Reconstruction of Atmospheric Circulation Patterns


The Northern Hemisphere Jet, a high altitude narrow path of strong winds that meanders around the globe, is a key factor in the strength and perseverance of mid-latitude extreme weather events. Some studies have suggested that the jet may slow and change trajectory with ongoing and future climate change. Such behavior has been exemplified by the California drought, the eastern US polar vortex, and the winter storms in England in the winter of 2013-2014. Arctic amplification (the stronger warming of high Northern Hemisphere latitudes compared to lower latitudes) may drive jet stream variability in recent decades, but the role of man-made climate change in jet stream variability is contested and the instrumental period of measurement is too short to draw firm conclusions about current and future jet stream trends. Our current projects aim at: 1) reconstructing the latitudinal position of the jet over the last 500+ years using tree-ring records that will be used to put current jet stream trends in a historical perspective; 2) Examine the influence of the jet stream position and specifically the North pacific Jet on California fire regimes.

1)  NSF-CAREER-Tree-ring based reconstruction of decadal to centennial-scale Northern Hemisphere Jetstream variability
2) USGS-SWCSC-Influence of interannual North Pacific Jet variability on Sierra Nevada Fire regimes

Tree-ring analysis on the African continent

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Atlas cedar. Algeria-NW Africa.
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Miombo Woodlands, Mozambique
A growing concern about ongoing and projected climate change and its socio-economic and environmental implications raises questions regarding long-term natural and anthropogenically forced climate variability.  Improving our understanding of the climate system, its variability, and its sensitivity to external forcings (including anthropogenic, volcanic, and solar forcings) requires a longer time frame than instrumental data alone can offer. The development of more regional-scale climate reconstructions has been a primary IPCC AR4 recommendation in order to elucidate climate variability patterns and their association with forcings at the regional scale, where changes are most relevant to ecosystems and society. 
Our lab is involved in various projects aiming at developing climate (temperature and precipitation) reconstructions on the African continent using tree-ring width and other tree-ring parameters (isotopes, blue intensity). We are specifically developing: a temperature reconstruction in NW Africa using carbon isotopes from Cedrus atlantica trees and exploring the dendroclimatic potential of Widdringtonia cedarbergensis in southern Africa based on oxygen and carbon isotopes. Our lab in collaboration with Dr. Matt Therrell from the University of Alabama and the Africamuseum in Tervuren, Belgium is developing a tree-ring network in southern Africa where we collected samples in the Miombo woodlands of Tanzania, Mozambique, and Zambia.

Past California hydroclimate and wildfire regimes

California has been experiencing a severe drought since 2012 that has culminated in a 500-year snowpack low in 2015.  The California drought poses persistent threats to its economy and environment, including water shortages for urban and agriculture consumers, low hydroelectric power generation, and enhanced risk of wildfire and tree mortality. We use tree-ring data to reconstruct hydroclimate in California as well as historical fire regimes and fire-climate interactions.  We further evaluate the potential role of the North Pacific Jet in the production of extreme drought and fire years in California.  Associations between the jet stream and California hydroclimate and wildfire allow evaluation of potential changes in the likelihood of extreme drought and fire years through the rest of the 21st century as anthropogenic climate change progresses. 
Funded by USGS-Southwest Climate Science Center and NSF -GEOS CAREER grant
 



Estimating carbon flux and storage


Temperate forests account for almost 35% of the carbon sequestered globally each year, but the sequestration response to combined effects of changing climate and ecological pressures remain poorly quantified. The resilience of these forests to climate change will determine the extent to which temperate forests act as carbon sinks or sources.  The DOE funded research project: Estimating carbon flux and storage: constraint of the Community Land Model using observations at different temporal scales (PIs: David Moore & Valerie Trouet) uses tree rings collected at forested eddy-covariance sites associated with the Ameriflux network to generate aboveground biomass estimates.  We then use these biomass estimates to help constrain current carbon cycling models to better understand how trees are using the available carbon and generate a more realistic picture of the carbon pools and fluxes within various forest types.