| Summary: | <p>Precipitation plays a fundamental role in transferring water and energy in the climate system. Human activities significantly impact the hydrological cycle, but the magnitude of global and regional changes remains uncertain due to the complex nature of microphysical-dynamical interactions. This thesis aims to improve the understanding of precipitation responses to climate change through a combination of bottom-up (process-driven) and top-down (energetic) approaches. </p>
<p>The relationship between aerosol-induced changes in atmospheric energetics and precipitation responses across different scales is studied in terms of fast (radiatively or microphysically mediated) and slow (temperature-mediated) responses. This thesis proposes an energetic framework to decompose rainfall changes into contributions from clouds, aerosols, and clear-clean sky (without aerosols and clouds). It provides a way to better interpret and quantify the precipitation changes caused by aerosol perturbations. Further investigations show that the increase rate in global-mean precipitation with temperature (hydrological sensitivity; η) is dependent on the spatial pattern of sea surface temperature (SST) change, which has been overlooked before. Warming in strong tropical ascending regions can produce large η via enhanced global circulation and atmospheric radiative cooling. The warming pattern effect significantly contributes to the spread of η in the current generation of general circulation models (GCMs). After accounting for the pattern effect, the reconstructed global-mean precipitation agrees much better with observations, evidencing the importance of SST patterns on global-mean precipitation changes. Furthermore, we extend the analysis to regional rainfall by showing that anthropogenic aerosols modulate the Sahel rainfall variability through their impacts on regional SST. We show analysis supporting the chain of processes whereby changes in anthropogenic aerosol emissions alter net radiative fluxes and sea surface temperature variability in the North Atlantic Ocean, leading to a shift of the intertropical convergence zone (ITCZ), changes in the West African monsoon, and eventually changes in Sahel rainfall. These findings highlight the importance of accurate representation of regional aerosol radiative effects for the simulation of Sahel rainfall variability.</p>
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