The Effects of Light-Absorbing Aerosols, Blocking, and Clouds on Greenland's Surface

Abstract

The Greenland Ice Sheet covers over 80% of Greenland’s land area and is the largest reservoir of frozen water in the Northern Hemisphere. Over the past few decades, the Greenland Ice Sheet has experienced significant summertime ice and surface snow loss. Much of this mass loss can be attributed to short-term processes that initiate surface energy changes. In this dissertation, we examine the effects of 1) light-absorbing aerosols on the Greenland Ice Sheet and in the overlying atmosphere, 2) the impacts of atmospheric block location over the Greenland Ice Sheet on cloud formation and atmosphere-surface energy exchanges, and 3) how these block-induced surface energy responses will change in the future. Aerosols are microscopic solid or liquid particles that can be transported long distances through the atmosphere. Light-absorbing aerosols like black carbon and certain types of dust are unique in that they absorb sunlight and warm the local environment. In the Arctic, black carbon reduces surface reflectivity and increases surface energy to induce or enhance melt. Light- absorbing aerosols suspended in the atmosphere warm the local air and change vertical circulation patterns. We use an idealized version of the Community Earth System Model (CESM) to understand how Greenland-specific local atmospheric and in-snow light-absorbing aerosols can affect Greenland Ice Sheet surface energy input and snowmelt processes. Overall, we find that the largest snowmelt and net surface energy responses occur in simulations containing only in-snow light-absorbing aerosols while atmospheric absorbing aerosols decrease incident sunlight on the surface and produce insignificant melt and energy changes. Atmospheric and in-snow aerosols have offsetting effects on the surface energy budget. Atmospheric blocks are mostly stationary high pressure systems that last for days to weeks at a time. Blocking over Greenland has been shown to enhance net surface energy by reducing cloud cover and transporting warm, moist air over the surface. The variability of the Greenland Ice Sheet topography impacts regional block-induced surface changes based on block location. We use a variety of observational datasets to understand the impacts of block location on cloud formation and surface energy fluxes over the Greenland Ice Sheet. Blocking over northern Greenland produces larger cloud reductions that enhance sunlight and decrease net longwave energy at the surface than southern Greenland blocking activity. Net surface energy increases for all block locations, but more so for blocks over eastern Greenland because of greater warm air transport in combination with enhanced sunlight absorption by the Greenland Ice Sheet. Five different global climate simulations from the Coupled Model Intercomparison Project, phase 6 (CMIP6) are examined to understand how summer Greenland blocking activity will affect net surface energy fluxes in the middle (2040-2059) and at the end (2080-2099) of the 21st century for the largest warming scenario. Net surface energy input increases with blocking over all simulations. However, atmospheric warming over the Greenland Ice Sheet produced by block circulation plays a larger role in total energy enhancement than solar energy input changes. Future net surface energy fluxes may be larger because historical (1986-2005) CMIP6 simulations underestimate component and total energy fluxes found in Modern Era Retrospective Analysis, version 2 (MERRA-2) observational data. These results indicate that Greenland blocking will continue to contribute to increased total surface energy in the future.