# Presentation: 451781

## 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

## 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**

**ENSO**

**Indian Ocean**

- instantaneous

- time lag

## Coupled Model Reproduces Observed Relationship

- IMR: 60-100E, 5-25N

## Coupled Model Reproduces Observed Relationship

- IMR: 60-100E, 5-25N

## Coupled Model Reproduces the Probability of ENSO

- Dry ISM

- Wet ISM

- Normal ISM

## Conditional Composite

- Classifications based on Niño-3.4 SST and IMR anomalies

|  |  | Niño-3.4 SST |  |  |
| --- | --- | --- | --- | --- |
|  |  | Cold
(SSTA<-0.43SST) | Normal
(|SSTA|<0.43SST) | Warm (SSTA>0.43SST) |
| 

IMR | Wet
(IMRA>0.43IMR) | 
Cold-Wet | 
Normal-Wet | 
Warm-Wet |
|  | Normal
(|IMRA|<0.43IMR) | 
Cold-Normal | 
Normal-Normal | 
Warm-Normal |
|  | Dry
(IMRA<-0.43IMR) | 
Cold-Dry | 
Normal-Dry | 
Warm-dry |

- monsoon anomalies un-related to ENSO

- monsoon anomalies related to ENSO

- the difference due to monsoon impacts

## Conditional Composite

- Number 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

**Indian Monsoon Impacts an Ongoing Warm Event**

**Model**

## Indian Monsoon Impacts an Ongoing Warm Event

**Observations**

**Indian Monsoon Impacts an Ongoing Warm Event**

## Indian Monsoon Impacts an Ongoing Cold Event

**Indian Monsoon Impacts an Ongoing Cold Event**

**Model**

## Indian Monsoon Impacts an Ongoing Cold Event

**Observations**

**Indian Monsoon Impacts an Ongoing Cold Event**

## Model

- Normal SST & Dry Monsoon

- Normal SST & Wet Monsoon

**Model**

## Observations

- Normal SST & Dry Monsoon

- Normal SST & Wet Monsoon

**Observations**

## Model

**Model**

**Indian Monsoon Impacts an Ongoing Warm Event**

**Warm-Dry minus Warm-Normal**

**Warm-Wet minus Warm-Normal**

**SST**

**hc**

**taux**

**Warm-Normal**

## Model

**Indian Monsoon Impacts an Ongoing Cold Event**

**Cold-Dry minus Cold-Normal**

**Cold-Wet minus Cold-Normal**

**SST**

**hc**

**taux**

**Cold-Normal**

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

## Summary IImpacts 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 Ocean

**DJF SST STD**

- Indian Ocean Coupled

- Indian Ocean De-Coupled

- Ratio De-Coupled/Coupled

## Indian Ocean Coupled

**Monthly SST STD 2S-2N**

- 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 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 |

|  | 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 ENSO

**Indian Ocean Impacts an Ongoing ENSO**

- PO warm

- PO cold

- IO normal

- IO cold minus normal

- IO warm minus normal

- Model

**Notes:**
Figure 4 Evolution of composite SST anomalies averaged over 2oS-2oN for warm&normal years (a), and warm&low minus warm&normal years (b) and warm&high minus warm&normal years (c) in the control run. (d)-(f) are similar to (a)-(c) except for cold years. Shading denotes SST anomalies over 0.1oC in (a) and (d), and significance levels as scaled by the color bar according to the Student t-test in (b)-(c) and (e)-(f). The time coordinate starts at January and ends at June in the following year. Refer to the text for details of the composite method.

## Warm Cases

- Cold Cases

- JJAS IO SSTA<0

- JJAS IO SSTA>0

- Observation

- JJAS

- DJF

## JJAS U850

- JJAS U200

- 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

- 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

**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 Winds

**Indian Ocean Impacts on Pacific Winds**

- Observed 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 IIImpacts 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

## Number of Cases for DJF NINO3.4 SSTA

|  | 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 |

- Warm Events

- Cold Events

- Coupled Model

- Observations

|  | high | normal | low |
| --- | --- | --- | --- |
| wet | 40 | 23 | 5 |
| normal | 19 | 14 | 6 |
| dry | 9 | 4 | 4 |

**IOSST**

**ISM**

**IOSST**

**IOSST**

**IOSST**

**ISM**

**ISM**

**ISM**

## Composite DJF NINO3.4 SSTA

|  | 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 |

- 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

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

- Forced Indian Ocean

- SST Variance Ratio (Forced/ Coupled)

## Coupled

- De-coupled/Coupled

- Forced/Coupled

- DJF SST Standard Deviation

## 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 IIIImportance 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.

## NINO3.4 SST Spectrum

- Coupled

- Indian Ocean De-coupled

- Indian Ocean Forced

- Observation

## Slide 42

## Slide 43

## Slide 44

## Slide 45

## Slide 46

## JJA(0)

- Strong Indian Monson

- Weak Indian Monson

- Enhanced Evaporation

- Enhanced Evaporation

- Reduced Evaporation

- Reduced Evaporation

- Reduced Moisture Convergence

- DJF(1)

- DJF(2)

- JJA(1)

- Cold Indian Ocean

- Warm Indian Ocean

- Warm Central Pacific

- Cold Central Pacific

- Warm Western Pacific

- Cold Western Pacific

- Air-Sea Interaction

- Air-Sea Interaction

- Anomalous Surface Easterly

- Anomalous Surface Westerly

- Strong WC

- Weak WC

- Weak WC

- Strong WC

- Warm Central Pacific

- Cold Central Pacific

**A**

- Enhanced Evaporation

- Reduced Short Wave

**C**

- ENSO Affects Indian Monsoon Transition in Two Ways:
- Modulation of Walker Circulation & A Rossby Wave Type Response

## C

- DJF

**C**

**C**

**A**

**A**

## Slide 49