Low-frequency variability of precipitation in the North American monsoon region as diagnosed through earlywood and latewood tree-ring chronologies in the southwestern US

Recent studies have pointed out the statistical occurrence of dual-season droughts detected in tree-ring chronologies over the southwestern US region that is not well described by instrumental observed records of the 20th century. In this study, a multi-statistical approach that evaluates persistent dual-season drought using a mode-of-variability oriented approach is proposed, considering a new network of tree-ring earlywood (EW)- and latewood-adjusted (LW_adj) chronologies from throughout southwestern North America. To determine dominant patterns of spatiotemporal variability, empirical, orthogonal functions, canonical correlation analysis, and multi-taper-method singular value decomposition analyses were applied, with focus on variability from inter-annual to centennial periods and highlighting the multi-decadal signals inherent to proxy record network. During the instrumental period, we demonstrate that EW and LW_adj networks of tree-ring chronologies are able to capture the associated precipitation responses of cool and warm season atmospheric teleconnections. Considering the four-century period of the complete tree-ring network, we explore the possibility of a dual summer-winter variability signal in the low-frequency climate regime. EW and LW_adj seem to be coherent in-phase at the very low-frequency scale (50-100 years spectral band). This provocative result is supported by major historic documented multi-year droughts of the region since 1650. Thus, the temporal variation of these chronologies time series and its associated spatial pattern strongly suggest that this low-frequency mode might represents an important spatiotemporal variation of droughts in the Southwest; however, the source of this signal is still an open question and of great interest for drought planning and resource management in the region.