Abstract
Seasonality and interannual variability in North American photosynthetic activity reflect potential patterns of climate variability. We simulate 24 yr (1983–2006) and evaluate regional and seasonal contribution to annual mean gross primary productivity (GPP) as well as its interannual variability. The highest productivity occurs in Mexico, the southeast United States and the Pacific Northwest. Annual variability is largest in tropical Mexico, the desert Southwest and the Midwestern corridor. We find that no single region or season consistently determines continental annual GPP anomaly. GPP variability is dependent upon soil moisture availability in low- and mid-latitudes, and temperature in the north. Soilmoisture is a better predictor than precipitation as it integrates precipitation events temporally. The springtime anomaly is the most frequent seasonal contributor to the annual GPP variability. No climate mode (i.e. ENSO, NAM) can be associated with annual or seasonal variability over the entire continent. We define a region extending from the Northeast United States through the midwest and into the southwestern United States and northernMexico that explains a significant fraction of the variability in springtime GPP. We cannot correlate this region to a single mechanism (i.e. temperature, precipitation or soil moisture) or mode of climate variability.
Highlights
Global atmospheric CO2 concentrations have been increasing over the past 250 yr in response to anthropogenic sources in the form of human burning of fossil fuel and land cover change (Keeling et al, 1995)
The goal of this study is multiple: Is it possible to partition the continent into regions that are dominant in terms of explaining large-scale interannual gross primary productivity (GPP) variability? Are there patterns in temporal behaviour that we can identify? Can we describe regions in NA that have identifiable reliance on particular atmospheric drivers of GPP? For example, can we identify regions where an early spring is indicative of an annual increase in GPP, or regions where anomalously high midsummer precipitation results in a large positive excursion in annual GPP? we will attempt to correlate North American GPP variability to modes of climate variability such as Northern Annular Mode (NAM) and El-Nino/Southern Oscillation (ENSO)
The parameterization of photosynthetic carbon assimilation is based on enzyme kinetics originally developed by Farquhar et al (1980), that are linked to stomatal conductance and thence to the surface energy budget and atmospheric climate (Collatz et al, 1991, 1992; Sellers et al, 1996a; Randall et al, 1996)
Summary
Global atmospheric CO2 concentrations have been increasing over the past 250 yr in response to anthropogenic sources in the form of human burning of fossil fuel and land cover change (Keeling et al, 1995). The net increase in CO2 concentration represents the residual CO2 from the anthropogenic contribution and large exchange of CO2 between the atmosphere and surface (oceans and land). It has been shown that on an annual basis, only about 50% of the CO2 emitted by fossil fuel burning and land cover change resides in the atmosphere; about half is taken up by the oceans and terrestrial biosphere (Oeschger et al, 1975; Tans et al, 1990; IPCC, 2007).
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