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

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, Wu, R., and B. P. Kirtman, 2004: Understanding the impacts of the Indian Ocean on ENSO variability in a coupled GCM. J. Climate, 17, Wu, R., and B. P. Kirtman, 2005: Roles of Indian and Pacific Ocean air-sea coupling in tropical atmospheric variability. Clim. Dyn., 25,

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

Coupled Model Reproduces Observed Relationship IMR: E, 5-25N

Coupled Model Reproduces Observed Relationship IMR: E, 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-WetNormal-WetWarm-Wet Normal (|IMRA|<0.43  IMR ) Cold-NormalNormal-NormalWarm-Normal Dry (IMRA<-0.43  IMR ) Cold-DryNormal-DryWarm-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 ColdNormalWarm Wet Normal Dry Observation ColdNormalWarm Wet Normal Dry 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 Event Warm-Dry minus Warm-Normal Warm-Wet minus Warm-Normal SST hctaux Warm-Normal

ModelIndian Monsoon Impacts an Ongoing Cold Event Cold-Dry minus Cold-Normal Cold-Wet minus Cold-Normal SST hctaux 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

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 composites based on JJAS IO SST and JJAS NINO-3.4 SST ColdNormalWarm High Normal Low ColdNormalWarm High71421 Normal20 13 Low28914 COLA Model Observation

DJF NINO-3.4 SST anomalies for different composites COLA Model Observation ColdNormalWarm High Normal Low ColdNormalWarm High Normal Low excluding 1877, 1982, 1997

Indian Ocean Impacts an Ongoing ENSO PO warm PO cold IO 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 U200 JJAS 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 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 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

highnormallow wet532 normal434 dry11610 Highnormallow wet51016 normal684 dry332 highnormallow wet456 normal4107 dry11236 Number of Cases for DJF NINO3.4 SSTA Warm Events Cold Events Coupled ModelObservations highnormallow wet40235 normal19146 dry944 IOSST ISM IOSST ISM

highnormallow wet normal dry highnormallow wet normal dry highnormallow wet normal dry Composite DJF NINO3.4 SSTA Warm Events Cold Events Coupled ModelObservations highnormallow wet normal dry 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 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

Strong Indian Monson Weak Indian Monson Enhanced Evaporation Reduced Evaporation Reduced Moisture Convergence JJA(0) DJF(1) DJF(2) JJA(1) Cold Indian Ocean Warm Indian OceanWarm Central Pacific Cold Central PacificWarm Western Pacific Cold Western Pacific Air-Sea Interaction Anomalous Surface Easterly Anomalous Surface Westerly Strong 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

DJF C C A A