Modulation of interdecadal Pacific oscillation on the relationship between El Niño–Southern Oscillation and the Southern Annular Mode during austral summer

Abstract The relationship between El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) during austral summer is examined. It is found that their relationship is nonstationary and depends on the phase of the Interdecadal Pacific Oscillation (IPO). A strong ENSO–SAM relationship is observed during the positive IPO phase, while this relationship is weak during the negative IPO phase. The effects of sea surface temperature anomalies (SSTA) in the equatorial central/eastern Pacific atmospheric stationary wave train, and synoptic‐scale high‐frequency eddies are found to be responsible for this interdecadal change in ENSO–SAM relationship. During the positive IPO phase, warm SSTA in the equatorial eastern Pacific associated with El Niño events induce a poleward‐propagating wave train and cause an anomalous anticyclone over Antarctica. The anomalous baroclinicity to the north of the anomalous anticyclone is conducive to the eastward extension of eddy activity within the entrance of the midlatitude jet stream, resulting in the development and maintenance of the negative SAM phase. However, during the negative IPO phases, the tropical SSTA centers during ENSO events shift toward the equatorial central Pacific, forcing the Rossby wave train that generates an anomalous anticyclone over the Ross/Amundsen Sea, to the north of that caused by ENSO during the positive IPO phase. Consequently, the anomalous baroclinicity does not align with the midlatitude jet stream core, and the eddy–mean flow interaction at the jet stream cannot be effectively triggered, inducing a meridionally arched pattern confined to the Pacific–South American sector. Additionally, when the IPO and ENSO are out of phase (in phase), the superposition effect tends to amplify (dampen) the ENSO–SAM connection.