Impact of Indo-Pacific Feedback Interactions on ENSO Dynamics Diagnosed Using Ensemble Climate Simulations

Abstract

The impact of Indo-Pacific climate feedback on the dynamics of El Niñ o-Southern Oscillation (ENSO) is investigated using an ensemble set of Indian Ocean decoupling experiments (DCPL), utilizing a millennial integration of a coupled climate model. It is found that eliminating air-sea interactions over the Indian Ocean results in various degrees of ENSO amplification across DCPL simulations, with a shift in the underlying dynamics toward a more prominent thermocline mode. The DCPL experiments reveal that the net effect of the Indian Ocean in the control runs (CTRL) is a damping of ENSO. The extent of this damping appears to be negatively correlated to the coherence between ENSO and the Indian Ocean dipole (IOD). This type of relationship can arise from the long-lasting ENSO events that the model simulates, such that developing ENSO often coincides with Indian Ocean basin-wide mode (IOBM) anomalies during non-IOD years. As demonstrated via AGCM experiments, the IOBM enhances western Pacific wind anomalies that counteract the ENSO-enhancing winds farther east. In the recharge oscillator framework, this weakens the equatorial Pacific air-sea coupling that governs the ENSO thermocline feedback. Relative to the IOBM, the IOD is more conducive for ENSO growth. The net damping by the Indian Ocean in CTRL is thus dominated by the IOBM effect which is weaker with stronger ENSO-IOD coherence. The stronger ENSO thermocline mode in DCPL is consistent with the absence of any IOBM anomalies. This study supports the notion that the Indian Ocean should be viewed as an integral part of ENSO dynamics.

Coherent Tropical Indo-Pacific Interannual Climate Variability

“…This suggests that a cool western IO is indeed followed by an enhanced WWV after a few months. This result cannot be explained by the IOBMbased mechanism proposed by Santoso et al (2012) and Izumo et al (2015), which only involves westerlies over the PO.…”

“…Annamalai et al (2005) suggest that a negative autumn IOD (including a cool western IO) does not by itself have a strong impact on the PO because its eastern and western poles are so close that their influence on PO wind cancels. However, Izumo et al (2015) suggest that a negative IOD is followed by a negative BWI in spring, which influences the PO by creating westerlies there (Santoso et al 2012;Annamalai et al 2005). According to Izumo et al (2015), the IOB-induced westerlies in spring are stronger if there was an IOD in the previous autumn.…”

“…Note that our concept of westward/eastward growth differs from that of Santoso et al (2013), who follow the propagation of the peak of El Niño in summer (year 1)-winter (year 1-2) whereas we are concerned with the initial stages of an event. The exact values of the thresholds were chosen to generate roughly equally large groups.…”

“…The concept of influence of the IO on the PO transmitted by large-scale atmospheric processes (atmospheric bridge) is more than a decade old (Clarke 2003;Annamalai et al 2005;Chen and Cane 2008) and has been thought to arise from the IOD (Izumo et al 2010b the IOBM (Santoso et al 2012), or a combination of both (Izumo et al 2015). Nevertheless it seems useful to investigate further the link between SD and ENSO because our correlation analysis in section 3 showed that the western IO contains more information on ENSO than the eastern IO.…”

“…Izumo et al (2010b) suggest that IO SST anomalies can influence large-scale wind patterns over the western PO (i.e., atmospheric bridge), which might explain the significant improvement of predictive skill of ENSO by taking the autumn Indian Ocean dipole (IOD) into account. In Santoso et al (2012) and Annamalai et al (2005), numerical experiments suggest that IO basinwide mode (IOBM) warming often associated with El Niño leads to easterly winds above the western PO, which in turn dampen the ongoing El Niño. Izumo et al (2015) use a simple numerical model to reconcile the IOD and IOBM mechanisms.…”