Spring-Time Zonal Advection of Dry Air in the Low-Level Free Troposphere: A Major Drought Onset Mechanism over the US Great Plains

The North American Great Plains (GP) have been hit by numerous intense droughts over the
past few decades and are projected to observe intensified dry conditions under different
climate change scenarios. Summer droughts are socioeconomically costly and there is a critical
need for reliable seasonal and sub-seasonal predictions in the region. However, dynamic
prediction models reveal virtually zero skill in GP summer drought prediction and the drought
onset mechanisms are not yet fully understood. Here, we study the onset of summer droughts
over the GP from a moisture budget perspective using both observation and reanalysis data
sets. Based on our composite analysis, we have identified a severe advection of dry moisture
tendency in the low-level free troposphere in spring as the most distinctive difference between
the climatological moisture budget terms and those of the dry composite. The anomalous dry
advection in spring sets the stage for summer droughts through a sharp drop of relative
humidity right above the planetary boundary layer, suppressing deep convection via
entrainment of dry air and reducing spring and early-summer precipitation. Further breakdown
of the moisture advection into zonal and meridional terms, as well as dynamic (caused by
circulation anomalies) versus thermodynamic (caused by moisture anomalies) contributions,
reveals that the dry advection preceding the summer droughts is entirely attributed to the
zonal tendency term and exclusively dominated by the thermodynamic contribution. The
thermodynamic nature itself is governed by the zonal gradient of specific humidity at the low-
level troposphere, and the spring-time dry conditions over the upwind regions (The Rockies and
western United States) in particular. Using a multivariate linear regression model, we show that
anomalies of the zonal thermodynamic term of moisture advection in spring alone can explain
up to 40% of summer precipitation variability in the US central plains. We also present our
analysis on the large-scale atmospheric and oceanic drivers of the anomalous moisture
conditions that initiate the summer droughts over GP. The results of this study can provide
useful information to further our understanding of drought onset mechanisms and to improve
summer drought prediction over the GP.