Flash droughts over the United Kingdom

Flash drought is a complex phenomenon distinguished by an unsual rapid development driven by severe precipitation deficits and/or anomalous increases in atmospheric evaporative demand (AED). While most research has focused on drier parts of the world, flash droughts can occur in temperate regions like the United Kingdom (UK). Historically most attention in the UK has focused on long, multiannual drought events driven by successive dry winters (e.g. 2004 – 2006). However, recent years have seen rapid onset flash droughts as part of exceptionally arid summers (e.g. 2018) that have had severe and widespread impacts on people and ecosystems alike. Here, we analysed the occurrence of this type of rapid-onset drought events in the UK for the period 1969-2021. Our results show that flash droughts affected both the wetter regions of north-west and the drier regions of south-east over the last five decades. Flash droughts frequency exhibit a high interannual variability, as well as a large spatial differences. Central and northern regions were the most frequently affected by flash droughts in comparison to southeastern region. Overall, positive trends were reported in eastern and northern regions, while negative and non-significant trends predominate over the western region. In UK, flash drought development responds primarily to precipitation variability, although AED is important as a secondary driver of flash drought triggering in the drier regions of southeastern England. Likewise, we found that flash droughts typically develop under remarkable positive anomalies in sea level pressure and 500 hPa geopotential height associated to the presence of high-pressure systems. This study presents a first detailed characterisation of flash drought in UK, providing useful information for drought assessment and management, and a baseline against which future changes in flash drought occurrence can be projected.