Compound hydrometeorologic extremes in Northern Europe: a case study on flooding and drought in Denmark

Climate projections indicate a general increase in precipitation in Northern Europe accompanied by an increase in variability across a range of hydroclimate variables. In Denmark, increases in the amount of winter precipitation have resulted in several winter seasons (e.g., 2019-2020 and 2022-2023) with above-average precipitation. As a result, these periods have been marked by a vast extent of surface flooding due to the successive nature of precipitation events. Recent occurrences of drought (2018 and 2022) in Denmark, which have historically been less of a concern, have reduced water availability and resulted in impacts across sectors, with particularly large impacts on agricultural systems. The intra- and/or inter-annual occurrence of wintertime compound precipitation events and summertime flash drought events may emerge as an important constraint on agricultural productivity in Denmark. This analysis uses the Standardized Antecedent Precipitation-Evapotranspiration Index (SAPEI) to quantify periods of excess rainfall and precipitation deficits. The SAPEI can capture both periods of excess soil moisture (due to preconditioning by successive storms) and soil moisture deficits since it serves as a proxy for soil moisture. In the study, SAPEI is applied to observational records and climate projections to assess changes in the frequency of hydrometeorologic events, and to localize areas, which are specifically at risk to these extremes under current and future climates. Recent events are highlighted within the context of natural climate variability. These events are detailed to illuminate the impacts of compound wet and dry events and illustrate their importance as an important compound event in need of further research.