Interdecadal Modulation Of AMO On Atmospheric Anomalies In The Tropical Pacific-Indian Ocean Region During ENSO Years

Previous studies have demonstrated that the Atlantic Multidecadal Oscillation(AMO)could affect the El Ni?o-Southern Oscillation(ENSO)through air-sea dynamic process,with a stronger ENSO sea surface temperature(SST)amplitude during a negative AMO phase than during a positive phase.In this study,reanalysis data and simulation data are used in this work to investigate the modulating effect of AMO on the ENSO-related atmospheric anomalies amplitude in the tropical Indo-Pacific.We find that the ENSO-related atmospheric anomaly amplitudes in the tropical Indo-Pacific during different AMO phases are not necessarily consistent with these ENSO SST changes.The primary conclusions are summarized as follows:(1)We emphasize an Atlantic multidecadal(i.e.,AMO)has a significant interdecadal modulation effect on the intensity of the tropical Pacific atmospheric anomaly in ENSO years.For El Ni?o episodes,the low-level wind and precipitation anomalies over the tropical Pacific in the boreal winter are more pronounced during the negative AMO phase than during the positive phase,corresponding well to the stronger SST anomalies.However,La Ni?a events during the negative AMO phase are accompanied by weaker atmospheric anomalies in the tropical Pacific,although their SST anomalies are stronger than those during the positive phase.We suggest that this mismatch between La Ni?a SST and atmospheric anomalies can be largely attributed to AMO decadal modulation.A positive(negative)AMO favors lead anomalous ascending(descending)flow over the tropical Atlantic and western tropical Pacific,anomalous descending(ascending)flow over the tropical central and eastern Pacific,which intensified(weakened)trade winds and weakened(intensified)precipitation in the central tropical Pacific by modifying Walker circulation.Therefore,when La Ni?a coincides with a positive AMO,the low-level easterly and negative precipitation anomalies are superimposed,which gives rise to stronger atmospheric perturbations.For the El Ni?o event,"superposition" and "cancellation" occurred in the negative phase and positive phase of AMO,respectively.Since the SST anomaly of El Ni?o is stronger when the AMO is in negative phase,the modulation effect of AMO is further amplified the difference of tropical Pacific atmospheric anomalies in El Ni?o years under different AMO phases.(2)Similarly,AMO also has a significant interdecadal modulation effect on the relationship between ENSO and CWIP in ENSO years.It is,for El Ni?o episodes,the CWIP anomaly in the boreal winter are more pronounced during the negative AMO phase than during the positive phase,corresponding well to the stronger SST anomalies.However,La Ni?a events during the negative AMO phase are accompanied by weaker CWIP anomaly,although their SST anomalies are stronger than those during the positive phase.It is suggested that the AMO's modulating can also influence ENSO's linkage with the CWIP in interdecadal time scale.AMO is associated with a multidecadal atmospheric variation in the Walker circulation.The circulation during the positive(negative)AMO phase favors an anomalous ascending(descending)flow over the tropical eastern Indian Ocean-west Pacific and an anomalous descending(ascending)flow over the central-western Indian Ocean,thereby favoring an enhanced CWIP there.Therefore,El Ni?o is accompanied by a more pronounced CWIP during the negative AMO phase compared to that during a positive AMO phase.For La Ni?a episodes,however,these two pathways have opposite modulation effects.Although AMO+/La Ni?a is weaker than AMO-/La Ni?a,the accompanied CWIP is relatively stronger as an multidecadal dry condition induced by the Atlantic warming reinforces the negative CWIP anomaly induce by La Ni?a.For the El Ni?o events,similar to the Pacific,the AMO-induced decadal modulation further amplifies the strength differences of the El Ni?o CWIP anomalies under different AMO phases.