Direct and semi-direct aerosol effects in the NASA GEOS-5 AGCM: aerosol-climate interactions due to prognostic versus prescribed aerosols

The present-day climate response to aerosol direct and semi-direct effects is investigated using the NASA Goddard Earth Observing System version 5 (GEOS-5) atmospheric general circulation model. We focus our investigation on aerosol-climate interactions by using either prognostic aerosols from an online aerosol module or aerosols from a climatology based on the prognostic aerosols. As found in previous studies, forcing from all aerosols cools the land surface, warms the troposphere, and impacts global mean circulation, affecting both the strength of the Hadley cell and the zonal mean wind. Less absorbing natural aerosol alone tends to have weaker impacts on global climate. We find that removing the feedback of meteorology on aerosol distributions can significantly impact the climate response depending on the parameter, region, and season considered. Much of the differing climate response to prognostic and prescribed aerosols occurs in regions remote from direct aerosol forcing, such as in the stratosphere and the northern and southern high latitudes. This suggests that aerosol-climate interactions may induce remote dynamical responses to aerosol forcing in global models. The largest effect of removing coupling is to enhance the aerosol optical depth globally over the oceans. This enhancement is due to the removal of the co-variability between aerosol mass and relative humidity on sub-monthly timescales in the high humidity oceanic environment.