Separate tree-ring reconstructions of spring and summer moisture in the northern and southern Great Plains

Abstract

The two most severe droughts to impact the Great Plains in the twentieth century, the 1930s Dust Bowl and 1950s Drought, were the result of multiyear moisture deficits during the spring and especially the summer season. Tree-ring reconstructions of the Palmer Drought Severity Index indicate similar droughts in magnitude have occurred in previous centuries, but these reconstructions do not capture the potential distinct seasonal drought characteristics like those of the 1930s and 1950s. Separate tree-ring reconstructions of the spring and summer Z-index based on earlywood, latewood, and adjusted latewood width chronologies have been developed for two regions in the northern and southern Great Plains of the US. The reconstructions extend from 1651 to 1990 and 1698–1990, respectively, with instrumental data added from 1991 to 2017. The four reconstructions explain from 39 to 56% of the variance during the 1945–1990 calibration interval and are significantly correlated with independent moisture balance observations during the 1900–1944 validation period. The reconstructions reproduce similar seasonal sea-surface temperature and 500 mb geopotential height spatial correlation patterns detected with the instrumental data. The 1930s is estimated to have been the most extreme decadal summer drought to impact the two regions concurrently in the last few centuries. On average, spring moisture deficits were more severe during the multidecadal droughts of the mid- to late-nineteenth century, but the timing of drought onset and termination differed between the study regions. In the recent two decades the spring moisture balances for the two study regions have largely been opposite, and this has been one of the most extreme periods of anti-phasing in the last few centuries. Seasonal moisture reversals are not randomly distributed in time based on the reconstructed estimates and are related to sea-surface temperature anomalies in the tropical Pacific and to mid-tropospheric circulation changes over the North Pacific–North American sector during May and June.