The Impact of Anthropogenic Aerosols on the Properties of the Indian Ocean Dipole

Abstract:

Over the twentieth century, the tropical Indian Ocean has exhibited a distinct boreal autumn warming that resembles a positive phase of the Indian Ocean dipole (IOD). The warming pattern is well captured by Coupled Model Intercomparison Project Phase 5 (CMIP5) models forced with long-lived greenhouse gases and anthropogenic aerosols. While greenhouse gases and aerosols generate offsetting warming and cooling patterns in the Indian Ocean, much less is known about their influence on IOD properties, particularly the negative sea surface temperature (SST) skewness in the eastern IOD (IODE) region. Using single forcing runs from nine CMIP5 models, we show that the SST negative skewness is more enhanced under aerosols than for greenhouse gases and is controlled by the SST response to thermocline variations. This coupling shows substantial asymmetry between warm and cool IODE conditions in simulations that only include anthropogenic aerosols, and is fed by strong thermocline coupling to equatorial winds. Vast intermodel differences in mean-state conditions exist such that the SST skewness is only moderately controlled by the climatological zonal gradients in SST, thermocline and precipitation across the tropical Indian Ocean. However, for the model that exhibits the strongest negative IODE SST skewness among the nine CMIP5 models, CSIRO-Mk3-6-0, the aerosol-induced mean-state conditions do control the strength of the Bjerknes feedback asymmetry and skewness. In future decades, the combination of a reduction in anthropogenic aerosol emissions and increasing greenhouse gases will most likely contribute to a weakening of the IODE SST skewness.