Recent studies suggest that the tropics have expanded from ~2-5° over the past few decades and that this widening may continue into the future in association with global climate change. This expansion has potentially important implications for subtropical societies that include profound changes in the hydrological cycle and large-scale atmospheric circulation. Consistent with theory, models forced by greenhouse gases (GHG's) produce this expansion, however, some measurements have showed that observed tropical expansion is significantly larger than simulations of the 20th and 21st century from the Coupled Model Intercomparison Project version 3 (CMIP3). Recent climate simulations have shown that direct heating of the troposphere and surface, such as that caused by absorbing aerosols can drive expansion.

In this study, we quantify future changes in the tropical belt width using CAM3 and the 4 Representative Concentration Pathway's (RCP's) through the end of the 21st century. The RCP scenarios all assume stringent emissions controls on aerosols and their precursors, and hence include progressive decreases through the 21st century. Changes in tropical belt width are analyzed using 5 metrics. We find that the global, annual expansion rates vary from .09 +/- .05º and .14 +/- .03º decade^-1 for GHG and SO2 forcings respectively under RCP 4.5. Furthermore, Northern hemisphere (NH) expansion rates vary from .05 +/- .04º and .12 +/- .02º decade^-1 for GHG and SO2 forcings respectively under RCP 4.5. As widening increases with RCP, SO2 still accounts for as much widening as GHG's. Given the large reductions in future sulfate emissions with all RCP's reaching preindustrial values by 2100, we find that sulfate aerosols may be just as dominant a driver of tropical expansion in the northern hemisphere as GHG's through the end of the 21st century.

This dissertation is motivated by the lack of research depth on the influence biomass burning produced aerosols impose on West African monsoon dynamics during daily time scales. The impacts of such aerosols on atmospheric thermodynamics and cloud microphysics are investigated using a combination of satellite observations and global climate model simulations.

Using observations, we find (1) suppression of precipitation rates over the Guinea coastline with elevated aerosol transport and (2) an increase in rates with decreased aerosol amounts. Decreases in the amount of shortwave radiation reaching the surface over land and ocean attributed to increases in low-level cloud coverage and the absorptive nature of biomass aerosols observed in nature on dirty conditions are matched with reductions in convective available potential energy (CAPE), vertical air mixing essential for deep-convection cloud cover. Increased aerosol content is also associated with decreases in cloud droplet effective radius, a defining feature of aerosol indirect effects.

Guided by available observations, model experiments are designed for NCAR’s CESM2 with meteorology being nudged to observations using MERRA2 reanalysis to visualize data not available through such observations. Simulations successfully reproduce aerosol transport and corresponding precipitation changes found in observations. However, this model is not in agreement with changes in low-level cloud fraction which leads to increases (decreases) in shortwave radiation reaching the surface (top of atmosphere) during dirty conditions. Our model simulations show that aerosol semidirect and indirect effects interact together to alter cloud formation processes and ultimately control the precipitation response.

Throughout the equatorial Atlantic, smoke aerosols impact the structure of stratocumulus to cumulus transition (SCT) through an alteration of atmospheric stability and moisture availability. Boundary layer deepening and increasing sea surface temperatures put the location of this transition within the Gulf of Guinea. Increased low-level clouds occur over the Atlantic cold tongue where aerosol layers reside above low cloud tops, reflecting a negative aerosol semidirect effect. Coupled with significant changes in cloud top height and tropospheric stability further South, these aerosol effects combine to extend in space during increased aerosol loading episodes.