Disentangling the influence of climate memory on multi-year droughts using model experiments

Multi-year droughts are severe natural hazards that have become more common due to climate change. Their longevity and resulting impact distinguish them from shorter seasonal or sub-annual drought events. This persistence in drought events suggests an important role for memory in the climate system but could also be caused by coincidental alignment of consecutive dry years. Since multi-year droughts are relatively rare and research has mostly focused on individual case studies, general multi-year drought drivers remain poorly understood. We identified different regional influences from the ocean, atmosphere and land surface correlating with multi-year droughts onset, with lags up to several months in prior to drought onset. Building on this, we here investigate how land-atmosphere and ocean-atmosphere interactions shape multi-year drought frequency, duration, and periodicity on annual to multi-decadal timescales. For this, we have set up a set of global climate model experiments performed with EC-Earth3, designed to selectively enable or suppress land-atmosphere and ocean-atmosphere coupling, both on global and regional scales. This allows us to directly assess the influences of ocean-atmosphere and land-atmosphere coupling, memory in the ocean and land, and the role of climate variability on different drought characteristics from annual to multi-decadal time scales. By comparing multi-year droughts to shorter drought events across these experiments, we can quantify the extent to which different interactions actively promote the occurrence of multi-year droughts. Our results will provide new insight into the influence of climate memory and variability on multi-year droughts and clarify the potential limits of predictability for multi-year droughts on regional and global scales.