ENSO impacts on summer monsoon seasonal aggregate and daily rainfall characteristics in southern Africa

The rainy season in most of southern Africa (SAF), defined here as Africa subcontinent poleward of 10S, is associated with the summer monsoon regime, occurring in the region between October and March with seasonal peak during December-January-February (DJF) months. The quality of this season, particularly the peak monsoon (DJF), is of great importance as it rains little during the rest of the year over most SAF. In this study, the influence of El Nio-Southern Oscillation (ENSO) episodes, El Nio (EN) and La Nia (LN), on the summer monsoon circulation, rainfall and the frequency of extreme rainfall events over SAF is analyzed with seasonal and monthly resolution, using data from monthly GPCC products version 2022 and three different daily datasets (CPC version 1.0, CHIRPS version 2.0 and GPCP version 2022) for the 1982-2019 period. Extreme events are defined as 3-day mean precipitation above the 90th percentile. The mean frequencies of extreme events are determined for each month and season, and for each ENSO category episodes (EN, LN, and neutral), and the differences between EN and neutral years and LN and neutral years are computed through anomaly composites superimposed with either outgoing long-wave radiation (OLR) or eddy streamfunction at either low or upper levels, to understand the circulation mechanisms associated with ENSO episodes anomalies. Although some little differences exists, the pattern of significant signals in the frequency of extreme events is found to be well coherent with that of seasonal total rainfall anomalies in both the EN and LN episodes. The analysis also shows that some anomalies, which are consistent and important during part of the season, are smoothed out in a seasonal analysis. All daily datasets considered here show similar results, but that of CHIRPS seems to outperform better the well known and expected pattern, probably due to its high resolution. There are changes of anomalies within the summer monsoon season, with positive (negative) anomalies associated with EN episodes in spring (summer) over almost all SAF, associated with a low-level cyclonic (anticyclonic) anomalies to the north-eastern (southwestern) SAF, connected with a Rossby wave train generated in central south Pacific, which passing over south America/south atlantic ocean suffer from it less interference in comparison with the same wave in late summer. The opposite is almost true for LN events. To our knowledge, such results have never been reported for SAF elsewhere before, and those suggest the prevalence of likely regional processes over remote influences during part of the season. Similar results has been found early for South America. These results have important implications for seasonal predictions improvement, and rain dependent activities.