The United States Seasonal Drought Atlas: Gridded Cool and Warm Season Moisture Balance Reconstructions from 1400 to 2016

The North American Drought Atlas (NADA) used the network of 1,936 climate-sensitive tree-ring chronologies from Canada, the United States, and Mexico to reconstruct the Palmer drought severity index (PDSI) at each of 11,396 grid points across the continent, providing a spatial and temporal history of drought and wetness for the past 2,000 years. Most North American tree-ring chronologies are correlated with the PDSI, which estimates the influence of monthly precipitation and temperature on the long-term soil moisture balance over a period of approximately 12 months. But some North American tree-ring chronologies also encode distinct seasonal climate signals and have been used to develop the new United States Seasonal Drought Atlas (USSDA), which reconstructs discrete cool (December-April) and warm season (May-July) moisture amounts over most of the country. All available tree-ring chronologies in and near the United States were first screened for “discrete” correlation with the atmospheric moisture balance for each season using Palmer’s “Z-index” to represent the moisture balance. Discrete correlation is used here to identify chronologies with a significant correlation with the Z-index at the closest grid point in one season, but not the other. The Z-index is an estimate of the short-term or atmospheric P-PE based on temperature and precipitation observations, without the prescribed monthly persistence intended to describe the slow accumulation or depletion of soil moisture. We use a new 0.5° gridded version of the Z-index for the contiguous United States (CONUS) based on the Penman-Monteith approximation of evapotranspiration and an improved and updated version of the NOAA Vose-Heim instrumental monthly precipitation and temperature dataset. Unlike the PDSI, monthly or seasonal averages of the Z-index do not tend to be strongly auto-correlated in time and can therefore provide a useful index of the discrete seasonal hydroclimate conditions that drive tree growth in a subset of North American tree-ring chronologies. The seasonal subsets of tree-ring chronologies with discrete Z-index signals were used to develop separate cool and warm season moisture balance reconstructions for the contiguous United States from 1400 to 2016, and extending back as early as 500 CE for portions of the western United States where long seasonally discrete tree-ring chronologies have been developed. These new moisture reconstructions provide insight into the seasonality of droughts and pluvials over the United States, including the persistent warm season droughts of the mid-17^th century and cool season droughts of the mid-19^th century over the Southeast. The cool season reconstructions faithfully reproduce the ENSO teleconnection to the United States detected in the instrumental Z-indices for the December-April season. The instrumental and reconstructed warm season Z-indices are more highly correlated with modes of ocean-atmospheric variability in the Atlantic. A web-based interface is being developed to facilitate the analysis and visualization of the new seasonal reconstructions.