Attribution study of the 2023-2024 Drought on the South of Africa.

In 2023-2024 austral summer (Dec-Mar), an intense drought caused severe economical and human losses in the South of Africa, resulting in a loss of 1/3 of the total crop harvest. Here we report on a fairly standard attribution study for the drought of 2023/24 summer to assess if human influence increased the occurrence and intensity of droughts in the region. We used HadGEM-GA6 data to assess the likelihood of observing these events in scenarios with/without anthropogenic activity using 3 month Standardized Precipitation Evapotranspiration index (SPEI3) to quantify drought intensity. The sensitivity to region choice was explored using definitions of South of 20S, South of 15S, the region analyzed in the last World Weather Attribution report as well as individual countries. Simulations (with and without human activity) for the climatological period (1970-2010) as well as for 2023-2024 specifically were compared. The influence of the El Niño Southern Oscillation (ENSO) on SPEI3 and associated attribution statements was considered by compositing simulations by year into El Niño and La Niña phases. When using HadGEM simulations for the historical period (1970-2010), results showed that simulations with human activity showed lower SPEI values compared to natural simulations, hence implying that South African is drier compared to a natural scenario. Nonetheless the probability of drought is sensitive to the region chosen for the analysis, for example, for the south of 20S the probability of drought is mostly between 1.1 - 2 times more likely in simulations with human activity, whereas in the WWA area this probability rises to 5.9 - 16.9. By contrast, in HadGEM simulations with the prescribed conditions of 2023-2024, the probability of drought is much higher but also shows more uncertainty.

In addition, human activity strengthened the intensity and frequency of the dry periods set up by El Niño conditions in most countries located in the South of Africa, but the occurrence of droughts changes with the region. For example, in Zimbabwe, drought occurrence is 1.8 more likely in simulations with human influence during El Niño events, whereas in South Africa and Zambia the drought occurrence is 1.6 and 3.2 times more likely respectively whereas in Malawi it remains unchanged. In addition, when considering the prescribed conditions of 2023-2024 the probability of drought rises drastically for all countries.