the el-nino southern oscillation (enso)

Tropical M. D. Eastin

The El-Nino Southern Oscillation(ENSO)

La Nina ImpactsEl Nino Impacts


Outline

History

Observed Structure and Evolution

What Causes an El-Nino?

ENSO Forecasting

Global Impacts


ENSO

History

• In the 1600s, Peruvian fisherman noticed their fish harvests failed every few years due to warmer-than-normal waters (upwelling provides nutrient-rich cold water for fish). The warming always occurred in December, so the phenomena was named El Nino, in reference to the Christ child.

• In 1899 the Indian Monsoon failed, leading to severe drought and famine. This lead Gilbert Walker, head of the Indian Met. Service, to search for a way to predict the monsoon. He identified a peculiar surface pressure oscillation: when the pressure is high over the maritime continent (Indonesia and Darwin), surface pressures are low over India and the central southern Pacific (Tahiti). He referred to this as the Southern Oscillation.

• In 1969, UCLA professor Jacob Bjerknes first recognized that El Nino and the Southern Oscillation were actually manifestations of the same physical phenomena that results from unstable interactions between the ocean and atmosphere, and referred to it as ENSO

• The 1982-83 El Nino event was the first to receive significant public and research interest → a relatively new (not well understood) phenomena

Sir Gilbert Walker

Jacob Bjerknes


ENSO: Observed Structure

Normal Conditions or La Nina:

• Strong easterly winds induce upwelling of cold water in the equatorial eastern Pacific• Shallow oceanic thermocline in the east Pacific (due to upwelling)• Warm SSTs confined to western Pacific with a deep thermocline• Low pressure and convection in west Pacific• High pressure and subsidence (clear air) in east Pacific



El-Nino Conditions:

• Weaker easterly winds result in less upwelling of cold water• Warm SSTs “spread” to east Pacific (also solar heating not offset by upwelling)• Increase in the east Pacific thermocline• Low pressure and convection shifts to the east Pacific• High pressure and subsidence shifts to the west Pacific



ENSO Indices

• Based on observed SST anomalies (difference from the long term mean) in the equatorial Pacific in four regions (observation from TAO moored buoys)• Based on surface pressure differences between Tahiti and (minus) Darwin, called the Southern Oscillation Index (SOI)

Nino 1+2

Region that often first warms during the

onset of an El Nino

Nino 3

Largest variabilityin SSTs over an

average ENSO cycle

SOI TahitiDarwin

SOI

Most highly correlated withNino 3.4 SST

Nino 3.4

Most highly correlated with eastward shift

of convection

Nino 4

Most highly correlated with global

weather patterns



NOAA’s Multivariate ENSO Index (MEI)

• Combines normalized anomalies of SST (in Nino3.4), surface pressures (the SOI), surface winds, surface air temperatures, and cloud fraction to obtain a “composite” view of the state of ENSO

• Definitions: El Nino = Standardized Departures > +1.0La Nina = Standardized Departures < -1.0

El Nino

La Nina

82-83

10-1188 98-99

97-98

Source: http://www.cdc.noaa.gov/ENSO/enso.mei_index.html



[Animation]

The 1995-1996 La-Nina EventJanuary 1995 – December 1996



[Animation]

The 1997-1998 El-Nino EventJanuary 1997 – December 1998


What Causes El Nino?Triggering Mechanism

• Not well understood• Deep thermocline in the western Pacific believed to be a necessary (not sufficient) condition

• “Westerly wind bursts” (WWBs) over a period of several days may be one trigger

• Most often associated with the Madden-Julian Oscillation (MJO)• Atmospheric Kelvin waves also generate sustained westerly winds• Twin TC’s straddling the equator can also generate sustained westerly winds

• Multiple sustained WWBs decrease the equatorial easterlies that induced cold upwelling• Less upwelling combined with a west-east ocean current (forced by the WWBs) increases the central and eastern Pacific SSTs and lowers the thermocline depth and initiates an El Nino event

Anomalous surface winds(i.e. a WWB) associated with MJO convection (centered in the box)


What Causes El Nino?Onset of the 1997-98 El Nino

Daily Mean Surface Values in the equatorial Pacific: 1 January 1997 thru 31 December 1998

Zonal Wind Anomalies (m/s) Mean Zonal Wind (m/s) SST Anomalies (ºC)

WWB

StrongEasterlies

WWB

El NinoWeakerEasterlies


What Causes El Nino?An Oceanic Component

• The WWBs acting alone would lead to a gradual eastward progression of SST anomalies (which is observed but the signal is weak)• In contrast, observations show a pronounced rapid “emergence” of warm SST anomalies in the equatorial east Pacific (along the Peruvian coast in the Nino1+2 region)

• What causes this rapid emergence?

• Delayed Oscillator Theory is one explanation for this rapid emergence

• Atmospheric WWBs generate equatorial Rossby and Kelvin waves in the ocean• Oceanic waves propagate along the density contrast of the thermocline

Oceanic Rossby waves: Move westward at slow speedsInduce upwelling (decreases the thermocline depth)Effectively cool the ocean mixed layer and SSTs

Oceanic Kelvin waves: Move eastward very rapidly (much faster than Rossby waves)

Induce downwelling (increases the thermocline depth)

Effectively warm the ocean mixed layer and SSTs

• This theory also provides an explanation for the ENSO oscillation every 4-5 years


What Causes El Nino?

The Delayed Oscillator in a Simple Ocean Model

Initial Time

Forcing fromsingle WWB

Thermocline Depth

UpwellingRossbyWaves

DownwellingKelvinWave

Thermocline Depth

25 days

Kelvin Wavereflects and becomes

a Rossby wave

Rossby Wave reflectsbecomes a Kelvin wave

50 days

75 days

100 days

175 days

225 days

Multiple reflections canlead to La Nina onset


What Causes La Nina?Onset of the 1998-99 La Nina

Daily Mean Surface Values in the equatorial Pacific: 1 January 1997 thru 31 December 1998

Zonal Wind Anomalies (m/s) Mean Zonal Wind (m/s) SST Anomalies (ºC)

Lack of StrongWWB

StrongEasterlies

La Nina

WeakerEasterlies

Lack of StrongWWB


ENSO: Global ImpactsGlobal Impacts

• ENSO variability alters convection in tropical Pacific• This convective variability produces zonal anomalies in the Walker and Hadley Circulations, which, in turn, influences mid-latitude synoptic-wave patterns and alters the global weather• Anomalous synoptic wave patterns lead to warmer/colder and wetter/drier conditions

La Nina Impacts (Winter)El Nino Impacts (Winter)


ENSO: U.S. Impacts

El Nino: Summer TemperaturesEl Nino: Winter Temperatures


ENSO: U.S. Impacts

El Nino: Summer RainfallEl Nino: Winter Rainfall


ENSO: U.S. Impacts

La Nina: Summer TemperaturesLa Nina: Winter Temperatures


ENSO: U.S. Impacts

La Nina: Summer RainfallLa Nina: Winter Rainfall


ENSO Forecasting

Forecast Models

• All models forecast SSTs in the equatorial Pacific (most often for the Nino3.4 region)

Statistical models

• Employ simple multiple regression techniques based on ENSO indices• Based on evolution of previous ENSO events (and historical records)• Quality of forecasts reliant on quality of historical data• Cannot forecast “record” events• No physical interpretation possible

Dynamical Models

• Most are complex coupled atmosphere-ocean models • Initialization requires 3-D observations of ocean and atmosphere (data sparse region)• Small scale features are parameterized• Can forecast record events (not bound by past events)


ENSO Forecasting

Source: http://iri.columbia.edu/climate/ENSO/currentinfo/SST_table.html


The El-Nino Southern Oscillation(ENSO)

Summary:

• History (basic timeline and rise to prominence) • Oceanic and atmospheric structure/flows during El Nino and La Nina• ENSO Indices (defining parameter, differences, and uses)

• Causes of El Nino• Westerly Winds Bursts (definition, origin, impact/forcing)• Delayed Oscillator Theory (role of waves, explain rapid onset)

• Global impacts of ENSO• Impact of ENSO in the U.S.

• ENSO Forecasting (difference in model types)


References

Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage( http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl )

Kindle, J. C. , and P. A. Phoebus, 1995: The ocean response to perational westerly wind bursts during the 1991-1992El Nino. J. Geophysical. Res., 100, 4893-4920.

Knaff, J. A., and C. W. Landsea, 1997: An El Nino-Southern Oscillation Climatology and Persistence (CLIPER) Forecasting

Scheme. Wea. Forecasting, 12, 633-652.

McPhaden, M. J., 2004: Evolution of the 2002/3 El Nino. Bull. Amer. Meteor. Soc., 85, 677-695.

the el-nino southern oscillation (enso)

Documents

east pacific high pressure

equatorial pacific

western pacific

nino event

surface pressure differences

surface winds

surface pressures

structureelnino conditions