Contrasting the roles of regional anthropogenic aerosols from the western and eastern Hemispheres in driving the 1980–2020 Pacific multi-decadal variations

Abstract. The multi-decadal variations of the Pacific climate are extensively discussed as being influenced by external forcings such as greenhouse gases (GHGs) and anthropogenic aerosols (AA). Unlike GHGs, the potential impacts of AA could be more complex because of the heterogeneity of spatial distribution during the past few decades. Here we show, using regional aerosol forcing large ensemble simulations based on CESM1, that the increasing fossil fuel-related aerosol emission over Asia (EastFF) and the reduction in aerosol emission over North America and Europe (WestFF) have remarkably different impacts in driving the Pacific circulations and SST changes since the 1980s. EastFF excites a typical El Niño-like SST pattern in the tropical Pacific and weakens the climatological Pacific Walker Circulation. WestFF induces a CP-type El Niño-like SST pattern with warming at middle region of the equatorial Pacific, which is consistent with the 2nd leading EOF pattern of the observation. Over the North Pacific region, EastFF, located at low-to-mid latitudes, favors an IPO-like SST pattern through a teleconnection pathway between tropical and extratropical Pacific but is overwhelmed by internal variability evolving from a positive phase to a negative IPO phase. In contrast, WestFF, located at mid-to-high latitudes, strongly affects the North Pacific via a west-to-east mid-latitude pathway and induces extensive warming. The competing effects of the heterogeneously distributed regional aerosol forcings are expected to be changed in the near future, which is likely to introduce opposite and more profound impacts of aerosol forcing on the Pacific multi-decadal changes.