Impacts of the Indian Summer Monsoon and Indian Ocean on ENSO Variability

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Impacts of the Indian Summer Monsoon and Indian
Ocean on ENSO Variability

• Renguang Wu1 and Ben Kirtman1,2
• 1Center for Ocean-Land-Atmosphere Studies
• 2George Mason University

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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.
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Indian Summer Monsoon-Indian Ocean-Pacific Ocean
Interactions
instantaneous
ISM
time lag
Indian Ocean
ENSO
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Coupled Model Reproduces Observed Relationship
IMR 60-100E, 5-25N
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Coupled Model Reproduces Observed Relationship
IMR 60-100E, 5-25N
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Coupled Model Reproduces the Probability of ENSO
Normal ISM
Dry ISM
Wet ISM
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Conditional Composite
Classifications based on Niño-3.4 SST and IMR
anomalies
Niño-3.4 SST Niño-3.4 SST Niño-3.4 SST
Cold (SSTAlt-0.43?SST) Normal (SSTAlt0.43?SST) Warm (SSTAgt0.43?SST)
IMR Wet (IMRAgt0.43?IMR) Cold-Wet Normal-Wet Warm-Wet
IMR Normal (IMRAlt0.43?IMR) Cold-Normal Normal-Normal Warm-Normal
IMR Dry (IMRAlt-0.43?IMR) Cold-Dry Normal-Dry Warm-dry
the difference due to monsoon impacts
monsoon anomalies un-related to ENSO
monsoon anomalies related to ENSO
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Conditional Composite
Number of years for different composites based on
JJAS IMR and NINO3.4 SST
Cold Normal Warm
Wet 40 41 28
Normal 32 41 32
Dry 23 36 49
COLA Model
Cold (warm) events tend to be more frequent in
wet (dry) Indian monsoon years
Cold Normal Warm
Wet 34 11 9
Normal 13 17 15
Dry 6 14 24
Observation
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Indian Monsoon Impacts an Ongoing Warm Event
Model
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Indian Monsoon Impacts an Ongoing Warm Event
Observations
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Indian Monsoon Impacts an Ongoing Cold Event
Model
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Indian Monsoon Impacts an Ongoing Cold Event
Observations
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Model
Normal SST Wet Monsoon
Normal SST Dry Monsoon
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Observations
Normal SST Dry Monsoon
Normal SST Wet Monsoon
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Model
Indian Monsoon Impacts an Ongoing Warm Event
Warm-Dry minus Warm-Normal
Warm-Normal
hc
taux
SST
Warm-Wet minus Warm-Normal
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Model
Indian Monsoon Impacts an Ongoing Cold Event
Cold-Dry minus Cold-Normal
Cold-Normal
hc
taux
SST
Cold-Wet minus Cold-Normal
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Simple model experiment with idealized SST
forcing in the North Indian Ocean
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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

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ENSO Variability is Reduced without Indian Ocean
DJF SST STD
Indian Ocean Coupled
Indian Ocean De-Coupled
Ratio De-Coupled/Coupled
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Monthly SST STD 2S-2N
Indian Ocean Coupled
Indian Ocean De-Coupled
Ratio De-Coupled/Coupled
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Indian Ocean Affects the Probability of ENSO
Warm (cold) ENSO events are more (less) frequent
and stronger (weaker) when the Indian Ocean SST
is low
Model
IOSST 60-90E, 5S-5N
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Number of years for different composites based 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
COLA Model
Cold Normal Warm
High 7 14 21
Normal 20 20 13
Low 28 9 14
Observation
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DJF NINO-3.4 SST anomalies for different
composites
Cold Normal Warm
High -0.47 -0.35 0.04
Normal -0.50 -0.16 0.51
Low -0.25 0.09 0.83
COLA Model
Cold Normal Warm
High -1.30 0.08 0.92
Normal -0.79 -0.17 0.77
Low -0.50 0.23 0.72
Observation
0.67 excluding 1877, 1982, 1997
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Indian Ocean Impacts an Ongoing ENSO
Model
IO normal
IO cold minus normal
IO warm minus normal
PO warm
PO cold
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Warm Cases
Cold Cases
Observation
JJAS
DJF
JJAS IO SSTAlt0
JJAS IO SSTAgt0
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JJAS U200
Warm PO
JJAS U850
IO cold
IO normal
IO warm
JJAS U200
JJAS U850
Cold PO
Cold IO SST induces LL westerly UL easterly
over EIO-WPO Warm IO SST induces LL easterly UL
westerly over EIO-WPO
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Cold IO SST induces LL westerly UL easterly
over EIO-WPO
Enhancing warm ENSO Weakening cold ENSO
COLD
Indian Ocean
Pacific Ocean
Warm IO SST induces LL easterly UL westerly
over EIO-WPO
Weakening warm ENSO Enhancing cold ENSO
WARM
Pacific Ocean
Indian Ocean
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Indian Ocean Impacts on Pacific Winds
Coupled Model SST
EOF1 38 (eof2 24)
Response to PO IO SST
Response to PO SST
Response to IO SST
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Indian Ocean Impacts on Pacific Winds
Observed SST
EOF2 11 (eof1 52)
Response to PO IO SST
Response to PO SST
Response to IO SST
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Process
Indian Ocean SSTA
Atmospheric Heating
Walker Circulation
Equatorial Pacific Wind
ENSO
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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

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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

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Number of Cases for DJF NINO3.4 SSTA
Coupled Model
Observations
Warm Events
IOSST
IOSST
high normal low
wet 4 5 6
normal 4 10 7
dry 1 12 36
high normal low
wet 5 3 2
normal 4 3 4
dry 11 6 10
ISM
ISM
Cold Events
IOSST
IOSST
High normal low
wet 5 10 16
normal 6 8 4
dry 3 3 2
high normal low
wet 40 23 5
normal 19 14 6
dry 9 4 4
ISM
ISM
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Composite DJF NINO3.4 SSTA
Coupled Model
Observations
Warm Events
IOSST
IOSST
high normal low
wet 0.51 0.64 0.51
normal 0.45 0.72 0.80
dry 0.73 0.86 0.89
high normal low
wet 0.89 0.50 0.54
normal 1.49 1.11 0.64
dry 1.30 1.41 1.14
ISM
ISM
Cold Events
IOSST
IOSST
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 -0.48 -0.50 -0.54
normal -0.48 -0.49 -0.39
dry -0.55 -0.56 -0.53
ISM
ISM
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Is specified Indian Ocean SST proper for ENSO?
Importance of Indian Ocean Coupling for ENSO
Variability
SST Variance Ratio (Forced/ Coupled)
Forced Indian Ocean
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DJF SST Standard Deviation
Coupled
De-coupled/Coupled
Forced/Coupled
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De-Coupled Indian Ocean
Forced Indian Ocean
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Lag-lead correlation wrt DJF NINO3.4 SST
Indian Ocean coupled
Indian Ocean forced
Interfering effects of the Indian monsoon
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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

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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.
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NINO3.4 SST Spectrum
Coupled
Indian Ocean De-coupled
Indian Ocean Forced
Observation
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ENSO Affects Indian Monsoon Transition in Two
Ways Modulation of Walker Circulation A Rossby
Wave Type Response
DJF(2)
Weak WC
Weak WC
Warm Indian Ocean
Warm Central Pacific
Cold Western Pacific
Enhanced Evaporation
Reduced Evaporation
Air-Sea Interaction
JJA(1)
Anomalous Surface Westerly
Weak Indian Monson
Warm Central Pacific
Enhanced Evaporation
Reduced Moisture Convergence
Reduced Short Wave
C
A
DJF(1)
Strong WC
Strong WC
Cold Indian Ocean
Cold Central Pacific
Warm Western Pacific
Air-Sea Interaction
Enhanced Evaporation
Reduced Evaporation
JJA(0)
Anomalous Surface Easterly
Strong Indian Monson
Cold Central Pacific
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DJF
C
A
A
C
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