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.