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

1. 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

2. 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.

3. Indian Summer Monsoon-Indian Ocean-Pacific Ocean Interactions instantaneous ISM time lag Indian Ocean ENSO

4. Coupled Model Reproduces Observed Relationship IMR: 60-100E, 5-25N

5. Coupled Model Reproduces the Probability of ENSO Normal ISM Dry ISM Wet ISM

6. 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

7. 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

8. Indian Monsoon Impacts an Ongoing Warm Event Model

9. Indian Monsoon Impacts an Ongoing Warm Event Observations

10. Indian Monsoon Impacts an Ongoing Cold Event Model

11. Indian Monsoon Impacts an Ongoing Cold Event Observations

12. Model Normal SST & Wet Monsoon Normal SST & Dry Monsoon

13. Observations Normal SST & Dry Monsoon Normal SST & Wet Monsoon

14. Indian Monsoon Impacts an Ongoing Warm Event Model Warm-Normal Warm-Dry minus Warm-Normal hc taux SST Warm-Wet minus Warm-Normal

15. Indian Monsoon Impacts an Ongoing Cold Event Model Cold-Normal Cold-Dry minus Cold-Normal hc taux SST Cold-Wet minus Cold-Normal

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

17. 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

18. ENSO Variability is Reduced without Indian Ocean DJF SST STD Indian Ocean Coupled Indian Ocean De-Coupled Ratio De-Coupled/Coupled

19. Monthly SST STD 2S-2N Indian Ocean Coupled Indian Ocean De-Coupled Ratio De-Coupled/Coupled

20. 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

21. Number of years for different composites based on JJAS IO SST and JJAS NINO-3.4 SST COLA Model Observation

22. DJF NINO-3.4 SST anomalies for different composites COLA Model Observation 0.67 excluding 1877, 1982, 1997

23. Indian Ocean Impacts an Ongoing ENSO Model IO normal IO cold minus normal IO warm minus normal PO warm PO cold

24. Warm Cases Cold Cases Observation JJAS DJF JJAS IO SSTA<0 JJAS IO SSTA>0

25. 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

26. 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

27. 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

28. 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

29. Process Indian Ocean SSTA Atmospheric Heating Walker Circulation Equatorial Pacific Wind ENSO

30. 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

31. 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

32. Number of Cases for DJF NINO3.4 SSTA Coupled Model Observations Warm Events IOSST IOSST ISM ISM Cold Events IOSST IOSST ISM ISM

33. Composite DJF NINO3.4 SSTA Coupled Model Observations Warm Events IOSST IOSST ISM ISM Cold Events IOSST IOSST ISM ISM

34. Is specified Indian Ocean SST proper for ENSO? Importance of Indian Ocean Coupling for ENSO Variability SST Variance Ratio (Forced/ Coupled) Forced Indian Ocean

35. De-Coupled Indian Ocean Forced Indian Ocean

36. Lag-lead correlation wrt DJF NINO3.4 SST Indian Ocean coupled Indian Ocean forced Interfering effects of the Indian monsoon

37. 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

38. 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.