Imprint of Historical Anthropogenic Emissions on the Subseasonal Variability of the Indian Summer Monsoon

Abstract:

The Indian Summer Monsoon system affects the lives of over a billion people, the majority of whom who depend on agricultural activities for their livelihood. During the monsoon season, the region experiences wet and dry spells associated with multiple modes of intraseasonal variability. Such subseasonal hydroclimatic extremes have important socio-economic implications. Based on 60 years of observational data, we will present evidence to show that the characteristics of these wet and dry spells have changed significantly over the historical period. We use targeted single forcing experiments with the NOAA Geophysical Fluid Dynamics Laboratory CM3 coupled model to investigate the separate effect of aerosols and greenhouse gases on the observed long-term trends. The simulations consist of three-member ensemble experiments forced only by time-evolving anthropogenic aerosols, greenhouse gases, and natural forcings, and a five-member experiment with all forcings (natural and anthropogenic). Using these simulations, we show that anthropogenic aerosols and greenhouse gases have had a substantial effect on total rainfall and subseasonal variability during the peak monsoon season, respectively. We will also discuss how increasing aerosols and greenhouse gas concentrations have influenced the wet and dry spell characteristics, and explain the physical mechanisms responsible for such changes.