Impacts of the Indian Summer Monsoon and Indian Ocean on

ENSO Variability

Renguang Wu1 and Ben Kirtman1,2

1Center for Ocean-Land-Atmosphere Studies2George Mason University

Wu, R., and B. P. Kirtman, 2003: On the impacts of the Indian summer monsoon on ENSO in a coupled GCM. Quart. J. Roy. Meteor. Soc., 129B, 3439-3468.

Wu, R., and B. P. Kirtman, 2004: Understanding the impacts of the Indian Ocean on ENSO variability in a coupled GCM. J. Climate, 17, 4019-4031.

Wu, R., and B. P. Kirtman, 2005: Roles of Indian and Pacific Ocean air-sea coupling in tropical atmospheric variability. Clim. Dyn., 25, 155-170.

Indian Summer Monsoon-Indian Ocean-Pacific Ocean Interactions

ISM

ENSOIndian Ocean

instantaneous

time lag

Coupled Model Reproduces Observed Relationship

IMR: 60-100E, 5-25N

Coupled Model Reproduces the Probability of ENSO

Dry ISMWet ISM

Normal ISM

Conditional CompositeClassifications based on Niño-3.4 SST and IMR anomalies

Niño-3.4 SST

Cold(SSTA<-0.43SST)

Normal(|SSTA|<0.43SST)

Warm (SSTA>0.43SST)

IMR

Wet(IMRA>0.43IMR) Cold-Wet Normal-Wet Warm-Wet

Normal(|IMRA|<0.43IMR) Cold-Normal Normal-Normal Warm-Normal

Dry(IMRA<-0.43IMR) Cold-Dry Normal-Dry Warm-dry

monsoon anomalies un-related to ENSO

monsoon anomalies related to ENSO

the difference due to monsoon impacts

Conditional CompositeNumber of years for different composites based on JJAS IMR and NINO3.4 SST

COLA Model

Cold Normal Warm

Wet 40 41 28

Normal 32 41 32

Dry 23 36 49

Observation

Cold Normal Warm

Wet 34 11 9

Normal 13 17 15

Dry 6 14 24

Cold (warm) events tend to be more frequent in wet (dry) Indian monsoon years

Indian Monsoon Impacts an Ongoing Warm Event

Model

Observations

Indian Monsoon Impacts an Ongoing Warm Event

Indian Monsoon Impacts an Ongoing Cold Event

Model

Observations

Indian Monsoon Impacts an Ongoing Cold Event

Normal SST & Dry Monsoon

Normal SST & Wet Monsoon

Model

Normal SST & Dry Monsoon

Normal SST & Wet Monsoon

Observations

ModelIndian Monsoon Impacts an Ongoing Warm EventWarm-Dry minus Warm-Normal

Warm-Wet minus Warm-Normal

SST hc tauxWarm-Normal

ModelIndian Monsoon Impacts an Ongoing Cold EventCold-Dry minus Cold-Normal

Cold-Wet minus Cold-Normal

SST hc tauxCold-Normal

Simple model experiment with idealized SST forcing in the North Indian Ocean

Summary IImpacts of Indian summer monsoon on ENSO

• A dry (wet) Indian summer monsoon enhances (weakens) an ongoing warm event

• The Indian monsoon impacts are via modulating surface wind stress anomalies over the western equatorial Pacific

• The impacts of an anomalous Indian summer monsoon on cold ENSO events are weaker

ENSO Variability is Reduced without Indian OceanDJF SST STD

Indian Ocean Coupled

Indian Ocean De-Coupled

Ratio De-Coupled/Coupled

Monthly SST STD 2S-2N

Indian Ocean Coupled

Indian Ocean De-Coupled

Ratio De-Coupled/Coupled

Indian Ocean Affects the Probability of ENSO

IOSST: 60-90E, 5S-5N

Warm (cold) ENSO events are more (less) frequent and stronger (weaker) when the Indian Ocean SST is low

Model

Number of years for different compositesbased on JJAS IO SST and JJAS NINO-3.4 SST

Cold Normal Warm

High 41 37 31

Normal 24 42 34

Low 30 35 44

Cold Normal Warm

High 7 14 21

Normal 20 20 13

Low 28 9 14

COLA Model

Observation

DJF NINO-3.4 SST anomalies for different composites

COLA Model

Observation

Cold Normal Warm

High -0.47 -0.35 0.04

Normal -0.50 -0.16 0.51

Low -0.25 0.09 0.83

Cold Normal Warm

High -1.30 0.08 0.92

Normal -0.79 -0.17 0.77

Low -0.50 0.23 0.72

0.67 excluding 1877, 1982, 1997

Indian Ocean Impacts an Ongoing ENSOP

O w

arm

PO

col

dIO normal IO cold minus normal IO warm minus normal

Model

Warm Cases Cold Cases

JJAS IO SSTA<0 JJAS IO

SSTA>0

Observation

JJAS

DJF

JJAS U850

JJAS U200JJAS U850

JJAS U200

Cold IO SST induces LL westerly & UL easterly over EIO-WPO Warm IO SST induces LL easterly & UL westerly over EIO-WPO

IO normal

IO warm

IO cold

Warm PO

Cold PO

Warm IO SST induces LL easterly & UL westerly over EIO-WPO

Cold IO SST induces LL westerly & UL easterly over EIO-WPO

Indian Ocean

Pacific Ocean

Pacific Ocean

Indian Ocean

Enhancing warm ENSO Weakening cold ENSO

Weakening warm ENSO Enhancing cold ENSO

COLD

WARM

Indian Ocean Impacts on Pacific Winds

Coupled Model SST

Response to IO SST

EOF1 38%

(eof2 24%)

Response to PO & IO SST

Response to PO SST

Indian Ocean Impacts on Pacific WindsObserved SST

Response to IO SST

EOF2 11%

(eof1 52%)

Response to PO SST

Response to PO & IO SST

Process

Indian Ocean SSTA

Atmospheric Heating

Walker Circulation

Equatorial Pacific Wind

ENSO

Summary IIImpacts of Indian Ocean on ENSO

• The ENSO variability is reduced when the Indian Ocean is de-coupled from the atmosphere

• Warm (cold) ENSO is stronger when the JJAS Indian Ocean SST is relatively low (high), but it is weaker when the Indian Ocean SST is relatively high (low)

• The impacts of the Indian Ocean on the ENSO variability is through modulating convective heating over the Indian Ocean and the Walker circulation over the tropical Indo-western Pacific region

• In observations, the Indian Ocean has significant impact on the intensity of cold ENSO events. There is also evidence for this impact during some warm ENSO events

Combined impacts of the Indian summer monsoon and Indian Ocean

• Dry (wet) Indian summer monsoon favors warm (cold) ENSO• Low (high) Indian Ocean JJAS SST favors warm (cold) ENSO

• Inferences:dry ISM-low IO SST mostly favorable for warm ENSO

wet ISM-high IO SST mostly favorable for cold ENSO

high normal low

wet 5 3 2

normal 4 3 4

dry 11 6 10

High normal low

wet 5 10 16

normal 6 8 4

dry 3 3 2

high normal low

wet 4 5 6

normal 4 10 7

dry 1 12 36

Number of Cases for DJF NINO3.4 SSTA

Warm Events

Cold Events

Coupled Model Observations

high normal low

wet 40 23 5

normal 19 14 6

dry 9 4 4

IOSST

ISM

IOSSTIOSST

IOSST

ISM

ISM

ISM

high normal low

wet 0.89 0.50 0.54

normal 1.49 1.11 0.64

dry 1.30 1.41 1.14

high normal low

wet -1.24 -1.06 -0.90

normal -0.89 -0.76 -0.70

dry -0.66 -0.51 -0.84

high normal low

wet 0.51 0.64 0.51

normal 0.45 0.72 0.80

dry 0.73 0.86 0.89

Composite DJF NINO3.4 SSTA

Warm Events

Cold Events

Coupled Model Observations

high normal low

wet -0.48 -0.50 -0.54

normal -0.48 -0.49 -0.39

dry -0.55 -0.56 -0.53

IOSST

ISM

IOSST

ISM

IOSST

ISM

IOSST

ISM

Is specified Indian Ocean SST proper for ENSO? Importance of Indian Ocean Coupling for ENSO Variability

Forced Indian OceanSST Variance Ratio (Forced/ Coupled)

Forced Indian Ocean

De-Coupled Indian Ocean

Lag-lead correlation wrt DJF NINO3.4 SST

Indian Ocean forced

Interfering effects of the Indian monsoon

Indian Ocean coupled

Summary IIIImportance of Indian Ocean coupling for ENSO

• Without Indian Ocean coupling, the Indian monsoon interferes with the Indian Ocean SST, leading to reduced ENSO variability

Final Remarks

While ENSO is essentially a phenomenon whose dynamics is determined by air-sea interaction within the Pacific Ocean, there is the possibility that its temporal evolution, frequency, and amplitude could be modified by Indian summer monsoon and Indian Ocean feedbacks.