A new atmospheric circulation tree-ring index (ACTI) derived from climate proxies: Procedure, results and applications

This paper introduces a new procedure to investigate the influence of weather types on spatio-temporal tree-ring growth patterns. Within the framework of the procedure a new proxy-based climate index termed as the atmospheric circulation tree-ring index (ACTI) is calculated that combines weather-type classification data with climate proxies. The new approach is tested using a Central European multi-species tree-ring width network consisting of 115 sites. The procedure used to calculate the ACTI time series includes: 1. Monte Carlo simulations, 2. multiple testing correction, and 3. selection and grouping algorithms. The tree-ring width series are grouped according to their weather-type responses, and the ACTI time series were computed for each group. The new procedure explains the climate signal preserved in tree-ring chronologies more comprehensively because the complex interaction between the different climate elements is taken into account by using weather-type classification data. Strong correlations between the ACTI and tree-ring width series (r = 0.69) and between the ACTI time series and climate gridded datasets (r > 0.6) were observed. Further a correlation between ACTI and northern hemisphere mean temperatures (r = 0.5) were found. These results indicate that the ACTI enables the investigation of weather-type growth relationships.