Congo Air Boundary Breakdown Timing and Its Connection to Southern African Rainfall Onset in Reanalyses and Historical CMIP6 Models

ABSTRACT Establishing the credibility in CMIP6 future projections of a delayed and drier southern African rainfall season requires evaluating how well their historical model simulations represent regional mechanisms of rainfall seasonality. Previous literature has suggested that rainfall season onset over the subcontinent is associated with the breakdown of the Congo Air Boundary (CAB), a near‐surface level convergence line separating the tropical rain belt from subsiding air over southern Africa. This study shows that the ERA5, ERA‐Interim and MERRA2 reanalyses have strong agreement over CAB climatology, CAB breakdown timing, and the relationship between CAB breakdown and rainfall onset timing. Using two novel metrics to quantify CAB breakdown timing, we find a clear statistically significant relationship showing CAB breakdown is associated with rainfall onset over southern Africa in the reanalyses at synoptic and climatological scales. Twenty‐seven CMIP6 model climatologies broadly reproduce the annual cycle of CAB length and frequency decreases during austral summer identified in reanalyses. Yet the models capture a much wider 56‐day range in the average CAB breakdown date across the two metrics relative to 11 days across the reanalyses. The models show the same association between CAB breakdown and rainfall onset timing seen in reanalyses: model climatologies with later breakdowns have later onsets ( r ‐value = 0.9). The strength of this association across CMIP6 ensemble members indicates that while intermodel variability of season timing can be high, individual models successfully simulate this core regional process linked to CAB breakdown. These results are important to future work considering projected delays to the southern African rainy season.