Impact of Indo-Pacific feedback interactions on ENSO

12 The impact of Indo-Pacific climate feedback on the dynamics of El Niño Southern Oscil13 lation (ENSO) is investigated using an ensemble set of Indian Ocean decoupling experiments 14 (DCPL), utilising a millennial integration of a coupled climate model. It is found that elim15 inating air-sea interactions over the Indian Ocean results in stronger ENSO variability with 16 various degrees of amplification across all experiments. This is accompanied by a shift in 17 the underlying ENSO dynamics toward a more prominent thermocline mode that stems 18 from stronger air-sea coupling in the equatorial Pacific. The stronger coupling results from 19 weaker western Pacific zonal wind anomalies that generally act as a negative feedback on 20 ENSO. The decoupling experiments reveal that the net effect of the Indian Ocean in the 21 control runs (CTRL) is a damping on ENSO. The extent of this damping appears to be 22 negatively correlated to the coherence between ENSO and the Indian Ocean Dipole (IOD). 23 During non-IOD years in CTRL, developing ENSO tends to coincide with Indian Ocean 24 basin-wide mode (IOBM) anomalies. As demonstrated via AGCM experiments, the IOBM 25 enhances western Pacific wind anomalies that weaken the ENSO-enhancing winds farther 26 east. Relative to the IOBM, the IOD is more conducive for ENSO growth. The net damping 27 by the Indian Ocean in CTRL thus stems from the dominating effect of the IOBM which 28 is weaker with stronger ENSO-IOD coherence. The stronger ENSO thermocline mode in 29 DCPL is consistent with the absence of any IOBM anomalies. This study supports the 30 notion that the Indian Ocean should be viewed as an integral part of ENSO dynamics. 31