Flash droughts
Springtime subseasonal meteorological drought development in the central US
Droughts can develop swiftly on subseasonal time scales, also called flash droughts, with the presence of extreme atmospheric anomalies persisting for only a few days to weeks. Rapidly intensifying droughts can be hazardous to agriculture, particularly during springtime (April-June) in the central U.S. when both rainfall and crop development are at their peak. Recently, increasing studies have focused on case studies aimed at capturing different aspects of these springtime flash droughts. Here, we aggregated these studies (see Table 1 in Jong et al. 2022) and examined some recurring characteristics of how subseasonal springtime U.S. droughts evolve.
Jong, B.-T., M. Newman, and A. Hoell (2022): Subseasonal meteorological drought development over the central United States during spring. J. Clim., 35, 2525-2547. http://doi.org/10.1175/JCLI-D-21-0435.1
Fig. 1 Hovmoller diagrams for the composite persistent P-ET deficit events for the Ohio Valley region. (From Fig. 4 in Jong et al. 2022)
Fig. 2 Evolution of the composite persistent P-ET deficit events averaged over the Ohio Valley region. (From Fig. 12 in Jong et al. 2022)
We focused on "meteorological drought", that is, persistent atmospheric moisture deficits, defined as precipitation minus evapotranspiration (P-ET). We found that :
(1) P-ET deficits persisting for ~ a week can rapidly dry soil moisture, decreasing ~40 percentiles in ~10 days. The anomalously low soil moisture then lingers a month or more, especially in late spring. (Fig. 2)
(2) These P-ET deficits are initiated by a quasi-stationary Rossby wave packet (RWP), propagating from the western North Pacific, which arises about a week prior to drought onset. (Fig. 1)
(3) The RWP imposes a persistent ridge west of the incipient drought region and strong subsidence over it. This subsidence is associated with low-level divergent flow that dries the atmosphere and suppresses precipitation. (See Figs. 6&7 in Jong et al. 2022)
We concluded that during spring in the central U.S., rapid decline in soil moisture (that is, development of flash drought) is driven dynamically by atmospheric drying due to large-scale subsidence and the resulting low-level divergent outflow, rather than thermodynamically by drying due to anomalously warm near-surface temperatures. Please refer to our paper for more details.