Drought self-propagation in drylands through moisture recycling

Soil dryness modulates the surface energy balance through a reduction in evaporation, and can in turn affect both local and downwind precipitation. But when evaporation is heavily constrained by soil moisture, there is also a reduced local water vapor supply to the atmosphere, manifesting as downwind moisture deficits. Soil moisture–precipitation feedbacks as a whole — including surface heating-induced boundary layer processes and interactions, as well as changes in tropospheric moistening — have already been extensively investigated, particularly at the local scale. However, little is known about the non-local impact of soil moisture on precipitation. Here, we focus on the impact of water vapor reductions instigated by already existing soil drought, estimate the downwind effect on precipitation and thus gauge the potential for drought self-propagation. A Lagrangian approach constrained by observational and reanalysis data is employed to reveal the origins of water vapor, establishing a causal link between upwind evaporation and downwind rainfall. We assess the self-propagation of the 40 largest soil drought events from 1980 to 2016, obtained with a novel mathematical morphology method. Specifically, we estimate the reduction in precipitation caused by drought-stricken areas in the direction of drought propagation, and isolate the effect of upwind soil moisture drought from the influence of potential evaporation and circulation variability. Our results show that droughts self-propagate in subtropical drylands, owing to the strong decline in evaporation in response to soil water stress. For entire events, the reduction in precipitation along the propagation front can be more than 15%, and up to 30% for individual months. Our findings highlight that terrestrial ecosystems reliant on their own evaporation supplying  rainfall are most affected, and underline the susceptibility of arid environments to self-inflicted drought expansion.