Meteorological Aspects of the El NioSouthern Oscillation Eugene M' Rasmusson and John M' Wallace

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Meteorological Aspects of the El
Niño/Southern Oscillation Eugene M. Rasmusson
and John M. Wallace

• Presented by Scott Melaragno
• Geog 622.01
• 2/11/09

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Introduction to El Niño/Southern Oscillation

• Regional climate anomalies such as droughts,
  record cold winters, and unusual numbers of storm
  are seen every year around the world.
• Some of these events can be very coherent over
  much of the globe.
• the recognition process has been somewhat like
  the assembly of a global-scale jigsaw puzzle
• How does this affect ENSO?

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What is ENSO?
El Niño

• Southern Oscillation
• A coherent pattern of pressure, temperature, and
  rainfall fluctuations discovered and named by Sir
  Gilbert Walker more than a half-century ago.
• It is the seesaw in atmospheric pressure at sea
  level between the SE Pacific subtropical high and
  the region of low pressure stretching across the
  Indian Ocean from Africa to northern Australia
• Also involves surface temperatures throughout the
  tropics and monsoon rainfall in S. Africa, India,
  Indonesia, and N. Australia
• Was a challenged theory.

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The Birth of the ENSO theory

• Descriptive studies of the 1957-1958 El Niño
  event, captured by merchant ship data from the
  tropical Pacific, were instrumental in revealing
  this link.
• The large-scale interaction between atmosphere
  and ocean was confirmed by statistical studies of
  past episodes.
• Vilhelm Bjerknes's investigations of the
  1957-1958, 1963-1964, and 1965-1966 ENSO
  episodes.
• He was among the first to use satellite imagery
  to define the different anomalously wet and dry
  regions and related this to episodes of warm sea
  surface temperatures (SST).
• Bjerknes showed that these fluctuations in SST
  and rainfall are associated with large-scale
  variations in the equatorial trade wind systems,
  which in turn reflect the major variations of the
  Southern Oscillation pressure pattern.

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What are the Identified Relationships?

• Since the studies of Bjerknes, ENSO can be
  considered the dominant global climate signal on
  time scales of a few months to a few years
  (7-10).
• Note the Different relationships seen in Figure
  1. (Especially A-C).
• The individual episodes range from 2-10 years.
• Does this explain everything about ENSO events
  and their impacts?

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What Causes These Events?

• The mechanisms through which SST anomalies in the
  equatorial Pacific might produce the worldwide
  climate anomalies have been a subject of intense
  debate.
• the primary way in which the tropical atmosphere
  responds to temperature contrasts at the earth's
  surface is through thermally direct circulation
• We can now use the Boundary Layer physics that we
  have learned to explain this phenomenon.

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Explanations using Boundary Layer Mechanics

• Warmest regions at the surface ascent of
  moisture-laden air from planetary boundary layer.
• This causes widespread cloudiness. (Roll Clouds,
  Open Cell clouds, Mikes lecture..)
• Elsewhere, subsidence of dry air from upper
  troposphere form a lid on the Boundary Layer.
• This prevents trade wind cumulous from growing to
  a size that can produce substantial rainfall.
• Over the equatorial Pacific there is usually a
  strong SST gradient, with relatively cool waters
  in the east and warmer (by 3 to 6 K) waters in
  the west.

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What does these Mechanics Produce?
This east-west gradient is associated with a
thermally direct circulation in the equatorial
plane

• Sinking motion in the eastern Pacific.
• Westward low-level flow (the equatorial extension
  of the trade winds)
• Rising motion and deep cumulus convection over
  the extreme western Pacific and Indonesia,
• Eastward return flow at the cirrus cloud level
  (10 to 15 km)
• Bjerknes named this Walker Circulation.

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ENSO Event
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Purpose of this Paper

• Rasmusson and Wallace want to expand the limited
  knowledge of ENSO events (limited at the time of
  1982-83).
• How ENSO may affect areas other than the Pacific
  Ocean.
• The Global Climate Models have a temporal problem
  and forecasting these events and authors want to
  better predict these events.
• Basically to analyze and provide better
  conclusion to the 1982-83 El Niño event, which
  was the strongest up to that point.

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Data and Methods
Data was compiled from the most recent ENSO
episode which was 1982-83

• In order to make accurate comparisons, the
  authors also compiled climatological data from
  six episodes between 1950-1975.
• They then compared this data to the 1940-41
  event, which was the strongest ENSO event before
  1982.
• Using this data and the current 1982-83 data, the
  authors could then compare the temporal and
  atmospheric dynamics

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Methods

• Rasmusson and Wallace give a blow by blow account
  of the 1982-83 ENSO episode to present the data
  that they encountered.
• Again because this was a very unique event and it
  impacts were felt worldwide, the detailed report
  of the event was critical in this analysis.
• Throughout this ENSO episode, Rasmusson and
  Wallace compare to both the composite and 40-41
  events.
• This is critical to the conclusions of this
  paper.

• Steps of the 82-83 Episode
• Delayed onset of the event (November/82)
• A swing in the Southern Oscillation.
  (April-August/82)
• A sharp rise in SST. (September and October/82)
• Peak of the 1982-1983 Episode. (December/82-May/83
  )
• Return to Normal Conditions (July-September/83).
• Discussion of Results

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Results Delayed Onset of the Event
(November/82)

• Scientists were caught off guard by the unusual
  timing of the onset of the 1982- 1983 episode
  relative to the climatological mean annual cycle.
  
• Normal ENSO events start occur early in the year
  after SST anomalies approach above 2 K.
• The winds weakened abruptly around November,
  coincident with a swing in the Southern
  Oscillation.
• The resulting change in surface wind stress along
  the equator was believed to be responsible for
  the onset of warm SST

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A swing in the Southern Oscillation
(April-August/82)

• Meteorologists refer to these pressure changes as
  a swing of the Southern Oscillation.
• This swing was very severe for the 1982-83 event.
  How?
• The Oscillation Index (SLP Darwin
  SLP Tahiti)
• April-August 1982 the sea-level pressure
  difference between Tahiti and Darwin dropped from
  near normal to about 3 mbar below normal.
• This produced the begins of the 82/83 event.

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A sharp rise in SST. (September and October/82)

• Dramatic rises in SST and sea level along the
  South American coast and throughout the eastern
  equatorial Pacific .
• Abrupt shift in low-level winds west of the date
  line from easterly to westerly and the
  corresponding east-west seesaw in sea-level
  pressure.
• The intertropical convergence zone (ITCZ), at
  this time of year, normally lies near 10N,
  shifted southward toward the equator.

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Peak of the 1982-1983 Episode.
(December/82-May/83)

• Further intensification and extension of the
  anomaly patterns of the previous months cause
  droughts in southern Africa, southern India and
  Sri Lanka, and the Australian-Indonesian region
  continued.
• The anomalies for 1982 to 1983 were so large that
  it was nearly as warm along the South American
  coast as in the western Pacific, and the warmest
  SST's in the equatorial belt crossed the date
  line and extended considerably farther to the east

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Important Factors of this Peak

• This gradual eastward shift of the rain area,
  surface westerly wind anomalies, and maximum SST
  is a striking characteristic of the time history
  of this episode.
• The important U.S. anomalies observed during this
  period were those associated with westerly wind
  anomalies extending across the subtropical
  Pacific, the southern border of the United
  States, and the Gulf of Mexico.
• Unusual number of winter storms in California.
  Flooding to parts of California and the Gulf
  states. SW Mountains battered with snow.
• Does not mean a direct relationship

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Return to Normal Conditions (July-September/83)

• Decay phase again was preceded by a dramatic
  swing in the Southern Oscillation.
• Also the delayed effect with relation of SST with
  a slight rise in SST during summer of 1983.
• This may have been a response to the second sharp
  downward swing of the SO which happened from
  Feb.-May/83
• Finally by September/83, SST shifted back to
  normal with winds following suit.
• Now that the episode was over, Rasmussen and
  Wallace began to make their conclusions.
• Many climatologic records were broken during this
  event. (From Peru to Australia)

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Conclusions

• An enhancement of the mean annual cycle in SST,
  largely confined to the eastern third of the
  Pacific, with maximum amplitude near the South
  American coast. (Oceanographer
  Viewpoint)
• A broader scale warming of the equatorial Pacific
  from near the date line eastward, with maximum
  amplitude from 90-150W, starting around the
  middle of the calendar year, peaking near the end
  of the year, and disappearing a few months later.
  (Meteorological Viewpoint).

• The 1982- 1983 ENSO episode invites speculation
  as to whether all such episodes might be viewed
  as a superposition of two, interrelated kinds of
  events, which have much in common and usually
  occur in combination about 6 months apart (El
  Niño/SO).
• Leads to several conclusions made by the Authors

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Chicken and the Egg
These type of questions concerning the nature of
atmosphere-ocean interactions in the tropics.

• What are the processes responsible for the
  pronounced and unexpected swings of the Southern
  Oscillation during this and other ENSO episodes?
• Why did the heaviest rainfall, the surface
  westerlies, and the warmest SST's gradually
  migrate eastward across the equatorial Pacific
  during the episode?
• What is the role of the annual cycle in the
  initiation, evolution, and decay of this and
  other episodes?

• Relationship Questions
• Were the unusually strong westerly flow and
  storminess over the southwestern United States
  and the unusual warmth over Eurasia related to
  the ENSO episode or did they occur for other
  reasons?
• Is it possible to distinguish between the 82-83
  episode, which is characterized by mild
  conditions over much of the N. U.S. , and those
  where the East was significantly colder than
  normal.

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Final Thoughts

• It is obvious where future directions of this
  paper were taken (Written in 1983).
• But considering the time this research was done,
  very thorough analysis.
• Rasmussen and Wallace introduced several new
  ideas about the relationships between El Niño and
  Southern Oscillation events.
• Papers since have further elaborated (Following
  Presentations) Especially with 1997-1998 episode.