Response of the northward heat transport depends on regional anthropogenic aerosol effects in CMIP6

Past decades witnessed strong spatial changes in the emissions of anthropogenic aerosols and their precursors resulting in a global redistribution of maxima in the anthropogenic aerosol optical depth. This study investigates the response of the circulation to the different anthropogenic aerosol patterns around the 1970s and 2000s with focus on the meridional heat transport. Our analysis uses 309 historical model experiments of the Coupled Model Intercomparison Project Phase 6 and 94 single-forcing experiments for anthropogenic aerosols from the Detection and Attribution Model Intercomparison Project (DAMIP). We substantially reduce the influence of internal variability by computing multi-model multi-realization means and additional averaging over time periods of 15 years. The results highlight the influence of anthropogenic aerosol radiative effects on the total northward heat transport. Around the 1970s, most anthropogenic aerosols were located over Europe, North America and the North Atlantic. At that time, the anthropogenic aerosol increase explains almost half of the total change in the summertime northward heat transport in the tropics compared to pre-industrial times. In polar regions, the anthropogenic aerosols around the 1970s counteracted the induced response of the northward heat transport to greenhouse gas forcing. It suggests that changes induced by the aerosol pattern until the 1970s delayed the increase in Arctic warming in CMIP6, later known as Arctic amplification. The later change in the anthropogenic aerosol pattern between the 1970s and the 2000s led to different hemispheric asymmetries in the anthropogenic aerosol optical depth and hence the reflected shortwave radiation. Due to the associated different regional radiative effect, the change in the summertime northward heat transport in the polar region is now qualitatively similar for anthropogenic aerosols and greenhouse gas forcings for the 2000s against the 1970s. Specifically, the heat transport to the Arctic during summer increases for the 2000s compared to the 1970s consistent with emergence of Arctic amplification in the late 1970s.