Propagation of drought through changed atmospheric circulation induced by land surface energy flux anomalies
Droughts can propagate spatiotemporally, but the underlying mechanisms are not fully understood. Given that changes in moisture transport are critical in this process, previous studies have focused more on the reduced ‘moisture’ due to dry upwind land surface conditions. Here, we highlight the importance of ‘circulation’ and show that existing droughts can modulate atmospheric circulation to induce dry conditions in adjacent regions. We find this is an effective mechanism for explaining drought propagation, especially during neutral ENSO phases. Three numerical models with varying complexity are used in this study: the Linear Baroclinic Model (LBM), NeuralGCM, and Community Earth System Model version 2 (CESM2). Using the LBM, we find that imposing low-level heating, replicating drought-like conditions, can perturb the atmosphere and generate circulation anomalies. Further, we identify the drought propagation ‘hotspots’, where imposed drought-like anomalies have a stronger potential to perturb the atmosphere compared to other locations. These include the Great Plains in North America, southeastern South America, and west and northeast of the Tibetan Plateau. In North America, we conduct cases studies for the years 2012, 1988, and 1956 using all three models. By imposing drought-like anomalies only within a small domain over the identified hotspot, the models generally agreed on the location of next drying, located east of the forcing region, facilitating eastward propagation consistent with observed subsequent drying patterns. Strong upper-level winds overlapping with induced strong vertical motion, rather than horizontal temperature advection to balance the imposed heating, lead to strong vorticity advection in upper-level. This could explain why hotspots can easily propagate droughts compared to other regions. Thus, land surfaces that exhibit strong positive coupling between soil moisture-temperature, and that also have a strong potential to perturb the atmosphere through imposed heating, playing a key role in enabling drought propagation.
ZOOM:
https://illinois.zoom.us/j/82767407475?pwd=7ca6OfzVOPneyOwAULFkntXaYWaBvo.1
Meeting ID: 827 6740 7475
Pass Code: 809774