Asymmetrical Modulation of the Relationship Between the Western Pacific Pattern and El Nino-Southern Oscillation by the Atlantic Multidecadal Oscillation in the Boreal Winter

Tropical atmospheric convection generated by the El Nino-Southern Oscillation (ENSO) plays a crucial role in affecting the western Pacific pattern (WP) in the boreal winter by triggering an atmospheric teleconnection. Here we show from analysis of observations and model simulations that the Atlantic Multidecadal Oscillation (AMO) asymmetrically modulates the relationship between ENSO and WP. We find a significant modulatory effect of AMO on the relationship between wintertime El Nino and WP. A robust El Nino-WP relation can be attributed to the negative AMO phase (-AMO), yet a weak relationship during the positive AMO phase (+AMO). In contrast, the relationship between La Nina and WP is independent of AMO modulation. Furthermore, during the -AMO period, stronger El Nino amplitudes lead to stronger atmospheric convection anomalies over the tropical western North Pacific, which excites stronger atmospheric teleconnection and thus has a more significant effect on WP than during the +AMO period.

Automated summary

The Atlantic Multidecadal Oscillation (AMO) asymmetrically modulates the relationship between El Nino and the Western Pacific pattern (WP). The modulatory effect is significant during the negative AMO phase, but weak during the positive AMO phase. The relationship between La Nina and WP, however, is independent of AMO modulation. Furthermore, stronger El Nino amplitudes during the negative AMO period have a more significant effect on WP due to stronger atmospheric convection anomalies over the tropical western North Pacific.