14 El Nino Other Oscillations(1)

8/10/2019 14 El Nino Other Oscillations(1)

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El Nio(and other natural climate oscillations)


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1. Historical Perspective Leading to our Current Understanding

El Nio

Southern Oscillation

ENSO = El Nio Southern Oscillation

2. Direct and Indirect Impacts of El Nio on Global WeatherPatterns

3. Other types of natural climate oscillation

Outline:


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History of the Term El NioThe original definition of El Nio goes back to 18th or 19th century when Peruviansailors coined the term to describe a warm southward current that appeared annuallynear Christmas time off the Peruvian coast. Hence the name El Nio = Spanish forThe Child = "Christ Child".

The warm, southward currentoccasionally (every 3 to 7 years)seemed much more intense thanusual and eventually the term El

Nio came to refer primarily to theseoccasional extreme events


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El Nino

The Roughly Periodic (3-7 years) Occurrence of Prolonged(ca. 8 months) Warming of Coastal Waters off of Peru andEcuador


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In the 1920s Sir Gilbert Walker Analyzed Atmospheric Pressure DataMeasured at Darwin Australia and Tahiti and Noticed Some RemarkableCorrelations in the Two Data Sets


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Why Are The Two WeatherStations Well Correlated?


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Recall the Idealized Versus Actual Pattern of Global Sea Level PressureAnd Resulting Surface Winds

1. Differences due to unequal heating of land and ocean by the sun

2. Land heats more strongly than adjacent ocean in summer and cools more thanadjacent ocean in winter - due to differences in specific heat of rock and seawater.


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Recall the Idealized Hadley Circulation

1. Poleward-directedair aloftis deflectedto the right (northern hemisphere) --

which is toward the east2. Equatorward-directedair at surfaceis deflectedto the right(northern

hemisphere) -- which is toward thewest- to create the Trade Winds


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1. Winds at the ocean surfacemovetoward the equator,

but are deflected by Coriolistoward the (right) westtoform surface trade winds.

2. Winds aloftmove awayfrom the equator, but aredeflected by Coriolistoward

the (right) eastto completethe Walker-Circulation

Walker-Circulation

East-West Atmospheric Circulation Cell


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Coherent Change (PeriodicReversal) in the East-West Circulation CellOver the Pacific (Walker Cell) Resulting in Coherent Changes in

Atmospheric Pressure Patterns, Precipitation Patterns and Wind Direction



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UnderNormalconditions in the Pacific the Trade Windsblow from east to west and in the process they do twothings...

1. The winds transport and pile

up warm surface water on thewestern side of the Pacific tocreate a thick (200 meters)

pool of very warm oceanwater.

2. The piled up water (warmpool) tilts the thermoclinedeeper in the west andshallower in the east.


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The Coherent Oscillation (i.e.,the Periodic Reversal)of theWalker Circulation Cell

El Nino ConditionsNormal Conditions

The Southern Oscillation

Low SLP High SLPHigh SLP High SLP


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El Nino ConditionsNormal ConditionsLa Nina Conditions

La NinaExceptionally StrongTrade Winds

Note:There is a third state to the Southern Oscillation not shown in thisfigure associated with enhanced strength in the Trade Winds

NormalTrade Winds

ReversedTrade Winds

Exceptionally StrongTrade Winds

The Coherent Oscillation (i.e.,the Periodic Reversal)of the WalkerCirculation Cell

The Southern Oscillation


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Southern OscillationIndex(SOI):1. A simple index (i.e., a single number) used to describe the severity of

El Nino conditions - i.e., the strength of the reversal in the Walker

Circulation

2. Traditionally defined as the normalized difference in atmospheric

pressure measured at Darwin Australia and Tahiti, but moreelaborate and robust metrics also exist.


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Southern Oscillation Index

Red = El Nino ConditionsBlue = La Nina Conditions


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El Nino Southern Oscillation (ENSO)

Coupled Ocean-Atmospheric Interactions

1. Roughly periodic (3-7 years) occurrence of prolonged (ca. 8 months) warming

of coastal waters off of Peru and Ecuador

2. Coherent change in the east-west atmospheric circulation cell over the Pacific

(Walker Cell) resulting in coherent changes in atmospheric pressure patterns,

precipitation patterns and wind direction

3. And Much Much More

In 1969Jacob Bjerknes proposed that there was a physical connection between the

oceanographic and atmospheric variations and now the oceanic (El Nino) and the

atmospheric (Southern Oscillation) aspects are combined in the single term 'El

Nio Southern Oscillation' (ENSO) that encompasses both the ocean and theatmosphere


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La NinaExceptionally Strong Trade

Winds, Equatorial Upwelling andCooling in the Eastern Pacific

El Nino

Exceptionally Weak or ReversedTrade Winds, Little or NoEquatorial Upwelling and Strong

Warming in the Eastern Pacific

NormalNormal Trade Winds, Upwellingand Cooling in the Eastern Pacific

Sea Surface Temperature Patterns During La Nina, El Nino and

Normal Conditions in the Pacific


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Warming of the surface water off of Peru during an El Nino Event is part of a

much greater warming event covering large parts of the eastern tropical Pacificand reaching down to the thermocline

El Nino ConditionsNormal Conditions


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Temperature Field in the Pacific Prior to El Nino(Normal Conditions)


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Temperature Field in the Pacific During the InitialProgression of El Nino


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Temperature Field in the Pacific During the Recoveryfrom El Nino


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El Ninos Progression March:Westerly Wind Burst

north of Australia

April:Kelvin Wave ReachesSouth America

May:Water Piles up off of SouthAmerica

June:High Sea-Level SpreadsNorth and South


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QuestionWhat should happen to global-averagetemperature when the thick warm pool of waterin the western Pacific spreads out across a largearea of the equatorial Pacific during an El Nino

period?

a) Global Temperature Should: Go Up

b) Global Temperature Should: Go Down

c) Global Temperature Should: Stay About the Same

The Correct Answer is (a) temperatures willgo upas thearea of warm water in contact with the atmosphere increasesand allows the ocean to give up some of its stored heat to theatmosphere


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Global Temperature Anomalies Since the Industrial Era

1880 1900 1920 1940 1960 1980 2000!

.4

!.2

0.

.2

.4

.6

12-month Running Mean (C)

Land!Ocean Temperature Index

Temperatures are Relative to Base Period 1950-1980

1997-1998El Nino

1999-2000

La Nina


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Environmental Change LinkedDirectly and Indirectly to El Nio


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1. Relative Cooling in the Western Tropical Pacific Creates Drought Conditionsin Australia/IndonesiaDuring an El Nino

2. Relative Warming in the Central and Eastern Tropical Pacific Heat Energy forFormation of Intense Precipitation and Storms

El Ninos DirectImpact on Pacific Precipitation and

Storm Patterns


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Draught and Fires in Indonesia


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Pacific Hurricanes Increase During an

El Nino, but Atlantic Hurricanes Decrease


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Flooding in the Western United States


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El Ninos DirectEffect on Equatorial Productivity

El Nino ConditionsNormal ConditionsHigh Equatorial Productivity Low Equatorial Productivity


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El Nios DirectEffect on Biological Productivity off of

Peru and Ecuador

NormalConditions

El NioConditions


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Environmental Changes Linked Indirectlyto El NioTeleconnections


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El Nino Related Changes in Global Climate


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The answer to how El Nino can influence weather patterns around to

globe lies in what El Nino can do the the Jet Streams that circle the globe


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Changes in the Jet Stream During El Nino and La Nina


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Prevailing Wind Direction at a Given Location is Strongly Influenced by the

Position of the Jet Stream


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Jet streams have wave-like properties and just like a disturbance in water causes a wave to

propagate outward, so too does a regional disturbance in the jet stream position propagate inwave-like fashion around the globe. ENSO produces such a disturbance that can thenpropagate around the world.


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More Info About El Ninohttp://www.pmel.noaa.gov/tao/elnino/nino-home.html


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Other Natural Climate Variations


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Other Natural Climate Variations

1. El Nino Southern Oscillation (ENSO)

2. North Atlantic Oscillation (NAO)

3. Atlantic Multi-Decadal Oscillation (AMO)

4. Pacific Decadal Oscillation (PDO)

5. Arctic Oscillation (AO)


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Positive NAOStrong pressure difference betweenIceland and Azores

Stronger Westerly Winds LocatedMore Northward

Negative NAOWeak pressure difference betweenIceland and Azores

Weaker Westerly WindsLocated More Southward

North Atlantic Oscillation (NAO)


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North Atlantic Oscillation Index


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Pacific Decadal Oscillation (PDO)

Positive Phase:

North Pacific cooler than normal

Equatorial Pacific warmer thannormal

Negative Phase:North Pacific warmer than normal

Equatorial Pacific cooler than normal


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Pacific Decadal Oscillation (PDO) Index


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Atlantic Multi-Decadal Oscillation


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Atlantic Multi-Decadal Oscillation Derived fromGlobal SST Data

Time Variation of AMO

similar colors rise and fall in unison fromyear to year or decade to decade.

purple and red colors are out of phase - yearsor decades of high temperatures for redmeans low temperatures for purple regionsfor the same year or decade

Spatial Coherence of AMO


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Atlantic Multi-Decadal (AMO)Index Derived by Removing the

Multi-Decadal Global SST Linear Trend


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The Atlantic Multi-Decadal Oscillation Causes Warming and Cooling of

Sea Surface Temperatures that, in turn, Cause Increases and Decreases inAtlantic Hurricanes

Hurricane Tracks1985-1994

Hurricane Tracks1995-2004


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Atlantic Multi-Decadal (AMO)Derived by Keeping the Multi-Decadal Global SST Linear Trend


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With the multi-decadal trend included it becomes more doubtful that

hurricane activity will ever decline in subsequent decades


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Summary

1. Besides El Nino Southern Oscillation (ENSO) there are a large number of

other natural climate oscillations that change phase (sea surface temperaturepattern and wind velocity pattern) on the order of decades

2. When examining the issue of global warming, one needs to take into accountnatural climate variability that can occur on the order of decades!

3. Global Warming is a Multi-Decadal Trend with natural decadal

oscillations superimposed on that trend!


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Global Temperature Anomalies Since the Industrial Era

1880 1900 1920 1940 1960 1980 2000!

.4

!.2

0.

.2

.4

.6

12-month Running Mean (C)

Land!Ocean Temperature Index

Temperatures are Relative to Base Period 1950-1980

1997-1998El Nino

1999-2000La Nina

Documents

14 El Nino Other Oscillations(1)