Decadal modulation of the ENSO–East Asian winter monsoon relationship by the Atlantic Multidecadal Oscillation

Abstract

This work investigates the decadal modulation of the El Nino-Southern Oscillation (ENSO)–East Asian winter monsoon (EAWM) relationship by the Atlantic Multidecadal Oscillation (AMO). A stable ENSO–EAWM relationship is found during the positive AMO phase but not during the negative phase. While the impact of El Nino events on the EAWM does not depend on the AMO phase, a different picture is observed for La Nina events. The La Nina boreal winter season coincides with a strengthened EAWM during a positive AMO phase and a weakened EAWM during a negative AMO phase. We suggest that the AMO’s modulating effect mainly comprises two pathways that influence ENSO’s impact on the EAWM. On one hand, when La Nina coincides with a positive AMO, the warm SST anomalies over the western North Pacific (WNP) are amplified both in intensity and spatial extent, which favors strengthened WNP cyclonic anomalies and an enhanced EAWM. During La Nina with a negative AMO, only very weak SST anomalies occur over the WNP with reduced WNP cyclonic anomalies that are confined to the tropics, thus having little effect on the EAWM. On the other hand, an eastward-propagating Rossby wavetrain across the mid-high latitudes of Eurasia during a warm AMO phase strengthens the Siberian high and thus leads to a strengthened EAWM, while during a cold AMO phase the Siberian high is weakened, leading to a reduced EAWM. In contrast, El Nino and its associated atmospheric responses are relatively strong and stable, independent of the AMO phase. These results carry important implications to the seasonal-to-interannual predictability associated with ENSO.