Southern African rainfall regimes in a warmer world: Insight from PlioMIP2 mid-Pliocene Warm Period (3.264-3.025 Ma) simulations

The mid-Pliocene warm period (mPWP; 3.264-3.025 Ma) was characterised by near-modern geography and atmospheric carbon dioxide concentrations (~400 ppmv). With temperatures ~3°C warmer than the pre-Industrial era, it is often considered an analogue for near-future climate change. While many paleoenvironmental reconstructions suggest wetter conditions across southern Africa, projections for the mPWP and future suggest drier conditions associated with warmer climate states. Southern Africa’s mPWP proxy record is, however, limited and to date no detailed study exists for the region using Pliocene Model Intercomparison Project (PlioMIP) model outputs. Thus, to further understand southern African climate during the mPWP and to add constraints to future warmer world scenarios for the subcontinent, we use outputs from 17 models participating in the second PlioMIP phase to explore changes in annual and seasonal rainfall and associated atmospheric circulation patterns during the mPWP, compared to pre-Industrial period simulations. Although patterns of change are diverse, robust signals are evident. An annual rainfall decline, of approximately -0.5 mm/day, is quantified for most regions. Overall, drier conditions of a similar magnitude for the summer (October-March) and winter (April-September) periods contribute to this change, but the winter period shows more consistent drying. Relatively strong consistency across individual models for the pattern of intensification and expansion of the subtropical high-pressure belt suggests increased subsidence is a dominant driver behind the drying evident. General poleward tendencies of large-scale atmospheric circulation belts cause a poleward shift in the subcontinent's rainfall zones, resulting in a farther (reduced) southwestern (northeastern) extent of the summer (winter) rainfall zone (SRZ and WRZ). Projections reveal reduced seasonality for WRZ regions due to relatively stable summer rainfall and substantially reduced winter rainfall, while a relatively large early (late) summer rainfall decline (increase) for October-December (January-March) for SRZ regions re-organises the summer wet-season to be more concentrated in the late summer period.