title: Short-term favorable weather conditions are an important control of interannual variability in carbon and water fluxes.
abstract: Ecosystem models often perform poorly in reproducing interannual variability in carbon and water fluxes, resulting in considerable uncertainty when estimating the land-carbon sink. While many aggregated variables (growing season length, seasonal precipitation, or temperature) have been suggested as predictors for interannual variability in carbon fluxes, their explanatory power is limited and uncertainties remain as to their relative contributions. Recent results show that the annual count of hours where evapotranspiration (ET) is larger than its 95th percentile is strongly correlated with the annual variability of ET and gross primary production (GPP) in an ecosystem model. This suggests that the occurrence of favorable conditions has a strong influence on the annual carbon budget. Here we analyzed data from eight forest sites of the AmeriFlux network with at least 7 years of continuous measurements. We show that for ET and the carbon fluxes GPP, ecosystem respiration (RE), and net ecosystem production, counting the "most active hours/days" (i.e., hours/days when the flux exceeds a high percentile) correlates well with the respective annual sums, with correlation coefficients generally larger than 0.8. Phenological transitions have much weaker explanatory power. By exploiting the relationship between most active hours and interannual variability, we classify hours as most active or less active and largely explain interannual variability in ecosystem fluxes, particularly for GPP and RE. Our results suggest that a better understanding and modeling of the occurrence of large values in high-frequency ecosystem fluxes will result in a better understanding of interannual variability of these fluxes.
authors: Institute for Atmospheric and Climate Science ETH Zurich Zurich Switzerland.
authors: Jakob
authors: J
authors: Zscheischler
authors: Institute of Environmental Engineering ETH Zurich Zurich Switzerland.
authors: Simone
authors: S
authors: Fatichi
authors: Institute of Agricultural Sciences ETH Zurich Zurich Switzerland.
authors: Sebastian
authors: S
authors: Wolf
authors: Department of Geography University of Colorado Boulder Boulder Colorado USA.
authors: Peter D
authors: PD
authors: Blanken
authors: Department of Civil, Environmental and Geodetic Engineering Ohio State University Columbus Ohio USA.
authors: Gil
authors: G
authors: Bohrer
authors: USDA Forest Service Northern Research Station New Lisbon New Jersey USA.
authors: Kenneth
authors: K
authors: Clark
authors: Department of Atmospheric and Oceanic Sciences University of Wisconsin-Madison Madison Wisconsin USA.
authors: Ankur R
authors: AR
authors: Desai
authors: USDA Forest Service Northern Research Station Durham New Hampshire USA.
authors: David
authors: D
authors: Hollinger
authors: Lawrence Berkeley National Lab Berkeley California USA.
authors: Trevor
authors: T
authors: Keenan
authors: School of Public and Environmental Affairs Indiana University, Bloomington Bloomington Indiana USA.
authors: Kimberly A
authors: KA
authors: Novick
authors: Institute for Atmospheric and Climate Science ETH Zurich Zurich Switzerland.
authors: Sonia I
authors: SI
authors: Seneviratne
keywords: ecosystem fluxes; eddy covariance; interannual variability; short time scales