climate variability ii (dl)

8/3/2019 Climate Variability II (DL)

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Climate Variability IIClimate Variability IIMechanisms and Influences on Synoptic WeatherMechanisms and Influences on Synoptic Weather

MET 171BMET 171B

SJSUSJSU

Lecture TopicsLecture Topics

IntraIntra--seasonal modes of climate variabilityseasonal modes of climate variability

Madden Julian OscillationMadden Julian Oscillation Pacific North American OscillationPacific North American Oscillation

Annular Modes (NAO, AO)Annular Modes (NAO, AO)

InterInter--annual modes of climate variabilityannual modes of climate variability

ElEl--Nino Southern Oscillation (ENSO)Nino Southern Oscillation (ENSO)

InterInter--decadal modes of climate variabilitydecadal modes of climate variability

Pacific Decadal OscillationPacific Decadal Oscillation

The WeatherThe Weather--Climate GapClimate Gap

Fundamental Question:Fundamental Question: Is weather predictable at lead times longer thanIs weather predictable at lead times longer than

those of deterministic numerical weather forecasts?those of deterministic numerical weather forecasts?

Need added predictive skill in the weatherNeed added predictive skill in the weather--climate gap.climate gap.

Predictability

synoptic 10 - 30 days seasonalWEATHER INTRASEASONAL CLIMATE

This Time: El Nino

Lecture OutlineLecture Outline

ElEl--Nino Southern Oscillation (ENSO)Nino Southern Oscillation (ENSO)

Definitions/CharacteristicsDefinitions/Characteristics

Dynamical MechanismsDynamical Mechanisms

TeleconnectionsTeleconnections -- Influence on Synoptic WeatherInfluence on Synoptic Weather

Pacific Decadal Oscillation (PDO)Pacific Decadal Oscillation (PDO)

El NinoEl Nino--Southern OscillationSouthern OscillationBig Gorilla: most influential mode of interannual climate variabBig Gorilla: most influential mode of interannual climate variab ilityility

El NiEl NiooAn anomalous warming in eastern Pacific ocean temperaturesAn anomalous warming in eastern Pacific ocean temperatures(warm water(warm water movesmoveseast)east)

La NiLa NiaaAn anomalous cooling in eastern and central Pacific oceanAn anomalous cooling in eastern and central Pacific oceantemperatures (warm watertemperatures (warm water staysstayswest)west)

Southern OscillationSouthern OscillationPressure fluctuations centers of action in the westernPressure fluctuations centers of action in the westernPacific/eastern Indian Oceans and the southeastern PacificPacific/eastern Indian Oceans and the southeastern Pacific

1997, El Nino year (warm)1997, El Nino year (warm)

1998, La Nina year (cold)1998, La Nina year (cold)

Ocean surface temperatures

El Nino/La Nina refers to Ocean, Southern Oscillation to Atmosphere

La Nada: TheLa Nada: The NormalNormal PacificPacific


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Characteristics of El NiCharacteristics of El Nioo

Anomalous low (high) pressure in the eastern (western) PacificAnomalous low (high) pressure in the eastern (western) Pacific

Weak or even reversed trade winds across PacificWeak or even reversed trade winds across Pacific Dry (Wet) conditions in the west (east) PacificDry (Wet) conditions in the west (east) Pacific

Deep thermocline in the eastDeep thermocline in the eastupwelling cappedupwelling capped

Characteristics of La NiCharacteristics of La Niaa

Anomalous low (high) pressure in the western (eastern) PacificAnomalous low (high) pressure in the western (eastern) Pacific

Stronger than normal trade winds across PacificStronger than normal trade winds across Pacific

Dry (Wet) conditions in the east (west) PacificDry (Wet) conditions in the east (west) Pacific

Deep thermocline in the westDeep thermocline in the westshallow in the east (cool watershallow in the east (cool water upwellsupwells))

Mean SLPMean SLP

Southern Oscillation Index (SOI)Southern Oscillation Index (SOI)Tracks seeTracks see--saw in pressure between eastern Pacific/Indiansaw in pressure between e astern Pacific/Indian

Ocean and central PacificOcean and central Pacific

Uses pressure observations from Tahiti and Darwin,Uses pressure observations from Tahiti and Darwin,

AustraliaAustralia

Determining Phase of the ENSODetermining Phase of the ENSO

Southern Oscillation Index (SOI)Southern Oscillation Index (SOI) Tracks seeTracks see--saw in pressure between eastern Pacific/Indiansaw in pressure between eastern Pacific/Indian

Ocean and central PacificOcean and central Pacific

Uses pressure observations from Tahiti and Darwin, AustraliaUses pressure observations from Tahiti and Darwin, Australia

)(diff

diffavdiff

PSD

PPSOI

=

Avg. Tahiti MSLP for month Avg. DarwinMSLP for month

Long term avg. of Pdiff forthat month (climatology)

Long-term standard deviation of Pdifffor the month

Determining Phase of the ENSODetermining Phase of the ENSO

SSTSST--based methods use measurements of sea surfacebased methods use measurements of sea surface

temperature in various averaging regions of the Pacifictemperature in various averaging regions of the Pacific

ONI (ONI (Oceanic NiOceanic Nio Index)o Index): Running 3 month average of: Running 3 month average ofNino 3.4 SSTNino 3.4 SST

El Nino/La Nina conditions declared when ONIEl Nino/La Nina conditions declared when ONI exceedsexceeds

0.5C anomaly for 5+ consecutive months0.5C anomaly for 5+ consecutive months


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ONI and SOI are the primary indexesONI and SOI are the primary indexes OLROLRAnomolyAnomoly

There are many other indexes used totrack ENSO

Where are we now?Where are we now?Last month SSTLast month SSTAnomoliesAnomolies show that EL Nino is fadingshow that EL Nino is fading..

Where are we going?Where are we going? Characteristics of ENSOCharacteristics of ENSO

Begins to develop in early summerBegins to develop in early summer

Maximum amplitude in DecMaximum amplitude in Dechence its namesakehence its namesake

Strongest atmosphericStrongest atmospheric

teleconnectionsteleconnections

in DJFin DJF

Decays rapidly FebDecays rapidly Feb--MarMar

Typically last 12Typically last 12--18 mo18 mo

Return Interval of 2Return Interval of 2--7 years : Why 27 years : Why 2--7 years?7 years?


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Evolution of 1997Evolution of 1997--1998 El Nino1998 El Nino

Dynamical Theories of ENSODynamical Theories of ENSO

BjerknesBjerknes OceanOcean--Atmosphere FeedbackAtmosphere Feedback Positive feedback loop that fosters developmentPositive feedback loop that fosters development

Chicken (ocean) vs. Egg (atmosphere) issueChicken (ocean) vs. Egg (atmosphere) issue

Delayed Oscillator TheoryDelayed Oscillator Theory Stochastic TheoryStochastic Theory


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HigherSSTineastPacific LowerSSTineastPacific

Winds SST

Bjerknes TheoryBjerknes TheoryBjerknes TheoryBjerknes Theory

Positive feedback between circulation and SSTPositive feedback between circulation and SST Explains the amplification of a warm or cold ENSO eventExplains the amplification of a warm or cold ENSO event

but does not explain the transition between the twobut does not explain the transition between the two Questions remain:Questions remain:

What causes transition to events of opposite sign?What causes transition to events of opposite sign? Why do events last 12Why do events last 12 --18 months on average?18 months on average?

Theory I: Delayed OscillatorTheory I: Delayed Oscillator

Idea: systematic coupled oceanIdea: systematic coupled ocean--atmosphere feedbackatmosphere feedback

Start w/ warm ENSO, apply forcing near datelineStart w/ warm ENSO, apply forcing near dateline

Relaxation of trade winds sends oceanicRelaxation of trade winds sends oceanic RossbyRossbywavewavewest (moves very slow ~ 300 days to cross Pacific)west (moves very slow ~ 300 days to cross Pacific)

Reflection of oceanicReflection of oceanic RossbyRossbywave off western boundarywave off western boundary

of Pacific sends rapid oceanic Kelvin wave (equatoriallyof Pacific sends rapid oceanic Kelvin wave (equatoriallytrapped westerly moving wave) eastward (70 days).trapped westerly moving wave) eastward (70 days).

Kelvin WavesKelvin Waves

3. Kelvin wave causes3. Kelvin wave causes thermoclinethermocline to rise across basin, having little effectto rise across basin, having little effecton SST except in eastern Pacific whereon SST except in eastern Pacific where thermoclinethermocline is shallowestis shallowest

4. Allows cool SST to return4. Allows cool SST to return

5. SST gradient5. SST gradient intensification of trade windsintensification of trade winds positive feedbackpositive feedback

6. Process repeats and switches phases periodically6. Process repeats and switches phases periodically

This idea holds that ENSO events with positive feedbacks betweenThis idea holds that ENSO events with positive feedbacks between tradetrade

wind, SST, and convection, this theory reproduce cycles of 3wind, SST, and convection, this theory reproduce cycles of 3 --5 years5 years

Theory I: Delayed OscillatorTheory I: Delayed Oscillator

4. Allows cool SST to return4. Allows cool SST to return5. SST gradient leads to intensification of trade winds5. SST gradient leads to intensification of trade winds positivepositive

feedbackfeedback

6. Process repeats and switches phases periodically6. Process repeats and switches phases periodically

This idea holds that ENSO events with positive feedbacks betweenThis idea holds that ENSO events with positive feedbacks between

trade wind, SST, and convection, this theory reproduce cyclestrade wind, SST, and convection, this theory reproduce cycles

of 3of 3--5 years5 years

Warm Water to West,

Shallow Thermocline to East,

cool SST

Oceanic waves force

thermocline to rise and fall,

but since only shallow in the

east, resulting impact in SST

is largest there


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Top: SST anomaly and sea level anomaly; Bottom: Thermocline Depth Anomaly

Theory II: StochasticTheory II: Stochastic

ENSO events triggered by randomENSO events triggered by random forcingsforcings from the atmospherefrom the atmosphere

Indeed ENSO cycles are not regular, therefore this theory betterIndeed ENSO cycles are not regular, therefore this theory betterconforms to observationsconforms to observations

Possible connection between burst the large scale tropicalPossible connection between burst the large scale tropicalconvective complexes associated with the MJO leading to westerlyconvective complexes associated with the MJO leading to westerly

wind bursts as a triggering mechanismwind bursts as a triggering mechanism

TeleconnectionTeleconnection

Defined:Defined: Interconnection between atmospheric circulation in oneInterconnection between atmospheric circulation in one

sector of the globe to influence directly or indirectly circulatsector of the globe to influence directly or indirectly circulationion

patterns that span large areaspatterns that span large areas

Low Frequency :Low Frequency : Longer than synoptic timescale (weeks to decades)Longer than synoptic timescale (weeks to decades)

Teleconnection toTeleconnection to MidlatitudesMidlatitudes

Low PressureTropical Convection

Example:Example: Variable 1:Variable 1: Changes in tropical Pacific SSTChanges in tropical Pacific SST

Variable 2:Variable 2: Changes in strength/location of subtropical jet overChanges in strength/location of subtropical jet over

Pacific and downstream precipitation patterns over Western USPacific and downstream precipitation patterns over Western US

Composite:Averages taken over all El Nino (La Nina) winters

Warmest SST

http://www.esrl.noaa.gov/psd/enso/images/u200.comp.gif


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http://www.esrl.noaa.gov/psd/enso/images/z200.comp.gif http://www.esrl.noaa.gov/psd/enso/images/u200.comp.la.gif

http://www.esrl.noaa.gov/psd/enso/images/z200.comp.la.gif

Which map shows average winterWhich map shows average winter

precipitation anomaly for El Ninoprecipitation anomaly for El Nino

conditions, and which for La Nina?conditions, and which for La Nina?

ENSO Signals (WinterENSO Signals (Winter PrecipPrecip)) ENSO Signals (Winter Temp)ENSO Signals (Winter Temp)


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ENSOENSOTeleconnectionsTeleconnections

ENSO phases alter synoptic patternsENSO phases alter synoptic patterns

Major US changes during warm events:Major US changes during warm events:

-- Subtropical Jet Extends eastward and southSubtropical Jet Extends eastward and south

-- Blocking patterns migrate eastwardBlocking patterns migrate eastward

-- PersistantPersistant positive PNA patternpositive PNA pattern

-- colder and wetter along the southern tiercolder and wetter along the southern tier

Major US changes during cold events:Major US changes during cold events:

-- subtopicalsubtopical jet weakensjet weakens

-- drier and warmer in southern tierdrier and warmer in southern tier

-- cold and wet in Pacific Northwest.cold and wet in Pacific Northwest.

-- more negative PNA patternsmore negative PNA patterns

ENSOENSOTeleconnectionsTeleconnections

Tropical Cyclone Frequency (warm phase)Tropical Cyclone Frequency (warm phase)

Reduction in TC over Australia & NW PacificReduction in TC over Australia & NW Pacific(warm water and convection shifts east)(warm water and convection shifts east)

Reduction in TC over Atlantic (increased verticalReduction in TC over Atlantic (increased vertical

shear)shear)

Increase in eastern PacificIncrease in eastern Pacific -- near Hawaii (increasednear Hawaii (increased

SST+ convection)SST+ convection)

ENSO and Global TemperatureENSO and Global Temperature

Atmosphere gains a tremendous amountAtmosphere gains a tremendous amount

of energy during ENSO events due to theof energy during ENSO events due to the

increased latent heat from the ocean.increased latent heat from the ocean.

Global temperatures spike noticeablyGlobal temperatures spike noticeably

during strong El Nino events (1C).during strong El Nino events (1C).

InsertInsert CoolingCooling Graphic HereGraphic Here

Decadal Climate VariabilityDecadal Climate Variability

InvolvesInvolves memorymemory of system: persistence of cool SST from oneof system: persistence of cool SST from onewinter to the nextwinter to the next

This means ocean processes play big role. Cool water in winterThis means ocean processes play big role. Cool water in winterduring deep mixed layer gets stored below mixed layer induring deep mixed layer gets stored below mixed layer in

summer only to reemerge next wintersummer only to reemerge next winterpersistencepersistenceStill not completely resolved whether PDO is real or artifact ofStill not completely resolved whether PDO is real or artifact of

climate systemclimate system

PDO: El NinoPDO: El Ninos Grandfathers Grandfather

Interplay between the PDO and the dominant ENSO cycles importantInterplay between the PDO and the dominant ENSO cycles important

-- 1983 and 1997 strong ENSO events occurred in years in which the1983 and 1997 strong ENSO events occurred in years in which the PDO was in a warm phasePDO was in a warm phase

leads to amplification of ENSO signal.leads to amplification of ENSO signal.

-- Switch in PDO phase in 1977 plays big role in climate patternsSwitch in PDO phase in 1977 plays big role in climate patterns across the Westacross the West

Stronger impact

2-7 yr quasi-periodic

Lasts 1-2 years

Tropical roots

Weaker impact

40 year cycles

Last ~ 2 decades

Extratropical roots

Warm Phase Warm Phase


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Back to the Cold Phase???Back to the Cold Phase???

climate variability ii (dl)

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