Drought in the Southeastern United States: Causes, Variability over the Last Millennium, and the Potential for Future Hydroclimate Change*

Abstract

An assessment of the nature and causes of drought in the southeastern United States is conducted as well as an assessment of model projections of anthropogenically forced hydroclimate change in this region. The study uses observations of precipitation, model simulations forced by historical SSTs from 1856 to 2007, tree-ring records of moisture availability over the last millennium, and climate change projections conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. From the perspective of the historical record, the recent drought that began in winter 2005/06 was a typical event in terms of amplitude and duration. Observations and model simulations are used to show that dry winter half-years in the Southeast are weakly associated with La Niñ as in the tropical Pacific but that this link varies over time and was possibly of opposite sign from about 1922 to 1950. Summer-season precipitation variability in the Southeast appears governed by purely internal atmospheric variability. As such, model simulations forced by historical SSTs have very limited skill in reproducing the instrumental record of Southeast precipitation variability and actual predictive skill is also presumably low. Tree-ring records show that the twentieth century has been moist from the perspective of the last millennium and free of long and severe droughts that were abundant in previous centuries. The tree-ring records show a 21-yr-long uninterrupted drought in the mid-sixteenth century, a long period of dry conditions in the early to mid-nineteenth century, and that the Southeast was also affected by some of the medieval megadroughts centered in western North America. Climate model projections predict that in the near term, future precipitation in the Southeast will increase but that evaporation will also increase. The median of the projections predicts a modest reduction in the atmospheric supply of water vapor to the region; however, the multimodel ensemble exhibits considerable variation, with a quarter to a third of the models projecting an increase in precipitation minus evaporation. The recent drought, forced by reduced precipitation and with reduced evaporation, has no signature of model-projected anthropogenic climate change.

A critical overview of progress in studies of migration of dragonflies (Odonata: Anisoptera), with emphasis on North America

“…Obviously, serious drought or changes in rainfall patterns, especially in destination regions, might affect the pattern and overall success of the migrant strategy. Seager et al (2009) modeled precipitation in the Southeast up to 2100 C.E. and found little likelihood of severe drying.…”

Tropical Cyclones and Drought Amelioration in the Gulf and Southeastern Coastal United States

“…While ENSO has been shown to influence TC activity (Gray et al 1994), our analysis did not find ENSO to have a significant influence on TCDB occurrence. We suspect this is because the primary drought-related outcomes in the southeastern United States associated with eastern tropical Pacific SSTs are operative principally in the winter months (December-February), outside of the hurricane season (Seager et al 2009;Ortegren et al 2011).…”

“…Land-falling tropical cyclones (TCs; tropical depressions, tropical storms, and hurricanes) and droughts have well-documented economic and ecological impacts in the southeastern United States (e.g., Seager et al 2009;Nogueira andKeim 2010, 2011;Ortegren et al 2011). Most of the societal impacts of these events are perceived negatively, as both TCs and droughts often impose high economic costs.…”

Do High-Elevation Northern Red Oak Tree-Rings Share a Common Climate-Driven Growth Signal?

“…Disentangling the apparent time-dependent mixed temperature-precipitation signal retained in high-elevation northern red oak ring-width represents a challenge that will likely require an intra-seasonal examination of vesselwood-latewood variance, especially relationships to seasonal climate variability and/or possible frequency-dependent climate responses contained therein (e.g., Gonzalez and Eckstein, 2003;Fonti et al, 2009). It is important to note that although future hydroclimate change remains uncertain for the southeastern United States in response to climatic warming (e.g., Seager et al, 2009), this study points out that temperature change across the Southern Appalachian region may have differential effects on winter and summer precipitation seasonality, and if so, tree-ring records may have difficulty resolving this change clearly.…”