Asymmetric Modulation of the Atlantic Multidecadal Oscillation on ENSO Amplitude

We investigate the asymmetric modulation of the Atlantic Multidecadal Oscillation (AMO) on El Niño-Southern Oscillation (ENSO) amplitude using long-term observational and reanalysis datasets. Results show that the modulation of the AMO on ENSO amplitude exhibits significant asymmetry in both eastern Pacific (EP) and central Pacific (CP) types and El Niño (El) and La Niña (La) events. Both CP-La and EP-El events are significantly strengthened during the negative AMO phase, while EP-La and CP-El events show no remarkable changes under different AMO phases. Using ocean mixed layer heat budget, we show that the thermocline feedback and net surface heat flux are crucial for the strengthening of CP-La, while zonal advective feedback and thermocline feedback favor the intensification of EP-El during the negative AMO phase. Further analyses reveal that during the negative AMO phase there are relatively colder sea-surface temperatures (SSTs) in the western equatorial Pacific with weakened trade wind, while warmer SSTs occur in the eastern equatorial Pacific with strengthened trade wind, leading to a weakening of zonal SST gradient. As a result, the thermocline deepens (shallows) with the negative (positive) Ekman pumping velocity (EPV) in the eastern (central) equatorial Pacific. These conditions collectively favor the strengthening of CP-La and EP-El events. During the negative AMO phase, the relatively dry background and the zonal wind over the tropical Pacific can lead to significant dry advection toward the central tropical Pacific, which enlarges the local sea-air humidity difference and promotes the release of latent heat flux from the ocean to strengthen CP-La.