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El Niño - PowerPoint PPT Presentation


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El Niño. What is El Niño?. Warming of sea surface waters in the central and eastern tropical Pacific Ocean. El Niño: the ocean part: Warm phase of ENSO: El Niño - Southern Oscillation Southern Oscillation : the atmospheric part; a global wave pattern

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what is el ni o
What is El Niño?
  • Warming of sea surface waters in the central

and eastern tropical Pacific Ocean.

  • El Niño: the ocean part: Warm phase of ENSO:

El Niño - Southern Oscillation

  • Southern Oscillation: the atmospheric part;

a global wave pattern

  • La Niña: is the cold phase of ENSO:

Cool sea temperatures in tropical Pacific

  • EN events occur about every 3-7 years
peak el ni o occurs about christmas season
Peak El Niño occurs about Christmas season ...
  • “Christ Child”
  • “The Boy”
  • “The Little One”
  • Cf: La Niña
  • “The Girl”
slide5

ENSO

A natural mode of the coupled ocean-atmosphere system

ENSO: EN and SO together:

Refers to whole cycle of warming and cooling.

ENSO events have been going on for centuries (records in corals, and in ice layers in glaciers in South America)

ENSO arises from air-sea interactions in the tropical Pacific

slide11

Excerpt: RIME OF THE ANCIENT MARINER

Samuel Taylor Coleridge

The fair breeze blew, the white foam flew,

The furrow followed free:(The trades)

We were the first that ever burst

Into that silent sea

Down dropt the breeze, the sails dropped down,

Twas sad as sad could be;

And we did speak only to break (The doldrums)

The silence of the sea!

All in a hot and copper sky

The bloody Sun, at noon,(Vicinity of the ITCZ)

Right up above the mast did stand,

No bigger than the moon.

Day after day, day after day,

We stuck, nor breath nor motion:

As idle as a painted ship

Upon a painted ocean.

slide12

SSTsandSSTanomalies

  • There is a close link between tropical SSTs and precipitation
  • Not simple: involves total SST and its gradients
  • Winds blow down pressure gradient toward warmest waters
  • Convergence over/near warmer waters
  • Subsidence in cooler regions
  • For the most part:
  • High SSTs determine where the action is!
  • Low SSTs determine where it isn't!
slide13

ENSO Impacts

Occur around the world

Droughts (e.g. Australia, Africa, Brazil, Indonesia)

Floods (e.g. Peru, southern USA)

slide19

0

200

400

0

200

400

Jan

July

April

Oct

Kessler subsurface ocean temperature climatology used for TAO array

slide20

Subsurface ocean temperatures

along equator from TAO array

Total (top) and anomaly (bottom)

Development of 1997-98

El Niño

slide21

lag

-8

-4

0

4

8

Evolution of ENSO

Ocean heat content

1980-1998

Trenberth et al. 2002

so enso is predictable
So ENSO is predictable
  • We can track the subsurface temperatures and their evolution
  • We can model these and make predictions for up to about a year ahead.
  • Statistical methods work quite well
  • But can not deal with the different flavors of El Niño
slide23

Processes involved in ENSO

Different flavors of El Niño

Links with global warming

slide24

A major source

of climate

variability from

year to year

is from the

El Niño

phenomenon

slide25

El Niño theories

  • El Niño: Start of, lifetime, and time between events
  • Previous El Niño sows the seeds for the next
  • Leads to quasi-cyclic behavior
  • El Niños occur every 3 to 7 years or so.
  • Bit like a pendulum
  • El Niños occur 31% of time, La Niñas 23%
  • The ocean is “primed” for an event to occur
  • The trigger may be random “weather”
  • “Delayed Oscillator”
  • Ocean waves are a key:
  • Rossby (off equatorial, westwards traveling)
  • Kelvin (equatorial, eastwards traveling)

Waves reflect off boundaries and return altered

slide26

Rainfall in thunderstorms and organized convection gives rise to latent heat release in the atmosphere, which determines atmospheric heating patterns.

The heating patterns set up low level convergence and upper level divergence that drives Rossby waves in the atmosphere.

These propagate to other regions (teleconnections) and determine the atmospheric circulation locally.

slide27

Increased SSTs

Enhanced convection

Upper troposphere:

outflow, divergence

Anticyclonic pair

Wave-train

Storm tracks:

southward, enhanced

northward, diminished

Trenberth et al. (1998)

slide29

Star Tribune Aug 2 1993

Minneapolis, MN

FLOODS

Upper Mississippi

Basin

Source: Trenberth

slide30

Different flavors of El Niño

Events come in different “flavors”

El Niño refers to a warming of the tropical Pacific but this does

not take account of surrounding areas and character of event.

Tropical winds and rainfall

respond to total SSTs and

so depend on time of year

and details. Surface wind

convergence is in vicinity

of warmest water, not

anomaly.

Small changes in SST can

change region of warmest

water by 1000 km or more!

slide35

Sources of variability in ENSO

  • There is evidence of changes in ENSO about 5000 years ago from paleo evidence: that ENSO ceased
  • Model studies also show this
  • Implies that small forcings are able to cause large alterations in the behavior of ENSO (nonlinear)
  • Changes can be abrupt
  • Also the 1976 shift
  • Timmermann et al. (1999) suggest that greenhouse warming will induce greater ENSO activity marked by more large amplitude cold (La Nina) events and the possibility of abrupt regime shifts.
  • The forced response is a shift in the probabilities of states preferred.
  • Character of change is model dependent
slide36

Quelccaya Ice Cap

Peru

Annual layers of ice reveal El Niños

Courtesy

Lonnie Thompson

slide37

Records of ENSO in the mid-Holocene

  • Modern drought-adapted pollen taxa not present before 5000 years ago; ENSO variability much weakened or absent (McGlone et al. 1992; Shulmeister and Lees 1995).
  • GBR coral indicates interannual hydrologic variability nearly absent compared to modern, background conditions warm; ENSO variability absent (Gagan et al. 1997).
  • New Guinea coral indicates interannual variability significantly damped; ENSO variability weak (Tudhope et al. 2001).
  • Flood deposits in lake occur less frequently before about 5000 years ago; strong El Niño events more rare (Rodbell et al. 1999).
  • Presence of warm-loving fauna suggests permanent warm state of eastern Pacific SST. Persistent El Niño; cool La Niña’s absent (Sandweiss et al. 1996). Summary from Cole 2001
slide38

El Niño and Global warming

1998 warmest year on record, 2001 second warmest

El Niño contributes to global warming

There is a pattern of more and bigger El Niños in past 20 years

El Niño years

La Niña years

Is global warming contributing to changes in El Niño?

Likely, yes, to some extent.

Which part is natural variability?

slide39

Global warming

Heating 

Temperature  & Evaporation 

water holding capacity 

atmospheric moisture 

 

greenhouse effect  & rain intensity 

Floods & Droughts

slide41

There is a mini global warming with El Niño:

0.24°C peak in 1998, 0.17°C for year

Trenberth et al. 2002

slide42

lag

-8

-4

0

4

8

mo

Evolution of ENSO:

surface temperatures

1950-78 left

1979-1998 right

Shows how and where warming is manifested.

Trenberth et al. 2002

slide43

How will El Niño events change with global warming?

  • El Niño involves a build up and depletion of heat as well as major redistribution of heat in the ocean and the atmosphere during the course of events.
  • Because GHGs trap heat, they interfere.
  • Possibly expand the Pacific Warm Pool.
  • Enhance rate of recharge of heat losses.
  • More warming at surface: enhanced thermocline  enhanced swings
  • More frequent El Niños?
  • Some models more El Niño-like with increased GHGs.
  • But models do not simulate El Niño well
  • Nor do they agree
  • The hydrological cycle is expected to speed up with increased GHGs. Increased evaporation enhances the moisture content of the atmosphere which makes more moisture available for rainfall. ENSO-related droughts are apt to be more severe and last longer, while floods are likely to be enhanced.