The Two Types of ENSO

Central-Pacific El Niño (related to atmospheric forcing)

Eastern-Pacific El Niño (related to thermocline variation)

(Yu and Kao 2007; Kao and Yu 2009)

Possible Forcing Mechanisms for CP ENSO

Monsoon

forcing

Extra-tropical

forcing

CP ENSO

(Yu et al. 2009)

(Yu et al. 2010)

CP-ENSO SST Variations

-14

-4

-10

-6

-12

-8

+2 0 -2

(Yu, Kao, and Lee 2010)

North Pacific Oscillation (NPO) and Associated SST

Anomalies

EO

F2

SST

NPO (SLP EOF mode) Correlated SST

NPO-Correlated SSTA Evolution

lag-0 Lag+2 Lag+4

+6 +8 +10

+12 +14 +16

(Yu and Kim 2011)

Three Evolution Patterns for CP El Nino

mean thermocline depth at equator

Prolonged Decay

Abruptly Decay

Symmetric Decay

(Yu and Kim 2010)

Transitions between CP and EP ENSOs

NPO

CP EP

Equatorial Thermocline

EP/CP-ENSO Correlates with SLP

EP ENSO

CP ENSO

Hadley Circulation

Walker Circulation

Walker and Hadley Circulations

Equator

20-30N

CP EP

When Walker Circulation Is Stronger…

Equator

20-30N

EP El Niño

NPO

When Hadley Circulation Is Stronger…

Equator

20-30N

CP El Niño

El Niño shifted from EP to CP (Yu, Lu, and Kim 2012)

Walker Circulation

before 1990 after

1990

Wal

ker

Cir

cula

tio

n S

tren

gth

(×

10

-1

Pa/s

ec)

Hadley Circulation

before 1990

after 1990

Had

ley

Cir

cula

tio

n S

tren

gth

(m

/sec

)

EP

before 1990

after 1990

NPO

CP

The increased extratropical forcing to the tropics after 1990

is a likely cause for the recent emergence of the Central-

Pacific El Niño.

Regressed US Winter (JFM) Temperature

NE

SW

NW

SE

With EP El Niño Index With CP El Niño Index

(1948-2010)

(Yu, Zou, Kim, and Lee, 2012)

Ensemble Forced AGCM Experiments

NCAR CAM4 Forced by the SST from

EP ENSO SSTA + Climatology CP ENSO SSTA + Climatology

(Yu, Zou, Kim, and Lee 2012)

ENSO SST Anomalies Prescribed for the AGCM (CAM4) Experiment

EP El Niño CP El Niño

Case Studies with EP/CP El Niño Events Strong EP

Events Strong CP

Events

Regressed US Winter (JFM) Precipitation

With EP El Niño Index With CP El Niño Index

CP Impact – EP Impact

CP El Niño enhances dry anomalies & weakens wet anomalies More droughts in the US

(Yu and Zou 2013)

Precipitation Anomalies during El Niño Winters

mean:6.7

mean:6.6

mean:1.5 mean:-4.3 mean:0.8

mean:-10.3

mean:4.2 mean:12.4

EP CP

EP

EP

EP

CP

CP

CP

Different Impacts of the Two Types of ENSO on US winter

Enhanced Drying Effects 90° Rotation of the Impacts

US Winter Temperature US Winter Rainfall

Summary • There may exist two distinct types of El Niño: an Eastern-Pacific type

and a Central Pacific type.

• Tropical Pacific Ocean interacts with the Walker circulation to produce

the EP El Niño but interacts with the Hadley circulation to produce the

CP El Niño.

• The increased influence from extratropial atmosphere over tropical Pacific is a likely cause for the shift of El Niño from the Eastern-Pacific type to the Central-Pacific type since 1990.

• The impacts produced by the Central-Pacific and Eastern-Pacific types of El Niño on US winter climate are different.

• The increasing occurrence of the CP El Niño in the recent decades may have also contributed to the US drought conditions.