Pacific control of the Atlantic Multidecadal Oscillation - El Niño relationship in the Community Earth System Model - Large Ensemble Simulation

This study investigates the impacts of the interaction between Interdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Oscillation (AMO) on El Niño frequency and the associated atmosphere and ocean dynamics, by using the Community Earth System Model - Large Ensemble Simulation (CESM-LES). CESM-LES well reproduces the individual impacts of IPO and AMO on El Nino frequency and the physical atmosphere-ocean processes, which are consistent with previous studies. However, the effects of AMO on El Niño frequency are robust only during the negative IPO phase and very weak during the positive IPO. We further investigate this asymmetric AMO - El Niño relationship with respect to the IPO phase, which is termed as the IPO state-dependent impact of AMO. We found that the AMO-induced inter-ocean teleconnections are facilitated by the enhanced eastern Pacific mean state during the negative IPO phase and hindered by the weakened eastern Pacific mean state during the positive IPO phase. 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 enhance the negative IPO, which is unfavorable for El Niño occurrence. Conversely, the negative AMO suppresses the negative IPO, thus diminishing the suppressing effect of the negative IPO on El Niño activity. During the positive IPO phase of the weakened atmospheric mean state, the AMO-induced inter-ocean atmospheric teleconnections are much weaker; thus neither the positive nor negative AMO has any significant impact on El Niño occurrence. The results from CESM atmospheric general circulation model (AGCM) sensitivity experiments, which are forced by prescribed sea surface temperature anomalies (SSTAs) associated with the four interactive IPO and AMO phases, well explain this underlying mechanism of the asymmetric inter-ocean teleconnection. Our findings emphasize the importance of the eastern Pacific mean state in the AMO-induced inter-ocean atmospheric teleconnections on the El Niño frequency at decadal timescale.