Quantifying onset period rather than single date reveals projected intensification of dry-to-wet season transition.

Marcia T Zilli , Jerry B Samuel, Fran Morris, Neil C. G. Hart

Recent years have seen increasingly frequent delayed rainfall onset, especially in South America (SAm) and Southern Africa (SAfr). This arrival of the wet season is critical to water resources, ecosystems, and rain-fed agriculture. Existing rainfall-based approaches define the rainfall onset by a single date, limiting their ability to characterise the progression into the wet season. We introduce a novel method to identify the dry-to-wet transition period (DWTP) in rainfall time series, which better reflects this seasonal progression. The DWTP is defined as the interval between the first significant rainfall after the dry season and the establishment of sustained wet-season conditions. Identifying an onset period enables the quantification of key transition characteristics, including intensity, precipitation rate, dry day frequency, and duration. The method has the flexibility to be applied across regions, variable data resolutions, and different data sources.

Applied to observations, the method reproduces the observed spatial progression of the rainy season across both SAm and SAfr, with the first rains of the DWTP occurring 4-8 weeks before conventional rainfall accumulation onset dates. Our approach shows substantial regional variability in the intensity and duration of the transition into the wet season that is not captured by single-date metrics. The method captures widely reported onset delays in high-emission end-of-century scenarios, unambiguously showing later arrival of the first rains of the season across both regions. However, the method reveals these delayed first wet spells are likely to be followed by more intense and therefore abrupt establishment of wet seasons. These future changes are diagnosed in pioneering km-scale convection-permitting regional climate simulations over SAm and SAfr, with similar changes projected in the convection-parametrised driving global climate model and HadGEM3 CMIP6 projections. As risk of delayed rainfall onset grows, the DWTP framework may also have important potential applications in seasonal-to-subseasonal forecasting for early warnings.