DATASETS
The automatic adaptive signature generalization (AASG) algorithm overcomes many of the limitations associated with classification of multitemporal imagery. By locating stable sites between two images and using them to adapt class spectral signatures from a high-quality reference classification to a new image, AASG mitigates the impacts of radiometric and phenological differences between images and ensures that class definitions remain consistent between the two classifications. Here, I provide source code (in the R programming environment), as well as a comprehensive user guide, for the AASG algorithm. See Dannenberg, Hakkenberg and Song (2016) for details of the algorithm.
Moisture delivery to western North America is closely linked to variability in the westerly storm tracks of midlatitude cyclones, which are, in turn, modified by larger-scale features such as the El Niño-Southern Oscillation system. Instrumental and modeling data suggest that extratropical storm tracks may be intensifying and shifting poleward due to anthropogenic climate change, but it is difficult to separate recent trends from natural variability because of the large amount of decadal and longer variation in storm tracks and their limited instrumental record. We reconstruct cool-season, midlatitude Pacific storm-track position and intensity from 1693 to 1995 CE using existing tree-ring chronologies along with a network of newly developed chronologies from the U.S. Pacific Northwest, where small variations in storm-track position can have a major influence on hydroclimate patterns. Our results show high interannual-to-multidecadal variability in storm-track position and intensity over the past 303 years, with spectral signatures characteristic of tropical and northern Pacific influences. Comparison with reconstructions of precipitation and tropical sea surface temperature confirms the relationship between shifting drought patterns in the Pacific Northwest and storm-track variability through time and demonstrates the long-term influence of El Niño. These results allow us to place recent storm-track changes in the context of decadal and multidecadal fluctuations across the long-term record, showing that recent changes in storm-track intensity likely represent a warming-related increase amplified by natural decadal variability.
Changes in moisture delivery to western North America are largely controlled by interrelated, synoptic-scale atmospheric pressure patterns. Long-term records of upper-atmosphere pressure and related circulation patterns are needed to assess potential drivers of past severe droughts and evaluate how future climate changes may impact hydroclimatic systems. Here we develop a tree-ring-based climate field reconstruction of cool-season 500 hPa geopotential height on a 2 x 2 degree grid over North America and the North Pacific to AD 1500 and examine the frequency and persistence of preinstrumental atmospheric pressure patterns using Self-Organizing Maps. Our results show extended time periods dominated by a set of persistent upper-air pressure patterns, providing insight into the atmospheric conditions leading to periods of sustained drought and pluvial periods in the preinstrumental past. A striking shift from meridional to zonal flow occurred at the end of the Little Ice Age and was sustained for several decades.