Understanding Drought Propagation in the US Midwest through Low-Level Heating Anomalies

This work explores the large-scale land-atmosphere mechanisms driving drought propagation in the US Midwest during the late spring and early summer months (April–May–June), when surface drying critically affects vegetation growth. To do this, we use a Linear Baroclinic Model (LBM) forced by low-level heating anomalies across various locations in North America to replicate the effect of surface drying on the atmosphere. The strongest signals emerge when these forcings are placed near the Rocky Mountains. Significant high geopotential height anomalies typically develop in the upper atmosphere northeast of the forcing location. Associated horizontal wind anomalies can induce divergent moisture flows, leading to reduced precipitation in adjacent areas and facilitating the expansion of drought conditions. This LBM-based drought propagation mechanism provides a qualitative explanation for drought propagation observed during two severe droughts in the Midwest in 1988 and 2012. In both cases, when low-level heating anomalies are present along with surface dry conditions, the model represents the eastward and northeastward expansion of surface dryness, consistent with the direction of the observed drought propagation. However, in the absence of heating anomalies, the drought propagation did not follow the patterns predicted by the model. Therefore, increased predictability of drought propagation is expected when low-level heating anomalies associated with dry surfaces are correctly represented in models.