
tropics to other parts of the Earth.
It is reasonable to ask whether el Niños occurred during the
ice ages.
El Niño-like climate patterns during the most recent ice age
have been found in New England (bad link).
Traces of el Niño-like climate patterns during the time since
the last glacial maximum have been found in the tropical
Pacific.
Analysis of Pacific coral reefs suggests
el Niños were occurring long before the
last ice age, but were somewhat weaker
during ice ages.
“Super” el Niños and ice age cycles?
Could el Niño have triggered the end of the last ice age?
Evidently, heat transfer mechanisms such as el Niño have
influenced Earth’s climate for as long as we can discern.
Let’s look at some of the more recent el Niños.
1982-1983. The el Niño that “the rest of the world” finally
noticed.
Four el Niños between 1982 and 1997.
1997, the last “big one.”
Comparison of 19821983 and 1996-1997.
I first saw this while I
was teaching Physics
6, Winter 1997. The
animations cut off at
about the date I
downloaded them.
Remember, the 19821983 event was huge
and deadly.
Guess what I said the
first time I saw this.
There are a number of different ways of measuring el Niño
“strength.” Most take some kind of weighted average of
anomalies in a number of measurable parameters.
Here’s a representation of the SOI (Southern Oscillation Index)
for the past 127 years:
Most web pages show el Niño going “up,” not “down” as in this
graphic. Of course, this is from an Australian web page.
Here’s a representation of the SOI (Southern Oscillation Index)
since 1950:
Do you see any trends?
Here’s a representation
of the sea surface
temperature anomalies
since about 1950:
Example of an el Niño forecast:
Remember the butterfly!
On the topic of future el Niños and global warming, the best I
can do is summarize information (8-9 years old, but still good)
from Kessler:
“Some say that the string of warm events during the 1990s are
evidence that a general warming trend is starting to change
the weather; others say that these variations are within normal
limits.”
“We won't have good statistics about El Niño for another
hundred years or so, perhaps even longer if it is truly chaotic.”
“One way that a general global warming could affect El Niño is
through the sensitivity of tropical deep convection (organized
thunderstorms involving strong upward motion over large
regions) to the water temperature beneath.”
“At around 27.5°C (81.5°F) [convection] suddenly becomes
much more efficient.”
“Since there are large regions of water with temperature close
to 27.5°C (either slightly warmer or slightly cooler), even a
small general temperature change could produce large
changes in the area in which convection takes place.”
“A mere one-half degree uniform increase could therefore
produce a significant change in the amount of tropical
convection, perhaps making the normal state appear more
like El Niño.”
“Tropical convection thus has the potential to amplify the
disruption of global climate, perhaps far out of proportion to a
seemingly small temperature change.”
“So, while the values of temperature rise proposed in the
global warming debate appear small, in the sense that in daily
life a 1-2 degree change hardly matters, the danger lies in the
reorganization of the large nonlinear systems that regulate the
entire weather machine.”
“There may well be feedbacks that would damp out some of
these influences on the tropical convection process (or possibly
amplify them even more). I don't know and I don't think
anyone does.”
“No one should argue that because the overall values of
temperature change seem small, or that a computer model of
the effects of greenhouse warming may be wrong by a
significant percentage, the effect on human society could not
be drastic.”
This web page is a good source of information about recent
Pacific Ocean changes (link dead 2012).
Something else interesting: the waters of the Pacific undergo
even longer-term oscillations, called the Pacific Decadal
Oscillation, than those associated with el Niño.
Over a 40 to 60 years period, this
horseshoe-shaped region of water
oscillates between warmer than
normal and cooler than normal.
It appears we are moving towards a time when in the US it is
cooler and wetter in the north and drier in the south (dead link).
In fact, there are a number of ocean-atmosphere cycles we
are just beginning to recognize. See this web page.
Reminders to me for next time I revise these lectures: Walker
cells and convection. More on PDO, Pacific Decadal Oscillation.
el Niño FAQ’s
http://www.bom.gov.au/lam/climate/levelthree/analclim/elnino.htm
http://www.usatoday.com/weather/resources/askjack/wfaqnino.htm
3http://www.pmel.noaa.gov/tao/elnino/nino-home.html
2http://www.aoml.noaa.gov/general/enso_faq/
1http://www.pmel.noaa.gov/~kessler/occasionally-asked-questions.html
http://gcmd.gsfc.nasa.gov/Resources/FAQs/elnino.html
4http://www.enn.com/specialreports/elnino/history.asp
http://www.elnino.noaa.gov/
http://www.pmel.noaa.gov/tao/elnino/faq.html
http://sealevel.jpl.nasa.gov/elnino/index.html
http://www.cdc.noaa.gov/ENSO/
http://www.vision.net.au/~daly/elnino.htm
Rossby waves
http://www.oce.orst.edu/po/research/rossby_waves/chelton.html
http://www.bangor.ac.uk/~oss110/oxp2003/lecture9.pdf
http://www.pmel.noaa.gov/~kessler/jclim95-abstract.html
Kelvin waves
http://sealevel.jpl.nasa.gov/elnino/970425.html
http://topex-www.jpl.nasa.gov/aviso/text/general/discover/results/kel_defn.htm