Drought Relief and Reversal over North America from 1500 to 2016

Season-to-season persistence of soil moisture drought varies across North America. Such interseasonal autocorrelation can have modest skill in forecasting future conditions several months in advance. Because robust instrumental observations of precipitation span less than 100 years, the temporal stability of the relationship between seasonal moisture anomalies is uncertain. The North American Seasonal Precipitation Atlas (NASPA) is a gridded network of separately reconstructed cool-season (December-April) and warm-season (May-July) precipitation series and offers new insights on the intra-annual changes in drought for up to 2000 years. Here, the NASPA precipitation reconstructions are rescaled to represent the long-term soil moisture balance during the cool season and 3-month-long atmospheric moisture during the warm season. These rescaled seasonal reconstructions are then used to quantify the frequency, magnitude, and spatial extent of cool-season drought that was relieved or reversed during the following summer months. The adjusted seasonal reconstructions reproduce the general patterns of large-scale drought amelioration and termination in the instrumental record during the twentieth century and are used to estimate relief and reversals for the most skillfully reconstructed past 500 years. Subcontinental-to-continental-scale reversals of cool-season drought in the following warm season have been rare, but the reconstructions display periods prior to the instrumental data of increased reversal probabilities for the mid-Atlantic region and the U.S. Southwest. Drought relief at the continental scale may arise in part from macroscale ocean-atmosphere processes, whereas the smallerscale regional reversals may reflect land surface feedbacks and stochastic variability.

Automated summary

The season-to-season persistence of soil moisture drought varies across North America, with some skill in forecasting future conditions. The North American Seasonal Precipitation Atlas (NASPA) offers new insights on intra-annual changes in drought over up to 2000 years. The reconstructions show patterns of drought amelioration and termination, with rare subcontinental-to-continental-scale reversals in the following warm season.