Future changes in the characteristics of the dry-to-wet transition period in the monsoonal regions of Southern Africa and South America

Successive CMIP model generations have indicated a future delay in the onset of the rainy season in some monsoonal regions worldwide, driven mostly by the reduction in the onset phase precipitation. These projections are in agreement with the observed drying trend in these regions, coupled with an increased likelihood of recurring drought-like conditions resulting from rising temperatures. Here, we use a novel methodology to characterise the present-day and future rainy season onset in monsoonal regions over Southern Africa (SAfr) and South America (SAm). The Dry-to-Wet Transition Period (DWTP) expands the current use single date onset methods to consider a period, incorporating more information about the transition, such as duration, precipitation intensity, and dry spells. The DWTP starts with the first significant rains of the season and ends when the rain becomes regular and sustained. The DWTP starts in the southeastern and northwestern SAfr regions between August and September and progresses towards central SAfr by mid-October. Over SAm, the DWTP starts in late August in the western Amazon progressing eastward to reach eastern Brazil in late October. In both regions, the onset date defined using established methodologies occurs within the DWTP. Future projections, based on global parameterised and regional convection-permitting simulations, confirm a delay in the DWTP of about 20 days over SAfr and 20-30 days over SAm. Future scenarios project a later start of the rains in both monsoon areas, resulting in a shorter DWTP. Over SAfr, the DWTP will see more dry days over the Congo basin while over eastern SAfr, the fraction of dry days will increase, resulting in a more abrupt start of the rainy season. Over SAm, the DWTP is projected to have lower rain rates and more dry days over the Amazon, resulting in a shorter but more abrupt transition into the rainy season. These results exemplify the advantages of using a period to better characterise the transition into the rainy season and identify observed and future trends in its characteristics. It provides a novel framework to better quantifying the diverse response to global warming that can modulate regional hydrological cycles and water availability. The methodology can be further expanded to account for different variables, such as temperature and soil moisture, and can be easily implemented in the seasonal forecast system as a tool to improve the overlook into the dry-to-wet transition periods.