The Indian Ocean Sea Surface Temperature Warming Simulated by CMIP5 Models during the Twentieth Century: Competing Forcing Roles of GHGs and Anthropogenic Aerosols

Abstract

Abstract The Indian Ocean exhibits a robust basinwide sea surface temperature (SST) warming during the twentieth century that has affected the hydrological cycle, atmospheric circulation, and global climate change. The competing roles of greenhouse gases (GHGs) and anthropogenic aerosols (AAs) with regard to the Indian Ocean warming are investigated by using 17 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The increasing GHGs are considered to be one reason for the warming. Here model evidence is provided that the emission of AAs has slowed down the warming rate. With AAs, the warming trend has been slowed down by 0.34 K century−1. However, the cooling effect is weakened when only the direct aerosol effect is considered. GHGs and AAs have competed with each other in forming the basinwide warming pattern as well as the equatorial east–west dipole warming pattern. Both the basinwide warming effect of GHGs and the cooling effect of AAs, mainly through indirect aerosol effect, are established through atmospheric processes via radiative and turbulent fluxes. The positive contributions of surface latent heat flux from atmosphere and surface longwave radiation due to GHGs forcing dominate the basinwide warming, while the reductions of surface shortwave radiation, surface longwave radiation, and latent heat flux from atmosphere associated with AAs induce the basinwide cooling. The positive Indian Ocean dipole warming pattern is seen in association with the surface easterly wind anomaly during 1870–2005 along the equator, which is produced by the increase of GHGs but weakened by AAs via direct aerosol effects.