Understanding the Southeast United States "Warming Hole": Forced Response or Internal Variability?

Abstract:

Observed summertime temperatures in the southeast United States did not change significantly from 1895 to 2011 despite the fact that temperatures averaged over the entire U.S. have increased by 0.65°C during this time. A variety of possible causes have been suggested to explain this southeast U.S. “warming hole”, including changes in Pacific and Atlantic sea surface temperatures (SSTs) due to internal variability, forcing from anthropogenic aerosols, and changes in local hydrology due to land-use change. Here we investigate some of these mechanisms using the GFDL-CM3 chemistry climate model, one of the CMIP5 models best able to capture the observed warming hole. We use the “aerosol only” and “greenhouse gas only” single forcing simulations to further develop our understanding of the potential drivers of the warming hole. We find that in both simulations, temperatures in the southeast U.S. exhibit a weaker response to anthropogenic forcing compared with the rest of the country, suggesting a preferred regional mode of response that is largely independent of the type of forcing. Increasing anthropogenic aerosols weaken the Bermuda High, reducing the transport of moisture into the southeast U.S. from the Gulf of Mexico. Accordingly, total cloud fraction, precipitation, and soil moisture content are also reduced. These changes are associated with an increase in the surface absorption of shortwave (SW) radiation, thus offsetting the expected cooling due to the radiative effects of the aerosols, and creating a “cooling hole” in the aerosol only simulation. In the greenhouse gas only simulation, there is an opposite-signed response, with increases in the Bermuda High strength and moisture flux into the southeast U.S, and decreases in the surface SW absorption. Finally, the impacts of Pacific and Atlantic SST changes, both forced and internal, on the simulated temperature in the southeast U.S. are also discussed.