The response of the large-scale ocean circulation to 20th century Asian and non-Asian aerosols

Multidecadal trends in the large-scale ocean circulation are influenced by changes in radiative forcings such as long-lived greenhouse gases, volcanic aerosols, and solar irradiance. Model simulations suggest that anthropogenic aerosols can also force circulation changes, including delaying the weakening of the North Atlantic thermohaline circulation, altering the interhemispheric sea surface temperature gradient, and inducing a pan-oceanic heat redistribution. The extent to which aerosols from different regions contribute to these oceanic changes is currently unknown. Using specifically designed 20th century coupled climate model experiments that separate Asian and non-Asian aerosol impacts, it is shown that the non-Asian aerosol component, predominantly sulfate aerosols, accounts for much of the simulated aerosol-induced oceanic changes. These include delaying the weakening of the global meridional circulation, increasing the northward heat transport across the equatorial Atlantic, and inducing a subsurface cooling in the subtropical southern Indian Ocean. As global sulfate aerosol levels peaked in the 1980s, these trends may be starting to reverse. This study highlights the importance of Northern Hemisphere non-Asian anthropogenic aerosols in driving remote changes in Southern Hemisphere subtropical and extratropical oceans.