Pacific Mean-State Control of Atlantic Multidecadal Oscillation-El Nino Relationship

We investigate the potential impacts of the interdecadal Pacific oscillation (IPO) and Atlantic multidecadal oscillation (AMO) on El Nino and the associated atmosphere and ocean dynamics by using the Community Earth System Model-Large Ensemble Simulation (CESM-LENS). The individual effects of IPO and AMO on El Nino frequency and the underlying atmosphere-ocean processes are well reproduced in CESM-LENS and agree with previous studies. However, the sensitivity of El Nino frequency to the AMO is robust mainly during the negative IPO phase and very weak during the positive IPO phase. Further analysis suggests that the atmospheric mean state in the Pacific is much amplified during the negative IPO phase, facilitating the AMO-induced interocean atmospheric teleconnections. More specifically, during the negative IPO phase of the amplified mean state, the positive AMO enhances ascending motion from the northeastern Pacific, which in turn increases subsidence into the southeast Pacific through local anomalous Hadley circulation. The associated low-level easterly wind anomalies in the central equatorial Pacific are also reinforced by amplified upper-level divergence over the Maritime Continent to enhance the negative IPO, which is unfavorable for El Nino occurrence. Conversely, the negative AMO nearly cancels out the suppressing effect of the negative IPO on El Nino occurrence. During the positive IPO phase of the weakened atmospheric mean state, however, the AMO-induced interocean atmospheric teleconnections are much weaker; thus, neither the positive nor the negative AMO has any significant impact on El Nino occurrence.