Enhanced joint impact of western hemispheric precursors increases extreme El Niño frequency under greenhouse warming

Sea surface temperature variability over the north tropical Atlantic (NTA) and over the subtropical northeast Pacific (SNP), which is referred to as the North Pacific Meridional Mode, during the early boreal spring is known to trigger El Nino-Southern Oscillation (ENSO) events. The future changes of the influence of those northwestern hemispheric precursors on ENSO are usually examined separately, even though their joint impacts significantly differ from the individual impacts. Here, we show that the impacts of both NTA and SNP on ENSO significantly increase under greenhouse warming and that the degrees of enhancement are closely linked. The wetter mean state over the off-equatorial eastern Pacific is a single contributor that controls the impacts of both NTA and SNP on ENSO. The enhanced joint impacts of the northwestern hemispheric precursors on ENSO increase the occurrences of extreme El Nino events and the ENSO predictability under greenhouse warming. Using output from climate models, this study shows that a wetter mean state over the off-equatorial eastern Pacific is the single key driver of the enhanced impact of northwestern hemispheric precursors on ENSO under anthropogenic global warming.