Forward modelling of tree-ring width and comparison with a global network of tree-ring chronologies

Abstract

Abstract. We investigate relationships between climate and tree-ring data on a global scale using the process-based Vaganov–Shashkin Lite (VSL) forward model of tree-ring width formation. The VSL model requires as inputs only latitude, monthly mean temperature, and monthly accumulated precipitation. Hence, this simple, process-based model enables ring-width simulation at any location where monthly climate records exist. In this study, we analyse the growth response of simulated tree rings to monthly climate conditions obtained from the CRU TS3.1 data set back to 1901. Our key aims are (a) to assess the VSL model performance by examining the relations between simulated and observed growth at 2287 globally distributed sites, (b) indentify optimal growth parameters found during the model calibration, and (c) to evaluate the potential of the VSL model as an observation operator for data-assimilation-based reconstructions of climate from tree-ring width. The assessment of the growth-onset threshold temperature of approximately 4–6 °C for most sites and species using a Bayesian estimation approach complements other studies on the lower temperature limits where plant growth may be sustained. Our results suggest that the VSL model skilfully simulates site level tree-ring series in response to climate forcing for a wide range of environmental conditions and species. Spatial aggregation of the tree-ring chronologies to reduce non-climatic noise at the site level yielded notable improvements in the coherence between modelled and actual growth. The resulting distinct and coherent patterns of significant relationships between the aggregated and simulated series further demonstrate the VSL model's ability to skilfully capture the climatic signal contained in tree-ring series. Finally, we propose that the VSL model can be used as an observation operator in data assimilation approaches to reconstruct past climate.

A monthly global paleo-reanalysis of the atmosphere from 1600 to 2005 for studying past climatic variations

“…We used a multiple regression approach that is informed by VS-Lite. First, all TRW series were modelled with VS-lite 19 . From this, the relevant variables were identified as temperature during 4 months JJAS (in the northern hemisphere) and precipitation during the 3 months AMJ.…”

“…We use monthly resolved documentary temperature information for the Carpathian basin 50 , the Czech Republic 51 , Germany 52 , Poland 53 and Switzerland 54 . We use 35 climate sensitive TRW records 8,19 . MXD records mainly stem from a previous collection 55 .…”

Assimilation of pseudo-tree-ring-width observations into an atmospheric general circulation model

“…3 correspond to the optimistic value SNR = 10, with the exception of the last sensitivity study, which analyzes the dependency on observational noise levels. Regarding the geographical distribution of the observations, we place a pseudo-TRW chronology at every grid box where at least one actual TRW chronology from the database of Breitenmoser et al (2014) is present. This strategy yields a realistic observational network comprising 257 stations (see Fig.…”

An additional step toward comprehensive paleoclimate reanalyses

“…Consequently, current proxy system models still have clear limitations. In particular, some of them are well adapted to specific regions but are not able to simulate adequately the observed changes everywhere [e.g., Breitenmoser et al, 2014]. Applying them in some areas may then be worse than considering simpler and more robust statistical assumptions.…”