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Impacts of the Indian Summer Monsoon and Indian Ocean on ENSO Variability Renguang Wu 1 and Ben Kirtman 1,2 1 Center for Ocean-Land-Atmosphere Studies 2 George Mason University

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impacts of the indian summer monsoon and indian ocean on enso variability

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

slide2

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.

slide3

Indian Summer Monsoon-Indian Ocean-Pacific Ocean Interactions

instantaneous

ISM

time lag

Indian Ocean

ENSO

conditional composite
Conditional Composite

Classifications based on Niño-3.4 SST and IMR anomalies

the difference due to monsoon impacts

monsoon anomalies related to ENSO

monsoon anomalies un-related to ENSO

conditional composite7
Conditional Composite

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

COLA Model

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

Observation

slide12

Model

Normal SST & Wet Monsoon

Normal SST & Dry Monsoon

slide13

Observations

Normal SST & Dry Monsoon

Normal SST & Wet Monsoon

slide14

Indian Monsoon Impacts an Ongoing Warm Event

Model

Warm-Normal

Warm-Dry minus Warm-Normal

hc

taux

SST

Warm-Wet minus Warm-Normal

slide15

Indian Monsoon Impacts an Ongoing Cold Event

Model

Cold-Normal

Cold-Dry minus Cold-Normal

hc

taux

SST

Cold-Wet minus Cold-Normal

summary i impacts of indian summer monsoon on enso
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
slide18

ENSO Variability is Reduced without Indian Ocean

DJF SST STD

Indian Ocean Coupled

Indian Ocean De-Coupled

Ratio De-Coupled/Coupled

slide19

Monthly SST STD 2S-2N

Indian Ocean Coupled

Indian Ocean De-Coupled

Ratio De-Coupled/Coupled

slide20

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

slide21

Number of years for different composites

based on JJAS IO SST and JJAS NINO-3.4 SST

COLA Model

Observation

slide22

DJF NINO-3.4 SST anomalies for different composites

COLA Model

Observation

0.67 excluding 1877, 1982, 1997

slide23

Indian Ocean Impacts an Ongoing ENSO

Model

IO normal

IO cold minus normal

IO warm minus normal

PO warm

PO cold

slide24

Warm Cases

Cold Cases

Observation

JJAS

DJF

JJAS IO SSTA<0

JJAS IO SSTA>0

slide25

Warm PO

JJAS U200

JJAS U850

IO cold

IO normal

IO warm

JJAS U850

Cold PO

JJAS U200

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

slide26

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

Indian Ocean

Pacific Ocean

slide27

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

slide28

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

slide29

Process

Indian Ocean SSTA

Atmospheric Heating

Walker Circulation

Equatorial Pacific Wind

ENSO

summary ii impacts of indian ocean on 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
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

slide32

Number of Cases for DJF NINO3.4 SSTA

Coupled Model

Observations

Warm Events

IOSST

IOSST

ISM

ISM

Cold Events

IOSST

IOSST

ISM

ISM

slide33

Composite DJF NINO3.4 SSTA

Coupled Model

Observations

Warm Events

IOSST

IOSST

ISM

ISM

Cold Events

IOSST

IOSST

ISM

ISM

slide34

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

SST Variance Ratio (Forced/ Coupled)

Forced Indian Ocean

slide35

De-Coupled Indian Ocean

Forced Indian Ocean

slide36

Lag-lead correlation wrt DJF NINO3.4 SST

Indian Ocean coupled

Indian Ocean forced

Interfering effects of the Indian monsoon

summary iii importance of indian ocean coupling for enso
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
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.