Greenhouse warming and internal variability increase extreme and central Pacific El Nino frequency since 1980

El Nino has been recorded to change its properties since the 1980s, characterized by more common extreme El Nino and Central Pacific (CP) El Nino events. However, it is still unclear whether such change is externally forced or part of the natural variability. Here, we find that the frequency of the extreme and CP El Nino events also increased during the period 1875-1905, when the anthropogenic CO2 concentration was relatively lower, but with a positive phase of the Atlantic Multidecadal Oscillation (AMO). Models and palaeoclimate proxies reveal that a positive AMO enhances the zonal sea surface temperature gradient in the CP, which strengthens zonal advective feedback, favoring extreme and CP El Nino development. Moreover, we estimate that internal variability contributed to similar to 65% of the increasingly extreme and CP El Nino events, while anthropogenic forcing has made our globe experience similar to 1 more extreme and similar to 2 more CP events over the past four decades. ability.