Flash Drought Occurrence in the Coastal Carolinas Could Increase in the Future.

Flash droughts have a high frequency of occurrence in the Southeastern United States. Historically, the Carolinas are not always included in regional studies of the Southeastern U.S. despite their shared vulnerability to the impacts of these droughts. For example, mariculture, or marine farming, is one economic sector in the Carolinas that is especially vulnerable to flash droughts. In recent years, the oyster farming community in the coastal regions of the Carolinas has experienced an increase in the number of “mass mortality events”, and the potential contribution of droughts to these events is not well known. This study will analyze drought as a potential factor in these mass mortality events through collaboration with North Carolina oyster farmers. We utilize historical drought data from 1980-2018 to characterize drought by its intensity and duration, and then compare the spatial distribution and temporal trends of flash drought and persistent drought across the region. Generally, flash droughts are defined as severe droughts lasting around 1-3 months, whereas persistent droughts could last months to over a year in duration. Analyzing both time scales allows patterns of the drought types to emerge and enables us to investigate the different impacts of both droughts types. When looking across the Carolinas, the importance of separating drought out by timescale becomes apparent, as the mountain region has the highest occurrence of persistent droughts, and the Piedmont region has the highest occurrence of flash droughts. The coastal plain is affected by both types of drought, and at the highest severity of both drought types, it is the predominant location of occurrence. Along the coast of the Carolinas, the number of flash drought days from 1980-2018 has shown no change or a slight decrease, whereas the number of persistent drought days has decreased. Over the past 39 years, the probability of flash drought events has increased in the coastal region, more so in South Carolina than North Carolina, but the probability of occurrence for persistent drought days has decreased across the entire coastal region. These findings indicate that over the past 39 years the Carolina coasts have witnessed shorter, more severe droughts, leading to a potentially greater impact on oyster farmers in the area through mass mortality events.