Distribution, trends and drivers of flash droughts in the United Kingdom

Abstract. Flash droughts have been the subject of a great deal of scientific attention in the last decade, but the greatest emphasis has been on relatively dry climates. Here, we characterised the occurrence of this type of rapid-onset drought events in a more humid setting, the United Kingdom (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 in every season over the last five decades. However, the spatiotemporal distribution of flash droughts is highly variable in UK, with important regional and seasonal contrasts. Central and northern regions were generally the most frequently affected by flash droughts in comparison to southeastern region. Overall, there are non-significant trends in flash drought frequencies in winter, summer, and autumn. Nevertheless, we found a significant and notable increase in the number of flash droughts recorded in spring months. In the UK, flash drought occurrence responds primarily to precipitation variability in all seasons, and particularly in winter and autumn. In spring and summer, the atmospheric evaporative demand (AED) is important as a secondary driver for triggering flash droughts, especially in the drier regions of the southeastern UK. Moreover, our findings evidenced that this relevance is rising significantly in spring and summer in the southeast, over the study period. The atmospheric and oceanic conditions controlling these anomalies in precipitation and AED that drive flash droughts were also analyzed. Remarkable anomalies in sea level pressure and 500 hPa geopotential height associated with the presence of high-pressure systems were noted over UK during the development of the most severe flash droughts in all seasons. Likewise, flash drought development typically occurred under negative phase of North Atlantic Oscillation phase in winter and autumn, while in summer and spring positive phase is dominant. We also found positive anomalies in sea surface temperature during the development of flash droughts in spring and summer, while mixed anomalies were reported in winter and autumn. This study presents a detailed characterisation of flash drought phenomenon in UK, providing useful information for drought assessment and management, and a climatology of flash droughts that can be used as a baseline against which future changes in flash drought occurrence can be assessed.