Drivers of multi-year droughts in large ensemble simulations

Multi-year droughts (MYDs) are severe natural hazards that have become more common due to climate change. Given their significant societal impact compared to normal droughts (ND) of shorter duration, it is crucial to better understand the drivers of MYDs. In this work we used a combination of a large-ensemble of climate models and reanalysis data to study the difference between MYDs and NDs. For six different climatic regions, chosen to be of similar size to the dominating regional atmospheric circulation patterns, we used reanalysis data of precipitation and potential evapotranspiration to show the regional characteristics and drivers of MYDs and contrast these with characteristics of NDs. Our findings reveal that MYD occurrence and duration varies significantly between the regions, where relatively larger differences in duration between MYD and NDs can indicate different drivers resulting in the different drought durations. Regions with distinctive seasonality in their precipitation climatology tend to experience faster drought onsets compared to regions with climatologically steady precipitation. Furthermore, our analysis shows that MYDs and NDs often start with similar conditions but diverge over time, and that longer-term memory is present in some regions, which might provide avenues for the predictability of MYDs. However, since MYDs are rare events (2 to 6 MYDs per region between 1950-2023 in this study), we supplement reanalyis data with that of CMIP6 climate models with a large number of ensembles to assess the drivers of MYDs with more statistical rigour. This creates the opportunity to study the contributions of oceans, soil moisture, snow, and other climate variables on the persistent circulation patterns leading to MYDs, and to find the influence of climate variability on the occurrence of MYDs.

DOI: http://dx.doi.org/10.2139/ssrn.4974995